Introduction to environmental engineering 0132347474, 9780132347471
InIntroduction to Environmental Engineering, First Edition,authors Richard Mines and Laura Lackey explain complicated en
4,024 352 4MB
English Pages xv-364 p.: ill.; 26 cm [381] Year 2009
Polecaj historie
Table of contents :
Cover......Page 1
Contents......Page 6
About This Book......Page 15
1.2 What’S Your Definition Of Environmental Engineering?......Page 18
1.3.2 A Lousy Report Card......Page 19
1.4 Duties And Important Characteristics Of Environmental Engineers......Page 20
1.4.4 Government......Page 21
1.5.1 Love Canal......Page 22
1.5.2 Milwaukee Cryptosporidiumoutbreak......Page 23
1.6.2 Professional Licensure......Page 24
1.6.3 Engineering Ethics......Page 25
1.6.4 Continuing Education......Page 26
1.7 Problem Solving......Page 27
Summary......Page 29
Exercises......Page 30
References......Page 31
2.2 Dimensions And Units......Page 32
2.3.1 Density......Page 35
2.3.2 Concentration......Page 37
2.3.3 Flow Rate......Page 43
2.3.4 Residence Time......Page 44
2.4 Precision, Bias, And Accuracy......Page 45
2.5 Significant Figures......Page 46
Exercises......Page 47
3.2.2 Elements......Page 49
3.3 Chemical Reactions......Page 54
3.3.1 Stoichiometry......Page 55
3.4.1 Acid-Base Chemistry......Page 59
3.4.2 Strong Acids And Bases Vs.Weak Acids And Bases......Page 62
3.4.3 The Carbonate System And Alkalinity......Page 64
3.5 Solid-Phase Equilibrium Reactions......Page 65
3.6.2 General Gas Laws—Boyle’S, Charles’, Gay-Lussac, Combined Gas......Page 68
3.6.3 Dalton’S Law Of Partial Pressure......Page 70
3.6.4 Raoult’S Law And Henry’S Law......Page 71
References......Page 73
Exercises......Page 74
4.2 Biological Systems......Page 76
4.2.2 Classification Of Organisms......Page 77
4.2.3 Major Groups Of Organisms......Page 78
4.2.4 Microbial Growth......Page 83
4.3 Ecological Systems......Page 86
4.3.1 Energy Flow In Ecosystems......Page 87
4.3.2 Food Chains......Page 88
4.3.3 Bioconcentration, Bioaccumulation, And Toxicity......Page 91
4.4.1 Carbon Cycle......Page 92
4.4.2 Nitrogen Cycle......Page 93
4.4.3 Phosphorus Cycle......Page 96
4.4.4 Sulfur Cycle......Page 97
4.5 Limnological Concepts And Eutrophication......Page 98
4.5.1 Stratification......Page 99
4.5.2 Lake Classification......Page 101
4.5.3 Dissolved-Oxygen Depletion In Streams......Page 102
References......Page 110
Exercises......Page 111
5.1 Concept Or Perception Of Risk......Page 115
5.2 Risk Assessment......Page 118
5.2.2 Toxicity Assessment......Page 119
5.2.3 Exposure Assessment......Page 124
5.2.4 Risk Characterization......Page 127
5.3 Risk Management......Page 129
5.4.2 Environmental Impact Statement......Page 130
Summary......Page 133
Exercises......Page 134
6.2 Chemical And Biochemical Reactions......Page 137
6.2.2 Rate Law And Order Of Reaction......Page 138
6.2.3 Zero-Order Reactions......Page 139
6.2.4 First-Order Reactions......Page 140
6.2.5 Second-Order Reactions......Page 142
6.2.6 Temperature Corrections......Page 145
6.3 Material Balances......Page 147
6.4.2 Reactors......Page 151
6.5.1 Definition Of Energy And Work......Page 160
Exercises......Page 167
7.1 Introduction......Page 171
7.2.2 Green Engineering......Page 172
7.2.3 Material Selection......Page 173
7.3 Nuclear Physics......Page 175
7.3.1 Radioactivity......Page 176
7.4.2 Paper Or Plastic Bags?......Page 182
7.4.3 Selection Of Materials For Beverage Containers......Page 183
7.4.5 Advanced Integrated Wastewater Pond System (Aiwps)......Page 184
References......Page 186
Exercises......Page 188
8.2 Beneficial Uses Of Water......Page 189
8.2.1 Public Supply......Page 190
8.2.6 Industrial......Page 191
8.4 Water Pollution......Page 192
8.5.1 Conventional Water Quality Assessment Parameters......Page 194
8.6.1 Drinking Water Standards......Page 211
8.6.2 Wastewater Effluent Standards......Page 212
8.6.3 Surface Water Quality Standards......Page 213
Summary......Page 214
References......Page 215
Exercises......Page 216
9.1 Introduction......Page 218
9.2.2 Drinking Water Standards......Page 219
9.3.1 Conventional Surface Water Treatment......Page 220
9.4 Overview Of Groundwater Treatment Systems......Page 222
9.4.2 Reverse Osmosis Treatment......Page 223
9.5.1 Coagulation And Flocculation......Page 224
9.5.2 Mixing......Page 227
9.5.3 Flocculation Tanks......Page 230
9.5.4 Water Softening......Page 232
9.5.5 Sedimentation......Page 237
9.5.6 Filtration......Page 243
9.5.7 Disinfection......Page 250
9.6 Treatment Of Water Treatment Plant Residuals......Page 255
References......Page 256
Exercises......Page 257
10.1 Introduction......Page 260
10.2.1 Secondary Wastewater Treatment......Page 262
10.3 Overview Of Wastewater Treatment Systems......Page 263
10.4 Preliminary Treatment......Page 264
10.4.2 Grit Removal......Page 265
10.5 Primary Treatment......Page 266
10.6 Secondary Treatment......Page 269
10.6.1 Activated Sludge......Page 270
10.6.2 Aerator Systems......Page 275
10.6.3 Trickling Filters......Page 277
10.7 Secondary Clarification......Page 279
10.8.3 Chlorine Contact Basin......Page 281
10.9.1 Sludge Weight And Volume Relationships......Page 283
10.9.2 Thickening Operations......Page 284
10.9.3 Stabilization......Page 286
Summary......Page 292
References......Page 293
Exercises......Page 294
11.1 Introduction......Page 296
11.2 History......Page 297
11.3 Regulatory Overview......Page 298
11.4.1 Particulates......Page 299
11.4.2 Nitrogen Oxides (Nox)......Page 300
11.4.4 Sulfur Oxides (Sox)......Page 301
11.4.6 Ozone (O3)......Page 302
11.4.8 The Greenhouse Effect And Global Climate Change......Page 303
11.5 Control Of Particulate Matter From Stationary Sources......Page 306
11.5.1 Mechanical Separators—Settling Chambers......Page 307
11.5.2 Electrostatic Precipitators (Esps)......Page 310
11.6.1 Incineration......Page 314
11.6.2 Absorption—Packed-Bed Scrubbers And Flue Gas Desulfurization......Page 315
References......Page 319
Exercises......Page 320
12.1.1 Highlights In Hazardous Waste History......Page 322
12.2.1 Contaminants Of Concern......Page 326
12.2.2 Contaminant Characteristics......Page 332
12.2.3 Darcy’S Law......Page 337
12.3.2 Environmental Factors Affecting Microbial Metabolism......Page 340
12.4.1 In Situremediation Schemes......Page 341
12.4.2 Ex Situremediation Schemes – Composting And Landfarming......Page 345
References......Page 348
Exercises......Page 349
13.1 Introduction......Page 350
13.2.1 Rcra Solid Waste Definition......Page 352
13.2.2 Rcra Hazardous Waste Definition......Page 353
13.3 Waste Generation (Quantifying Msw Generation)......Page 354
13.4.1 Types Of Collection Systems......Page 357
13.4.3 Problems And Concerns......Page 358
13.5 Landfill Containment And Monitoring Systems......Page 360
References......Page 362
Exercises......Page 363
A......Page 366
B......Page 367
C......Page 368
D......Page 369
E......Page 370
G......Page 371
H......Page 372
M......Page 373
N......Page 374
P......Page 375
R......Page 376
S......Page 377
U......Page 379
W......Page 380
Z......Page 381
Citation preview
Introduction to Environmental Engineering
This page intentionally left blank
Introduction to Environmental Engineering
RICHARD O. MINES, Jr., Ph.D., P.E. Mercer University
LAURA W. LACKEY, Ph.D., P.E. Mercer University
Prentice Hall New York • Boston • San Francisco • London • Toronto Sydney • Tokyo • Singapore • Madrid • Mexico City Munich • Paris • Cape Town • Hong Kong • Montreal
CIP information can be obtained by contacting the Library of Congress
Editorial Director, ECS: Marcia Horton Senior Editor: Holly Stark Associate Editor: Dee Bernhard Editorial Assistant: William Opaluch Managing Editor: Scott Disanno Production Editor: Clare Romeo Art Director: Jayne Conte Cover Designer: Bruce Kenselaar Art Editor: Greg Dulles Manufacturing Manager: Alan Fischer Manufacturing Buyer: Lisa McDowell Marketing Manager: Tim Galligan
© 2009 Pearson Education, Inc. Upper Saddle River, New Jersey 07458. All rights reserved. Printed in the United States of America. This publication is protected by copyright and permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, 1 Lake Street, Upper Saddle River, NJ 07458. The author and publisher of this book have used their best efforts in preparing this book. These efforts include the development, research, and testing of theories and programs to determine their effectiveness. The author and publisher make no warranty of any kind, expressed or implied, with regard to these programs or the documentation contained in this book. The author and publisher shall not be liable in any event for incidental or consequential damages with, or arising out of, the furnishing, performance, or use of these programs. Pearson Education Ltd., London Pearson Education Singapore, Pte. Ltd Pearson Education Canada, Inc. Pearson Education—Japan Pearson Education Australia PTY, Limited Pearson Education North Asia, Ltd., Hong Kong Pearson Educación de Mexico, S.A. de C.V. Pearson Education Malaysia, Pte. Ltd. Pearson Education, Inc., Upper Saddle River, New Jersey
10 9 8 7 6 5 4 3 2 1 ISBN 10: 0-13-234747-4 ISBN 13: 978-0-13-234747-1
Contents
ABOUT THIS BOOK 1 • ENVIRONMENTAL ENGINEERING AS A PROFESSION 1.1 Welcome 1 1.2 What’s Your Definition of Environmental Engineering? 1 1.3 Environmental Engineers Make a Difference 2 1.3.1 Jobs in Demand 2 1.3.2 A Lousy Report Card 2 1.3.3 Basic Necessities—New Challenges 3 1.4 Duties and Important Characteristics of Environmental Engineers 3 1.4.1 Multidisciplinary Teams 4 1.4.2 Work Outdoors 4 1.4.3 Consulting Firms 4 1.4.4 Government 4 1.4.5 Regulatory Agencies 5 1.4.6 Industry 5 1.4.7 Academia 5 1.4.8 Does Environmental Engineering Match My Interests? 5 1.5 Environmental Calamities 5 1.5.1 Love Canal 5 1.5.2 Milwaukee Cryptosporidium Outbreak 6 1.5.3 Chernobyl Nuclear Disaster 7 1.6 Becoming an Environmental Engineer: A Lifelong Process 7 1.6.1 College Education 7 1.6.2 Professional Licensure 7 1.6.3 Engineering Ethics 8 1.6.4 Continuing Education 9 1.7 Problem Solving 10 1.8 Environmental Management 12
xiv 1
v
vi
Contents
Summary Key Words Exercises References
12 13 13 14
2 • INTRODUCTION TO ENVIRONMENTAL ENGINEERING CALCULATIONS: DIMENSIONS, UNITS, AND CONVERSIONS
15
2.1 Introduction 15 2.2 Dimensions and Units 15 2.3 Essential Derived Units and Conversion Practice 18 2.3.1 Density 18 2.3.2 Concentration 20 2.3.3 Flow rate 26 2.3.4 Residence time 27 2.4 Precision, Bias, and Accuracy 28 2.5 Significant Figures 29 Summary 30 Key Words 30 References 30 Exercises 30
3 • ESSENTIAL CHEMICAL CONCEPTS
32
3.1 Introduction 32 3.2 Fundamentals 32 3.2.1 Atoms 32 3.2.2 Elements 32 3.3 Chemical Reactions 37 3.3.1 Stoichiometry 38 3.4 Solution Chemistry—Aqueous Phase 42 3.4.1 Acid-Base Chemistry 42 3.4.2 Strong Acids and Bases vs. Weak Acids and Bases 45 3.4.3 The Carbonate System and Alkalinity 47 3.5 Solid-Phase Equilibrium Reactions 48 3.6 Solution Chemistry—Gas Phase 51 3.6.1 Ideal Gas Law 51 3.6.2 General Gas Laws—Boyle’s, Charles’, Gay-Lussac, Combined Gas 51 3.6.3 Dalton’s Law of Partial Pressure 53 3.6.4 Raoult’s Law and Henry’s Law 54 Summary 56 Key Words 56 References 56 Bibliography 57 Exercises 57
4 • BIOLOGICAL AND ECOLOGICAL CONCEPTS 4.1 Introduction 59 4.2 Biological Systems 59
59
Contents
4.2.1 Cell Structure 60 4.2.2 Classification of Organisms 60 4.2.3 Major Groups of Organisms 61 4.2.4 Microbial Growth 66 4.3 Ecological Systems 69 4.3.1 Energy Flow in Ecosystems 70 4.3.2 Food Chains 71 4.3.3 Bioconcentration, Bioaccumulation, and Toxicity 74 4.4 Nutrient Cycles 75 4.4.1 Carbon Cycle 75 4.4.2 Nitrogen Cycle 76 4.4.3 Phosphorus Cycle 79 4.4.4 Sulfur Cycle 80 4.5 Limnological Concepts and Eutrophication 81 4.5.1 Stratification 82 4.5.2 Lake Classification 84 4.5.3 Dissolved-Oxygen Depletion in Streams 85 Summary 93 Key Words 93 References 93 Exercises 94
5 • RISK ASSESSMENT
98
5.1 Concept or Perception of Risk 98 5.2 Risk Assessment 101 5.2.1 Data Collection and Evaluation 102 5.2.2 Toxicity Assessment 102 5.2.3 Exposure Assessment 107 5.2.4 Risk Characterization 110 5.3 Risk Management 112 5.4 Environmental Impact Analysis 113 5.4.1 Overview of Environmental Impact Statement 113 5.4.2 Environmental Impact Statement 113 Summary 116 Key Words 117 References 117 Exercises 117
6 • DESIGN AND MODELING OF ENVIRONMENTAL SYSTEMS 6.1 Introduction 120 6.2 Chemical and Biochemical Reactions 120 6.2.1 Rates of Reaction 121 6.2.2 Rate Law and Order of Reaction 121 6.2.3 Zero-Order Reactions 122 6.2.4 First-Order Reactions 123 6.2.5 Second-Order Reactions 125 6.2.6 Temperature Corrections 128 6.3 Material Balances 130
120
vii
viii Contents
6.4 Flow Regimes and Reactors 134 6.4.1 Flow Regimes 134 6.4.2 Reactors 134 6.5 Energy Balances 143 6.5.1 Definition of Energy and Work 143 Summary 150 Key Words 150 References 150 Exercises 150
7 • SUSTAINABILITY AND GREEN DEVELOPMENT
154
7.1 Introduction 154 7.2 Sustainable Development and Green Engineering 155 7.2.1 Sustainable Development 155 7.2.2 Green Engineering 155 7.2.3 Material Selection 156 7.3 Nuclear Physics 158 7.3.1 Radioactivity 159 7.4 Case Histories 165 7.4.1 New Chairs from Haworth and Steelcase 165 7.4.2 Paper or Plastic Bags? 165 7.4.3 Selection of Materials for Beverage Containers 166 7.4.4 Coal versus Nuclear Energy 167 7.4.5 Advanced Integrated Wastewater Pond System (AIWPS) 167 Summary 169 Key Words 169 References 169 Exercises 171
8 • WATER QUALITY AND POLLUTION 8.1 Importance of Water 172 8.2 Beneficial Uses of Water 172 8.2.1 Public Supply 173 8.2.2 Domestic 174 8.2.3 Irrigation 174 8.2.4 Livestock 174 8.2.5 Aquaculture 174 8.2.6 Industrial 174 8.2.7 Mining 175 8.2.8 Thermoelectric Power 175 8.3 Hydrologic Cycle 175 8.4 Water Pollution 175 8.5 Water Quality Parameters 177 8.5.1 Conventional Water Quality Assessment Parameters 177 8.6 Water Quality Standards 194 8.6.1 Drinking Water Standards 194 8.6.2 Wastewater Effluent Standards 195 8.6.3 Surface Water Quality Standards 196
172
Contents
8.7 Kepone Contamination of the James River 197 Summary 197 Key Words 198 References 198 Exercises 199
9 • WATER TREATMENT
201
9.1 Introduction 201 9.2 General Considerations for Selecting Technology 202 9.2.1 Water Source and Quality 202 9.2.2 Drinking Water Standards 202 9.2.3 Microconstituents 203 9.3 Overview of Surface Water Treatment Systems 203 9.3.1 Conventional Surface Water Treatment 203 9.3.2 Membrane Treatment 205 9.4 Overview of Groundwater Treatment Systems 205 9.4.1 Conventional Lime-Soda Ash Treatment 206 9.4.2 Reverse Osmosis Treatment 206 9.5 Surface Water Treatment Processes 207 9.5.1 Coagulation and Flocculation 207 9.5.2 Mixing 210 9.5.3 Flocculation Tanks 213 9.5.4 Water Softening 215 9.5.5 Sedimentation 220 9.5.6 Filtration 226 9.5.7 Disinfection 233 9.6 Treatment of Water Treatment Plant Residuals 238 Summary 239 Key Words 239 References 239 Exercises 240
10 • DOMESTIC WASTEWATER TREATMENT 10.1 Introduction 243 10.2 Wastewater Treatment Categorization 245 10.2.1 Secondary Wastewater Treatment 245 10.2.2 Advanced Wastewater Treatment (AWT) 246 10.3 Overview of Wastewater Treatment Systems 246 10.4 Preliminary Treatment 247 10.4.1 Screening 248 10.4.2 Grit Removal 248 10.5 Primary Treatment 249 10.6 Secondary Treatment 252 10.6.1 Activated Sludge 253 10.6.2 Aerator Systems 258 10.6.3 Trickling Filters 260 10.7 Secondary Clarification 262 10.8 Disinfection of Wastewater 264
243
ix
x
Contents
10.8.1 Disinfectants Used in Wastewater Treatment 264 10.8.2 Chlorination of Wastewater 264 10.8.3 Chlorine Contact Basin 264 10.9 Sludge Treatment and Disposal 266 10.9.1 Sludge Weight and Volume Relationships 266 10.9.2 Thickening Operations 267 10.9.3 Stabilization 269 10.9.4 Dewatering 275 10.9.5 Sludge Disposal 275 Summary 275 Key Words 276 References 276 Exercises 277
11 • AIR POLLUTION
279
11.1 11.2 11.3 11.4
Introduction 279 History 280 Regulatory Overview 281 Sources and Effects 282 11.4.1 Particulates 282 11.4.2 Nitrogen Oxides (NOx) 283 11.4.3 Carbon Monoxide (CO) 284 11.4.4 Sulfur Oxides (SOx) 284 11.4.5 Lead 285 11.4.6 Ozone (O3) 285 11.4.7 Volatile Organic Compounds (VOCs) 286 11.4.8 The Greenhouse Effect and Global Climate Change 286 11.5 Control of Particulate Matter From Stationary Sources 289 11.5.1 Mechanical Separators—Settling Chambers 290 11.5.2 Electrostatic Precipitators (ESPs) 293 11.6 Gas and Vapor Control Technology 297 11.6.1 Incineration 297 11.6.2 Absorption—Packed-Bed Scrubbers and Flue Gas Desulfurization 298 Summary 302 Key Words 302 References 302 Exercises 303
12 • FUNDAMENTALS OF HAZARDOUS WASTE SITE REMEDIATION 305 12.1 The Problem 305 12.1.1 Highlights in Hazardous Waste History 305 12.2 Contaminant Characteristics and Phase Distribution 309 12.2.1 Contaminants of Concern 309 12.2.2 Contaminant Characteristics 315 12.2.3 Darcy’s Law 320 12.3 Overview of Microbial Processes 323 12.3.1 An Overview of Bacteria 323 12.3.2 Environmental Factors Affecting Microbial Metabolism 323
Contents
12.4 Introduction to Engineered Remediation Processes 324 12.4.1 In situ Remediation Schemes 324 12.4.2 Ex situ Remediation Schemes – Composting and Landfarming 328 Summary 331 Key Words 331 References 331 Exercises 332
13 • INTRODUCTION TO SOLID WASTE MANAGEMENT 13.1 Introduction 333 13.2 Regulations and Solid Waste Management 335 13.2.1 RCRA Solid Waste Definition 335 13.2.2 RCRA Hazardous Waste Definition 336 13.2.3 RCRA Nonhazardous Waste 337 13.3 Waste Generation (Quantifying MSW Generation) 337 13.4 Solid Waste Collection 340 13.4.1 Types of Collection Systems 340 13.4.2 Equipment 341 13.4.3 Problems and Concerns 341 13.5 Landfill Containment and Monitoring Systems 343 Summary 345 Key Words 345 References 345 Exercises 346 Index 349
333
xi
ESource Reviewers We would like to thank everyone who helped us with or has reviewed texts in this series. Naeem Abdurrahman, University of Texas, Austin Stephen Allan, Utah State University Anil Bajaj, Purdue University Grant Baker, University of Alaska–Anchorage William Beckwith, Clemson University Haym Benaroya, Rutgers University John Biddle, California State Polytechnic University Tom Bledsaw, ITT Technical Institute Fred Boadu, Duke University Gregory Boardman, Virginia Tech Tom Bryson, University of Missouri, Rolla Ramzi Bualuan, University of Notre Dame Dan Budny, Purdue University Betty Burr, University of Houston Fernando Cadena, New Mexico State University Joel Cahoon, Montana State University Dale Calkins, University of Washington Linda Chattin, Arizona State University Harish Cherukuri, University of North Carolina–Charlotte Arthur Clausing, University of Illinois Barry Crittendon, Virginia Polytechnic and State University Donald Dabdub, University of CA Irvine Kurt DeGoede, Elizabethtown College John Demel, Ohio State University James Devine, University of South Florida Heidi A. Diefes-Dux, Purdue University Jerry Dunn, Texas Tech University Ron Eaglin, University of Central Florida Dale Elifrits, University of Missouri, Rolla Timothy Ellis, Iowa State University Christopher Fields, Drexel University Patrick Fitzhorn, Colorado State University Susan Freeman, Northeastern University Howard M. Fulmer, Villanova University Frank Gerlitz, Washtenaw Community College John Glover, University of Houston John Graham, University of North Carolina–Charlotte Ashish Gupta, SUNY at Buffalo Otto Gygax, Oregon State University Malcom Heimer, Florida International University
xii
http://www.prenhall.com/esource/
Donald Herling, Oregon State University Thomas Hill, SUNY at Buffalo A. S. Hodel, Auburn University Kathryn Holliday-Darr, Penn State U Behrend College, Erie Tom Horton, University of Virginia James N. Jensen, SUNY at Buffalo Mary Johnson, Texas A & M Commerce Vern Johnson, University of Arizona Jean C. Malzahn Kampe, Virginia Polytechnic Institute and State University Autar Kaw, University of South Florida Kathleen Kitto, Western Washington University Kenneth Klika, University of Akron Harold Knickle, University of Rhode Island Terry L. Kohutek, Texas A&M University Bill Leahy, Georgia Institute of Technology John Lumkes, Purdue University Mary C. Lynch, University of Florida Melvin J. Maron, University of Louisville James Mitchell, Drexel University Robert Montgomery, Purdue University Nikos Mourtos, San Jose State University Mark Nagurka, Marquette University Romarathnam Narasimhan, University of Miami Shahnam Navee, Georgia Southern University James D. Nelson, Louisiana Tech University Soronadi Nnaji, Florida A&M University Sheila O’Connor, Wichita State University Matt Ohland, Clemson University Paily P. Paily, Tennessee State University Kevin Passino, Ohio State University Ted Pawlicki, University of Rochester Ernesto Penado, Northern Arizona University Michael Peshkin, Northwestern University Ralph Pike, Louisiana State University Andrew Randall, University of Central Florida Dr. John Ray, University of Memphis Stanley Reeves, Auburn University Larry Richards, University of Virginia Marc H. Richman, Brown University
ESource Reviewers
Christopher Rowe, Vanderbilt University Liz Rozell, Bakersfield College Heshem Shaalem, Georgia Southern University Tabb Schreder, University of Toledo Randy Shih, Oregon Institute of Technology Howard Silver, Fairleigh Dickenson University Avi Singhal, Arizona State University Tim Sykes, Houston Community College Toby Teorey, University of Michigan Neil R. Thompson, University of Waterloo
xiii
Dennis Truax, Mississippi State University Raman Menon Unnikrishnan, Rochester Institute of Technology Michael S. Wells, Tennessee Tech University Ed Wheeler, University of Tennessee at Martin Joseph Wujek, University of California, Berkeley Edward Young, University of South Carolina Garry Young, Oklahoma State University Steve Yurgartis, Clarkson University Mandochehr Zoghi, University of Dayton
About this Book
Introduction to Environmental Engineering aims to excite freshman-level students about the field of environmental engineering. It presents engineering concepts in an engaging way to high school graduates who have limited skills in introductory calculus, chemistry, and physics. To understand some of the more advanced material presented here, students may need to complete a course in general chemistry and calculus I; most engineering-school freshmen meet this requirement after their first term of college-level study. Some topics covered are appropriate for sophomores, and portions of the text could be used as supplemental reading in upper-level environmental engineering courses typically taken by juniors and seniors. Primarily, this book is intended for use in the first course that civil and environmental engineers take in their curriculum; but many of its chapters could well be assigned in an Introductory Engineering survey course to expose students to various areas of environmental engineering. The book is written in a straightforward manner for easy reading, and it presents numerous examples in each chapter. A Solutions Manual is available that includes complete solutions to the end-of-chapter exercises. A brief description of the topics presented covered in the book may be summarized as in the text follows. Chapter 1 presents a synopsis of the exciting field of environmental engineering and the need for environmental engineers. Chapter 2 focuses on mathematical equations and proper units used by engineers. A review of essential chemical concepts, such as calculating molecular and equivalent weights, balancing chemical equations, and acid/base reactions, is presented in Chapter 3. Chapter 4 describes the major types of microorganisms encountered in environmental engineering, along with biological and ecological concepts. Energy balances are explained, and the carbon, nitrogen, phosphorus, and sulfur cycles are introduced. The definition of risk, the calculation of risk, risk assessment, and environmental impact analysis (EIS) are presented in Chapter 5. Chapter 6 discusses zero-, first-, and second-order reactions encountered in environmental systems. Batch, completely mixed, and plug-flow reactors used for treating water, wastewater, and air are described along with the equations used for modeling these systems. Chapter 7 defines sustainable development and green engineering and introduces these current and relevant xiv
About This Book
topics. It presents case studies illustrating designs that are sustainable while minimizing adverse affects on the environment. A review of nuclear physics is included, since nuclear reactors will certainly be more widely used in the future to reduce greenhouse-gas emissions. The major parameters used by environmental engineers for assessing water quality are presented in Chapter 8. Chapter 9 introduces the major unit operations and processes used for treating sources of potable drinking water. Chapter 10 follows up with an introduction to the unit operations and processes used for treating municipal wastewater and the sludge that is produced. Chapter 11 presents an overview of air pollution, global warming theory, regulatory issues, and the major technologies used for treating particulate and gaseous air pollutants. Solid-waste generation, collection, and disposal issues are discussed in Chapter 12. Finally, Chapter 13 presents techniques used for remediating hazardous wastes, along with background information on the Resource Conservation and Recovery Act (RCRA) and Superfund, which encompasses the Comprehensive Environmental Response, Compensation and Liability Act (CERCLA) and Superfund Amendments and Reauthorization Act (SARA).
ACKNOWLEDGMENTS We extend our sincere appreciation and gratitude to Dr. W. Jack Lackey (Professor Emeritus, George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology) for his thorough review of the manuscript and his ideas for improvement. Laura’s Dad has far exceeded his role as a fabulous father by spending long hours in reviewing our work and formulating invaluable suggestions. We are also highly appreciative of the feedback from our students at Mercer University, who used the manuscript in EVE 290, Introduction to Environmental Engineering. And finally, Rich would be remiss if he did not acknowledge his major professor, mentor, colleague, and friend, Dr. Joseph H. Sherrard (1942–2004), who inspired and encouraged him through doctoral studies at Virginia Tech. We are most grateful for the patience exhibited and support provided by our families.
xv
This page intentionally left blank
CHAPTER
1
Environmental Engineering as a Profession Objectives In this chapter, you will learn about: The demand for environmental engineers, and the ways environmental engineering enhances the health and well-being of society Duties and important characteristics of environmental engineers Three major environmental calamities that have occurred since 1947 Professional licensure and lifelong learning as an environmental engineer A five-step, structured approach for solving engineering problems
1.1 WELCOME As you begin your exploration of the environmental engineering profession, you may be curious, excited—perhaps even a little fearful of what will be expected of you. You are encountering the first challenge of your professional journey. This book will give you tools to develop the skills you need for solving environmental engineering problems as a college student and later as a practicing engineer. This chapter will show you some of the exciting things you can do in environmental engineering. Your adventure with learning will not stop when you graduate. During your career, you will continue to learn, as technology and regulations change and new environmental dilemmas evolve. Environmental engineering is a dynamically growing field, and you will be a part of its future. This book is an excellent source for students in the field of Civil Engineering, and for students in biology, chemistry, and geology. It provides a good background to the practical and valuable field of Environmental Engineering. Welcome!
1.2 WHAT’S YOUR DEFINITION OF ENVIRONMENTAL ENGINEERING? When we asked three people “on the street” what they thought an environmental engineer would do, they replied: “Take care of garbage and stuff like that.” “Keep the air and water clean so we can use it.” “Take care of pollution.” While these descriptions may be accurate to some degree, they only scratch the surface of the responsibilities of an environmental engineer today.
2
Chapter 1
Environmental Engineering as a Profession
1.3 ENVIRONMENTAL ENGINEERS MAKE A DIFFERENCE Environmental engineering has traditionally been a subspecialty of Civil Engineering, and at some colleges and universities it is housed in the Chemical Engineering department. Environmental engineers protect the health and well-being of the public by minimizing the release and impact of pollutants into the air, land, and water. Professionals in the field have studied chemistry, biology, mathematics, and engineering sciences. They work each day to design control and treatment systems that reduce or limit the negative effects that humans have on the many ecosystems of the world. 1.3.1 Jobs in Demand The demand for environmental engineers continues to grow. The Bureau of Labor (http://www.bls.gov/oco/ocos027.htm) indicates that environmental engineers should have employment growth of 25% during the projected decade (2006–2016); this growth is much faster than the average for all occupations. The same data suggest that Computer Hardware, Electrical, and Mechanical engineering jobs are expected to have slower-than-average rates of employment growth through 2016. Civil and industrial engineers are expected to experience employment growth of 18% and 20%, respectively, over the projections decade. An article in Fortune Magazine (Fisher, 2005) quoted David Levy (Chairman, Jerome Levy Forecasting Center) as declaring: “The greatest increase in demand by far will be folks who know how to clean up spaceship earth. That’s because an increasingly health-conscious public is eager to find environmental engineers who can prevent problems rather than simply control those that already exist.” Indeed, future professionals in this field will be in demand because they have skills and knowledge that governments, businesses, and industries will need to solve and prevent environmental problems. 1.3.2 A Lousy Report Card Since 1998, the American Society of Civil Engineers (ASCE) has been rating the state of America’s infrastructure. Table 1.1 shows scores from several recent report cards. The grades assigned to our systems that treat drinking water, wastewater, solid waste, and hazardous waste are alarming, as it appears that our country is barely passing! For nearly a decade, little to no progress has been made to improve our nation’s deteriorating infrastructure. This report is unacceptable, and improving these grades will require the expertise of environmental engineers. ASCE (2005) estimates that about $1.6 trillion must be invested to upgrade and renovate the environmental infrastructure. Additionally, Table 1.1 ASCE Report Card on America’s Infrastructure 1998 grade
2001 grade
2005 grade
Drinking water
D
D
D-
Wastewater
D+
D
D-
Solid waste
C-
C+
C+
Hazardous waste
D-
D+
D
Source: ASCE (2005), ASCE (2001), ASCE (1998).
Section 1.4
Duties and Important Characteristics of Environmental Engineers
America’s airports, bridges, dams, national power grid, public parks and recreation areas, railroads, security, schools, transit systems, and waterways will encounter problems to be solved by a team of professionals including environmental engineers. The Water Infrastructure Network (WIN, a broad-based coalition of elected officials, water and wastewater providers, regulators, administrators, engineers, and environmentalists dedicated to preserving and protecting America’s water and wastewater infrastructure) also cited the enormity of the work ahead. They forecasted unprecedented financial problems over the next 20 years for America’s water and wastewater infrastructure. WIN (2004) projects that an annual investment of $23 billion over current allocations will be needed to meet the priorities as stated in the Clear Water Act (passed by Congress in 1977). The purpose of the act was to establish the “best available technology economically achievable” for wastewater discharges containing toxic substances and to provide the “best conventional pollutant control technology” for discharges containing conventional pollutants such as biochemical oxygen demand (BOD) and suspended solids (SS). 1.3.3 Basic Necessities—New Challenges There will always be a high demand for qualified professional environmental engineers, since there is a continuous need for high-quality drinking water, clean air, and uncontaminated ground and surface water and land. Moreover, changes in population growth, habits, and lifestyles of people around the world create new challenges for environmental engineers. As the world’s population approaches 7 billion, the auxiliary problems associated with providing sanitary living conditions are enormous and ever changing. Here’s a brief look at some changes that will impact future work in the field: • Population increases create greater amounts of waste in terms of human excrement, discarded products, and food waste. • Citizens tend to become more wasteful as their economic well-being increases. • Modern lifestyles require considerable amounts of energy and resources from the earth. The liquids, solids, and gaseous wastes that are produced must be properly treated before being reintroduced into the environment. • Endocrine disruptors such as human and veterinarian antibiotics, anti-inflammatory medicines, blood lipid regulators, and sex and steroidal hormones are being found in water and wastewater. Environmental engineers must be proactive to develop alternative technologies and processes that minimize the production of pollutants and the needless wasting of the earth’s resources. Sustainable processes are also required to enable us to meet not just our present needs but also those of future generations. Challenges. Dilemmas. Ethical problems. Legal concerns. Political battles. Demands for cost-effective solutions. Balance between needs of the earth and needs of people. Remediation. Innovation. New technology. New problems. Each of these will require a team of new professionals ready to meet a future of environmental engineering!
1.4 DUTIES AND IMPORTANT CHARACTERISTICS OF ENVIRONMENTAL ENGINEERS Being an environmental engineer is tremendously rewarding, because your skills can improve the quality of life and help sustain the ecological balance for future generations. The profession is a challenging one, requiring a strong foundation in
3
4
Chapter 1
Environmental Engineering as a Profession
science, math, and engineering. Professionals integrate knowledge in these areas to create engineered systems that treat pollutants in the atmospheric, aquatic, and terrestrial environments. Most environmental problems are complex, because the treatment and transport of pollutants involve multiple mediums. For example, using a scrubber for removing particulate matter and gases from the discharge of a fossil-fuel power plant involves working with pollutants in three different phases. It requires knowledge of the chemical reactions involved in the combustion process and in the production of the air contaminants. The pollutants in the exhaust gas are transferred to the liquid phase during scrubbing, wherein the gases are neutralized and particles separated by gravity. Ultimately, the particulate matter removed will be disposed of in a landfill. There are other complex systems such as groundwater contamination from leaking underground storage tanks (LUSTs) in which physical, chemical, and biological processes must be understood and manipulated to engineer appropriate solutions. Environmental engineers work in a variety of settings, managing an array of tasks. 1.4.1 Multidisciplinary Teams Environmental engineering jobs are stimulating, since you get to work on a wide range of projects and on multidisciplinary teams consisting of structural, mechanical, electrical, and geotechnical engineers and biologists, chemists, planners, economists, lawyers, and politicians. As a team player, you must have good communication skills and be able to convey your thoughts and designs to nontechnical and technical audiences in both written and oral forms. You will need to be able to deliver good oral presentations as well as make sketches and use appropriate software to prepare schematic diagrams and other types of engineering drawings. 1.4.2 Work Outdoors Environmental engineering is especially appealing to those who enjoy outdoor activities and wish to sustain a healthy environment. It is not unusual for environmental engineers to develop and implement protocols for collecting air, water, and soil samples. On occasion, they will actually supervise the collection of the samples. Where construction projects involve building new water or wastewater treatment facilities or managing and supervising the cleanup of a contaminated hazardous waste site, environmental engineers may serve as resident observers. Performing soil surveys, environmental assessments, water-quality surveys, and hydraulic and hydrologic surveys all involve activities outdoors. 1.4.3 Consulting Firms Many environmental engineers work for engineering consulting firms to solve a variety of environmental problems. Newly graduated environmental engineers tend to focus on technical issues related to the selection and design of appropriate technologies to treat water, wastewater, air pollution, groundwater, and contaminated soil. After several years of perfecting their technical expertise, they may progress to project management or to formation of their own firms as entrepreneurs. 1.4.4 Government Some environmental engineers work for city or county governments and municipalities. They oversee the operation of public works departments, engineering departments, water treatment plants, and wastewater reclamation facilities. Planning and budgeting of major projects is their prime responsibility.
Section 1.5
1.4.5 Regulatory Agencies Environmental engineers also work at federal and state regulatory agencies. They are responsible for reviewing and permitting of new water and wastewater treatment plants, air-pollution control technologies, cleanup of contaminated land sites, and the siting and operation of sanitary landfills. 1.4.6 Industry Industries such as pulp and paper manufacturing, textile processing, petroleum and petrochemical, pharmaceutical, and meat processing employ environmental engineers to ensure compliance with environmental regulations. Some environmental engineers develop ways to reduce and minimize the quantity of water used and the amount of pollutants produced at such facilities; while others are responsible for operating on-site industrial wastewater treatment plants or pretreatment systems, baghouses and electrostatic precipitators for air emissions, and disposal of residuals and sludges in landfills. 1.4.7 Academia Environmental engineers employed in academia teach and serve as mentors to the next generation of engineers. They are rewarded and motivated by seeing their students learn and become productive citizens in society. Professors also engage in scholarly activities, such as research to explore innovative and cost-effective ways to reduce the effects of pollution on the environment. They also develop new teaching strategies and paradigms to enhance learning in the classroom. An important aspect of being a professor is presenting ideas and research results on technical and education-related activities at conferences and in peer-reviewed journals. 1.4.8. Does Environmental Engineering Match My Interests? The foregoing information should help you understand whether environmental engineering suits your personality. At this point in your career, you are just beginning to develop the skills needed by a professional environmental engineer. The environmental engineering curriculum has been designed to provide both the soft and hard skills you will require. Your faculty advisor and instructors will mentor and assist you in your development along the way. Your commitment and perseverance will lead you to your goals.
1.5 ENVIRONMENTAL CALAMITIES A number of environmental calamities have occurred in recent decades. Table 1.2 provides references for these tragedies, three of which are described below. In some instances, defining the effects associated with the release of certain pollutants (such as synthetic organic compounds, SOCs) into the environment took several years, since new technology was needed to detect and monitor the compounds at low levels, and the links between SOCs and their metabolites to related health effects were difficult to identify. 1.5.1 Love Canal One stark example of chemical contamination of the environment occurred at Love Canal near Niagara Falls, New York. During the 1890s, William T. Love envisioned building a canal to connect the upper and lower Niagara Rivers to facilitate the production of hydroelectricity. Because of economic fluctuations, Love’s project was doomed. In 1920, Love’s land was sold to the City of Niagara Falls in a public auction
Environmental Calamities
5
6
Chapter 1
Environmental Engineering as a Profession
Table 1.2 Environmental Calamities in Recent History Pollutant Released
Date
Reference
Milwaukee
Cryptosporidium in drinking water
March 23, 1993–April 8, 1993
http://en.wikipedia.org/wiki/Milwaukee_ Cryptosporidium_outbreak
Chernobyl
Radiation
April 26, 1986
www.chernobyl.com/info.htm
Love Canal
Toxic wastes
1920 to 1950s
www.epa.gov/history/topics/lovecanal/01.htm
Bhopal, India
Methyl isocyanate
December 2–3, 1984
www.bhopal.org/whathappened.html
Exxon Valdez
Crude oil
March 23, 1989
www.bytesurgery.com/missionpossible/mp/text/exxon.html
Smog in Great Smoky Mountains
Ozone and acid rain
1991 to 2001
www.aldha.org/smoky2.htm
Hurricane Katrina (Natural Calamity)
Pathogens, pesticides, dioxins, and debris
August 29, 2005
Environmental Health Perspectives, Vol 114, No 1, January 2006.
Name
and was later used as a chemical waste disposal site. The Hooker Chemical Company (a subsidiary of Occidental Petroleum) acquired the site in 1947 and buried 21,800 tons of toxic waste in the area in a five-year period. In 1952, Hooker closed the site and covered the canal with earth. The company later sold the site to the Niagara Falls Board of Education, and a school and 100 homes were built on it. During the intervening years, residents complained of strange odors and substances percolating to the surface in their yards. Numerous miscarriages and birth defects were reported by the New York State Health Department. In 1978, after Lois Gibbs and an associate held two Environmental Protection Agency (EPA) officials hostage in her home, President Carter declared a federal emergency at Love Canal. Finally on May 21, 1980, because of to the persistence of Lois Gibbs and other residents, President Carter and the Environmental Protection Agency agreed to relocate all families living at Love Canal. Occidental Petroleum eventually spent more than $200 million to clean up the site. This calamity led Congress to pass the Superfund Law, which holds companies responsible for the cleanup of hazardous waste sites. Was it ethical for Hooker Chemical Company to sell the Love Canal to the Niagara Falls Board of Education? 1.5.2 Milwaukee Cryptosporidium Outbreak From March 23 through April 8, 1993, ineffective filtration at one of Milwaukee’s water treatment plants caused an estimated 403,000 residents to become ill due to the inadequate removal of the protozoan parasite, Cryptosporidium parvum (Corso et al., 2003). Turbidity levels in the treated water exceeded normal levels. Over 100 deaths, mainly of the elderly and those with compromised immune systems, were attributed to this outbreak. Could anything have been done to prevent this parasite from entering the water distribution system?
Section 1.6
Becoming an Environmental Engineer: A Lifelong Process
1.5.3 Chernobyl Nuclear Disaster On April 26, 1986, for a period of 10 days, radioactive material was released into the atmosphere from Number 4 nuclear reactor at the Chernobyl Power Plant (www. chernobyl.com/info.htm). Numerous safety precautions had not been followed, resulting in a meltdown of the reactor core. Estimates indicate that local residents were exposed to radioactivity 100 times greater than that of the Hiroshima bomb. Thirty people were killed immediately, and estimates suggest that over 15 million people were impacted by this disaster. Babies are still being born in this area with no arms, no eyes, and only stumps for limbs. What actually caused the meltdown, and is there an antidote for radiation poisoning?
1.6 BECOMING AN ENVIRONMENTAL ENGINEER: A LIFELONG PROCESS Becoming an environmental engineer is a lifelong journey. First, one must receive four years of formal training and meet the requirements for professional licensure. Then, one must continue one’s education in order to remain technically competent and current. 1.6.1 College Education Traditionally, environmental engineers have been trained as civil engineers and pursued graduate degrees in “Sanitary” or “Environmental” Engineering. Formal training normally consisted of obtaining a four-year, Bachelor of Science (BS) degree in civil engineering from an ABET, Inc. accredited engineering program; and then specializing at the graduate level in a particular area such as water, wastewater, air pollution, solid wastes, or hazardous wastes. Obtaining a Master of Engineering (ME) or Master of Science (MS) in Environmental Engineering normally takes one or two years past the baccalaureate. Two or three more years beyond the master’s are normally required to obtain the Doctor of Philosophy (PhD) or Doctor of Science (DSc). Several colleges and universities now offer four-year undergraduate degrees in environmental engineering. Table 1.3 shows a typical undergraduate environmental engineering curriculum by semester. 1.6.2 Professional Licensure During the senior year, most environmental engineering students take the Fundamentals of Engineering (FE) examination. This nationally normalized test, administered by the National Council of Examiners for Engineering and Surveying (NCEES), leads to becoming a licensed professional engineer (PE). The 8-hour FE examination tests environmental engineers on their understanding of the basic sciences, math, ethics, engineering economy, statics, dynamics, thermodynamics, electrical, and engineering sciences. Upon passing the FE, graduates work four years under the mentorship of a licensed PE before they become eligible to take the 8-hour Professional Engineering (PE) exam, which focuses on design and the application of environmental engineering principles. Passing the PE exam permits one to become a registered or licensed professional engineer, allowed to sign and seal engineering drawings, plans, and specifications and essentially to practice environmental engineering. It is anticipated by 2015 that students graduating from civil and environmental engineering programs will have an additional 30-credit-hour requirement for professional licensure imposed by the National Council of Examiners for Engineering and Surveying (NCEES).
7
8
Chapter 1
Environmental Engineering as a Profession
Table 1.3 Undergraduate Environmental Engineering Curriculum
Freshman
Sophomore
Junior
Senior
Fall
Spring
Chemistry I
Chemistry II
Computer Programming
Introduction to Design
Calculus I
Calculus II
Humanities I
Humanities II
Freshman Experience
Ethics
Physics I
Physics II
Statics
Dynamics
Electrical I
Electrical II
Intro Environmental Engineering
Probability & Statistics
Differential Equations
Thermodynamics
Engineering Economy
Engineering Systems Analysis
Biology
Geology
Technical Communication
Hydrology
Hydraulics
Wastewater Treatment
Humanities III
Solid Waste Management
Air Pollution Control
Air Chemistry
Public Health
Remediation
Water Treatment
Environmental Engineering Lab
Senior Design I
Senior Design II
Groundwater Hydrology
Technical Elective
Humanities IV
Humanities V
1.6.3 Engineering Ethics The engineering occupation consists of professionals who must make ethical decisions. Ethics is the study of morals, which guide our conformance to a standard of what is right and wrong. By definition, professionals possess sophisticated skills, formal education, and standards that have been established by societies or organizations for admission to the profession. Each person makes moral choices in his or her relationships. Engineering ethics involves rules and standards governing the conduct of engineers in their professional lives. Many professional societies have prepared codes of ethics to provide guidance for the working engineer. The National Society of Professional Engineers (NSPE) has sought to promote professional licensure and a code of ethics across the engineering field. Listed below are the fundamental canons promulgated by NSPE. Note how each canon reflects a specific precedence with respect to the engineer’s consideration for clients, employers, the profession, and the public.
Section 1.6
Becoming an Environmental Engineer: A Lifelong Process
Engineers, in the fulfillment of their professional duties, shall (http://www.nspe. org/index.html): 1. 2. 3. 4. 5. 6.
Hold paramount the safety, health, and welfare of the public. Perform services only in areas of their competence. Issue public statements only in an objective and truthful manner. Act for each employer or client as faithful agents or trustees. Avoid deceptive acts. Conduct themselves honorably, responsibly, ethically, and lawfully so as to enhance the honor, reputation, and usefulness of the profession.
1.6.4 Continuing Education Environmental engineers must remain knowledgeable of current technologies. Professional engineering societies and organizations play a vital role in helping environmental engineers maintain their technical competency and currency. They promote training seminars, workshops, and conferences to keep their constituents informed. To promote lifelong learning, each professional society publishes journals with peer-reviewed articles on technical issues, engineering ethics, regulatory updates, and innovative technologies. The American Academy of Environmental Engineers is the primary society that promotes environmental engineering. A variety of organizations, however, focus on environmental issues. The American Society of Civil Engineers (ASCE) is probably the engineering society most familiar to many environmental engineers, as historically most have been trained as civil engineers. Municipal and industrial wastewater treatment and sludge treatment and disposal issues are promoted by the Water Environment Federation (WEF). Environmental engineers interested in water treatment and related water issues are normally members of the American Water Works Association (AWWA). For those interested in air pollution and solid and hazardous waste management, the Air and Waste Management Association (AWMA) is the society to contact. The Association of Environmental Engineering and Science Professors (AEESP) is an environmental organization for those affiliated with academia. Table 1.4 lists the websites for some of the most prominent societies associated with environmental issues.
Table 1.4 Website Addresses for Organizations with Environmental Engineers Organization
Web Address
Air and Waste Management Association
www.awma.org
American Academy of Environmental Engineers
www.aaee.net
American Society of Civil Engineers
www.asce.org
American Society for Engineering Education
www.asee.org
American Water Works Association
www.awwa.org
Association of Environmental Engineering and Science Professors
www.aeesp.org
Solid Waste Association of North America
www.swana.org
Water Environment Federation
www.wef.org
9
10
Chapter 1
Environmental Engineering as a Profession
1.7 PROBLEM SOLVING Environmental engineers solve a variety of problems. One must first identify and understand the problem or system, then define the parameters used to model the system; and finally solve the problem. For many environmental problems, you will need to perform energy and material balances around the system to solve them. Most engineers use a structured approach to solve problems or to develop computer algorithms. The following five-step method is recommended for solving most engineering problems. 1. Identify and define the problem clearly and concisely. List all pertinent data, the knowns and unknowns, along with the appropriate units. 2. Make a sketch or diagram of the system and label all parts. 3. Select the appropriate theory or equations, estimate values where data are missing, and make assumptions to simplify the solution. 4. Solve the problem using mathematical equations, graphical procedures, or trial-and-error methods. In many cases, an energy and materials balance must be completed. 5. Check your answer to see if it makes sense, is reasonable/correct, and verify results by an alternative method if possible.
EXAMPLE 1.1 Computing velocity of flow in a pipe Calculate the average velocity of water flowing in a 6-inch-diameter pipe if the flow rate is 3.0 cubic feet per second. Solution 1. Find the average velocity of water in the pipe. Q = volumetric flow rate = 3.0 ft3/s pipe diameter = D = 6 inches 2. Draw a figure. Q 3.0 cfs
Q 3.0 cfs 6 in
3. Assume that the pipe is flowing full. area of a circle: A = pD2/4 A = cross-sectional area of pipe, ft2 (cross-hatched area in figure) D = pipe diameter, ft continuity equation: Q = A * V Q = volumetric flow rate, ft3/s V = average flow velocity in pipe, ft/s 4. Solve for the cross-sectional area of the pipe. A = pD2/4 p16 in22 1 ft2 = 0.196 ft2 = 28.27 in2 * A = 4 144 in2
Section 1.7
Problem Solving
11
Solve for velocity by rearranging the continuity equation. Q = AV Q V = A 3.0 ft3>s ft V = = 15.3 2 s 0.196 ft 5. Check by comparing the original Q 13.0 ft3/s2 in the problem statement with the calculated Q 12.99 ft3/s2 based on the area and the calculated velocity. Q = AV
ft ft3 Q = 0.196 ft2 * a15.3 b = 2.99 ⬵ 3.0 s s
Checks
Calculating mass removal rates Groundwater from a deep well is being pumped to a small water treatment plant at 5 m3/min. The water contains 0.05 mg/L of arsenic (As), and the primary drinkingwater standard is set at 0.01 mg/L of arsenic. Calculate the number of grams of arsenic that must be removed from the water each day to meet the standard. Solution 1. Find the mass of arsenic that must be removed daily. Q = volumetric flow rate = 5 m3/min C1 = concentration of arsenic in raw water = 0.05 mg/L C2 = concentration of arsenic in treated water = 0.01 mg/L 2. Draw a figure. Q 5 m3/min C1 0.05 mg/L
Water Treatment Plant
Q 5 m3/min C2 0.01 mg/L
3. Assume that the volumetric flow rate is the same into and out of the water treatment plant. mass removed = mass in - mass out mass of arsenic = Q * C Q = volumetric flow rate, m3/min C1 = concentration of arsenic in raw water = 0.05 mg/L C2 = concentration of arsenic in treated water = 0.01 mg/L 4. Solve for the mass of arsenic entering and exiting the water treatment plant. mass in = Q * C1 = 5
mg 1000 L 60 min 24 h 1g m3 a0.05 b¢ ba ba b ≤a 3 min L 1h 1d 1000 mg 1m
mass in = Q * C1 = 360 g>d mass out = Q * C2 = 5
mg 1000 L 60 min 24 h 1g m3 a 0.01 b¢ ba ba b ≤a 3 min L 1h 1d 1000 mg 1m
EXAMPLE 1.2
12
Chapter 1
Environmental Engineering as a Profession
mass out = Q * C2 = 72 g>d mass of arsenic removed = mass in - mass out = 360 - 72 = 288 g>d 5. Check by subtracting C2 from C1 and multiplying this difference by the volumetric flow rate. mass of arsenic removed = Q 1C1 - C22 Q 1C1 - C22 = ¢ 5
mg 1000 L 60 min m3 b¢ b ≤ a0.05 - 0.01 ≤a min L 1h 1 m3 1g 24 h ba b a 1d 1000 mg
Q 1C1 - C22 = 288 g>d, which is equal to the mass of arsenic removed in Step 4.
1.8 ENVIRONMENTAL MANAGEMENT Environmental issues related to our air, land, and water resources will continue as high-quality water sources diminish; global warming affects climate, causing drought in some areas and flooding in others; and with the loss of rain forests and fertile land for growing crops continues. Environmental engineers must work closely with the civil engineering profession to develop sustainable processes and designs to maintain worldwide infrastructure (transportation systems, wastewater collection and treatment systems, water distribution and treatment systems, sanitary landfills, etc.) while minimizing adverse affects on the ecosystem. Environmental management involves managing human interaction and impact on the living (biotic) and nonliving (abiotic) environment. The International Organization for Standardization (ISO) has developed ISO 14000 environmental management standards to help companies minimize adverse affects from their operations on the environment. In the United States, all federal agencies must prepare a detailed statement known as an Environmental Impact Statement (EIS), as required by the National Environmental Policy Act (NEPA). The U.S. Environmental Protection Agency (EPA) reviews EISs prepared by all federal agencies and ensures that its own actions are in compliance with NEPA. To sustain our ecosystem and our modern way of life, environmental engineers will be called upon to develop innovative and better solutions with regard to management of air resources, water supply and treatment systems, wastewater collection and treatment systems, and solid and hazardous waste collection, treatment, and disposal systems. Problems associated with pollution must be prevented and minimized by using alternative processes and fuels. Companies must produce products that have long life and the potential to be reused, and that require less energy and water to manufacture.
S U M M A RY Environmental engineering is an engineering specialty that involves identifying, designing, and implementing systems to protect human health and well-being and the environment. Environmental engineers have a prominent role in maintaining a high quality of life by providing potable water and clean air, remediating contaminated land, and minimizing the adverse effects of pollution on the environment.
Exercises 13
Graduating from an ABET-accredited engineering program provides students with the formal training necessary to become environmental engineers. Traditionally, environmental engineers were trained as civil engineers at the undergraduate level and pursued graduate work at the master’s or doctoral level to become environmental engineers. Several colleges and universities now offer undergraduate and graduate environmental engineering programs. Completing the requirements for the bachelor’s degree in environmental engineering is just the first step in becoming an environmental engineer. Learning is a lifelong process, and environmental engineers must remain current with new technologies and ever-changing regulations. This may be accomplished by actively participating in professional societies and reading technical literature to keep abreast of state-of-the-art science and technology. Most often, environmental engineers work on multidisciplinary teams along with lawyers, chemists, biologists, planners, regulators, and other engineers to solve environmental problems. Employment opportunities for environmental engineers are diverse, ranging from engineering consulting to environmental law, from academic to governmental, and from municipal to industrial. It is anticipated that the strong demand for environmental engineers will continue into the next decade. environmental engineer ecological balance pollution
life long learning professional engineer problem solving
air, land, and water environments
KEY WORDS
EXERCISES For Exercises 1 through 6, use the Internet to perform a Google search for help in answering the questions. If necessary, seek out a professor or environmental engineer to discuss the topics presented.Then prepare a written paragraph or two for each topic, including the ideas discussed with the professor or engineer. Discuss the role each topic plays in environmental engineering. Include comments on the types of problems that the engineer may confront and how to solve these problems for each topic. 1.1 1.2
1.3
1.4
1.5
How are the principles from microbiology used in the design of wastewater treatment processes and the remediation of contaminated soil? List and discuss two equations from fluids and hydraulics that are used by environmental engineers in the design of water distribution and wastewater collection systems. [Hint: Continuity and Bernoulli’s equation.] Environmental engineers and scientists have reported that global warming may be a real problem. List and discuss the major causes and the air pollutants that are associated with this phenomenon. Environmental engineers are responsible for designing water treatment plants that provide safe, potable drinking water to the public. What chemical principles are used for removing pollutants from water? Give some examples of unit processes used for treating water. [Hint: Oxidation/reduction, gas transfer, precipitation.] Many environmentalists promote the idea of recycling solid wastes. Collection, separation, and recycling systems are often designed by environmental engineers. Discuss the advantages and disadvantages of recycling metals, glass, and paper discarded in solid wastes. Some politicians advocate recycling and reusing 50% of the materials discarded. Is this realistic or even possible? Why or why not?
14
Chapter 1
Environmental Engineering as a Profession
1.6
1.7
1.8
1.9
Hydrology is another area in which environmental engineers are highly involved. During storm events, rain that falls to the surface either percolates into the soil to replenish ground water supplies or becomes surface runoff. Stormwater runoff from impervious areas in cities or from agricultural areas contributes large volumes of water of poor quality, due to pollutant entrapment and dissolution. List and discuss some of the contaminants that might be found in urban stormwater runoff versus agricultural runoff. Typically, the rational method or equation is used for estimating the volume of runoff. Write down the equation and list the variables along with the appropriate units for each parameter in the equation. A long, rectangular settling basin is used for removing suspended solids during water treatment. The basin’s length-to-width ratio is 4:1 and its width-to-depth ratio is 1:1. Determine the basin’s volume in cubic feet if its depth is 25 feet. A 25-meter-diameter circular tank 10 meters deep is used for storing liquid sodium hydroxide solution at a wastewater treatment plant. Determine the tank’s cross-sectional area in square meters and its circumference in meters. Bituminous coal containing 5% (weight basis) sulfur is burned at a power plant to provide energy for generating electricity. Assume the combustion of coal is complete and the following equation can be used for modeling the oxidation of sulfur into sulfur dioxide. S + O2 : SO2
1.10
Determine the kilograms of sulfur dioxide produced daily if 20,000 kilograms of coal are combusted each day. Chlorine is the most widely used disinfectant for killing pathogens during water treatment. Determine the kilograms of chlorine used daily at a water treatment plant handling 10,000 cubic meters per day of flow at a chlorine dosage of 10 mg/L. [Hint: Multiply the flow rate by chlorine dosage and make appropriate conversions.]
REFERENCES ASCE (2005). 2005 Report Card for America’s Infrastructure. Accessed May 21, 2008 from http://www.asce.org/reportcard/2005/page.cfm?id=103&print=1m. ASCE (2001). 2001 Report Card for America’s Infrastructure. Accessed May 21, 2008 from http://www.asce.org/pressroom/news/pr030801_reportcard.cfm. ASCE (1998). 1998 Report Card for America’s Infrastructure. Accessed May 21, 2008 from http://www.asce.org/pdf/reportcard.pdf. Bureau of Labor (2008). Occupational Outlook Handbook, 2006–07 Edition. Accessed May 21, 2008, 2007 from http://www.bls.gov/oco/ocos027.htm. Corso, P.S., Kramer, M.H., Blair, K.A., Addiss, D.G., Davis, J.P, and Haddix, A.C. (2003). Cost of illness in the 1993 waterborne Cryptosporidium outbreak, Milwaukee: Emerging Infectious Diseases, Vol. 9, No. 4, 426–431. Fisher, Anne (2005). Hot careers for the next 10 years, Fortune Magazine, March 21, 2005. WIN (2004). Water Infrastructure Network Recommendations for Clean and Safe Water in the 21st Century. Accessed October 16, 2008 from http://www.win-water.org/reports/winow.pdf.
CHAPTER
2
Introduction to Environmental Engineering Calculations: Dimensions, Units, and Conversions Objectives In this chapter, you will learn about: Units and dimensions Units of concentration for liquid and gas phases Density Concept of flow rate and residence time Precision and accuracy
2.1 INTRODUCTION A number without an associated dimension has no meaning in engineering; a number with an improper unit has a misleading meaning. The importance of proper units was highlighted by the loss of the 1999 NASA Mars Climate Observer, a 125-million-dollar spacecraft that was destroyed when it entered the Mars atmosphere traveling too fast. The error was blamed partially on the failed translation of English units into metric units (Isbell et al., 1999). This chapter introduces the units and dimensions commonly used by environmental engineers and presents practice problems that require you to master the manipulation and conversion of these units.
2.2 DIMENSIONS AND UNITS Dimensions can be used to describe the measurable aspects of any object or system, while units provide the increments necessary to quantify the dimensions. All numerical engineering measurements should be dimensioned with proper units. Common dimensions associated with environmental engineering problems include length, mass, time, and temperature. Associated units that scale the measured quantity for these dimensions include meters (m), kilograms (kg), seconds (s), and degrees Fahrenheit (°F), respectively. Notice that dimensions and units only qualitatively describe a measurement and that a numerical value matched properly with units is required for a quantitative description (e.g., 2.0 meters, 4 minutes). Four systems of units are commonly used: the International System of Units, the Imperial Units (closely related to the British engineering system), the U.S. Customary Units, and the Absolute or cgs (centimeter-gram-second) system. The International System of Units is commonly referred to as the SI, metric, or MKS (meter-kilogram-second) system. Because the SI hierarchy of units is founded on a decimal basis, such that the addition of a prefix to a unit simply changes its value by a factor of 10, it has been widely adopted internationally. Many seasoned engineers, as well as American citizens in general, have continued to use the U.S. Customary Units, so it is important to master both the U.S. Customary and the SI systems of units.
16
Chapter 2
Introduction to Environmental Engineering Calculations: Dimensions, Units, and Conversions
The list of fundamental dimensions includes length, mass, time, and often temperature. All other quantities are derived from these dimensions. Derived units commonly used by environmental engineers include those of volume 1length32, density 1mass/length32, and concentration 1mass/length32. Notice the similarity between density and concentration as this will become important in subsequent sections of this chapter. Some specific units include the newton 1N = mass * length/time22, which quantifies the force on an object, the joule 1N * length2, which quantifies the energy of a system, and the pascal 1Pa = N * m-22, which describes a system pressure.Table 2.1 shows fundamental dimensions, derived dimensions, and associated units for the four common systems of units. Common prefixes used to describe units in the SI system are defined in Table 2.2, and some frequently used conversions are listed in Table 2.3.
Table 2.1 Commonly Used System of Units—Fundamental and Derived Units System
Length
Mass
Time
Temperature Force*
International System of Units (SI)
meter
kilogram
second
K, °C
newton
joule
U.S. Customary Units (commonly referred to as English or Standard Units)
foot
pound mass second, 1lbm2 hour
°R, °F
pound force 1lbf2
BTU
Imperial Units (derived from English units; often referred to as the British system)
foot
slug*
second
°R, °F
pound force 1lbf2
Absolute System (cgs)
centimeter
gram
second
K, °C
dyne
*Derived unit defined as follows: newton 1N2 =
joule 1J2 = N # m =
kg # m
冫s2
# pound force 1lbf2 = 32.2 lbm ft冫s2 #2 slug = lbf s 冫ft # dyne = g cm冫s2
kg # m2
冫s2
British thermal unit 1BTU2 = 252 cal = 1.054 J pound weight = lbm # g 1where g = 9.8 m/s22 calorie 1cal2 = 4.184 J
Table 2.2 Commonly Used SI Unit Prefixes Prefix name
Symbol
Factor
tera
T
1012
giga
G
109
mega
M
106
kilo
k
103
deka
da
deci
d
10-1
centi
c
10-2
milli
m
10-3
micro
m
10-6
nano
n
10-9
pico
p
10-12
10
Energy*
BTU ft # lb joule or calorie
Section 2.2
Table 2.3 Common Conversion Factors Value known with this initial unit
Multiply by
To obtain
Length m
3.281
ft
m
1.094
yd
mile
5280
ft
in
2.54
cm
ft
12
in
mile
1.609
km
Å
10-10
m
Mass lbm
453.6
g
kg
2.2
lbm
lbm
16
oz
ton (short)
2000
lbm
ton (metric)
1000
kg
Volume gal
3.785
L
ft3
7.481
gal
m3
1000
L
3
ft
28.32
L
m3
35.31
ft3
m3
1.308
yd3
cm3
1
ml Pressure
atm
760
mm Hg
atm
101.37
kPa
atm
1.013
bar
atm
14.70
psi
psi
2.307
ft H2O @4°C
atm
1.01325 * 105
N/m2
(continued)
Dimensions and Units
17
18
Chapter 2
Introduction to Environmental Engineering Calculations: Dimensions, Units, and Conversions
Table 2.3 Common Conversion Factors (continued) Value known with this initial unit
Multiply by
To obtain
Energy kWh
3412.8
BTU
kWh
8.6057 * 105
cal
kWh
1.341
hp # hr
1 cal
4.184
J
BTU
1055
J
Power kW
1000
J/s
kW
1.34
hp
BTU/sec
1.055
kW
kW
737.56
ft # lbf/s
Temperature °F = 1.81°C2 + 32 K = °C + 273.15 °R = °F + 459.67
2.3 ESSENTIAL DERIVED UNITS AND CONVERSION PRACTICE 2.3.1 Density The density of a substance is defined as the ratio of its mass to a unit volume and can be shown mathematically as: r =
m V
(2.1)
where: r = density, kg/m3, m = mass, kg, and V = volume, m3. From this definition, we can conclude that the density of a substance is an intrinsic property and therefore independent of sample size. Take note, however, that density varies with temperature as well as with the concentration of any impurities dissolved in a solution. For example, the density of water at 0°C and 80°C is 0.9998 g/cm3 and 0.9718 g/cm3, respectively. If water at 0°C contains 1% sodium chloride (table salt), the density of the aqueous solution is 1.0075 g/cm3, noticeably higher than that of pure water. Possibly you have noticed that floating on your back in ocean water seems easier than floating in a lake. This is because the salt content of the ocean is higher than that of most lakes, causing the density of ocean water at the
Section 2.3
Essential Derived Units and Conversion Practice 19
Table 2.4 Densities of Common Substances Substance
Density (g/cm3)
Air @ STP
0.001293
Southern pine
0.65
Gasoline
0.72
Ethanol
0.802
Ice @ 0°C
0.92
Water @ 4°C
1.00
Rubber
1.5
Glass
2.6
Aluminum
2.7
Wrought iron
7.6
Uranium
18.7
sea surface to be approximately 1.1027 g/cm3. The two main factors that control ocean water’s density are its temperature and its salinity. The density of some common items is shown in Table 2.4.
EXAMPLE 2.1 Density practice Recall the story of Archimedes, a Greek mathematician who lived in the third century BCE. Hiero II, the King of Syracuse on the Ionian Sea, charged Archimedes to determine whether his newly acquired crown was made of pure gold. While bathing in a public bath, the legend tells us, the famous mathematician discovered a method to test the purity of the crown and was so enamored with his discovery he ran naked through the streets yelling “Eureka!” Archimedes’ principle can be simply stated as follows: When an object is submerged in a liquid, the amount of liquid displaced is equal to the volume of the object. Use this idea to solve the following problem. A 2-liter graduated cylinder initially contains 500 ml of water. An irregularly shaped object having a mass of 105 g is submerged into the water. The final volume of the water in the cylinder is 1025 ml. Determine the density of the object. Solution Calculate the volume of the object by determining the difference in water-volume readings within the graduated cylinder. V = 1025 ml - 500 ml = 525 ml Now, determine the density of the object by using the mass that was given and substituting into Equation (2.1). r =
105 g m = = 0.20 g/ml V 525 ml
20
Chapter 2
Introduction to Environmental Engineering Calculations: Dimensions, Units, and Conversions
EXAMPLE 2.2 Problem solving with density During an environmental assessment of a natural wetland in the Everglades, it was determined that 1.5 cubic meters of the dry native soil had a mass of 420 kg. Determine the mass of a cubic foot of that same soil. Solution Calculate the density of the wetland soil. r =
420 kg 1000 g m 1m 3 = * * a b = 0.28 g/cm3 V 1 kg 100 cm 1.5 m3
Use the density found to calculate the mass associated with 1 ft3 of the Everglades soil. m = rV =
0.28 g
* a
2.54 cm 3 12 in 3 b * a b * 1 ft3 = 7929 g 1 in 1 ft
cm3 1 kg = 7.9 kg m = 7929 g * 1000 g
2.3.2 Concentration Concentrations of substances dissolved in a liquid phase are often described on a mass-per-unit-volume basis such that C =
m V
(2.2)
where: C = concentration of dissolved substance, mg/L, m = mass of solute or dissolved substance, mg, and V = total volume, L. Notice the similarity between Equations (2.1) and (2.2), which describe, respectively, the density of a substance and the gravimetric-based definition for concentration. The most commonly used mass-based concentration units include milligrams per liter (mg/L) and micrograms per liter 1mg/L2. Although not as common, micrograms per cubic meter 1mg/m32 may also be used to express a massbased contaminant concentration. In contrast, the concentration of dissolved substances in liquid phases may alternatively be described on a mass-per-unit-mass basis (the mass of the solute dissolved per the mass of solution). The most common mass-based units are parts per million (ppm) and parts per billion (ppb). For perspective, the maximum contaminant level goal (MCLG) of mercury in drinking water allowed by the Environmental Protection Agency (EPA) is 0.002 ppm (40 CFR, §141.51). This means that there can be no more than 0.002 grams of mercury in every million grams of drinking water purified and distributed by your local municipality. Assuming the density of water is 1.0 g/ml, this is equivalent to allowing only 2 grams 10.0044 lbm2 of mercury in approximately 264,000 gallons of drinking water.
Section 2.3
Essential Derived Units and Conversion Practice 21
EXAMPLE 2.3 Determine relationship between mg/L and ppm In many environmental aqueous systems, it is appropriate to make the simplifying assumption that 1 mg/L is equal to 1 ppm, provided that the fluid in question is primarily water, so that the density of the solution essentially is 1 g/ml. (This assumption is often made by environmental engineers.) Show that 1 mg/L = 1 ppm. Solution The solution requires that the concentration on a mass-per-unit-volume basis (mg/L) be divided by the solution density, followed by simple unit conversions. 1
1g mg 1 ml 1L 1 * * * = = 1 ppm L 1000 mg 1g 1000 ml 106
The concentration unit molarity 1M = moles per liter2 is defined as one mole per unit volume of substance. Avogadro’s number is used to define a mole (mol) such that there are 6.02 * 1023 molecules of any material per mole. By definition, the atomic mass of an element or substance is equal to the mass of one mole of that substance. Example 2.4 applies this concept to a solution of water containing sodium nitrate. Concentrations may also be described on a molar, mass, or volume percentage basis. For example the mass percent of species A in solution is defined as follows: mass A 1%2 =
MA * 100 MA + MB
(2.3)
where: MA = mass of compound A, lbm 1g2, and MA + MB = total mass of the solution, lbm 1g2. The volume percent concentration is defined similarly, with the volumes of substances A and B substituted into Equation (2.3) for mass. The molar concentration or the mole fraction of species A is defined as xA =
moles A in solution total moles of solution
EXAMPLE 2.4 Determining solution concentration A researcher has added 2.06 g of the salt sodium nitrate, NaNO3 , to a cylinder containing water. The cylinder diameter is 2 inches and the depth of the water in the cylinder is 4 inches (refer to Figure 2.1). Determine the concentration of salt in solution, showing your answer in the units specified. a. mg/L b. mass % c. molarity
22
Chapter 2
Introduction to Environmental Engineering Calculations: Dimensions, Units, and Conversions
D 2 in.
h 4 in.
Figure 2.1 Schematic of cylinder described in Example 2.4.
Solution First, calculate the molecular weight of NaNO3 by summing the atomic weights of the individual compound elements. The atomic weights of sodium, nitrogen, and oxygen are 23, 14, and 16 g/mole, respectively. MW NaNO3 = 23 + 14 + 31162 = 85 g/mole Assume the density of water is 1 g/ml. Part a.
Calculate the volume of water stored in the cylinder. V =
pD2h 4 p B 12 in.2a
V =
2.54 cm 2 bR 1 in.
* 4 in. a
2.54 cm b = 206 cm3 1 in.
4 1 ml 1L V = 1206 cm32a ba b = 0.206 L 3 1000 ml 1 cm 2.06 g 1000 mg CNaNO3 1mg/L2 = a ba b = 10,000 mg/L 0.206 L 1g
Recognize that if the solution density is assumed to be 1 g/ml, then 10,000 mg/L is equal to 10,000 ppm. Part b.
Mass A 1%2 =
MA * 100 MA + MB
Find the mass of solution (MA + MB) by multiplying the solution volume found in Part a above with the density. mass A 1%2 =
2.06 g 206 ml *
1g ml
* 100 = 1.0%
The solutions to Parts a and b guide us to a good “rule of thumb” to remember! Referring back to this example, we notice that if a solution has a density near 1 g/ml, then: 1% = 10,000 mg/L = 10,000 ppm
Section 2.3
Essential Derived Units and Conversion Practice 23
Part c. Brackets [ ] are often used to symbolize concentration in molarity. Brackets will be used in this problem and throughout this book. moles NaNO3 L
3NaNO34 =
12.06 g NaNO32 ¢ 3NaNO34 =
1 mol ≤ 85 g NaNO3
0.206 L
= 0.118 M
Molecular weight can be used to convert between mass- and molar-based concentrations. Using the information from Example 2.4, we can show that: CNaNO3 = MWNaNO3 3NaNO34 g g mol CNaNO3 = 85 * 0.118 = 10 mol L L Gas-phase concentration units are usually presented on a mass-per-unit-volume-of-air basis 1mg/m32 at a known temperature and pressure or as parts per million on a volume basis (ppmv). The ppmv is simply the volume or mole fraction of the pollutant in the gas multiplied by 106. ppmv =
volume of pollutant gas moles of pollutant gas 11062= 11062 volume of gas mixture total moles of gas mixture
(2.4)
Conversion between mass per unit volume and ppmv concentration requires knowledge of the ideal gas law: PV = nRT
(2.5)
where: P V n R T
= = = = =
absolute pressure, atm, volume occupied by n moles of gas, L, number of moles of gas, # universal gas constant, 0.08206 L atm冫gmol # K, and absolute temperature, K.
R, the “universal gas constant,” has the same value for all gases. The values for R with different pressure, volume, and temperature units are shown in Table 2.5. Notice that a common set of units must be used to solve Equation (2.5). Table 2.5 Universal Gas Constant, R 0.08206 L
3# # atm 冫gmol # K = 0.08206 m atm冫kgmol # K
8.314 J冫gmol # K = 8.314 kPa
# m3 冫kgmol # K
1.987 cal冫gmol # K = 1.987 BTU冫lbmol # °R 3
# atm 冫gmol # K
3
# atm 冫lbmol # °R
82.06 cm
0.7302 ft
24
Chapter 2
Introduction to Environmental Engineering Calculations: Dimensions, Units, and Conversions
EXAMPLE 2.5 Practice with the ideal gas law Use the ideal gas law to calculate the volume of one mole of gas at standard temperature and pressure (STP), defined as 0°C and 1 atmosphere. Solution Convert 0°C into Kelvin: T = 0°C + 273 = 273 K and recognize that 1 gram mole = 1 gmol. Solving for the volume of the gas sample by rearranging Equation (2.5), the ideal gas law becomes V =
nRT P
Since we are interested in the volume of just one mole of gas, substitution provides L # atm b1273 K2 gmol # K = 22.4 L 1 atm
11 gmol2a0.08206 V =
So, for a gaseous system at STP, one mole of gas occupies 22.4 L.A similar calculation shows that at 25°C and atmospheric pressure, one mole of gas occupies 24.5 L. Consider committing both of these numbers to memory.
EXAMPLE 2.6 Unit-conversion practice At standard conditions, the exhaust gas from a well-controlled coal-burning power plant contains 5 ppmv sulfur dioxide 1SO22. At STP, determine the concentration of SO2 emitted on a mg/m3 basis. MW SO2 32 2(16) 64 g/mol. Solution SO2 ¢
mg
≤ = 3
m
5 m3 SO2 6
3
10 m exhaust gas *
*
mol 1000 L * 22.4 L m3
64 g SO2 106 mg mg * = 14,286 3 g mol m
Simplifying the above solution, we obtain Equation (2.6), which can be used to convert between mg/m3 and ppmv in gaseous environments. mg m3
=
ppmv * MW * 103 22.4
1at STP2
(2.6)
Section 2.3
Essential Derived Units and Conversion Practice 25
In Equation (2.6), MW represents the molecular weight of the contaminant, and the number 22.4 is the volume of one mole of an ideal gas at standard conditions. This number is a variable in the conversion equation but can easily be calculated for any environmental condition, as shown in Example 2.5 above. For example, if the system temperature were 25°C at atmospheric conditions, Equation (2.6) would become: mg m3
=
ppmv * MW * 103 24.5
1T = 25°C; standard P = 1 atm2
EXAMPLE 2.7 Conversion of gas-phase concentration units The concentration of carbon dioxide exiting a car’s tailpipe is approximately 12% on a volume basis. Assuming the exhaust-gas temperature is 500°C, determine the concentration of CO2 in the gas stream in the units and temperature specified. a. ppmv at 500°C and 25°C, respectively b. mg/m3 at 500°C and 25°C, respectively Solution Part a. The tailpipe’s concentration is 12% at 500°C. This can easily be made into a ratio. X parts 12 X = = 6 100 million parts 10 6 12 * 10 ppmv = X = = 120,000 ppmv 100 12% =
To determine the exhaust-gas concentration in ppmv at a temperature of 25°C, it is important to notice that the ratios of different gas volumes in a system remain the same at any temperature, as long as condensation does not occur, and that all gases retain an ideal character. So, the concentration of the tailpipe exhaust at 25°C is 120,000 ppmv, equal to the value at 500°C. Part b. Use Equations (2.5) and (2.6) to solve for the gravimetric concentrations of CO2 . Calculate the volume of 1 mole of ideal gas at 500°C. 11 gmol2a0.08206 V =
L # atm b1273 + 5002 K gmol # K = 63.4 L 1 atm
Modify Equation (2.6), such that 63.4 L is the volume representing a mole of gas at 500°C and solve for the mass-based concentration. The molecular weight of CO 2 is 44 g/mol. g 120,000 * 44 * 103 3 mg ppmv * MW * 10 mol = = 63.4 63.4 m3 gCO2 7 mgCO2 = 8.33 * 10 = 83.3 m3 m3
26
Chapter 2
Introduction to Environmental Engineering Calculations: Dimensions, Units, and Conversions
In a similar fashion, the gravimetric concentration of carbon dioxide at 25°C, where the volume of one mole of gas occupies 24.5 L, was found to be approxg CO2 . imately 215 m3
Gas-phase concentration can also be expressed as a mole fraction, yA . Gasphase mole fractions are simply defined as the ratio of the moles of compound A 1nA2 to the total moles (n) in the gas phase or mathematically as yA = nA>n. Furthermore, if we assume an ideal system, the partial pressure of compound A 1PA2 is determined using the ideal gas law such that: PA =
nA RT V
The partial pressure 1Pi2 is defined as the pressure that would be exerted by a single component in a gas mixture if it alone occupied the same volume at the same temperature of the mixture. Combining this knowledge with Dalton’s law, which explains that the total system pressure is equal to the sum of all the partial pressures 1Pi2 of the parts (mathematically, Dalton’s law is expressed RT ni ), we can now express yA as a ratio of either moles as P = a Pi = V a i i or pressures as follows: yA =
nA PA = n P
(2.7)
2.3.3 Flow rate The concept of flow rate appears in many types of environmental engineering applications, such as stream flow, pipe flow for water distribution, groundwater flow, flue gas flow, and many (many) others. Flow rate can be presented on either a mass or volumetric basis. The mass-based flow rate, Qm , has dimensions of mass per unit time, with typical units being kg/s and lbm/hr. Similarly, when flow rate is described on a volumetric basis, Q, typical units are million gallons/day (MGD), m3/d, ft3/s, L/s, and gal/min (gpm). The density of the fluid flowing is used to relate mass flow to volumetric flow, as shown in Equation (2.8). Typical units for each quantity are included to verify the relationship between mass and volumetric flow. Qm = r * Q kg kg m3 = 3 * s s m
(2.8)
Earlier in the chapter we noted the similarity between Equations (2.1) and (2.2), which define density and gravimetric concentration, respectively. This knowledge lets us replace r in the above equation with C from Equation (2.2), allowing us to calculate the mass flow of a substance in a stream or pipe if we know the associated volumetric flow rate and the compound concentration.
Section 2.3
Essential Derived Units and Conversion Practice 27
EXAMPLE 2.8 Illustration of flow-rate calculation Drinking water delivered to homes from a municipality contains a residual freechlorine concentration of approximately 0.5 mg/L. If a sprinkler system delivers 6 gpm of water, calculate the mass flow rate of free chlorine released into the yard. Solution Assume that no residual chlorine evaporates (this is a convenient but probably not a very good assumption). Modify Equation (2.8) by substituting C for to calculate the mass flow rate of chlorine spread onto the lawn. Qm = C * Q mg g gal 60 min 24 h 3.785 L = a0.5 * b * a6 * * * b L 1000 mg min h d gal g = 16.3 d Let’s work another example and pay particular attention to conversions. Assume that a wastewater treatment system effluent flow is 20 million gallons per day (MGD) and it contains 10 mg/L of solids. Calculate the mass flow rate of solids discharged with units of lbm/day. Qm = C * Q mg kg 2.2 lbm 106 gal MG 3.785 L = 10 * 20 * * * b ¢ 6 L d kg MG gal 10 mg = 1665
lbm d
All unit conversions are done within the parentheses in the equation above. Canceling all units inside the parentheses and proceeding with the multiplication of the conversion factors, we derive the following well-known conversion factor: 8.34
lbm * L mg * MG
This conversion factor makes it simple to determine the mass flow rate in lbm/day when concentration and volumetric flow data are given in mg/L and MGD, respectively. We now solve the above example as Qm = 10
mg lbm * L lbm * 20 MGD * 8.34 = 1668 L mg * MG d
2.3.4 Residence time The term residence time ( ) has two meanings, but we need only one equation for both. By definition, residence time (also known as “detention time” and “retention time”) is: 1. the time required to fill a container, and 2. the average time a fluid spends in a given reactor or control volume (V).
28
Chapter 2
Introduction to Environmental Engineering Calculations: Dimensions, Units, and Conversions
Residence time, u, is easily calculated for both definitions as follows: u =
V Q
(2.9)
EXAMPLE 2.9 Residence-time calculation Consider a lagoon that can be modeled as a perfect hemisphere having a diameter of 50 m (refer to Figure 2.2). If river water flows into (and out of) the reservoir at 7.5 m3/min, calculate the residence time of the lagoon. Solution 4 3 pr 3 Use Equation (2.9) combined with unit-conversion skills to solve for the lagoon retention time. sphere volume =
u =
V = Q
1 4 ¢ p125 m23 ≤ 2 3 m3 60 min 24 h ba b ¢ 7.5 ≤a min h d
= 3.0 d
D = 50 m Q 7.5 m3/min
Figure 2.2 Schematic of pond described in Example 2.9.
Q 7.5 m3/min
2.4 PRECISION, BIAS, AND ACCURACY Most of us have been taught in mathematics courses how to deal with exact numbers. In this section we consider how to interpret measured or approximated values that might be obtained in a laboratory or industrial environment. The discussion first introduces the concepts of precision and accuracy, which are used to describe the quality or “truth” of a measurement. Although these terms are often used interchangeably, they should not be, because they have distinctly different meanings. Accuracy describes how close a measurement is to the true value, while precision indicates how well the measurement is made. So, precision depends on the reproducibility or repeatability of the measurement. Reproducibility, in turn, depends on the user’s ability and on the precision of the measuring device. Calibration or standardization influences accuracy. One way to communicate the precision of a measurement is by properly assigning and then manipulating significant figures. Unfortunately, being precise does not insure that the average of the measurements is correct. Bias is the difference between the average of the measured values and the true value.
Section 2.5
(a)
(b)
Significant Figures
(c)
Figure 2.3 Illustrating the concepts of accuracy and precision: (a) accuracy, (b) precision, (c) accuracy with precision.
The concepts of bias, accuracy, and precision can be represented in terms of the results obtained by marksmen of differing degrees of skill, as shown in Figure 2.3. If the bull’s-eye represents the truth, the marksman firing the three shots shown in Figure 2.3(a) was accurate but not very precise. Figure 2.3(b) shows the results from a precise but inaccurate shooter: the shots are close together, but far from the bull’seye. Notice that the bias was caused by a systematic (built-in) error. Consider target practice with a rifle equipped with a scope. If the scope is not properly aligned with the rifle barrel, the shots of even the most precise shooter will be directed to one side of the bull’s-eye. The marksman producing the results shown in Figure 2.3(c) was both accurate and precise.
2.5 SIGNIFICANT FIGURES Engineers and scientists use significant figures to identify the precision of a measured number. Appropriately applying significant figures to laboratory measurements allows to describe the level of confidence we have in our measurements. Use the following guidelines to determine which digits are significant. 1. When the number contains a decimal, the leftmost digit that is not a zero is considered significant. All other digits within the number are considered significant. For example, the numbers 0.254, 0.0254, and 2.54 all contain three significant figures, but 0.025 and 0.0025 each contain only two. 2. Terminal zeros placed to the right of a decimal point are assumed to be significant. Thus, the numbers 10.40 and 10.00 each have four significant figures, while 10.0 has three. 3. If the number does not contain a decimal, the rightmost nonzero digit is considered significant, as are all other digits in the number. This rule can cause uncertainty. For example, the number 400 has only one significant figure, while the number 400. has three. It is recommended that scientific notation be used to reduce this ambiguity. For example, 3000 is expressed as 3 * 103 if one digit is significant or as 3.00 * 103 if three digits are significant. 4. Internal zeros are assumed to be significant, such that 30.24 has four significant figures.
29
30
Chapter 2
Introduction to Environmental Engineering Calculations: Dimensions, Units, and Conversions
When measured values are used in calculations, the precision of the result obtained should not exceed the precision of the measured values. In general, the following rules should be applied: 1. For addition and subtraction, the solution should contain the same number of decimal places as the measurement that contained the least number of significant figures. For example, 3.23 + 1.0 = 4.2. Notice that the solution is given to only one decimal place and two significant digits—the same number of significant digits as in 1.0. 2. When a value is obtained by multiplying or dividing measured quantities, the result should contain only as many significant figures as there were in the measurement with the fewest significant digits. Applying this rule, the product of 3.07 * 3.1 should be reported with only two significant digits—that is, as 9.5.
S U M M A RY Analytical problem solving is a skill all engineering students need to practice. This chapter introduced some basic concepts that the competent problem solver must master. The ideas associated with units and dimensions were introduced. Several schemes used to classify units were presented, including the SI and U.S. Customary systems. Four derived units (density, concentration, flow rate, and residence time) were defined. The concept of unit conversion was explored and a variety of examples were completed that exemplify this skill. The precision and accuracy of measurements taken in the laboratory were discussed.
KEY WORDS
accuracy concentration Dalton’s law density dimension
flow rate ideal gas law mass percent molarity precision
residence time significant figures volume percent
REFERENCES Code of Federal Regulations, Title 40: Protection of Environment, Chapter 1: Environmental Protection Agency, Subchapter D: Water Programs, Part 141: National Primary Drinking Water Regulations, §141.51: Maximum contaminant level goals for inorganic contaminants. Isbell, D., Hardin, M., and Underwood, J. (1999). Mars Climate Orbiter Team Finds Likely Cause of Loss. Retrieved July 20, 2006 from http://mars.jpl.nasa.gov/msp98/orbiter/.
EXERCISES 2.1
Perform the following unit conversions by hand. You may use the unitconversion feature on your calculator to check your work. (a) Convert 4000 grams into pounds. (b) Convert 237.4 feet to meters. (c) Convert 80 years to seconds. (d) Convert 55 mph into a speed of nm per second. (e) Convert 0.004 gallons into ml. (f) Convert 10.8 mm to angstroms.
Exercises 31
(g) (h) (i) (j) (k) (l) 2.2 2.3 2.4 2.5
2.6 2.7 2.8 2.9
2.10
2.11 2.12 2.13 2.14
2.15 2.16
2.17
Convert 87ºF into Celsius and Kelvin. Convert 54ºC to degrees Fahrenheit. Convert 13.2 atmospheres to newtons per square meter (pascals). Convert 20.4 kilocalories to joules. Convert 1000 megawatts to horsepower. Convert 20.8 ft/s to mi/hr.
A cube occupies 0.3358 ft3. What is its volume in mm3? A box measures 4.35 ft in length, 0.03899 yd in width, and 8.12 inches in height. Determine its volume in cubic centimeters. The density of a given object is known to be 3.25 * 10-4 lbm /in3. Determine its density in g/ml. A Styrofoam cooler is made with outside dimensions of 4.00 ft * 3.00 ft * 2.00 ft and inside dimensions of 3.50 ft * 2.50 ft * 1.50 ft. Assume the average density of the Styrofoam is 0.03 g/cm3. Determine the volume of Styrofoam used in cubic meters and the weight in pounds. Consider a piece of platinum having a volume of 13.4 ml and mass of 286.8 g. Calculate the density of platinum in g/ml. A liter of water was found to contain 5 mg of benzene 1C 6H 62. What is the concentration of benzene in (a) mg/l, (b) ppm, and (c) molarity? The density of ethanol is 0.789 g/ml. Find the mass of 20 ml of ethanol. A mass of 0.525 g of sodium chloride 1NaCl2 has been added to a cylinder containing water.The cylinder diameter is 1.5 inch and the depth of the water in the container is 5 inches. Determine the concentration of the sodium ion in solution, showing your answer in units of (a) mg/L, (b) mass %, and (c) molarity. The sulfur dioxide 1SO22 stack-gas concentration from fossil-fuel combustion is 2 ppmv. Determine the stack-gas SO2 concentration in units of mg>m3. Assume 0ºC and 1 atm pressure. Calculate the volume of 1 mole of SO2 at 25ºC and 1 atm. Express your answer in m3. The smoke inhaled from a cigarette contains approximately 400 ppmv carbon monoxide. Express this concentration as a percentage of air inhaled. The average exhaled carbon dioxide 1CO22 concentration from a human breath is 4%. Convert this concentration value to ppmv. Fill a rigid cylinder having a volume of 25.0 L with nitrogen gas to a final pressure of 20,000 kPa at 27ºC. Determine the number of moles of N2 gas the cylinder contains. A 1-kg block of dry ice (solid CO2) vaporizes to gas at room temperature. Determine the volume of gas produced at 25ºC at 975 kPa. Assume the discharge from a wastewater treatment plant has a flow of 30 MGD with a solids concentration of 5 mg/L. Determine the mass flow rate of solids discharged in unit of lbm/day. Consider a rectangular wastewater treatment cell having a length of 100 ft, width of 20 ft, and depth of 20 ft. If the flow into the cell is 50 ft3/min, calculate the residence time of the treatment cell.
CHAPTER
3
Essential Chemical Concepts Objectives In this chapter, you will learn about: Fundamental concepts in atomic theory Chemical reactions Acid-base chemistry Carbonate system and alkalinity Solid-phase reactions Gas laws
3.1 INTRODUCTION A solid foundation in chemical concepts is vital to both the environmental engineering practitioner and the student. Chemistry is central to your understanding water and wastewater treatment processes and the design of treatment strategies for mitigating hazardous waste. You will need a knowledge of chemistry to understand issues associated with anthropogenic emissions causing tropospheric ozone generation, acid rain, and the greenhouse effect. This chapter gives an overview of essential chemistry concepts.
3.2 FUNDAMENTALS 3.2.1 Atoms The atom is the smallest part of an element that can exist and still retain all the chemical properties associated with the element. The most current model of the atom, provided by nuclear physicists and often referred to as the Standard Model, includes components such as mesons, baryons, and quarks. The model developed by Niels Bohr in 1913, however, is still useful. Bohr postulated that the atom maintains a neutral charge and consists of a positively charged nucleus and a negatively charged atomic shell that contains electrons. The nucleus is composed of positively charged protons and of neutrons that have no charge. For an atom to maintain a neutral charge, the number of protons and electrons must be equal. 3.2.2 Elements All matter is composed of 103 fundamental substances called elements, which are distinguished by the number of protons within the atom.The number of protons in the nucleus, Z, is the element’s atomic number. The sum of protons and neutrons in an atom’s nucleus is the element’s mass number, A. Atoms of the same element contain the same number of protons, but the number of neutrons may vary to form isotopes. Isotopes are atoms of the same element that contain a different number of neutrons and thus have different weights.
Periodic Table of the Elements
1 1A 1 1
11
H
3
4
5
6
7
Atomic number Element symbol Element name
He
13 3A 5
14 4A 6
15 5A 7
16 6A 8
17 7A 9
Helium
B
C
N
O
F
Ne
3
2 2A 4
Li
Be
Lithium
Beryllium
Boron
Carbon
Nitrogen
Oxygen
Fluorine
Neon
6.94
9.01
10.81
12.01
14.01
16.00
19.00
20.18
Hydrogen
1.01
2
Na
18 8A 2
Sodium
22.99
Average atomic mass*
4.00
10
11
12
13
14
15
16
17
18
Na
Mg
Al
Si
P
S
Cl
Ar
Sodium
Magnesium
22.99
24.31
19
20
3 3B 21
4 4B 22
5 5B 23
6 6B 24
7 7B 25
K
Ca
Sc
Ti
V
Cr
Mn
Potassium
Calcium
Scandium
Titanium
39.10
40.08
44.96
47.87
50.94
52.00
54.94
37
38
39
40
41
42
43
Rb
Sr
Y
Zr
Nb
Mo
Tc
Rubidium
Strontium
Yttrium
Zirconium
85.47
87.62
88.91
91.22
Vanadium Chromium Manganese
12 2B 30
Aluminum
Silicon
Phosphorus
Sulfur
Chlorine
Argon
26.98
28.09
30.97
32.07
35.45
39.95
28
11 1B 29
31
32
33
34
35
36
Co
Ni
Cu
Zn
Ga
Ge
As
Se
Br
Kr
Iron
Cobalt
Nickel
Copper
Zinc
Gallium
Germanium
Arsenic
Selenium
Bromine
Krypton
55.85
58.93
58.69
63.55
65.39
69.72
72.61
74.92
78.96
79.90
83.80
44
45
46
47
48
49
50
51
52
53
54
Ru
Rh
Pd
Ag
Cd
In
Sn
Sb
Te
I
Xe
Rhodium
Palladium
Silver
Cadmium
Indium
Tin
Antimony
Tellurium
Iodine
Xenon
102.91
106.42
107.87
112.41
114.82
118.71
121.76
127.60
126.90
131.29
8
10
26
9 8B 27
Fe
Niobium Molybdenum Technetium Ruthenium
92.91
95.94
(98)
101.07
55
56
57
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
Cs
Ba
La
Hf
Ta
W
Re
Os
Ir
Pt
Au
Hg
Tl
Pb
Bi
Po
At
Rn
Cesium
Barium
Lanthanum
Hafnium
Tantalum
Tungsten
Rhenium
Osmium
Iridium
Platinum
Gold
Mercury
Thallium
Lead
Bismuth
Polonium
Astatine
Radon
132.91
137.33
138.91
178.49
180.95
183.84
186.21
190.23
192.22
195.08
196.97
200.59
204.38
207.2
208.98
(209)
(210)
(222)
87
88
89
104
105
106
107
108
109
Rf
Db
Fr
Ra
Ac
Francium
Radium
Actinium
(223)
(226)
(227)
Rutherfordium Dubnium
(261)
(262)
Hs
Mt
Hassium
Meitnerium
(266)
(264)
(269)
(268)
58
59
60
61
62
63
64
65
66
67
68
69
70
71
Pr
Nd
Pm
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Praseodymium Neodymium Promethium Samarium
Thulium
Ytterbium
Lutetium
140.91
144.24
(145)
150.36
151.96
157.25
158.93
162.50
164.93
167.26
168.93
173.04
174.97
90
91
92
93
94
95
96
97
98
99
100
101
102
103
Th
Pa
U
Np
Pu
Am
Cm
Bk
Cf
Es
Fm
Md
No
Lr
232.04
Protactinium Uranium
231.04
238.03
Europium Gadolinium
Neptunium Plutonium Americium
(237)
(244)
(243)
Curium
(247)
Terbium
Dysprosium Holmium
Berkelium Californium Einsteinium
(247)
(251)
(252)
Fermium Mendelevium Nobelium Lawrencium
(257)
(258)
(259)
(262)
Section 3.2
Erbium
140.12
Thorium
Fundamentals 33
Figure 3.1 Periodic table.
Bh Bohrium
Ce Cerium
* If the average atomic mass number is in parentheses it refers to the atomic mass of the most stable isotope.
Sg Seaborgium
34
Chapter 3
Essential Chemical Concepts
The atomic weight (AW) of an element is relative to the weight of carbon-12 and is equal to the mass of one mole of that element. The atomic weight of each element is listed in the periodic table (Figure 3.1) and is the weighted average AW based on the abundance of individual isotopes found in nature. The molecular weight (MW) of a compound is the sum of the AWs of the individual elements in the compound. Sometimes we incorrectly use the term MW to refer to the AW of an element, even though single atoms of elements are not molecules. Strictly speaking, the term AW should be used for a single atom and MW for molecules. Consider hydrogen: the AW is 1 g/mol, and the MW for H 2 is 2 g/mol. EXAMPLE 3.1 Determine atomic and molecular weights Find the AW of each element and the MW of each compound in grams per mole (g/mol) for each item in the list below. Use AW information from the periodic table. Solution After using the periodic table to determine the AW of each element, calculate the molecular weight of calcium carbonate and sodium bicarbonate by summing the atomic weights of the elements in each of these compounds. In most environmental engineering problems, it is acceptable to round AWs and/or MWs to two (and oftentimes fewer) decimal places. Element or compound
Symbol
Atomic or molecular weight (g/mol)
Carbon Hydrogen Calcium Sodium Oxygen Calcium carbonate
C H Ca Na O CaCO3
12.01 1.01 40.08 22.99 16.00 40.08 + 12.01 + 3 1162 = 100.09
Sodium bicarbonate
NaHCO3
22.99 + 1.01 + 12.01 + 31162 = 84.01
A radical or free radical is typically a very reactive element, compound, or ion (such as H +, F -, or Cl-) that has an unpaired electron in its outermost electron shell. Table 3.1 identifies some radicals frequently observed within the environment that are generally treated as a single unit, because they are assumed not to dissociate in solution. The equivalent weight (EW) of an element or radical provides information on its reactivity. EW is calculated by dividing the element’s AW or radical’s MW by its assumed valence (or charge), z: EW =
MW z
(3.1)
In Equation (3.1), z represents the valence1 of the reactive unit. Although commonly used to describe acid-base reactions, the reactive unit in any chemical reaction is often 1
Note that the concept introduced in Equation (3.1) has been simplified. Equivalent weight is commonly defined using three methods. As discussed above, the first method relies on use of the valence the element or radical assumes in a given compound. In acid-base reactions, the equivalent weight depends on the number of protons or hydroxyl radicals that react. The equivalent weight for oxidation-reduction reactions is based on the number of moles of electrons transferred. Further discussion of these topics is beyond the scope of this book.
Section 3.2
Fundamentals 35
referred to as an equivalent. (A reactive species can have multiple EWs because of its involvement in multiple types of reactions.) Typical units for EW include both grams per equivalent (g/eq) and milligrams per milliequivalent (mg/meq). Normality, N, relates the mass of solute per volume of solution, as presented in Equation (3.2). A 1 N solution contains 1 equivalent weight of a substance per liter of solution. mass of material>L of solution
number of equivalents (3.2) = EW L of solution Recalling that the concentration unit molarity (M) is defined as the moles of solute per liter of solution, the relationship between N and M is a simple one, indicating that the normality of a solution is never less than the concentration of a molarity-based solution. N =
N = z * M
(3.3)
Table 3.1 Common Ions and Radicals Associated with Environmental Chemistry Radical
Chemical formula
MW (g/mol)
EW (g/eq)
Bicarbonate
HCO3-
61
61
Carbonate
CO23
60
30
Ammonium
NH4+
18
18
-
17
17
Hydroxyl
OH
Hypochlorite
OCl-
51.5
Nitrate
NO3-
62
62
Nitrite
NO2-
46
46
Orthophosphate
PO34
95*
31.7*
Sulfate
SO24
96
48
Hydrogen sulfate
HSO4-
97
97
-
51.5
Cyanide
CN
26
26
Permanganate
MnO4-
119
119
Chromate
CrO24
116
58
Dichromate
Cr2O27
216
108
*Example calculations for both molecular and equivalent weight of the free radical orthophosphate follows: Molecular weight for phosphorus (P) is 31 and for oxygen (O) is 16. So, the MW of PO34 = 31 + 41162 = 95 g>mol. The equivalent weight for the radical is simply the MW divided by its 95 g>mol valence of 3, giving EW of PO3= 31.7 g>eq. 4 = 3 eq>mol
Calculation of equivalent weight Determine the equivalent weight for the following species: a. magnesium ion, Mg 2+ b. calcium carbonate, CaCO3 c. carbon dioxide, CO2
EXAMPLE 3.2
36
Chapter 3
Essential Chemical Concepts
Solution part a From the periodic table (Figure 3.1), the AW of Mg 2+ is 24.3 g/mol, and the valence is z = 2 in solution. Calculate the EW using Equation (3.1). EW for Mg 2+ =
24.3 g>mol g MW = = 12.15 z eq 2 eq>mol
Solution part b AW for Ca = 40, for C = 12, and for O = 16 g/mol, so the MW of CaCO3 is 100 g/ mol. In solution, calcium carbonate dissociates into calcium and the free radical carbonate, as follows: CaCO3 Δ Ca2+ + CO23 Notice that the valence associated with the reaction is z = 2, so EW for CaCO3 =
100 g>mol g MW = 50 = z eq 2 eq>mol
Solution part c The MW of CO2 is 44. In solution, carbon dioxide forms carbonic acid and has the potential of giving up 2 H + ions. CO2 + H2O Δ H2CO3 Δ H+ + HCO3- Δ 2H+ + CO23 EW for CO2 =
44 g>mol g MW = 22 = z eq 2 eq>mol
EXAMPLE 3.3 Calculate concentration in normality Find the normality (N) of the following solutions: a. 0.25 M solution of CaCO3 . mg b. 588 solution of K 2Cr2O7 L Solution part a Using Example 3.2, Part b, the equivalent weight of CaCO3 is 50 g/eq and z = 2. Use Equation (3.3). N = z * M = 2 * 0.25 M = 0.50 N solution of CaCO3 Solution part b AW for K = 39, and for Cr2O27 MW = 112 * 522 + 17 * 1622 = 216 g/mol. In solution, potassium dichromate dissociates into potassium (K) and the free radical dichromate 1Cr2O27 2 as follows: K 2Cr2O7 Δ 2 K+ + Cr2O27
Section 3.3
Notice the valence associated with the reaction is z = 2. MW for K 2Cr2O7 = 112 * 392 + 2162
g g = 294 mol mol
Convert the solution concentration to M: g mol K 2Cr2O7 588 mg * * = 0.002 M = 2 mM K 2Cr2O7 L 1000 mg 294 g Using Equation (3.3) to determine normality gives: N = z * M = 2 * 0.002 M = 0.004 N solution of K 2Cr2O7
3.3 CHEMICAL REACTIONS Chemical reactions describe the transformation of one or more elements or compounds (reactants) into different products. A chemical equation shows what happens during a chemical reaction. Chemical reactions traditionally are grouped into five classifications: 1. Synthesis or combination reactions––two or more reactants combine to form a third compound. In general, this type of reaction can be written as A + B : AB, where A and B represent either elements or compounds. 2. Decomposition reactions––a reactant decomposes or breaks down into multiple elements or compounds. A decomposition reaction is the opposite of a synthesis reaction and can be represented as AB : A + B. 3. Single-replacement reactions––an element reacts with a compound and trades places with another element in that compound such that A + BC : AC + B. 4. Double-replacement reactions (metathesis reaction)—reacting compounds exchange cations (positively charged ions) such that AB + CD : AD + CB. 5. Combustion reactions––an organic substance reacts with oxygen to form carbon dioxide and water. Such reactions are typically exothermic (giving off heat) and can be represented, for example, by the burning of propane: C 3H 8 + 5 O2 : 3 CO2 + 4 H 2O. The following equations present two simple synthesis reactions involving lime or calcium oxide (CaO). Equation (3.4) examines the reaction often referred to as slaking that involves heating a mixture of water and lime to produce calcium hydroxide. Equation (3.5) presents the reaction where lime is added to sewage to remove phosphates by precipitation. CaO1s2 + H 2O1l2 : Ca1OH221s2
(3.4)
3 CaO1s2 + 3 H 2O1l2 + 2 PO34 1aq2 : Ca 31PO4221s2 + 6 OH 1aq2
(3.5)
Notice, in both reaction equations, that the reactants and products are separated by an arrow, indicating that products to the right are yielded by or formed from the specified reactants located to the left of the arrow. Although not required, the state of the reactant and product is given in parentheses as a subscript. The subscripts (s), (l), and (aq) refer to solid, liquid, and aqueous, respectively. Finally, the equations must be balanced such that the mass of each element involved is conserved throughout the reaction, so each side of the equation must contain the same number of moles of each element. From Equation (3.4), notice that 1 mole of CaO and 1 mole of water
Chemical Reactions
37
38
Chapter 3
Essential Chemical Concepts
are required (for a total of 2 moles of reactant) to produce 1 total mole of product, Ca1OH22 . Similarly, Equation (3.5) requires 3 moles of CaO to form one mole of product, Ca31PO422 . Although balancing an equation is often an arduous task for students as they begin their chemistry endeavors, when it is done properly the ratio of the number of moles produced to reactants involved is apparent. Recognize that essentially all reactions are reversible and that both the forward and reverse reactions are taking place simultaneously. Consider the reversible reaction shown in Equation (3.6). aA + bB Δ cC + dD
(3.6)
At equilibrium, the quantitative relationship between reactants and products present in solution, or the extent of the reaction, is described by the equilibrium constant, K. For sufficiently diluted solutions, as commonly found in environmental systems, K, as defined in Equation (3.7), is a constant for a given temperature and is independent of the initial concentrations of reactants and products. Pure solids are excluded from the equilibrium-constant equation, because they have a constant tendency to dissolve, and by definition their activity (or concentration in dilute solutions) is equal to 1. Water, or the solvent, is not included, as it has no effect on equilibrium. K =
3C4c 3D4d
(3.7) 3A4a 3B4b where a, b, c, and d are the number of moles of constituents A, B, C, and D, respectively. The brackets, [ ], denote concentration of a constituent in moles per liter. 3.3.1 Stoichiometry Environmental engineers often use chemical equations for estimating the quantity of chemicals that must be added to water or wastewater to achieve a specific result. For example, the quantity of sludge that will be produced can be determined from a balanced chemical equation. An equation is balanced when the law of the conservation of mass is reflected: elements are not created or destroyed, just rearranged by the reaction. Mass and charge are conserved in all chemical reactions, such that the quantity of each type of element is equal on both sides of the equation and each compound has a net charge of zero. When mass and charge are properly balanced, the chemical equation should represent the stoichiometry observed in a reaction. The concept of balancing reactions is fundamental to chemistry. Although equations can be balanced algebraically using linear systems, usually they are balanced by inspection. To balance an equation by inspection, follow these simple rules: 1. Insure that all products and reactants are represented by the correct chemical formula, appropriately written on the proper side of the equation, and are separated by plus signs. 2. Identify the most complicated compound in the equation. Assume that only one unit or mole of this compound is in the reaction. Balance the elements in this compound one at a time, placing coefficients appropriately in front of products and reactants in the equation. 3. Focus your initial attention on those atoms present in the fewest compounds. 4. Continue the trial-and-error process for the remaining compounds. As a general rule, balance the atoms that are most prevalent (i.e., hydrogen and oxygen) near the end of the inspection. 5. Remove fractions and simplify such that the group of coefficients remaining are the smallest whole numbers possible. Use these rules of inspection to balance the equations in the example that follows.
Section 3.3
Chemical Reactions
39
EXAMPLE 3.4 Balancing chemical reactions a. Because of the promulgation of the 1970 Clean Air Act, tetraethyl lead was phased out of gasoline, and methyl tertiary-butyl ether (MTBE) was added instead to serve as an octane booster. The aim was to make the internal combustion engine “knock” less and burn “cleaner,” emitting fewer unburned hydrocarbons into the atmosphere. Balance the combustion reaction of MTBE 31CH 323COCH 34 in a pure oxygen environment. b. The formation of rust is a single-replacement reaction (it is also an oxidation-reduction reaction) that commonly occurs in the environment. Rust is formed when the iron (Fe) present in steel is oxidized to a hydrated form, iron(III) oxide or ferric oxide, in the presence of oxygen and water. Balance the reaction that describes the formation of rust. [Hint: Hydrous ferric oxide 1Fe2O3 # 3 H 2O2 is the primary component in red-brown rust.] Solution part a 1. Recognize that carbon dioxide and water will be formed in the combustion of a hydrocarbon and write appropriate products and reactants. 1CH 323 COCH 3 + O2 : CO2 + H 2O 2. Assume only 1 mole of MTBE is reacting, and balance CO2 production by observing that 5 moles of C are reacting for each mole of MTBE. 1CH 323 COCH 3 + O2 : 5 CO2 + H 2O 3. Count the number of H atoms present on the left-hand side of the equation and add the coefficient 6 to the H 2O on the right, forcing both sides of the equation to have 12 moles of hydrogen. 1CH 323 COCH 3 + O2 : 5 CO2 + 6 H 2O 15 in front of the O2 on the left-hand 2 side of the equation. Now each side has 16 moles of oxygen.
4. Balance the O by adding the coefficient
15 O : 5 CO2 + 6 H 2O 2 2 5. Remove the fractions by multiplying through by 2. 1CH 323 COCH 3 +
2 1CH 323 COCH 3 + 15 O2 : 10 CO2 + 12 H 2O Solution part b 1. Write the reactant and products present in the reaction. Fe + O2 + H 2O : Fe2O3 # 3 H 2O
40
Chapter 3
Essential Chemical Concepts
2. Recognize that 2 moles of Fe are needed to form the product. Add the coefficient 2 before the reactant Fe. 2 Fe + O2 + H 2O : Fe2O3 # 3 H 2O 3. Balance the hydrogen molecules on each side of the reaction. 2 Fe + O2 + 3 H 2O : Fe2O3 # 3 H 2O 4. Balance the oxygen molecules on each side of the reaction. 2 Fe +
3 O + 3 H 2O : Fe2O3 # 3 H 2O 2 2
5. Remove the fractions by multiplying through by 2. 4 Fe + 3 O2 + 6 H 2O : 2 Fe2O3 # 3 H 2O
EXAMPLE 3.5 Reaction stoichiometry Methane is a greenhouse gas that is about 23 times more effective on a molecular basis than carbon dioxide in causing global warming. Methane is emitted by a variety of anthropogenic sources, including rice fields and cattle (IPCC, 2001). According to a recent study by Neue (1993), rice fields annually produce 20–100 teragrams (1 Tg = 1 million metric tons) of methane through the anaerobic decomposition (methanogenesis) of organic materials that are either applied to or native to the fields. Estimating annual methane emissions of approximately 500 Tg (Cicerone and Oremland, 1988), rice growing contributes 5% to 20% of annual anthropogenic methane emissions. Using formic acid as an example of an organic that is anaerobically reduced, a. Calculate the molar ratio of methane production to carbon dioxide production when formic acid (CHOOH) is reduced. Reaction products will include methane, carbon dioxide, and water. b. Calculate the mass and volume of methane emitted from reducing 161 g of formic acid. Assume conditions of standard temperature and pressure (STP; T = 25°C and P = 1 atm) and eventual emission of all products to the gas phase. Solution part a 1. Write the reactants and products participating in the microbially mediated reaction: CHOOH : CH 4 + CO2 + H 2O 2. Balance the equation by confirming that all quantities of atoms are equal on both sides of the equation. 4 CHOOH : CH 4 + 3 CO2 + 2 H 2O
Section 3.3
Chemical Reactions
41
3. From reaction stoichiometry, recognize that 1 mole of methane and 3 moles of carbon dioxide are produced per 4 moles of formic acid. mol CH 4 1 = mol CO2 3 Solution part b 1. Determine the mass of both methane and carbon dioxide produced from 161 g of formic acid reacting. Begin by determining the number of moles of formic acid available to react by dividing by the molecular weight of the compound. 161 g CHOOH *
1 mol CHOOH = 3.5 mol CHOOH 46 g
2. Determine the mass of methane and carbon dioxide produced. Remember that the molecular weights of methane and carbon dioxide are 16 and 44, respectively. mass of CH4 1g2 = 3.5 mol CHOOH *
1 mol CH4 produced 16 g CH 4 * 4 mol CHOOH reacting 1 mol CH 4
= 14 g CH 4 And similarly for carbon dioxide: mass of CO2 1g2 = 3.5 mol CHOOH *
3 mol CO2 produced 44 g CO2 * 4 mol CHOOH reacting 1 mol CO2
= 115.5 g CO2 To find the volume of methane and carbon dioxide gas produced at STP (25°C and 1 atm), utilize the ideal gas law (Section 3.6.1), PV = nRT. Rearrange and solve for the volume of methane:
V =
nRT = p
14 g CH 4 * g 16 mol CH 4
a0.08206
L # atm b1273 + 252 K mol # K = 21.4 L 1 atm
Similarly for CO2 , 64.2 L of gas are produced for every 161 g of formic acid reduced. Notice that the volume of CO2 produced is three times that for CH 4 , as expected from the stoichiometry observed in Part a.
EXAMPLE 3.6 Reaction stoichiometry Calculate the moles of oxygen required to react with 1 mole of glucose 1C 6H 12O62 according to the following balanced chemical equation. Also determine the mass of oxygen required to oxidize 1 gram of glucose. 1 C 6H 12O6 + 6 O2 : 6 CO2 + 6 H 2O MW = 180 192
42
Chapter 3
Essential Chemical Concepts
Solution: The stoichiometric or molar ratio of O2 to glucose 1C 6H 12O62 is 6:1. This means that 6 moles of oxygen are required for the oxidation of 1 mole of C 6H 12O6 to form carbon dioxide and water. On a mass basis, 192 grams of oxygen are required to react with 180 grams of glucose to completely oxidize it to carbon dioxide and water.Therefore, 1.07 g of oxygen are required to oxidize 1 g of glucose 1192/180 = 1.072.
EXAMPLE 3.7 Practice with reaction stoichiometry a. Calculate the grams of oxygen required to oxidize 1 mole of C 5H 7O2N (general formula representing the composition of a bacterial cell) according to the following balanced chemical equation. 1 C5H7O2N + 5 O2 : 5 CO2 + 2 H2O + NH3 MW = 113 160 b. Determine the concentration of oxygen required in mg/L to oxidize a 500 mg/L solution of C 5H 7O2N. Solution part a 160 grams of oxygen are required per mole of C 5H 7O2N, or 160 grams of oxygen are required per 113 grams of C 5H 7O2N. Solution part b The concentration of oxygen required to oxidize 500 mg/L of C 5H 7O2N is given as: O2 concentration =
500 mg C 5H 7O2N 160 g 708 mg O2 a b = L 113 g L
Recognize that this is also the theoretical chemical oxygen demand (COD) of a 500-mg/L solution of C 5H 7O2N.
3.4 SOLUTION CHEMISTRY—AQUEOUS PHASE Water covers approximately 70% of the earth’s surface. Air at 75ºC having a relative humidity of 70% contains 0.013 pounds of water per pound of dry air. Clearly, understanding the basic concepts of reactions that occur in the aqueous phase is important to environmental engineers. 3.4.1 Acid-Base Chemistry The concept of pH describes acidity by providing a logarithmic relationship between the H + and OH - ion concentrations in a solution. For aqueous solutions, the pH scale is often cited as 0 to 14 without explanation. This range provides an arbitrary convenience, because it places the value for neutrality of pure water at 25ºC precisely in the middle with a pH = 7. At 25°C, if pH 6 7, the system is acidic, while a pH 7 7 indicates a more basic solution. Values of pH less than 0.0 and
Section 3.4
Solution Chemistry—Aqueous Phase
greater than 14.0 are possible, however, and are frequently prepared in laboratory settings, and also exist in nature. For example, acid mine waters characterized with a pH = - 3.6 have been encountered underground in the Richmond Mine at Iron Mountain, CA (Nordstrom et al., 2000). By definition, the pH is the negative of the logarithm of the activity or the relative hydrogen-ion concentration. For dilute solutions, pH is well defined mathematically as pH = - log 3H +4
(3.8)
where 3H +4 represents the molar 1moles H +/L2 concentration of the hydrogen ions in solution. Similarly, pOH can be defined as pOH = - log 3OH -4 such that pH + pOH = 14. The pH is useful when evaluating surface-water quality, regional and global acid-rain effects, and industrial effluent and makeup waters. Surface-water pH is influenced naturally by photosynthesis and respiration processes as well as by regional geology, such as limestone or glacial deposits. Anthropogenic sources such as acid-rain deposition and wastewater discharge also modify stream and lake pH. Although some aquatic life species can thrive under extreme pH conditions, the United States. Environmental Protection Agency (EPA) has identified that lakes having a pH 6 5 are stressed and considered acidic. In particular, fish prefer a pH between 6 and 9. Figure 3.2 shows pH ranges of common items. As defined by J.N. Brønsted and T.M. Lowry in 1923, a Brønsted-Lowry acid is a compound that donates a hydrogen ion or proton to another substance, while a base is any substance that accepts a proton. A generic acid-base reaction is shown in Equation (3.9), where on the left-hand side, HA represents an acid and B- is the base. HA B
Acid
A HB
Base
Base
Acid
(3.9) Acid-conjugate base pairs
Increasing Acidity pH Scale 0
1
2
3
4
5
6
Rain water
1M HCl
Stomach Acid
Raw apples Strawberries Grapefruit Raspberry jam
Peanut butter
Household white vinegar Wheaties Black coffee
Figure 3.2 pH Estimates for common household items.
7
8
9
10
11
12
Antacid tablets
Blood Baking soda
Household ammonia
13
14
1 M NaOH Household bleach Drain
Glass cleaner openers
43
44
Chapter 3
Essential Chemical Concepts
According to the Brønsted-Lowry theory, every time a proton is donated by an acid (HA), the conjugate base is formed 1A-2. The donated proton is accepted by a base, and an acid is formed (HB). In solution, amphoteric compounds can react as either an acid or a base. Amphoteric compounds commonly found reacting in environmental systems include NH 3 , HCO3- , HSO 4- , and H 2O. The autoionization of water is an example. H2O H2O
HO H3O
Acid
Base
Base
Acid
(3.10)
In aqueous systems, the H + ion does not actually exist, because it readily combines with water to form H 3O +, but throughout this text and many others, H + and H 3O + are used interchangeably. Equation (3.11) describes the weak acid-base reaction that occurs within a simple aqueous system. H 2O Δ OH - + H +
(3.11)
The equilibrium constant for the dissociation of water is: K =
3OH -43H +4
(3.12)
3H 2O4
For dilute solutions, the molar concentration of water is 55.6 mol/L at 25°C, allowing Equation (3.12) representing the ionization of water to simplify as follows: Kw = 3OH -43H +4 = 10-14
1at 25°C2
(3.13)
where Kw is the dissociation constant for water at 25°C. This equation can also be written as pH + pOH = 14.
EXAMPLE 3.8 Calculation of system pH and pOH Determine the pH, pOH, and 3OH -4 of an aqueous system known to have a hydrogen-ion concentration of 10-3.5 M. Assume the system is at 25°C. Solution From Equation (3.8), we know that pH = - log3H +4 Substitute 10-3.5 M for the concentration of hydrogen and solve for pH. pH = - log110-3.52 pH = 3.5 Calculate pOH and then 3OH -4 by applying Equation (3.13). pH + pOH = 14 pOH = 14 - 3.5 = 10.5
Section 3.4
Solution Chemistry—Aqueous Phase
Solve for the hydroxide-ion concentration. pOH = - log3OH -] 10.5 = - log3OH -4 3OH -4 = 3.16 * 10-11 M
3.4.2 Strong Acids and Bases vs. Weak Acids and Bases Strong acids and bases will almost completely dissociate in water. The strength of an acid or base is quantified by its dissociation constant. The dissociation constant (often called equilibrium constant) for acids is Ka and for bases is Kb and can be generalized by Equations (3.14) and (3.15), respectively. These equations provide generic acid and base dissociation reactions, followed by their associated equation, which determines the equilibrium constant. In the equations, HA is a generic acid and BOH is a generic base. HA Δ H + + AKa =
3H +43A-4
(3.14)
3HA4
BOH Δ B+ + OH Kb =
3B+43OH -4
(3.15)
3BOH4
Similar to the log-based system used to describe water acidity (pH), acid and base strength is described by the negative logarithm of the dissociation constant, as shown below: pKa = - log Ka
(3.16)
pKb = - log Kb
(3.17)
In general, if the pKa 6 1, the acid is considered strong, and it is appropriate to assume that this acid will completely dissociate in water. Similarly, base strength, or the ability to accept protons, increases as the Kb increases. The pH of a system containing a strong acid or base is relatively independent of the dissociation constant and depends instead on the concentration of acid or base added to the system. Table 3.2 shows equilibrium constants for acids and their conjugate bases that commonly participate in environmentally engineered and natural systems. The strongest acids are in the upper portion of the left column, the strongest bases in the lower portion of the right column. Notice that the product of a Ka with its conjugate Kb always equals the dissociation constant for water: Ka * Kb = Kw = 10-14
at 25°C
(3.18)
45
46
Chapter 3
Essential Chemical Concepts
Table 3.2 Equilibrium Constants for Acids and Their Conjugate Base That Are Commonly Found in the Environment (T ⴝ 25°C) Acid
pK a
Base
pKb
-
Chloride ion
17
HCl
Hydrochloric acid
-3
Cl
H2SO4
Sulfuric acid
-3
HSO4-
Bisulfate ion
17
HNO3
Nitric acid
0
NO3-
Nitrate
14
HSO4-
Bisulfate ion
2
SO24
Sulfate
12
HF
Hydrofluoric acid
3.4
F-
Fluoride ion
10.8
Nitrate
9.5
Bicarbonate
7.7
Bisulfide
6.9
HNO2
Nitrous acid
4.5
NO3-
H2CO…3
Carbon dioxide and carbonic acid
6.3
HCO3-
H2S
Hydrogen sulfide
7.1
HS
NH4+
Ammonium
9.3
NH3
Ammonia
4.7
10.3
CO23
Carbonate
3.7
12.3
PO34
Phosphate
1.7
S2-
Sulfide
0
Hydroxide
0
HCO3-
Bicarbonate
HPO24
Monohydrogen phosphate
HS-
Bisulfide
14
-
H2O
Water
14
OH
OH-
Hydroxide
24
O2-
Oxide
-10
Source: CRC, 2006
Table 3.2 shows that both HCl and H 2SO4 are strong acids, and CO23 and OH are considered strong bases. Note that the stronger the acid, the weaker its conjugate base, and the weaker the acid, the stronger its conjugate base. -
EXAMPLE 3.9 pH calculation for a strong acid Calculate the pH of a 0.01 M HNO3 solution at 25°C. Solution From Table 3.2, notice that nitric acid is a strong acid with pKa 6 1. Therefore, it is reasonable to assume that the acid completely dissociates. HNO3 : H + + NO3Stoichiometry provides that 0.01 M H + and 0.01 M NO3- are in solution, so pH = - log3H +4 pH = - log10.012 = 2
Section 3.4
Solution Chemistry—Aqueous Phase
3.4.3 The Carbonate System and Alkalinity The chemical compounds associated with the carbonate system include carbonate solids such as CaCO3(s), carbonate 1CO23 2, bicarbonate 1HCO 3 2, carbonic acid (H2CO3), aqueous carbon dioxide 1CO21aq22, and gaseous carbon dioxide 1CO21g22. In environmental water chemistry, the carbonate species participate in some of the most important acid-base reactions. For example, the carbonate system provides surface and ground waters with an acid-conjugate base pair for a natural buffer. Carbon dioxide is significant because it is produced during microbial respiration and consumed by the photosynthesis process. Carbonate species enter the atmosphere through two major pathways • By the dissolution of carbonate- or bicarbonate-containing complexes into solution. Equation (3.19) shows the equilibrium reaction involving calcium carbonate. • By the absorption of CO21g2 into the aqueous phase, shown in Equation (3.20). Ksp represents the solubility product and KH represents Henry’s Law constant. Both are similar to an equilibrium constant and will be introduced in detail in Sections 3.5 and 3.6.4. CaCO31s2 Δ Ca2+ + CO23 ; CO21g2 Δ CO21aq2 ;
-8.34 Ksp = 3Ca2+43CO23 4 = 10
KH =
3CO21aq24 CO21g2
= 10-1.5
mol L # atm
(3.19)
(3.20)
Aqueous phase carbon dioxide reacts with water to form carbonic acid. CO21aq2 + H 2O Δ H 2CO3 Because it is analytically difficult to distinguish between H 2CO3 and CO21aq2 , H 2CO…3 is commonly used to represent the sum of the two species. 3H 2CO…34 = 3H 2CO34 + 3CO21aq24
(3.21)
The term alkalinity quantifies the buffering capacity of a system, meaning how well the system can resist changes in pH with the addition of an acid. Typically, alkalinity is a function of the carbonate system and additional weak acids and bases such as ammonia, silicates, and phosphates. In natural systems, alkalinity depends strongly on regional bedrock composition. For instance, if a region’s bedrock contains dolomite 1CaMg1CO3222 or calcite, i.e., limestone 1CaCO32, —high alkalinity carbonates—the buffering capacity of surface waters is expected to be high. In contrast, surface waters in areas containing primarily igneous bedrock typically exhibit low alkalinity and therefore have a minimal capacity to buffer the system. Wastewaters may contain additional compounds such as hydroxide, ammonia, phosphates, organic acids, and silicates that contribute to alkalinity. The susceptibility of surface water to pH variations resulting from acid rain and/or industrial effluents are classified by the EPA. Table 3.3 summarizes this classification. The table indicates that surface waters with an alkalinity concentration of less than 20 mg/L as CaCO3 are deemed susceptible to acidity.
47
48
Chapter 3
Essential Chemical Concepts
Table 3.3 Classification of Surface Water Susceptibility to Acidity, Based on Alkalinity Classification
Alkalinity concentration (mg/L as CaCO3)
Critical
62
Endangered
2–5
Highly sensitive
5–10
Sensitive
10–20 7 20
Not sensitive Source: Godfrey et al.,1996; Van Loon and Duffy, 2005.
Equation (3.22) represents total alkalinity for most natural water systems and holds true for all waters having proton activity from CaCO3 only. In the equation, the carbonate concentration 3CO23 4 is multiplied by 2 because it can consume 2 moles of protons or acidity per mole of CO23 . alk a
eq + b = 3HCO 3-4 + 23CO23 4 + 3OH 4 - 3H 4 L
(3.22)
where brackets, [ ], denote concentration in moles/liter.
3.5 SOLID-PHASE EQUILIBRIUM REACTIONS Solids participate in a variety of environmental reactions. The impact of calcite dissolution on the alkalinity of natural waters has already been discussed. Equation (3.23) shows a generic equilibrium reaction for an ionic compound A aBb . Dissolution is the forward reaction, when the solid dissolves, and precipitation is the reverse reaction, when the solid is formed by ions A and B combining. A aBb Δ aA + bB
(3.23)
Equation (3.24) is an equilibrium equation based on the reaction shown in Equation (3.23). 3A4a3B4b = Ksp
(3.24)
There is no denominator in Equation (3.24) [compare to Equation (3.14) for acid-base equilibrium systems] because the concentration of a solid in solution remains effectively constant. The equilibrium constant representing a product’s solubility is Ksp , the solubility product. The relationship between Ksp and solubility indicates that sparingly soluble salts have small Ksp values while soluble salts have relatively large Ksp values. Unsurprisingly, table salt has a large Ksp value, and silver nitrite is sparingly soluble. Reviewing the solubility products of several common salts shown in Table 3.4 provides more examples. Comparing the values of Ksp for sparingly soluble solids does not predict relative solubility if the number of ions produced by the dissolution is different. Example 3.10 examines the relationship between solubility product and solubility. Recognize that the ions in the evaluated solutions are participating in only one equilibrium reaction. This is typically an inappropriate assumption for environmental samples, so the common ion effect must be considered.
Section 3.5
Solid-Phase Equilibrium Reactions
49
Table 3.4 Typical Solubility Product Constants, Ksp , for Solids Commonly Found in the Environment (T ⴝ 25°C) Compound
*Solubility product (Ksp)
NaCl – sodium chloride
36
C12H22O11 – sucrose (common table sugar)
1.97
Al1OH23 – aluminum hydroxide
1 * 10-32
AgNO2 – silver nitrite
6.0 * 10-4
AgCl – silver chloride
1.8 * 10-10
Ca31PO422 – calcium phosphate
1 * 10-27
Ca1OH22 – calcium hydroxide
5 * 10-9
CaCO3 – calcium carbonate
5 * 10-9
Cr1OH23 – chromium(III) hydroxide
6.3 * 10-31
Fe1OH23 – iron(III) hydroxide
6 * 10-38
FeS – iron(II) sulfide
6 * 10-18
MgCO3 – magnesium carbonate
4 * 10-5
Mg1OH22 – magnesium hydroxide
9 * 10-12
Ni1OH22 – nickel hydroxide
2 * 10-16
PbCO3 – lead carbonate
7.4 * 10-14
PbCrO4 – lead(II) chromate
2.8 * 10-13
*Ksp values for many of these solids widely vary among sources. The values presented represent typical of those observed. Source: Sawyer et al., 2003.
EXAMPLE 3.10 Solubility of solids Calculate the solubility of the following ionic compounds. Assume a chunk of each solid such that an excess exists, and assume T = 25°C. Present results in g/L. a. Ca1OH221s2 b. CaCO31s2 Solution part a Referring to Table 3.4, the Ksp for Ca1OH22 is 5 * 10-9. The dissolution reaction for Ca1OH22 is Ca1OH221s2 Δ Ca 2+ 1aq2 + 21OH 21aq2
The equilibrium expression can be expressed as Ksp = 5 * 10-9 = 3Ca2+4 3OH -42
50
Chapter 3
Essential Chemical Concepts
Within the equilibrium expression, both 3Ca2+4 and 3OH -4 concentrations are unknown. Mole ratios observed in the dissolution reaction show that for every 1 mole of Ca2+ formed, 2 moles of OH - are formed. If x represents the concentration of Ca2+ ions, then 2x moles of OH - will be produced at equilibrium. So the equilibrium expression is: 5 * 10-9 = 1x212x22 Use algebra to solve for x: 4x3 = 5 * 10-9 5 * 10-9 B 4
x =
3
x = 1.08 * 10-3 M So, the solubility of Ca2+ and the Ca1OH22 solid is equal to 1.08 * 10-3 M. Knowing the molecular weight of Ca1OH22 is 74 grams per mole, convert to a mass-per-volume concentration: 1.08 * 10-3 mol Ca 1OH22 L
*
74 g Ca 1OH22 1 mol Ca 1OH22
= 0.080
g Ca1OH22 L
Solution part b The process for determining the solubility of CaCO3 is similar. From Table 3.4, the solubility product is 5 * 10-9. Notice that the Ksp values for both Ca salts analyzed in this example are the same. Write the equilibrium dissolution reaction and the solubility-product expression. Ksp = 5 * 10-9 = 3Ca2+43CO23 4 Allow x to represent the equilibrium concentration of Ca2+ in solution and rewrite the equilibrium expressions. 5 * 10-9 = 1x21x2 5 * 10-9 = x2 -5 x = 3Ca2+4 = 3CO2M 3 4 = 7.07 * 10 Converting to a mass basis, the solubility of CaCO3 in solution is 7.07 * 10-3 g/L. Notice that for both salts represented in the example, their Ksp was the same but their solubilities differ. The take-home message: it is infeasible to compare or predict solubility by observing only the quantity of solubility products for sparingly soluble solids, because they may form different numbers of ions in solution.
EXAMPLE 3.11 U s i n g s o l u b i l i t y k n o w l e d g e t o d e t e r m i n e Ksp The solubility of AgCl is 1.34 * 10-5 M in pure water. Calculate the value of Ksp . AgCl Δ Ag + + Clx = solubility = 3Ag +4 = 3Cl-4 = 1.34 * 10-5 M Ksp = 3Ag +4 3Cl-4 = 1x2 1x2 = x2 = 11.34 * 10-5 M22 = 1.8 * 10-10
Section 3.6
Solution Chemistry—Gas Phase
51
3.6 SOLUTION CHEMISTRY—GAS PHASE An understanding of gas-phase chemistry is imperative to the study of atmospheric chemistry, including global issues such as climate change, hazardous air pollutants, acid deposition, photochemical smog production, and stratospheric ozone depletion. Gas-phase chemistry is also important to an environmental engineer focused on Title V permitting, the design engineer focused on unit operations used to remove particulates or hazardous volatile organic compounds from industrial emissions, or the engineer responsible for supplying oxygen to an aerobic digester found at a wastewater treatment facility. An introduction to broadly useful gas-phase chemistry concepts is provided in this section. 3.6.1 Ideal Gas Law In 1834, Benoît Paul Émile Clapeyron, an engineer and physicist, discovered that the ratio produced when (pressure * volume) is divided by (number of moles PV absolute temperature) in gas systems was a constant, such that = R. The ideal nT gas law, shown in Equation (3.25), relates system pressure, volume, temperature, and number of moles. Gases that can be well characterized by this equation are termed ideal gases. In general, the ideal gas law gives a good characterization of monatomic gases (all the noble gases are monotonic, and at sufficiently high temperatures all other elements are considered monotonic in their gaseous state) and provides a basis for reasonable approximation for most gases. PV = nRT
(3.25)
where: P V n R T
= = = = =
absolute pressure, atm, volume occupied by gas, L, moles of gas, universal gas law constant; 0.08206 atm # L>mol # K, and temperature, K 1273.15 + °C2.
EXAMPLE 3.12 Practice with the ideal gas law Calculate the volume that one mole of an ideal gas occupies at standard conditions of 1 atm of pressure and 0°C.
V =
nRT = P
atm # L b13273 + 04K2 mol # K = 22.4 L 11 atm2
11 mol2a0.08206
3.6.2 General Gas Laws—Boyle’s, Charles’, Gay-Lussac, Combined Gas Rearranging the ideal gas law produces a variety of general gas laws relating P, V, n, and T. For example, consider two gas systems where temperature and the number of moles are kept constant but the pressures and volumes are
52
Chapter 3
Essential Chemical Concepts
different. Writing the law for each case with numerical subscripts to the variables produces: P1V1 = nRT for system 1 and, P2V2 = nRT for system 2. The quantity nRT is the same for both, so P1V1 = P2V2
(3.26)
This relationship is Boyle’s law and shows that for an isothermal system containing a finite amount of gas, pressure and volume are inversely proportional such that as pressure increases, the volume must decrease. Similarly, Charles’ law is easily derived when the molar quantity of gas and system pressure remain constant, and volume and temperature vary. Writing the ideal gas law and assigning subscripts to the system variables produces: PV1 = nRT1 PV2 = nRT2 Algebraically collecting the constant terms in both equations produces: V1 V2 nR = = = constant T1 T2 P
(3.27)
Charles’ law implies that volume and temperature are directly proportional; when temperature increases, volume increases. V1 V2 = T1 T2
(3.28)
Similarly, the Gay-Lussac law relates pressure and temperature for ideal gases such that when temperature increases, pressure increases. P1 P2 = T1 T2
(3.29)
The combined gas law is easily derived when system pressure, temperature, and volume are all allowed to vary while the quantity of gas stays constant: P1V1 P2V2 = T1 T2
(3.30)
EXAMPLE 3.13 Illustrating Charles’ law A stack gas leaving a paint-drying oven has a volumetric flow rate of 1000 standard cubic feet per minute (scfm) and an operating temperature of 400ºF. a. Find the actual volumetric flow rate exiting the stack at 400ºF. b. Determine the actual volumetric flow rate if the process temperature is altered such that the temperature of the stack gas as it exits is 600ºF.
Section 3.6
Solution Chemistry—Gas Phase
Solution part a Charles’ law, Equation (3.28), relates the volume and temperature of a gas such that V1 V2 = T1 T2 Allow variable 1 to represent current operating conditions of 1000 scfm and 400ºF and variable 2 to coincide with standard temperature and pressure (STP). As defined by the U.S. Environmental Protection Agency, the temperature for standard conditions is 68ºF (20ºC) and the pressure is 1 atm (101.325 kPa). Always use either the Kelvin or Rankin absolute temperature scale when solving problems with the gas laws. Recall that T1K2 = T1°C2 + 273 and that T1R2 = T1°F2 + 460. Begin by calculating the absolute temperature for both conditions. T1 = 168 + 4602 = 528 R T2 = 1400 + 4602R = 860 R V1 = 1000 scfm Apply Charles’ law to find the volumetric flow rate at operating conditions. V2 1000 scfm = 528 R 860 R actual volumetric flow rate = V2 =
1000 cfm * 860 R = 1629 cfm 528 R
Solution part b Apply Charles’ law again to determine the volumetric flow rate for the gas phase, once process conditions have changed such that the exit gas temperature is 600°F = 1060 R. V1 V2 = T1 T2 V2 1629 cfm = 860 R 1060 R V2 = 2008 cfm
3.6.3 Dalton’s Law of Partial Pressure In a mixture of ideal gases, each gas exerts pressure independently of the others. The partial pressure of each gas is proportional to the percentage by volume of that gas in the mixture, or, the partial pressure is equal to the pressure that gas would exert if it were the sole occupant of the available volume. Dalton’s law shows that the sum of all the partial pressures is equal to the total pressure of the system. PT = P1 + P2 + P3 + Á + Pi
(3.31)
PT is equal to the total system pressure and P1 , P2 , etc. represent partial pressures of each component in the system. Recall that the partial pressure
53
54
Chapter 3
Essential Chemical Concepts
of component i is equal to the mole fraction of component i times the total pressure: Pi = yiPT
(3.32)
ni Pi ; = n PT yi = mole fraction of component i in the gas phase, ni = the number of moles of gaseous component i, and n = the total number of moles of gas in the system.
where: yi =
3.6.4 Raoult’s Law and Henry’s Law Both Raoult’s law and Henry’s law relate the amount of gas that at equilibrium can dissolve in a liquid to the vapor pressure of the gas. Raoult’s law provides a relationship between the partial pressure and the vapor pressure of a pure component. More specifically, Raoult’s law [Equation (3.33)] relates the partial pressure of a gas component present above a liquid to the vapor pressure of the pure component and the mole fraction of the component in the liquid phase. Pi = P…i xi
(3.33)
Table 3.5 Henry’s Constants Provided for Common Environmental Gases in an Aqueous System at 25ºC Common Presentations of Henry’s Law Henry’s law expression
PA = k HA xA atm
kH dimension
PA = kCH 3A4 L soln # atm molsoln
Common Gases of Environmental Concern O2
4.26 * 104
769
N2
8.5 * 104
1540
CO2
1.66 * 103
29.9
NH3
-1
0.017
Cl2
6.08 * 102
11.0
H2
7.1 * 104
128
H2S
5.53 * 10
2
10
CH4
4.257 * 104
769
SO2
4.61 * 101
0.83
O3
4.26 * 103
76.9
9.38 * 10
3A4 = aqueous-phase concentration of A, moles/L. PA = partial pressure of A, atm xA = mole fraction of A in the liquid phase
Section 3.6
Solution Chemistry—Gas Phase
55
P…i = pure component vapor pressure, atm, Pi = partial pressure of component i, atm, and xi = mole fraction of i in the liquid phase. Similarly, Henry’s law, written in terms of a liquid-phase mole-fraction concentration 1xi2, states that the equilibrium aqueous and gas-phase concentrations are related. Pi = kiHxi
(3.34)
where kiH = Henry’s constant based on liquid-phase mole fraction, atm. For ideal solutions, P…i = kiH . For most solutions, unfortunately, the relationship is not this simple. In general, Raoult’s law best models the solution solvent and Henry’s law best describes the solute behavior. A variety of references provide Henry’s law constants, but caution should be used when applying the law, because it is presented in a variety of forms, and constants vary as different concentration units are applied. Table 3.5 provides a brief overview of two common formulations of Henry’s law and associated appropriate units.
EXAMPLE 3.14 U s e H e n r y ’s l a w t o d e t e r m i n e e q u i l i b r i u m l i q u i d - p h a s e concentrations The concentration of oxygen in the atmosphere is approximately 20.9% by volume. Oxygen, however, is only slightly soluble in water, and its solubility is a function of temperature. As temperature increases, oxygen solubility in water decreases. For many aquatic systems that rely on the presence of oxygen, this poses a potential problem during summer months, when both temperature and the rate of biological activity are at their highest. Use Henry’s law to estimate the maximum oxygen concentration available to aquatic life during both summer and winter months in Norris Lake located in Maynardville, Tennessee, assuming 1 atm of pressure. Assume that average winter and summer high temperatures in Maynardville are 43.5ºF and 87.2ºF, L # atm respectively. The Henry’s law constant for oxygen in water is 527.3 at 43.5°F mol L # atm and 857.7 at 87.2°F. mol Solution Use the following form for Henry’s law (from Table 3.5), PA = kC H 3A4, to determine the oxygen solubility in water at 43.5ºF (6.4ºC) and 87.2ºF (30.7ºC). First, determine the partial pressure of oxygen in the environment. PO 2 = 1 atm *
20.9 = 0.209 atm 100
Now, solve for the aqueous-phase concentration at 43.5ºF 3O2143.5°F24 =
PO2 kC H
=
mol O2 0.209 atm = 3.96 * 10-4 # L atm L 527.3 mol
56
Chapter 3
Essential Chemical Concepts
Converting to a mass-per-unit-volume basis gives: O2143.5°F2 = ¢ 3.96 * 10-4
mol O2 32.0 g 1000 mg mgO2 b = 12.7 ≤¢ ≤a L 1 mol O2 1g L
Similarly at 87.2ºF, the solubility of oxygen is O2187.2°F2 = 7.8
mg O2 . L
S U M M A RY Fundamental chemistry concepts often used by environmental engineers were provided. The concepts of elements and molecules were introduced and units of concentration and chemical reactions were reviewed. Reaction stoichiometry was defined and practice in balancing reactions was provided. A working definition of pH was introduced and its importance in acid-base chemistry discussions was illuminated. The chemical compounds associated with the carbonate system were highlighted in a summation on alkalinity that outlined its importance in both natural and engineered environments. Concepts of gas-phase chemistry were introduced. The behavior of ideal gases was characterized using the ideal gas, Boyle’s, Charles’, and Henry’s laws. Practical exercises were provided to highlight key ideas.
KEY WORDS
atom acid alkalinity amphoteric atomic number atomic shell atomic weight base Boyle’s law Charles’ law chemical equation chemical reactions combined gas law
common ion effect Dalton’s law dissociation constant dissolution electrons elements equivalent equivalent weight free radicals Gay-Lussac law Henry’s law ideal gas law isotope
law of conservation of mass mass number molecular weight neutrons normality nucleus pH precipitation protons Raoult’s law stoichiometry valence
REFERENCES Cicerone, R.J., and Oremland, R.S. (1988). Biogeochemical Aspects of Atmospheric Methane. Global Biochem Cycles 2/4: 299–327. CRC Handbook of Chemistry and Physics, 87th ed., Ed. D.L. Lide, Taylor & Francis, Boca Raton, FL, 2006. Godfrey, P.J., M.D. Mattson, M.F. Walk, P.A. Kerr, O.T. Zajicek, and A. Ruby III (1996). The Massachusetts Acid Rain Monitoring Project: Ten Years of Monitoring Massachusetts Lakes and Streams with Volunteers. Publication No. 171. University of Massachusetts Water Resources Research Center. Available online at: http://www.umass.edu/tei/wrrc/ WRRC2004/pdf/ARMfinalrpt.PDF. IPCC (2001). Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change, Ed. Houghton, J.T., Ding,Y., Griggs, D.J., Noguer, M., van der Linden, P.J., Dai, X., Maskell, K., and Johnson, C.A., Cambridge University Press, Cambridge, United Kingdom, and New York.
Exercises 57
Neue, H. (1993). Methane Emission from Rice Fields: Wetland Rice Fields May Make a Major Contribution to Global Warming. BioScience 43:7, 466–73. Nordstrom, D.K, C.N. Alpers, C.J. Ptacek, and D.W. Blowes (2000). Negative pH and Extremely Acidic Mine Waters from Iron Mountain, California, Environ. Sci. Technol., 34 (2):254–258. Sawyer, C. L., McCarty, P. L., and Parkin, G. F. (2003). Chemistry for Environmental Engineers, McGraw-Hill, New York. VanLoon, G.W., and S.J. Duffy (2005). Environmental Chemistry a Global Perspective, 2nd ed, Oxford University Press, Great Britain.
BIBLIOGRAPHY Benefield, L.D., Judkins, J.F., and Weand, B.L. (1982). Process Chemistry for Water and Wastewater Treatment, Prentice-Hall, Englewood Cliffs, NJ. Reynolds, T.D., and Richards, P.A. (1996). Unit Operations and Processes in Environmental Engineering. PWS Publishing Company, 20 Park Plaza, Boston, MA. Sander, R. (1999). Compilation of Henry’s Law Constants for Inorganic and Organic Species of Potential Importance in Environmental Chemistry (Version 3). Available online at http://www.henrys-law.org. Sanks, R.M. (1982). Water Treatment Plant Design for the Practicing Engineer, Ann Arbor Science, 230 Collingwood, Ann Arbor, Michigan. Snoeyink, V.L., and Jenkins, D. (1980). Water Chemistry, John Wiley & Sons, New York. Viessman,W., and Hammer, M.J. (2005). Water Supply and Pollution Control, Pearson/Prentice Hall, Upper Saddle River, NJ.
EXERCISES 3.1
In the accompanying table, calculate the atomic or molecular weight in grams per mole for each of the elements or compounds listed. Also provide the chemical symbol or formula for each element or compound.
Element or compound
Symbol or formula
Atomic or molecular weight (g/mole)
Nitrogen Potassium Magnesium Iron Nitrate Sodium carbonate Manganese 3.2
In the accompanying table, calculate the equivalent weight in grams per equivalent (g/eq) for each of the elements or compounds listed. Also provide the chemical symbol or formula for each element, radical, or compound.
Element or compound Sulfate
Symbol or formula SO24
Bicarbonate Magnesium Iron (III)
Fe3+
Nitrate Sodium carbonate Phosphate
PO3-4
Equivalent weight (g/mole)
58
Chapter 3
Essential Chemical Concepts
3.3
This problem involves an understanding of molarity and normality. (a) Calculate the grams of hydrochloric acid (HCl) that must be diluted to a volume of 1 liter to produce a concentration of 0.5M. (b) Calculate the normality of 1 L of solution containing 45 grams of sodium hydroxide (NaOH).
3.4
Balance the following reactions. (a) CH 3OH + NO3- : N2 + CO2 + H 2O + OH (b) C 6H 14O2N + O2 + H + : CO2 + NH 4+ + H 2O Calculate the pH and pOH of a 0.5N solution of hydrochloric acid (HCl) at 25ºC Calculate the pH and pOH of a 0.001M solution of sodium hydroxide (NaOH) at 25ºC. Calculate the solubility of the following ionic compounds. Assume T = 25°C.
3.5 3.6 3.7
(a) Mg1OH22 1s2 (b) FeS1s2 3.8 3.9 3.10
Determine the volume in cubic feet occupied by 120 pounds of carbon dioxide at 1.5 atm and 40ºC. What volume of oxygen at 30ºC and 0.21 atm is required for combustion of 20 g of propane gas 1C3H82? A VOC incinerator is to be designed to oxidize an off-gas containing 1600 ppmv of toluene vapor and 12,000 actual cubic feet per minute (acfm) of air at a temperature of 250ºF. (a) Determine the standard volumetric air flow rate (scfm) that must be treated by the VOC incinerator. (b) Determine the actual volumetric air flow rate that must be treated by the incinerator if the temperature is increased to 400ºF.
CHAPTER
4
Biological and Ecological Concepts Objectives In this chapter, you will learn about: The abiotic and biotic components of planet Earth Cell structure and classification of organisms The major groups of organisms found in biological and ecological systems Microbial growth and the equations used in modeling the growth rate of microorganisms The flow of energy in ecological systems and through the food chain The cycling of carbon, nitrogen, phosphorus, and sulfur in ecological systems Lake classification and stratification Streeter-Phelps Dissolved Oxygen Sag Model for streams
4.1 INTRODUCTION When evaluating natural and engineered systems that involve living organisms, it is important to consider biological and ecological concepts, especially the role that microorganisms play with regard to the flow of energy and nutrients in the biosphere. We begin with a brief introduction to ecology, the branch of biology that deals with the way living organisms (biota) interrelate and interact with the environment. Ultimately, the radiant energy of the sun sustains all life on Earth; initiating the flow of energy through the biosphere. Through photosynthesis, green plants and algae utilize the sun’s energy to transform inorganic compounds and nutrients from the soil into organic compounds that are used in the synthesis of starches, sugars, amino acids, and proteins. Plants, algae, and phytoplankton are primary producers that are ultimately consumed by animals and humans (consumers). When plants and animals die, their remains are used by decomposers, consisting primarily of bacteria and fungi, allowing nutrients and organic matter to be recycled. The consumption of plants by animals that are in turn eaten or consumed by higher animal forms is called a food chain. Each step in the food chain is called a trophic level, with energy being transferred up through the levels. The transfer of energy through the food chain is very inefficient, and, unlike the nutrients, the energy is not recyclable. At each trophic level, much of the energy is dissipated and irrecoverably lost as heat.
4.2 BIOLOGICAL SYSTEMS Planet Earth consists of abiotic and biotic components. The abiotic or nonliving component is represented by the atmosphere (air), lithosphere (soil), and hydrosphere (water). It includes, for example, organic and inorganic compounds, nonliving elements, climate, and the hydrologic cycle. The biotic component (biosphere) contains all the living organisms on Earth and includes plants, animals, microorganisms, and humans.
60
Chapter 4
Biological and Ecological Concepts
DNA
Cell wall
Lipid
Cell membrane
Figure 4.1 Schematic of a rod-shaped bacterial (prokaryotic) cell.
Ribosomes Flagellum
Capsule or slime layer
1–2 mm in diameter and 10 mm long
4.2.1 Cell Structure The basic functional and structural unit of all living organisms is the cell. There are two types of cells, prokaryotic and eukaryotic, categorized primarily according to their genetic material and the complexity of their structure. Prokaryotes’ cells are the simplest. They have no nucleus or nuclear membrane; their genetic material, deoxyribonucleic acid (DNA) is contained in a circular loop called a plasmid. Figure 4.1 is a simple schematic of a prokaryotic cell. Eukaryotes’ cells are more complex, containing a true nucleus with a nuclear membrane, several chromosomes, and a respiratory system located in mitochondria. Figure 4.2 shows a schematic of a eukaryotic cell. The Acaryote, a third category sometimes mentioned, contains viruses (Lester and Birkett, 1988). Viruses are entities that carry the information necessary for replication but must invade and rely upon a living host cell in order to replicate. Figure 4.3 shows a schematic of a virus. 4.2.2 Classification of Organisms Biologists and microbiologists have developed several methods of classifying organisms. Conventional taxonomic classification methods rely on organisms’ observable properties, such as appearance or morphology, metabolic characteristics, and how they interact with dyes or staining.A widely used system proposed by Whittaker (1969)
Figure 4.2 Schematic of a eukaryotic cell.
Mitochondrion
Nuclear membrane
Cytoplasm
Nucleolus Nucleus
Ribosomes
Vacuole
Cytoplasmic membrane 2 mm to > 100 mm in diameter
Section 4.2
Biological Systems
Hexagonal head Nucleic acid core Contractile sheath Base plate Tail fiber 20 nm up to 400 nm in diameter
is made up of five kingdoms: Monera, Protista, Fungi, Plantae, and Animalia. Each kingdom is further classified in the following order: phylum, class, order, family, genus, and species. In the 1970s, microbiologists developed a new system that relies on phylogeny. Phylogeny classifies organisms according to their genetic characteristics and is based on their evolutionary history. Under the phylogenetic classification (Madigan et al., 2000), three domains (kingdoms) comprise all organisms: Archaea, Bacteria, and Eukarya. All prokaryotic organisms are found in the Archaea or Bacteria domain. Microbiologists and environmental engineers need to identify and classify microbes for many reasons. For example, the identification and enumeration of pathogens or disease-causing microorganisms in drinking water is essential, so that the proper disinfectant can be selected and added to the water to prevent disease. Engineers are also interested in optimizing biological processes associated with wastewater treatment and remediation of contaminated groundwater and soil; therefore, it is imperative to identify and classify the microorganisms involved. The basic taxonomic classification unit is species. Species are groups of individual organisms or strains that have similar characteristics or attributes. Groups of species that have major similarities are called genera. For example, the bacterium that causes cholera is Vibrio cholera (Brock, 1979). The species name is cholera and the genus (singular for genera) is called Vibrio. Two genera of microorganisms typically encountered during the biological treatment of wastewater using the activated sludge process are responsible for nitrification. Nitrification is modeled as a two-step, sequential process, mediated by the genera Nitrosomonas and Nitrobacter. 4.2.3 Major Groups of Organisms The major groups of organisms applicable to environmental problems are algae, bacteria, crustaceans, fish, fungi, helminths, macrophytes, metazoa, protozoa, rotifers, stalk ciliates, and viruses. These organisms are found in natural systems such as lakes, rivers, wetlands, and soils as well as engineered systems such as wastewater treatment plants, landfills, and bioremediation sites. Each group will be briefly discussed in the paragraphs that follow. Algae
Algae are protists that range in size from unicellular phytoplankton to large multicellular seaweeds. All algal cells contain plastids or chloroplasts and are capable of photosynthesis. With the exception of the blue-green algae (cyanobacteria), all algal
Figure 4.3 Schematic of a virus.
61
62
Chapter 4
Biological and Ecological Concepts
Anabaena
Figure 4.4 Schematics of Anabaena and Chlorella algae.
Chlorella 0.5–1.0 mm up to 60 mm in diameter
cells are eukaryotes. Algae are autotrophic organisms that utilize inorganic carbon as their carbon source. Algae are the primary producers in the aquatic food chain. A major concern to environmental engineers is excessive algal growth or blooms in water-supply lakes and reservoirs, where they can clog intake structures and cause taste and odor problems in water. In wastewater treatment lagoons, algae play a significant role by producing oxygen, which bacteria and other microbes use for the oxidation and degradation of organics in the wastewater. Figure 4.4 shows two types of algae: Anabaena and Chlorella. Bacteria
Bacteria are essential in the recycling of nutrients through ecosystems. They are used in treating contaminated water and wastewater, and some species cause plant and animal diseases. Figure 4.5 shows three bacterial cellular forms and arrangements: Micrococcus, Streptococcus, and Bacillus. They are classified as Monerans, normally reproducing by binary fission (splitting into two identical cells). Individual bacteria are shaped spherically, cylindrically, or spirally. Most bacteria range in size from 0.5 to 14 mm (Sullia and Shantharam, 1998). All bacteria have a rigid cell wall that maintains the shape of the cell. The genetic material of bacteria is not contained in a true nucleus but just incorporated into their cytoplasm.
Figure 4.5 Three bacterial cellular forms and arrangements.
Micrococcus
Streptococcus
Bacillus
0.5–1.0 mm in diameter up to 25 mm long
Section 4.2
Biological Systems
Daphnia 0.2 to 5 mm long
Diaptomus 0.5 to 5 mm long
Crustaceans and Microcrustaceans
Crustaceans are multicellular organisms that possess a hard body or shell belonging to the arthropod group. Large crustaceans include shrimp and lobsters. Microscopic crustaceans (microcrustaceans), which serve as food for fish, include Cyclops, Daphnia (water flea), ostracods, and copepods. They feed on other microorganisms, algae, and organic matter. Figure 4.6 shows examples of the Diaptomus and Daphnia microcrustaceans. Fish
Fish are among the animals that have a spine or backbone (vertebrates). They are a key unit in natural food webs and have a significant impact on plankton, macrophytes (aquatic plants), and other aquatic organisms. Bioassays using fish and minnows are used in toxicity testing of wastewater effluent. Change in the composition of a fish may be attributed to pollution or to a change in its habitat.The diversity of fish species is another indicator of environmental change. Figure 4.7 shows Dr. W. Jack Lackey holding a rockfish. Fungi
Fungi are nonphotosynthetic, eukaryotic protists. They are typically aerobic and referred to as saprophytes, since they degrade and utilize decaying organic matter from plant and animal remains. Fungi can tolerate low-pH or acidic conditions, lowmoisture, and low-nitrogen conditions better than most other microorganisms. They are subdivided into molds, yeasts, and mushrooms. Molds are filamentous; yeasts are nonfilamentous and are unicellular. Mushrooms are a more highly differentiated form of fungi, forming basidia (the structures we call mushrooms) above the ground. Molds and mushrooms reproduce asexually (by budding or spores) and sexually (by spores). Yeasts reproduce asexually by binary fission or budding and sexually through the formation of ascospores; they are facultative organisms that can grow either aerobically or anaerobically. Yeasts are used in fermentation processes such as beer and wine production, in making bread, and in producing antibiotics. Fungi are important in the degradation of cellulose and in the composting of sludge. Fungi cause athlete’s foot and ringworm. Two examples of fungi are shown in Figure 4.8.
Figure 4.6 Examples of a Diaptomus and Daphnia.
63
64
Chapter 4
Biological and Ecological Concepts
Figure 4.7 Photograph of Dr. W. Jack Lackey holding a rockfish (up to 200 cm long and 57 kg in weight).
Helminths
Worms collectively are described as helminths. They are macroinvertebrates— that is, they lack a spine or backbone, and they are large enough to see with the naked eye. Helminths are among the principal causative agents of disease in the world and fall into three major phyla: Nematoda (roundworms), Platyhelminthes (flatworms), and Annelida (segmented worms). Their primary significance is that the species that cause most human diseases are infective as either adults or larvae, while in other species it is the eggs that are infective. Helminth eggs have been found to survive in oxidation pond sediments up to 10 years (Metcalf and Eddy, 2003). Chlorine disinfection and anaerobic digestion have been shown to be ineffective at inactivating helminth eggs. This raises questions regarding the application of biosolids or sludge on land. Figure 4.9 shows an example of a leech and flatworm. Macrophytes
Macrophytes are large aquatic plants that may be attached to lake bottoms or may be free floating; some are totally submersed, and others emergent. They provide habitat for microorganisms and other aquatic life. Examples of macrophytes include duckweed, watercress, hydrilla, water lilies, water hyacinth, and water lettuce. Eutrophic lakes generally have an abundance of macrophytes; these can cause
Section 4.2
Ascus 8–20 mm
Sporangiospores 1–5 mm
Biological Systems
65
Figure 4.8 Examples of fungi.
nuisance conditions in lakes used for recreational purposes. When these aquatic plants die, aerobic bacteria consume large quantities of oxygen in degrading and oxidizing the carbon and nitrogen that becomes available.This may lead to low dissolvedoxygen levels in the water column. Examples of macrophytes are presented in Figure 4.10. Figure 4.9 Examples of a leech and flatworm.
Leech 3–6 cm
Planaria 5–13 mm
Figure 4.10 Examples of macrophytes.
Ceratophyllum
Lily pad
1–3 m in length
Up to 25 cm in diameter
66
Chapter 4
Biological and Ecological Concepts
Protozoa
Amoeba 30–600 mm
Figure 4.11 Example of a protozoan.
Protozoans are motile, eukaryotic protists that typically range in size from 10 to 300 mm. Most are aerobic, nonphotosynthetic, and reproduce by binary fission. Protozoa feed on bacteria and particulate matter, serving as polishers of the effluent from biological wastewater treatment facilities. Well-known protozoans include Entamoeba histolytica, which causes amebic dysentery; Giardia lamblia, which is responsible for beaver’s disease or Giardiasis; and Cryptosporidium parvum, which causes cryptosporidiosis. Giardia and Cryptosporidium form cysts and oocysts that are resistant to traditional chlorine disinfection (Metcalf and Eddy, 2003). Figure 4.11 shows an amoeba. Rotifers
Rotifers are microscopic eukaryotic animals. Figure 4.12 shows a rotifer. Rotifers are aerobic heterotrophs that consume bacteria and particulate organic matter by ingestion. The presence of rotifera and/or stalk ciliates indicates a highly efficient aerobic system. Viruses
Viruses are submicroscopic particles ranging in size from 10 to 250 nm (1nm 10-3 mm2 (Henry and Heninke, 1996). According to cell theory, viruses are not living organisms and are considered as parasites, since they can reproduce only in a living host.Viruses are composed of a nucleic core consisting of either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA), surrounded by a protein capsid. All viruses are pathogenic. Each type of virus can infect only a specific type of host cell. There are specific viruses that infect bacteria, plants, animals, and humans. Viruses that infect bacteria are called bacteriophages. Human enteric viruses that cause waterborne disease include Norwalk viruses, rotaviruses, reoviruses, and adenoviruses (Metcalf and Eddy, 2003). A diagram of the Herpes Simplex virus is presented in Figure 4.13. 0.4 mm
Figure 4.12 Example of a rotifer.
Herpes simplex 180–200 nm
Figure 4.13 Diagram of a virus.
4.2.4 Microbial Growth Microorganisms play a critical role in recycling nutrients in the biosphere and in the treatment of wastewater, contaminated groundwater, and hazardous wastes. We need to understand their nutritional requirements for growth and their phases of growth in order to develop technologies that can be implemented to treat wastes biologically. Microorganisms known as decomposers are especially important in breaking down dead plants and animals, and excreta, thus enabling the recycle of organics and nutrients in the biosphere. Bacteria are ubiquitous in nature and are found in water, air, and soil. Hoover and Porges (1952) proposed the following formula as the composition of a bacterium: C5H7O2N. Another well-known formula (McCarty, 1970) includes phosphorus: C60H87O23N12P. Based on the latter composition, a bacterial cell has a total formula weight of 1374 and consists of approximately 52.4% carbon, 12.2% nitrogen, and 2.3% phosphorus by dry weight. It is easy to recognize that bacteria and other microbes require carbon, nitrogen, and phosphorus so that their growth will not be limited. Other trace nutrients such as sodium, iron, and potassium are also required. In order to flourish, all microorganisms require acceptable environmental conditions, including proper moisture, a pH in the range of 6 to 8.5, and a temperature ranging from 15° to 30°C. Various species of organisms can withstand extreme pH and temperature conditions, and even a lack of moisture; however, this is not routinely the case.
Section 4.2
Log mass of microbes
1
2
3
4
5
En
gen
ou
Stationary
Declining growth
Ex po ne nt ial
do
Lag
s
Figure 4.14 Bacterial growth curve.
Time
If a pure culture of bacteria were grown in the laboratory in a batch reactor under proper environmental conditions, the growth curve depicted in Figure 4.14 would result. Normally, microbiologists measure the number of organisms grown, while environmental engineers and scientists quantify the mass of organisms grown, expressed as a concentration in milligrams of dry mass per liter of solution. Suspended and volatile solids analyses are used in most environmental engineering applications for measuring the concentration of microorganisms. During Phase 1 of the growth curve, called the “lag phase,” the bacteria become acclimated to their new environment. It takes time for organisms to develop enzymes to synthesize from the media the compounds they need for growth—thus the lag. During Phase 2, the “exponential growth phase,” the bacteria are growing at their maximum rate. Excess substrate (food) and nutrients exist, so there is nothing to limit growth. Phase 3 is the “declining growth phase”; the growth rate slows and the bacterial death rate increases. Substrate becomes growth limiting, and accumulation of metabolic waste products may inhibit growth. Phase 4 is the “stationary phase,” where the growth rate equals the death rate. As the death rate starts to exceed the growth rate, the mass or concentration of bacteria starts to decline. Exogenous or external substrate has been exhausted by this time. Finally, Phase 5, the “endogenous phase” occurs, in which the death rate exceeds the growth rate. Bacteria that are still alive oxidize their own cellular components and feed on the remains of dead bacteria. The concentration of biomass decreases at an exponential rate. Microbial Growth Equations
Microbial growth in a batch reactor, as depicted in Figure 4.14, can be expressed by the following equation. a
where: a
Biological Systems
dX b = mX dt G
dX b = microorganism growth rate, mass/(volume # time), dt G m = specific growth rate of microorganism, time-1, and X = microorganism concentration, mass/volume.
(4.1)
67
68
Chapter 4
Biological and Ecological Concepts
The specific growth rate of a microorganism 1m2 is associated with a particular species. In most environmental engineering applications, heterogeneous cultures of microbes are used, and so biokinetic coefficients for the overall heterogeneous culture are used in designing biological treatment systems. The French microbiologist Monod (1949) found that the specific growth rate of a microorganism depends on some growth-limiting substrate or nutrient. He developed Equation (4.2), which indicates that the microorganism’s specific growth rate is a function of both the maximum specific growth rate and the concentration of the limiting substrate. m =
mmax S KS + S
(4.2)
where: S = growth limiting substrate or nutrient concentration, mass/volume, mmax = maximum specific growth rate, time-1, and Ks = half-saturation constant, concentration of limiting substrate or nutrient at which half the maximum specific growth rate occurs, mass/volume. Combining Equations (4.1) and (4.2) yields Equation (4.3), which represents microbial growth rate under batch operating conditions. a
mmax XS dX b = dt G KS + S
(4.3)
Most full-scale engineered biological treatment systems use continuous-flow reactors rather than a batch reactor. Equation (4.3) must be modified to account for the portion of biomass or microorganisms lost through death and decay (endogenous decay). Endogenous decay is represented by Equation (4.4). a
where: a
dX b = - kdX dt ED
(4.4)
dX b = endogenous decay rate, mass/(volume·time), and dt ED kd = endogenous decay-rate constant, time-1.
The net growth rate of microorganisms in a biological reactor can then be expressed as Equation (4.5). a
dX dX dX b = a b + a b dt NG dt G dt ED
(4.5)
Substituting Equations (4.3) and (4.4) into the above expression yields: a
mmax XS dX b = - kdX dt NG KS + S
(4.6)
Section 4.3
Cell yield or yield coefficient of a microbe is another useful biological term. Qualitatively, it is defined as the quantity of biomass produced per unit of substrate oxidized. Mathematically, the yield coefficient is expressed as follows: Y =
1dX>dt2G
(4.7)
1dS>dt2U
where: Y = microbial yield coefficient, mass of biomass produced/mass of substrate utilized, dX a b = microbial growth rate, mass/(volume # time), and dt G dS a b = substrate utilization rate, mass/(volume # time). dt U The specific substrate utilization rate, U, with units of inverse time 1time-12 is defined by the following equation. a
dS b = UX dt U
(4.8)
Equation (4.9) may be developed by substituting Equations (4.1) and (4.8) into Equation (4.7). Y =
1dX>dt2G 1dS>dt2U
=
m U
(4.9)
The specific substrate utilization rate (U) may be expressed as a Monod-type function; Equation (4.10) shows substrate utilization as a function of both the maximum specific substrate utilization rate 1Umax2 and the limiting substrate concentration (S). U =
Umax S Ks + S
(4.10)
Equation (4.11) is another way of expressing net microbial growth rate by substituting Equations (4.4) and (4.7) into Equation (4.5). a
dS dX b = Ya b - kdX dt NG dt U
(4.11)
The net growth rate of a heterogeneous culture is normally expressed as Equation (4.11). This derived equation has been successfully demonstrated in actual studies performed by Heukelikian et al. (1951) and is used in many environmental texts: Peavy et al., 1985; Mihelcic, 1999; Reynolds and Richards, 1996; Metcalf and Eddy, 2003; and Viessman and Hammer, 2005. Equation (4.11) is used in the design and operation of activated sludge treatment processes.
4.3 ECOLOGICAL SYSTEMS The study of the interrelationships among plants and animals (biota) and their interactions with the environment is the branch of biology called ecology. Ecological systems or ecosystems consider the organisms living together in a specific
Ecological Systems
69
70
Chapter 4
Biological and Ecological Concepts
environment, such as a pond, field, forest, or desert, and their dependence upon each other and the abiotic environment for their survival. As an example, in a lake ecosystem, the biotic components could consist of phytoplankton, Daphnia, fish, submergent plants, and emergent plants. The abiotic components would include temperature, inorganic and organic compounds, and nutrients in the water column in addition to those found in the lake sediments. In an ecosystem, a population denotes a group of organisms belonging to one species. A community is defined as a number of populations living together. 4.3.1 Energy Flow in Ecosystems Organisms that live in the biosphere use solar radiation along with organic and inorganic compounds found in the Earth’s crust for energy. Table 4.1 lists the scientific names for organisms based on their energy source, carbon source, and electron acceptor. Both energy and mass flow through ecosystems. The sun provides the energy for sustaining all biological life on earth through the process of photosynthesis, which is carried out by green plants in terrestrial ecosystems and phytoplankton (algae) in aquatic ecosystems. Organisms that utilize sunlight as a source of energy are called phototrophs. Green plants and algae use inorganic carbon in the form of carbon dioxide 1CO22, bicarbonate 1HCO3-2, and/or carbonate 1CO32-2. Organisms that use inorganic carbon for synthesizing cellular components are called autotrophs. Green plants and algae are called primary producers and are at the bottom of the food chain. Equation (4.12) is a simple representation of the photosynthesis reaction carried out by green plants and any organisms that contain chlorophyll, which allows them to convert sunlight into chemical energy. 6 CO2 + 6 H2O + 2800 kJ energy from sun
chlorophyll
" C H O + 6 O (4.12) 6 12 6 2
Sunlight provides the energy for driving the reaction, and carbon dioxide provides the carbon source for the synthesis of glucose 1C6H12O62. The standard free energy for the synthesis of glucose is 2800 kJ per mole. The glucose is then used for synthesizing other organic compounds and plant biomass with the uptake of nitrogen, phosphorus, sulfur, and other trace nutrients as needed by the plant. An
Table 4.1 Classification of Organisms Based on Nutrient Requirements Function
Source
Name
Energy
Organic compounds
Chemoorganotroph
Inorganic compounds
Chemoautotroph
Sunlight
Phototroph
Organic compounds
Heterotroph
Inorganic compounds
Autotroph
Oxygen
Aerobic
Nitrites, nitrates, sulfates
Anoxic
Organic compounds
Anaerobic, fermentative
Carbon source
Electron acceptor
Section 4.3
Primary producers (Autotrophs)
Primary consumers (Herbivores)
Secondary consumers (Carnivores)
Trophic level 1
Trophic level 2
Trophic level 3
Ecological Systems
Figure 4.15 Representation of a food chain.
important by-product of photosynthesis is the production of oxygen, which is released to the atmosphere by the terrestrial plants or dissolved in water by the aquatic plants. During the night, when energy from the sun is not available, plants and algae use a process called aerobic respiration to meet their energy needs. Organic compounds such as glucose and starches are oxidized to carbon dioxide and water with the release of energy. In essence, aerobic respiration is the reverse of Equation (4.12). 4.3.2 Food Chains Energy is transferred through the biosphere via food chains. The food chain involves a sequence of steps or trophic levels by which a smaller consumer is eaten by a larger consumer. A simplified representation of a food chain is presented in Figure 4.15. Trophic Level 1 is represented by the primary producers (plants and algae) that are eaten by the primary consumers. The primary consumers (herbivores) are chemotrophic animals that eat or consume the primary producers, and they represent Trophic Level 2. Trophic Level 3 is represented by the secondary consumers, or carnivores, including humans, which are flesh eating animals. The “decomposers” are secondary consumers that derive their energy from dead and decaying animal and plant remains. The decomposers, consisting of bacteria and fungi (chemoheterotrophic organisms), are primarily responsible for the recycling of nutrients through the ecosystem. In the oceans, phytoplankton are the primary producers, consumed by fish at a higher trophic level, with whales or sharks consuming at the highest trophic level. In the terrestrial environment, humans are at the highest trophic level. Figure 4.16 shows an example of a simple food chain. A food web is a representation of the complex relationships between organisms and their food chains. Figure 4.17 is an example of a food web showing the interdependence of a variety of species, including humans, fish, and microorganisms. As energy flows through the ecosystem, it is lost at each trophic level due to inefficiency. Most of the loss takes the form of waste heat that cannot be recovered. Henry and Heinke (1996) present an example of the flow of energy in a midwestern U.S. cornfield. The energy utilization by the cornfield was only 1.6%. They also report that most natural ecosystems operate with overall energy-utilization efficiencies of 0.1% to 2%, with the most efficient agricultural systems operating around 3%. A general rule of thumb is that only 10% of the energy consumed is converted to biomass (Davis and Masten, 2004). Examples 4.1 and 4.2 show the loss of energy through the food chain.
Trophic level 1
2
3
4
Grass
Grasshopper
Mouse
Hawk
Figure 4.16 Simple food chain.
71
72
Chapter 4
Biological and Ecological Concepts
Humans Eagle
Herring gull Cormorant
Snapping Turtle
Salmon/lake trout
Forage Fish Sculpin Chub
Alewife
Invertebrates
Smelt
Plankton
Waterfowl
Mineral nutrients
Bacteria and fungi
Vegetation
Dead animals and plants
Figure 4.17 Example of a food web. Source: http://www.epa.gov/glnpo/atlas/images/big05.gif
Note: This is a simplified representation of the food web showing the main pathways. Food (energy) moves in the direction of the arrows. The driving force is sunlight. Depictions of the various organisms are not to scale.
Section 4.3
Ecological Systems
73
EXAMPLE 4.1 Energy balance and efficiency A cow has consumed 100 kJ of energy in the form of grass, with 63% of it being wasted in the form of excrement, urine, and gas. a. If 5 kJ of energy is stored in the form of body tissues, how much energy was used in respiration? b. What is the energy efficiency of conversion for this trophic level? Solution part a energyin = energyout + energyrespiration + energyexcreta energyexcreta = 0.63 1100 kJ2 = 63 kJ 100 kJ = 5 kJ + energyrespiration + 63 kJ energyrespiration = 100 - 5 - 63 = 32 kJ Solution part b The energy efficiency of conversion is calculated using the following equation. energy efficiency =
useful energy output 5 kJ 1100%2 = 1100%2 = 5.0% total energy input 100 kJ
EXAMPLE 4.2 Energy use through food chain Using the rule of thumb that 10% of the energy consumed is used in the production of biomass, calculate the amount of energy passing up through the food chain depicted in Figure 4.16, assuming that the grass contains 1000 joules of energy. Solution First, determine the energy passing through each trophic level by using the 10% rule. For trophic level 1 (Grass), 100 J 11000 J * 0.10 = 100 J2 are passed to trophic level 2 (Grasshopper). From trophic levels 2 to 3 and 3 to 4, 10 J 1100 J * 0.10 = 10 J2 and 1 J 110 J * 0.10 = 1 J2, respectively, are passed up through the food chain. 1000 J
100 J
10 J
1J
Grass ¡ Grasshopper ¡ Mouse ¡ Hawk At trophic level 4 (Hawk), only 1 J of energy from the grass goes into production of biomass or body tissue in the hawk. The overall energy efficiency through this food chain is calculated as follows: energy efficiency =
useful energy output 1J 1100%2 = 1100%2 = 0.10% total energy input 1000 J
74
Chapter 4
Biological and Ecological Concepts
4.3.3 Bioconcentration, Bioaccumulation, and Toxicity Bioconcentration and bioaccumulation are important terms dealing with the accumulation of toxic compounds in organisms. Bioconcentration relates to the sorption of a toxic compound into an organism from the aqueous phase. For example, fish take in harmful substances from the water as it passes their gills. Bioaccumulation is a serious problem that results in the accumulation of a toxic substance in the lipids and fatty tissues of animals; the concentration of the compound or toxicant increases as it passes up through the food chain. At the highest trophic level, humans may be exposed to a contaminant that is several orders of magnitude higher in concentration than was observed in the water column and in the organisms at the lowest trophic level. Table 4.2 shows the bioaccumulation or biomagnification of polychlorinated biphenyls (PCBs) in the aquatic environment. Mercury and dichlorodiphenyl-trichloroethane (DDT) are examples of toxic compounds that biomagnify as they pass up through the food chain. This biomagnification depends on the specific compound ingested, its concentration, the length of exposure to the compound, and the metabolism of the organism. In some instances, the organism may be able to degrade and metabolize the toxic compound. Toxicity effects of various chemical compounds on humans and animals may be manifested as acute or chronic symptoms leading to death or serious illness. Acute toxicity effects are normally expressed in periods of hours to days, whereas chronic or long-term effects are manifested over weeks to years. Chemicals that cause cancer or tumors in animals are called carcinogens. Certain other chemicals, even if they do not cause cancer, may cause specific problems affecting the kidneys, liver, or other vital organs. Studying the effects of chemical compounds is complex, because a person may be exposed to the contaminant in several different pathways or venues: ingestion with food and water; inhalation during breathing; or absorption through the skin (dermal). If a person or animal is exposed to several different chemicals or toxic compounds, how is it possible to identify which one is causing the detrimental effect? Exposure to some compounds individually may have a mild effect or none. However, if an animal is exposed to a mix of chemical compounds simultaneously, a synergistic effect may occur which causes a significant adverse effect more severe than that of a compound acting by itself. Antagonistic effects may be realized for other compounds, where the adverse effect of one compound is negated by a positive effect from another. The toxicological effects of various chemicals depend on several factors. Not only the type of compound, but also the duration of the exposure and the Table 4.2 An Example of the Biomagnification of PCBs in an Aquatic Food Chain (Great Lakes) Organism
PCB concentration, ppm
Phytoplankton
0.025
Zooplankton
0.123
Smelt (fish)
1.04
Lake trout
4.83
Herring gull eggs
124
Source: http://www.epa.gov/glnpo/atlas/glat-ch4.html#3
Section 4.4
concentration of the contaminant may be important. In general, babies, children, the elderly, and individuals with compromised immune systems are susceptible to adverse effects at low concentration and short duration (i.e., low exposure). Environmental toxicologists study the effects of suspected toxicants in laboratory animals and extrapolate the results, so that regulatory agencies may establish standards to minimize the adverse effects on the public. A group of chemicals called “endocrine disruptors” pose a serious threat to humans and wildlife. These compounds interfere with hormones responsible for growth, metabolism, and reproduction. Suspected organic compounds identified as endocrine disruptors include pesticides and prescription drugs, as well as bloodpressure medication and antibiotics. The next generation of environmental engineers will develop the analytical tools for quantifying and monitoring these compounds in addition to providing innovative technologies for removing them from surface and groundwaters and wastewater.
4.4 NUTRIENT CYCLES The principal sources of nutrients to ecosystems are the soil and geologic formations. Oxygen, nitrogen, and carbon dioxide are abundant in the atmosphere. Most organisms are not capable of fixing atmospheric nitrogen into an organic form and must consume other plants or animals for their source of nitrogen. Plants take up nitrogen, phosphorus, and sulfur for synthesis of biomass. Plant-eating animals (herbivores) consume plants that in turn are consumed by herbivore-consuming animals (carnivores) to complete the food chain. Animal excreta and the remains of plants and animals undergo degradation and decomposition by the decomposers. The decomposers, consisting primarily of bacteria and fungi, facilitate the decomposition process, making residual organics and nutrients available for recycling in the biosphere. These organisms break down dead plant and animal materials, releasing nutrients to the environment. Nutrients can be recycled in the biosphere, whereas energy flows through the ecosystem. As energy flows through the food chain, it is converted to heat and is lost as useful energy. The next subsection briefly describes the carbon, nitrogen, phosphorus, and sulfur cycles with particular emphasis on the environmental engineering significance of each. 4.4.1 Carbon Cycle Organic chemistry deals with various carbon-containing compounds. The term organic matter, or organic carbon, has been applied to substances that originated from plants and animals. All organic compounds contain carbon in combination with one or more elements. Hydrogen is almost always a constituent. Organic compounds consisting of only hydrogen and carbon are called hydrocarbons. The majority of organic compounds contain carbon, hydrogen, and oxygen. Minor elements found in combination with carbon include nitrogen, phosphorus, and sulfur. Uniquely, carbon has the ability to bond with other carbon atoms to form chains or rings. Carbon is the building block of life on earth. Compounds that contain carbon— excluding carbon dioxide, carbonic acid, carbonates, cyanides, cyanates, and carbides— are considered organic. Inorganic forms of carbon encountered in environmental engineering include carbon dioxide (CO2), aqueous carbon dioxide (CO2)aq , carbonic acid (H2CO3), carbonate (CO 23 - ), and bicarbonate 1HCO3- 2. The inorganic forms of carbon are used by plants for synthesizing carbohydrates; autotrophic organisms use them for synthesizing lipids, amino acids, and proteins.
Nutrient Cycles
75
76
Chapter 4
Biological and Ecological Concepts
Carbon is cycled through the ecosystem as producers are eaten by consumers, which in turn are eaten by other consumers in the food chain. When plants and animals die, decomposers facilitate the decomposition process, releasing nutrients and organic matter to be utilized by other organisms. In the process, some of the organic matter and residual nutrients will remain in place where the plant or animal has expired. A portion of the organic matter is also oxidized to carbon dioxide and released into the atmosphere. In aquatic ecosystems, the decomposition of plant and animal matter may occur in the water column or in the sediments at the bottom of a lake or river. The carbon dioxide released by the decomposers dissolves in the water, reaching an equilibrium concentration according to Henry’s law, or escapes to the atmosphere.Algae, in turn, during photosynthesis use the carbon dioxide as their carbon source for the production of starches and other organic compounds, releasing oxygen as a by-product into the water to be used by other aquatic organisms. Organic carbon forms found in nature include fossil fuels (crude oil, natural gas, coal, and peat), organic matter found in detritus, and organic compounds found in living plants and animals. When fossil fuels are combusted for the production of energy, carbon dioxide, water vapor, particulates, nitrogen oxides, and sulfur oxides are discharged into the atmosphere. Carbon dioxide and water vapor are considered “greenhouse” gases and are involved in the theory of global warming. Two prevalent forms of inorganic carbon are found in natural geological deposits of limestone 1CaCO32 and dolomite [CaMg1CO322]. Figure 4.18 shows a simplified diagram of the carbon cycle. 4.4.2 Nitrogen Cycle Nitrogen and nitrogenous compounds are very important in biological systems. Nitrogen is a macronutrient required by all life forms. Microorganisms prefer ammonium/ammonia 1NH4+/NH32 as their source of nitrogen, however; nitrates 1NO3-2 can be used at the cost of higher energy expenditure in converting them into ammonium. Algae and plants prefer to use nitrates as their source of nitrogen. Some bacteria are capable of fixing nitrogen from the atmosphere 1N22 and live in a symbiotic relationship with plants to provide them with the nitrogen they need. Figure 4.19 is a simplified schematic of the nitrogen cycle in surface water. As was the case for carbon, nitrogen has gaseous components, with diatomic nitrogen 1N22 being the predominant form. The control of nitrogen species entering water bodies from municipal and industrial wastewater treatment plant discharges, as well as in stormwater runoff Figure 4.18 Simplified diagram of the carbon cycle.
CO2
Atmosphere CO2
CO2 O2 Photosynthesis
CO2 aqueous Aquatic Plants and Organisms Water Carbonate Rocks
Fossil fuel combustion, Volcanic activity, Forest firess
O2 CO2 Respiration
Plants
Peat, Coal, Oil, Natural Gas
Animals
Decomposition, Accumulation, Compaction of Dead Plants Land
Section 4.4
Wastewater Effluent
Precipitation Dustfall
Runoff
Atmosphere
NH3/NH4 NO3 Organic N
NH3/NH4 NO3 Organic N
NH3/NH4 NO3 Organic N
N2
Fixation NH3/NH4 NO3
Water Column
Synthesis
Organic N
Nutrient Cycles
N2
NH3 Volatilization
NO NO33-
+ NH33/NH /NH44 NH Sedimentation
Sediment Layer
Ammonification
Denitrification
Nitrification
Ammonification NH3/NH4
Organic N
Denitrification
Synthesis
Figure 4.19 The nitrogen cycle in surface water. Source: EPA Nitrogen Control Manual (1993), p. 7.
from agricultural and urban areas, must be properly managed if cultural eutrophication is to be curtailed. The discharge of ammonia to receiving waters promotes the consumption of large quantities of oxygen by the nitrification process, leading to anaerobic conditions and the death of fish and other aquatic life. Free undissociated ammonia is toxic to fish at a concentration of approximately 0.02 mg/L as nitrogen (Benefield and Randall, 1980). Nitrification
Nitrification is an aerobic transformation process that uses autotrophic microorganisms to oxidize ammonium 1NH4+2 into nitrate 1NO3-2. Biological nitrification is modeled as a two-step sequential reaction mediated by bacteria consisting of the genera Nitrosomonas and Nitrobacter (EPA, 1993). Equations (4.13) and (4.14) show the oxidation of ammonium and nitrite, respectively. Equation (4.15) is the overall nitrification reaction excluding the synthesis of biomass. NH4+ + 1.5 O2
Nitrosomonas
NO2- + 0.5 O2 NH4+ + 2 O2
Nitrifiers
" NO- + 2 H+ + H O 2 2
Nitrobacter
" NO3
" NO- + 2 H+ + H O 3 2
(4.13) (4.14) (4.15)
77
78
Chapter 4
Biological and Ecological Concepts
Equation (4.15) indicates that 4.57 g of O2 are required per g of ammonium nitrogen 12 * 32/14 = 4.572 that is oxidized to nitrate. Using the equivalent weight of CaCO3 as 50 g per equivalent, 7.14 grams of alkalinity expressed as CaCO3 are required per gram of ammonium nitrogen oxidized 312 * 1/142 * 50 = 7.144. If the alkalinity concentration is too low, nitrification will be inhibited, since the nitrifiers require inorganic carbon as their carbon source for synthesizing biomass. Homework Problem 3 presents the nitrification reaction, including the synthesis of biomass from carbon dioxide.
EXAMPLE 4.3 Nitrification stoichiometric coefficients Calculate the stoichiometric coefficients for oxygen consumption and alkalinity consumption during nitrification using Equation (4.15). Solution For oxygen consumption:
¢
2 moles O2 1 mole NH4+ - N = 4.57
≤¢
32 g O2 1 mole NH4+ - N ≤¢ ≤ 1 mole O2 14 g N
g O2 consumed g NH4+ - N oxidized
For alkalinity consumption:
¢ ¢
1 g H+ 1 mole NH4+ - N 1 eq 2 moles H+ ≤ ¢ ≤ ¢ ≤¢ ≤ + + 14 g N 1 mole NH4 - N 1 mole H 1 g H+
50 g alkalinity as CaCO3 g alkalinity as CaCO3 consumed ≤ = 7.14 eq g NH4+ - N oxidized
Denitrification
Denitrification is a biologically mediated, anoxic process that involves the reduction of nitrate 1NO3-2 into nitrogen gas 1N22. It is widely used in advanced wastewater treatment for the removal of nitrogen. A carbon source is required, since the denitrifiers are heterotrophic organisms. Biological dissimilatory denitrification is typically modeled as a two-step sequential reaction, as follows, when synthesis of biomass is excluded and methanol 1CH3OH2 is used as the carbon source. 6 NO3- + 2 CH 3OH ¡ 6 NO2- + 2 CO2 + 4 H 2O
(4.16)
6 NO2- + 3 CH 3OH ¡ 3 N2 + 3 CO2 + 3 H 2O + 6 OH -
(4.17)
The overall denitrification reaction is summarized in Equation (4.18). 6 NO 3- + 5 CH 3OH
Denitrifiers
" 3 N + 5 CO + 7 H O + 6 OH 2 2 2
(4.18)
Section 4.4
Nutrient Cycles
79
A significant point about denitrification is that a portion of the alkalinity destroyed or consumed during the nitrification process is restored. Based on the stoichiometric equation (4.18) for the overall denitrification process, 3.57 g of alkalinity as CaCO3 are produced per g of nitrate nitrogen reduced (see Example 4.4).
EXAMPLE 4.4 Denitrification stoichiometric coefficients Calculate the stoichiometric coefficient for alkalinity production during denitrification using Equation (4.18). Solution
¢
17 g OH1 mole NO3- - N 1 eq 6 moles OH≤ ¢ ≤ ¢ ≤¢ ≤ 6 moles NO3 - N 1 mole OH 14 g N 17 g OH-
¢
50 g alkalinity as CaCO3 g alkalinity as CaCO3 produced ≤ = 3.57 eq g NO3- - N reduced
Deamination
Deamination or ammonification is the conversion of organic nitrogen to ammonium/ammonia nitrogen 1NH4+/NH32. When plants and animals die, proteins are first hydrolyzed by hydrolytic enzymes produced by bacteria. Specific types of bacteria are capable of removing the amino group 1NH22 from amino acids under either aerobic or anaerobic conditions, releasing ammonia nitrogen and making it available for recycling in the biosphere. 4.4.3 Phosphorus Cycle Organic compounds containing phosphorus are found in all living organisms. Unlike the carbon, nitrogen, and sulfur cycles, the phosphorus cycle has no gas-phase component. Both inorganic and organic forms of phosphorus exist. Because of its relatively low concentration in natural waters, as compared to the available quantities of carbon and nitrogen, it is often the growth-limiting nutrient. Figure 4.20 is a simplified schematic of the phosphorus cycle. Phosphorus occurs naturally in soils and rocks as calcium phosphate 3Ca31PO4224 and calcium hydroxyapatite 3Ca51PO4231OH24. These calcium compounds are only slightly soluble in water, resulting in phosphorus concentrations of approximately 1 part per billion (ppb) in most natural aquatic systems. Concentrations of phosphorus in natural waters would remain low if they were not impacted by human activity. Wastewater discharges from industrial and municipal wastewater treatment plants (WWTPs) and agricultural runoff from fertilizers are the major contributors of phosphorus release to the environment. Orthophosphate 1PO43-2, a soluble form of inorganic phosphorus, is readily used by microorganisms and plants. Enhanced biological phosphorus removal (EBPR) wastewater treatment processes utilize phosphorus accumulating organisms (PAOs) that can take up excess phosphorus in the orthophosphate form. Chemical precipitation of phosphorus from wastewater relies on the use of aluminum and iron salts, such as aluminum sulfate and ferric chloride, for precipitating orthophosphate to form insoluble forms, such as AlPO4 and FePO4 .
80
Chapter 4
Biological and Ecological Concepts
Water Column
Zooplankton
Algae phytoplankton
Fish
Particulate organic phosphorus
Bacteria
Dissolved inorganic phosphate and dissolved organic phosphorus
Insoluble inorganic phosphates
Organic phosphorus Sediments
Figure 4.20 The phosphorus cycle.
4.4.4 Sulfur Cycle Sulfur is essential for the synthesis of animal protein. In nature, the primary inorganic form of sulfur is the sulfate ion A SO24 B found in natural waters. Sulfate is taken up by plants and microorganisms for the production of cell tissue. Then animals consume plants and microorganisms for synthesizing their own cell tissue. Upon death, sulfur is released to the environment through degradation of proteins contained in the plant and animal tissues. Degradation is microbially mediated and may occur either under aerobic or anaerobic conditions. Elemental sulfur 1S02 and combined forms of sulfur such as pyrite 1FeS22 are found in geological deposits. Figure 4.21 shows a schematic of the sulfur cycle. Several important environmental problems are associated with sulfur and its combined compounds. Odor and corrosion problems frequently occur in sanitary sewer systems and at the headworks of wastewater treatment facilities, when sulfate is reduced under anaerobic conditions to sulfides and hydrogen sulfide gas. Hydrogen sulfide is a malodorous gas that smells like rotten eggs and potentially causes death to humans at a concentration Ú 100 ppm (EPA, 1985). The following equations show the reduction of sulfates as described above. SO24 + organic matter
Anaerobic Bacteria
S2- + H+ 4 HS-
+
HS + H 4 H2S
" S2- + H O + CO 2 2
(4.19) (4.20) (4.21)
Section 4.5
Oxidation: Beggiotoa, Thiobacillus, Thiothrix
Limnological Concepts and Eutrophication
Oxidation: Beggiotoa, Thiobacillus, Thiothrix Elemental Sulfur S0
H2S Sulfide S2
Reduction: Desulfovibrio
Oxidation: Thiobacillus
SO3 Sulfates SO32
Organic S Degradation/Mineralization
R-SH
Assimilation
In the presence of oxygen, sulfur bacteria (Thiobacillus) oxidize hydrogen sulfide, converting it to sulfuric acid according to Equation (4.22). H2S + 2 O2
Sulfur Bacteria
" H SO 2 4
(4.22)
Metallic components of screens and grit-removal equipment located at the front of wastewater treatment facilities, and concrete, cast iron, and steel pipes, may be corroded by the sulfuric acid produced. Acid Mine Drainage
A serious environmental problem resulting from the mining of coal and precious metals is acid mine drainage. When pyrite and other sulfide-bearing deposits are exposed to water under aerobic conditions, bacteria will convert the sulfide to sulfate, releasing hydrogen ions and causing the pH of the water to drop. This phenomenon results in streams that are devoid of aquatic life. Equations (4.23) through (4.25) illustrate the oxidation of pyrite by microorganisms such as Thiobacillus thiooxidans, Thiobacillus ferrooxidans, and Ferrobacillus ferrooxidans: + 4 FeS2 + 14 O2 + 4 H2O : 4 Fe2+ + 8 SO24 + 8H
(4.23)
4 Fe2+ + 4 H+ + O2 : 4 Fe3+ + 2 H2O
(4.24)
4 Fe3+ + 12 H2O : 4 Fe1OH231S2 + 12 H+
(4.25)
Sulfur Dioxide Emissions
Although not shown in Figure 4.21, sulfur dioxide 1SO22 and other gaseous oxides of sulfur are emitted to the atmosphere from the combustion of fossil fuels. The primary sources are electrical power generating facilities and industries that burn fossil fuels for energy. In the atmosphere, sulfur oxides form sulfuric acid causing “acid rain.”
4.5 LIMNOLOGICAL CONCEPTS AND EUTROPHICATION Limnology is the study of the biological, chemical, and physical characteristics of fresh-water lakes and rivers. Free-floating or suspended organisms that live in a body of water are called plankton. Plankton in turn are subdivided into phytoplankton
Figure 4.21 Schematic of simplified sulfur cycle.
81
Chapter 4
Biological and Ecological Concepts
DO Concentration, mg/L 0
0
1
2
3
4
5
6
7
8
20
40 Depth, ft
82
60
80
100
120
Figure 4.22 DO profile for Norris Lake, September 5, 2007. Source: http://tnfish.org/WaterQualitySampling_TWRA/.
(plant species such as algae) and zooplankton (animal species such as crustacean, rotifers, and protozoa). Macrophytes are large aquatic plants that may be free floating or attached to the bottom (emergent). Water-quality data for various lakes are accessible by the internet. Typical data collected include: conductivity, dissolved-oxygen (DO) concentration, pH, and temperature as a function of depth. Figures 4.22 and 4.23 show the DO and temperature profile of Norris Lake near Knoxville, Tennessee. Other water-quality data for Norris Lake and other lakes in Tennessee may be accessed at the following website: http://tnfish.org/ WaterQualitySampling_TWRA/. 4.5.1 Stratification Twice annually in temperate latitudes, normally during summer and winter, lakes undergo stratification—i.e., formation of layers of water at different temperature and density. Three distinct zones develop in a lake from the surface to the bottom: the epilimnion, metalimnion, and hypolimnion, as shown in Figure 4.24. This phenomenon is attributed to the affect of temperature on the density of water. Water has a unique characteristic, in that the solid form (ice) has a lower density than the liquid at 0°C. The maximum density occurs at a temperature of approximately 4°C. At freezing, or 0°C, the ice that forms has a lower density than water at 4°C, so the ice floats. Therefore, lakes freeze from the top down, making it possible for fish and other aquatic life to live when ice covers the surface. During the spring, energy from the sun heats the upper layer of water in the lake (epilimnion), warming it from 0°C toward 4°C. The denser water sinks to the bottom, creating currents that bring colder water to the surface. During this spring
Section 4.5
Limnological Concepts and Eutrophication
Temperature, F 0
0
10
20
30
40
50
60
70
80
20
Depth, ft
40
60
80
100
120
Figure 4.23 Temperature profile for Norris Lake, September 5, 2007. Source: http://tnfish.org/WaterQualitySampling_TWRA/.
turnover, because of the mixing action, sediments along with nutrients re-enter the water column. During both the spring and the fall turnover, the contents of the lake are well mixed, resulting in uniform levels of temperature, dissolved oxygen, and nutrients throughout the water column. As spring turns into summer, the top layer or epilimnion continues to be heated, so that a warm layer of water overlies the cooler, denser layers. The epilimnion is well mixed, well oxygenated, and essentially has the same temperature throughout. Below the epilimnion is a layer of more dense water known as the metalimnion. During summer stratification, a significant change in the temperature occurs with depth ( Ú 1°C per meter of depth) in the metalimnion, and this temperature gradient is known as the thermocline. Below the metalimnion is the bottom layer of water, the densest and coolest one, known as the hypolimnion. The temperature in the hypolimnion is essentially the same throughout. Figure 4.24 also shows the temperature and DO profile during summer and winter stratification in a typical lake. Note that these profiles are similar in shape to those presented in Figures 4.22 and 4.23. Figure 4.25 shows the mixing patterns during spring and fall turnover, and the temperature profile during winter and summer stratification of a typical lake. During the fall, the temperature in the epilimnion decreases, resulting in denser water overriding warmer water beneath it. Eventually it cools and sinks to the bottom, causing the warmer water to be forced to the surface. This is known as the fall turnover. It causes complete mixing of the lake, allowing sediments, organics, and nutrients to re-enter the water column. As temperatures drop during the winter months, the lake continues to cool, and ice forms at the surface that is less dense than the cooler water below it. The temperature profile increases from 0°C at the surface to 4°C, causing winter stratification of the lake.
90
83
84
Chapter 4
Biological and Ecological Concepts
Epilimnion
Epilimnion Metalimnion
Metalimnion
Hypolimnion
Hypolimnion
4C
Lake Stratification
0 mg/L DO
Temperature and DO Profile of Lake During Summer Stratification
0C
9.5 mg/L DO Epilimnion
Metalimnion
Hypolimnion
44˚C C
0 mg/L DO
Temperature and DO Profile of Lake During Winter Stratification
Figure 4.24 Lake stratification during summer and winter.
4.5.2 Lake Classification Lakes are classified according to nutrient and organic enrichment as oligotrophic, eutrophic, or mesotrophic. Oligotrophic lakes are typically found in colder climates, have low concentrations of nutrients and organics, low levels of algae and macrophytes, abundant dissolved oxygen, and good transparency. Eutrophic lakes have the opposite characteristics: high levels of organics and nutrients, low dissolved oxygen in the hypolimnion, high concentrations of algae and macrophytes, and poor transparency. Intermediate between these two types are mesotrophic lakes, which have intermediate levels of organics, nutrients, and oxygen, but generally have an abundance of fish. Over time, a build up of nutrients, organics, and sediments leads to natural aging or eutrophication of a lake or body of water. This may take several hundred years, ultimately turning the lake into a swamp or bog. Cultural eutrophication accelerates this natural aging process due to the discharge of organics, nitrogen, and phosphorus from wastewater treatment facilities and stormwater runoff that contains nutrients and sediment. Both nitrogen and phosphorus stimulate algal growth, and the concentration of these nutrients should be limited in wastewater discharges and from surface runoff. Scientists and engineers tend to agree that
Section 4.5
Limnological Concepts and Eutrophication
Ice Cover 0C
4C
2C
4C
4C
4C
4C
4C
Winter Stratification 25C 22C
Spring Turnover
4C
4C
20C 8C
8C
8C
7C
8C 6C
Summer Stratification
6C
Fall Turnover
Figure 4.25 Temperature and mixing profiles during turnover and stratification.
phosphorus is the growth-limiting nutrient in the prevention of eutrophication and excessive algal growth, since there are several organisms that are capable of fixing nitrogen from the atmosphere. According to Liebig’s law of the minimum, the yield of any organism is directly related to the concentration of the least abundant nutrient or substrate necessary for growth. With regard to phosphorus, a total phosphorus (TP) concentration of 0.01 g/m3 1mg/L2 is recommended as the boundary condition between an oligotrophic and mesotrophic lake. The boundary TP concentration between a mesotrophic and eutrophic water body is 0.02 g/m3 1mg/L2. 4.5.3 Dissolved-Oxygen Depletion in Streams The discharge of wastes into a stream or lake can lead to unfavorable conditions for the aquatic life when the water body’s assimilative capacity is exceeded. Streams have natural assimilative abilities to oxidize organic and nitrogen compounds by indigenous heterotrophic and autotrophic bacteria. In this process, oxygen serves as the electron acceptor and is utilized by the bacteria in the oxidation of carbonaceous (organic) and nitrogenous compounds. Therefore, the dissolved-oxygen
85
86
Chapter 4
Biological and Ecological Concepts
Table 4.3 Solubility of Oxygen in Fresh Water at Atmospheric Pressure Temperature (°C)
Dissolved-oxygen concentration (mg/L)
0.0
14.62
1.0
14.22
2.0
13.83
3.0
13.46
4.0
13.11
5.0
12.77
6.0
12.45
7.0
12.14
8.0
11.84
9.0
11.56
10.0
11.29
11.0
11.03
12.0
10.78
13.0
10.54
14.0
10.31
15.0
10.08
16.0
9.87
17.0
9.67
18.0
9.47
19.0
9.28
20.0
9.09
21.0
8.92
22.0
8.74
23.0
8.58
24.0
8.42
25.0
8.26
Source: Standard Methods (1998), pp. 4–132.
(DO) concentration will decrease with time and distance downstream from the point of discharge. This process is known as deoxygenation; typically it is modeled by a first-order reaction using the parameter, ultimate biochemical oxygen demand (L): RDEOXYGENATION = kDL
(4.26)
Section 4.5
Limnological Concepts and Eutrophication
where: RDEOXYGENATION = rate of deoxygenation, equal to the rate at which oxygen is being removed from a stream, mg/L # d, kD = deoxygenation rate coefficient (base e), equal to the BOD rate constant (k), d-1, and L = ultimate biochemical oxygen demand (BOD), mg/L. The ultimate BOD represents the total quantity of oxygen consumed by bacteria at 20°C for the oxidation of organic compounds to carbon dioxide and water. As dissolved oxygen is being removed from the stream due to deoxygenation, another process called reaeration is occurring simultaneously. Reaeration is the process in which oxygen is transferred by diffusion across the water surface exposed to the atmosphere. Water traveling over rocks and through rapids entrains more oxygen than does a slower-moving river. The equilibrium or saturation concentration of dissolved oxygen is estimated using Henry’s law and is a function of temperature. Table 4.3 lists the DO concentrations in fresh water as a function of temperature. A listing of reaeration-rate coefficients for various types of water bodies is presented in Table 4.4. The reaeration process is modeled as a first-order reaction: RREAERATION = - k2D
(4.27)
where: RREAERATION = rate of reaeration which is equal to the rate at which oxygen is transferred into the stream or water body, mg/L # d, k2 = reaeration rate coefficient (base e), d-1, and D = dissolved-oxygen deficit, mg/L. The DO deficit is the difference between the DO saturation concentration and the actual or measured dissolved-oxygen concentration in the water: D = DOsat - DO
(4.28)
where: D = DO deficit, mg/L, DOsat = DO saturation concentration at a specified temperature and pressure, mg/L, and DO = actual or measured DO concentration in the water, mg/L. Figure 4.26 shows a plot of DO versus time of travel in a river.The concepts of deoxygenation and reaeration are illustrated in this figure. Table 4.4 Reaeration Coefficients 1k22 Water Body Small ponds and backwaters
k2 @ 20°C, d-1 0.1 – 0.23
Sluggish streams and large lakes
0.23 – 0.35
Large streams of low velocity
0.35 – 0.46
Large streams of normal velocity
0.46 – 0.69
Swift streams
0.59 – 1.15
Rapids and water falls Source: Peavy, Rowe, and Tchobanoglous (1985), p. 87.
Greater than 1.15
87
88
Chapter 4
Biological and Ecological Concepts
Dissolved Oxygen Concentration
DO Saturation Concentration
Do
Figure 4.26 DO versus time.
Dc
D
Critical Deficit
Reaeration Rate Exceeds Deoxygenation Rate Deoxygenation Rate Exceeds Reaeration Rate Time of Travel
Should the deoxygenation process exceed the reaeration process in a stream, the DO level will drop, which may lead to anaerobic conditions. Anaerobic conditions in a water body will result in the death of most fish and aquatic organisms, along with the production of obnoxious odors and floating solids. A DO concentration of greater than 4 or 5 mg/L is necessary to support game fish. Point sources of pollution (such as industrial and municipal wastewater treatment plant discharges) along with nonpoint sources (agricultural and urban stormwater runoff) can overwhelm the assimilative capacity of a stream or lake. Heterotrophic and autotrophic bacteria indigenous to these water bodies will degrade the carbonaceous and nitrogenous compounds, consuming large quantities of oxygen, as discussed earlier in this chapter. It is imperative that environmental engineers understand the mechanisms of deoxygenation and reaeration and the implications of discharging organic and nitrogenous compounds into surface waters. Streeter-Phelps DO Depletion Model
The concept of “dissolved-oxygen depletion” was first described by Streeter and Phelps (1925). They provided what is now considered the classical approach for modeling the impact of organic wastes discharged into receiving streams. Their model describes the simultaneous transfer and uptake of oxygen in a river by the following differential equation: dD = kDL - k2D dt
(4.29)
where: dD = change in the DO deficit (D) with time, mg/L # d. dt All other terms have been previously defined. The ultimate BOD (L) as a function of time (t) is modeled as a first-order reaction. L = Lo e-kD t
(4.30)
where: Lo = ultimate BOD concentration after the stream and wastewater discharge have mixed, mg/L, t = time of travel of wastewater discharge downstream, days, and e = base “e”, 2.71828.
Section 4.5
Limnological Concepts and Eutrophication
The ultimate BOD concentration 1Lo2 is calculated using Equation (4.31). Lo =
Qstream 1Lstream2 + Qww 1Lww2 Qstream + Qww
(4.31)
where: Qstream = flow rate in stream, volume/time, Qww = flow rate of wastewater discharged into stream, volume/time, Lstream = ultimate BOD concentration of stream prior to wastewater discharge, mg/L, and Lww = ultimate BOD concentration of wastewater, mg/L. Substituting Equation (4.30) into Equation (4.29) results in Equation (4.32). dD = kDLoe-kD t - k2D dt
(4.32)
Equation (4.32) may be integrated using the boundary conditions at t 0, D D0, and L = Lo , and at t = t, D = Dt , and L = Lt , resulting in the general form of the Streeter-Phelps Equation (4.33) used for estimating the DO deficit at downstream locations from the point of discharge: Dt =
kDLo C e1-kDt2 - e1-k2t2 D + Doe1-k2t2 k2 - kD
(4.33)
where: Dt = DO deficit at any time t, downstream of the discharge point, mg/L, t = time of travel of wastewater discharge downstream, days, and Do = DO deficit at the point of discharge, mg/L. To calculate the DO deficit at the point of discharge, the DO concentration at the point of discharge must be calculated using Equation (4.34). DOo =
Qstream 1DOstream2 + Qww 1DOww2 Qstream + Qww
(4.34)
where: DOstream = DO concentration of stream prior to wastewater discharge, mg/L, and DOww = DO concentration of wastewater, mg/L. The DO deficit 1Do2 at the point of discharge can then be calculated using Equation (4.35). Do = DOsat - DOo
(4.35)
Temperature variations affect biological and chemical reactions. Equation (4.36) is the general form of the equation used for making temperature corrections to any type of rate constant or coefficient. It was derived from the Arrhenius-van’t Hoff equations. k1T°C2 = k120°C2 1u21T°C - 20°C2
(4.36)
where: k1T°C2 and k120°C2 = rate constant or coefficient at specified temperature in °C and at 20°C, respectively, and u = temperature-correction coefficient, dimensionless.
89
90
Chapter 4
Biological and Ecological Concepts
Both the reaeration 1k22 and deoxygenation 1kD2 rate coefficients must be corrected for temperature variations other than 20°C. Metcalf and Eddy (1991) recommend the following temperature-correction coefficients 1u2 for adjusting these rate coefficients for temperature variations. A theta 1u2 value of 1.024 should be used for correcting k2 . The deoxygenation 1kD2 rate coefficient should be corrected with a 1u2 value of 1.145 when the temperature is … 20°C and 1.056 when the temperature is between 20 and 30°C. The maximum dissolved-oxygen deficit will occur where the reaeration rate equals the deoxygenation rate. This point is known as the critical point, and the time required to reach it can be determined by differentiating Equation (4.33) and setting the derivative equal to zero. Solving for time gives: tc =
Do 1k2 - kD2 k2 1 ln B ¢1 ≤R k2 - kD kD kDLo
(4.37)
where tc = time of travel to the critical deficit point in the stream, days. The distance traveled (x) for a river flowing at a constant velocity (u) can be determined by multiplying the stream velocity by the travel time (t). Example 4.5 illustrates how to use the Streeter-Phelps equations for locating the critical deficit point in a stream.
EXAMPLE 4.5 Calculating critical deficit point in a stream A municipal WWTP discharges 22.5 million gallons per day of secondary effluent containing 30 mg/L of ultimate BOD at 26.7°C with 1.5 mg/L of dissolved oxygen. The stream flow is 161 cubic feet per second (cfs) at a velocity of 1.5 feet per second (fps) and an average depth of 5 ft. The temperature of the stream before the wastewater enters is 21°C. The stream is 85% saturated with oxygen and has an ultimate BOD of 2.0 mg/L. The reaeration 1k22 and deoxygenation 1kD2 rate coefficients are 0.35 d-1 and 0.20 d-1, respectively at 20°C. Determine the following: a. the wastewater flow rate in cfs; b. the temperature of the combined wastewater and stream; c. the dissolved-oxygen concentration of the mixture of wastewater and stream; d. the DO deficit of the mixture of wastewater and stream; e. the ultimate BOD concentration of the mixture of wastewater and stream; f. the distance downstream 1xc2 to the point of minimum DO concentration, and g. the minimum DO concentration in stream. Also: h. Plot the DO deficit versus time starting at 0 and going to 3.0 days in increments of 0.1 day. Solution part a The wastewater flow rate is converted from million gallons per day to cubic feet per second to be consistent with the flow units for the stream. Alternatively, the streamflow units of cfs could be converted to million gallons per day. Qww = 22.5
MG 106 gal 1d 1h 1 min 1 ft3 ba ba b¢ ¢ ≤a ≤ = 34.8 cfs d MG 24 h 60 min 60 s 7.48 gal
Section 4.5
Limnological Concepts and Eutrophication
Solution part b The temperature of the mixture of the stream and wastewater is determined as follows: To =
Qstream 1Tstream2 + Qww 1Tww2 Qstream + Qww
=
161 cfs121°C2 + 34.8 cfs126.7°C2 161 cfs + 34.8 cfs
= 22°C
Solution part c Determine the DO concentration of the combined water and wastewater at the point of discharge. The DO concentration in the stream above the discharge is estimated as follows.The DO saturation in water at 21°C is 8.92 mg/L; however, the stream is only 85% saturated, therefore the saturated concentration must be multiplied by 0.85, as follows: DOstream = 0.8518.92 mg>L2 = 7.6 mg>L DOo = =
Qstream 1DOstream2 + Qww 1DOww2 Qstream + Qww 161 cfs17.6 mg>L2 + 34.8 cfs11.5 mg>L2 161 cfs + 34.8 cfs
= 6.5 mg>L
Solution part d Calculate the initial oxygen deficit 1Do2 at the point of discharge. First, look up in Table 4.3 the DO saturation concentration of water at 22°C (the temperature of the mixture of water and wastewater), which is 8.74 mg/L. Do = DOsat - DOo = 8.74 mg>L - 6.5 mg>L = 2.2 mg>L Solution part e Determine the ultimate BOD concentration 1Lo2 of the mixture of water and wastewater at the point of discharge. Lo =
Qstream 1Lstream2 + Qww 1Lww2
Qstream + Qww = 7.0 mg>L
=
161 cfs12.0 mg>L2 + 34.8 cfs130 mg>L2 161 cfs + 34.8 cfs
Solution part f Next, correct the reaeration and deoxygenation rate coefficients for the temperature of 22°C. Use theta values 1u2 of 1.024 and 1.056 for correcting k2 and kD , respectively. k1T°C2 = k120°C21u21T°C - 20°C2 k2122°C2 = k2120°C211.0242122°C - 20°C2 = 0.35d -111.0242122°C - 20°C2 = 0.37d -1 kD122°C2 = kD120°C211.0562122°C - 20°C2 = 0.20d-111.0562122°C - 20°C2 = 0.22d -1 Determine the time of travel 1tc2 to reach the point of minimum DO in the stream. tc =
tc =
Do1k2 - kD2 k2 1 ln B ¢ 1 ≤R k2 - kD kD kDLo -1
0.37 d
-1
1 0.37 d ln C £1 -1 - 0.22 d 0.22 d-1
2.2
mg 10.37 - 0.22 d-12 L
0.22 d-117.0 mg>L2
≥ S = 1.9 days
91
Chapter 4
Biological and Ecological Concepts
The distance downstream to the point of minimum DO concentration is determined by multiplying the critical time of travel 1tc2 by the stream velocity. xc = tc * velocity = 1.9d * 1.5
24hr ft 60 min 60s * * * s 1d 1hr min
xc = 246, 240 ft = 46.6 miles Solution part g The value of the minimum deficit is found by substituting the critical time 1tc2 into Eq. (4.33). Dc = Dc =
kDLo C e1-kDtc2 - e1-k2tc2 D + Doe1-k2tc2 k2 - kD 0.22 d-1 17.0 mg>L2 -1
-1
C e 1-0.22d
-1
* 1.9d2
0.37 d - 0.22 d mg 1-0.37d-1 * 1.9d2 + 2.2 e = 2.8 mg>L L
- e 1- 0.37d
-1
* 1.9d2
D
The minimum DO concentration in the stream may be calculated by substituting into Eq. (4.28) and rearranging it. Dc = DOsat - DOc DOc = DOsat - Dc = 8.74 mg /L - 2.8 mg /L = 5.9mg /L Solution part h A plot of the dissolved oxygen deficit versus time from 0 to 3.0 days in increments of 0.1 days is presented in the following figure. Notice the deficit at the critical time of 1.9 days is 2.8 mg/L. 3.00 Dc 2.8 2.50 Dissolved oxygen deficit, mg/L
92
2.00
1.50
1.00
0.50
0.00
0.0
0.5
1.0
1.5 tc 1.9 2.0 Time of travel, days
2.5
3.0
3.5
References 93
S U M M A RY This chapter introduced the basic concepts of biology and ecology that are important to environmental engineers. Ecology was defined as the branch of biology dealing with the interrelationships between living organisms and the interactions between these organisms and their environment. Biological concepts were presented in which the cell was described as the basic functional and structural unit of all living organisms. The taxonomic classification of organisms based on genus and species was discussed. This was followed by a description of the major groups of organisms encountered in environmental problems: algae, bacteria, crustaceans, fish, fungi, helminths, macrophytes, protozoa, rotifers, and viruses. Microbial growth rate and the equations used for modeling microbial growth were presented. The importance of photosynthesis and the flow of energy from the sun through ecosystems was explained. Food chains consisting of primary producers (plants and algae) that are consumed by primary and secondary consumers (herbivores and carnivores) allow energy to be transferred through the biosphere. Bioconcentration of a toxic compound involves the uptake of the compound into an organism from the aqueous phase whereas bioaccumulation is the magnification in concentration of a compound as it passes up through food chains. The use and recycling of nutrients in ecosystems was discussed by presenting the carbon, nitrogen, phosphorus, and sulfur cycles. The chapter concluded with an introduction to limnology, the study of the physical, chemical, and biological characteristics of fresh water lakes and rivers. The Streeter-Phelps Dissolved Oxygen Sag Model was presented along with an example problem to illustrate how the discharge of municipal wastewater effluent can lead to low dissolved oxygen levels in receiving streams. abiotic component bioaccumulation bioconcentration biotic component consumers denitrification deoxygenation dissolved oxygen sag
ecological systems eukaryotic cell eutrophication food chain limnology microbial growth nitrification nutrient cycles
photosynthesis prokaryotic cell producers reaeration stratification turnover
REFERENCES American Public Health Association, American Water Works Association, Water Environment Federation (1998). Standard Methods for the Examination of Water and Wastewater, 20th ed., American Public Health Association, Washington, DC. Benefield, L.D., and Randall, C.W. (1980), Biological Process Design for Wastewater Treatment, Prentice Hall, Englewood Cliffs, NJ. Brock, T.D. (1979). Biology of Microorganisms, Prentice Hall, Englewood Cliffs, NJ. Crites, R., and Tchobanoglous, G. (1998). Small and Decentralized Wastewater Management Systems, WCB/McGraw-Hill, New York, NY. Davis, M.L., and Masten, S.J. (2004). Principles of Environmental Engineering and Science, McGraw Hill, New York, NY. Environmental Protection Agency (1985). Design Manual Odor Control and Corrosion Control in Sanitary Sewerage Systems and Treatment Plants, EPA/625/1-85/0187, Office of Research and Development, Washington, DC. Environmental Protection Agency (1993). Manual Nitrogen Control, EPA/625/R-93/010, Office of Research and Development, Washington, DC.
KEY WORDS
94
Chapter 4
Biological and Ecological Concepts
Environmental Protection Agency (2007). http://www.epa.gov/glnpo/atlas/images/big05.gif, accessed October 20, 2008. Henry, J.G., and Heinke, G.W. (1996). Environmental Science and Engineering, Prentice Hall, Upper Saddle River, NJ. Heukelekian, H., Oxford, H.E., and Manganelli, R. (1951). Factors Affecting the Quantity of Sludge Production in the Activated Sludge Process, Sewage and Industrial Wastes,Vol. 23, 945. Hoover, S.R., and Porges, N. (1952). Assimilation of Dairy Wastes by Activated Sludge, II: The Equation of Synthesis and Oxygen Utilization, Sewage and Industrial Wastes, Vol. 24. Lester, J.N., and Birkett, J.W. (1988). Microbiology and Chemistry for Environmental Engineers, E & FN Spon, London, England. Madigan, M.T., Martinko, J.M., and Parker, J. (2000). Brock Biology of Microorganisms, 9th ed., Prentice Hall, Upper Saddle River, NJ. McCarty, P.L. (1970). Phosphorus and Nitrogen Removal by Biological Systems, in Proceedings, Wastewater Reclamation and Reuse Workshop, Lake Tahoe, CA, June 25–27, 1970. Metcalf and Eddy (1991). Wastewater Engineering, McGraw-Hill, New York. Metcalf and Eddy (2003). Wastewater Engineering: Treatment and Reuse, McGraw-Hill, New York. Monod,J.(1949).The Growth of Bacterial Cultures,Annual Review of Microbiology,Vol. 3,371–394. Mihelcic, James R. (1999). Fundamentals of Environmental Engineering, John Wiley & Sons, Inc., Danvers, MA. Peavy, H.S., Rowe, D.R., and Tchobanoglous, G. (1985). Environmental Engineering, McGrawHill Book Company, New York. Reynolds, T.D., and Richards, P.A. (1996). Unit Operations and Unit Processes in Environmental Engineering, PWS Publishing, Boston. Streeter, H.W., and Phelps, E.B. (1925). A Study of the Pollution and Natural Purification of the Ohio River, U.S. Public Health Service Bulletin No. 146. Sullia, S.B., and Shantharam, S. (1998). General Microbiology, Science Publishers Inc., Enfield, NH. Viessman, W., and Hammer, M.J. (2005). Water Supply and Pollution Control, Pearson Prentice Hall, Upper Saddle River, NJ. Whittaker, Robert (1969). New Concepts of Kingdoms or Organisms. Evolutionary Relations Are Better Represented by New Classifications than by the Traditional Two Kingdoms, Science, Vol. 163, 150–160.
EXERCISES 4.1
4.2
The following chemical formula is often used for representation of a bacterial cell, C60H87O23N12P1 . (a) Determine the mass (mg) of each element in 3000 mg of bacterial cells. (b) Suppose 25 mg/L of ammonia nitrogen and 5 mg/L of orthophosphate as phosphorus are available for growing bacteria. If other nutrients are in abundance, which is the limiting nutrient—nitrogen or phosphorus? (c) What mass of bacterial cells could be produced in terms of milligrams of bacterial cells per liter of water based on the limiting nutrient in Part b? (d) Suppose the nitrogen source was cut to 15 mg/L of ammonia nitrogen. How much bacterial cell mass (mg/L) could be produced? (e) Suppose the phosphorus source was cut to 4 mg/L of orthophosphate as phosphorus. How much bacterial cell mass (mg/L) could be produced? The term C10H19O3N1 is often used to represent the biodegradable organic material in wastewater. In the past, most biological denitrification processes occurred after aerobic treatment of the wastewater, necessitating the addition of a carbon source such as methanol 1CH3OH2. If the denitrification process precedes aerobic treatment, the organic matter in the influent wastewater can serve as the carbon source for denitrification. The following stoichiometric
Exercises 95
equation shows the overall denitrification reaction when the biodegradable organic matter in the incoming wastewater is used rather than adding an external source such as methanol or acetate. C10H19O3N1 + 10 NO3- : 5 N2 c + 10CO2 + 3 H2O + NH3 + 10 OH(a) Using the above equation, list the organic donor (species being oxidized) and the electron acceptor (species being reduced). (b) Calculate the grams of alkalinity as CaCO3 produced per gram of nitrate nitrogen 1NO3- - N2 utilized. (c) Determine the grams of organic matter required per gram of 1NO3- - N2 converted to nitrogen gas. 4.3
Nitrification is an aerobic process in which ammonium/ammonia nitrogen is transformed into nitrate. The genera Nitrosomonas and Nitrobacter are the two bacterial organisms that mediate the process. Equations (4.13), (4.14), and (4.15) show the reactions when synthesis of biomass is neglected. Crites and Tchobanoglous (1998) show the following overall nitrification reaction, including synthesis of biomass. Because of rounding of the coefficients, the equation does not balance exactly; however, this error is negligible. NH4+ + 1.863 O2 + 0.098 CO2 : 0.0196 C5H7O2N + 0.98 NO3+ 0.941 H2O + 1.98 H+
4.4
(a) Calculate the quantity (grams) of alkalinity as CaCO3 consumed per gram of ammonium nitrogen 1NH4+ - N2 oxidized. (b) How does this compare with the alkalinity consumption from Equation (4.15)? (c) Calculate the quantity (grams) of oxygen required per gram of NH4+ - N oxidized. (d) How does this compare with the oxygen consumption from Equation (4.15)? A continuous-flow, completely mixed reactor was operated at various flow rates to collect specific growth-rate data. The specific microorganism growth rate and soluble substrate concentration data are presented below. S (mg/L)
M(h-1)
20.0
0.66
10.0
0.50
6.6
0.40
5.0
0.33
4.0
0.28
(a) Determine the maximum specific microorganism growth rate 1mm2 and the half-velocity rate constant (Ks) by making a double-reciprocal plot (Lineweaver-Burke Plot) of Equation (4.2). Plot 1 versus S1. See linearized form of Equation (4.2) below: KS 1 1 1 = + m mmax S mmax Alternatively, the above coefficients may be found by using the Solver function in Excel. (b) Give the magnitude and units for each of the coefficients determined.
96
Chapter 4
Biological and Ecological Concepts
4.5
A pure culture of bacteria was grown at 25°C under batch conditions. The following experimental data were collected. Time (h)
Bacterial concentration, X (mg/L)
2.0
3470
3.4
3700
4.8
4100
6.1
4400
7.7
4780
Integration of Equation (4.1) results in the following equation, which can be used for determining the specific growth rate of the culture. ln1X2 = ln1Xo2 + mt
4.6
A plot of the ln(X) versus time (t) will yield a straight line with the slope equal to m and the y-intercept equal to the ln1Xo2 or Xo = eY-intercept. (a) Determine the magnitude and give the units for m. (b) Determine the magnitude and give the units for Xo . Equation (4.11) is used in the design and operation of activated sludge wastewater treatment processes. The term 1dX/dt2NG/X is equal to SRT-1, where SRT is the solids retention time that represents the average unit of time that the microorganisms remain in the system. The substrate utilization rate 1dS/dt2U is equal to Q1Si - Se2/V for a completely mixed reactor. Dividing both sides of Equation (4.11) by X, the microorganism concentration, results in the following equation: Q3Si - Se4 1 = Y¢ ≤ - kd SRT XV A plot of 1/SRT versus Q1Si - Se)/1XV2 yields a slope equal to Y, the yield coefficient, and the endogenous decay coefficient 1kd2 as the Y-intercept. Determine the magnitude and units for Y and kd , given the following data collected on a continuous-flow activated sludge bench-scale unit.
SRT (days)
Q (gallons/day)
V (gallons)
X (mg/L)
Si (mg/L)
Se (mg/L)
2.94
20
20
4165
2462
546
5.13
20
20
6566
2406
305
7.15
20
20
8966
2383
224
20
20
10781
2436
172
10.5 4.7 4.8
4.9 4.10
Draw a simplified food chain with at least four trophic levels of organisms in a fresh-water pond feeding on plankton. Draw a sketch of a lake in the Northern Hemisphere, and label and describe the epilimnion, hypolimnion, and thermocline. Explain what happens during spring turnover and summer stratification. Draw a simplified diagram showing the major carbon forms comprising the carbon cycle. An industrial WWTP discharges into a large river 0.5 m3/s of industrial wastewater containing an ultimate BOD concentration of 220 mg/L at a temperature
Exercises 97
of 26°C and a DO concentration of 2.0 mg/L. The velocity of the river is 0.85 m/s. Upstream of the point of discharge, the river has a temperature of 12°C, an ultimate BOD of 15 mg/L, and flow rate of 2.5 m3/s. The deoxygenation- and reaeration-rate coefficients are 0.2 d-1 and 0.4 d-1 at 20°C. (a) Calculate the dissolved-oxygen concentration (mg/L) at the point of discharge. (b) Calculate the dissolved-oxygen deficit 1Do2 in mg/L at the point of discharge. (c) Calculate the ultimate biochemical oxygen demand 1Lo2 in mg/L at the point of discharge. (d) Calculate the dissolved-oxygen deficit (mg/L) 50 km downstream, assuming that the temperature in the river remains the same as at the point of discharge. (e) Calculate the dissolved-oxygen concentration (mg/L) 50 km downstream, assuming that the temperature in the river remains the same as at the point of discharge.
CHAPTER
5
Risk Assessment Objectives In this chapter, you will learn about: The concept or perception of risk How to calculate risk The steps involved in performing risk assessments Environmental Impact Statements (EIS)
5.1 CONCEPT OR PERCEPTION OF RISK Risk and safety are complementary concepts used in making decisions concerning environmental matters. Risk relates to the probability that an adverse effect or outcome will occur, whereas safety is the probability that no adverse effect will occur. Risk is expressed numerically and has no units. It is reported either as a fraction or as a percentage. If expressed as a fraction, it will have a value between 0 and 1. The closer the risk is to 1, the higher the probability or likelihood of an event’s happening. Risk may be calculated as a lifetime risk or an annual risk. It also may be calculated with regard to certain groups or individuals that engage in specific activities, or it may be expressed as an average risk for the general public. The best way to explain risk is to present a few examples illustrating highand low-level-risk activities. Risk is a part of everyday life, and certain activities incur a higher level of risk than others. Piloting a plane, skydiving, and NASCAR racing are examples of high-risk activities. Low-risk activities include walking on a trail, watching TV, and sleeping. Other activities that have high risk associated with them include: smoking, binge drinking, unsafe sex, sleep deprivation, and unwise use of prescription and nonprescription drugs. Although the riskiness of such activities is often overlooked, they can result in severe consequences. A stark reminder of reality is to calculate the probability of death. We all know that the Grim Reaper will come for us some day. Each of us will eventually die, so the risk of death can be expressed as 100% or 1.0. The risk associated with specific activities or exposure to toxic chemicals can also be estimated. There is a lot of uncertainty in calculating risk. There may be insufficient data or no data available. Some procedures may have doubtful validity, such as extrapolating results from animal studies and applying them to humans. The Environmental Protection Agency (EPA) establishes specific concentration limits for various types of pollutants in different media to minimize the risk to the public. Normally, the concentration of a specific contaminant is set so that the risk of developing cancer ranges from one additional cancer per million people 110-62 to one hundred additional cancers per million people 110-42. More often than not, the one-in-a-million scenario is typically used in risk-analysis calculations.
Section 5.1
Concept or Perception of Risk 99
Table 5.1 Fifteen Leading Causes of Death in the United States, 2004 Cause
Annual deaths
Risk (%)
1. Heart disease
654,092
27.3
2. Malignant neoplasms (cancer)
550,270
22.9
3. Cerebrovascular (strokes)
150,147
6.3
4. Chronic low respiratory disease
123,884
5.2
5. Accidents (unintentional injury)
108,694
4.5
6. Diabetes mellitus
72,815
3.0
7. Alzheimer’s disease
65,829
2.7
8. Influenza and pneumonia
61,472
2.6
9. Nephritis
42,762
1.8
10. Septicemia
33,464
1.4
11. Suicide
31,647
1.3
12. Chronic liver and cirrhosis
26,549
1.1
13. Essential hypertension and hypertensive renal disease
22,953
1.0
14. Parkinson’s disease
18,018
0.8
15. Pneumonitis due to solids and liquids
16,959
0.7
418,810
17.5
2,398,365
100.0
All other causes Total deaths
Source: Centers for Disease Control, http://webappa.cdc.gov, final 2003 and preliminary 2004.
Table 5.1 lists the fifteen leading causes of death in the United States during 2004. Cardiovascular or heart disease was the leading cause of death and cancer was the second leading cause. Neglecting age, gender, and ethnicity, an American’s risk of dying from heart disease is estimated at 27.3% (or, expressed first as a fraction, 654,092/2,398,365 = 0.273). According to Wilson and Crouch (2001), there are about 40,000 fatalities per year from automobile accidents. The annual risk of dying in a motor vehicle accident is approximately 40,000/2,398,365 = 0.017 or 1.7%.
EXAMPLE 5.1 Calculating risk Using the data in Table 5.1, estimate the risk or probability of an American dying from influenza (flu) or pneumonia in 2004. Express the risk as a fraction and as a percentage. Solution First, identify the number of individuals who died from influenza and pneumonia in 2004 from Table 5.1. There were 61,472 deaths attributed to this category, which ranked eighth. Second, from Table 5.1, identify the total number of deaths recorded in the United States during 2004, which was 2,398,365.
100 Chapter 5
Risk Assessment
Next, calculate the risk or probability of dying from the flu or pneumonia by dividing the number of deaths attributed to flu or pneumonia by the total number of deaths recorded for 2004: risk =
61,472 = 0.026 2,398,365
or 2.60 %
EXAMPLE 5.2 Additional risk calculation Bath County, Virginia, is located in the northwest portion of the state. The county population is approximately 5000. Using the data from Table 5.1, estimate the number of persons in Bath County who died from cancer in 2004. Assume that 100 persons passed away in Bath County in 2004. Solution First, in Table 5.1, look up the risk or percentage of Americans dying from cancer in the United States. In 2004, approximately 22.9% 1risk = 550,270/ 2,398,365 * 100 = 22.9%2 of the deaths in the United States were attributed to some form of cancer. Next, estimate the number of persons dying from cancer in Bath County by multiplying the number of deaths in Bath County for 2004 (100) by the risk or fraction of deaths attributed to cancer in the entire United States 122.9/100 = 0.2292. 100 * 0.229 = 22.9 ⬵ 23 The number of persons dying from cancer in Bath County is estimated to be approximately 23. Activities that increase mortality risk by one in a million 110-62 are presented in Table 5.2. Each of these activities produces essentially the same amount of risk. Table 5.2 Activities that Increase Mortality Risk by One in a Million Activity
Type of risk
Smoking 1.4 cigarettes
Cancer, heart disease
Drinking 1冫2 liter of wine
Cirrhosis of the liver
Spending 1 hour in a coal mine
Black lung disease
Living 2 days in Boston or New York
Air pollution
Traveling 484 km (300 mi) by car
Accident
Flying 1613 km (1000 mi) by jet
Accident
Flying 9677 km (6000 mi) by jet
Cancer by cosmic radiation
Traveling 16 km (10 mi) by bicycle
Accident
Traveling 6 minutes by canoe
Accident
Living 2 summer months in Denver on vacation from New York
Cancer by cosmic radiation
Section 5.2
Risk Assessment
101
Table 5.2 (continued) Activity
Type of risk
Living 2 months with a cigarette smoker
Cancer, heart disease
Eating 40 tablespoons of peanut butter
Liver cancer caused by aflatoxin B
Eating 100 charcoal-broiled steaks
Cancer from benzopyrene
Risk of accident by living within 8 km (5 mi) of a nuclear reactor for 50 years
Cancer caused by radiation
Source: Wilson, 1979.
Smoking 1.4 cigarettes has the same probability or risk of causing death as driving 484 km (300 miles) in a car. A person has a one-in-a-million chance of dying from drinking 0.5 liters of wine or from living for 50 years within an 8-km (5-mile) radius of a nuclear reactor. In this chapter, we will focus on the risk to human health due to the release of chemicals and pollutants into the environment.
5.2 RISK ASSESSMENT Risk assessment is a scientific methodology used by engineers, scientists, and regulators to quantify the risk or perceived risk of a contaminant’s adverse effect on human health and the environment. By definition, risk is a function of hazards and exposures. Anything that produces an adverse effect on human health and the environment or ecosystem is defined as a hazard. Exposure as defined by the U.S. EPA is the qualitative or quantitative assessment of contact to the skin or orifices of the body by a chemical. Figure 5.1 shows the four-step risk-assessment process adopted by the EPA (1989): data collection and evaluation, toxicity assessment, exposure assessment, and risk characterization. Risk management follows this riskassessment process. Figure 5.1 Four-step risk assessment process followed by risk management.
Data Collection and Evaluation
Exposure Assessment
Toxicity Assessment
Risk Characterization
Risk Management
102 Chapter 5
Risk Assessment
5.2.1 Data Collection and Evaluation The first steps in the risk-assessment process are the collection and evaluation of data at the proposed site. The types of site data required for the baseline risk assessment include the following: the identification of potential contaminants; concentration of contaminants in the major sources and media; and characterization of sources and environmental setting with regard to contaminant fate and transport, release potential, and persistence. A sampling plan for determining background levels of naturally occurring and anthropogenic levels of chemicals must be developed and implemented. After completion of the site sampling investigation, the large quantity of analytical data collected must be evaluated. Major steps in this phase are evaluating the analytical methods used, evaluating quality of data with respect to sample quantitation limits (SQL) and detection limits (DL), comparing concentrations in blanks to concentrations in samples, and developing a set of data to be used in the risk assessment. 5.2.2 Toxicity Assessment Toxicology involves studying the adverse effects of exposure of living organisms to chemicals or toxicants. Toxic effects may range from mild allergic reactions to death. Whether or not a chemical produces a toxic effect, and how severe it is, depends on the susceptibility of the exposed individual. An individual’s susceptibility is a function of sex, age, diet, genetics, prior exposure to the chemical or other toxicants, and health. The toxicity component of the baseline risk assessment considers the types of adverse health effects associated with the exposure to specific chemicals, the relationship between the magnitude of exposure and adverse effects, and the related uncertainties of a compound’s carcinogenicity (cancer-causing ability) to humans. Toxicity assessment is accomplished in two steps: hazard identification and doseresponse assessment. Hazard Identification
Hazard identification is the process of determining whether exposure to a chemical, biological, or physical agent (toxicant) is linked to a particular health effect such as cancer or birth defect. This stage of the process relies on toxicity studies performed on animals, such as mice and rats, or animal tissue. Since living organisms are used, such tests are called bioassays. Once in the body, the toxicant is absorbed by the blood and transported throughout the body, where it may accumulate in tissues and organs, be transformed into less innocuous compounds or more harmful metabolites, and/or excreted in urine and feces. For example, both the liver and kidneys are susceptible to chlorinated organic compounds and heavy metals. Some toxicants promote acute toxicity, whereas others accumulate over time, causing chronic toxicity. Chronic toxicity may result in mutagenesis—mutation of the genetic material, deoxyribonucleic acid (DNA). This may lead to cancer, tumors, birth defects, or miscarriages and other reproductive failure. Toxicants capable of causing cancer are called carcinogens and those causing birth defects are called teratogens. Dose-Response Assessment
Dose-response assessment is the process of evaluating the relationship between the level of exposure to the toxicant and the incidence and extent of adverse effect on the exposed individual (animal, human, cell, tissue). The response to the agent may be carcinogenic or noncarcinogenic and the tests conducted may be short-term acute tests or long-term chronic tests.
Section 5.2
Risk Assessment
103
Cumulative Percent Mortality
100
50
0
1
10
LD50 100
1000
10000
Dose (mg/kg)
Figure 5.2 Typical dose-response mortality curve and LD50.
The dose of a toxicant is a major factor determining the level of harm caused. Dose is defined as the mass of chemical received by the exposed individual or animal, with units expressed in milligrams per kilogram (mg/kg) of body weight. It is not unusual to express the dose with units of time if the individual or animal receives the compound over time. Typical units would be expressed as milligrams per kilogram per day 1mg/kg # d2. (Although they share similar units, the concentration of a compound or toxicant is not the same as the dose. Concentration is related to the mass of the compound per unit volume in the ambient air, soil, or water.) One way to predict the toxicity of a chemical is to perform acute-toxicity tests on rats or mice. The quantity of a specific chemical that is required to kill the organism is determined. The relationship between dose and response of a population of living organisms normally follows a logistic or S-shape curve, as shown in Figure 5.2. This curve is developed by exposing the test organisms to increasing doses of a specific toxic compound. As the dose is increased, the number of deaths also rises. Dose expressed in mg of toxicant per kilogram of body weight (mg/kg) is plotted on the abscissa scale versus the cumulative mortality (number of deaths). The dose at which 50% of the organisms die is called the median lethal dose (LD50). Carcinogenic Effects Instead of mortality, the response could be reported as the probability or risk of causing cancer. Figure 5.3 shows dose-response curves for
Figure 5.3 Typical carcinogenic dose-response curves.
Carcinogen Linear Cumulative Response or Risk Carcinogen Non-linear
Dose
104 Chapter 5
Risk Assessment
Non-Carcinogen Non-Linear Cumulative Response or Risk
Figure 5.4 Typical noncarcinogenic dose-response curve.
Dose
carcinogenic agents. This type of response curve is typically used for evaluating the risks associated with carcinogens. For carcinogens, it is assumed that a threshold level does not exist, since there is a cancer risk associated with all doses. Doseresponse curves can be developed for both acute and chronic toxicity; however, we are generally more interested in the long-term, chronic studies, as they are generally linked to cancer. There is no absolute scale for toxicity; therefore it is only possible to state that one compound is more or less toxic than another, due to its effects on animals and humans. Noncarcinogenic Effects Figure 5.4 shows a dose-response curve for a noncarcinogenic agent. The dose-response curves for noncarcinogens indicate there is some exposure threshold to the toxicant that results in an adverse effect on the animal or human. Below this threshold level, no adverse effect will result. Toxicity tests are performed to identify and determine the magnitude of these thresholds. The lowest dose administered during testing that elicits an adverse response is called the lowestobserved-adverse-effect level (LOAEL), while the highest dose administered that does not create an adverse effect is termed no-observable-adverse-effect level (NOAEL). Figure 5.5 illustrates a dose-response curve for a noncarcinogenic toxicant along with the NOAEL, LOAEL, and reference dose (RfD). The reference dose is estimated by dividing the NOAEL by an uncertainty factor, which typically is 10,
Figure 5.5 Reference dose, NOAEL, LOAEL for noncarcinogenic compound.
Non-Carcinogen
Cumulative Response or Risk
NOAEL RfD
Actual Threshold LOAEL Dose (mg/kg·day)
Section 5.2
Table 5.3 Reference Dose (RfD) for Chronic Noncarcinogenic Effects of Selected Chemicals Chemical
RfD (mg/kg.day)
Acetone
0.900
Arsenic
0.0003
Benzene
0.004
Chlorine
0.100
Chloroform
0.010
Fluorine (soluble fluoride)
0.060
Mercuric chloride 1HgCl22
0.0003
Methyl mercury (MeHg)
0.0001
Nitrate
1.60
Phenol
0.300
Strontium
0.600
Vinyl chloride
0.003
Xylenes
0.200
Source: U.S. Environmental Protection Agency, http://www.epa.gov/IRIS.
100, or 1000. These numbers reflect the uncertainty associated with the variability of response to the toxicant by humans and animals, extrapolation from high compound doses in animals to a low dose in humans, and lacking or incomplete data sets. An uncertainty factor of 10 is used if there is minimal uncertainty, and the value increases to 1000 with less certainty. Table 5.3 lists the oral reference doses for chronic, noncarcinogenic effects of several chemicals. Mathematical Modeling of Dose-Response Curves
Dose-response curves are developed from bioassays performed on animals at high doses. The data obtained must then be extrapolated to the human population at low doses. Various models have been proposed, including the one-hit model and multistage model (Crump, 1984). The one-hit model assumes that a single chemical or toxicant will result in the formation of a tumor in the exposed individual. The multistage model is based on the multistage formation of tumors that is caused by a sequence of biological events. The one-hit model is actually a special case of the multistage model and will be presented below. The model shows the relationship between the dose of a toxicant (d) and the lifetime risk (probability) of cancer, P(d), and is presented in Equation (5.1). P1d2 = 1 - e-1q0 + q1 d2 where: P1d2 = lifetime risk or probability of cancer, dimensionless, d = dose of toxicant or chemical, and q0 and q1 = parameters that are picked to fit the data set.
(5.1)
Risk Assessment
105
106 Chapter 5
Risk Assessment
The background rate for the incidence of cancer, P(0), may be estimated by substituting a value of zero for d into Equation (5.1): P102 = 1 - e-1q0 + q1 * 02 = 1 - e-q0
(5.2)
Recall that exponentials can be represented as an expansion: ex = 1 + x +
x2 x3 x4 xn + + + Á 2! 3! 4! n!
(5.3)
For small values of x, this expansion can be expressed as: ex ⬵ 1 + x
(5.4)
Substituting this result into Equation (5.2), the background rate for cancer incidence, P(0), can be expressed as: P102 = 1 - e-q0 = 1 - 31 + 1- q024 = q0
(5.5)
For small toxicant doses, Equation (5.1) can be approximated as: P1d2 ⬵ 1 - 31 - 1q0 + q1d24 = q0 + q1d = P102 + q1d
(5.6)
At low doses, then, the additional risk of cancer above the background level can be expressed as follows: additional risk = A1d2 = P1d2 - P102
(5.7)
Substituting Equation (5.6) into Equation (5.7) results in Equation (5.8), which may be used to calculate the additional risk incurred by an organism when exposed to a toxicant at a dosage of d. additional risk = A1d2 = P1d2 - P102 = P102 + q1 d - P102 ⬵ q1d
(5.8)
The multistage model based on tumor formation is shown as Equation (5.9). P1d2 = 1 - e-1q0 + q1 d + q2 d
2
+ q3 d3 + Á + qn dn2
(5.9)
where the qi are positive constants selected to yield the best fit to the response-dose data. It is easy to see that Equation (5.1) for the one-hit model is a special case of the multistage model. When we perform toxicological tests on carcinogenic compounds in animals, two major problems arise. First, how do we extrapolate the data collected at high doses down to low doses? Second, will the response exhibited by the animal population be the same, lower, or higher than in humans? The Environmental Protection Agency uses a modified multistage model called the linearized multistage model, replacing the linear term in the polynomial function with its 95% confidence limit to achieve a more stable estimate. This model assumes a linear relationship between risk and dose at low doses. The slope of the dose-response curve is called the slope factor (SF) or potency factor (PF). Slope factors for several chemical compounds are presented in Table 5.4. The EPA maintains a database with background information on the toxicological properties of selected compounds called the Integrated Risk Information System (IRIS). The slope factors for oral and inhalation exposure routes for these compounds may be found there.
Section 5.2
Table 5.4 Slope Factors for Selected Chemicals for Oral Route Chemical
Slope factor (kg.day/mg)
Arsenic
1.5
Benzene
1.5 * 10-2 to 5.5 * 10-2
Carbon tetrachloride
1.3 * 10-1
Chloroform
6.1 * 10-3
DDT
3.4 * 10-1
Dieldrin
1.5 * 101
Heptachlor
4.5
PCB
4 * 10-2 low risk; 2.0 high risk
2,4,6-trinitrotoluene
3 * 10-2
Vinyl chloride
7.2 * 10-1 to 1.5
Source: U.S. Environmental Protection Agency, http://www.epa.gov/IRIS.
The slope factor can be defined mathematically as: slope factor =
incremental lifetime cancer risk CDI
(5.10)
where: slope factor = slope of dose-response curve, kg # d/mg or 1mg/kg # d2-1; Incremental Lifetime Cancer Risk = incremental risk of cancer in a lifetime above the background rate, dimensionless; and CDI = Chronic Daily Intake, the average dose of toxicant absorbed per kilogram of body weight over an entire lifetime (70 years), mg/(kg # d). 5.2.3 Exposure Assessment The third step in performing a baseline risk assessment is an exposure assessment, delineating the magnitude of the actual or potential dose that an exposed individual receives, its frequency and its duration, along with the potential exposure pathways. A toxicant can enter the body in three pathways: inhalation (air), ingestion (food and liquids), and absorption (dermal through skin or eyes). Exposure media include surface water, groundwater, sediment, air, soil and dust, and food. Conducting an exposure assessment involves evaluating toxicant releases; identifying exposed populations and potential pathways; estimating exposure-point toxicant concentrations from actual monitoring data and computer modeling; and estimating toxicant intakes for specific pathways. The exposure period over which the dose of the toxicant is or has been administered in humans is classified as: acute (1 day), subacute (10 days), subchronic (2 weeks to 7 years), and chronic (7 years to lifetime). The EPA uses the point-estimate method for estimating the reasonable maximum exposure (RME). Both contaminant concentration and contaminant intake rate are required for estimating the RME.
Risk Assessment
107
108 Chapter 5
Risk Assessment
Table 5.5 Residential Exposure Equations for Various Pathways Exposure route
Intake equation
Ingestion of drinking water
CDI =
CW * IR * EF * ED BW * AT
Ingestion of surface water while swimming
CDI =
CW * CR * EF * ED BW * AT
Dermal contact with water
CDI =
CW * SA * PC * EF * ED * CF BW * AT
Ingestion of chemicals in soil
CDI =
CS * IR * CF * FI * EF * ED BW * AT
Dermal contact with soil
CDI =
CS * CF * SA * AF * ABS * EF * ED BW * AT
Inhalation of airborne (vapor phase) chemicals
CDI =
CA * IR * ET * EF * ED BW * AT
Ingestion of contaminated fish, shellfish, fruits, and vegetables CDI =
CF * IR * FI * EF * ED BW * AT
Source: U.S. Environmental Protection Agency (1989). ABS = absorption factor, dimensionless; AF = soil-to-skin adherence factor, mg/cm2; AT = averaging time, period over which exposure is averaged, days; AT = equal to exposure duration for noncarcinogens and 70 years for carcinogens; BW = body weight, the average body weight over the exposure period, kg; C = chemical concentration, the average concentration contacted over the exposure period (e.g., mg/L for water); CA = chemical concentration in air, mg/m3; CDI = chronic daily intake, mg/kg # day; CF = conversion factor (e.g.,1L = 1000 cm3, 10-6 mg/kg); CR = contact rate, L/h; CS = chemical concentration in soil, mg/kg; CW = chemical concentration in water, mg/L; ED = exposure duration, years; EF = exposure frequency, days/year; ET = exposure time, h/day; FI = pathway specific, fraction ingested, dimensionless; IR = intake or contact rate, the amount of contaminated medium taken in or contacted per unit time or event (e.g., L/day, mg/day, kg/meal,m3/h); PC = chemical specific dermal permeability constant, cm/h; and SA = skin surface area available for contact, cm2.
A generic equation for estimating chronic daily intake (CDI) is given as Equation (5.11). Table 5.5 lists some of the equations used for calculating CDI for various exposure routes. Definitions and units for each parameter in Table 5.5 are listed after Equation (5.11). CDI =
C * IR * EF * ED BW * AT
(5.11)
where: AT = averaging time, the period over which exposure is averaged, d, BW = body weight, the average body weight over the exposure period, kg, C = chemical concentration, the average concentration contacted over the exposure period (e.g., mg/L for water), CDI = chronic daily intake, mg/kg # d, ED = exposure duration, yr,
Section 5.2
Risk Assessment
109
EF = exposure frequency, d/yr, and IR = intake or contact rate, the amount of contaminated medium taken in or contacted per unit time or event (e.g., L/d, mg/d, kg/meal,m3/h). The CDI must be determined for each route of exposure and each medium. Recall that individuals may be exposed to toxicants by ingestion, dermal contact, and inhalation through various mediums such as air, water, dust, sediment, soil, and food. Default values used in risk calculations by EPA are presented in Table 5.6. Table 5.6 Summary of Standard Default Exposure Factors
Land Use Residential
Exposure pathway
Recreational
Exposure duration (years)
Body weight (kg)
Ingestion of potable water
2L
350
30
70 (adult) 15 (child)
Ingestion of soil and dust
200 mg (child) 100 mg (adult)
350
6 (child) 24 (adult)
15 (child) 70 (adult)
Inhalation of contaminants
20 m3 1total2 15 m3 1indoor2
350
30
70
1L
250
25
70
Ingestion of soil and dust
50 mg
250
25
70
Inhalation of contaminants
20 m3 1workday2
250
25
70
Ingestion of potable water
2L
350
30
70
Ingestion of soil and dust
200 mg (child) 100 mg (adult)
350
6 (child) 24 (adult)
15 (child) 70 (adult)
Inhalation of contaminants
20 m3 1total2 15 m3 1indoor2
350
30
70
Consumption of homegrown produce
42 g (fruit) 80 g (vegetable)
350
30
70
Consumption of locally caught fish
54 g
350
30
70
Commercial/ Ingestion of potable industrial water
Agricultural
Daily intake
Exposure frequency (days/year)
Source: U.S. Environmental Protection Agency (1991).
EXAMPLE 5.3 Calculating chronic daily intake Estimate the chronic daily intake (CDI) of arsenic from exposure to a city water supply that contains an arsenic concentration of 0.010 mg/L, the maximum contaminant level (MCL) established by the Environmental Protection Agency. Suppose that a 70-kg person consumes 2 L of water each day for 70 years. Determine the chronic daily intake (CDI) of arsenic in mg/kg # d.
110 Chapter 5
Risk Assessment
Solution Substitute the above values into Equation (5.11) to calculate the CDI. mg L 365 d b * 170 yr2 b * a2.0 b * a yr L d 365 d 170 kg2 * 170 yr2a b yr mg = 2.86 * 10-4 1kg # d2
C * IR * EF * ED = CDI = BW * AT
a0.010
5.2.4 Risk Characterization During risk characterization, the data collected from the toxicity assessment and exposure assessment are used to calculate the risk associated with each medium (air, water, soil, etc.) and exposure route (ingestion, dermal, and inhalation). Risk Characterization for Carcinogens
Equation (5.10) may be arranged as Equation (5.12) so that we can estimate the incremental lifetime risk of cancer by multiplying the CDI by the slope factor obtained from the IRIS toxicological database. For carcinogenic compounds at low doses (with risk below 0.01), risk is calculated using Equation (5.12). The slope factor is obtained from Table 5.4 or by using IRIS on the Environmental Protection Agency website (http://www.epa.gov/IRIS). incremental lifetime cancer risk = CDI * slope factor
(5.12)
At high doses of carcinogenic compounds (with risk above 0.01), risk is calculated using a modified form of Equation (5.2):. P102 = 1 - e-q0 = 1 - e-11CDI21slope factor22
(5.13)
If several carcinogenic compounds are involved, the risks are added together to calculate the overall risk to the individual.
EXAMPLE 5.4 Risk calculations using CDI and slope factors
Using the chronic daily intake (CDI) value of 2.86 * 10-4 mg # kg-1 # d-1 determined in Example 5.3 along with the other data provided, determine: a. The upper-bound cancer risk per person. b. The number of extra cancers that would be expected per year in a city of 100,000 people if they consumed the same water containing arsenic. Solution part a Determine the upper-bound cancer risk for this person by substituting into Equation (5.12). First, obtain from Table 5.4 the slope factor for arsenic by the oral route, which is 1.5 kg # d # mg-1.
Section 5.2
Risk Assessment
111
incremental lifetime cancer risk = CDI * Slope Factor mg kg # d b = 2.86 * 10-4 a1.5 mg kg # d incremental lifetime cancer risk = 4.29 * 10-4 Therefore, over a 70-year period, the chance of a person getting cancer from drinking this water would be approximately 429 in a million. Solution part b Finally, the number of extra cancers that would be expected per year in a city of 100,000 is calculated by multiplying the incremental lifetime cancer risk by the population of the city and dividing by the 70-year period. city population cancers = incremental lifetime cancer risk * year exposure duration 100,000 people b = 0.61 = 4.29 * 10-4 a 70 yr The extra number of cancers expected per year in the city described is 0.61.
Risk Characterization for Noncarcinogens
The EPA uses the hazard index (HI) for noncarcinogenic compounds. The hazard index is calculated by dividing the chronic daily intake (CDI) by the reference dose (RfD), as shown in Equation (5.14). Since reference doses are associated with a specific exposure pathway, they must be used only with exposure data from the same pathway. HI =
CDI RfD
(5.14)
Where more than one non-carcinogenic compound is present, the sum of the HIs is determined. A hazard index value of less than 1.0 is normally considered acceptable; whereas a value equal to or greater than 1.0 indicates unacceptable risk.
EXAMPLE 5.5 Calculating hazard index Suppose a 70-kg adult drinks 2.0 liters per day of water for a 20-year period. Assume that the water contains 0.05 mg/L of chloroform and 0.6 mg/L of nitrates. a. Determine the hazard index and state whether this is acceptable or unacceptable. b. Chloroform is a potential carcinogen. Determine the carcinogen risk associated with drinking water containing 0.05 mg/L of chloroform. Would it meet the goal of 1 * 10-6 that is usually recommended by EPA? Solution part a First, the CDI for chloroform is estimated. From Table 5.5, use the following equation for ingestion of drinking water.
112 Chapter 5
Risk Assessment
CDI = CDI =
CW * IR * EF * ED BW * AT
10.05 mg>L2 * 12.0 L>d2 * 1365 d>yr2 * 120 yr2 170 kg2 * 1365 d>yr2 * 120 yr2
= 1.43 * 10-3 mg>kg # d
Second, locate in Table 5.3 the reference dose (RfD) for chloroform, which is 0.010 mg/kg # day. Third, calculate the HI for chloroform using Equation (5.14). HI =
1.43 * 10 -3 mg>kg # d CDI = = 0.14 RfD 0.010 mg>kg # d
Use the same procedure above for nitrate. From Table 5.5 use the equation for ingestion of drinking water to determine the CDI for nitrate. CDI =
10.6 mg>L2 * 12.0 L>d2 * 1365 d>yr2 * 120 yr2 170 kg2 * 1365 d>yr2 * 120 yr2
= 1.71 * 10-2 mg>kg # d
Next, locate the reference dose for nitrate from Table 5.3 as 1.6 mg/kg # d. Now, calculate the HI for nitrate using Equation (5.14). HI =
1.71 * 10-2 mg>kg # d CDI = = 0.011 RfD 1.6 mg>kg # d
Finally, calculate the total HI by summing the individual HI value for each toxicant total HI = ©HI = 0.14 + 0.011 = 0.151 6 1.0 Since the total hazard index is 6 1.0, the water is considered safe to drink. Solution part b First, recall that incremental lifetime cancer risk is estimated using Equation (5.12). Second, the slope factor for chloroform for the oral exposure route is determined from Table 5.4 as 6.1 * 10-3 kg # d # mg-1. Third, calculate the incremental cancer risk of drinking this water over a lifetime using Equation (5.12). Use the CDI value of 1.43 * 10-3 mg # kg-1 # d-1 calculated in Part a for chloroform incremental lifetime cancer risk = CDI * slope factor
incremental lifetime cancer risk = 11.43 * 10-3 mg # kg-1 # d-12 * (6.1 * 10-3 kg # d # mg-12
incremental lifetime cancer risk = 8.72 * 10-6 From a cancer-risk perspective, this drinking water would not meet the acceptable risk goal of 1 * 10-6 normally used by EPA, since 8.72 * 10-6 7 1 * 10-6.
5.3 RISK MANAGEMENT Risk management follows the four-step risk assessment process previously discussed. Risk managers make decisions using the results of the risk assessment to minimize the adverse effects of a toxicant, process, or technology on human health and the environment. Risk is evaluated in terms of the cost associated with reducing risk. Factors that are considered include the feasibility of technology to reduce the
Section 5.4
Environmental Impact Analysis
risk, the potential to create more risk (e.g., where dredging sediments may result in the release of contaminants into the water column), statutory requirements, and public opinion. Ultimately, risk analysis is the basis for decisions on implementing and enforcing drinking-water and wastewater effluent standards; defining acceptable exposure doses; locating industries, landfills, and wastewater treatment facilities; and establishing the desired level of cleanup at a hazardous waste site.
5.4 ENVIRONMENTAL IMPACT ANALYSIS 5.4.1 Overview of Environmental Impact Statement The National Environmental Policy Act (NEPA) was signed into law by President Richard Nixon on January 1, 1970.As outlined in Section 1502 of the Code of Federal Regulations (40 CFR 1502), NEPA requires an environmental impact statement (EIS) when a project is federally controlled and significantly affects the environment. Federally controlled projects are those that are federally funded, undertaken by the federal government, or licensed by the federal government. A detailed report must be submitted to the Environmental Protection Agency for review and comments. NEPA also requires that EISs be made available to the public and local, state, and federal authorities. A “Google” search on the internet will provide numerous examples of Environmental Impact Statements. These examples are generally several hundred pages in length; sometimes have more “gloss” than substance; and it may be difficult to “see the forest for the trees.” The standard format of an EIS recommended by EPA is presented below, along with a brief discussion of three critical sections of the report. 5.4.2 Environmental Impact Statement Vesilind and Morgan (2004) suggest three steps in writing an environmental impact statement: inventory, assessment, and evaluation. Inventory
The first step is to prepare an inventory of the environmental conditions at the proposed site and alternate sites that exist at the time of the report, along with recent trends. Collection of hydrological and meteorological data, animal and plant species at and near the vicinity of the site, along with demographic and socioeconomic data should be included in the inventory phase. Assessment
The assessment portion of the EIS involves analysis and number crunching of the data collected for the environmental inventory. Several assessment methods exist, each generally involving listing all potential areas of concern that might impact the environment. First, an importance factor (IF) and a magnitude factor (MF) are established for each area of concern. A Likert scale from 0 to 5 is normally used for each of these. On a typical Likert scale, a value of 0 would indicate no importance for IF and minimal effect for MF. Likewise, a value of 5 indicates high importance and maximum effect for the IF and MF factors, respectively. Next, the effect of each area of environmental concern is assigned a value. If the effect (E) is beneficial, a positive one 1 +12 is assigned or if detrimental, a negative one 1-12. The final environmental impact (EI) is estimated by summing the products of the importance factor (IF), the magnitude factor (MF), and the effect term (E): n
EI = a 1IFi21MFi21Ei2 i-1
(5.15)
113
114 Chapter 5
Risk Assessment
where: EI = environment impact (dimensionless), IFi = importance factor for ith environmental concern (ranges from 0 to 5), MFi = magnitude factor for ith environmental concern (ranges from 0 to 5), Ei = effect term for the area of environmental concern, +1 for beneficial or positive effect, and -1 for detrimental or negative effect, and n = number of areas of environmental concern. Example 5.6 illustrates the assessment procedure for an overly simplified environmental impact analysis using Equation (5.15) and the procedure just discussed. Evaluation
The final step in writing the environmental impact statement is to analyze the results obtained from performing the assessment. Final conclusions are developed and presented in this step. These conclusions will be used by the public, politicians, and regulatory agencies in making the final decision to proceed, or not, with the proposed project. Often, common sense does not win out.
EXAMPLE 5.6 Simplified environmental assessment problem A city is considering two alternatives for handling sludge that will be produced at its new wastewater treatment facility (WWTF). The first alternative involves incineration of the stabilized sludge, with disposal of the ash to a landfill. The second alternative is to truck the stabilized sludge offsite and apply it to agricultural land near the new proposed WWTF. Solution The best way to solve this problem is to use a spreadsheet. First, set up two tables, listing environmental concerns in Column 1. Second, select Importance Factors (IF) and Magnitude Factors (MF) ranging from 0 to 5 for each concern. Third, place the effect term (E) in Column 4. Recall that E has a value of +1 for a beneficial effect and -1 for a detrimental effect. Fifth, calculate the environmental impact (EI) for each area of concern by using Equation (5.15), as presented in Column 5. Finally, calculate the overall EI for each alternative by summing the values in Column 5. This procedure is repeated for the second alternative, truck sludge offsite and apply to land.
Alternative 1: Incineration of Sludge with Ash Disposed of in Landfill Area of concern
Importance factor
Magnitude factor
Effect term
Environmental impact
(Column 2)
(Column 3)
(Column 4)
(Column 5)
Air pollution
5
5
-1
-25
Groundwater pollution
4
2
-1
-8
Noise
1
1
-1
-1
Odor
2
2
-1
-4
Traffic congestion
2
1
-1
(Column 1)
-2 Sum = - 40
Section 5.4
Environmental Impact Analysis
115
Alternative 2: Truck Sludge Off Site and Apply to Agricultural Land. Area of concern
Importance factor
Magnitude factor
Effect term
Environmental impact
(Column 2)
(Column 3)
(Column 4)
(Column 5)
(Column 1) Air pollution
5
4
-1
-20
Groundwater pollution
4
5
-1
-20
Noise
3
2
-1
-6
Odor
4
5
-1
-20
Traffic congestion
2
1
-1
-2 Sum = - 68
The overall environmental impact (EI) value for Alternative 1 is - 40, versus -68 for Alternative 2. Based on this simplified assessment procedure, Alternative 1, incineration of the stabilized sludge with the remaining ash disposed of in a landfill, would be selected to minimize adverse effects on the environment. Keep in mind that these types of analyses are subjective and the results will vary significantly, depending on the individual performing the assessment. A cost-benefit analysis will eventually be performed for the two alternatives, and it is highly likely that the cost of implementing Alternative 2 will be significantly less than that for Alternative 1. Ultimately, capital costs along with operating and maintenance (O & M) costs, rather than environmental impact assessments, often end up driving political decisions.
Format of Environmental Impact Statement
A standard format for an EIS is presented in Table 5.7. Agencies should use this format or one that covers each category listed. The sections listed below are explained in more detail, as they are the critical portions of the EIS. Affected Environment This section of the EIS describes the environment of the areas(s) that will be affected or created by the proposed project, along with all of the alternatives being considered. Only data and analyses commensurate with the importance of their impact on the environment should be presented. Table 5.7 Recommended Format for Environmental Impact Statement a)
Cover Sheet
b)
Summary
c)
Table of Contents
d)
Purpose of and Need for Action
e)
Alternatives Including Proposed Actions
f)
Affected Environment
g)
Environmental Consequences
h)
List of Preparers
i)
List of Agencies, Organizations, and Persons to Whom Copies of EIS Sent
j)
Index
k)
Appendices
116 Chapter 5
Risk Assessment
Environmental Consequences
The scientific and analytical basis for the proposed project and alternatives is presented in this section. Major items to be covered include: the environmental impact of the proposed project and alternatives; adverse effects that cannot be avoided if the proposed project is implemented; the relationship between short-term uses of the environment and the sustainability of long-term productivity; and any irreversible or irretrievable commitments of resources that would be involved if the proposed project were implemented. Alternatives Including the Proposed Action This section is the heart of the environmental impact statement. The environmental impacts of the proposed project and alternatives are compared, so that decision makers and the public have a clear choice among the options. Specific areas that must be addressed are listed below: • All reasonable alternatives must be rigorously explored and objectively evaluated in detail. Reasons for eliminating an alternative must be presented and discussed. • Detailed documentation and substantial treatment of each alternative and proposed action must be provided, so that reviewers can judge for themselves the merit of each. • Reasonable alternatives outside the jurisdiction of the lead agency should be included. • The “no action” alternative should be evaluated. • Unless prohibited by another law, the lead agency should list the preferred alternative or alternatives in the draft EIS and final document. • Appropriate mitigation measures should be included that have not been included already for the proposed project or alternatives.
S U M M A RY Risk relates to the probability that an adverse effect or outcome will occur. Risk has no units; it is expressed as a fraction or a percentage. The Environmental Protection Agency normally sets contaminant concentration levels in air, water, and soil so that the risk of developing cancer is one additional cancer per million 110-62 or one hundred additional cancers per million people 110-42. In most scenarios, a risk of onein-a-million is normally used. Risk assessment is the scientific methodology used by engineers, scientists, and regulators to quantify risk of a contaminant’s adverse effect on human health and the environment. Four major steps involved in risk assessment include: data collection and evaluation, toxicity assessment, exposure assessment, and risk characterization. Two types of chemicals or toxicants evaluated by EPA are carcinogenic and noncarcinogenic compounds. Dose-response curves are developed from acute- and chronic-toxicity studies. For noncarcinogens, a threshold level exists such that no adverse effect is produced at concentrations below this value. Threshold values for carcinogenic compounds do not exist, since exposure to even small doses might induce a cancer response. The EPA uses a linearization of the multistage model for modeling the effect of a toxicant from animal studies to human populations. A toxicant can enter the body by three pathways: inhalation (air), ingestion (food and liquids), and absorption (dermal through skin or eyes). Exposure media include surface water, groundwater, sediment, soil and dust, air, and food. Various equations were presented for calculating the chronic daily intake (CDI) value through the various exposure routes. These values are used in conjunction with a reference dose to calculate the
Exercises 117
hazard index (HI) for noncarcinogenic compounds. If the HI is equal to or greater than 1.0, the risk of an adverse or toxic effect is significant. An HI value less than 1.0 is acceptable. Risk managers make decisions based on the four-step risk-assessment process by translating the risk into the costs associated with reducing it. Environmental impact statements (EISs) were discussed and an example illustrating how to perform a simple environmental impact (EI) analysis was presented. bioassays carcinogen chronic daily intake (CDI) concentration dose dose-response curve environmental impact analysis (EIS)
exposure hazard hazard index (HI) LOAEL mutagen NOAEL probability reference dose (RfD)
risk safety slope factor teratogen toxicant
KEY WORDS
REFERENCES Centers for Disease Control (2007). Death and Death Rates for 2004 and Age-Adjusted Death Rates and Percent Changes in Age-Adjusted Rates from 2003 to 2004 for the 15 Leading Causes of Death: United States, Final 2003 and Preliminary 2004. http://webappa.cdc.gov, accessed June 20, 2007. Crump, K.S. (1984). An Improved Procedure for Low-Dose Carcinogenic Risk Assessment from Animal Data, Journal of Environmental Pathology, Toxicology, and Oncology, 5-4(5): 339–348. Environmental Protection Agency (2007). http://www/epa/gov/IRIS, accessed June 24, 2007. Environmental Protection Agency (1991). Risk Assessment Guidance for Superfund.Volume 1. Human Health Evaluation Manual, Supplemental Guidance “Standard Default Exposure Factors” Interim Final. Office of Emergency and Remedial Response, Toxics Integration Branch, U.S. Environmental Protection Agency, Washington, DC. Environmental Protection Agency (1989). Risk Assessment Guidance for Superfund.Volume 1. Human Health Evaluation Manual (Part A), EPA/540/1-89/002. Vesilind, P.A., and Morgan, S.M. (2004). Introduction to Environmental Engineering, 2nd ed., Brooks/Cole, Belmont, CA. Wilson, R. (1979). Analyzing the Daily Risks of Life, Technology Review, 81(4): 41–46. Wilson, R., and Crouch, E.A.C. (2001). Risk Benefit Analysis, Harvard University Press, Digital Design Group, Newton, MA.
EXERCISES 5.1
5.2
5.3
Estimate the number of persons who would die annually from smoking a pack of cigarettes daily in a city of 200,000 people. Assume the risk of smoking a pack of cigarettes each day is 3.6 * 10-3. Calculate the risk of dying per year from cigarette smoking if a person lives 70 years. Consider the lifetime health risk of drinking water containing 0.02 mg/L of DDT for a 70-kg adult. The slope factor for DDT as estimated by EPA is 3.4 * 10-1 1kg # day/mg2. Estimate the annual number of additional cancers per year one might expect in a population of 150,000 people. A new wastewater treatment plant (WWTP) is to be constructed to service a part of the Knoxville, Tennessee, metropolitan area. The anticipated design capacity of the facility is 113,550 m3/day (30 million gallons per day). Two alternatives have been proposed for disposing of the effluent from the facility.
118 Chapter 5
Risk Assessment
Alternative 1 recommends discharging the effluent to the Tennessee River; Alternative 2 suggests spraying the effluent on a pine forest (silvaculture). Use data in the tables provided below to assess the alternatives. Alternative 1: Discharge to River Area of concern
IF
MF
E
Air pollution
0
0
-1
Groundwater pollution
0
0
-1
Noise
1
1
-1
Odor
3
2
-1
Surface water pollution
5
5
-1
Tree farming
0
0
+1
Wildlife
5
4
-1
Area of concern
IF
MF
E
Air pollution
4
3
-1
Groundwater pollution
4
5
-1
Noise
1
2
-1
Odor
2
3
-1
Surface water pollution
4
3
-1
Tree farming
5
4
+1
Wildlife
4
4
+1
EI
Alternative 2: Silvaculture
5.4
5.5
EI
Calculate the chronic daily intake (CDI) for the various exposure routes for a young adult male who is exposed to drinking water containing toluene at a concentration equal to the drinking-water standard (MCL) of 1 mg/L. Assume that he weighs 70 kg, consumes 2 liters of water each day, showers for a total of 12 minutes each day, and swims for 60 minutes, 5 days a week, for 20 years while training for the Olympics. During showering, assume that the average toluene concentration in the air is 2 mg/m3, that the dermal uptake rate from water (PC) is 10 * 10-6 m/h, and that dermal absorption during showering is only 10% of the available dose. Assume the surface area of an adult male to be 1.94 m2, use an inhalation rate of 20 m3/day, and assume that 50 ml of water are ingested per hour during swimming. The U.S. drinking-water standards for the following carcinogens are given below. For each carcinogen, determine the lifetime individual cancer risk and the incremental cancer risk per year in a population of 300 million people. Calculate risk based on residential exposure factors recommended by EPA. (a) Arsenic, MCL = 0.01 mg/L, and SF = 1.5 kg # day/mg (b) Benzene, MCL = 0.005 mg/L, and SF = 5.5 * 10-2 kg # day/mg (c) Carbon tetrachloride, MCL = 0.005 mg/L, and SF = 1.3 * 10-1 kg # day/mg (d) Polychlorinated biphenyls (PCBs), MCL = 0.0005 mg/L, and SF = 2.0 kg # day/mg, upper-bound, high-risk. (e) Trichloroethylene, (TCE) MCL = 0.005 mg/L, and SF = 1.1 * 10-2 kg # day/mg (f) Vinyl chloride, MCL = 0.002 mg/L, and SF = 1.5 kg # day/mg
Exercises 119
5.6
5.7
5.8
5.9
5.10
Determine the number of additional deaths per year occurring in a population of 300 million people from motor-vehicle accidents. Assume the annual risk of dying in a motor vehicle accident is 2.4 * 10-4. Calculate the number of deaths over an average lifetime of 70 years. Suppose drinking water contains 5.0 mg/L of phenol, 0.5 mg/L of xylene, and 0.35 mg/L of acetone. If a 70-kg adult drinks 2.0 liters per day of this water for 20 years, calculate the hazard index and discuss whether this is a safe level of exposure. Suppose your campus water supply was contaminated with trichloroethylene (TCE) at a concentration of 10 mg/L. Determine the total intake over a fouryear academic program. Assume students live on campus and attend classes 9 months each year, and therefore, use 275 days per academic year and a 70-kg adult male in your calculations. Estimate the lifetime incremental cancer risk for a 50-kg woman exposed to an airborne carcinogen in the work place. Exposure is based on inhaling 1 m3/h of air containing 0.1 mg/m3 of carcinogen, 8 hours per day, 5 days per week, 50 weeks per year, for 20 years.Assume the carcinogen has an absorption factor of 85% and the inhalation potency or slope factor is 2 * 10-3 1mg/kg # day2-1. Also estimate the lifetime incremental cancer risk for a 70-kg adult male exposed to the same contaminant under similar circumstances. Georgia Power is considering building a new 1000-megawatt power plant to provide additional electricity to its customers in the southeast. Two alternatives are being considered for providing the energy necessary to drive the steam turbines: coal-fired versus nuclear. Use the tables below to perform an environmental impact assessment (determine the EI value of each alternative) and select the best alternative to implement. Discuss political factors that might influence the decision. Alternative 1: Coal-Fired Alternative Area of concern
IF
MF
E
Air pollution Groundwater pollution
5
5
-1
3
2
-1
Noise
2
1
-1
Odor
3
3
-1
Residuals disposal
2
3
-1
Radiation
1
1
-1
Surface-water pollution
4
5
-1
Area of concern
IF
MF
E
Air pollution (no greenhouse gases)
5
4
+1
Groundwater pollution
3
2
-1
Noise
1
1
-1
Odor
2
1
+1
Residuals disposal
5
5
-1
Radiation
5
4
-1
Surface-water pollution
4
5
-1
EI
Alternative 2: Nuclear Alternative EI
CHAPTER
6
Design and Modeling of Environmental Systems Objectives In this chapter, you will learn about: Rates and order of reactions Performing material and flow balances on control volumes or processes Completely mixed, plug, and dispersed plug flow regimes The design and modeling of batch, completely mixed, and plug flow reactors with and without reactions occurring within Performing energy balances around simple environmental engineering systems
6.1 INTRODUCTION The design and modeling of environmental systems requires a thorough understanding of material, energy, and flow balances. Material balances are performed on “black boxes” or control volumes that serve as the boundaries of the system being studied. Usually, the control volume represents a unit process in which a chemical or biochemical reaction is taking place. It is important to know the order of reaction and whether materials are conserved, produced, or removed during the process. Batch, completely mixed, and plug flow reactors, each with and without reactions, are used extensively in environmental and chemical engineering.These reactors are utilized to treat water, wastewater, and contaminated air and soil. Thermodynamic principles that involve changes in free energy (G), enthalpy (H), and entropy (S) are used by scientists and engineers to determine whether chemical and biochemical reactions will proceed as written. More importantly, engineers and chemists are concerned about how fast a reaction takes place. Kinetics is the study of the rate of a process or reaction.
6.2 CHEMICAL AND BIOCHEMICAL REACTIONS Environmental engineers manipulate chemical and biological reactions. Some of these reactions, like radioactive decay, occur naturally or spontaneously. Others will not proceed unless energy is added to the system. We will focus on chemical and biological (biochemical) reactions. Environmental engineers design water and wastewater treatment processes, air pollution abatement technologies, and groundwater treatment systems. These processes, technologies, and systems utilize biochemical and chemical reactions to oxidize, reduce, precipitate, and/or transform pollutants into harmless forms. In some applications no reactions are taking place. For example, water flowing into and out of a water storage tank and water flowing through a pipe exhibit no chemical or biological reaction. In these examples, the water or substance that flows through the system is conserved or considered as a “nonreactive” substance. In other systems, a reaction such as oxidation or reduction may occur, involving the
Section 6.2
Chemical and Biochemical Reactions
transformation of some of the chemical species. The rate of reaction, or time required for a reaction to run to completion, is a significant parameter used in the design of reactors and environmental treatment systems. 6.2.1 Rates of Reaction For the simple, irreversible reaction shown as Equation (6.1), the chemical reaction rate can be expressed as the change in concentration of a specific component with respect to time. The stoichiometric coefficients for this reaction are unity and they provide the quantitative relationship between substances in a chemical reaction. 1A + 1B:1C
(6.1)
The rate (r) of the above chemical reaction can be expressed as differentials as follows: r = -
dA dB dC = = dt dt dt
(6.2)
where: dA mass = change in species A concentration with time, , dt volume # time dB mass = change in species B concentration with time, , dt volume # time dC mass = change in species C concentration with time, , and dt volume # time r = rate of reaction,
mass . volume # time
A negative sign is placed before the differentials for A and B, since they are being dA dB removed during the reaction. In other words, and are negative. A positive sign dt dt dC is inferred before differential , since C is being produced during the reaction. dt When the stoichiometric coefficients are not all unity, the rate of change in the concentration of a given species with respect to time must be modified. This is done by multiplying by the reciprocal of the stoichiometric coefficient. Equation (6.3) is a chemical reaction where the stoichiometric coefficients have a value other than one. 2A + 4B:3C
(6.3)
The rate for this chemical reaction can be expressed in a generalized form as: r = -
1 dA 1 dB 1 dC = = 2 dt 4 dt 3 dt
(6.4)
6.2.2 Rate Law and Order of Reaction Determination of the rate law is important to understanding the kinetics of a reaction. The rate law for any reaction must be determined by experiment and cannot be inferred from the reaction stoichiometry. For Equation (6.1), experimentation may
121
122 Chapter 6
Design and Modeling of Environmental Systems
show that the rate of the reaction is linear with respect to the concentrations of A and B. Knowing this, we could write the rate law for Equation (6.1) as follows: r = where:
dC = kA B dt
(6.5)
k = reaction rate constant, volume/time # mass, A = concentration of A, mass/volume, and B = concentration of B, mass/volume.
A general form of Equation (6.5) is: r =
dC = kAaBb dt
(6.6)
The superscripts a and b are used to define the reaction order. Since in our example a and b are both equal to 1, the reaction is said to be first order with respect to A and first order with respect to B. The sum of all the exponents 1a + b2 is called the total reaction order. In this case, the total reaction order is 2. Fractional reaction orders are possible for biological systems, but often an integer value for reaction order is determined or assumed in order to solve kinetic problems. Table 6.1 presents the general form of the equation for zero-, first-, and secondorder removal and production reactions. These equations are applicable only to unimolecular reactions that can be described as A : P. The reaction-rate constant is denoted as k and the reaction time is t. The concentration of a given constituent is denoted as C. A “removal” reaction denotes the removal or reduction in concentration of a given species during the reaction or process. Similarly, the increase in concentration of a given species would signify a “production” reaction. 6.2.3 Zero-Order Reactions A zero-order reaction is one that proceeds at a rate that is independent of the concentration of any reactant. For example, consider the irreversible conversion of a single reactant (A) to a single product (P) according to Equation (6.7). A:P
(6.7)
If Equation (6.7) is a zero-order removal reaction, the rate-law equation is written as follows: r = -
dC = kC 0 = k dt
(6.8)
Table 6.1 General Form of Zero-, First-, and Second-Order Equations Removal
Production
Zero
Ct = C0 - kt
Ct = C0 + k t
First
Ct = C0 e-kt
Ct = C0 ek t
Second
1 1 = + kt Ct C0
1 1 = - kt Ct C0
Section 6.2
Chemical and Biochemical Reactions
123
where: dC mass = rate of change in the concentration of A with time, , and dt volume # time k = reaction-rate constant, volume/(mass # time). Equation (6.8) is a differential equation. Using calculus, Equation (6.8) can be rearranged by integrating over the limits, where C0 = initial concentration of A at time equal to 0 and Ct = concentration of A at any time, t, as shown in Equation (6.9). The integrated form of Equation (6.9) is Equation (6.10). Ct
LC0
dC = - k
t
L0
dt
(6.9)
Ct - C0 = - kt
(6.10)
Notice that Equation (6.10) corresponds to a zero-order removal reaction shown in Table 6.1. A zero-order removal reaction will plot as a straight line with a negative slope of k on arithmetic paper (Figure 6.1).A zero-order production reaction will plot as a straight line with a positive slope of k on arithmetic paper (Figure 6.2). 6.2.4 First-Order Reactions First-order reactions are those that proceed at a rate which is directly proportional to the concentration of one of the reactants. Since the concentration of the reactant Figure 6.1 Plot of zero-order removal reaction.
Y-intercept C0
Ct
Slope k
Time (t)
Figure 6.2 Plot of zero-order production reaction. Slope k Ct
Y-intercept C0 Time (t)
124 Chapter 6
Design and Modeling of Environmental Systems
changes with time, an arithmetic plot of product concentration for a first-order production reaction will be curvilinear, as shown in Figure 6.3. Consider the irreversible conversion of a single reactant (A) to a single product (P), as shown by Equation (6.11). A:P
(6.11)
Writing the rate law equation for a removal reaction, assuming a first-order reaction, results in: r = -
dC = kC 1 dt
(6.12)
Rearranging differential Equation (6.12) and establishing the following integration limits, where C0 = initial concentration of A at time equal to zero and Ct = concentration of A at any time t, results in: Ct
LC0
t 1 dC = - k dt C L0
(6.13)
The integrated form of Equation (6.13) is: ln1Ct2 - ln1C02 = - kt
(6.14)
Taking the antilog of both sides of Equation (6.14) results in the familiar form of a firstorder or exponential equation, Equation (6.15). Ct = C0 e -kt
(6.15)
where k = reaction-rate constant, time-1. By taking the natural log of both sides of Equation (6.15), we see that a firstorder removal reaction will plot as a straight line with negative slope of k on a semilog plot of the natural log of the concentration versus time, as shown in Figure 6.4. Another important equation that involves first-order kinetics is the half-life equation, which applies to several environmental engineering applications. The most familiar application is in the nuclear field involving the half-lives of radioactive
Figure 6.3 Arithmetic plot of first-order production reaction.
Ct
Time (t)
Section 6.2
Chemical and Biochemical Reactions
125
Y-intercept ln[C0]
ln[Ct]
Slope k
Figure 6.4 Semilog plot of first-order removal reaction.
Time (t)
materials. The half-life is the time required for a compound’s concentration to be reduced to 50% of its initial concentration. A generalized form of the half-life equation can be derived by substituting into Equation (6.15), t1>2 for t and Ct = 0.5C0 : 0.5 C0 = C0 e -kt1>2
(6.16)
Rearranging Equation (6.16) results in: t1>2 =
-ln10.52 k
=
0.693 k
(6.17)
Thus, the half-life is easily calculated when k is known. 6.2.5 Second-Order Reactions A second-order reaction proceeds at a rate that is proportional to the second power of a single reactant. An irreversible reaction involving the conversion of a single reactant to a single product is: A:P
(6.18)
Assuming a second-order removal reaction, the rate-law equation can be written as Equation (6.19). r = -
dC = kC 2 dt
(6.19)
where C is the concentration of reactant A, mg/L or g/L. Rearranging differential Equation (6.19) and establishing the following integration limits where C0 is the concentration of A at time zero and Ct is the concentration of A at any time t, results in: Ct
LC0
- 1
t 1 dC = k dt C2 L0
(6.20)
The integrated form of Equation (6.20) is 1 1 = + kt Ct C0
(6.21)
A second-order removal reaction will plot as a straight line with slope of k, as shown in Figure 6.5.
126 Chapter 6
Design and Modeling of Environmental Systems
Slope k Ct Y-intercept C0
Figure 6.5 Plot of second-order removal reaction.
Time (t)
EXAMPLE 6.1 Reaction order and k determination Consider the irreversible conversion of a single reactant (A) to a single product (P) for the following reaction: A : P. Evaluate the following data to determine whether the reaction is zero order, first order, or second order. Also determine the magnitude of the rate constant k and list the appropriate units. Time (minutes)
Concentration of A (g/L)
0
1.00
11
0.50
20
0.25
48
0.10
105
0.05
Solution First, make an arithmetic plot of the concentration of A (g/L) on the ordinate scale versus time (minutes) on the abscissa scale. If the plot produces a straight line, the reaction is zero order and the slope of the line is equal to the value of the rate constant k. 1.20
Concentration of A, g/L
1.00
0.80
0.60
0.40
0.20
0.00
0
20
40
60 Time, minutes
80
100
120
Section 6.2
Chemical and Biochemical Reactions
Since a plot of the concentration of A versus time produces a curve, the reaction is probably not best modeled as zero order. Therefore, make a semilog plot of the natural log (ln) of the concentration of A on the ordinate scale versus time (minutes) on the abscissa scale. If the plot produces a straight line, the reaction is first order. The natural log of the concentration of A must be determined before plotting, as shown below. The use of Excel or similar spreadsheet software is recommended for completing such repetitive calculations. Time (minutes)
Concentration of A (g/L)
ln [A]
0
1.00
0.00
11
0.50
- 0.69
20
0.25
- 1.39
48
0.10
- 2.30
105
0.05
- 3.00
0.00 0
20
40
60
80
100
120
ln(Concentration of A)
0.50 1.00 1.50 2.00 2.50 3.00 3.50 Time, minutes
A semilog plot of the concentration of A versus time produces a curve, indicating that the reaction is not first order. Next, plot the reciprocal of the concentration of A (g/L) on the ordinate scale versus time (minutes) on the abscissa scale. If a straight line is produced, the reaction is second order.The reciprocal of the concentration of A must be determined before plotting, as shown below. Time (minutes)
Concentration of A (g/L)
1/A (L/g)
0
1.00
1.0
11
0.50
2.0
20
0.25
4.0
48
0.10
10.0
105
0.05
20.0
127
Design and Modeling of Environmental Systems
A plot of the reciprocal of A versus time yields a straight line, so the reaction is second order. The slope of the line is equal to the magnitude of the reaction-rate constant k, which in this case is 0.1865. The units for k are L/1g # min2. 25.0 y 0.1865x 0.5357 R2 0.9963 20.0 (Concentration of A)1
128 Chapter 6
15.0
10.0
5.0
0.0
0
20
40
60
80
100
120
Time, minutes
If none of the above plots produces a straight line, the reaction order may be fractional or variable. Other texts, such as Levenspiel (1972) or Metcalf and Eddy (2003), should be consulted.
6.2.6 Temperature Corrections The reaction rate for many chemical and biochemical reactions increases rapidly (exponentially) with an increase in temperature, as shown by the Arrhenius relationship: k = Ae -1Ea/RT2
(6.22)
where: k A Ea R T
= = = = =
reaction rate constant, a constant that is independent of temperature for a specific reaction, activation energy, cal/mol, ideal gas constant, 1.98 cal/(mol # K), and reaction temperature, K.
At a given temperature, if the reaction-rate constant is known, the Arrhenius relationship can be used to predict the rate constant at another temperature. For example, consider the Arrhenius equation for temperatures identified as T1 and T2 . For temperature T1 : k1 = Ae -1Ea/RT12
(6.23)
k2 = Ae -1Ea/RT22
(6.24)
For temperature T2 :
Section 6.2
Chemical and Biochemical Reactions
129
Dividing Equation (6.24) by Equation (6.23) yields: k2 Ae -1Ea/RT22 = k1 Ae -1Ea/RT12
(6.25)
By taking the natural log of both sides of Equation (6.25) and rearranging yields: ln ¢
k2 -Ea Ea + ≤ = k1 RT2 RT1
(6.26)
Simplifying Equation (6.26) yields: ln ¢
k2 Ea 1T2 - T12 ≤ = k1 R T2 T1
(6.27)
Ea may be considRT2T1 ered a constant (C) and Equation (6.27) can be rearranged as follows: For most situations in environmental engineering, the term
ln ¢
k2 ≤ = C1T2 - T12 k1
(6.28)
Taking the antilog of both sides of Equation (6.28) results in: k2 = eC1T2 - T12 k1
(6.29)
Replacing eC with the temperature correction factor 1u2 results in the following equation, which is commonly used for correcting biochemical and chemical reactions for temperature variations: k2 = u1T2 - T12 k1
(6.30)
where: k2 = reaction-rate constant at temperature T2 , and k1 = reaction-rate constant at temperature T1 . Absolute temperature values must be used in other forms of the Arrhenius equation, but the use of Celsius is acceptable in Equation (6.30), since only a difference in temperature is involved.
EXAMPLE 6.2 Te m p e r a t u r e e f f e c t s o n r a t e c o n s t a n t s A u of 1.047 is typically used for making temperature corrections to the biochemical oxygen demand (BOD) rate constant k. If the BOD rate constant at 25ºC is 0.20 d-1, determine the value of the BOD rate constant k for a temperature of 20ºC.
130 Chapter 6
Design and Modeling of Environmental Systems
Solution First, solve Equation (6.30) for k2 . Make appropriate substitutions and solve as follows. k2 = k1u1T2 - T12 0.20 d-1 = k111.0472125°C - 20°C2 k1 = 0.20 d-1/11.04725°C = 0.16 d-1 The value of the BOD rate constant k at a temperature of 20ºC is equal to 0.16 d-1. The rate of reaction for many systems is strongly dependent on temperature. In this example, lowering the temperature by 5ºC decreased the reaction rate by 20%.
6.3 MATERIAL BALANCES Material or mass balances are routinely encountered in environmental engineering problems and generally focus on reactors or specific processes. Material balances are an important way of analyzing water and wastewater treatment processes, air pollution control systems, and stream modeling applications to name a few examples. The quantity of material or mass entering and exiting the system should balance, along with all flows into and out of the system. Both the law of conservation of matter and the law of conservation of energy must be obeyed. The law of conservation of matter states that with the exception of nuclear reactions, matter can neither be created nor destroyed. Conservation of energy means that energy cannot be created or destroyed; however, the form of energy may change. In most environmental engineering applications mass balances are performed on flows and materials entering and exiting the control volume. Energy balances are usually performed separately and in many instances are not required. A qualitative or word equation for a materials balance is: 3accumulation4 = 3inputs4 - 3outputs4 + 3reaction4
(6.31)
The accumulation term is similar to the balance in a checkbook. Under steady-state conditions, when the flow and concentration in each stream remain constant, the accumulation term is equal to zero. Inputs (deposits) represent all flows and materials entering the system, whereas outputs (withdrawals) represent those exiting the system. Depending on the type of system or process, a chemical or biochemical reaction may be taking place, resulting in the production or removal of a substance. The positive sign in front of the reaction term may actually be negative if the reaction involves the removal or destruction of a constituent. If the reaction results in the formation of a product, then it remains positive. In biological treatment systems, microorganisms are grown or produced, and the reaction term would be positive. Simultaneously, substrate in the form of biochemical oxygen demand (BOD) is being removed from these systems. This results in a negative sign for the reaction term when material balances are performed on substrate in biological processes. Example 6.3 shows a simple mass balance for a manhole in a sewer system. This is an example of a “conservative” or nonreactive substance, since no biochemical or chemical reaction occurs in the control volume (manhole).
Section 6.3
Material Balances
131
EXAMPLE 6.3 Mass balance on flows and solids A manhole receives inflows from two sewer laterals and has one outflow as seen in the diagram below.The flow rates and suspended solids (SS) concentrations in each of the flow streams are given on the diagram. Perform a materials balance on flows and suspended solids concentrations around the manhole to determine the unknown flow and suspended solids concentration. Q2 ? L/h SS2 150 mg/L
Control volume
Q3 56 L/h
Q1 40 L/h Manhole SS1 200 mg/L
SS3 ? mg/L
Solution First, perform a flow balance around the manhole by substituting into Equation (6.31). 3accumulation4 = 3inputs4 - 3outputs4 + 3reaction4 The steady-state assumption will be used; therefore, the accumulation term will be zero. Since there are no reactions occurring in the manhole, the last term on the right side of Equation (6.31) is also zero. 0 = Q1 + Q2 - Q3 + 0 Q1 + Q2 = Q3 40
L L + Q2 = 56 h h Q2 = 56
L L L - 40 = 16 h h h
Next, perform a materials balance on suspended solids (SS) concentration around the manhole as follows. The mass of solids is calculated by multiplying the flow by the SS concentration: mass 1M2 = Q * 1SS concentration2 Start the solids balance using Equation (6.31). 3accumulation4 = 3inputs4 - 3outputs4 + 3reaction4
132 Chapter 6
Design and Modeling of Environmental Systems
The steady-state assumption will be used, so the accumulation term will be zero. Since no reactions are occurring in the manhole that produce or remove solids; the reaction term is also zero. 0 = M1 + M2 - M3 + 0 M1 + M2 = M3 40
mg mg L L L a200 b + 16 a150 b = 56 1SS32 h L h L h
56
mg mg mg L 1SS32 = 8000 + 2400 = 10,400 h h h h
SS3 =
10,400 mg>h 56 L>h
= 186
mg L
Example 6.4 illustrates the mass-balance procedure for an air-pollution problem in which a reaction is taking place (e.g., the formation of particles). It is an example of a completely mixed-flow reactor, which will be discussed in more detail in Section 6.4. EXAMPLE 6.4 Mass balance on air flow and particulates Estimate the concentration of particulate matter, a form of air pollution, in the ambient air above the city of Atlanta, Georgia, assuming the following data. The average annual wind velocity is 20 miles per hour and the assumed mixing height above the city is approximately 4000 feet. Also assume that the city of Atlanta is 10 miles long and 10 miles wide. The annual quantity of particles discharged into the air is approximately one million tons. Solution The solution of this problem requires the use of the continuity equation, where Q = AV. It will also be necessary to perform a materials balance on air flow and particulate matter entering and exiting the volume of air above the city. The control volume in this example is assumed to be a rectangular box 4000 feet in height and 10 mi * 10 mi in surface area. The diagrams below help to envision the problem.
10 mi 10 mi
Airout Qout Cout
4000 ft
Pollutantsin
Airin Qin Cin
Section 6.3
Airin Qin Cin
Airout Qout Cout
Pollutantsin
First, estimate the air flow rate entering the side of the control volume using the continuity equation: Q = AV. Q = a4000 ft * 10 mi * 5280
ft mi ft ft3 b a20 b a5280 b = 2.230 * 1013 mi h mi h
The air flow rate into and out of the control volume (rectangular box) over the city is equal to 2.230 * 1013 ft3/h. Next, estimate the quantity of pollutants or particles that are discharged into the atmosphere in micrograms per hour as follows: 1.0 * 106 * a
tons 2000 lb 454 g 1.0 * 106 mg a ba b¢ ≤ yr g ton lb
1 yr mg 1d ba b = 1.037 * 1014 365 d 24 h h
One mg is equal to 1 * 10-6 grams. Next, perform a materials balance on particles, starting with Equation (6.31): 3accumulation4 = 3inputs4 - 3outputs4 + 3reaction4 The steady-state assumption is used in this case, since particles are not accumulating in the control volume; therefore, the accumulation term goes to zero. Particles are being produced within the control volume, so the third term on the right side of the above equation is positive and has a magnitude equal to the mass of particles produced. We will also assume that the concentration of particles in the air entering the control volume is zero 1Cin = 02. Recall that mass is calculated by multiplying the flow rate by the concentration. Maccum = Min - Mout + Mproduced Maccum = QinCin - QoutCout + Mproduced 0 = Qin 102 - QoutCout + 1.037 * 1014 mg>h 12.23 * 1013 ft3>h2 Cout = 1.037 * 1014 mg>h Cout =
1.037 * 1014 mg>h 2.23 * 10 ft >h
Cout = 4.65
13
mg ft3
¢
3
= 4.65
mg ft3
1 gal mg 1 ft3 1000 L ≤¢ ≤¢ ≤ = 164 3 3 7.48 gal 3.785 L m m
If steady-state and complete-mix conditions are assumed, the particle concentration within and exiting the control volume representing Atlanta is 164 mg/m3.
Material Balances
133
134 Chapter 6
Design and Modeling of Environmental Systems
6.4 FLOW REGIMES AND REACTORS The detention time and flow rate are significant parameters in most environmental engineering unit operations and processes. A flow model or regime is used to evaluate the effects of detention time and flow rate on a given system. Reactors are tanks or vessels in which chemical and biological reactions occur. The type of reactor used impacts the effectiveness of treatment or the degree of conversion of a given process. 6.4.1 Flow Regimes There are three types of flow regimes related to reactor design and degree of mixing encountered in most environmental engineering applications. In continuous flow systems, the flow regime will approach either ideal plug flow or ideal completely mixed flow. For ideal plug flow to occur, all the elements of the fluid that enter the system (or control volume) at a given time pass through the system at the same velocity, remain in the system the same amount of time, and exit the system at the same time. Plug flow occurs in long, narrow basins with length-to-width (L:W) ratios of 50:1 or greater. Water flowing through a pipe or hose is an example of plug flow. Ideal completely mixed flow is approached when fluid elements that enter the system are instantaneously and uniformly dispersed throughout the system. The concentration of the constituents within the reactor is identical to that discharged from the reactor. In actual reactor systems, the flow regime will be somewhere between these two extreme idealized cases and is often referred to as dispersed plug flow. Levenspiel (1972) and Reynolds and Richards (1996) provide more details about these flow regimes. 6.4.2 Reactors Environmental engineers design reactor systems for treating water, wastewater, air, and solid and hazardous wastes. Three types of reactors encountered in environmental engineering include: batch, completely mixed, and plug flow. Batch reactors do not have a continuous input and output of materials. Both completely mixed and plug flow reactors generally have continuous inputs and outputs, with chemical and/or biochemical reactions occurring within. Kinetically, batch and plug flow reactors are evaluated similarly. Completely mixed reactors are less efficient than either batch or plug flow reactors. In reality, it is difficult to achieve ideal completely mixed flow or ideal plug flow. Dispersed plug flow reactors can be designed which result in a flow regime inbetween plug flow and completely mixed. Environmental engineers often use a series of completely mixed reactors which approximate the efficiency of plug flow reactors while providing the benefit of resisting shock loadings. Exercise 1 at the end of this chapter involves using a series of completely mixed reactors to simulate plug flow. Detailed design of reactor systems is beyond the scope of this text. The reader should consult the following texts to learn more about reactor design: Levenspiel, 1972; Reynolds and Richards, 1996; and Metcalf and Eddy, 2003. A brief explanation for each type of reactor will be presented along with derivations of zero-, first-, and second-order reactions. Ideal Batch Reactors
Batch reactors are typically used in situations where the flow to the reactor is less than one million gallons per day 13785 m3/d2 and operation involves a sequence of events. Batch reactors do not have a constant input and output, but normally their
Section 6.4
Flow Regimes and Reactors
135
contents are completely mixed. First, the reactor is filled with the process stream containing the constituents to be processed. Next, the flow to the reactor is stopped, and air or chemicals are added to the reactor so that treatment may begin. The reactor is operated in this mode until the desired degree of treatment or conversion has been accomplished. Processing time may be several hours to several months, depending on the application. Once processing or treatment of the flow has been accomplished, the contents of the reactor are removed and a new batch of influent is added to the reactor for processing. The advantage of using a batch reactor is that it allows flexibility of operation, since the reaction time can be varied. The main disadvantage is that multiple reactors may be required, and they are generally limited to small flows. A simple schematic diagram of a batch reactor is presented in Figure 6.6. Batch Reactor: Zero-Order Removal Reaction
This section shows the derivation of a zero-order removal reaction in a completely mixed, batch reactor (CMBR). As seen in Figure (6.6), the dotted lines for the influent and effluent flows to and from the reactor indicate that they are not continuous. Other important parameters used in the derivation are defined below. Q = volumetric flow rate into and out of the batch reactor, volume/time, V = liquid volume in the batch reactor, volume, C0 = concentration of a given constituent in the influent to the reactor, mass/volume, and Ct = concentration at some specified time of a given constituent within the reactor. Recall from Equation (6.8) that a zero-order removal reaction takes the following form: r = -
dC = kC0 dt
or
dC = -k dt
To derive the detention time equation for a batch reactor, we begin with the materials balance equation, Equation (6.31). A materials balance will be performed on constituent “C”: 3accumulation4 = 3inputs4 - 3outputs4 + 3reaction4 A materials balance involves calculating the mass of a constituent per unit of time as it enters, exits, accumulates, increases, or decreases within the reactor. Equation (6.31) Figure 6.6 Schematic of a batch reactor.
Q
Q
C0
Ct
V
Ct
136 Chapter 6
Design and Modeling of Environmental Systems
is rewritten in differential equation form as Equation (6.32). The accumulation term dC represents the concentration of a specific constituent that accumulates a b dt accum within the reactor. It must be multiplied by the reactor volume to yield the proper units, mass/time:
a
dC dC b V = QC0 - QCt + a bV dt accum dt
(6.32)
Since batch reactors do not have continuous flows into and out of them, the input and output terms are zero. Because we are assuming that a zero-order removal reaction occurs in the reactor, the reaction term is replaced with Equation (6.8). Substituting Equation (6.8) into Equation (6.32) yields: a
dC b V = 0 - 0 + 1- k2V dt accum
(6.33)
Equation (6.33) can be simplified to obtain: a
dC b = -k dt accum
(6.34)
Equation (6.34) is rearranged and integration limits established as follows: Ct
LCo
dC = - k
t
L0
dt
(6.35)
Integrating Equation (6.35) between the limits for t from zero to t, and for the concentration C which varies from C0 , the concentration of C at time zero, to Ct , the concentration C at any time t, yields: Ct - C0 = - kt
(6.36)
Rearranging Equation (6.36) and solving for t results in Equation (6.37), the detention time equation for an ideal batch reactor with a zero-order removal reaction taking place within. t =
C0 - Ct k
(6.37)
where: t = hydraulic detention time, time, and k = zero-order removal rate constant, mass/volume # time. Knowing the rate constant, along with the influent and effluent characteristics, the design engineer can determine the detention time or reaction time necessary for the desired degree of treatment. Table 6.2 lists the equations for detention times for zero-, first-, and second-order reactions in a completely mixed batch reactor. The Greek letter theta (u) is used rather than t to denote detention time in most chemical and environmental engineering texts.
Section 6.4
Flow Regimes and Reactors
137
Table 6.2 Detention Times for Zero-, First-, and Second-Order Removal and Production Reactions in a Completely Mixed Batch Reactor Removal Zero
u =
First
u =
Second
u =
Production
C0 - Ct k ln1C02 - ln1Ct2 k 11>Ct - 1>C02 k
u = u = u =
Ct - C0 k ln1Ct2 - ln1C02 k 11>C0 - 1>Ct2 k
EXAMPLE 6.5 Completely mixed batch-reactor design A CMBR is to be designed to treat pharmaceutical wastewater that contains 600 mg/L of chemical oxygen demand (COD). A treatability study on the pharmaceutical wastewater determined the kinetics for COD removal to be zero order with a rate constant of 55 mg/(L # h). Determine: a. Detention time necessary to reduce the COD of the wastewater to 50 mg/L so it can be discharged into the municipal sewerage system. b. The volume of the reactor to treat 3000 m3 of wastewater daily. Solution part a First, use Equation (6.37) to calculate the required detention time, since the batch reactor uses a zero-order removal reaction. The required detention time is 10 hours. t =
1600 - 502 mg>L C0 - Ct = = 10 h k 55 mg>(L # h)
Solution part b Next, assume that two batches of wastewater will be processed each day with 1500 m3 of wastewater per batch.This will allow the reactor to be filled in 1 hour and drained in 1 hour, resulting in a total time of 12 hours for each batch of processed wastewater. Use a reactor volume that is approximately 10% more than the required volume to allow additional depth for freeboard—in this case, 1650 m3 11.10 * 1500 m32. Freeboard is extra height added to the depth of the reactor so that the reactor contents do not overflow.
Ideal Completely Mixed Reactors
An ideal, completely mixed flow reactor (CMFR) changes the influent constituent to the final effluent constituent concentration as it enters the reactor. The contents of the reactor are uniformly mixed, so that the constituents within the reactor have the same concentration as those that exit the reactor. Mechanical means may be used to mix the contents, but usually air is used. In some situations, mechanical mixing is
138 Chapter 6
Design and Modeling of Environmental Systems
combined with aeration. Completely mixed reactors normally operate as continuous flow units. Often, completely mixed reactors are called continuously stirred tank reactors (CSTR). Figure 6.7 is a schematic diagram of a completely mixed, continuous flow reactor. Notice the similarity with the batch reactor; however, the flow into and out of a CMFR is continuous, not on a batch basis. Completely mixed reactors are advantageous because the influent is diluted to the final concentration on entering the reactor. Therefore they are not affected by shock loadings of organics or other toxic substances or by dramatic changes in pH. CMFRs, however, are not as efficient as plug flow reactors. Completely Mixed Flow Reactor: First-Order Removal Reaction
This section shows the derivation of the detention-time equation for a first-order removal reaction in a CMFR. Recall from Equation (6.12) that a first-order removal reaction takes the following form: r = -
dC = kC1 dt
or
dC = - kC dt
To derive the detention-time equation for the CMFR, we must first use Equation (6.31) to perform a materials balance on constituent C. 3accumulation4 = 3inputs4 - 3outputs4 + 3reaction4 Equation (6.31) is written in differential form to show mass flow through the reactor. a
dC dC b V = QC0 - QCt + a bV dt accum dt
(6.38)
The steady-state assumption is made, so the accumulation term goes to zero. Since we are assuming that a first-order removal reaction occurs in the reactor, the reaction term is replaced with Equation (6.12), dC/dt = - kCt . 0 = QC0 - QCt - kCtV
(6.39)
Figure 6.7 Schematic of a continuous flow, completely mixed reactor.
Q
Q
C0
Ct
V
Ct
Section 6.4
Flow Regimes and Reactors
139
Rearranging Equation (6.39) results in: kCtV = Q1C0 - Ct2
(6.40)
The detention time 1u2 is defined as the volume divided by the volumetric flow rate. When substituted into the equation above, we get: u =
1C0 - Ct2 1C0>Ct - 12 V = = Q kCt k
(6.41)
Table 6.3 lists the detention equations for continuous-flow, completely mixed reactors for zero-, first-, and second-order removal and production reactions.
Table 6.3 Detention Times for Zero-, First-, and Second-Order Removal and Production Reactions in a Continuous Flow, Completely Mixed Reactor Removal u =
Zero First
u = u =
Second
Production
C0 - Ct
u =
k 1C0>Ct - 12 k 1 C0 - 1≤ ¢ k Ct Ct
u = u =
Ct - C0 k 11 - C0>Ct2 k C0 1 ¢1 ≤ Ct kCt
EXAMPLE 6.6 Completely mixed reactor design A completely mixed flow reactor is to be designed to treat an influent stream containing 150 mg/L of chemical oxygen demand (COD) at a flow rate of 100 gallons per minute (gpm). COD represents the total quantity of oxygen required to oxidize organic matter to carbon dioxide and water. Assume COD removal follows a firstorder removal reaction with a rate constant k of 0.40 h-1. Determine: a. The detention time in hours. b. The volume of the reactor in ft3 if the effluent is to contain 20 mg/L of COD. Solution part a First, determine the required detention time by substituting the appropriate values into Equation (6.41). The detention time is equal to 16.25 hours. u =
1C0 - Ct2 kCt
=
1150 - 202 mg>L 0.40 hr -1 120 mg>L2
= 16.25 h
140 Chapter 6
Design and Modeling of Environmental Systems
Solution part b Next, calculate the volume of the reactor by rearranging the equation for detention time 1u2. Recall that the definition of detention time is volume divided by volumetric flow rate. V = uQ = 16.25 h a100
gal 60 min 1 ft3 ba b¢ ≤ = 13,035 ft3 min h 7.48 gal
The reactor volume is approximately 13,000 ft3.
Ideal Plug Flow Reactors
An ideal plug flow reactor (PFR) is one in which the influent constituent concentration decreases or increases along the length of the reactor. Plug flow is also called “pipe” flow. For a removal reaction, Figure 6.8 shows how the concentration of a constituent such as biochemical oxygen demand (BOD) would decrease along the length of the reactor. (BOD is a quantitative measure of the oxygen utilized by bacteria for the oxidation of degradable organic matter.) In ideal plug flow, longitudinal mixing does not occur and the particles that enter the reactor pass through it in the same sequence in which they enter. Ideal plug flow reactors are more efficient than completely mixed reactors. From a kinetic analysis they yield the same results as batch reactors. The major drawback to plug flow reactors is that they are susceptible to shock loadings of organics or other toxic substances or to dramatic pH shifts that can kill the microorganisms within the reactor. Plug Flow Reactor: Second-Order Removal Reaction
This section presents the general form of the equation for one-dimensional plug flow. The detention-time equation for a second-order removal reaction occurring within a continuous plug flow reactor is developed. Figure (6.9) is a schematic of an ideal, continuous plug flow reactor. The general form of the equation for one-dimensional plug flow is presented as Equation (6.42). Remember that r is equal to the rate of reaction. Either a production or a removal reaction may take place. The sign will be positive if a substance is produced and negative if a reactant is removed. The reaction type may be zero order, first order, or second order. a
Figure 6.8 Schematic of a continuous plug flow reactor showing concentration decreasing along length of reactor.
Q
V Volume
C0
C0 CONC. Ct
Distance X
Q ¢C b = ;r A ¢X
Q Ct
(6.42)
Section 6.4
L
Flow Regimes and Reactors
141
Q Ct
Q
C X
C0
Figure 6.9 Schematic of a continuous plug flow reactor.
CX X
Recall from Equation (6.19) that a second-order removal reaction takes the following form: r =
dC = - kC 2 dt
Substituting for r into Equation (6.42) and replacing ¢C/¢X in Equation (6.42) with dC/dX results in: a
Q dC b = - kC 2 A dX
(6.43)
Rearranging Equation (6.43) and establishing integration limits yields: -
A L 1 Ct 1 dX = dC Q L0 k LC0 C 2
(6.44)
Integrating Equation (6.44) for X between the limits of 0 and L and for C between C0 , the concentration of C at time zero, and Ct , the concentration of C at time t, produces: -
AL V 1 1 1 = = -u = B - ¢ ≤ - ¢ - ≤ R Q Q k Ct C0
(6.45)
Solving Equation (6.45) for detention time 1u2 yields: u =
1 1 1 B¢ ≤ - ¢ ≤R k Ct C0
(6.46)
Table 6.4 lists the detention time equations for continuous plug flow reactors for zero-, first-, and second-order removal and production reactions. Notice the detention-time Table 6.4 Detention Times for Zero-, First-, and Second-Order Removal and Production Reactions in a Continuous Plug Flow Reactor Removal Zero
u =
First
u =
Second
u =
Production
C0 - Ct k ln1C02 - ln1Ct2 k 11>Ct - 1>C02 k
u = u = u =
Ct - C0 k ln1Ct2 - ln1C02 k 11>C0 - 1>Ct2 k
142 Chapter 6
Design and Modeling of Environmental Systems
equations shown in Table 6.4 for PFRs are identical to those shown in Table 6.2 for batch reactors.
EXAMPLE 6.7 Plug flow reactor design A continuous PFR is to be designed to treat an influent stream containing 200 mg/L of acetic acid at a flow rate of 400 liters per minute (lpm). A second-order removal reaction is occurring, where the rate constant k is 0.0085 L/1mg # h2. Determine: a. The detention time in hours. b. The volume of the reactor in m3 to achieve 90% removal of acetic acid. Solution part a First, it is necessary to calculate the concentration of acetic acid in the effluent. This may be accomplished using the definition of percent removal (%): percent removal 1%2 =
1Cin - Cout2100 Cin
(6.47)
Cin = Concentration of constituent in the influent, mass/volume Cout = Concentration of constituent in the effluent, mass/volume 90% removal =
1200 mg>L - Cout2100 200 mg>L
Cout = 20 mg/L Next, the detention time required to achieve 90% removal of acetic acid for a second-order removal reaction can be determined using Equation (6.46). u =
1 1 1 1 1 1 B¢ ≤ - ¢ ≤R = B¢ ≤ - ¢ ≤R # k Ct C0 0.0085 L>(mg h) 20 mg>L 200 mg>L
= 5.29 h Solution part b Finally, determine the volume of the plug flow reactor by rearranging the equation for detention time 1u2. Recall that detention time is equal to volume divided by the volumetric flow rate. Therefore, the volume of the reactor is equal to u * Q. V = u * Q = 5.29 ha
60 min 1 m3 400 L b a b¢ ≤ = 127 m3 min h 1000 L
Section 6.5
6.5 ENERGY BALANCES Analogous to material and flow balances, energy balances must be performed when dealing with thermal pollution from coal-fired power plants and nuclear reactors, potential climate changes resulting from the discharge of greenhouse gases, and the combustion of fossil fuels (coal, natural gas, and gasoline) to produce energy. This section will present a brief introduction to thermodynamics and give examples illustrating how to perform energy balances. 6.5.1 Definition of Energy and Work Thermodynamics is the study of energy changes resulting from physical and chemical processes. Changes in energy associated with biological and chemical processes are very important in environmental engineering. Energy is defined as the capacity for doing work. There are many forms of energy, such as chemical, electrical, kinetic, potential, and thermal (heat). Heat and work are related forms of energy. Thermal energy can be converted into work, and work can be converted into heat energy. Various units are used for measuring energy: British thermal unit (BTU), calorie (cal), and joule (J). The BTU is defined as the energy required to raise the temperature of one pound of water one degree Fahrenheit (ºF). The basic unit of thermal energy, the calorie, is defined as the quantity of energy required to raise the temperature of one gram of water one degree Celsius (ºC). The joule is defined as the amount of work done by a force of one newton to raise an object one meter. Work is defined as transferring energy to an object by applying force and causing motion. Work is measured as force multiplied by the distance displaced. The units of work are expressed in foot-pounds 1ft # lb2 or joules. One calorie of thermal energy is equivalent to 4.184 joules, and one BTU is equivalent to 778 ft # lb of work. Another important term is power, which is defined as the rate of doing work. Power has units of energy per unit of time. Typical units for power include joules per second (J/s) or watts (W). One watt is equivalent to one J/s or 3.412 BTU/h. Producing work requires energy, and energy has many forms. Chemical energy in organic compounds can be released as heat during combustion. This energy can then be used for heating or producing electrical energy through steam-driven turbines. Chemical energy is a form of internal energy (U). Kinetic energy (KE) can produce electricity through windmills or water flowing through turbines. Potential energy (PE) results from a change in elevation above the earth. Equation (6.48) shows that the total energy (E) a substance possesses is the sum of the internal, kinetic, and potential energies. E = U + KE + PE
(6.48)
The first law of thermodynamics states that energy cannot be created or destroyed, excluding nuclear reactions. Only the form of energy will change. Like material and flow balances, energy balances can be performed when energy flows through the ecosystem or a particular process. The general form of an energy balance is similar to Equation (6.31) for material and flow balances: 3energy accumulated4 = 3energy input4 - 3energy outputs4 + 3energy generated4
(6.49)
Energy Balances 143
144 Chapter 6
Design and Modeling of Environmental Systems
Energy generated is actually comprised of two terms such that: 3energy generated4 = 3energy produced4 - 3energy consumed4
(6.50)
Regardless of the process, energy conversion is not 100% efficient, and a loss of useful energy occurs, normally through waste heat. The second law of thermodynamics states that there will always be some waste heat released during energy conversions. Heat is a form of internal energy expressed as the thermodynamic property enthalpy (H), which is a function of temperature, pressure, and volume. The enthalpy of a substance is defined by: H = U + PV
(6.51)
where: U = the internal energy of the substance, P = pressure of the system, and V = volume of the system. When a process occurs without a change in volume, the change in internal energy can be calculated as follows:
Δ U = mcv ΔT
(6.52)
where: m = mass of substance, cv = specific heat or heat capacity of the substance at constant volume, and
ΔT = temperature change. For constant-pressure systems, thermal or heat energy changes can be estimated using the equation:
Δ H = mcp Δ T
(6.53)
where:
ΔH = change in enthalpy or thermal (heat) energy, m = mass of substance, and cp = specific heat or heat capacity of the substance at constant pressure. For incompressible substances, such as solids under normal environmental conditions and most liquids, cv and cp are nearly the same and are replaced with c.Therefore, ΔU = ΔH, and Equation (6.52) is rewritten as follows, where c replaces cv:
Δ U = mc Δ T
(6.54)
where: c = specific heat or heat capacity of the substance. For most environmental applications, we are concerned with the rate of energy change across boundaries of systems. Therefore, Equation (6.54) is modified to # account for the mass flow rate 1m2: # 3rate of change in stored energy4 = mc ΔT
(6.55)
Section 6.5
Energy Balances 145
For thermal pollution problems in environmental engineering, the heat energy for a given water body is calculated by multiplying the mass of the water by the absolute temperature and by the specific heat of water. We assume that the specific heat of water does not vary significantly with temperature. At 15°C, the specific heat of water is 4.18 kJ/(kg # °C), 1.0 kcal/(kg # °C), or 1.0 BTU/(lb # °F).
EXAMPLE 6.8 Thermal discharges to river An Industrial WWTP in Covington, Virginia, discharges approximately 2.0 million gallons per day (MGD) of treated effluent to the Jackson River at an average temperature of 80°F. If the temperature and flow rate of the Jackson River upstream of the discharge are 50°F and 100 ft3/s, respectively, determine the temperature in the river downstream of the industrial discharge. WWTP Discharge Q 2 MGD T 80ºF
Jackson river Q 64.6 MGD T 50ºF Q 66.6 MGD T ? ºF
Solution First, we must convert the river flow rate from cubic feet per second to MGD. 100
ft3 7.48 gal 60 s 60 min 24 h 1 MG ba ba b¢ 6 ¢ ≤a ≤ = 64.6 MGD 3 s min h d ft 10 gal
Next, we calculate the heat energy in the wastewater and then in the river upstream prior to the discharge, using a modified form of Equation (6.53): 3heat energy4 = mcT The heat energy in the wastewater stream is estimated as follows:
B
heat MG 106 gal 8.34 lb 1.0 BTU R = 2.0 ¢ ≤¢ ≤¢ ≤ 180°F2 energy ww d MG gal lb # °F =
1.334 * 109 BTU d
The heat energy in the Jackson River is estimated as follows: 3heat energy4.river = 64.6 =
MG 106 gal 8.34 lb 1.0 BTU ¢ ≤¢ ≤¢ ≤ 150°F2 d MG gal lb # °F
2.694 * 1010 BTU d
146 Chapter 6
Design and Modeling of Environmental Systems
Performing an energy balance on the two flows using Equation (6.49) results in the following equation. Steady-state conditions are assumed, so the accumulation term goes to zero. 304 = B ¢
1.334 * 109 BTU 2.694 * 1010 BTU heat + 304 ≤ + ¢ ≤R - B R d d energy ww + river
Therefore, we determine the actual temperature in the Jackson River downstream of the industrial discharge by dividing the heat energy calculated above by the total flow of the wastewater and river, and converting appropriately using the specific heat of water.
B
heat 2.827 * 1010 BTU = R energy ww + river d
2.827 * 1010 BTU 66.6 MG 106 gal 8.34 lb 1.0 BTU = ba b1T2 ¢ ≤a d d MG gal lb # °F T = 50.9°F We can also determine the same answer by calculating the flow weighted average of the two streams with the following equation: T3 =
T1Q1 + T2Q2 Q1 + Q2
where T1 , T2 , and T3 represent the temperature of the wastewater, river, and combined streams, respectively. Q1 and Q2 are the flow rates for the wastewater discharge and the river upstream. The flow in the river downstream of the discharge is the sum of the two streams: T3 =
80°F 12.0 MGD2 + 50°F 164.6 MGD2 2.0 MGD + 64.6 MGD
= 50.9°F
EXAMPLE 6.9 Coal-Fired power plant mass and energy balances A 2000-megawatt (MW) coal-fired power plant is only 33.5% efficient at converting the coal’s energy into electrical energy. Assume that the coal has an energy content of 25 kJ/g and contains 60% carbon, 2% sulfur, and 9% ash. Perform a materials and energy balance around the coal-fired power plant. Assume that 65% of the ash is released as fly ash and 35% of the ash settles outside of the firing chamber and is collected as bottom ash. Approximately 15% of the waste heat is assumed to exit in the stack gases, and the cooling water dissipates the remaining heat. Air emission standards restrict sulfur and particulate quantities to 260 g SO2 per 106 kJ of heat input and 13 g particulates per 106 kJ of heat input into the coalfired power plant.
Section 6.5
a. Calculate the quantity of heat loss to the cooling water (MW). b. Calculate the quantity of cooling water (kg/s) and flow 1m3/s2 assuming a 10ºC increase in the temperature of the cooling water. c. Calculate the efficiency of the sulfur-dioxide removal system to meet air emission standards. d. Calculate the efficiency of the particulate removal system to meet air emission standards. Solution part a Perform the energy balance around the coal-fired power plant using Equation (6.49). Power is the correct term when expressing energy per unit of time in the balances. Energy is expressed in units of BTU (joules), whereas power has units of BTU/h (watts).
B
energy energy in energy out energy out R = B R - B R - B R accumulated coal stack gases cooling water - B
energy out useful R electrical power
At steady state, the energy accumulated is zero and the equation reduces to the following form.
B
energy in energy out energy out energy out useful R = B R + B R + B R coal stack gases cooling water electrical power
We estimate the energy in the coal 1MWt2 or thermal power by dividing the useful energy produced as electrical power 1MWe2 by the efficiency of the coal-fired power plant as follows: input power =
output power 2000 MWe = = 5970 MWt efficiency 0.335
Determine the total energy or power losses in the system as follows: total losses = energy input - energy output = 5970 - 2000 = 3970 MWt Estimate the stack losses, assuming 15% of the total energy lost is through stack-gas emissions: stack losses = 0.15 13970 MWt2 = 595.5 MWt Calculate the energy loss in the cooling water.
B
Energy In Energy Out Energy Out Energy Out Useful R = B R + B R + B R Coal Stack Gases Cooling Water Electrical Power 5970 MWt = 595.5 MWt + B
B
energy out R + 2000 MWe cooling water
energy out R = 3374.5 MWt cooling water
Energy Balances 147
148 Chapter 6
Design and Modeling of Environmental Systems
Solution part b Calculate the mass flow of water required to cool the process using Equation (6.55) and a specific heat (c) equal to 4.18 kJ/kg # °C. # 3rate of change in stored energy4 = mc ¢T # 3374.5 MWt = m a4.18
kJ 1 MW 1000 J b110°C2 ¢ 6 b ≤a # kg °C kJ 10 J>s
# m = 8.07 * 104 kg>s Knowing that 1 m3 of water weighs 1000 kg, we determine the cooling-water flow rate: Q = 8.07 * 104 kg>s ¢
1 m3 ≤ = 80.7 m3>s 1000 kg
Solution part c Determine the quantity of sulfur dioxide and particulates that can be emitted per unit of heat input into the coal-fired power plant. First calculate the heat input into the plant. 5970 MWt ¢
106 W 1 kW 24 h 1 kJ>s 60 s 60 min kJ ba ba ba ba b = 5.16 * 1011 ≤a 1 MW 1000 W d kW min h d
Next, calculate the quantity of sulfur dioxide that is permitted to be discharged daily. 260 g 6
10 kJ
a5.16 * 1011
1 kg kJ ba b = 1.34 * 105 kg SO2>d d 1000 g
Next, calculate the quantity of particulates (fly ash) that is permitted to be discharged daily. 13 g 6
10 kJ
a5.16 * 1011
1 kg kg kJ ba b = 6708 d 1000 g d
Determine the quantity of coal burned daily. 1 J>s 86,400 s 1 g coal 1 kg 106 W 1 kJ ba ba b a ba b ≤a 1 MW 1W 1000 J d 25 kJ 1000 g kg = 2.06 * 107 d
5970 MW ¢
The molecular weight of sulfur dioxide 1SO22 is 32 + 21162 = 64. Therefore, 32 grams of sulfur will produce 64 grams of sulfur dioxide, assuming that the sulfur is completely oxidized. Now, calculate the quantity of SO2 produced daily. SO2 produced = 2.06 * 107
kg coal kg S 64 kg SO2 kg a0.02 ba b = 8.24 * 105 d kg coal 32 kg S d
Section 6.5
Energy Balances 149
The removal efficiency for the air pollution control equipment can be calculated using the following equation: removl efficiency =
1Cin - Cout2 * 100 Cin
=
1Min - Mout2 * 100 Min
Estimate the removal efficiency for sulfur dioxide to meet air standards. SO2 removal efficiency =
18.24 * 105 - 1.34 * 1052kg>d * 100 8.24 * 105 kg>d
= 83.7 %
Solution part d Next, knowing that the coal is 9% ash, calculate the quantity of particulates or fly ash released: fly ash produced = 2.06 * 107
kg coal kg ash kg a 0.09 b10.652 = 1.21 * 106 d kg coal d
Estimate the removal efficiency for particulates to meet air standards. particulate removal efficiency =
11.21 * 106 kg>d - 6708 kg>d2 * 100 1.21 * 106 kg>d
= 99.4 %
A schematic summarizing the mass and energy flow through the plant is given in Figure 6.10.
595.5 MW Energy loss Particulates 6708 kg/d SO2 1.34 105 kg/d
2000 MW Electrical output
5970 MW Energy input 5.16 1011 kJ/d 2.06 107 kg Coal/d
Stack 33.5 % Efficient power plant
Fly ash 1.21 106 kg/d SO2 8.24 105 kg/d
3374.5 MW 1.86 106 kg/d Energy loss Bottom ash to cooling water
Air pollution control system
Figure 6.10 Mass and energy (power) flows in a typical coal-fired power plant.
150
Chapter 6
Design and Modeling of Environmental Systems
S U M M A RY A brief introduction to chemical and biochemical reactions with specific emphasis on the rates of reactions and order of reactions was presented. Zero-, first-, and secondorder removal and production reactions were derived for batch, plug, and completely mixed reactor systems. Material balances were performed around these three reactor systems with and without reactions occurring within. Examples illustrating how to perform material and flow balances were presented. The design of batch, plug flow, and completely mixed reactors was presented. The concepts and definitions necessary to perform simple energy balances were introduced. Designing and modeling environmental engineering systems requires that the engineer have a good understanding of kinetics of reactions in addition to being able to perform material and energy balances around these systems.
KEY WORDS
batch reactor completely mixed flow completely mixed reactor continuous flow control volume detention time dispersed plug flow energy
first law of thermodynamics half-life heat kinetics material balance plug flow power
rate law reaction order second law of thermodynamics thermodynamics work
REFERENCES Levenspiel, O. (1972). Chemical Reaction Engineering, John Wiley & Sons, New York. Metcalf and Eddy (2003). Wastewater Engineering: Treatment and Reuse, McGraw-Hill, New York. Reynolds, T.D., and Richards, P.A. (1996). Unit Operations and Processes in Environmental Engineering, PWS Publishing, Boston, MA.
EXERCISES 6.1
Several reactor configurations are to be considered for reducing the influent substrate concentration from 100 mg/L to 20 mg/L at a design flow rate of 1.0 million gallons per day. Assume that substrate removal follows first-order kinetics, and the first-order rate constant is 0.8 days-1. The following equation may be used for estimating the removal efficiency for completely mixed reactors operating in series, assuming first-order removal reactions. n Sn 1 = a b S0 1 + ku
where: S0 and Sn = influent substrate concentration and effluent substrate concentration from the nth completely mixed reactor in series, mass/ volume, n = number of completely mixed reactors in series, k = first-order removal rate constant, d-1, and u = detention time in each of the completely mixed reactors in series.
Exercises 151
Determine and compare the volume required for the following reactor configurations. (a) One continuous flow, ideal completely mixed reactor (b) One continuous flow, ideal plug flow reactor (c) Two continuous flow, ideal completely mixed reactors in series (d) Four continuous flow, ideal completely mixed reactors in series 6.2
6.3
During a chemical reaction, the concentration of Species A was measured as a function of time. The observed concentration at various time intervals is presented below. Determine the reaction order and rate constant, k. Is Species A being removed or produced? Time (min)
Concentration of A (mg/L)
0
100
10
80
20
60
30
40
40
20
50
0
The concentration of Species C was measured as a function of time during a chemical reaction. Its observed concentration at various time intervals is presented below. Determine the reaction order and rate constant, k. Is Species C being removed or produced? Time (hr) 0
6.4
100
1.0
80
3.5
50
6.5
26
11.0
10
The concentration of Species D was measured as a function of time during a chemical reaction. Its observed concentration at various time intervals is presented below. Determine the reaction order and rate constant, k. Is Species D being removed or produced? Time (hr)
6.5
Concentration of C (mg/L)
Concentration of D (mg/L)
0
200
1.0
142
2.0
111
3.0
90
4.0
77
5.0
67
During a chemical reaction, the concentration of Species B was measured as a function of time. The observed concentration of Species B at various time
152
Chapter 6
Design and Modeling of Environmental Systems
intervals is presented below. Determine the reaction order and rate constant, k. Is Species B being removed or produced?
6.6 6.7
Time (hr)
Concentration of B (mg/L)
0
100
5.0
125
10.0
150
15.0
175
20.0
200
If the half-life of a chemical compound is 30 days under anaerobic conditions, determine the first-order removal-rate constant, k. Three wastewater streams are combined at a food-processing facility to equalize the pH prior to biological treatment. The flow rate and pH of each of the wastewater streams is presented in the accompanying table. Perform a mass balance on flow and the hydrogen-ion concentration [H +] so that the pH of the three combined streams may be estimated. The pH of a solution is equal to the negative logarithm of the hydrogen-ion concentration 1pH = - log 3H +42. Wastewater Stream
6.8
6.9
6.10
6.11
Flow (liters per minute)
pH
1
5
5.5
2
20
6.5
3
25
8.5
A sanitary landfill receives 600 ft3 of municipal solid waste 5 days per week at a density of 500 lb/yd3. If the solid waste is compacted to 1000 lb/yd3 and the average depth of each cell is 10 feet, estimate the expected life of the landfill in years if 25 acres of space are still available 11 acre = 43,560 ft22. Draw a materials-balance diagram to solve the problem. Perform a materials balance on substrate (S) around a chemostat (completely mixed reactor without recycle) assuming a first-order removal 1dS/dt = - kS2 for substrate with a rate-constant k value of 0.5 h-1. The influent substrate concentration is 150 mg/L, and 90% removal is desired. Determine the detention time in hours for the chemostat, assuming steady-state conditions. Calculate the volume of an ideal plug flow reactor for the following scenario. The volumetric flow rate is 6500 m3/day and Species A is being removed or converted according to a first-order reaction as follows: dCA/dt = - kCA , where CA is the concentration of Species A and k = 9000 d-1. A 95% removal or conversion of Species A is required. A 1000 MW coal-burning power plant is burning West Virginia bituminous coal with 8% ash content. The power plant is 33% efficient, with 35% of the ash settling out in the firing chamber as bottom ash. A simplified schematic diagram is shown below. Assume 3.5 kWh per pound of coal. (a) Draw an energy diagram for the facility and calculate the rate of heat emitted to the environment in kJ/s; (b) Determine the rate of coal input to the furnace in kg/day; and (c) Assuming that the electrostatic precipitator (ESP) is 99% efficient, calculate the rate of fly ash emitted to the atmosphere in kg/day.
Exercises 153
Exit gases and fly ash
Gases and fly ash
Coal
6.12
6.13
Furnace
ESP
Bottom ash
Fly ash
Stack
A pristine stream flowing at 100 cubic feet per second (cfs) in the Rocky Mountains contains 5 mg/L of suspended solids (SS). During the spring, ice melt conveys an additional 250 mg/L of SS at a rate of 20 cfs into the stream. Determine the concentration of SS in the stream during the spring. Calculate the minimum rate at which 15°C make-up water from a river must be pumped to evaporative cooling towers for a 1000-MW nuclear power plant. The efficiency of the plant is 32%, and all of the waste heat is assumed to be dissipated through evaporative cooling with no direct heat lost to the atmosphere.
CHAPTER
7
Sustainability and Green Development Objectives In this chapter, you will learn about: The importance of sustainable development Green engineering Case histories on sustainable development and green engineering
7.1 INTRODUCTION Environmental consciousness and safety have surfaced as primary initiatives during the last half of the 20th century and into the 21st. The negative effects of modern society on the earth’s natural environment are the result of numerous anthropogenic environmental calamities and the misuse of earth’s natural resources. Industry as well as individuals have exploited and wasted natural resources for convenience and economics at the expense of future generations. Sustainable development and green engineering are two relatively new, interrelated topics that are being integrated into engineering courses to help increase awareness and to promote the design of engineering systems that have minimum impact on the environment. In addition, sustainable development is focused on technology, processes, and resources that can be used for long time spans while conserving earth’s natural resources. The American Society of Civil Engineers (ASCE, 2004) has recently added sustainability to the first fundamental canon of its Code of Ethics: Engineers shall hold paramount the safety, health and welfare of the public, and shall strive to comply with the principles of sustainable development in the performance of their professional duties. Processes, designs, and systems that minimize or eliminate the generation of pollutants should be used because it is the ethical or right thing to do; not just because it’s mandated by law or is good business. Sustainable development and green engineering go hand in hand, and the sections that follow will present definitions for each, followed by case histories that illustrate some of the dilemmas that environmental engineers, lawyers, planners, and societies must ameliorate in the future.
Section 7.2
Sustainable Development and Green Engineering
7.2 SUSTAINABLE DEVELOPMENT AND GREEN ENGINEERING 7.2.1 Sustainable Development What is sustainable development? The most popular definition is from the World Commission on Environment and Development (1987): “development that meets the needs of the present without compromising the ability of future generations to meet their own needs.” Hawkins (1993) expresses it best in simple terms, “leave the world better than you found it, take no more than you need, try not to harm life or the environment, make amends if you do.” ASCE (2004) adopted the following definition for sustainable development: Sustainable development is the challenge of meeting human needs for natural resources, industrial products, energy, food, transportation, shelter, and effective waste management while conserving and protecting environmental quality and the natural resource base essential for future development. For example, natural processes and systems should be selected where appropriate when treating wastewater or other types of contamination. The use of algae, aquatic plants, and bacteria in a symbiotic relationship to treat industrial or municipal wastewater in a stabilization pond is an example of sustainable development. The algae and aquatic plants produce oxygen using sunlight through photosynthesis. In turn, heterotrophic bacteria use the oxygen produced by the algae and plants to oxidize the organic compounds in the wastewater to carbon dioxide and water. Periodically, some of the algae and aquatic plants are harvested and used for animal feed or digested to produce “biogas.” Sustainable designs are those that do not deplete the earth of all its current natural resources; leaving them available for future generations. Many of our natural resources, such as fish and forests, are renewable. Other renewable resources include solar, tidal, wave, hydro, and wind power. Natural resources that have finite quantities (depletable resources) include coal, natural gas, and oil. Sustainability also ensures that systems, processes, or developments have minimal impact on the environment and ecological systems. Not only is it important that engineered systems have a positive impact on people and their well-being, but they also must preserve wildlife, plants, and animals for our descendants. The biosphere must be maintained and sustained because, ethically, it is the right choice. 7.2.2 Green Engineering The United States Environmental Protection Agency (2007) defines green engineering as “the design, commercialization, and use of processes and products, which are feasible and economical while minimizing generation of pollution at the source and risk to human health and the environment.” Nine principles of green engineering may be stated as follows: 1. Engineer processes and products holistically (functional relationship between individual parts and the whole or entire system), use systems analysis, and integrate environmental impact assessment tools. 2. Conserve and improve natural ecosystems while protecting human health and well-being. 3. Use life-cycle thinking in all engineering activities.
155
156
Chapter 7
Sustainability and Green Development
4. Ensure that all material and energy inputs and outputs are as inherently safe and benign as possible. 5. Minimize depletion of natural resources. 6. Strive to prevent waste. 7. Develop and apply engineering solutions, while being cognizant of local geography, aspirations, and cultures. 8. Create engineering solutions, beyond current or dominant technologies; improve, innovate, and invent (technologies) to achieve sustainability. 9. Actively engage communities and stakeholders in development of engineering solutions. The key to green engineering is to apply these principles when it will be most effective and economical to do so—in the early design and developmental phases of a process or product. Diwekar (2003) recommends introducing green engineering principles as early as possible into the engineering decision-making process. Each year, billions of tons of waste are created worldwide that potentially could adversely affect human health and well-being and the environment. Many of these wastes are associated with industry and the production of chemical compounds. Production costs along with treatment and disposal costs continue to escalate. The continual rise in production costs occurs as economical sources of raw materials in the earth’s crust become depleted and more difficult sources must be used. Treatment and disposal costs rise as larger quantities of wastes are removed at treatment facilities, due to more stringent regulations requiring higher removal efficiencies. Engineers must use alternative chemicals and processes to minimize the toxicity of specific chemicals and to reduce the quantity of pollutants discharged to the ecosystem. Risk assessment is being applied to pollution prevention to help ameliorate the effects of harmful chemicals. Risk can be expressed mathematically as a function of hazard and exposure. Anything that produces an adverse effect on human health and the environment or ecosystem is defined as a hazard. Exposure as defined by the EPA is the qualitative or quantitative assessment of contact to the skin or orifices of the body by a chemical. Risk-assessment software tools are available to both academia and industry that can be used to prioritize, design, and select “green” engineering processes and products. Figure 7.1 is a schematic diagram showing the relationship between sustainable development, green engineering, and sustainability. According to Vallero and Vesilind (2007), sustainability is achieved by applying green engineering principles to ideal sustainable development. 7.2.3 Material Selection Engineers decide which material to use when designing equipment, machines, roads, bridges, airplanes, etc. Traditionally, the designer selects the best material after considering such factors as the material’s strength, stiffness, density, resistance to corrosion, cost, and many other material properties (Ashby, 2005). The need to also consider the
Sustainable development
Green engineering
Sustainability
Figure 7.1 Relationship between sustainable development, green engineering, and sustainability.
Section 7.2
Sustainable Development and Green Engineering
severity of the impact on the environment has been recognized for many years, but doing this accurately is very complex, and a generally accepted methodology has not been devised. Nevertheless, progress is being made, and a good introductory review of the topic is available (Ashby, 2005). First, consider one of the simpler approaches for including the impact on the environment when performing materials selection. This method compares the energy expended in making the item of interest from various possible materials. The material requiring the least energy would be selected, since, presumably, less environmental impact would result. It is assumed, not always accurately, that the pollution caused by particulate and undesired gases emitted during production of the material is proportional to the energy consumption. Consider, for example, a component made from plain carbon steel. Mining the ore and converting it to steel requires an expenditure of energy. It has been estimated that the energy required per kilogram of steel produced is about 25 MJ/kg. Forming the steel into the desired shape also requires energy, but the amount is typically insignificant compared to the above value. Table 7.1 gives the total energy required to produce and manufacture several common materials (Ashby, 2005). Notice that aluminum requires much more energy (200 MJ/kg) than steel, primarily because the electrolysis process used to reduce alumina ore 1Al 2O32 to aluminum metal (Al) requires an extremely large amount Table 7.1 Approximate Energy Requirements and CO2 Created During Production of Common Materials Energy (MJ/kg)
CO2 (kg/kg)
Plain carbon steel
25
2.0
Silicon
60
3.1
Stainless steel
80
5.1
Aluminum
200
12.7
Titanium
900
59
Concrete
1.1
0.16
Glass
14
0.8
Aluminum oxide
55
3.0
Aluminum nitride
220
12
85
2.1
Epoxy
100
3.6
Nylon
110
4.4
Foam
120
5.0
Teflon
150
7.5
Material
Metals
Ceramics
Polymers Polyethylene
157
158
Chapter 7
Sustainability and Green Development
of electricity. If recycled aluminum were used, rather than aluminum produced from ore, only about 12% as much energy would be needed. The table also gives the amount of CO2 emitted per kilogram of material produced. These values were obtained assuming that fossil fuels were the source of energy. Since CO2 is a greenhouse gas, it could be used as a measure of the environmental impact. It is not always the energy required to produce or manufacture a product that is important. For some products, such as a car or vacuum cleaner, the energy consumed during use of the product is large. In order to minimize the environmental impact for such products, the appropriate energy to consider when selecting materials of construction is that consumed during use of the product. Actually, the total energy required for production, manufacture, use, and disposal or recycling should be minimized. In summary, when choosing among materials that have appropriate properties and cost, pick the one that minimizes total energy consumption. A more comprehensive approach to materials selection recognizes that many factors in addition to energy consumption or CO2 burden should be considered. Some of these factors are: undesirable emission of gases such as NOx and SOx and particulates, toxicity, resource depletion, and potential for recycling. To further complicate the problem, recognize that it is necessary to quantify how each of the above factors impacts human health, damages the ecosystem, and depletes resources. This task is clearly very difficult. In an effort to simplify this problem, importance or weighting factors could be assigned. Unfortunately, while one person or group might, for example, rate human health and ecosystem quality about equal in importance and consider resources half as important, another group might have a different rating, making the selection of weighting factors very subjective. Because of the obvious complexity of selecting the best material for a specific application, an attempt has been made to provide the designer with a simpler approach. The method assigns a single measure or indicator to each material, so that the designer can use this numeric ranking when performing materials selection. The numeric values are called eco-indicators. They are determined by evaluating the various impacts on the environment, normalizing the impacts to a common scale (0–100 for example), and assigning weighting factors to reflect the perceived extent of the environmental impact. The end result is a single value for each material, which simplifies the designer’s selection task. A search for eco-indicators on the internet will reveal additional information on the procedure, including software. While the idea of such a single-valued approach is appealing, keep in mind that determination of eco-indicators is very complex and subjective, and thus their accuracy is questionable and their use controversial.
7.3 NUCLEAR PHYSICS The brief introduction to nuclear physics given here provides a basic understanding of radioactivity and the reactions that occur in nuclear reactors. Nuclear physics deals with transformations in the nucleus of atoms. Sawyer and McCarty (1978) present a good review of the basic concepts. One does not generally think of nuclear energy as “green” engineering. It is likely, however, that nuclear reactors will be used to offset the anticipated reduction in coal-fired power plants to help mitigate the release of greenhouse gases. Major advantages of nuclear reactors are that they do not produce particulates and acidforming effluents or greenhouse gases such as carbon dioxide and water vapor, and they are economical to operate in comparison to coal-fired electrical generating
Section 7.3
Nuclear Physics
159
facilities. Two significant disadvantages of using nuclear energy are high capital costs to build nuclear reactors and their auxiliary systems, and concerns over the disposal of high-level radioactive wastes. An atom of a specific element contains neutrons, electrons, and protons. The atomic number of an element represents the number of protons, which are positively charged particles in the nucleus. Neutrons are uncharged particles in the nucleus. The total number of both protons and neutrons represents the mass number. Elements with the same atomic number but different mass number are called isotopes. One way of identifying isotopes is to write the chemical symbol with the mass number written as a superscript to the upper left and the atomic number as a subscript to the lower left. Three of the numerous isotopes of uranium (U) are uranium-235 (U-235), uranium-238 (U-238) and uranium-239 (U-239). They each contain 92 protons, and their chemical symbols, mass numbers, and atomic numbers are written as follows: 235 92U
uranium-235
238 92U
uranium-238
239 92U
uranium-239
EXAMPLE 7.1 Identifying radioactive elements What are the elements 147X and 226 88X (where X represents the element)? Solution A periodic table can be used to solve this problem. Recall that the upper left number represents the mass number and the lower left number the atomic number. The first element is nitrogen (N), which has an atomic number of 7 and a mass number of 14. It is the most abundant isotope of nitrogen and is stable—that is, it does not undergo radioactive decay. The second element is radium (Ra), which has an atomic number of 88 and a mass number of 226. As discussed below, it does undergo radioactive decay to radon.
7.3.1 Radioactivity We live in a radioactive world with naturally occurring radioactive atoms or radionuclides. Some atoms are unstable, and those elements that have more than 83 protons are considered naturally radioactive. Radionuclides emit various forms of radiation as they decay to more stabilized forms. The three major forms of radiation are alpha particles, beta particles, and gamma rays. An alpha particle is essentially a helium nucleus, consisting of two protons and two neutrons. Alpha particles are relatively massive, slow moving, and easily stopped. Because of their short penetration range in tissue, the alpha emitter tends to be harmful only if ingested. When alpha particles are emitted by an unstable atom, the atomic number decreases by two units and the mass number decreases by four units. The transformation or decay of plutonium-239 illustrates what happens when alpha radiation is released. 239 94Pu
4 : 235 92U + 2a + g
(7.1)
160 Chapter 7
Sustainability and Green Development
where: a = alpha particle or alpha radiation, and g = gamma radiation or gamma ray. A beta particle 1b2 is a free electron emitted from an unstable nucleus during the spontaneous transformation of a neutron into a proton and an electron. During this transformation, the atomic number of the element increases by one and the mass number remains the same.A gamma ray 1g2 may or may not accompany the transformation.An example of b radiation being emitted from an element is the transformation or decay of strontium-90 into yttrium-90. 90 38Sr
: 90 39Y + b
(7.2)
Beta particles are negatively charged, and they travel further and penetrate deeper than alpha particles. Concrete, steel, lead, or water must be used to protect individuals from being exposed to beta radiation. Gamma (G) radiation or gamma rays are electromagnetic waves or photons that do not have mass or charge and travel at the speed of light. This is a damaging type of radiation because it is highly penetrating. Several centimeters to several feet of lead, concrete, or lead-glass must be used to shield individuals from gamma radiation. The energy associated with the release of radiation from radionuclides can be estimated from Einstein’s energy-mass-equivalence equation: E = mc2 where:
(7.3)
E = energy expressed in units of grams # cm/s 1ergs2; m = mass of particle, grams; and c = speed of light 12.998 * 1010 cm/sec2.
When radioactive decay occurs, there is a decrease in the mass of the system. The energy produced, which may be calculated using Equation (7.3), is present as kinetic energy of any alpha or beta particles emitted plus the energy of the gamma rays.These particles are slowed and the gamma rays are absorbed as the radiation travels through matter and the energy is converted into thermal energy—that is, the matter is heated.
EXAMPLE 7.2 Determining mass number and atomic number Determine the values of a and b, where a is the mass number and b the atomic number of elements X and Y. a. b.
226 a 88X : a + bY a 132 55X : b + bY
Solution part a Recall that when an alpha particle is emitted from an unstable nucleus, the atomic number decreases by two units and the mass number by four. For the reaction in Part a, the new element “Y” that is formed has an atomic number of 86 188 - 2 = 862 and the mass number is 222 1226 - 4 = 2222. Therefore, the element Y is radon-222 with a = 222 and b = 86. X is radium-226.
Section 7.3
Radon gas is a naturally occurring radioactive gas that is invisible and odorless. It forms from the radioactive decay of small amounts of uranium and thorium naturally present in rocks and soils. Certain rock types, such as black shale and selected igneous rocks, can have uranium and thorium in amounts higher than is typical for the earth’s crust. Solution part b Recall that when a beta particle is emitted from an unstable nucleus, the atomic number of the element increases by one and the mass number remains the same. For the reaction in Part b, the element “Y” that is formed has an atomic number of 56 155 + 1 = 562 and a mass number of 132; therefore, a = 132 and b = 56. “X” is cesium-132 and “Y” is barium-132.
Sources of Radioactivity
Naturally occurring radioactive atoms are found in the earth, air, vegetation, and our bodies. According to Johansen (2007), naturally occurring radionuclides in the earth bombard us with approximately 15,000 photons each second. There are two main classes of natural radioactive elements: primordial and cosmogenic. Primordial radionuclides have been present since the formation of the earth. Uranium (U) and thorium (Th) are the best known and have no stable isotope. They eventually decay to stable lead (Pb) isotopes. Potassium-40 is another example of a primordial radionuclide; its stable form is nonradioactive potassium. Cosmogenic radionuclides are those produced by cosmic-ray bombardment of earth’s atmosphere. There are 22 different cosmogenic radionuclides that eventually are incorporated into plants and animals. The best known are carbon-14 (C-14), hydrogen-3 (H-3), and beryllium-7 (Be-7). Units of Radioactivity
The basic unit of radioactivity is the curie (Ci). One curie corresponds to approximately 3.7 * 1010 disintegrations per second, the decay rate of one gram of radium (Ra). The curie is used for defining the quantity of radioactive material. One curie of an alpha emitter is the quantity that releases 3.7 * 1010 alpha particles per second, whereas one curie of a gamma emitter is the quantity of material that releases 3.7 * 1010 photons per second. As mentioned earlier, there are several sources of natural radioactivity. Radioactive materials enter our bodies through the food we eat and the air we breathe.The unit in which concentration of radioactivity in naturally occurring radioactive material (NORM) is expressed is the picoCurie (pCi). A picoCurie is one-trillionth of a curie and is equal to 2.22 disintegrations per minute. The standard 70-kilogram adult contains the following amounts of NORM: 30 pCi of uranium, 3 pCi of thorium, 30 pCi of radium, 110,000 pCi of K-40, and 400,000 pCi of C-14 (ICRP, 1985; NCRP, 1994). The energy released by radionuclides can be measured. The amount of energy deposited in the human body from radioactive decay is called the “dose.” The internal dose is the amount of radioactivity deposited within our bodies from the radionuclides we ingest from breathing and eating. The external dose comes primarily from gamma rays emitted by terrestrial sources, such as the ground and buildings, along with cosmic rays. The unit of gamma or X-ray radiation intensity—the roentgen (R)—is used in the study of the biological effects induced by radiation within cells. The roentgen is defined as the quantity of gamma or X-ray radiation that will produce one electrostatic
Nuclear Physics
161
162 Chapter 7
Sustainability and Green Development
unit (esu) of electricity in one cubic centimeter of dry air at 0ºC and 760 mm of pressure. This unit represents an exposure dose in air and must be translated into an absorbed dose. The term radiation absorbed dose (rad) corresponds to the absorption of 100 ergs of energy per gram of any substance and can be used for any type of radiation—a, b, g, or X-ray. In the International System of Units (SI), the unit for absorbed dose is the gray (Gy). One gray is equivalent to the absorption of 1 joule of energy per kilogram (kg) of substance. One gray is equal to 100 rads and can be used to quantify any type of ionizing radiation. The effect that different types of radiation have on human beings is measured in roentgen equivalent man (rem).The SI unit, sievert (Sv), is a unit of biological dose equal to the radiation dose having the same biological effect as one gray of gamma radiation. One sievert is equal to 100 rem. In the United States, the standard absorbed dose unit is the rem, and the average annual dose is 300 millirem (mrem), (Johansen, 2007).A millirem is one-thousandth of a rem. Therefore, 300 mrem is equal to 0.003 Sv or 0.3 mSv. All persons who work at nuclear plants are directed to keep their radiation exposure as low as reasonably achievable (ALARA). Adult workers are restricted to 5 rem per year; minors to 0.5 rem per year. Women who are pregnant are restricted during the term of the pregnancy to 0.5 rem (Nuclear Tourist, 2007). Nuclear Fission and Nuclear Reactors
Nuclear reactions may be induced by bombardment with various types of particles (alpha particles, protons, and neutrons) within either a nuclear reactor or particle accelerator. In a nuclear reactor, neutrons bombard uranium or plutonium atoms, causing splitting (fission) of their nucleus. Two fission fragments (elements), gamma rays, and two or three neutrons are produced for each atom that is fissioned (Masters, 1997). Nuclear reactors obtain their energy primarily from the conversion of the kinetic energy of the fission fragments into thermal energy. This energy is used to heat water to produce steam that drives turbines and generators to produce electricity. The same type of process is used by a coal fired steam-electric plant except for the source of the thermal energy. Tremendous amounts of energy are released during nuclear fission. The energy released during the fission of one gram atom of uranium-233, uranium-235, or plutonium-239 is approximately 5.3 * 106 kWh (Sawyer and McCarty, 1978). Since the atomic weights of these elements are nearly the same, the energy generated per kilogram of fissionable material is approximately 4.1 * 109 kcal. For comparative purposes, the energy released from the combustion of a kilogram of coal and a kilogram of vehicle gasoline is approximately 6930 and 12,000 kcal/kg, respectively. In nuclear reactors, fuel rods fabricated from fissionable material such as U-233, U-235, and/or P-239 are arranged so that a sustainable chain reaction is maintained. A critical mass of fissionable material is required to maintain such a reaction. Fuel rods generally remain in the reactor for about two years. Moderators or control rods containing boron or cadmium are inserted or removed from the reactor as necessary to control the nuclear reaction. These control rods capture excess neutrons produced during nuclear reactions. An appealing aspect of nuclear power is the fact that certain types of reactors, called breeder reactors, are capable of producing more fissionable material than they consume (Ma, 1983). In other words, while electrical power is being produced, more nuclear fuel is created.This is possible because, when a uranium atom fissions, only one of the two or three neutrons released is needed to sustain the nuclear reaction. Some of the extra neutrons are absorbed by the U-238 (a nonfissionable, very abundant isotope) present in the fuel. When U-238 absorbs a neutron, U-239 is formed. The U-239
Section 7.3
undergoes b-decay, forming neptunium-239 (Np-239), which also undergoes b-decay to form Pu-239. Since Pu-239 is fissionable, it is useful for fueling a future reactor. Natural uranium contains only a small fraction of fissionable U-235, but the remainder is U-238. Use of a breeder reactor to transform U-238 to Pu-239 offers an exceedingly large supply of nuclear fuel that can be used to create energy. Similarly, thorium-232 (Th-232) is very abundant in the earth’s crust and, like U-238, is a fertile isotope. Again, with breeder reactors, addition of a neutron changes it to Th-233, which undergoes b-decay to form Pu-233, which via b-decay is converted to U-233, a fissionable isotope. The quantities of naturally occurring U-238 and Th-232 are sufficiently large to permit fueling of breeder reactors for a very long time. The fission fragments are radioactive, and they, along with other radioactive waste components such as Pu-239, present significant handling, transport, and disposal problems. The half-life represents the time it takes to reduce the final concentration of a substance to 50% of the original concentration. Equation (7.4) shows the first-order removal reaction used to model radioactive substances and decay rates. The half-lives of several common radionuclides are presented in Table 7.2. Ct = C0e -kt
(7.4)
where: Ct = concentration or quantity of element at time equal to t, mg/L, g; C0 = concentration or quantity of element at time equal to 0, mg/L, g; k = first-order removal or decay rate constant with units of inverse time, min-1, hr -1; and t = decay time, min, hr. The half-life equation (Equation 7.5) is obtained by substituting 0.5 C0 for Ct and t1/2 for t into Equation (7.4) and rearranging as follows. 0.5C0 = C0e -k t1/2 0.5 C0 = e -k t1/2 C0 Table 7.2 Half-Lives of Common Radionuclides Radionuclide
Half-life (years)
Type of radiation
Hydrogen-3
12.3
b
Carbon-14
5730
b
1.28 * 109
b
Potassium-14 Cobalt-60
5.3
b and g
Strontium-90
28.1
b
Cesium-137
30
b
Radium-226
1600
Plutonium-239 Uranium-238 Source: Sawyer and McCarty (1978)
a and g a
24,000 4.51 * 10
9
a
Nuclear Physics
163
164 Chapter 7
Sustainability and Green Development
-kt1/2 = ln10.52 t1/2 =
- ln10.52 k
=
0.693 k
(7.5)
EXAMPLE 7.3 Radioactive decay calculations 24
A waste contains 100 mg/L of 15.0 hours. Calculate the following:
Na. The half-life for sodium-24 is approximately
a. The decay constant k. b. The time required to reduce the concentration to 10 mg/L. Solution part a First, substitute into Equation (7.5) and rearrange to solve for the decay constant (k). 0.693 = 15 h k 0.693 k = = 0.0462 h-1 15 h
t1/2 =
Solution part b Next, substitute C0 = 100 mg/L, Ct = 10 mg/L, and k = 0.0462 h-1 into Equation (7.4) and solve for time (t). Ct = C0e -k t 10
mg mg -0.0462 h-11t2 = 100 e L L
10 mg>L 100 mg>L
= e -0.0462 h
-1
1t2
Taking the natural log (ln) of both sides of the above equation and then solving for time (t) yields: ln a
10 b = - 0.0462 h-11t2 100
- 2.3026 = - 0.0462 h-11t2 t = 50 h
Nuclear Waste Immobilization and Storage
When a nuclear fuel rod is removed from a reactor, it contains the U-235 that did not fission, much of the original U-238, fission products, products formed by the radioactive decay of some of the fission products, and transuranics, which are elements with atomic numbers greater than that of uranium. Chemical processes have been developed for separating the isotopes that are useful for fueling future nuclear reactors (U-235, U-238, and Pu-239) from the radioactive waste (the transuranics, fission
Section 7.4
products, and decay products) (Ma, 1983). The proposed procedure for disposing of the radioactive waste is to immobilize it by incorporating it in a specially formulated glass that has a high resistance to leaching if inadvertently exposed to water. The raw materials for making the glass and the waste are mixed and melted, and the molten glass is poured into stainless steel canisters.The canisters are cooled to allow the glass to solidify and then welded shut. Each canister is 24 inches in diameter and almost 10 feet long and holds over 3000 pounds of glass. This process has been performed on a large scale since 1996 at the Department of Energy’s Savannah River facility in South Carolina. Over 7 million pounds of glass have been processed. The plan, which remains to be approved, is to transport the immobilized waste to a geologic repository for long-term storage. Storage times on the order of a thousand years are required to permit the waste to decay to an acceptable level of radioactivity— a level similar to that of the original uranium ore. Initially, deep salt mines were chosen as storage sites, since the presence of salt indicates the absence of water. The aim was that any future leakage of radioactive material would be retained in the salt deposit rather than contaminating groundwater. Most recently, Yucca Mountain in Nevada has been extensively evaluated for future use as a geologic repository.The proposed repository would cover 1159 acres, and the waste canisters would be located in tunnels 1000 feet below the surface and 1000 feet above the water table.
7.4 CASE HISTORIES Five brief case histories are presented to illustrate some of the dilemmas that confront the public, companies, and environmental engineers in the 21st century. 7.4.1 New Chairs from Haworth and Steelcase Ogando (2006) reported on the development and environmentally intelligent design of two types of office chairs manufactured by Haworth and Steelcase. The design engineers at each company used sustainable materials and environmentally conscious design practices throughout the cradle-to-cradle program. A cradle-to-cradle approach differs from a cradle-to-grave approach in that the former considers recycle and reuse, whereas the latter does not. Every aspect of these products was scrutinized for its environmental impact, considering not only what went into the product but also the potential to recover and recycle components of the product at the end of its useful life. Both chairs are made from 50% recycled materials. Although engineers at each company will consider using recycled plastics in the future; both companies chose to use virgin plastic along with recycled aluminum and steel. A significant selling point is that 98% of these chairs’ components can be recycled at the end of their intended life. Another important issue that drove design focused on disassembly. Each chair can be disassembled in approximately 5 minutes. Furthermore, the impact of these products on the environment was reduced by lowering transportation and shipping costs, since the chairs weigh less than 50 pounds and occupy less space than conventional chairs. 7.4.2 Paper or Plastic Bags? When you step up to the cash register, you hear the inevitable question, “Paper or plastic?” Which is the greener choice? Ultimately, there is no clear winner between paper and plastic. Each material has advantages and disadvantages, as discussed below. Performing a “Google search” on the internet will provide more than enough reading material for several hours. Both types of bags can be used. Regardless of which type is selected, it is important to reuse and recycle them.
Case Histories 165
166 Chapter 7
Sustainability and Green Development
The manufacturing of paper and plastic bags consumes valuable natural resources. Paper comes from trees, and plastic is derived from a petroleum product. Although trees are a renewable resource, a significant amount of energy is spent in growing, harvesting, and transporting the trees, and in making paper. The energy estimates to produce original bags are 594 British thermal units (BTUs) for plastic and 2511 for paper (reusablebags.com, 2007). From a pollution viewpoint, paper production seems to have a more detrimental impact on the environment. Harvesting of trees requires machinery that uses fossil fuels, releasing greenhouse gases to the environment as well as destroying animal habitat. Pulp and paper mills use toxic chemicals in the pulping process, releasing air pollutants and liquid-waste discharges that negatively affect our ecosystem. Plastic production and processing also requires the use of toxic chemicals. Five of the top six chemicals (propylene, phenol, ethylene, polystyrene, and benzene) whose production generates hazardous wastes are used by the plastic industry (Eberle, 2007). A life-cycle energy analysis conducted by Franklin and Associates found that at current recycling rates, two plastic bags use less energy and produce less solid, atmospheric, and waterborne waste than a single paper bag (ILEA, 2007). Two other important factors to be considered in choosing between paper and plastic bags are recyclability and degradability. Paper bags are easier to recycle, but they weigh ten times as much as plastic bags and occupy more space in a landfill (Kushner, 2007). It takes 91% less energy to recycle a pound of plastic than it takes to recycle a pound of paper (reuseablebags.com, 2007). Paper degrades faster than plastic, but in current landfills, because of a lack of moisture, both degrade at approximately the same rate (Eberle, 2007). Legislation will affect the use of paper, plastics, and other products in the future. Seven countries have plans to tax plastic bags (Kushner, 2007). The Board of Supervisors for the City of San Francisco passed an ordinance banning the use of plastic shopping bags (Goodyear, 2007). As you can see, it’s difficult to discern which type of material is the preferred choice. Both types can and should be used and recycled. 7.4.3 Selection of Materials for Beverage Containers We are all familiar with beverage containers. For soft drinks they are made of aluminum, polyethylene terephthalate (PETE), or glass. Milk jugs are typically made of polyethylene. Steel could be used as a beverage container. It is used extensively for canned goods. Each of these materials can be recycled. Which of these five materials, though, makes the most environmentally friendly container? One way to answer this question is to determine which material requires the least energy to produce and form into the desired shape (Ashby, 2005). The five possible materials are compared in Table 7.3. The last column lists the energies to produce a 1-liter container and was obtained by multiplying the first two columns. Using this as our ranking criterion, steel is the preferred material. The energy required to produce a steel container is less than one-third that required for aluminum. Of the materials considered, steel is the only one not commonly used for containing beverages. Obviously, non-environmental factors are also being considered. These include: retention of beverage flavor and freshness, ease of opening the container, packaging and transportation issues, product appearance, and customer preference. Certainly there is no simple procedure for considering all of these when selecting the “best” beverage container material. From an environmental viewpoint, though, steel is probably preferred.
Section 7.4
Case Histories 167
Table 7.3 Beverage Container Materials
Material
Mass per container (kg)
Production plus forming energy (MJ/kg)
Energy per container (MJ)
Aluminum
0.045
200
9.0
PETE
0.062
87
5.4
Glass
0.433
19
8.2
Polyethylene
0.038
85
3.2
Steel
0.102
25
2.6
7.4.4 Coal versus Nuclear Energy The nuclear power industry is now at a crossroads. There is great potential for increased nuclear capacity as electrical demand continues to grow; especially if nuclear’s environmental benefits are recognized by the public (Ux Weekly, 2000). Nuclear power has received a lot of bad press because of the incident at Pennsylvania’s Three Mile Island facility and the meltdown at Chernobyl. Other factors that create a negative image are concerns related to nuclear weapons ambitions in certain countries and the uncertainty surrounding nuclear waste management (Deutch and Moniz, 2006). Nuclear power is cost competitive with other forms of electricity generation, except where low-cost fossil fuels are available. If the social, health, and environmental costs of fossil fuels are also taken into account, nuclear power is outstanding (Uranium Information Centre, 2007). Miller et al. (2006) reported that nuclear power must dominate as the source of carbon-dioxide-free energy, since it is proven, dependable, available on a large scale, and economically viable. They also suggest that social objection to nuclear energy in some countries is well-meaning but constitutes a misguided distraction from solving the environmental and energy crisis that now faces world sustainability. The combustion of coal to generate electricity results in the discharge of gases and particulate matter into the atmosphere. The gases are greenhouse gases (carbon dioxide and water vapor) and sulfur and nitrogen oxides (SOx and NOx) that cause acid rain. Particles with diameters less than 10 micrometers 1PM 102 may contribute to sudden infant death syndrome (SIDS) and are known to cause numerous human cancers per year (Vallero and Vesilind, 2007). Nuclear power emits less than 1% of the greenhouse gases of fossil-fuel power sources (Sevior, 2006). Li and Wang’s (2006) study indicated that nuclear power was higher in technological efficiency, pure technology efficiency, and scale efficiency than coal power. A cost comparison of using nuclear energy compared to coal indicated that they are essentially equal (Nuclear Tourist, 2006). Evaluating fuel, capital, operationand-maintenance, waste-related, and decommissioning costs, nuclear energy costs $30 per megawatt hour 1MWh2 versus $29.1 per MWh for coal. 7.4.5 Advanced Integrated Wastewater Pond System (AIWPS) The Advanced Integrated Wastewater Pond System (AIWPS) was developed by Dr. William Oswald, late Professor of Public Health and Sanitary Engineering at the University of California, Berkeley. The system offers a sustainable means of treating wastewater by reducing the generation of biosolids and the associated risk to human
168 Chapter 7
Sustainability and Green Development
health and the environment. The AIWPS uses a series of ponds to treat either domestic or industrial wastewater to produce a high-quality effluent. The major components of a type 2 AIWPS system are described briefly below. Additional information on AIWPS may be found in the following references: Downing et al., 2002; Green et al., 2003; Green et al., 1995; and Green et al., 1995. Another advantage of these pond systems is the ability to harvest algal cells that may be used for animal feed or the production of biogas (Oswald, 1994; Oswald, 1991). Type 2 AIWPS
A schematic of a type 2 Advanced Integrated Wastewater Pond System is presented in Figure 7.2. The system consists of four ponds in series including a facultative pond, a high rate pond (HRP), a settling pond, and a maturation pond. Domestic or industrial wastewater must be pretreated before entering the system. Typically it passes through bar racks and coarse screens to remove large objects such as tree limbs, tires, debris, paper, and plastic. Facultative Pond
The upper layer of the facultative pond is aerobic, the lower layer anaerobic. Typically, four deep anaerobic pits are constructed in the bottom of the facultative pond. All incoming wastewater must pass through these pits. where anaerobic digestion of the incoming solids occurs. Under anaerobic conditions, complex solids are hydrolyzed by extracellular enzymes produced by the microorganisms indigenous to the process. The soluble organic matter produced during hydrolysis, as well as with the soluble organics in the influent wastewater, are metabolized by the facultative and anaerobic microorganisms into volatile fatty acids (VFAs) and alcohols. Methanogenic (strict anaerobes) bacteria convert the VFAs and alcohols to carbon dioxide gas and methane, which are released to the atmosphere. In the upper aerobic layer of the facultative pond, aerobic bacteria oxidize organics to carbon dioxide, and hydrogen sulfide gas is oxidized to sulfate.
High-Rate Pond
In the high-rate pond algae and bacteria grow in a symbiotic relationship. Algae utilize the sunlight in this shallow pond for producing carbohydrates and oxygen. Bacteria use the soluble and particulate organic material remaining in the wastewater exiting from the facultative pond for synthesizing new biomass (growing new bacteria) and for oxidizing some organic material to provide for their energy needs. Aerobic bacteria use the oxygen produced by the algae, because it serves as an electron acceptor during the oxidation of the organic matter. The dissolved oxygen (DO) level may be supersaturated during the day and may become anaerobic during the early morning hours as the oxygen produced by alga respiration is depleted. Settling Pond
Agglomeration of the algae and bacterial flocs occurs in the settling pond. The settling pond provides additional time for disinfection of the wastewater to occur by ultraviolet radiation from the sun.
Maturation Pond
The major function of the maturation pond is to allow additional time for bacteria to die as a result of exposure to ultraviolet light. Protozoa and rotifers will be present for polishing off the final effluent in efficient systems.
Figure 7.2 Schematic of type 2 advanced integrated wastewater pond system.
Facultative pond
High rate pond
Settling pond
Maturation pond
Aeration Influent
Effluent
References 169
Process Performance
St. Helena, California
The St. Helena AIWPS is a four-pond system. It consists of facultative ponds with deep fermentation pits, high-rate ponds without paddlewheel mixers, settling ponds, and maturation ponds. Based on the annual average values, the percent removal for biochemical oxygen demand (BOD), total nitrogen (TN), and total phosphorus (TP) are 97%, 90%, and 64%, respectively. Most of the organics and nitrogen are removed in the facultative ponds.
Hollister, California The AIWPS at Hollister, California, is a four-pond system that treats wastewater from a paper reclamation facility. It has facultative ponds, high rate ponds, settling ponds, and maturation ponds. The HRP does not have a paddlewheel mixer. Based on the annual average values, the percent removals for BOD and total volatile solids (TVS) are 96% and 42%, respectively. A five-log (99.999%) removal of Escherichia coli occurs through the pond system.
S U M M A RY Sustainable development was introduced as meeting the needs of the present without compromising the ability of future generations to meet their own needs. Green engineering involves designs of processes, systems, and technologies that minimize the generation of pollutants at the source while reducing risk to human health and the environment. Applying green engineering principles leads to sustainability. A brief introduction to nuclear physics was presented, because the production of electrical energy by nuclear reactors is anticipated to increase significantly, since it does not produce greenhouse gases. Radioactive atoms or radionuclides were defined as unstable atoms that release alpha, beta, or gamma radiation so they can assume stable forms. An example of nuclear fission indicated that the bombardment of uranium-235 with a neutron produces two fission fragments and energy, which may be used for steam production and the generation of electricity. Five case histories were discussed to show some of the challenges and dilemmas that citizens, politicians, and environmental engineers face in the 21st century as we strive to provide sustainable development using green engineering principles.
alpha particle beta particle curie environmentally intelligent design fission fission fragments
gamma radiation gray green engineering half-life radiation absorbed dose (RAD) radionuclides
radioactivity roentgen roentgen equivalent man (REM) sievert sustainable development
REFERENCES Ashby, M. F. (2005). Materials Selection in Mechanical Design, 3rd ed., chap. 16, pp. 417–437, Elsevier, New York. ASCE (2004). The Committee on Sustainability of Technical Activities Committee, Sustainable Engineering Practice: An Introduction, Reston, Virginia. Diwekar, U.M. (2003). Greener by Design, Environmental Science and Technology, 37, 5432–5444. Deutch. J.M., and Moniz, E.J. (2006). The Nuclear Option, Scientific American, 295(3), 76–83.
KEY WORDS
170 Chapter 7
Sustainability and Green Development
Downing, J.B., Bracco, E., Green, F.B., Ku, A.Y., Lundquist, T.J., Zubieta, I.X., and Oswald, W.J. (2002). Low Cost Reclamation Using the Advanced Integrated Wastewater Pond Systems® Technology and Reverse Osmosis, Water Science and Technology, 45(1), 117–125. Eberle, W.M. (2007). Paper or Plastic Bags? http://www.healthgoods.com/education/ environment_information/solid_waste/paper_or_plastic.htm, accessed September 22, 2008. Gabbard, A. (2007). Coal Combustion: Nuclear Resource or Danger, http://www.ornl.gov.info/ ornlreview/rev26-34/text/colmain.html, accessed June 14, 2007. Goodyear, C. (2007). S.F. First City to Ban Plastic Shopping Bags, http://www.sfgate.com/ cgi-bin/article.cgi?file=/c/a/2007/03/28/MNGDROT5QN1.DTL, accessed June 14, 2007. Green, F.B., Lundquist, T.J., Quinn, N.W.T., Zarate, M.A., Zubieta, I.X., and Oswald, W.J. (2003). Selenium and Nitrate Removal from Agricultural Drainage using AIWPS® Technology, Water Science and Technology, 48(2), 299-305. Green, F.B., Bernstone, L., Lundquist, T.J., Muir, J., Tresan, R.B., and Oswald, W.J. (1995). Methane Fermentation, Submerged Gas Collection, and the Fate of Carbon in Advanced Integrated Wastewater Pond Systems, Water Science and Technology, 31(12), 55–65. Green, F.B., Lundquist, T.J., and Oswald, W.J. (1995). Energetics of Advanced Integrated Wastewater Pond Systems, Water Science and Technology, 31(12), 9–20. Hawkins, P. (1993). The Ecology of Life. Harper Business, New York. International Commission on Radiological Protection (ICRP) (1985). Determination of Dose Equivalents Resulting from External Radiation Sources, ICRP Publication 39, Oxford, England, Pergamon Press. Institute for Lifecycle Environmental Assessment (ILEA) (2007). Paper vs. Plastic Bags, Franklin Associates, Ltd., 1990, http://www.ilea.org.lcas/franklin1990.html, accessed June 14, 2007. Johansen, K. (2007). Radioactivity in Coal and Fly Ash, We Energies Coal Combustion Products Utilization Handbook, Appendix B. Kushner, A. (2007). Paper or Plastic? What’s the Environmentally Correct Choice? http://www .bankrate.com/brm/news/cheap/20041215a1.asp, accessed June 14, 2007. Li, Y., and Wang, D. (2006). Technology Efficiency Study on Nuclear Power and Coal Power in Guangdong Province Based on DEA, Proceedings International Conference on Nuclear Engineering, July 17–20, 2006, Miami, FL. Ma, B.M. (1983). Nuclear Reactor Materials and Applications, Van Nostrand Reinhold Company Inc., New York. Masters, G. M. (1997). Introduction to Environmental Engineering and Science, Prentice Hall, Upper Saddle River, NJ. Miller,A.I., Suppiah, S., and Duffey, R.B. (2006). Climate Change Gains More from Nuclear Substitution than from Conservation, Nuclear Engineering and Design, 236(14–16), 1657–1667. NCRP (1994). Exposure of the Population in the United States and Canada from Natural Background Radiation. NCRP Report No. 94. National Council on Radiation Protection and Measurements, Bethesda, Maryland. Nuclear Tourist (2007). Radiation Safety, The Virtual Nuclear Tourist, January, 18, 2006, http://www .nucleartourist.com/systems/rad.htm, accessed August 8, 2007. Nuclear Tourist (2006). Cost Comparison for Nuclear vs. Coal, The Virtual Nuclear Tourist, October 22, 2006, http://www.nucleartourist.com/basics/costs.htm, accessed June 14, 2007. Ogando, Joseph (2006). Green Engineering, Design News, 01.09.06, 65–70. Oswald, W.J. (1994). Advanced Integrated Wastewater Pond Systems, NSF Undergraduate Faculty Enhancement Program, Colorado State University, Fort Collins, CO, July 18–23, 1994. Oswald, W.J. (1991). Introduction to Advanced Integrated Wastewater Ponding Systems, Water Science and Technology, 24(5), 1–7. Paper Bags Are Better Than Plastic, Right?, http://www.reusablebags.com/facts.php?id=7, accessed June 14, 2007. Sawyer, C.N., and McCarty, P.L. (1978). Chemistry for Environmental Engineering, McGrawHill, New York. Sevior, M. (2006). Considerations for Nuclear Power in Australia, International Journal of Environmental Studies, 63(6), 859–872. United States Environmental Protection Agency (2007). Green Engineering, http://www.epa .gov/oppt/greenengineering/pubs/whats_ge.html, accessed June 4, 2007.
Exercises 171
Uranium Information Center (2007) The Economics of Nuclear Power, http://www.uic.com. au/nip08.htm, accessed June 14, 2007. Ux Weekly (2000). UXC The Ux Consulting Company, LLC, Coal vs. Nuclear in an Uncertain Future, January , 2000, http://www.uxc.com/cover-stories/uxw_14-04-cover.html, accessed September 22, 2008. Vallero, Daniel A., and Vesilind, P. Aarne (2007). Socially Responsible Engineering, Justice in Risk Management, John Wiley & Sons, Hoboken, NJ. World Commission on Environment and Development (1987). United Nations, Our Common Future, Oxford Paperbacks, Oxford.
EXERCISES 7.1
Gabbard (2007) presented a startling account of the detrimental impact that electrical utilities have on the environment. Disturbing findings presented in his paper include the following: Americans living near coal-fired power plants are exposed to higher radiation doses than those living near nuclear power plants that meet government regulations. The population effective dose equivalent from coal plants is 100 times that from nuclear plants. The energy content of nuclear fuel released in coal combustion is greater than that of the coal consumed.
7.2
7.3 7.4
7.5
7.6
How can one validate or repudiate the claims made by Gabbard? Should the Environmental Protection Agency ban coal-fired electrical generating power facilities? The half-life of iodine-131 is approximately 8 days. Determine the decay constant k for iodine-131. Calculate the amount of iodine-131 that will remain after a year if the initial quantity of iodine-131 is 64 grams. If “X” represents an element, determine the elements for 168X and 207 82X. What “Y” is formed from the following disintegration reactions where a is the mass number and b is the atomic number? a (a) 230 90Th : a + bY 40 a (b) 19K : b + bY Several fast-food restaurants use paper containers for their food products while others use Styrofoam (polystyrene). Perform a Google search on the internet to find articles that present the advantages and disadvantages of using each. As a concerned citizen who believes in sustainable development, list some things that can be done in our lives to reduce waste, recycle, conserve, etc. to help minimize adverse affects on the environment and our natural resources. For example, take shorter showers to decrease the volume of gray water generated and save energy costs associated with heating the water. Recycle glass, aluminum, paper, plastics, newspapers, etc. to decrease the quantity of materials that are disposed of in landfills and to allow recovery and reuse of these materials, thereby saving our natural resources.
CHAPTER
8
Water Quality and Pollution Objectives In this chapter, you will learn about: The importance and beneficial uses of water The hydrologic cycle and the water-wastewater cycle The major sources of water pollution The physical, chemical, biological, and radiological parameters used for assessing water quality Drinking water standards, wastewater effluent standards, and surface water quality standards
8.1 IMPORTANCE OF WATER The quality and quantity of water have long been a major concern of every civilization. Water is an essential component of life. Humans are composed of 65% water and on the average require 2.3 liters of water daily to survive (Swanson, 2001). People can survive only 5 to 7 days without water (AWWA, 2005). Humans did not always recognize the connection between disease and impure water. It was assumed that if water tasted good, it was “safe” to drink. Civilizations not only used rivers as their water supply, but depended on them for food, navigation, hydropower, irrigation, and disposal of wastes. Early treatment systems focused on removing taste and odor contaminants, thereby, making the water “safe.” As technology developed, people were able to see microorganisms in water, measure their concentration, and link them to disease. Approximately 97% of the earth’s waters are brackish or saline and are found in the oceans. The remaining 3% comprise fresh waters; however, 2% of these are inaccessible in the form of snow, ice, or glaciers (Van der Leeden et al., 1990). Accessible sources of fresh water from surface and groundwater sources make up only 0.62% of the total volume of water on earth. This translates into approximately 8.54 * 1018 liters 12.25 * 1018 gallons2 of fresh water available for various beneficial uses, as described in detail below. Table 8.1 shows worldwide water distribution. Finding high-quality, abundant fresh-water sources is increasingly difficult. In the United States, the most recent survey of assessed water bodies indicates that between 40% and 50% of those waters surveyed are impaired or threatened (EPA, 2005). As water passes through the hydrologic cycle, it becomes contaminated.
8.2 BENEFICIAL USES OF WATER Water has a number of beneficial uses, and its quality helps to determine how it is used. In the United States, the U.S. Geological Survey publishes a water-use report every five years (USGS, 2006).This report lists eight primary water-use categories: public supply, domestic, irrigation, livestock, aquaculture, industrial, mining, and
Section 8.2
Beneficial Uses of Water 173
Table 8.1 Estimated World Water Supply and Budget
Water item
Volume in cubic miles (thousands)
Volume in cubic kilometers (thousands)
Percent of total water
Water in land areas: Freshwater lakes
30
125
0.009
Saline lake and inland seas
25
104
0.008
Rivers (average instantaneous volume)
0.3
1.25
0.0001
Soil moisture and vadose water
16
67
0.005
Groundwater to depth of 4,000 meters
2,000
8,350
0.61
Ice caps and glaciers
7,000
29,200
2.14
9,100
37,800
2.8
3.1
13
0.001
317,000
1,320,000
97.3
326,000
1,360,000
100
From world oceans
85
350
0.025
From land areas
17
70
0.005
102
420
0.031
On world oceans
78
320
0.024
On land areas
24
100
0.007
102
420
0.031
9
38
0.003
Groundwater outflow to oceans2
0.4
1.6
0.001
Total runoff and outflow
9.4
39.6
0.0031
Total in land areas (rounded) Atmosphere World oceans Total water in all items (rounded) Annual evaporation1
Total evaporation Annual precipitation
Total precipitation Annual runoff to oceans from rivers and icecaps
Source: Van Der Leeden et al., 1990. 1 Evaporation is a measure of total water participating annually in the hydrologic cycle. 2 Groundwater outflow to oceans arbitrarily set equal to about 5% of surface runoff.
thermoelectric power. Figure 8.1 is a pie chart showing the percent by category of water used in the United States during 2000. Each of these categories will be discussed briefly, and the quantities of water used will be summarized. 8.2.1 Public Supply Public water supply is defined as water withdrawn by public and private water suppliers that is distributed to at least 25 people or to water systems that have a minimum of 15 connections. Public supply water may also be furnished to commercial, domestic, industrial, and thermoelectric power suppliers.
174 Chapter 8
Water Quality and Pollution
Public supply (10.6%) Domestic (0.9%) Irrigation (33.6%) Livestock (0.4%) Aquaculture (0.9%) Industrial (4.8%) Mining (0.9%) Thermoelectric power (47.9%)
Figure 8.1 Percent by category of water used in the United States for 2000.
164,000 megaliters per day (ML/d), or 43,300 million gallons per day (MGD) of water withdrawals went to public supply. 8.2.2 Domestic Water used both for indoor and outdoor household purposes is called domestic. Within the home, water is used for bathing, cooking, drinking, flushing toilets, and washing clothes. Outdoor water use primarily involves watering gardens and lawns. Water withdrawals in 2000 for domestic usage in the United States were 13,600 ML/d (3,590 MGD). 8.2.3 Irrigation Irrigation water use includes providing water for all agricultural and horticultural activities, for golf courses, cemeteries, and turf farms, and for preplant irrigation, frost protection, and dust suppression. This is the second highest category of water withdrawal. In 2000, approximately 519,000 ML/d (137,000 MGD) were withdrawn for irrigation use in the United States. 8.2.4 Livestock Livestock water use is associated with dairy operations, feedlots, livestock watering operations, and other on-farm usages. In 2000, approximately 6,660 ML/d (1,760 MGD) of water went for these purposes. 8.2.5 Aquaculture Water used for raising finfish and shellfish for food, conservation, restoration, or sport is classified as aquaculture use. In the United States, about 14,000 ML/d (3,700 MGD) of water were used in 2000 for aquaculture activities. 8.2.6 Industrial Industrial water use includes water used to cool, dilute, fabricate, transport, and wash. Examples of industries that consume large quantities of water include: chemical, food, paper, petroleum, and primary metal processing. In 2000, 74,900 ML/d (19,800 MGD) of water were used by industry.
Section 8.4
Water Pollution 175
Atmosphere Condensation
Precipitation
Evaporation Evapotranspiration
Surface runo
ff
Soil mantle
Aquifers
Ocean Groundwater
flow
8.2.7 Mining Mining water is used to extract minerals such as coal, iron, sand, and gravel, crude petroleum, and natural gas. During 2000, an estimated 13,200 ML/d (3,500 MGD) of water were used for mining activities. 8.2.8 Thermoelectric Power Water for thermoelectric power is used to generate electricity by steam turbines, primarily in coal-fired steam plants. The thermoelectric power industry in the United States for 2000 was the category with the highest water withdraws at approximately 740,000 ML/d (196,000 MGD).
8.3 HYDROLOGIC CYCLE Water is a renewable resource. Figure 8.2 shows a diagram of the hydrologic cycle. Ultimately, all surface and groundwater comes from precipitation, in the form of either rain or snow. Precipitation that percolates into the ground becomes groundwater and the remainder becomes surface waters, making rivers, lakes, and oceans. Water returns to the atmosphere by evaporation and transpiration from plants and trees. As water passes through the hydrologic cycle, it picks up both natural and anthropogenic (man-made) contaminants. Water percolating through the soil and geologic formations dissolves minerals, increasing the concentration of dissolved species such as calcium, magnesium, iron, and manganese. Surface waters tend to have higher concentrations of suspended materials, such as clay, soil, sand, silt, leaves, and color-producing substances associated with humic and fulvic acids resulting from decaying vegetation. Humans use water for a variety of reasons, as previously discussed, often causing the degradation of water quality.
8.4 WATER POLLUTION Pollution of earth’s water occurs both naturally and by anthropogenic means. One simple definition of water pollution is the discharge or release of contaminants into water at concentrations that have an adverse effect on human health, well-being, and the environment. Figure 8.3 is a block diagram showing four major types of water pollution.
Figure 8.2 Hydrologic cycle.
176 Chapter 8
Water Quality and Pollution
Agricultural runoff -Soil erosion -Nitrogen -Phosphorus -Animal wastes -Pesticides Nonpoint sources
Urban runoff -Oil, Gasoline -Nitrogen -Phosphorus -Leaves -Pesticides
River
Figure 8.3 Four major types of water pollution.
Point sources Domestic wastewater discharge Industrial wastewater discharge -Suspended solids -Suspended solids -Organics -Organics -Phosphorus -Phosphorus -Oil & Grease -Nitrogen -Pathogens -Metals
Point sources of pollution are those that emanate from a single point. Examples include domestic and industrial wastewater treatment facilities that discharge treated wastewater into surface waters. Nonpoint or diffuse sources of pollution are land areas such as fields, parking lots, and roads that generate surface runoff containing various types of contaminants. In the past, control of water pollution focused mainly on point sources of pollution from publicly owned treatment works (POTWs). These are domestic wastewater treatment facilities that treat sewage that may contain some industrial wastewater. More recently, regulatory agencies are targeting nonpoint sources of pollution, both agricultural and urban runoff, since the pollutional strength of these types of wastewaters may be two to three times that of domestic wastewater. Management of both water and wastewater flows in entire watersheds is now being practiced to encourage a holistic approach to pollution control. This allows regulatory agencies to better manage these resources and to develop total maximum daily loads (TMDLs) for receiving streams, thus being able to maintain high water quality. Understanding the importance and interrelationship between drinking water and wastewater treatment is an integral part of managing a particular watershed. Figure 8.4 shows a schematic of this relationship. A city uses a river and wells to provide water to its citizens. Water withdrawn from these sources must be transported to a water treatment plant (WTP), where various chemicals are added and several types of treatment processes are used to produce safe and palatable water. The water is pumped from the treatment plant throughout the city’s water distribution system. Consumers, businesses, and industries use the water and return it to the city’s wastewater collection system or sewerage system in the form of gray water and wastewater. Raw sewage or domestic wastewater is then conveyed through gravity sewers and pressurized force mains to the wastewater treatment plant (WWTP). At the WWTP, physical, chemical, and biological processes transform the sewage into reclaimed water that can be used for irrigation, drinking, or aquifer recharge, or discharged back into a river. Upstream and downstream of a city, other municipalities and stakeholders use the water in the stream the same way, and some use it for other purposes.
Section 8.5
Wells
Water Quality Parameters 177
Water distribution system
Water treatment plant
Wastewater collection system
Raw sewage Wastewater treatment plant
Deep well injection
Wastewater effluent
River
8.5 WATER QUALITY PARAMETERS Water quality is normally classified by physical, chemical, biological or microbiological, and radiological parameters. There are hundreds of other parameters that may be used. Some analyses measure specific constituents or parameters in the water, such as sodium, chloride, pH, and temperature. The results of other types of analysis, such as biochemical oxygen demand (BOD) and chemical oxygen demand (COD), are nonspecific, since these tests only differentiate between biodegradable organics and chemically oxidizable organics. The specific types of organic matter found in the sample must be determined through other means. For instance, if the sample came from a sugar-processing facility, specific testing for glucose could be conducted to quantify the actual concentration found in the sample. A book published by the American Public Health Association (APHA), Standard Methods for the Examination of Water and Wastewater (1998), presents over 500 parameters and the procedures for performing their analysis in water, wastewater, and sludge. Table 8.2 lists some of the most familiar physical, chemical, biological, and radiological analyses performed on water samples. Physical characteristics of water are those that affect the senses: sight, taste, smell, and touch. Chemical parameters are those that relate to the actual composition of the water and are subdivided into inorganic and organic categories. Many chemical species are toxic to humans and therefore their concentrations in water are regulated. Primary drinking water standards exist for 16 inorganic chemicals and 53 organic chemicals. Biological and microbiological parameters are those associated with disease-causing organisms or pathogens that directly relate to public health. Radiological parameters focus on radionuclides that may be found in certain waters that have been exposed to radioactivity.These, too, have severe health implications and are regulated by the federal drinking water standards. 8.5.1 Conventional Water Quality Assessment Parameters Several parameters have traditionally been used to assess water quality, especially as it pertains to water and wastewater treatment. This section discusses briefly the physical, chemical, and biological parameters that environmental engineers and scientists
Figure 8.4 Integrated watershed management.
178 Chapter 8
Water Quality and Pollution
Table 8.2 Physical, Chemical, Biological, and Radiological Characteristics of Water Physical
Chemical–inorganic
Chemical-organic
Microbiological
Radiological
Absorbance
Aluminum
Alachlor
Bacteria
Alpha-particle emitters
Color
Arsenic
Aldicarb
Helminths or worms
Beta-particle emitters
Solids
Barium
Atrazine
Protozoa
Radium-226 and -228
Taste and odor
Beryllium
Benzene
Viruses
Uranium
Temperature
Cadmium
Carbon tetrachloride
Turbidity
Calcium
Ethlybenzene
Cesium
Heptachlor
Chloride
Lindane
Chromium
Polychlorinated biphenyls (PCBs)
Cobalt
Toluene
Copper
Trichloroethylene
Fluoride
Toxaphene
Iron
Vinyl chloride
Lead
Xylenes
Magnesium
2,4-D
Manganese Mercury Nickel Nitrate Nitrite Phosphorus Potassium Strontium Zinc
use to assess water quality and the effectiveness of treatment at water and wastewater treatment plants. Turbidity
Turbidity is a measure of water clarity.Turbidimeters measure the intensity of light that is scattered by suspended matter, primarily colloidal and fine dispersions. Clay, silt, finely divided organic and inorganic matter, plankton, and microorganisms are examples of matter causing turbidity. Turbidity is expressed in nephelometric turbidity units (NTUs). The procedure for measuring turbidity is found in Section 2130 of Standard Methods (1998). As of January 14, 2005, drinking water systems cannot exceed 1 NTU and must not exceed 0.3 NTU in 95% of the daily samples in any given month.
Section 8.5
Solids
Total solids (TS) in a water or wastewater sample consist of suspended and dissolved solids. The procedures for measuring the various types of solids are found in Section 2540 of Standard Methods (1998). To determine total solids, a known volume of sample is placed in a preweighed crucible or aluminum tare and then evaporated and dried to a constant weight in an oven at 103–105°C. The crucible or aluminum tare is cooled in a desiccator and reweighed.The difference in the weight divided by the volume of sample yields the total solids concentration, expressed mathematically as: TSa
WTS WDS - WTare mg b = = L V V
(8.1)
where: TS WTS WDS WTare V
= = = = =
total solids concentration, mg/L, mass of total solids, mg, mass of dried solids and tare, mg, mass of tare, mg, and volume of sample, L.
Total suspended solids (TSS) are the portion of total solids that can physically be removed by passing through a filter with a specific pore size opening. Settleable solids relate to the fraction of suspended solids that will settle out of solution by the force of gravity. Suspended solids are determined by passing a specified volume of sample through a preweighed 0.45-mm fiberglass filter pad and then drying the pad at 103–105°C. The concentration of suspended solids is then determined by dividing the weight of the dry solids accumulated on the filter pad by the volume of the sample: TSS a
mg WTSS WDSFP - WFP b = = L V V
(8.2)
where: TSS WTSS WDSFP WFP V
= = = = =
concentration of total suspended solids, mg/L, mass of total suspended solids, mg, mass of dried solids, filter pad, and tare, mg, mass of filter pad and tare, mg, and volume of sample, L.
The total dissolved solids (TDS) concentration, which relates more to the chemical quality of water, is determined as: TDS = TS - TSS
(8.3)
where: TDS = concentration of total dissolved solids, mg/L, TS = concentration of total solids, mg/l, and TSS = concentration of total suspended solids, mg/L. Each of the above solids species has a volatile fraction (related to organic content) and a fixed fraction (related to mineral content). The volatile fraction is determined by igniting the dry solids in a muffle furnace at 550°C. The total volatile solids (TVS) concentration can be calculated from: TVS a
WTVS WDS - WFS mg b = = L V V
(8.4)
Water Quality Parameters 179
180 Chapter 8
Water Quality and Pollution
where: TVS WTVS WDS WFS V
= = = = =
concentration of total volatile solids, mg/L, mass of total volatile solids, mg, mass of dried solids and tare, mg, mass of fixed solids and tare after ignition at 550°C, mg, and volume of sample, L.
Equation (8.5) is used to calculate the total volatile suspended solids (TVSS) concentration. TVSS a
WTVSS WDSFP - WFSFP mg b = = L V V
(8.5)
where: = = = =
concentration of total volatile suspended solids, mg/L, mass of total volatile suspended solids, mg, mass of dried solids, filter pad, and tare, mg, mass of fixed solids, filter pad, and tare after ignition at 550°C, mg, and V = volume of sample, L.
TVSS WTVSS WDSFP WFSFP
Equation (8.6) is used to calculate the total fixed solids (TFS) concentration (mg/L): TFS = TS - TVS
(8.6)
Example 8.1 illustrates the determination of total solids, total volatile solids, total fixed solids, total suspended solids, and total dissolved solids.
EXAMPLE 8.1 Solids analyses The following test results were obtained for a sample of river water. All solids analyses were performed using a sample volume of 100 ml. Tare mass of evaporating dish =
25.6039 g
Mass of evaporating dish plus residue after evaporation @ 105°C =
25.6289 g
Mass of evaporating dish plus residue after ignition @ 550°C =
25.6119 g
Tare mass of Whatman glass fiber filter =
1.6469 g
Residue on Whatman glass fiber filter after drying @ 105°C =
1.6544 g
Determine: a. b. c. d. e.
The total solids concentration. The total suspended solids concentration. The total dissolved solids concentration. The total volatile solids concentration. The total fixed solids concentration.
Section 8.5
Solution part a First, calculate the total solids concentration using Equation (8.1). TS a
WTS WTS WDS - WTare mg b = = L V V V 125.6289 g - 25.6039 g2 1000 mg 1000 ml mg a b = 250 = ba g 100 ml L L
Solution part b Next, calculate the total suspended solids concentration using Equation (8.2). TSS a
mg WTSS WDSFP - WFP b = = L V V 11.6544 g - 1.6469 g2 1000 mg 1000 ml mg ba = a b = 75 g 100 ml L L
Solution part c Now, using Equation (8.3), calculate the total dissolved solids (TDS) concentration. TDS = TS - TSS = 250 - 75 = 175
mg L
Solution part d Next, calculate the total volatile solids (TVS) concentration using Equation (8.4). TVS a
mg WTVS WDS - WFS b = = L V V 125.6289 g - 25.6119 g2 1000 mg 1000 ml mg ba = a b = 170 g 100 ml L L
Solution part e Finally, using Equation (8.6), calculate the total fixed solids (TFS) concentration. TFS = TS - TVS = 250 - 170 = 80
mg L
Color
True color is due to colloidal and dissolved substances. The decomposition of leaves, conifer needles, and wood produces tannins, humic acid, fulvic acid, and humates, which are the organic compounds causing color in water. Inorganic compounds that cause color include iron and manganese. The procedure for measuring the color concentration is found in Section 2120 of Standard Methods (1998). Apparent color is due to suspended materials in the water, such as suspended solids. When color is measured on unfiltered samples, it is called “apparent color,” whereas color measured on filtered water samples 10.45-mm filter2 is termed “true color.”
Water Quality Parameters 181
182 Chapter 8
Water Quality and Pollution
Color in water is measured by visual comparison with standards consisting of established platinum-cobalt solutions. Solutions of potassium chloroplatinate 1K 2PtCl 62 tinted with small amounts of cobalt chloride yield colors that are very much like natural colors. The pH of the water should be reported along with the color measurement, since color “intensity” generally increases as pH increases. The standard color unit (c.u.) is the color produced by 1 mg/L K 2PtCl 6 . Taste and Odor
Taste and odor compounds in surface waters are mainly attributed to algae. Bacterial degradation of algae, algal waste products, and the algae themselves may cause taste and odor problems. In addition, ferrous ions (Fe2), manganous ions (Mn2), and sulfide ions (S2) often cause taste and odor problems. The taste and smell of chlorine in water is also a common complaint among customers. Taste and odor compounds generally found in groundwater include hydrogen sulfide 1H 2S2, reduced iron and manganese, and high total dissolved solids (TDS), which is frequently caused by a high salt content. Chemicals that often contaminate groundwater and that may cause taste and odor problems include trichloroethylene (TCE), other synthetic organic compounds, and leachate from landfills. Taste and odors are measured by dilution of the sample with distilled water until no taste or odor is detected. The procedure for measuring taste in water is found in Section 2160 of Standard Methods (1998), and Section 2150 lists the procedure for determining odor. The greatest dilution of sample at which tastes can be detected determines the flavor threshold number (FTN) as follows: FTN =
A + B A
(8.7)
and the greatest dilution of sample at which odors can be detected determines the threshold odor number (TON) as follows: TON =
A + B A
(8.8)
where: FTN TON A B
= = = =
flavor threshold number, dimensionless, threshold odor number, dimensionless, volume of sample tested, and volume of taste and odor dilution water used.
EXAMPLE 8.2 Calculating threshold odor number A 100-ml sample of water is diluted with 200 ml of dilution water so that there is no detection of the odor. Calculate the threshold odor number (TON). Solution Substitute the appropriate values into Equation (8.8) to solve for TON. TON =
100 ml + 200 ml = 3 100 ml
Section 8.5
Temperature
Temperature is an important physical parameter that affects the density, viscosity, vapor pressure, and surface tension of water. Biological and chemical reactions increase with an increase in temperature. A 10ºC increase often doubles the rate of biological and chemical reactions. Gas solubility is affected by temperature, and oxygen’s solubility increases with a decrease in temperature. The solubility of certain chemical compounds, such as sodium chloride, increases with an increase in water temperature. Temperature is normally measured with a thermometer. The procedure for measuring temperature is found in Section 2550 of Standard Methods (1998). Nitrogen (N)
Nitrogen is a nutrient required by all living organisms. In excess amounts in surface waters it contributes to excessive algal growth, which results in eutrophication. Nitrogen enters the water supply in agricultural and urban runoff as fertilizers, and in industrial and domestic wastewater treatment discharges. The most common and important forms of nitrogen are listed in Table 8.3. Oxidized nitrogen 1NOx2 is the sum of NO2- + NO3- , whereas total Kjeldahl nitrogen (TKN) is the sum of the ammonia and organic nitrogen. Organic nitrogen is nitrogen found in protein, amino acids, and urea 1NH 2CONH 22. The primary drinking water standard for nitrate is 10 mg/L as N, because it is linked to infant methemoglobinemia (blue baby syndrome). Most organisms use ammonium nitrogen as their nitrogen source, but algae normally assimilate nitrate. Some bacteria are capable of fixing nitrogen gas from the atmosphere to form ammonium nitrogen. Nitrogen found in most organic compounds exists with a negative-3 valence 1N 3-2. The total nitrogen concentration in a sample consists of the total Kjeldahl nitrogen and oxidized nitrogen: total nitrogen 1TN2 = TKN + NOx
(8.9)
The ammonium ion exists in equilibrium with ammonia according to the following equilibrium equation. At pH values normally encountered in environmental engineering, the ammonium ion is the predominant species, whereas ammonia occurs at pH values greater than 10. NH 4+ 4 NH 3 + H +
K a = 5.6 * 10-10 @ 25°C
(8.10)
3NH 3 - N4 = 3NH 4+4 + 3NH 34
(8.11)
where 3NH 3 - N4 = concentration of ammonium and ammonia obtained from an ammonia nitrogen analysis according to Standard Methods. Table 8.3 Important Forms of Nitrogen Nitrogen species
Chemical formula
Valence of N
Ammonia
NH3
-3
Ammonium
NH4+
-3
Nitrogen gas
N2
Nitrite
NO2-
+3
Nitrate
NO3-
+5
0
Water Quality Parameters 183
184 Chapter 8
Water Quality and Pollution
3NH 3 - N4 is measured by colorimetry using Nessler’s reagent (potassium mercuric iodine, K 2HgI 4). Nessler’s reagent combines with NH 3 to form a yellowishbrown colloid. The procedures for performing the various types of nitrogen analyses are found in Section 4500-N of Standard Methods (1998). Phosphorus (P)
Phosphorus is an essential nutrient required by all living organisms. It, like nitrogen, is linked to eutrophication of surface waters if present in excessive amounts. Phosphorus enters the water supply from phosphorus-based detergents, corrosion inhibitors added to drinking water systems, and domestic and industrial wastewater discharges: total phosphorus 1TP2 = inorganic-P + organic-P
(8.12)
Organic-P is found in proteins and amino acids and is normally only a minor consideration. Inorganic-P species include orthophosphates, polyphosphates, and metaphosphates. Polyphosphates and metaphosphates are also known as condensed phosphates. Condensed phosphates and organic phosphorus must be converted to orthophosphate before they can be measured. Major orthophosphates include: trisodium phosphate 1Na 3PO42; disodium phosphate 1Na2HPO42; monosodium phosphate 1NaH 2PO42; and diammonium phosphate 1NH 422HPO4 . Orthophosphates are used by microorganisms and can be precipitated out of solution by adding chemicals such as lime or alum. Orthophosphate is typically measured by one of three colorimetric methods. The phosphate ion 1PO4-32 combines with ammonium molybdate under acidic conditions to form molybdophosphate complex according to Equation (8.13). Orthophosphate
PO4-3 + 121NH422 MoO4 + 24 H+ : 1NH423 PO4 # 12 MoO3 + 21 NH4+ + 12 H2O
(8.13)
The blue-colored sol produced by the addition of stannous chloride is proportional to the amount of phosphate present as shown by Equation (8.14). 1NH 423 PO4 # 12 MoO3 + Sn+2 : 1molybdenum blue2 + Sn+4
(8.14)
Condensed Phosphate
Major polyphosphates include: sodium hexametaphosphate, Na31PO426 ; sodium tripolyphosphate, Na 5P3O10 ; and tetrasodium pyrophosphate, Na4P2O7 . Condensed phosphates are converted to orthophosphates by boiling samples that have been acidified with H 2SO4 for 90 minutes. The orthophosphate formed from the condensed phosphates is measured in the presence of the original orthophosphate concentration present in the sample by one of the colorimetric procedures, yielding the total inorganic phosphate concentration. The concentration of condensed phosphate is then determined by difference: condensed phosphate = total inorganic phosphate - orthophosphate (8.15)
Organic Phosphate Prior to quantification, organic phosphate must be destroyed by wet oxidation or digestion using one of the following chemicals: perchloric acid, nitric-sulfuric acid, or persulfate. All forms of phosphorus are measured in an organic determination (total phosphorus). First the sample is digested, followed by
Section 8.5
Water Quality Parameters 185
colorimetric measurement of orthophosphate. The organic-P concentration is then determined by difference: organic phosphorus = total phosphorus - total inorganic phosphorus (8.16) The procedures for performing the various types of phosphorus analyses are found in Section 4500-P of Standard Methods (1998). Dissolved Oxygen (DO)
The dissolved-oxygen (DO) concentration is a critical parameter in assessing water quality. DO is the amount of molecular oxygen 1O22 dissolved in water and is a function of temperature, salinity, and pressure. The concentration of DO increases as the temperature of the water decreases. For example, the concentration of DO is 9.09 and 7.56 mg/L at 20°C and 30°C, respectively. Lakes and rivers should contain more than 5 to 6 mg/L of DO to ensure aerobic conditions. Fish and other aquatic organisms require oxygen to survive. If insufficient oxygen is available, anaerobic conditions may result, leading to fish kills and obnoxious odors from the production of hydrogen sulfide and methane. Typically, wastewater discharged into receiving waters must contain a minimum DO concentration of 5 mg/L to ensure that aerobic conditions prevail. During wastewater treatment, a DO concentration of 2.0 mg/L is usually maintained in aeration basins to sustain the microorganisms involved in treating wastewater. The dissolved-oxygen content in water affects its corrosiveness and taste. Water containing DO is more palatable than water without DO. Historically, DO was measured by titration using the azide modification of the Winkler method (Standard Methods, 1998). An oxygen meter and probe are now most commonly used to measure DO in the laboratory and in the field. They must be properly calibrated before measurements are taken. Biochemical Oxygen Demand (BOD)
BOD is a biological assay in which aerobic bacteria oxidize the biodegradable organic compounds in a water or wastewater sample to carbon dioxide and water. BOD actually measures the quantity of oxygen necessary to oxidize the organic matter and therefore gives an indirect measurement of the quantity of organic matter. The greater the oxygen demand, the higher the concentration of biodegradable organics in the sample. The theoretical basis for BOD is based on stoichiometry and first-order removal kinetics. Knowing the chemical formula of an organic compound, we can write the stoichiometric equation showing the compound being oxidized to carbon dioxide and water. The example that follows illustrates the procedure for calculating the theoretical oxygen demand for a chemical compound such as C 5H 7O2N (C 5H 7O2N is commonly used to represent the organic fraction of microorganisms). Since nitrogen is present in the compound, ammonia will be released as a by-product. For compounds consisting of only carbon, hydrogen, and oxygen, the end products are carbon dioxide and water.
EXAMPLE 8.3 Calculating theoretical oxygen demand Calculate the theoretical oxygen demand of a water sample containing 1000 mg/L of C 5H 7O2N.
186 Chapter 8
Water Quality and Pollution
Solution Start by writing the stoichiometric equation showing the oxidation of the chemical compound to carbon dioxide, water, and ammonia. Make sure that the equation is properly balanced. C 5H 7O2N + 5 O2 : 5 CO2 + 2 H 2O + 1 NH 3 Next, calculate the gram-molecular and atomic weights of C 5H 7O2N and O2, respectively. MW C 5H 7O2N = 51122 + 7112 + 21162 + 11142 = 113 g AW O2 = 11322 = 32 g From the stoichiometric equation, 5 * 32 = 160 grams of oxygen are required to react with 113 grams of C 5H 7O2N. As shown below, 1416 mg/L of oxygen are required to oxidize 1,000 mg/L of C 5H 7O2N. 1000
mg C 5H 7O2 N 160 g O2 mg O2 ¢ ≤ = 1416 L 113 g C 5H 7O2N L
If the total oxygen demand is to be determined, another stoichiometric equation is written for the oxidation of NH 3 to nitrate. This process, known as nitrification, consumes approximately 4.57 g O2 per g of NH 3-N oxidized. Derivation of BOD Equation
Bacteria oxidize organic material to produce energy that will be used for synthesizing new bacterial cells and for maintenance functions. Figure 8.5 shows a generalized graph of the amount of oxygen consumed (Y) and the amount of organic material remaining (L) as a function of time. The rate at which organic matter, depicted in Figure 8.5, is being oxidized can be described mathematically as a first-order removal reaction: dL = - kL dt
(8.17)
where: L = concentration of organic matter, mg/L, k = BOD rate constant, d-1, and t = oxidation period, days.
BOD remaining (L)
Lu Oxygen consumed (Y ) or BOD exerted (BODt)
Figure 8.5 Oxygen consumed and BOD exerted versus time.
Y
Lt
Time
Section 8.5
Water Quality Parameters 187
Rearranging Equation (8.17) gives 1 dL = - k dt L
(8.18)
Integration limits for Equation (8.18) are shown as follows, where L goes from the ultimate BOD 1Lu2 to the BOD at any time t, and t goes from zero to t: Lt
LLU
t 1 dL = - k dt L L0
(8.19)
Integrating Equation (8.19) results in: ln1Lt2 - ln1Lu2 = - kt
(8.20)
Rearranging Equation (8.20) and taking the antilog of both sides of the equation yields: Lt = Lue -k t
(8.21)
From Figure 8.5, the ultimate BOD 1Lu2 is equal to Y plus the BOD at any time t, Lt , or: Lu = Y + Lt
(8.22)
Rearranging Equation (8.22) and solving for Lt yields: Lt = Lu - Y
(8.23)
Substituting Equation (8.23) into Equation (8.21) results in Equation (8.24). Lt = Lu - Y = Lue -kt
(8.24)
Rearranging Equation (8.24) and solving for Y (BOD exerted) results in: Y = Lu A 1 - e -kt B
(8.25)
Equation (8.25) is expressed in a more useful form as: BODt = BODu A 1 - e -kt B
(8.26)
where: Y = BODt = amount of oxygen consumed or BOD exerted at some time, t, mg/L, and Lu = BODu = ultimate BOD of the sample, mg/L.
EXAMPLE 8.4 C a l c u l a t i n g B O D g i v e n BODu a n d r a t e c o n s t a n t Calculate the 5-day and 17-day BOD of a domestic wastewater, given the BOD rate constant of 0.22 d-1 and an ultimate BOD of 300 mg/L.
188 Chapter 8
Water Quality and Pollution
Solution First, substitute a time of 5 days and an ultimate BOD of 300 mg/L into Eq. (8.26), which yields a 5-day BOD of 200 mg/L: BODt = BODu A 1 - e -kt B BOD5 = 300
mg mg -1 A 1 - e -0.22 d 15 d2 B = 200 L L
Next, substitute a time of 17 days and an ultimate BOD of 300 mg/L into Eq. (8.26) which yields a 17-day BOD of 293 mg/L: BOD17 = 300
mg mg -1 A 1 - e -0.22 d 117 d2 B = 293 L L
BOD Laboratory Procedure
The procedure for performing the biochemical oxygen demand analysis is found in Section 5210 of Standard Methods (1998). The BOD test involves mixing a milliliters of sample (water or wastewater) with b milliliters of dilution water, which is distilled water to which nutrients and oxygen have been added. If the sample contains bacteria, then it is unnecessary to add “seed” or inoculum to the dilution water. The mixture is placed in a minimum of four 300-ml BOD bottles. Figures 8.6 and 8.7 are schematics illustrating the procedure for performing a “seeded” BOD analysis. The dissolved-oxygen (DO) concentration of the mixture in two of the bottles is determined using a DO meter and oxygen probe or by titration using the azide modification of the Winkler method. Two of the bottles are incubated in the dark at 20°C for five days, or some other desired time period, and then the DO is measured. For the results of the BOD test to be valid, at least 1 mg/L of DO should remain after incubation and at least 2 mg/L
Figure 8.6 BOD laboratory apparatus.
“a” ml Water or wastewater sample
“b”ml Dilution water
Nutrients Bacteria or seed
Dilution water Air
Section 8.5
BOD bottle B1
Water Quality Parameters 189
BOD bottle D1
Measure DO on day 1
BOD bottle B2
Mixture of sample and dilution water
BOD bottle D2
Figure 8.7 BOD bottle arrangement.
Measure DO on day 5
of DO should be used during the incubation period. The BOD for a “seeded” test is calculated according to Equation (8.27). Average values are used for D1 , D2 , B1 , and B2 . BODt 1mg>L2 =
1D1 - D22 - 1B1 - B22 f P
(8.27)
where: BODt = biochemical oxygen demand exerted for an incubation period of t days, mg/L, t = incubation period at 20°C, days, D1 and D2 = initial and final DO concentrations in the diluted sample, mg/L, B1 and B2 = initial and final DO concentrations in the blanks, mg/L, Blanks are BOD bottles containing dilution water and seed, P = decimal fraction of sample used, and f = ratio of the seed in sample to seed in blank or control. P =
ml of sample volume of BOD bottle
(8.28)
f =
% seed in D1 % seed in B1
(8.29)
When the sample already contains bacteria, Equation (8.27) is modified as follows for calculating the BOD for an “unseeded” test: BODt a
1D1 - D22 mg b = L P
(8.30)
190 Chapter 8
Water Quality and Pollution
EXAMPLE 8.5 Calculating BOD for a “seeded” sample A 5-day BOD test is performed on a sample of chlorinated wastewater effluent. Sixty milliliters of wastewater sample and 2 ml of “seed” are added to 300-ml BOD bottles and mixed with enough dilution water to fill the bottles. “Seed” must be added, since wastewater effluent is chlorinated, which kills pathogens and bacteria. For the blanks or controls, 2 ml of “seed” are added to 300-ml BOD bottles and mixed with enough dilution water to fill the bottles. The average dissolved-oxygen concentrations of the diluted wastewater samples and blanks on day 1 of the test were 6.8 and 7.4 mg/L, respectively. After incubation at 20°C for 5 days, the average DO concentrations of the diluted wastewater samples and blanks were 2.5 and 7.1 mg/L, respectively. Using Equation (8.27), calculate the 5-day BOD of the chlorinated effluent. Solution First, calculate the decimal fraction (P) of sample used according to Equation (8.28). P =
ml of sample 60 ml wastewater sample = = 0.20 volume of BOD bottle 300 ml
Next, calculate the ratio of seed in the sample to seed in the blanks according to Equation (8.29). The “seed” inoculates the dilution water, resulting in 240 ml being seeded for D1 and 300 ml for B1 . f =
240 ml>300 ml % seed in D1 = = 0.80 % seed in B1 300 ml>300 ml
Finally, determine the 5-day BOD using Equation (8.27). BOD5 a
1D1 - D22 - 1B1 - B22 f 16.8 - 2.52 - 17.4 - 7.1210.802 mg b = = L P 60 ml>300 ml = 20.3
mg L
EXAMPLE 8.6 Calculating BOD for an “unseeded” sample A 5-day BOD test is performed on a sample of raw wastewater. Six milliliters of wastewater sample are added to 300-ml BOD bottles and mixed with dilution water. The average dissolved-oxygen concentration of the diluted wastewater samples on day 1 of the test was 7.6 mg/L and after incubation at 20°C for 5 days was 2.6 mg/L. Calculate the 5-day BOD of the raw wastewater. Since raw wastewater already contains bacteria, it is not necessary to add “seed” to the sample or to the dilution water. Therefore, Equation (8.30) must be used for determining the 5-day BOD concentration.
Section 8.5
Solution First, determine the decimal fraction of sample (P) used according to Equation (8.28). P =
ml of sample 6 ml wastewater sample = = 0.02 volume of BOD bottle 300 ml
Next, determine the 5-day BOD using Equation (8.30). BOD5 a
1D1 - D22 17.6 - 2.62 mg mg b = = = 250 L P 16 ml>300 ml2 L
Chemical Oxygen Demand (COD)
The COD test measures the oxygen equivalent of the organic matter using a strong oxidizing agent (potassium dichromate) in an acidic environment (sulfuric acid) to which silver sulfate has been added to serve as a catalyst. Excess dichromate is added according to the following reaction (Sawyer et al., 1994): + CnHaObNc + d Cr2O27 + 18d + c2 H
where d =
" 2d Cr3+ + n CO (8.31) 2 a + 8d - 3c + a b H2 O + c NH4+ 2
Heat & Catalyst
2n a b c + - . 3 6 3 2
The difference in the amount of dichromate present at the beginning and that remaining at the end is reported in terms of oxygen equivalents or organic matter oxidized after appropriate conversions. A titration with standard ferrous ammonium sulfate (FAS) is used for determining the quantity of dichromate used. COD ¢
1A - B2 * M * 8000 mg as O2 ≤ = L ml sample
(8.32)
where: COD 8000 A B M
= = = = =
chemical oxygen demand, mg O2/L, conversion factor, ml FAS used for blank, ml FAS used for sample, and molarity of FAS.
The COD of a sample of water or wastewater will always be less than the theoretical oxygen demand, since certain compounds may be refractory or difficult to treat. On the other hand, the BOD value will always be less than the COD value, since BOD measures only the biodegradable organic content. For domestic wastewater, environmental engineers often assume that the ultimate BOD 1Lu2 is equivalent to the actual COD of the wastewater and that the 5-day BOD value is approximately two-thirds of the COD value. The procedure for performing the chemical oxygen demand analysis is found in Section 5220 of Standard Methods (1998). Pathogens and Indicator Organisms
Numerous pathogenic organisms that cause disease may be found in water and wastewater. The four primary groups include bacteria, protozoa, viruses, and helminths or
Water Quality Parameters 191
192 Chapter 8
Water Quality and Pollution
worms. Bacteria are single-cell microorganisms, and certain strains are pathogenic. They may be spheroid, rod, or spiral in shape and consist of prokaryotic cells with a single strand of DNA. Escherichia coli is an example of a bacterium. Protozoa are motile, microscopic eukaryotes that are usually single cell. Two examples of protozoa are Cryptosporidium parvum and Giardia lamblia. Viruses are obligate intracellular parasites that can replicate only in a living host cell. Examples of viruses encountered in water and wastewater are the Hepatitis A virus and the Enteroviruses (polio, echo, coxsackie). Helminths or worms are parasites that are usually found in wastewater and contaminated water. Examples are tapeworm, stomach worm, and whip worm. Isolating and identifying pathogens in water samples is difficult and expensive. Large volumes must be processed in order to find certain pathogens, and special analytical equipment is needed to grow and identify them. Working with pathogenic organisms is dangerous, so indicator organisms that are nonpathogenic are used to determine the presence of potential pathogens in water and wastewater. Indicator organisms such as total coliform bacteria and fecal coliform bacteria are used to indicate the presence of pathogenic or harmful microorganisms. Coliform bacteria are found in the digestive tract of warm-blooded animals, and most are nonpathogenic. They are numerous in water and wastewater and easily detected. Their survival times are similar to those of pathogenic organisms. Enumeration and identification of these indicator organisms is accomplished by direct microscopic counts, pour and spread plates, the membrane-filter technique, and the multiple-tube fermentation method. The latter two are the traditional procedures used by engineers to enumerate indicator organisms. Membrane-Filter Technique The membrane-filter technique involves filtering samples of water or wastewater through sterilized filter pads (0.45-mm openings) for collecting indicator organisms.The filter pads are then placed in specified media (M-Endo) in petri dishes. These are incubated (35°C) for approximately 24 hours, the number of colonies formed are counted, and the results are reported in colony-forming units (CFUs) per 100 ml of sample. Typical coliform bacteria colonies appear pink to dark red in color with a green metallic sheen. Absence of bacteria growth is considered a negative test. Figure 8.8 shows a photograph of a membrane-filtration apparatus and associated filter
Figure 8.8 Photograph of membrane filtration apparatus and filter pad.
Section 8.5
10 ml Sample
Water Quality Parameters 193
1 ml Sample
0.1 ml Sample Positive results 4-3-2
Figure 8.9 Multiple-tube fermentation test.
pad. The procedure for performing the membrane-filter technique for members of the coliform group is found in Section 9222 of Standard Methods (1998). Another method for detecting coliform bacteria is the multiple-tube fermentation technique. This procedure involves inoculating a series of test tubes filled with lactose broth with varying amounts of sample. Figure 8.9 shows a typical multiple-tube fermentation setup. Normally, 15 test tubes are used, with sample volumes of 10 ml, 1.0 ml, and 0.1 ml being added. Coliform bacteria ferment lactose broth under anaerobic conditions, resulting in gas production 1CO22 and bacterial growth, as indicated by a cloudy broth. Equation (8.33) or statistical tables in Standard Methods (1998) can be used for calculating the most probable number (MPN) per 100 ml of sample. The MPN is a statistical estimate of the number of coliforms that are potentially present, since a number of other microbial species can also ferment lactose. The procedure for performing the multipletube fermentation filter technique for members of the coliform group is found in Section 9221 of Standard Methods (1998).
Multiple-Tube Fermentation Technique
number of positive tubes * 100 MPN = 100 ml 31ml sample neg. tubes2 * 1ml sample all tubes240.5
(8.33)
EXAMPLE 8.7 Calculating the most probable number Calculate the MPN in a set of 15 test tubes with the following number of positive tubes: 4, 3, and 2 for sample sizes of 10.0, 1.0, and 0.1 ml, respectively. Solution First, determine the volume of sample used in all the test tubes. ml of sample in all tubes = 5110.02 + 511.02 + 510.12 = 55.5 Next, determine the volume of sample used in the test tubes that were negative. ml of sample in neg. tubes = 1110.02 + 211.02 + 310.12 = 12.3
194 Chapter 8
Water Quality and Pollution
Finally, estimate the most probable number (MPN) per 100 ml of sample by substituting into Equation (8.33). number of positive tubes * 100 MPN = 100 ml 31ml sample neg. tubes2 * 1ml sample all tubes240.5 19 * 1002 MPN = = 34.4 100 ml 3112.3 ml2 * 155.5 ml240.5 The Total Coliform Rule in the Safe Drinking Water Act specifies a maximum contaminant level goal (MCLG) of zero for total coliforms, fecal coliforms, and Escherichia coli.
pH
The pH of water is a function of the hydrogen ion 1H +2 concentration. Mathematically, pH is defined as the negative logarithm of the hydrogen ion concentration: pH = - Log3H +4. The pH scale goes from 0 to 14 with 7 being neutral. A pH value above 7 indicates alkaline or basic conditions, and a value below 7 indicates acidic conditions. Both water and wastewater are treated to produce a pH between 6 and 8.5. Besides determining the solubility of various chemical species, pH affects biological and chemical reactions. A calibrated pH meter with a glass electrode is routinely used for measuring pH.
8.6 WATER QUALITY STANDARDS Three types of water quality standards have been developed to maintain the required quality of various water bodies. They include: drinking water standards, wastewater effluent standards, and surface water standards. 8.6.1 Drinking Water Standards Federal drinking water standards were promulgated by the U.S. Environmental Protection Agency under the Safe Drinking Water Act. State supremacy allows each state either to adopt the federal standards or develop its own standards, which must be equivalent or more stringent. There are primary and secondary standards for various types of contaminants. Maximum contaminant levels (MCLs) are the maximum concentrations of a specific contaminant allowed in treated drinking water. Maximum contaminant level goals (MCLGs) are concentrations that have been established for specific contaminants, such that no known or anticipated health effects will result. MCLGs are not enforceable, but municipalities are encouraged to try to meet them. Primary Drinking Water Standards
Primary drinking water standards are enforceable, approved standards based on health. They are specified for inorganic chemicals, organic chemicals, radionuclides, and turbidity. Table 8.4 lists some of the more important primary drinking water standards. A complete up-to-date list of primary standards may be found at www.epa.gov.
Section 8.6
Water Quality Standards
195
Table 8.4 Primary Drinking Water Standards Contaminant
MCL or TT (mg/L)
Cryptosporidium Giardia lamblia Total trihalomethanes (TTHMs) Arsenic
Potential health effects from ingestion of water
Sources of contaminants in drinking water
TT1
Gastrointestinal illness (e.g., diarrhea, vomiting, cramps)
Human and fecal animal waste
TT1
Gastrointestinal illness (e.g., diarrhea, vomiting, cramps)
Human and fecal animal waste
Liver, kidney, or central nervous system problems; increased risk of cancer
By-product of drinking water disinfection
Skin damage or problems with circulatory systems, and possible increased risk of cancer
Erosion of natural deposits; runoff from orchards, runoff from glass and electronics production wastes
0.08
0.010 as of 01/23/06
Cadmium
0.005
Kidney damage
Corrosion of galvanized pipes; erosion of natural deposits; discharge from metal refineries; runoff from waste batteries and paints
Alachlor
0.002
Eye, liver, kidney, or spleen problems; anemia; increased risk of cancer
Runoff from herbicide used on row crops
Benzene
0.005
Anemia; decrease in blood platelets; increased risk of cancer
Discharge of factories; leaching from gas storage tanks and landfills
Increased risk of cancer
Erosion of natural deposits
Radium-226 and radium-228 (combined)
5 pCi/L
1 Treatment technique: a required process intended to reduce the level of contaminant in drinking water. Source: www.epa.gov.
Secondary Drinking Water Standards
Secondary drinking water standards are non-enforceable standards based primarily on aesthetics. They are specified for inorganic chemicals, dissolved salts, corrosivity, color, and odor. Table 8.5 lists these standards. This may be found at www.epa.gov. 8.6.2 Wastewater Effluent Standards Under the Clean Water Act, the U.S. Environmental Protection Agency established the national pollution discharge elimination system (NPDES) permit, requiring all industrial and municipal wastewater treatment facilities (point sources of pollution) to have a permit to discharge into receiving waters. Cities with populations over 100,000 people are now required to also have an NPDES permit for stormwater discharges. NPDES permits regulate the flow, concentration, and mass of pollutant that can be discharged from a facility. Typical effluent standards that would be found in an NPDES permit for a secondary wastewater treatment facility are listed in Table 8.6.
196 Chapter 8
Water Quality and Pollution
Table 8.5 Secondary Drinking Water Standards Contaminant
Secondary standard
Aluminum
0.05 to 0.2 mg/L
Chloride
250 mg/L
Color
15 c.u.
Copper
1.0 mg/L
Corrosivity
Noncorrosive
Fluoride
2.0 mg/L
Foaming agents
0.5 mg/L
Iron
0.3 mg/L
Manganese
0.05 mg/L
Odor
3 threshold odor number (TON)
pH
6.5 to 8.5
Silver
0.10 mg/L
Sulfate
250 mg/L
Total dissolved solids (TDS)
500 mg/L
Zinc
5 mg/L
8.6.3 Surface Water Quality Standards Surface water quality standards or stream standards must be maintained for lakes, rivers, or streams. Water quality standards are established by each state. Most surface water quality standards contain beneficial-use categories, numerical levels, and narrative statements about the specific use designations. The highest classification or category is normally associated with the highest water quality and reserved for sources of drinking water. Water quality standards are used to determine whether the designated uses of a water body are being protected. Stream standards are established to maintain the highest beneficial use for a given water body. Table 8.7 shows a listing of the various categories used by the State of Florida.
Table 8.6 Typical NPDES Permit Limits for Secondary Wastewater Treatment Annual average
Monthly average
Weekly average
BOD (mg/L)
20
30
45
TSS (mg/L)
20
30
45
6–8.5
N.A.
N.A.
pH N.A.- Not applicable.
Summary
Table 8.7 Water Classification Based on Best Beneficial Use for the State of Florida Class or category
Description
Class I
Potable water supplies
Class II
Shellfish propagation or harvesting
Class III
Recreation, propagation, and maintenance of a healthy, well-balanced population of fish and wildlife
Class IV
Agricultural water supplies
Class V
Navigation, utility, and industrial use
8.7 KEPONE CONTAMINATION OF THE JAMES RIVER The toxic organochlorine insecticide kepone (decachlorooctahydro-1, 3, 4-metheno2H-cyclobuta [cd]-pentalen-2-one) was manufactured by Allied Chemical and Life Science Products (LSP) in Hopewell, Virginia, from 1966 until 1975. Approximately 1532 metric tons of kepone were produced. In late July 1975, the Commonwealth of Virginia issued an order to LSP to cease production of kepone after learning from the Center for Disease Control in Atlanta that a plant worker at LSP showed symptoms of neurological illness and kepone concentration in his blood in the parts per million range (Huggett and Bender, 1980). Its use has since been banned, because it is a suspected endocrine disruptor. A preliminary survey conducted by the Environmental Protection Agency revealed high concentrations of kepone in the soil and dust in Hopewell and in biota of the James River to which industrial effluent from the kepone manufacturers had been discharged. Governor Mills Godwin ordered the entire tidal James River and its tributaries closed to commercial and sport fishing. Numerous studies have been conducted on the toxic effects of kepone on biota and are referenced in the article by Huggett and Bender (1980). The U.S. Food and Drug Administration adopted action levels for kepone on a wet-weight basis of 0.1 mg/g in the edible portion of finfish, 0.3 mg/g in mollusks, and 0.4 mg/g in the edible portion of blue crabs. Kepone is a very stable and persistent contaminant that accumulated in the bottom sediments of the James River. From there it cycles into the water column to be taken up by plants and animals, eventually being returned to the sediments upon death of these organisms. It entered all levels of the food chain (Huggett et al., 1980) and contaminated 37 * 106 metric tons of sediment extending 120 km seaward from its source (Nichols, 2003). In 1988, 13 years after the closure of LSP, the fishing ban was lifted. The contaminated sediments have been diluted and covered by uncontaminated sediment that diminishes the kepone flux back into the water column. Remedial actions to remove kepone would be expensive and environmentally unwise (Huggett, 1989). Natural sedimentation provided the best way of decontaminating the estuary. S U M M A RY This chapter introduced the importance and beneficial uses of water. Ninety-seven percent of the earth’s waters are saline and are found in the oceans; the remaining 3% comprises fresh water. Only 0.62% of the earth’s water is both fresh and accessible, since 2% is in the form of snow, ice, or glaciers. The eight major beneficial uses of water in the United States are public supply, domestic, irrigation, livestock,
197
198 Chapter 8
Water Quality and Pollution
aquaculture, industrial, mining, and thermoelectric power. During the year 2000, the thermoelectric category withdrew the most water in the United States, followed by irrigation. As water passes through the hydrologic cycle, it becomes contaminated. Four major sources of water pollution include domestic wastewater discharges, industrial wastewater discharges, agricultural stormwater runoff, and urban stormwater runoff. Water quality parameters are divided into four major categories: physical, chemical, microbiological, and radiological. The procedures for conducting analyses on water, wastewater, and sludge are found in Standard Methods for the Examination of Water and Wastewater (1998). Parameters that traditionally have been used for assessing water quality include: turbidity, solids, color, taste and odor, temperature, nitrogen, phosphorus, biochemical oxygen demand (BOD), chemical oxygen demand (COD), and indicator-organisms testing, using either the membrane-filter technique or multiple-tube fermentation technique. A brief introduction to primary drinking water standards (health related) and secondary drinking water standards (aesthetics) was presented along with typical maximum contaminant levels (MCLs).Wastewater effluent standards were briefly addressed by describing the National Pollution Discharge Elimination System (NPDES) permit in addition to typical wastewater effluent standards for a secondary wastewater treatment facility. Surface water quality standards or stream standards were discussed and a water classification system based on the best beneficial use for the State of Florida was presented. In conclusion, a brief case history of the kepone contamination of the James River in Virginia was presented.
KEY WORDS
agricultural runoff beneficial use biochemical oxygen demand (BOD) chemical oxygen demand (COD) color domestic wastewater hydrologic cycle industrial wastewater kepone
maximum contaminant level (MCL) most probable number (MPN) national pollution discharge elimination system (NPDES) phosphorus primary standard secondary standard standard methods
stream standards suspended solids taste and odor total Kjeldahl nitrogen (TKN) total maximum daily loads (TMDL) turbidity ultimate BOD urban runoff
REFERENCES APHA, AWWA, WEF (1998). Standard Methods for the Examination of Water and Wastewater, American Public Health Association, 1015 Fifteenth Street, NW, Washington, DC. American Water Works Association (2005). http://www.awwa.org/Advocacy/learn/info/ HistoryofDrinkingWater.cfm. Environmental Protection Agency (2005). EPA 841—B-05-005, Handbook for Developing Watershed Plans to Restore and Protect Our Waters, October 2005. Huggett, R. J. (1989). Kepone and the James River, Contaminated Marine Sediments: Assessment and Remediation, The National Academy of Sciences. Huggett, R.J., and Bender, M.E. (1980). Kepone in the James River, Environmental Science & Technology, 14 (8), 918–920. Huggett, R. J., Nichols, M., and Bender, M.E. (1980). Kepone Contamination of the James River, American Chemical Society, Division of Environmental Chemistry, 19:1, 341–342. Nichols, M.M. (2003) Sedimentologic Fate and Cycling of Kepone in an Estuarine System: Example from the James River Estuary, The Science of the Total Environment, 97–98, 407–440.
Exercises 199
Sawyer, Clair N., McCarty, Perry L, and Parkin, Gene F. (1994). Chemistry for Environmental Engineers, McGraw-Hill, New York. Swanson, P. (2001). Water: The Drop of Life, Minnetonka, Minnesota, Northword Press. U.S. Geological Survey (2006). Estimated Water Use in the United States, 2000, http://water. usgs.gov/watuse/, accessed October 11, 2006. Van Der Leeden, F., Troise, F.L., and Todd, D.K. (1990). The Water Encyclopedia, 2nd ed., Lewis Publishers, Chelsea, Michigan.
EXERCISES 8.1 8.2
8.3
8.4
There are eight categories of beneficial uses of water. List four of them and briefly describe the types of water uses in each. As water passes through the hydrologic cycle, it becomes contaminated with natural and anthropogenic contaminants. List the four major types of water pollution. Give a brief qualitative description of the water quality of each. Explain the difference between a point source and nonpoint source of pollution and give an example of each type of source. Water quality is classified by physical, chemical, biological or microbiological, and radiological parameters. Go to the United States Environmental Protection Agency’s website at www.epa.gov under the Office of Drinking Water. List three water quality parameters in each of the four categories (physical, chemical, microbiological, and radiological) along with the current maximum contaminant level (MCL) concentration or the required treatment technique (TT). Select from both primary and secondary standards. Solids analysis is one of the most widely used parameters for assessing water quality. Use the following data for calculating total solids (TS), volatile solids (VS), dissolved solids (DS), total suspended solids (TSS), and total volatile suspended solids (TVSS). A sample volume of 150 ml was used in performing all solids analyses.
Tare mass of evaporating dish
= 24.3520 g
Mass of evaporating dish plus residue after evaporation @ 105ºC
= 24.3970 g
Mass of evaporating dish plus residue after ignition @ 550ºC
= 24.3850 g
Mass of Whatman filter and tare
= 1.5103 g
Mass of Whatman filter and tare after drying @ 105ºC
= 1.5439 g
Residue on Whatman filter and tare after ignition @ 550ºC
= 1.5199 g
8.5
8.6 8.7
8.8
An ammonia nitrogen analysis performed on a wastewater sample yielded 30 mg/L as nitrogen. If the pH of the sample was 8.5, determine the ammonium nitrogen concentration (mg/L) in the sample, assuming a temperature of 25ºC. Calculate the theoretical oxygen demand (mg/L) of a solution containing 450 mg of glucose 1C 6H 12O62 in 2 liters of distilled water. A result of a 7-day BOD test performed on a sample from an oligotrophic lake was 10 mg/L. The base e BOD rate constant determined from previous studies was estimated to be 0.10 d-1. Determine the ultimate BOD and 5-day BOD of the sample taken from the lake. A 5-day BOD test is performed on an industrial wastewater sample that contains no bacteria; therefore, a “seeded” BOD test is run. Ten ml of “seed” are added to 20 liters of dilution water. Thirty milliliters of industrial wastewater are added to a 300-ml BOD bottle, and the remaining volume consists of
200 Chapter 8
Water Quality and Pollution
8.9
8.10
“seeded” dilution water. The average dissolved-oxygen concentrations of the diluted wastewater samples and blanks (seeded dilution water) on the first day of the test were 7.5 mg/L and 9.0 mg/L, respectively. After incubation of separate BOD bottles at 20ºC for 5 days, the average DO concentrations of the diluted wastewater BOD bottles and seeded dilution water BOD bottles were 3.1 and 8.5 mg/L, respectively. Calculate the 5-day BOD of the industrial wastewater. A multiple-tube fermentation test was performed on a highly eutrophic reservoir water sample. A set of 15 test tubes with sample sizes of 10, 1.0, and 0.1 ml were used in the analysis, resulting in the following number of positive tubes: 5–2–1. Estimate the most probable number (MPN) per 100 ml of sample, and compare your answer to the value presented in Standard Methods (1998). Discuss in your own words what is meant by an indicator organism. Which group of organisms is used as an indicator organism and why? List three groups of pathogenic organisms that may be found in water and wastewater.
CHAPTER
9
Water Treatment Objectives In this chapter, you will learn about: The selection of appropriate unit operations and unit processes that are used for treating drinking water Primary and secondary drinking water standards Typical water treatment flow diagrams for treating surface and groundwater How to design mixing systems, flocculation basins, settling basins, and granular media filters Water softening and the lime-soda ash method for removing hardness from water The importance of disinfection and selection of the appropriate disinfectant
9.1 INTRODUCTION The removal of substances from water is accomplished by using biological, chemical, and physical methods. For treatment of potable water sources, chemical and physical techniques are more widely used than biological ones. A water treatment plant uses a combination of several methods, normally operated in series. If the treatment method involves physical forces, it is called a unit operation. Treatment technologies that are biological or chemical are called unit processes. Each sequence of unit operations and/or unit processes represents a “liquid” treatment train. Multiple “liquid” treatment trains are normally operated in parallel so that high-quality water can be produced in the event a unit operation or process fails. Parallel treatment trains are also necessary so that equipment maintenance and basin drainage can be accomplished without shutting down the entire plant. The residuals, or sludge, produced during water treatment must be collected, thickened, dewatered, and ultimately disposed. The unit operations and processes involved with sludge handling are called the “sludge” trains. Reliability and redundancy issues for sludge handling, treatment, and disposal are not quite as critical as those for the liquid treatment train. It is not uncommon, however, to provide dual units and pumps so that the sludge can be processed with the largest unit out of service. Typical unit operations commonly encountered in water treatment include screening, mixing, aeration, sedimentation, filtration, and ultraviolet-light disinfection. Chemical unit processes in which chemicals are added for removing contaminants include coagulation, chemical oxidation, chemical disinfection, and chemical precipitation. Biological unit processes in water treatment are less prevalent than those encountered in wastewater treatment. Biological nitrification (conversion of ammonia to nitrate under aerobic conditions) and biological denitrification (reduction of oxidized nitrogen under anoxic conditions) are the two primary biological processes practiced in water treatment.
202 Chapter 9
Water Treatment
9.2 GENERAL CONSIDERATIONS FOR SELECTING TECHNOLOGY There are three primary considerations for selecting the appropriate treatment technology: (1) the water quality of the source, (2) the required and/or desired quality of the treated water, (3) the emerging contaminants and the types of technologies that will be required for removing them from water. 9.2.1 Water Source and Quality The appropriate unit operations and processes for treating the water cannot be determined until a sustainable source is identified. In the past, surface water supplies such as rivers, lakes, and reservoirs or underground aquifers were the primary sources of water. During recent decades, alternative sources such as brackish waters and sea water, along with reclaimed wastewater, have become viable sources. With the increased popularity of living along the Florida and Gulf coasts and in the southwestern desert (Texas, New Mexico, Arizona, and California), membrane processes are being utilized more frequently for treating lower-quality water sources. As high-quality water sources become limited, municipalities will be encouraged to blend reclaimed wastewater effluent with low-quality sources. A potential treatment scheme for this blended type of water resource might consist of conventional coagulation, flocculation, sedimentation, and filtration, followed by membrane filtration. Water quality in the United States varies from region to region. Both the climate and geological formations in each region impact water quality. Sulfur is a predominant constituent found in groundwaters of the southeast. High chloride concentrations are found in the water of coastal communities due to saltwater intrusion. Water hardness also varies by region in the United States.The softest waters are found in New England, the south Atlantic, and Pacific Northwest. States with some of the hardest waters include Iowa, Illinois, Indiana, Arizona, New Mexico, and the Great Plains. If water is soft, soap will tend to lather in it rather easily, whereas in hard water it will not. Normally, the quality of groundwater is higher than that of surface water, unless the aquifer has been contaminated due to direct influence of surface waters or leaking underground storage tanks. Another advantage of using groundwater is that its characteristics are more consistent and the volume of water available does not change appreciably with time, unless the aquifer is overpumped. Overpumping may lead to intrusion of saltwater or transport of other contaminants into the aquifer. Surface water quality and volume are highly variable, changing with time. A lake or reservoir provides a water source with more consistent characteristics than a stream; however, seasonal changes and lake turnovers can result in variable quality. A main concern in using brackish and salt water is not the quality of the source, but the cost of treating the water with membranes. The permitting and disposal of the brine reject from membrane-treatment facilities may pose additional economic and political concerns. Using reclaimed wastewater as the source has the advantage that it can be pumped directly from the wastewater treatment plant to a water treatment facility. The concentration of various constituents in the wastewater effluent are well documented, but additional research is necessary to study the long term health effects of drinking reclaimed wastewater effluent. Public sentiment will undoubtedly favor using alternative water sources other than reclaimed wastewater. 9.2.2 Drinking Water Standards The required and/or desired quality of the treated water will place specific constraints on which technologies can be used in the process train. The treated water must meet primary drinking water standards (health related) in addition to those adopted by the
Section 9.3
Overview of Surface Water Treatment Systems
municipality, which relate to secondary standards. Primary standards are enforceable regulations promulgated by the Environmental Protection Agency (EPA). Maximum contaminant levels (MCLs) have been established for chemical, biological, and radiological constituents in water. Secondary standards with established MCLs for various parameters related to aesthetics have been adopted by most state regulatory agencies; however, they are not enforceable. It is certain that new standards will be promulgated as emerging contaminants are identified and maximum contaminant levels established. Standards for existing contaminants will likely be tightened, requiring higher levels of removal and the implementation of new and improved technologies. Primary and secondary drinking water standards may be found at the following website (Office of Drinking Water): www.epa.gov/safewater/contaminants/index.html#mcls. 9.2.3 Microconstituents A number of microconstituents are likely to be regulated by the EPA in the future. The list includes the following: perchlorate, N-nitrosodimethylamine (NDMA), endocrine disrupters, pharmaceuticals, personal care products, methyl tertiary butyl ether (MTBE), and nanoparticles. Additional information on these contaminants can be found on the EPA and American Water Works Association (AWWA) home websites (www.epa.gov and www.awwa.org). Endocrine disruptors, pharmaceuticals, and personal care products found in sewage and eventually in our water supplies are of major concern. Those that may cause serious health effects include antibiotics, hormones, blood lipid regulants, and antidepressants. The effects of these substances, even at low concentrations, are still unknown, but they are believed to cause problems with human reproduction systems, the thyroid, and the hypothalamus and pituitary systems (MWH, 2005). Nanoparticles may also pose significant health threats, since these particles range in size from 1nm 110-9 m2 to 100 nm and are capable of crossing the blood-brain barrier.
9.3 OVERVIEW OF SURFACE WATER TREATMENT SYSTEMS The quality of surface water is highly variable and the characteristics are not as consistent as those associated with groundwater. This section discusses surface water treatment facilities using conventional methods and membrane filters. 9.3.1 Conventional Surface Water Treatment Figure 9.1 is a flow diagram of a conventional water treatment plant (WTP) used for treating surface water.The depicted process train (MWH, 2005) is normally employed for surface waters with high turbidity ( 720 nephelometric turbidity units, NTU), high color ( 720 color units, c.u.), and high total organic carbon, 1TOC 7 4 mg/L2. Treatment begins with bar racks, or coarse screens, followed by fine screens (traveling screens), to remove tree limbs, rags, and finer debris from the water at the intake structure. For pH adjustment an acid or base may be added to the influent, along with an oxidant or disinfectant if high coliform bacteria concentrations are observed. The main unit operations and processes that follow in sequential order include rapid or flash mixing with coagulant addition, flocculation, sedimentation, and granular media filtration. This sequence allows turbidity and colloidal particles to be destabilized and removed from the water. Sometimes polymer and disinfectant are added to the filter influent. The addition of polymer will enhance turbidity removal through the filter and adding disinfectant will control biological growth on the filter.
203
204 Chapter 9
Water Treatment
An oxidant and/or disinfectant will be added to the filtered effluent to oxidize any remaining substances and ensure that any remaining pathogens are killed. The water is then sent to a clear well (storage tank) to allow the oxidant and/or disinfectant sufficient detention time to react with the constituents in the water. From the clear well, the water will be pumped throughout the distribution system by high-service pumps (high capacity, high head). Supplemental disinfectant, normally in the form of chloramines, is added to the water pumped into the distribution system to ensure a chlorine residual of 0.1–0.2 mg/L is provided at the furthest water tap in the system. A similar process train known as direct filtration consists of the same unit operations and processes presented in Figure 9.1, except that sedimentation is not used. Another variation of the conventional process train is known as in-line or contact filtration, where both the flocculation and sedimentation processes are omitted. Both direct and in-line filtration processes are used for higher-quality surface waters having low turbidity 1615 NTU2, moderate to low color 1620 c.u.2, and low TOC 164 mg/L2.
Figure 9.1 Flow diagram of conventional surface water treatment plant.
Raw water
Bar rack
Screens
Flow measurement pH adjustment Oxidant/disinfectant Coagulant Mixing
Flocculation
Sedimentation
Sludge Oxidant/disinfectant
Filtration
Backwash to washwater recovery pond Oxidant/disinfectant and fluoride
Clear well
High-service pumps Oxidant/disinfectant To distribution system
Section 9.4
Overview of Groundwater Treatment Systems
205
Raw water
Bar racks
Screens
Flow measurement pH adjustment Oxidant/disinfectant Catridge filters or microscreens
Membrane filtration modules
Washwater recovery or disposal pH adjustment Oxidant/disinfectant/fluoride
Clear well
High-service pumps
To distribution system
9.3.2 Membrane Treatment Figure 9.2 is a flow diagram of a membrane filtration water treatment plant for treating surface water. The first unit operation in the treatment train consists of bar racks and fine traveling screens followed by self backwashing cartridge filters or microscreens. The cartridge filters or microscreens are necessary for removing turbidity, which otherwise would foul the membranes. Next, microorganisms, hardness, and colloidal particles are removed as they pass through the membrane filters. An acid or base, in addition to an oxidant or disinfectant, is added to the filtered water. The acid or base maintains proper pH, whereas the oxidant/disinfectant kills pathogens and oxidizes any remaining substances. Disinfectant in the form of chloramines is normally added prior to pumping throughout the distribution system to ensure that proper chlorine residual is maintained. Membrane filtration plants require the highest-quality surface water with low turbidity 1610 NTU2, moderate to low color 1610 c.u.2 and low TOC 1 64 mg/L2.
9.4 OVERVIEW OF GROUNDWATER TREATMENT SYSTEMS Frequently, groundwater is of such high quality that very little treatment is required. This section briefly discusses both conventional water softening, using the lime-soda ash process, and reverse osmosis (RO) membrane treatment.
Figure 9.2 Flow diagram of membrane filtration water treatment plant.
206 Chapter 9
Water Treatment
9.4.1 Conventional Lime-Soda Ash Treatment Numerous WTPs in Florida have process treatment trains like that presented in Figure 9.3. Treatment consists of air-stripping to remove carbon dioxide 1CO22 and hydrogen sulfide 1H 2S2, and to add oxygen to the water. If volatile organic compounds (VOCs) are present in the groundwater, the off-gas from the air-stripping process must be collected and treated. Air stripping is generally followed by limesoda ash treatment in a solids-contact unit and granular media filtration. The limesoda ash process is used for removing hardness (Ca2+ and Mg2+) from the water in the form of CaCO3 and Mg1OH22 precipitates. Adding lime and soda ash raises the pH of the water to above 11. Recarbonation involves adding CO2 to lower the pH to a range of 9.5 to 8.5. This converts the excess lime to CaCO3 . Additional settling is required to remove the CaCO3 solids that form during recarbonation. Recarbonation and sedimentation generally follow the solids-contact unit. Granular media filtration is required to remove solids that remain after settling in the solids-contact unit.An oxidant/disinfectant is added to the filtered water as it enters the clear well. High-service pumps then pump the water throughout the distribution system. Supplemental disinfectant (normally chloramines) is added on the discharge side of the high-service pumps to provide chlorine residual throughout the distribution system. Conventional lime-soda ash treatment can yield a product water with a total hardness of approximately 40 mg/L as CaCO3 (Viessman and Hammer, 2005). 9.4.2 Reverse Osmosis Treatment Membrane softening is being used more frequently as a viable means of treating water. In addition to providing softened water, membranes offer the advantage of Figure 9.3 Flow diagram for lime-soda ash treatment plant.
Wells
Flow measurement
Aeration
CO2, H2S, VOCs O2 Lime and/or soda ash
Solids contact unit
Sludge CO2
Recarbonation & sedimentation
Sludge Oxidant/disinfectant
Filtration
Backwash to washwater recovery pond Oxidant/disinfectant and fluoride
Clear well
High-service pumps Oxidant/disinfectant To distribution system
Section 9.5
Surface Water Treatment Processes
207
Wells
Flow measurement pH adjustment Oxidant/disinfectant Antiscalant Catridge filters or microscreens
Reverse osmosis modules
Concentrate to disposal Base and corrosion inhibitor Oxidant/disinfectant and fluoride
Aeration
Clear well
High-service pumps
To distribution system
removing color and natural organic matter (NOM), which are precursors for disinfection and disinfection by-products (DDBPs). Figure 9.4 presents a flow diagram of a typical reverse-osmosis (RO) WTP equipped for hardness removal. One major drawback to using any type of membrane process is the disposal of concentrate generated during treatment. Regulatory agencies are sometimes unwilling to permit deep-well injection of these waste streams.
9.5 SURFACE WATER TREATMENT PROCESSES This section describes major unit operations and processes used for treating surface water for drinking. The coagulation process is discussed first. Then, examples are given to illustrate the design procedure for the following systems: mixing, flocculation, water softening, sedimentation, filtration, backwashing, and filter sizing. Disinfection and treatment of water treatment plant residuals are briefly introduced. 9.5.1 Coagulation and Flocculation Coagulation is a unit process consisting of the addition and mixing of a chemical reagent (coagulant or flocculant) to destabilize colloidal and fine solids suspended in water. Figure 9.5 is a schematic of the coagulation process. Flocculation involves slow stirring or gentle agitation to promote agglomeration of the destabilized particles formed during coagulation, so that heavy, rapid-settling flocs are formed. The destabilized particles are subsequently removed through sedimentation and filtration.
Figure 9.4 Flow diagram of reverse osmosis water treatment plant.
208 Chapter 9
Water Treatment
Coagulation process Coagulant
Figure 9.5 Schematic of coagulation process.
Solids/Liquid separation
Direct Filtration
Rapid Flocculation mixing Agglomeration Destabilization
Settling
Filtration Sludge
Colloidal Particles
A large portion of the solids in surface water and groundwater cannot be removed by sedimentation because they are colloidal particles. Colloidal particles are extremely small and have negligible mass and large surface area per unit volume. Colloidal particles are defined by their size, generally ranging from 1 nm 110-9 m2 to 1 mm 110-6 m2. Because of their large surface area, colloidal particles tend to acquire a negative surface charge. Turbidity in water is caused primarily by colloidal and suspended matter such as clay, silt, nonliving organic particles, plankton, and microbes. Clays are a major portion of natural turbidity. Kaolinite, montmorillonite, and other clay suspensions can be removed by adding alum. Color in water is usually attributed to colloidal forms of iron and manganese or to organic compounds (humic and fulvic acids) resulting from the decay of vegetation. Coagulation is effective in removing color and many organic macromolecules and particles from water. Iron (III) and aluminum (III) salts are effective at removing humic acids, but not as effective with fulvic acids. Coagulation and filtration readily remove algae and amoebic cysts and can achieve bacterial removals of 99%. Toxic organic compounds such as polychlorinated biphenyls (PCB) and dichloro-diphenyl-trichloro-ethane (DDT) and many inorganic toxic materials are adsorbed, and thus removed, on naturally occurring inorganic and organic particulates. Removal of the particulates removes the toxic substances. Electrical Double Layer
According to the double-layer electrical theory, a negative colloidal particle will be surrounded by a layer of counter positive ions known as the fixed or Stern layer. Surrounding this is another layer consisting primarily of counter-ions and co-ions called the Diffuse layer. Figure 9.6 is a schematic diagram of the double layer. The electrical potential decreases as the distance from the colloidal particle increases. The electrical potential at the colloid surface is called the Nerst potential and at the surface of shear it is called the zeta potential. The electrical potential drops linearly across the Stern layer and exponentially from the Stern layer through the Diffuse layer on into the bulk solution. The greater the magnitude of the zeta potential, the greater the stability of the colloidal particles. Colloidal Destabilization
Stability of the colloidal particles depends both on the repulsive forces associated with the electrical charge of the double layer and on the attractive forces associated with van der Waals forces. If the electrostatic charge on the particles
Section 9.5
Fixed or stern layer __ __ Colloid
__
Surface Water Treatment Processes
209
Diffuse layer
__ - - - - __ __ Plane of shear
Electrical potential Zeta potential Distance from surface
can be reduced or neutralized, the attractive van der Waals forces cause the destabilized colloids to agglomerate or coalesce when particles get close enough together. Chemicals called coagulants or flocculants are added to water for destabilizing colloidal particles. Destabilization can be accomplished by one of four methods: 1. 2. 3. 4.
lowering the surface potential by compressing the double layer; lowering the surface potential by adsorption and charge neutralization; enmeshment in precipitate; and adsorption and interparticle bridging.
Coagulants decrease the magnitude of the zeta potential by compressing the double layer or by adsorption and charge neutralization. At certain coagulant dosages, precipitates will form, and the colloidal particles will either serve as condensation nuclei for the precipitates or become enmeshed in the precipitate. The addition of polymers results in floc formation due to interparticle bridging. Overdosing of coagulants and polymer can lead to restabilization of the colloidal suspensions. Coagulants
Aluminum and iron salts are the major types of coagulants used in water treatment, primarily aluminum sulfate, ferrous sulfate, ferric chloride, ferric sulfate, and sodium aluminate. Alkalinity is consumed with the addition of inorganic metallic coagulants. Stoichiometric equations showing the formation of hydroxide precipitates and the consumption of alkalinity for the various coagulants may be found elsewhere (MWH, 2005; Viessman and Hammer, 2005). Only aluminum sulfate will be discussed in this section, since its low cost makes it the coagulant of choice in the United States (MWH, 2005). Aluminum sulfate or filter alum 3Al2 1SO423 # 14 H2O4 is the major coagulant used for coagulating turbidity in surface waters. Alum readily dissolves in water, with sulfate ions (SO 2 4 ) being dispersed throughout the liquid. Aluminum ions exist in water as hydrated ions. There are no free aluminum ions in the form of Al3+. The simplest hydrated form of aluminum is Al(H 2O)3 6 . Other forms of aluminum include: 4 5 1 2 Al1OH22+, Al 21OH24+ and 2 , Al7(OH)17 , Al13(OH) 34 , and Al(OH) 4 . Al(OH) Al1OH24 are called monomers, since they have only one Al atom. Al1OH231S2 is a solid, amorphous, gelatinous precipitate that forms. Each mg/L of alum decreases
Figure 9.6 Schematic of the electrical double layer.
210 Chapter 9
Water Treatment
water alkalinity by 0.50 mg/L as CaCO3 and produces 0.44 mg/L of CO2 (Viessman and Hammer, 2005). Alum causes coagulation in two ways: adsorption and charge neutralization or “sweep coagulation.” Adsorption and charge neutralization results when positively charged aluminum monomers and polymers are adsorbed onto negatively charged colloids, neutralizing the charge. The alum dose required is less than the solubility limit of the metal hydroxide. For “sweep coagulation,” a sufficient alum dose is added such that the solubility limit of the metal hydroxide is exceeded, allowing the precipitation of Al1OH23 . Colloidal matter and suspended matter become covered with a gelatinous, sticky sheath, causing them to settle out of solution. Coagulant Aids
Coagulant aids include acids and bases that may be added to the water to maintain a specific pH. Alkalinity is often added in the form of lime (CaO), soda ash 1Na2CO32, or sodium hydroxide (NaOH) to increase the buffering capacity of the water. Recall that alkalinity is consumed when inorganic metallic coagulants are used. Therefore, during water treatment, engineers must design chemical feed and storage facilities to provide for alkalinity addition during treatment. Other coagulant aids include anionic, cationic, or nonionic polymers, powdered activated carbon (PAC), and clays. PAC is added for removing color and the sorption of specific organic compounds. Clays may be added to provide turbidity or targets for coagulants. Clays should be added before alum addition. Polymers are normally added after alum or coagulant, if they are used together. Polymers are long chains of small subunits or monomers. Polymers may be cationic or positively charged 1+2, anionic or negatively charged 1-2, or non-ionic (neutral). The charge is related to functional groups and pH. Major functional groups associated with polymers include carboxyl (COOH); amines 1NH3+2, and hydroxyl 1OH-2. Polymers must be adsorbed onto the turbidity particles. Charge neutralization normally occurs with low-molecular-weight polymers. High-molecular-weight polymers are used to “bridge” between particles that would otherwise repel each other. Polymer dosages are normally 61 mg/L. Polymers are ineffective at very low turbidities; therefore, alum or clay is often added to the water to provide targets or nuclei for precipitation reactions to occur. If used, the targets should be added prior to adding the polymer. The quantity of coagulant that must be added to a particular type of water must be determined through experimentation. This may be accomplished by performing jar tests in the laboratory using a 6-gang stirring mechanism. Optimum dose is dependent on pH, temperature, turbidity, and alkalinity. Figure 9.7 is a photo of a typical jar test apparatus. 9.5.2 Mixing Mixing is the process whereby chemicals are added and instantaneously dispersed into the water. Mixing of coagulants has primarily been accomplished by mechanical units. Mechanical mixing is often called rapid or “flash” mixing, since the detention times used are generally less than 2 minutes and more frequently on the order of 20 to 30 seconds. Actual chemical reaction times are usually less than 1 second. Rapid-mix tanks are either square or circular. Stator blades and/or baffles along the sides of the walls should be used to prevent vortexes from forming. The design of rapid-mix units is based on detention time and the velocity gradient, G. Detention
Section 9.5
Surface Water Treatment Processes
Figure 9.7 Photograph of jar test apparatus.
time (U) is mathematically defined as the volume of the basin divided by the volumetric flow rate: u =
V Q
(9.1)
where: u = detention time, min, V = volume of the mix tank, ft3 1m32, and Q = volumetric flow rate into mix tank, ft3/min 1m3/min2. The velocity gradient, G, is a measure of the relative velocity between two fluid particles divided by the distance between them. For example, if two fluid particles are traveling at 3 feet per second relative to each other and the distance between the two is 1.5 feet; the velocity gradient would be calculated as follows: G =
3 fps 2 fps = = 2 s-1 1.5 ft ft
In the design of rapid-mix and flocculation basins, the velocity gradient is defined by Equation (9.2). G = a
W 0.5 P 0.5 b = a b m mV
where: G W V m P
= = = = =
velocity gradient, s-1 or fps/ft (mps/m), power input per unit volume, ft # lb/s # ft3 1N # m/s # m32, volume, ft3 1m32, viscosity of water, lb # s/ft2 1Pa # s or N # s/m22, and power input, ft # lb/s 1W or N # m/s2.
(9.2)
211
212 Chapter 9
Water Treatment
Chemical feed
Chemical feed a) Back-mix impeller
b) Flat-blade mixer
Chemical feed
Figure 9.8 Schematic for three methods of injecting coagulant.
c) In-line blender
The rate of particle collisions is proportional to the velocity gradient. A G value must be sufficient to produce the desired rate of particle collisions but not so large as to produce shear forces that prevent proper floc formation. In rapid-mix tanks, detention times vary from 20–50 seconds and velocity gradients vary from 700–1000 s-1 (Reynolds and Richards, 1996). Other ways to mix coagulants include in-line mixers or blenders, Parshall flumes, hydraulic jumps, over and under baffles, and mixing with air. Figure 9.8 is a schematic of three methods for coagulant injection.
EXAMPLE 9.1 Design of a rapid-mix basin A water treatment plant processes 30,000 cubic meters of water each day. A square rapid-mix tank with vertical baffles and flat impeller blades will be used. The design detention time and velocity gradient are 30 seconds and 900 s-1, respectively. Determine: a. The dimensions of the rapid-mix tank. b. Power input, if the temperature of the water is 20°C. Solution part a Solve for the volume of the basin by rearranging Equation (9.1). V = u * Q = 30 s * 30000
m3 min h d * * * = 10.4 m3 d 60 s 60 min 24 h
Assuming the depth of the rapid-mix basin is 6 meters, determine the length and width of the unit. Solve for the area by dividing the volume by the depth. A =
V 10.4 m3 = = 1.74 m2 D 6m
For a square rapid-mix basin, length (L) is equal to width (W), so A = W2. A = 1.74 m2 = W2
that is, W = L = 1.32 m
Section 9.5
Surface Water Treatment Processes
Solution part b The power input to the rapid-mix basin is determined by rearranging Equation (9.2). The absolute viscosity of water at 20°C is 1.002 * 10-3 N # s/m2 11.002 * 10-3 kg/m # s2. The volume of the rapid-mix basin is: 11.32 m2 * 11.32 m2 * 16 m2 = 10.45 m3. P = G2 mV = 1900 s-122 ¢ 1.002 * 10-3
P = 8481
N#m N#s 3 b110.45 m 2 = 8481 s m2
1W N#m a b = 8481 W = 8.5 kW s 1 N # m/s
To meet reliability/redundancy criteria, a spare 8.5-kW motor should be provided, or multiple tanks could be used.
9.5.3 Flocculation Tanks Tapered flocculation tanks employing three separate compartments, each operating with a different velocity gradient, normally are used in order to produce a dense floc that settles well. Typical G values in the first, second, and third compartments, respectively, are 50, 20, and 10 s-1. The detention time recommended by the Great Lakes Upper Mississippi River Board of State Sanitary Engineers (1992) is 30 minutes at a flow-through velocity not less than 0.5 fpm or greater than 1.5 fpm. The velocity gradient varies from 10 to 100 s-1. The Gt value is an important design parameter, because the total number of particle collisions is proportional to the product of G and the detention time. Gt varies from 104 to 105. The power dissipated in the water by a paddle flocculator is calculated using the following equation: P = where: P CD A r vr
= = = = =
CDArvr 3 2
(9.3)
power dissipated in the water, ft # lb/sec 1N # m/sec2, coefficient of drag, equal to 1.8 for flat blades, dimensionless, area of paddles, ft 2 (m2), mass density of water, lb # s2/ft4 1kg/m32, and velocity of the paddles relative to water, fps (mps).
Normally vr is 0.50 to 0.75 of the paddle velocity 1vp2. The velocity of the paddle blade is calculated from the following equation: vp = 2prN
(9.4)
where: vp = velocity of the paddle blade, fps (mps), r = distance from the center of the shaft to the center of the blade, ft (m), N = rotational speed, revolutions/second.
213
214 Chapter 9
Water Treatment
EXAMPLE 9.2 Design of a flocculation basin A four-arm paddlewheel with the following configuration will be used in a flocculation basin. Each arm has two paddles 3.0 meters long by 0.1 meter wide.The distance from the shaft to the center of the blade is 1.0 meter for the inner paddle and 1.5 meters for the outer paddle.The minimum anticipated temperature of the water is 20°C with an absolute viscosity, m = 1.002 * 10-3 N # s/m2 and density, r = 998.2 kg/m3. The angular or rotational speed is 4 revolutions per minute (rpm). The flocculation basin treats 6.0 * 106 liters per day. Assume: vr = 0.75vp and CD = 1.8. a. Determine how much power (P) is dissipated into the water. b. Determine the Gt value for the flocculation basin if the dimensions of the flocculation basin are 4 meters long by 4 meters wide by 4 meters deep.
R2 1.5 m
R1 1 m
Solution part a vr = 0.75vp = 0.75 * 2prN Area of paddle blades = 0.1 m * 3 m * 4 blades = 1.2 m2 For the inner paddle: vr1 = 0.75 * 2p11 m2a
4 rpm b = 0.314 m/s 60 s/min
1.811.2 m221998.2 kg/m3210.314 m/s23 CDArvr 3 P1 = = = 33.4 W 2 2 For the outer paddle: vr2 = 0.75 * 2p11.5 m2a P2 =
4 rpm b = 0.471 m/s 60 s/min
1.811.2 m221998.2 kg/m3210.471 m/s23 CDArvr 3 = = 112.6 W 2 2
Total power dissipated to water is P1 + P2 = 146 W.
Section 9.5
Surface Water Treatment Processes
Solution part b Calculate the velocity gradient G using Equation (9.2) so that the Gt value can be estimated. V = 4 m * 4 m * 4 m = 64 m3
G = a
W b m
0.5
= a
P b mV
0.5
= §
146 W N#s 1.002 * 10-3 2 * 164 m32 m
0.5 N#m s ¥¥ * § 1W
1
G = 47.7 s-1 u =
V 64 m3 1000 L 24 h 60 min 60 s * ¢ = ≤¢ ≤¢ ≤¢ ≤ = 922 s 6 3 Q 1d 1h 1 min 6 * 10 lpd 1m
Gt = 47.7 s-1 * 922 s = 4.4 * 104
OK because between 104 and 105
9.5.4 Water Softening Water softening is a unit process involving the addition of chemicals to water for the removal of ions that cause hardness. Although water softening is primarily used for treating groundwater, it is discussed here because the process is similar to adding coagulants to surface water for the removal of color and turbidity. Water hardness is caused by polyvalent metallic cations, principally calcium 1Ca2+2 and magnesium 1Mg2+2. Other divalent metallic cations that contribute to hardness include iron 1Fe2+2, manganese 1Mn2+2, and strontium 1Sr2+2. Hardness is not related to any health issue. It is undesirable because it produces scale in water heaters and pipes and increases consumption of soap and detergent. Hardness is expressed in units of mg/L of CaCO3 . Total hardness is generally calculated by adding the milliequivalents (meq) of the calcium and magnesium ions together and then multiplying by the equivalent weight of calcium carbonate, 50. There are 50 milligrams of CaCO3 per milliequivalent or 50 grams of CaCO3 per equivalent. Classification of water hardness is as follows (Sawyer et al., 1994): soft, 0–17 mg/L as CaCO3 ; moderately hard, 75–150 mg/L as CaCO3 ; hard, 150–300 mg/L as CaCO3 ; and very hard, 7300 mg/L as CaCO3 . Chemical Precipitation
Chemical precipitation through the addition of lime (CaO) and soda ash 1Na2CO32 is the primary means of removing hardness from water. The extent of chemical precipitation of hardness is based on the solubilities of calcium carbonate 1CaCO32 and magnesium hydroxide 1Mg1OH222. The solubility products of CaCO3 and Mg1OH22 are presented in Equations (9.5) and (9.6) below (Sawyer et al., 1994). 3Ca2+43CO32-4 = Ksp = 5 * 10-9
(9.5)
3Mg2+43OH-42 = Ksp = 9 * 10-12
(9.6)
The solubility, and therefore the precipitation of CaCO3 and Mg1OH22 , is pH dependent. The optimum pH for CaCO3 precipitation is from 9 to 9.5, whereas for
215
216 Chapter 9
Water Treatment
CO2
Lime
Flocculator/ clarifier
Recarb
Filtration
CO2 CaCO3 Mg(OH)2
Figure 9.9 Schematic of two-stage lime-soda ash softening WTP.
Soda ash
Flocculator/ clarifier
Recarb
CaCO3
Mg(OH)2 precipitation, the pH must be greater than 11.0. Typically, 1.25 meq/L of lime are added above the stoichiometric quantity to raise the pH above 11. Adding lime increases the Ca2+ concentration in the water, thus forcing the concentration of CO23 to be lowered according to Equation (9.5), which is accomplished by the precipitation of CaCO3 . Also, lime addition raises the pH of the water, causing a shift in the solubility of Mg1OH22 according to Equation (9.6), resulting in the precipitation of Mg1OH22 . The addition of soda ash also raises the pH causing the precipitation of Mg1OH22 as well. When excess lime treatment is used, the residual total hardness is about 40 mg/L as CaCO3 . Approximately, 30 mg/L as CaCO3 is associated with CaCO3 solubility, and 10 mg/L as CaCO3 is associated with Mg1OH22 solubility. Figure 9.9 is a schematic of a two-stage softening process using chemical precipitation. Solids contact units are shown in lieu of the conventional coagulation, flocculation, sedimentation process that uses rapid mixers and flocculation followed by settling basins. Solids contact units provide all three functions in one compact unit. Lime-Soda Ash Softening Equations
The stoichiometric equations involved in the lime-soda ash water-softening process are presented below. Although the reactions are presented sequentially, in reality, they proceed simultaneously. The arrow 1T2 denotes the removal of a solid precipitated either as CaCO3 or Mg1OH22 . When quicklime (CaO) is added to water, it forms hydrated or slaked lime 1Ca1OH222 according to Equation (9.7). This is an exothermic reaction (it generates heat). CaO + H2O : Ca1OH22
(9.7)
For groundwaters, carbon dioxide is often present, and it will be in equilibrium with carbonic acid 1H2CO32: CO2 + H2O : H2CO3
(9.8)
Before any hardness removal can occur, any lime that is added will first neutralize any carbonic acid present according to Equation (9.9). The carbonic acid is precipitated as CaCO3 . H 2CO3 + Ca1OH22 : CaCO3 T + 2 H 2O
(9.9)
Lime is added for the precipitation of calcium and magnesium carbonate hardness. Equations (9.10) through (9.12) show reactions associated with the removal of
Section 9.5
Surface Water Treatment Processes
carbonate hardness. Equation (9.10) indicates that carbonate hardness, Ca1HCO322 , is precipitated as CaCO3 by adding lime, Ca1OH22 . Ca1HCO322 + Ca1OH22 : 2 CaCO3 T + 2 H2O
(9.10)
Equation (9.11) shows that carbonate hardness due to magnesium bicarbonate, Mg1HCO322 , is converted to magnesium carbonate 1MgCO32, which is soluble, so the hardness of the water does not change. Mg1HCO322 + Ca1OH22 : CaCO3 T + MgCO3 + 2 H2O
(9.11)
The magnesium associated with MgCO3 can then be precipitated as Mg1OH22 and the calcium associated with lime is precipitated as CaCO3 according to Equation (9.12). MgCO3 + Ca1OH22 : CaCO3 T + Mg1OH22 T
(9.12)
Equations (9.13) through (9.16) show how noncarbonate hardness is precipitated and therefore removed. Noncarbonate hardness due to MgSO4 can be removed only by adding both lime and soda ash. Equation (9.13) indicates that magnesium associated with MgSO4 is converted to Mg1OH22 and that the calcium associated with lime is converted to CaSO4 with the net result of no hardness being removed. MgSO4 + Ca1OH22 : CaSO4 + Mg1OH22 T
(9.13)
Noncarbonate hardness in the form of MgCl2 is removed by adding both lime and soda ash according to Equations (9.14) and (9.16). Equation (9.14) indicates that the magnesium associated with MgCl2 is converted to Mg1OH22 and that the calcium associated with lime remains in solution as CaCl2 . Therefore, the hardness of the water remains unchanged. Soda ash then must be added according to Equation (9.16) so that the calcium associated with CaCl2 is precipitated as CaCO3 . MgCl 2 + Ca1OH22 : Mg1OH22 T + CaCl 2
(9.14)
Noncarbonate hardness in the form of CaSO4 can be removed only by adding soda ash 1Na2CO32, which results in the precipitation of CaCO3 : CaSO4 + Na21CO23 : CaCO3 T + Na2 SO4
(9.15)
CaCl 2 + Na21CO23 : CaCO3 T + 2 NaCl
(9.16)
Excess lime treatment is often practiced, wherein 1.25 milliequivalents per liter (meq/L) of lime are added above the amount predicted by the stoichiometric equations to ensure that the pH of the water is raised above 11. Recarbonation is the process of adding carbon dioxide to lower the pH and stabilize the water. Equation (9.17) shows that excess lime, Ca1OH22 , is converted to CaCO3 by adding carbon dioxide. This reaction lowers the pH from around 11 to about 10.2 (Viessman and Hammer, 2005): Ca1OH22 + CO2 : CaCO3 T + H2O
(9.17)
217
218 Chapter 9
Water Treatment
Further recarbonation of the water according to Equation (9.18) converts any remaining CaCO3 that did not precipitate to Ca1HCO322 . This reaction results in a final pH in the range of 9.5 to 8.5 (Viessman and Hammer, 2005). CaCO3 + CO2 + H 2O : Ca1HCO322
(9.18)
EXAMPLE 9.3 Excess-lime soda-ash softening problem Calculate the lime and soda-ash requirements to achieve the practical limits of hardness removal given the following water analysis. Develop a bar graph showing the original theoretical species in the water. CO2 = 8.8 mg/L
alkalinity 1HCO3-2 = 135 mg/L as CaCO3
Na+ = 13.7 mg/L
Ca2+ = 40 mg/L
SO24 = 29 mg/L
Mg2+ = 14.7 mg/L Cl- = 17.8 mg/L
The following table presents concentrations, equivalent weights, and milliequivalents of all species analyzed. Component
Concentration (mg/L)
Ca
2+
Mg
+
Alkalinity
1HCO3-2
SO24 Cl
-
Concentration (meq/L)
22
8.8/22 =
0.40
20
40/20 =
2.00
14.7
12.2
14.7/12.2 =
1.20
13.7
23
13.7/23 =
0.60
135
50
135/50 =
2.70
29
48
29/48 =
0.60
17.8
35.5
17.8/35.5 =
0.50
8.8
2+
Calculation
40
CO2/H2CO3
Na
Equivalent weight
Solution Figure 9.10 is a bar graph showing the theoretical combination of species in the raw water. If CO2 is present, it is placed on the left side of the bar graph. Cations are always placed at the top of the bar graph; calcium, magnesium, and sodium are shown in this order. Anions are shown at the bottom of the bar graph and should be sequenced as follows: hydroxide, carbonate, bicarbonate, sulfate, and chloride. The water should be electrically neutral, so the concentration of cations
Figure 9.10 Bar graph of raw water for Example 9.3.
0.4
0.0
2.0 Ca2
3.2 Mg2
3.8 Na
CO2 SO42
HCO3 0.4
0.0
2.7
Cl 3.3
3.8
Section 9.5
Surface Water Treatment Processes
219
must balance the anions. If not, the water analysis is suspect. Viessman and Hammer (2005) present the details of developing bar graphs. The accompanying table presents the theoretical species, along with the dosages of lime as calcium oxide (CaO) and soda ash as sodium carbonate 1Na2CO32 that must be added to achieve a residual hardness of approximately 40 mg/L as CaCO3 . Excess-lime treatment is assumed, which involves adding 1.25 meq/L of lime as CaO (equivalent to 35 mg/L as CaO) beyond the stoichiometric dosage. Equations (9.11) and (9.12) must both be used to determine the quantity of lime required for precipitating magnesium bicarbonate. For each mole of Mg1HCO322 , 2 moles of lime are required. Similarly, Equations (9.13 and 9.15) must be used to determine the quantity of lime and soda ash required for removing magnesium sulfate.
Component (1)
Equation (2)
Concentration (3) (meq/L)
CaO (4) (meq/L)
Na2CO3 (5) (meq/L)
CO2
(9.9)
0.40
0.40
0
Ca1HCO322
(9.10)
2.00
2.00
0
Mg1HCO322
(9.11)
0.70
0.7
0
(9.12) MgSO4
0.7
(9.13)
0.50
(9.15)
0.5
0
0
0.5
Excess lime CaO
1.25
0
Total
5.55
0.5
The total lime dosage as CaO, assuming 100% purity, is the sum of all species in Column 4:
5.55
lb>MG meq mg CaO lb * 28 * 8.34 = 1296 meq L mg>L MG
where MG denotes million gallons. The total soda-ash dosage as Na2CO3 , assuming 100% purity, is the sum of all species in Column 5:
0.5
lb>MG meq mg Na2CO3 lb * 53 = 221 * 8.34 meq L mg>L MG
If the purities of CaO and Na2CO3 are both 90%, the total dosages for each are as follows: Lime:
1296 lb>MG 0.90
= 1440
lb MG
Soda ash:
221 lb>MG 0.90
= 246
lb MG
220 Chapter 9
Water Treatment
Other Softening Methods
Ion exchange is another technique used for removing hardness from water. It is extensively used in households for removing hardness. Calcium (Ca2) and magnesium 1Mg2+2 ions are replaced by sodium ions 1Na+2 by passing water through a kaolinite or montmorillonite bed, or through a synthetic material such as a polymeric resin. Softening can also be accomplished when water is passed through a semipermeable membrane, such as those used in reverse osmosis, ultrafiltration, or other membrane processes. Information on these methods may be found in MWH, 2005, and Reynolds and Richard, 1996. 9.5.5 Sedimentation Sedimentation is a unit operation involving solids-liquid separation by gravitational settling to remove suspended solids. In water treatment, two types of settling are encountered: Type I—free or discrete settling, and Type II—flocculent settling. Each will be discussed, and examples will be presented illustrating the procedure for designing settling basins. Type I—Free or Discrete Settling
Free or discrete settling is the settling of discrete, nonflocculent particles whose size, shape, and density do not change with time as they settle. Particles settle as individual entities, and there is no interaction between particles. Examples of discrete settling include grit and sand particles in grit-removal systems and plain sedimentation (no coagulation) of surface waters. The settling velocity of a spherical particle (Newton’s law) is calculated as follows: Vs = B
0.5 4 g rp - rw ¢ ≤dR rw 3 CD
(9.19)
where: Vs g CD m rp rw d
= = = = = = =
settling velocity, fps (m/s), acceleration of gravity, ft/s2 1m/s22, coefficient of drag (dimensionless), mass of particle, lb (kg), density of particle, lb # s2/ft4 1kg/m32, density of liquid, lb # s2/ft4 1kg/m32, and diameter of particle, ft (m).
The coefficient of drag 1CD2 is a function of the flow regime, which is estimated by calculating the Reynolds number 1NR2. NR = where:
rwVsd Vsd = m v
(9.20)
m = absolute or dynamic viscosity of water, lb # s/ft2 1kg/m # s2, and v = kinematic viscosity of water, ft2/s 1m2/s2.
When the Reynolds number is 61, laminar flow conditions exist, and CD is calculated using Equation (9.21). CD =
24 NR
(9.21)
Section 9.5
Surface Water Treatment Processes
During transitional flow between laminar and turbulent, NR = 1 to 104, Equation (9.22) is used to determine CD . CD =
24 3 + + 0.34 NR 1NR20.5
(9.22)
The coefficient of drag is assumed to be equal to 0.4 for turbulent flow when NR 7 104. For laminar flow conditions, Newton’s law simplifies to Stokes’ law by substitution of 24/NR for the coefficient of drag, resulting in Equations (9.23) and (9.24). Vs =
Vs =
g1rp - rw2d2 18m g1SGp - 12d2 18v
(9.23)
(9.24)
Type II—Flocculent Settling
Flocculent settling occurs in primary clarifiers and during settling of coagulated water and wastewater. As flocculent particles settle, their size, shape, density, and settling velocity will change over time. Newton’s law or Stokes’ law cannot be used for modeling flocculent settling. Normally, a settling-column analysis is performed at a specific suspended solids concentration. Samples are withdrawn at various depths along the column, and the percent removal of suspended solids is plotted as a function of depth and time. Suspended solids (SS) removal is calculated using Equation (9.25). SS removal 1%) =
1SSinitial - SSfinal2 * 11002 SSinitial
(9.25)
Figure 9.11 shows a settling column with sampling ports. The curves in the plot represent isoremoval lines (SS removal %). Equation (9.26) is used for calculating the overall percent removal of solids that may be achieved at a given detention time. RT 1%2 = R0 + 1©¢R * Zi2/Z0
(9.26)
where: RT = total fraction of suspended solids removed at a given detention time, %, R0 = isopercent removal line intersecting the time axis at specified settling column depth, %, ¢R = difference between adjacent isopercent removal lines, %, Zi = distance from top of settling column or water level in column to the midpoint between two adjacent isopercent removal lines, ft (m), and Z0 = total depth of settling column, ft (m). Ideal Settling Basin
Figure 9.12 shows an ideal settling basin without the inlet, outlet, and sludge settling zones. The derivation of the overflow rate for settling basins is presented in this section. Overflow rate is both a design and operational parameter. It is used by engineers
221
222 Chapter 9
Water Treatment
0.0
0
10
20
30
40
50
60
Depth, m
0.5 70%
1.0
1.5 60% 2.0
20%
2.5 0.0
10
30%
20
40%
30 Time, min
50%
40
50
Figure 9.11 Settling column and plot.
Q
Q Vh
Influent
Effluent
H Vs
Figure 9.12 Ideal settling basin.
L
to determine the size of settling basins. Water treatment plant operators regulate the overflow rate of settling basins to ensure optimal removal of suspended solids. The overflow rate 1Vo2 is just another way of expressing the settling velocity 1Vs2 associated with a particle having a specific diameter. Recall that the settling velocity of a particle is primarily a function of the particle diameter, as expressed in Equations (9.23) and (9.24). Engineers select an overflow rate (which actually relates to a particle having specific size or diameter) and assume that particles equal or larger in diameter will theoretically be 100% removed. The derivation of the overflow rate follows. Recall that distance traveled is equal to the rate or velocity times time 1D = R * t2. The time that a particle remains in a settling basin is expressed by Equations (9.27) and (9.28). t = H>Vs
(9.27)
t = L>Vh
(9.28)
Section 9.5
Surface Water Treatment Processes
The continuity equation states that the volumetric flow rate (Q) is equal to the cross-sectional area multiplied by the velocity of flow: Q = AV
(9.29)
The horizontal flow-through basin velocity is expressed as:
Vh =
Q Q = A WH
(9.30)
where: t H L W Q V Vh Vs A
= = = = = = = = =
settling time, min, hours, depth of settling basin, ft (m), length of settling basin, ft (m), width of settling basin, ft (m), flow rate through basin, ft3/s 1m3/s2, average flow velocity, fps (mps), horizontal flow-through basin velocity, fps (mps), settling velocity, fps (mps), and cross-sectional area = W * H, ft2 1m22.
To derive the overflow rate equation, Vo is substituted into Equation (9.27) for Vs as follows: t =
H H = Vs Vo
(9.31)
Next, substitute Vh from Equation (9.30) into Equation (9.28), yielding: t =
L L = Vh Q>1WH2
(9.32)
Equating Equations (9.31) and (9.32) yields: t =
H L = Vo Q>1WH2
(9.33)
Rearranging Equation (9.33) results in the following: Vo =
HQ Q Q = = LWH LW As
(9.34)
The overflow rate equation is presented as Equation (9.35). Vo =
Q As
where: As = surface area of settling basin = L * W, ft2 1m22.
(9.35)
223
224 Chapter 9
Water Treatment
The detention time is defined as the time that the water remains in the settling basin. Normally the Greek letter theta 1u2, rather than t, is used for denoting detention time. Substituting Vh from Equation (9.30) into Equation (9.28) results in the detention-time equation as presented below: u = t =
L V LHW = = Vh Q Q
(9.36)
where V = volume of the tank or basin, ft3 1m32. Weir Loading Rate
The weir loading rate is defined as the volumetric flow rate divided by the length of the effluent weir. Regulatory agencies sometimes specify weir loading rates that must be maintained in settling basins. Mathematically, weir loading rate is given by: weir loading rate =
Q weir length
(9.37)
Settling-Basin Design Criteria
Typically, in water treatment, long rectangular or square settling basins are used, but circular basins also may be used. Solids contact basins in which mixing, coagulation, flocculation, and settling occur all in the same unit are now being used extensively in water treatment facilities.The actual design of a settling basin includes not only the ideal settling zone, but an inlet zone to equally distribute the incoming water and dissipate flow velocity; a sludge zone in which particles that settle out of solution accumulate and are subsequently removed for further processing; and an outlet zone in which baffles and weirs are used to ensure that no short circuiting occurs and quiescent conditions are maintained. Figure 9.13 shows a schematic of a long rectangular basin with the four zones.The dashed line represents the trajectory of a particle settling at the design overflow rate 1Vo2. In designing settling or sedimentation basins, environmental engineers use three main parameters. In order of importance, these are overflow rate, detention time, and weir loading rate. The overflow rate is the critical parameter that must be selected in order to properly design a settling basin. Overflow rates vary depending on the type of chemicals used during treatment. Precipitates or flocs from softening (lime and soda ash) are denser than those from coagulation (alum) and thus have a higher settling velocity. Knowing the design flow rate (the maximum daily flow for the plant) and selecting an overflow rate, the engineer can determine the surface area. Q
Q
Settling zone Vh
H
Vs Sludge zone L
Outlet zone
Influent Inlet zone
Figure 9.13 Schematic of settling basin with inlet, settling, outlet, and sludge zones.
Effluent
Section 9.5
Surface Water Treatment Processes
225
Table 9.1 Design Criteria for Settling Basins Parameter Overflow rate 1Vo2, m3/1d # m22 Overflow rate 1Vo2, gpd/ft2 Detention time, h Weir loading rate, m3/1d # m2 Weir loading rate, gpd/ft
Alum flocs
Iron flocs
Lime-soda flocs
20 to 33
29 to 41
29 to 61
500 to 800
700 to 1000
700 to 1500
4 to 6
4 to 6
4 to 8
150 to 220
200 to 270
270 to 320
12,000 to 18,000
16,000 to 22,000
22,000 to 26,000
Source: Reynolds and Richards (1996)
Next, the detention time is chosen and the depth determined by dividing the volume by the surface area. The weir loading rate is calculated after the planning of the weir configuration. Table 9.1 lists typical design criteria for settling basins. Rectangular basins normally have a length-to-width (L:W) ratio of 2:1 to 4:1 and a length-to-depth (L:D) ratio of 10:1 to 20:1. Bottom slopes are 1/100. Settling-basin depths range from 8 to 10 feet (2.5 to 3 m) for discrete particles and from 10 to 13 feet (3 to 4 m) for flocculent particles. Square tanks have dimensions ranging from 35 to 200 feet (11 to 91 m) and depths ranging from 6 to 19 feet (2 to 6 m). Where circular tanks are used, tank diameters of 15 to 300 feet (4.5 to 91 m) may be used. Normally, however, diameters … 200 feet 1… 61 m2 are used, and depths are usually 6 to 16 feet (2 to 5 m). All depths reported are in terms of side water depth (SWD), which is the depth of water at the side of the tank from the top of the effluent weir to the bottom of the tank. An additional 1 to 2.5 feet (0.3 to 0.8 m) must be added to the SWD to account for flow variations, which are relatively insignificant in water treatment plant design but are appreciable in wastewater applications. Figure 9.14 is a photo of a long rectangular settling basin.
Figure 9.14 Photo of long rectangular settling basin.
226 Chapter 9
Water Treatment
EXAMPLE 9.4 Rectangular settling basin example Two rectangular settling basins, each 90 ft long, 16 ft wide, and 12 ft deep, are operated in parallel to settle 1.5 MGD of water (0.75 MGD to each basin). The effluent weir length in each basin is equal to 3 tank widths. Determine: a. b. c. d. e.
Detention time. Horizontal flow velocity. Surface overflow rate. Weir length. Weir loading rate.
Solution part a The detention time is determined using Equation (9.1). u =
V 90 ft * 16 ft * 12 ft 7.48 gal 24 h = b = 4.14 h ¢ ≤a Q d 0.75 * 106 gal>d ft3
Solution part b The horizontal flow-through velocity is calculated using Equation (9.30). Vh =
0.75 * 106 gal>d Q ft3 1d 1h = ¢ ≤¢ ≤¢ ≤ = 0.36 fpm A 16 ft * 12 ft 7.48 gal 24 h 60 min
Solution part c The overflow rate 1Vo2 is determined from Equation (9.34). Vo =
0.75 * 106 gal>d Q gpd = = 521 2 As 90 ft * 16 ft ft
Solution part d Next, the weir length is estimated so that the weir loading rate may be determined: weir length per basin = W * L = 3 * 16 ft = 48 ft Solution part e Finally, the weir loading rate is calculated. weir loading rate =
Q 0.75 * 106 gpd gpd = = 15,625 WL 48 ft ft
9.5.6 Filtration Filtration is a unit operation that involves the separation of nonsettleable solids from water or wastewater by passing the water through a porous medium. In conventional filtration, water must be coagulated, flocculated, and settled
Section 9.5
Surface Water Treatment Processes
227
before passing through the filter. Direct filtration of water is sometimes practiced in which coagulated water is applied directly to filters without having undergone sedimentation. Flocculation may or may not be included in the treatment train. Filters are generally classified according to the types of media used. Single, or monomedia filters, have only one type of media, which is usually sand or crushed anthracite coal. Dual-media filters typically consist of a layer of coarse anthracite coal above a layer of sand. Multimedia filters, such as trimedia filters, typically consist of a layer of coarse anthracite coal above a layer of sand and a layer of garnet below the sand. Gravity-Type Granular Media Filtration
Gravity filters consisting of dual or multiple layers of coarse granular media are the type most widely used in water treatment. Rapid sand filters consisting of 24 to 30 inches (61 to 76 cm) of sand supported by 15 to 24 inches (38 to 61 cm) of layered gravel are commonly used in the United States. As the water passes through under gravity flow, coagulated colloidal and fine solids are removed by interception, straining, flocculation, or sedimentation. The primary goal is to achieve indepth filtration, wherein the coagulated particles penetrate and use the entire filter bed for solids removal rather than being strained at the surface. Eventually the head loss through the bed will build up to 8 to 10 feet (2.4 to 3 m) of water column, at which time the filter must be backwashed to remove the particles and turbidity that have accumulated. Backwashing involves reversing the flow of water back up through the bed, causing it to expand and thereby release the trapped particles. Auxiliary air and water jets may be used during backwashing to help scour the filter media, releasing more particles. Figure 9.15 is a cross-sectional view of a rapid sand filter. Coagulated water flows down through the filter (sand, gravel, and underdrains) and into the clear well. Chlorine is added to the water as it enters the clear well, so that sufficient contact time is available for the chlorine to inactivate any pathogens that may remain in the water. The flow-control valve regulates the flow rate through the filter. Typically, filters are operated to maintain a constant flow or production of water. This is
Figure 9.15 Cross-sectional view of a rapid sand filter during filtration.
V-1
Wash-water troughs
Coagulated water
Water depth 3 to 4 ft
Sand 24-30 in V-2
Gravel 15-20 in
To washwater recovery ponds
V-3
FC V-4 Underdrains Clear well
228 Chapter 9
Water Treatment
accomplished by partially opening the flow-control valve (FCV) and, as the filter run progresses, opening the valve wider to allow the same amount of flow as the head loss increases. Although not shown, water from the clear well is pumped throughout the water distribution system using high-service pumps. Other valves shown in Figure 9.15 are either open or closed during the filtration and backwashing cycles. The underdrain system is approximately 1 foot (0.3 m) in depth. The static water head on the sand media generally varies from 3 to 4 feet (0.9 to 1.2 m). Head Loss Through a Clean Filter
The Carman-Kozeny equation (Carman, 1937; Kozeny, 1927) gives the head loss for a bed of porous media of different particle sizes as follows: hL =
X L 11 - e2 V2a œ ij ©f w g di j e3
(9.38)
Equation (9.39) is used to calculate the friction factor, fœ : fœ = 150 ¢
1 - e ≤ + 1.75 NR
(9.39)
where: hL L e w Va
= = = = =
g di j Xi j NR fœ
= = = = =
frictional head loss, ft (m), depth of filter bed, ft (m), porosity of bed, dimensionless, dimensionless shape factor for different types of media, filtration velocity or velocity of approach, total flow applied to the filter divided by the filter area, fps (mps), gravitational acceleration, ft/s2 1m/s22, particle size = geometric mean of adjacent sieve sizes, ft (m), weight fraction of media retained on adjacent sieve sizes, dimensionless Reynolds number, and Carman-Kozeny dimensionless friction factor.
Shape factors for several types of media are as follows: pulverized coal = 0.73, angular sand = 0.73, rounded sand = 0.82, and Ottawa sand = 0.95.
EXAMPLE 9.5 Head loss across a filter of nonuniform particles Determine the head loss for a clean filter bed using the Carman-Kozeny equation for a stratified bed with uniform porosity of 0.45. The rapid sand filter is made up of a 24-inch-deep bed of sand, specific gravity 2.65, shape factor ( ) 0.82, temperature of 50°F, and filtration rate of 2.5 gpm/ft2. A sieve analysis is presented below.
Section 9.5
Surface Water Treatment Processes
f¿ X ij ij
% Sand retained
di j
(2) 1Xi j2
(3) 1ft * 10-32
NR
(1)
(4)
(5)
14–20
0.87
3.2883
1.07E +00
7.92E +01
20–28
8.63
2.333
7.56E -02
1.11E +02
4103
28–32
26.30
1.779
5.76E -01
1.45E +02
21422
32–35
30.10
1.500
4.86E -01
1.72E +02
34420
35–42
20.64
1.258
4.08E -01
2.04E +02
33501
42–48
7.09
1.058
3.43E -01
2.42E +02
16248
48–60
3.19
0.888
2.88E -01
2.89E +02
10365
60–65
2.16
0.746
2.42E -01
3.43E +02
9935
65–100
1.02
0.583
1.89E -01
4.39E +02
7673
Sieve
f¿ ij
Solution At a temperature of 50°F, v = 1.410 * 10-5 ft2/sec. First, calculate the approach velocity as follows: Va = 2.5
gpm 2
ft
¢
1 ft3 1 min ≤¢ ≤ = 5.57 * 10-3 fps 7.48 gal 60 s
For sieves 14–20, the Reynold’s number is calculated as follows, using Equation (9.20). The shape factor 1w2 must be included in Equation (9.20) to account for particles that are not completely spherical. 0.8215.57 * 10-3 fps213.2883 * 10-3 ft2 fVd = = 1.065 v 1.410*10-5 ft2>s
The new friction factor, fœ, is calculated using Equation (9.39) as follows: fœ = 150 ¢
1 - e 1 - 0.45 b + 1.75 = 79.2 ≤ + 1.75 = 150a NR 1.065
Calculate the following quantity: a
fiœ jXi j di j
=
fiœ jXi j di j
=
79.210.87>1002 3.2883*10-3 ft
= 210
The values in the table were calculated using a spreadsheet; hand calculations may differ slightly, because of round-off error. Next calculate the head loss in the clean filter using Equation (9.38) as follows. hL = hL =
210
© = 137,876
100
NR =
di j (6)
L11 - e2V2a fe3 g
a
fiœ jXi j di j
2 ft11 - 0.45215.57 * 10-3 fps22 10.822 0.453 132.2 ft>s22
1137,8762 = 1.96 ft
The head loss of a clean filter typically ranges from 0.5 to 1.5 feet (0.15 to 0.46 m), depending on filtration rate and depth of media.
229
230 Chapter 9
Water Treatment
Backwashing of Filters
Granular media filters must be backwashed to remove particulates and turbidity that build up during filter operation.Typically, operators backwash a filter when: a) the head loss through the filter reaches 8 to 10 feet (2.4 to 3 m), or b) the turbidity in the effluent reaches a specific level, such as 0.25 NTU, or c) the filter has been in operation for a specified amount of time (such as 24 to 48 hours). The water used for the backwashing process is filtered, disinfected, and stored in a tank that is either elevated, at ground level, or underground. Figure 9.16 is a cross-sectional view of a rapid sand filter during the backwashing process. High-pressure backwash pumps are necessary if an elevated backwash storage tank is not used. Backwash water at a rate of around 15 to 20 gpm/ft2 (611 to 815 lpm/m2) for 15 to 20 minutes is required to properly expand and clean a dirty filter. Approximately 1% to 5% of the filtered water is used during the backwashing process. The dirty washwater is collected in the backwash water troughs and transported to the washwater recovery pond, where the solids are settled out of solution and the supernatant is returned to the head of the water treatment plant. Filter Media
Filter media is characterized by its effective size and uniformity coefficient. The effective size 1de2 is defined as the 10-percentile diameter, or the sieve size in mm that will pass 10% (by weight) of the filter media. The uniformity coefficient is the ratio of the 60-percentile to 10-percentile diameters. Table 9.2 lists the effective size, uniformity coefficient, depth of media (D), and media or bed depth to effective size 1D/de2 ratios for various types of filter media. The specific gravities of sand, anthracite, and garnet are 2.65, 1.4 to 1.6, and 4 to 4.1, respectively. Silica sand and anthracite coal are the most widely used media types. The D/de ratio is used to estimate the depth of each layer of media, once the effective size has been selected. Figure 9.17 is a schematic showing the media configurations of sand, dual-media, and mixed-media filters. Number of Filters
For water treatment plants processing 62.0 MGD 188 L/s2, a minimum of two filters should be used. If the capacity of the WTP exceeds 2.0 MGD, the minimum should be four filters. Equation (9.40) is used for estimating the number of filters (N) required as a function of flow.
Figure 9.16 Cross-sectional view of a sand filter during backwash.
V-1 Backwash troughs
Backwash water
V-2 Washwater to recovery pond
V-3 FC V-4
Clear well
Section 9.5
Surface Water Treatment Processes
231
Table 9.2 Filter Media Specifications Effective size (de), mm
Depth, inches
Uniformity coefficient
1.0 0.5
20 10
1.45 1.30
1016
Anthracite, sand
1.48 0.5
30 15
1.50 1.20
1023
Large dual-media
Anthracite, sand
2.0 1.0
40 20
1.50 1.25
1016
Mixed-media
Anthracite, sand, garnet
1.0 0.42 0.25
18 9 3
1.45 1.50 1.25
1306
Monomedia
Anthracite
1.0
40
1.4
1016
Filter type
Material
Small dual-media
Anthracite, sand
Intermediate dual-media
D/de
Source: Kawamura (1991) Page 200.
Influent
Influent
Influent
Fine
Coarse
Coarse
Coal 28-48 in
Sand
30-40 in
24-36 in
Coal
Sand Finer
Sand Garnet Coarse
Fine
Conventional sand
Dual media
N = 1.2Q0.5
Finest
Figure 9.17 Filter media configurations.
Triple media
(9.40)
where: N = total number of filters, and Q = maximum daily flow rate, MGD. For large WTPs processing over 20 MGD 175,700 m3/day2, filters have 2 cells each with a gullet running down the center. Figure 9.18 shows a layout of a single filter for a large WTP. Size of Filters
According to Kawamura (1991), conventional gravity filters use length-to-width (L:W) ratios of 2:1 to 4:1. Surface areas of gravity filters typically range from 250 to 1000 ft2 (25 to 100 m2), and filter depths range from 12 to 20 feet (3.2 to 6 m).
232 Chapter 9
Water Treatment
Cell 1
Cell 1
Cells 1 and 2 comprise one filter
Cell 1
Settled water
Gullet
Cell 2
Figure 9.18 Filter layout for large water treatment plant.
Cell 2
Cell 2
Filtration Rates
Filtration rates depend on the type of media used in design. Kawamura (1991) recommends the following filtration rates: • • • •
Medium sand filters: 2 to 3 gpm/ft2 (5 to 7.5 m/h) Coarse sand filters: 4 to 12 gpm/ft2 (10 to 30 m/h) Dual and multimedia filters: 4 to 10 gpm/ft2 (10 to 25 m/h) Granular activated-carbon (GAC) filters: 3 to 6 gpm/ft2 (7.5 to 15 m/h)
EXAMPLE 9.6 Filter dimensions and backwash quantity A water treatment plant has four rapid sand filters. Each filter is designed for a capacity of 1 MGD. Backwashing is accomplished for 9 minutes at a rate of 15 gpm/ft2 once every 24 hours. The terminal head loss prior to backwashing averages 4 to 10 feet. Determine: a. The filter dimensions if an application or filtration rate of 3.5 gpm/ft2 is used. b. The quantity of water required for backwashing and percentage of filtered water used in backwashing. Solution part a Determine the surface area 1As2 of the filters by dividing the design flow rate by the filtration rate 13.5 gpm/ft22 and making appropriate conversions. As =
1 * 106 gal/d
13.5 gal/min # ft2211440 min/d2
= 198 ft2
For a square filter, L = W, so: W2 = 198 ft2 W = 2198 = 14.1 ft
Use W = L = 14 ft
Section 9.5
Surface Water Treatment Processes
Solution part b The backwash flow rate 1Qb2 is estimated by multiplying the backwash velocity or rate by the filter surface area 1As2 times the backwash duration (9 min/day). Qb = ¢
15 gal>min 2
ft
≤ 114 ft * 14 ft2a
9 min b = 2.65 * 104 gpd d
The percentage of water used in the backwashing process is estimated by dividing the backwash flow 1Qb2 by the product flow of 1.0 MGD: percentage =
2.65 * 104 gpd 1 * 106 gpd
1100%2 = 2.7%
The amount of water used for backwashing varies from 2% to 4% of the filtered water.
9.5.7 Disinfection Disinfection is a unit process in which a chemical is added to the treated water to oxidize residual organics and pathogens. The objective of disinfection is to kill pathogenic (disease-causing) microorganisms, whereas the objective of sterilization is to destroy all microorganisms. Conventional water treatment, which includes coagulation, flocculation, sedimentation, and filtration, removes over 90% of the pathogens found in water. Pathogens are also killed at high pH levels, therefore, the excess lime-soda ash process is effective. The conventional water treatment scheme described above along with disinfection as the last process will produce safe, potable water that will meet regulatory requirements. Selection of the appropriate disinfectant is essential, because some types are more effective at killing viruses versus bacteria or vice versa. The type of disinfectant used also affects the production of potential or suspected carcinogens, such as trihalomethanes (THMs) and haloacetic acids. The disinfectant selected should act as a powerful oxidizing agent in addition to providing a residual in the water distribution system to eliminate regrowth of pathogens. There are four major categories of human enteric pathogens: bacteria, viruses, protozoa, and helminths. Some of the diseases and associated organisms for each group are presented below. 1. Bacteria: typhoid fever is caused by Salmonella typhi; paratyphoid by Salmonella paratyphi; and cholera by Vibrio cholerae. 2. Protozoan parasites: amoebic dysentery is caused by Entamoeba histolytica; diarrhea and gastrointestinal problems by Cryptosporidium parvum and Giardia lamblia. 3. Helminth parasites: hookworm is caused by Necator americanus; roundworm by Ascaris lumbricoides; and whipworm by Trichurus trichiura. 4. Viruses: poliomyelitis is caused by the Poliovirus species; and hepatitis or inflammation of the liver by the Hepatitis A or Hepatitis E strain. Additional information on these pathogens may be found in MWH, 2005; Masters, 1997; and Viessman and Hammer, 2005.
233
234 Chapter 9
Water Treatment
Regulatory Requirements
The Surface Water Treatment Rule (SWTR) and Disinfection and Disinfection ByProducts Rule (D-DBP) are two important regulations promulgated by the EPA that have a significant impact on the disinfection process. These regulations specify certain removal levels for Giardia lamblia and viruses and therefore must be considered when selecting the appropriate disinfectant. Surface Water Treatment Rule Primary drinking water regulations have been established for surface waters and ground waters under the direct influence of surface water. Under these regulations, filtration and disinfection are required for all surface water sources and those groundwaters that are directly influenced by surface water. A 3-log reduction or 99.9% removal in Giardia lamblia and a 4-log reduction or 99.99% in viruses must be achieved. Disinfectants and Disinfection By-Products Rule: D-DBP
Under the Disinfection and Disinfection By-Products Rule there are limits required for haloacetic acids and trihalomethanes. The five haloacetic acids regulated (HAA5) include: monochloroacetic acid, dichloroacetic acid, trichloroacetic acid, monobromoacetic acid, and dibromoacetic acid. The maximum contaminant level (MCL) for HAA5 is 0.06 mg/L. Total trihalomethanes (TTHMs) consist of chloroform, dibromochloromethane, and bromoform. The MCL for TTHMs is 0.08 mg/L.
Chick’s Law
An effective disinfectant should act quickly (fast rate of kill) and provide a residual. The rate of kill is modeled as a first-order removal equation known as Chick’s law: Nt = N0e-kt
(9.41)
where: Nt N0 k t
= = = =
number of microorganisms at time t, number of microorganisms at time zero, rate constant, time-1, and contact time, time.
The rate constant (k) and contact time (t) necessary to achieve a specified kill are a function of the type of pathogen and the type of disinfectant used. The Ct parameter is useful for selecting the proper disinfectant and for designing tanks and reactors to accomplish a specified level of kill. Ct values are calculated by multiplying the concentration of disinfectant (mg/L) by the contact time (minutes). Research results on various types of disinfectants at different contact times have been published for a variety of pathogens (Reynolds and Richards, 1996). Figure 9.19 shows a plot of chlorine concentration as hypochlorous acid (HOCl) versus contact time (minutes) for various types of pathogens (three viruses and one bacterium). These types of figures allow environmental engineers several options when designing disinfection systems. For instance, to achieve a 99% kill of Escherichia coli, the design engineer could use a chlorine dosage of 0.3 mg/L at a contact time of 1 minute or use a chlorine dosage of 0.02 mg/L at a contact time of 10 minutes. Engineers must provide a flexible design, since a tank or reactor’s size is fixed upon construction. The only variable that can change is the disinfectant dosage. Therefore, the disinfectant feed system must be capable of achieving a wide range of dosages.
Section 9.5
Surface Water Treatment Processes
235
1.0
O
l
0.1
Cl H2
C
N
Chlorine concentration, mg/L
10.0
H
O
Cl
0.01
1
10
100
1000
Contact time, minutes
Mechanisms of Disinfectants
The effectiveness of the disinfection process, as previously discussed, is determined by both the concentration and contact time. Other parameters that influence pathogen kill include: pH, temperature, type of disinfectant, type of microorganism, and turbidity. Several mechanisms are proposed for killing pathogenic microorganisms. They include: 1. 2. 3. 4.
Destroying the cell walls Altering cell permeability Penetrating the cell, causing reactions with enzymes and protoplasm Damaging the cell’s DNA and RNA
Details on these mechanisms can be found in Crites and Tchobanoglous, 1998; Peavy, Rowe, and Tchobanoglous, 1985; and MWH, 2005. Chemistry of Chlorination
At most water and wastewater treatment plants, chlorine gas is injected into water under pressure to ensure dissolution of the gas and the formation of hypochlorous acid (HOCl). The hypochlorous acid dissociates into the hydrogen ion and the hypochlorite ion 1OCl-2. Both hypochlorous acid and the hypochlorite ion are strong disinfectants, producing free chlorine residual. However, they do not provide a long chlorine residual. The following two reactions illustrate the formation of HOCl and OCl-. HOCl is a more powerful oxidant than OCl-. The proportion of HOCl and OCl- is a function of pH. At 20ºC and a pH of 7.5, both HOCl and OClrepresent 50% of the chlorine concentration. Cl2 + H2O 4 HOCl + H+ + ClHOCl 4 H+ + OCl-
(9.42) (9.43)
Adding chlorine to water causes the pH to drop due to the production of hydrogen ions 1H+2. Calcium Hypochlorite and Sodium Hypochlorite
Calcium hypochlorite 3Ca1OCl224 and sodium hypochlorite [NaOCl] are two other forms of chlorine that can be added to water. When added to water according to the dissociation reactions listed below, they both produce the hypochlorite ion.
Figure 9.19 Chlorine concentration versus contact time for 99% kill of E. coli by various forms of chlorine.
236 Chapter 9
Water Treatment
Ca1OCl22 : Ca2+ + 2 OCl-
(9.44)
Na OCl : Na+ + OCl-
(9.45)
Chloramines
Frequently, ammonia is added to finished drinking water so that a long-lasting chlorine residual can be maintained through the formation of chloramines. Chloramines are not as strong of a disinfectant as hypochlorous acid and hypochlorite ion, but they maintain much longer residuals in the water distribution network. Analytical measurement of ammonia nitrogen measures both the ammonium nitrogen 1NH4+2 and ammonia gas 1NH32 dissolved in the sample. Ammonium dissociates according to the following reaction. NH 4+ Δ
H + + NH 3
(9.46)
The dissociation constant Ka for Equation (9.46) is 5.6 * 10-10 at 25°C. When chlorine is added to water containing ammonia, the following reactions occur and produce chloramines. The type of chloramine produced is pH dependent and is affected by the quantity of chlorine that is added. Trichloramine 1NCl32 is primarily produced below a pH of 4.4. Mono-chloramine 1NH2Cl2 and di-chloramine 1NHCl22 are produced between pH values of 4.5 and 8.5. At room temperature and a pH greater than 8.5, di-chloramine is the major form of chloramine produced. Chloramines are less powerful oxidants compared to HOCl and OCl-, however, they provide longer-lasting chlorine residual (combined chlorine residual). HOCl + NHCl2 : H2O + NCl3
(9.47)
HOCl + NH2Cl : H2O + NHCl2
(9.48)
HOCl + NH 3 : H 2O + NH 2Cl
(9.49)
The use of chloramines may result in the formation of N-nitrosodimethylamine (NDMA), a probable human carcinogen (MWH, 2005). Nitrification may cause problems in water distribution systems when chloramines are used rather than free chlorine residual. Breakpoint Chlorination
Some facilities practice breakpoint chlorination, which involves adding high dosages of chlorine to achieve a free chlorine residual, rather than a combined chlorine residual, as associated with chloramines production. Figure 9.20 shows the chlorine residual curve as a function of the chlorine dose applied. In Figure 9.20, moving from point A to B indicates that the chlorine added to the water reacts with reducing compounds such as nitrite 1NO2-2, ferrous iron 1Fe2+2, and hydrogen sulfide 1H2S2, resulting in a negligible chlorine residual. From point B to C, addition of more chlorine results in chloramine production and formation of combined residual. From point C to D, further chlorine addition results in the oxidation of chloramines, thereby reducing chlorine residual. Point D is known as the breakpoint, and addition of more chlorine results in free chlorine residual.
Section 9.5
Surface Water Treatment Processes
Chlorine residual
Applied Chlorine
C
Combined Cl2 residual D
Free Cl2 residual
B A
Chlorine dosage
Types of Chlorine Residual
There are two types of chlorine residual: free available chlorine residual and combined chlorine residual. A description of each type follows: a. Free available chlorine residual: Sum of the HOCl plus OCl- plus chlorine gas that may be dissolved in the water as aqueous chlorine 1Cl22aq . Aqueous chlorine is usually negligible. b. Combined chlorine residual: Chlorine that reacts with organic nitrogen and ammonia. Free available chlorine forms have greater oxidation potentials than combined chlorine forms. Rankings of chlorine species by disinfecting power and long-lasting residual are presented in Table 9.3. Problems in Using Chlorine
Chlorine is a toxic gas, poisonous to humans and animals, and there are safety concerns in handling and transporting it. Chlorine reacts with organic species in water to produce trihalomethanes and haloacetic acids, which are suspected carcinogens. Consumers also dislike the taste and odor associated with chlorine. Alternative Disinfectants
Four alternative means of disinfecting water are described below, together with advantages and disadvantages of each. Ozone Ozone 1O32 is an allotropic form of oxygen. It is a powerful oxidant that reacts with reduced inorganics and organic materials. Ozone is produced in a highstrength electrical field from oxygen in pure form or from the ionization of clean, dry air. Typical dosages range from 1 to 5 mg/L. No residual is produced, so following ozonation chlorine is added to provide one. A major advantage of using ozone is that trihalomethanes (THMs) are not formed.
Table 9.3 Ranking of Chlorine Species Disinfecting power
Long-lasting residual
1. HOCl
1. Chloramines
2. OCl-
2. OCl-
3. Chloramines
3. HOCl
Figure 9.20 Breakpoint chlorination curve.
237
238 Chapter 9
Water Treatment
Chlorine Dioxide Chlorine dioxide 1ClO22 is a powerful oxidizing agent that does not form chloroform or chloramines. It provides a stable chlorine residual, and typical dosages are 0.10 to 3.0 mg/L for terminal disinfection and up to 10 mg/L for controlling taste and odor problems. It is easily removed from water by aeration and readily decomposes by being exposed to UV radiation. A potential disadvantage is that it may reduce to chlorate, a potential carcinogen. UV Radiation
UV radiation with wavelengths in the range 245 to 285 nm can be produced by mercury-vapor arc lamps. UV mercury-vapor lamps are placed in quartz sleeves, which are then placed into the water. UV radiation is effective against bacteria and viruses, but turbidity and suspended solids inhibit it and should be removed prior to UV exposure. A typical dosage is 24,400 mwatts # sec/cm2. Two disadvantages are that (1) there is no residual, so chlorine or alternative disinfectants must be added; (2) the quartz sleeves must be cleaned periodically.
High-pH Treatment
Lime addition to raise the pH of water to a range of 11.2 to 11.3 with a contact time of 1.56 to 2.4 hours has proven effective in killing pathogens.
9.6 TREATMENT OF WATER TREATMENT PLANT RESIDUALS Residuals or sludge that is produced during water treatment is normally thickened in a gravity thickener and then dewatered by use of lagoons, drying beds, centrifuges, and/or plate and frame presses. Figure 9.21 shows a generalized schematic Figure 9.21 Schematic of water treatment residuals.
Raw water Recycled Mixing
Supernatant or decant
Flocculation Recycle Sedimentation
Gravity thickening
Filtration
Washwater recovery pond
Thickened sludge
Clear well
High-service pumps Lagoons
Drying beds
Centrifuges
P&F presses
To distribution system
Dewatered sludge To ultimate disposal or reuse
References 239
of sludge handling, dewatering, and disposal of water treatment residuals. The types of chemicals used during water treatment have a significant impact on the treatment and ultimate disposal of water treatment sludges. Water-softening residuals may have beneficial impacts on soil and crop yields, whereas there are concerns about the application of aluminum and iron sludges to crops (MWH, 2005). An excellent resource on water-residuals management is Water Treatment Principles and Design, Chapter 20 (MWH, 2005). Dewatered sludge has primarily been disposed of in sanitary landfills. Some municipalities such as the Macon Water Authority blend it with stabilized wastewater biosolids, compost it, and then sell it for soil amendment for application on agricultural lands.
S U M M A RY Providing safe, potable water to the public is one of the major responsibilities of environmental engineers. If primary and secondary drinking water standards are to be achieved, the selection of appropriate unit operations and unit processes for treating water is critical. This chapter presented and discussed typical water treatment flow diagrams that have been successfully used for treating surface water and groundwater. Examples were given of how to design rapid-mix basins, flocculation basins, settling basins, and filters. Although not a health concern, hardness removal using the lime-soda ash softening process was discussed, and an example was presented of how to calculate the quantity of lime and soda ash required to achieve a residual hardness of 40 mg/L as CaCO3 . Also discussed were the importance and selection of the appropriate disinfectant, as well as the technologies used for handling and treating water treatment plant residuals.
backwashing coagulation Ct value destabilization disinfection filtration rate
flocculation granular media head loss maximum contaminant level membrane treatment
mixing Reynolds number sedimentation settling velocity velocity gradient
REFERENCES Carman, P.C. (1937) Fluid Flow Through Granular Beds. Transactions of Institute of Chemical Engineers (London) Vol. 15, p. 150. Crites, R. and Tchobanoglous, G. (1998) Small and Decentralized Wastewater Management Systems, WCB/McGraw-Hill, Boston, MA. Great Lakes – Upper Mississippi Board of State Public Health and Environmental Managers. (1992) Recommended Standards for Water Works. Ten State Standards. Albany, NY. Kozeny, G. (1927) Sitzber. Akad. Wiss. Wein. Math. – Naturw. Kl,. Abt. Ila 136. Kawamura, S. (1991) Integrated Design of Water Treatment Facilities, John Wiley & Sons, Inc., New York, NY. Masters, G.M. (1997) Introduction to Environmental Engineering and Science, Prentice-Hall, Upper Saddle River, NJ. MWH (2005) Water Treatment Principles and Design, John Wiley & Sons, Inc., Hoboken, NJ. Peavy, H.S., Rowe, D. R., and Tchobanoglous, G. (1985) Environmental Engineering, McGrawHill, New York, NY.
KEY WORDS
240 Chapter 9
Water Treatment
Reynolds, T. D. and Richards, P. A. (1996) Unit Operations and Processes in Environmental Engineering. PWS Publishing Company, 20 Park Plaza, Boston, MA., pp. 284–318. Sawyer, C. L., McCarty, P. L., and Parkin, G. F. (1994) Chemistry for Environmental Engineers, McGraw-Hill, New York, NY. Viessman, W. and Hammer, M. J. (2005) Water Supply and Pollution Control, Pearson/Prentice Hall, Upper Saddle River, NJ, pp. 384–407.
EXERCISES 9.1
9.2
9.3
9.4
9.5
9.6
Draw a schematic diagram of a water treatment plant that uses a river as the source. Briefly discuss the purpose of each unit operation or unit process shown on the schematic. Draw a schematic diagram of a water treatment plant that uses a groundwater as the source. Briefly discuss the purpose of each unit operation or unit process shown on the schematic. A circular rapid-mix basin with stator blades is to be designed to treat 2.0 million gallons of water per day. The detention time should be 60 seconds with a velocity gradient of 900 s-1. The minimum temperature anticipated is 60°F and the absolute viscosity 1m2 is 2.359 * 10-5 lb # s/ft2. Determine the diameter (ft) if two rapid-mix basins are operating in parallel with a depth of 10 feet. Also calculate the power input to the basin (HP). 1 HP = 550 ft # lb/sec. A flocculation basin is to be designed with a 30-minute detention time and a mean velocity gradient of 40 s-1. Determine the power (W) required if the average design flow to the flocculation basin is 30,000 m3/d. m = 1.002 * 10-3 N # s/m2.. A flocculation basin 60 feet long, 45 feet wide, and 14 feet deep treats a coagulated water flow of 10 million gallons per day (MGD) at a temperature of 50°F (absolute viscosity, m = 2.735 * 10-5 lb # s/ft2). The power (P) input to the paddlewheel is 930 ft # lb/sec, resulting in a paddle blade-tip velocity of 1.4 and 1.0 feet per second (fps) for the outer and inner blades, respectively. Determine the following: (a) The detention time in hours. (b) The horizontal flow-through velocity (Vh) in feet per minute. (c) The mean velocity gradient (G) in s1. (d) The Gt value (dimensionless). A groundwater was analyzed and found to have the following constituents: CO2 = 17 mg/L Ca2+ = 80 mg/L Mg2+ = 29 mg/L
Bicarbonate = 180 mg/L as CaCO3 Na+ = 25.3 mg/L Cl- = 138.5 mg/L
(a) Draw a bar graph in milliequivalents per liter (meq/L) for determining lime and soda-ash requirements to soften the water to the practical limits of hardness removal (40 mg/L as CaCO3 and 10 mg/L magnesium hardness as CaCO3). (b) Calculate the quantity of lime in kilograms per day as calcium oxide (CaO) with a 95% purity to treat 18,000 m3/day of flow using excess-lime treatment (1.25 meq/L). (c) Calculate the quantity of soda ash 1Na2CO32 needed in kilograms per day with a 90% purity to treat 18,000 m3/day of flow.
Exercises 241
9.7
A water analysis yielded the following results: CO2 = 8.8 mg/L Ca2+ = 70 mg/L
HCO3- = 115 mg/L as CaCO3 SO24 = 96 mg/L
Mg2+ = 9.7 mg/L Na+ = 6.9 mg/L
Cl- = 10.6 mg/L
(a) Draw a bar graph in milliequivalents per liter (meq/L) for determining lime and soda-ash requirements to soften the water to the practical limits of hardness removal (40 mg/L as CaCO3 and 10 mg/L magnesium hardness as CaCO3). (b) Calculate the quantity of lime in pounds per day as calcium oxide (CaO) with a 98% purity needed to treat 5.0 MGD of flow using excess-lime treatment (1.25 meq/L). (c) Calculate the quantity of soda ash 1Na2CO32 in pounds per day with a 95% purity needed to treat 5.0 MGD of flow. 9.8
9.9
9.10
9.11
9.12
Use Stokes’ law [Equation (9.23) or (9.24)] to calculate the settling velocity (meters per second) of a spherical particle with a diameter of 1.0 mm and a specific gravity (S.G.) of 3.0 in water at a temperature of 30°C. The absolute and kinematic viscosities of water at 30°C are 0.798 * 10-3 N # s/m2 and 0.8 * 10-6 m2/s, respectively. The water density 1r2 is 995.7 kg/m3. A spherical particle with a diameter of 0.02 mm and a specific gravity (S.G.) of 2.8 settles in water having a temperature of 20°C. Use Stokes’ law [Equation (9.23) or (9.24)] for calculating the settling velocity in meters per second. The absolute and kinematic viscosities of water at 20°C are 1.002 * 10 -3 N # s/m2 and 1.003 * 10-6 m2/s, respectively. The water density 1r2 at 20°C is 998.2 kg/m3. Convert the settling velocity from meters per second to gallons per day per square foot 1gpd/ft22, which represent the units commonly used for expressing the overflow rate. Two long rectangular settling basins operate in parallel to treat 5 MGD of coagulated water. A length-to-width (L:W) ratio of 4:1 and length-to-depth (L:D) ratio of 10:1 will be used in design. Overflow rate = 1000 gpd/ft2 (a) Determine the dimensions of the settling basin in feet, assuming a depth of 10 feet. (b) Calculate the detention time of each basin in hours. (c) Determine the length of the effluent weir in each basin if the weir loading rate is not to exceed 12,000 gpd/ft. Two circular settling basins operating in parallel are to be designed for treating 40,000 m3/d of coagulated water. Each basin is 2.3 meters deep and has a single peripheral weir attached to its outside wall. Vo = 0.6 m/h (a) Determine the diameter of each basin in meters. (b) Calculate the detention time of each basin in hours. (c) Calculate the weir loading rate 1m3/d # m2. A rapid sand filter has a sand-bed depth of 762 mm. Other pertinent data include: sand specific gravity 2.65, porosity ( ) 0.41, filtration rate 1.53 lps/m2 shape factor ( ) 0.80, and temperature 15ºC. A sieve analysis is presented below. Determine the head loss for a clean filter using the Carman-Kozeny equation [Equation (9.38)] for a stratified bed. m = 1.139 * 10-3 kg/m # s; r = 999.1 kg/m3.
242 Chapter 9
Water Treatment
9.13
9.14
Sieve
% Sand retained
di j1m * 10-42
14–20
0.87
10.006
20–28
8.63
7.111
28–32
26.30
5.422
32–35
30.10
4.572
35–42
20.64
3.834
42–48
7.09
3.225
48–60
3.19
2.707
60–65
2.16
2.274
65–100
1.02
1.777
A new water treatment plant is to be constructed for treating 30 MGD. Estimate the number of filters required and calculate the area of each filter 1ft22, assuming that a length-to-width ratio (L:W) of 4:1 is employed in constructing the rapid sand filters and using a filtration rate of 6 gpm/ft2. Chlorine gas, 70% granular calcium hypochlorite 3Ca1OCl224, and 12% sodium hypochlorite solution are being considered as the primary disinfectants for a 6000-m3/d water treatment plant. The anticipated chlorine dose is 2.5 mg/L. Calculate the quantity of each disinfectant required in kilograms per month (30 days) to supply the dosage of 2.5 mg/L. Which disinfectant is the most cost effective if chlorine gas, Ca1OCl22 , and NaOCl cost $0.50/kg, $1.50/kg, and $0.95/kg, respectively?
CHAPTER
10
Domestic Wastewater Treatment Objectives In this chapter, you will learn about: The objectives of domestic wastewater treatment and the major ways of classifying wastewater treatment plants How to design major unit operations and processes for treating wastewater Microbial growth in heterogeneous cultures Sludge volume and weight relationships How to design sludge thickening systems How to design anaerobic and aerobic sludgedigestion systems
10.1 INTRODUCTION Wastewater is water that has been contaminated to the degree that it is no longer beneficial, and therefore must be treated before it can be used or released back into the environment. The four major types of wastewater are domestic or municipal, industrial, urban runoff, and agricultural runoff. Domestic and industrial wastewaters are considered point sources of pollution, since they emanate from a single point of discharge. Urban and agriculture runoff are associated with stormwater that does not percolate into the ground. They are nonpoint sources of pollution, since they emanate from an area. Domestic or municipal wastewater consists of sewage generated from residential, commercial, and public facilities. Table 10.1 presents typical characteristics of domestic wastewater. Industrial wastewaters are those generated from companies such as pulp and paper mills, steel mills, pharmaceutical facilities, and food-processing plants. In some cities, domestic wastewater may contain a significant proportion of industrial wastes. Stormwater runoff in urban areas is a major source of pollution that generates wastewater containing nitrogen, phosphorus, heavy metals, organics, and pathogens. Stormwater runoff from agricultural areas contains high concentrations of suspended solids, nitrogen, phosphorus, sediment, and pathogens. This chapter deals only with the treatment of domestic wastewater. Treatment and disposal of human excrement has been handled in numerous ways throughout history. In rural areas, on-site wastewater treatment systems have varied from low-tech facilities such as “outhouses” to cesspools and septic tanks. Outhouses, privies, or Johnny houses consist of a wooden structure built over a pit. A wooden seat with a hole is used by the patron. Sometimes lime is dumped into the pit to reduce odors. A cesspool system is similar to a septic system, but instead of a concrete or fiberglass septic tank for holding sewage generated in a home, a pit is dug and logs or pieces of lumber are placed over it. Human wastes are then flushed down the toilet and drained into the cesspool.
244 Chapter 10
Domestic Wastewater Treatment
Table 10.1 Typical Characteristics of Influent Domestic Wastewater Contaminant
Units
Weak concentration
Medium concentration
Strong concentration
Total solids (TS)
mg/L
350
720
1200
Total dissolved solids (TDS)
mg/L
250
500
850
Fixed dissolved solids (FDS)
mg/L
145
300
525
Volatile dissolved solids (VDS)
mg/L
105
200
325
Total suspended solids (TSS)
mg/L
100
220
350
Fixed suspended solids (FSS)
mg/L
20
55
75
Volatile suspended solids (VSS)
mg/L
80
165
275
5-d biochemical oxygen demand (BOD) @ 20°C
mg/L
110
220
400
Total organic carbon (TOC)
mg/L
80
160
290
Chemical oxygen demand (COD)
mg/L
250
500
1000
Total nitrogen (as N)
mg/L
20
40
85
Organic nitrogen (as N)
mg/L
8
15
35
Free ammonia (as N)
mg/L
12
25
50
Total phosphorus (as P)
mg/L
4
8
15
Organic phosphorus
mg/L
1
3
5
Inorganic phosphorus
mg/L
3
5
10
Chlorides
mg/L
30
50
100
Sulfate
mg/L
20
30
50
Alkalinity (as CaCO3)
mg/L
50
100
200
Grease
mg/L
50
100
150
Total coliform
#/100 ml
106 –107
107 –108
108 –109
Volatile organic compounds (VOCs)
mg/L
6 100
100–400
7 400
Source: Metcalf and Eddy (2003).
In the septic tank, anaerobic processes occur to reduce the solids and oxidize the organics. The effluent (the liquid that exits the septic tank) receives further treatment in the drain field, where aerobic facultative microorganisms decompose remaining organics as the water percolates through the soil. Septic tanks and cesspools do not perform well where the groundwater table is high or the soil permeability is very low. Septic tanks and cesspools must be pumped periodically to remove inert solids.Typically, rather than being maintained by removal of excrement, outhouses are abandoned and new ones constructed to replace them. Today, cesspools are no longer permitted. Instead, septic-tank systems should be used.
Section 10.2
Wastewater Treatment Categorization
In towns and communities that have sewerage systems, wastewater treatment plants (WWTPs) are constructed to treat wastewater before it is reused or discharged into surface waters. Wastewater is collected throughout the community by a pipe network called the sewerage system or wastewater collection system. This network consists of gravity sewers, pumping stations, and force mains. Gravity sewers make up most sewerage systems and contain various sizes of pipes that collect the wastewater from homes, businesses, and public areas. The wastewater flows through these systems under the influence of gravity. Pumping stations are necessary for moving the wastewater from various portions of the sewerage system. Pipes hooked up to the pumping stations are known as force mains, since the water flows under pressure and not by gravity.
10.2 WASTEWATER TREATMENT CATEGORIZATION Wastewater treatment plants have traditionally been classified as primary, secondary, or advanced. Each WWTP is made up of a series of unit operations and processes. A unit operation involves physical treatment of the wastewater, whereas unit processes involve biological or chemical treatment. A primary WWTP consists of primary clarification and chlorination. Secondary wastewater treatment facilities use biological processes, such as activated sludge, trickling filters, or rotating biological contactors (RBCs) as the major means of treating wastewater. Advanced wastewater treatment (AWT) facilities include unit operations and unit processes for removing nitrogen, phosphorus, suspended solids, recalcitrant organics, and other potential toxic compounds that cannot be removed by secondary WWTPs. Primary WWTPs are no longer being constructed in the United States, so they will not be discussed further here. All wastewater treatment plants must have a National Pollutant Discharge Elimination System (NPDES) permit in order to discharge to surface waters. Table 10.2 lists typical effluent parameters and concentrations that must be achieved for compliance with NPDES permits issued to secondary WWTPs. Permit requirements for AWT facilities are much more stringent, with effluent limits set at levels such as 5 mg/L for biochemical oxygen demand (BOD), 5 mg/L for total suspended solids (TSS), 3 mg/L for total nitrogen (TN), and 1 mg/L for total phosphorus (TP). Many of the advanced wastewater treatment facilities operating in Florida must achieve effluent limits of 5, 5, 3, and 1 mg/L or less for BOD, TSS, TN, and TP, respectively (Thabaraj, 1993). 10.2.1 Secondary Wastewater Treatment The objectives of conventional wastewater treatment systems are to remove suspended solids and organic matter (as measured by BOD) and to kill pathogens. Secondary WWTPs were developed so that higher levels of removal of TSS and BOD
Table 10.2 Typical Secondary Effluent NPDES Permit Requirements Annual average (mg/L)
Monthly average (mg/L)
Weekly average (mg/L)
BOD
20
30
45
TSS
20
30
45
pH
6.5–8.5
6.5–8.5
6.5–8.5
Parameter
245
246 Chapter 10
Domestic Wastewater Treatment
could be realized beyond those accomplished by primary treatment. In secondary WWTPs, biological systems convert soluble and colloidal organic matter into biomass. Biomass is a term for the heterogeneous culture of microorganisms (primarily bacteria) that are grown in the system and is represented by the chemical formula 1C 60H 87O23N12P2. Some particulate organic matter is hydrolyzed and oxidized during biological treatment as well. 10.2.2 Advanced Wastewater Treatment (AWT) Advanced wastewater treatment systems are used for removing nitrogen and phosphorus from the wastewater along with additional BOD and TSS that secondary treatment systems cannot remove. AWT systems may utilize both chemical and biological processes in conjunction with physical unit operations such as dual-media filters. Advanced biological wastewater systems that use nitrification followed by denitrification are widely used for removing nitrogen from the wastewater. Nitrification is an aerobic process that transforms ammonium nitrogen (reduced state) into an oxidized form 1NO3-2. This process is carried out by autotrophic bacteria of the genera Nitrosomonas and Nitrobacter. Equation (10.1) shows the overall nitrification reaction, neglecting the synthesis of autotrophic biomass. NH 4+ + 2 O2
Nitrifiers
" NO - + 2 H + + H O 3 2
(10.1)
To remove the nitrate that is formed during nitrification, a second biological process known as denitrification must be used in an anoxic environment (void of dissolved oxygen). Denitrifying, heterotrophic microorganisms reduce nitrate into nitrogen gas, which is released into the atmosphere. The following stoichiometric equation shows what happens during denitrification when methanol 1CH 3OH2 serves as the energy source. 6 NO 3- + 5 CH 3OH
Denitrifiers
" 3 N c + 6 OH - + 5 CO + 7 H O 2 2 2
(10.2)
Biological phosphorus removal may be accomplished by providing alternating anaerobic/aerobic treatment of the wastewater. Such systems promote the growth of phosphorus-accumulating organisms (PAOs) such as Acinetobacter. PAOs take up excess amounts of phosphorus from wastewater during the aerobic phase of biological treatment. These processes are known as enhanced biological phosphorus removal (EBPR) systems. AWT systems may also use chemical systems for removing nitrogen and phosphorus. Ammonium nitrogen can be converted to ammonia gas 1NH 32 by adding lime or sodium hydroxide to raise the pH above 11 and then passing the wastewater through a stripper to remove the ammonia. Phosphorus, in the form of orthophosphate 1PO34 2, is easily removed from wastewater by adding alum or lime for precipitation as aluminum phosphate or calcium phosphate. Advanced biological and chemical processes for removing nutrients from wastewater can be found in Metcalf and Eddy (2003), WEF (1998), and EPA (1993).
10.3 OVERVIEW OF WASTEWATER TREATMENT SYSTEMS Figure 10.1 shows a schematic diagram of a typical secondary WWTP that uses the activated sludge process. Bar racks are used at the beginning of the plant to remove tree limbs and other large objects from the influent. Screens (having smaller openings) are used
Section 10.4
Preliminary Treatment
247
Bar racks and screens Influent
Grit removal
Flowmeter
Aeration basin
Secondary clarifier
Return activated sludge
Figure 10.1 Secondary WWTP schematic.
for removing paper, plastics, and debris that may inhibit subsequent processes. The next unit operation, grit removal, removes inorganic particles such as sand, silt, grit, coffee grinds, and eggshells that can accumulate in clarifiers and digesters if not removed from the wastewater. It is necessary to measure the flow rate of the wastewater so that proper dosing of chemicals can be accomplished. Then the wastewater enters the aeration basin, a biological process, where air is injected into the wastewater to provide microorganisms with the correct amount of oxygen necessary for oxidizing the organic matter as measured by BOD or chemical oxygen demand (COD) and to keep the microorganisms in suspension. The TSS in the influent, along with the microorganisms that are grown in the aeration basin, are separated from the liquid portion of the wastewater in the secondary clarifier. The treated liquid wastewater flows over the effluent weir of the secondary clarifier.Thickened suspended solids and microorganisms (biomass) that accumulate at the bottom of the clarifier are returned to the aeration basin (return activated sludge). Chlorine is added to the secondary clarifier effluent, and the chlorine contact basin provides sufficient time for the chlorine to kill potential pathogens in the wastewater. Although not shown in the diagram, the waste activated sludge must be thickened and stabilized before ultimate disposal. The design capacity of most WWTPs is based on either the annual average daily flow (AADF) or maximum monthly flow (MMF).This means that all unit processes are designed for either the AADF or MMF.All unit operations, especially pumps, pipes, and conveyance structures, are typically designed to handle the peak hourly flow (PHF). The annual average daily flow can be estimated by multiplying 120 gallons of wastewater generated per person per day by the design population (Viessman and Hammer, 2005). This value includes nominal infiltration into the sewerage system. Maximum monthly, peak hourly, and minimum design flows can then be determined by assuming appropriate peaking factors. Detailed information regarding design flows is provided by Reynolds and Richards, 1996; Viessman and Hammer, 2005; and Metcalf and Eddy, 2003.
10.4 PRELIMINARY TREATMENT Raw sewage or influent to a WWTP normally passes through a number of preliminary treatment steps before being processed by the major unit operations and unit processes. Preliminary treatment systems that may be encountered include: bar racks and screens, grit removal, pumping, flow measurement, equalization, pre-aeration,
Chlorine contact basin
Waste activated sludge
Effluent
248 Chapter 10
Domestic Wastewater Treatment
pre-disinfection, shredding, and flotation. Detailed design procedures for most of these systems can be found in Metcalf and Eddy (2003) and WEF MOP #8 (1998). Rags, tree limbs, hair, paper, and plastics are some of the more important items that must be removed during preliminary treatment, because they can clog pumps, flowmeters, and valves. Screening and grit removal will be the only preliminary treatment unit operations discussed in this chapter. 10.4.1 Screening Mechanically cleaned bar racks are used first, followed by bar screens with smaller openings. Bar racks have clear openings ranging from 1.5 to 6 inches (38 to 150 mm). Bar racks are used for removing large objects such as logs or other debris. Mechanically cleaned coarse screens with openings from 1 to 2 inches (30 to 50 mm) are used for removing rags, paper, and other debris that may clog pipes, pumps, and valves. Bar racks and bar screens are made up of parallel bars or rods, while fine screens consist of wires, grating, wire mesh, or perforated plates. Figure 10.2 is a photo of a coarse bar screen. 10.4.2 Grit Removal Another unit operation normally performed during preliminary treatment is grit removal, which follows screening. Grit consists of sand, silt, small gravel, cinders, coffee grounds, eggshells, and other inert materials, which typically have a specific gravity around 2.65. These materials are abrasive and will cause pump impellers to wear excessively, and they will accumulate in tanks, digesters, and pipes. The three major types of grit removal systems in use today are aerated chambers, horizontal flow through basins, and vortex removal systems (Metcalf and Eddy, 2003). Horizontal flow-through and aerated grit chambers are generally designed to remove all particles that are retained on a 65 mesh screen ( 7 0.21-mm-diameter particle), whereas vortex type systems use centrifugal and gravitational forces to separate the grit from the wastewater using proprietary equipment. Vortex grit removal systems are frequently installed at newly constructed WWTPs. There are two primary manufacturers of such systems. Smith & Loveless Figure 10.2 Photograph of a bar screen.
Section 10.5
Primary Treatment 249
Figure 10.3 PISTA® Grit Removal System (manufactured by Smith & Loveless, Inc.).
Figure 10.4 High-performance SLURRYCUP™ grit washing and classification unit mounted on a GRIT SNAIL™ Quiescent Dewatering Escalator (manufactured by EUTEK® Systems™, Inc.).
makes the PISTA® system and EUTEK® makes the Teacup. Figures 10.3 and 10.4 show typical vortex removal systems. Once grit has been separated from the wastewater, it must be washed to remove organic matter that may be attached to the grit particles. For this purpose, both inclined-rake and inclined-screw conveyor washer systems are used. In some cases, hydrocyclones are used prior to the grit washer to enhance separation of the grit and organics. Detailed design information for these systems can be found in Metcalf and Eddy (2003), WEF MOP #8 (1998), and Reynolds and Richards (1996).
10.5 PRIMARY TREATMENT Clarification, or separation of suspended solids from the liquid portion of the wastewater, is accomplished during primary treatment. Primary treatment, which follows preliminary treatment, is a unit operation that involves the settling of suspended solids and the removal of oil, grease, and scum that float on the surface of the wastewater. Since these species contain organic matter, BOD removal is accomplished.
250
Chapter 10
Domestic Wastewater Treatment
Primary treatment does not remove soluble or colloidal organic materials. Lightweight organics that float to the surface are skimmed off and pumped to the digesters for treatment. The sludge that accumulates at the bottom of a primary clarifier is normally stabilized by anaerobic digestion before disposal. Primary sludge is called “raw” sludge and is objectionable, since it contains pathogens and organics and produces odors, especially if it becomes anaerobic. Typical removal efficiencies observed during primary treatment for BOD and TSS range from 30% to 40% and 50% to 70%, respectively (Metcalf and Eddy, 2003). Primary clarifiers are normally circular by design. Where land space is limited, however, long, rectangular basins are often used. Typical dimensions and design criteria for primary clarifiers are presented by Metcalf and Eddy, 2003. Depths of primaries range from 10–16 ft (3–4.9 m). Rectangular primary clarifiers have lengths ranging from 50–300 ft (15–90 m) and widths varying from 10–80 ft (3–24 m). The diameters of circular clarifiers range from 10–200 ft (3–60 m). Detention times in primaries range from 1.5–2.5 hours.Average overflow rates vary from 800–1200 gpd/ft2 [30–50 m3/(d # m224. Weir loading rates vary from 10,000–40,000 gpd/ft2 [125–500 m3/(d # m224. The design of primary clarifiers is based on the overflow rate, detention time, and weir loading rate. The overflow rate 1Vo2 is defined as the flow rate divided by the surface area of the clarifier: Vo = where:
Q AS
(10.3)
Vo = overflow rate gpd/ft2 1m3/d # m22, Q = design flow rate, MGD 1m3/d2, and A S = surface area of the clarifier, ft2 1m22.
The surface area of the clarifier is determined by dividing the design flow rate by the overflow rate. This area calculated is then converted into either a circular or a rectangular area. Once the detention time is selected and the total volume of the clarifier is calculated, clarifier depth may be determined. It is calculated by dividing the clarifier volume by the surface area of the clarifier. Detention time 1u2 is the average unit of time that the wastewater remains in the clarifier and is determined as follows: u =
V Q
(10.4)
where: V = volume of the primary clarifier in ft3 1m32, and Q = design flow rate, MGD 1m3/h2. Weir loading rate (q) is the third parameter that must be determined when designing primary clarifiers. Mathematically, q is defined as the design flow rate divided by the length of the weir: q = where:
Q weir length
q = weir loading rate, gpd/ft 1m3/d # m2, Q = design flow rate, gpd 1m3/d2, and weir length = length of primary clarifier effluent weir, ft (m).
(10.5)
Section 10.5
Primary Treatment 251
The weir loading rate is the last parameter to be checked. For circular clarifiers, a peripheral weir which extends around the entire circumference of the clarifier is used. For rectangular clarifiers, the design engineer uses inboard box weirs to provide the necessary weir length to meet the weir loading rate. Example 10.1 illustrates the procedure for designing primary clarifiers.
EXAMPLE 10.1 Primary clarifier design A municipal WWTP receives 20,000 m3/day of flow. Two primary clarifiers operating in parallel will handle the flow. The state’s regulatory agency’s criteria are as follows: average overflow rate = 20 m3/(d # m2 ), and the average detention time = 3.0 hours. Determine: a. Clarifier diameter. b. Side water depth. c. Weir loading rate if an inboard peripheral weir is used that is 0.25 m from the edge of the clarifier. Solution part a 10,000 m3/day
10,000 m3/day
AS =
10,000 m3>d Q = = 500 m2 Vo 20 m3>d # m2
AS =
pD2 = 500 m2 4
D = 32000>p40.5 = 25.2 m Go with a diameter of 25 meters. Circular primary clarifier diameters typically range from 3 to 60 meters. Solution part b Next, calculate the volume of the clarifier so the side water depth may be determined by dividing the volume by the surface area: V = u Q = 3.0 h ¢
d m3 ≤ ¢ 10,000 ≤ = 1250 m3 24 h d
252
Chapter 10
Domestic Wastewater Treatment
Since we have selected a diameter of 25 rather than 25.2 meters, calculate a new surface area: AS = depth =
p125 m22 4
= 491 m2
1250 m3 V = = 2.55 m AS 491 m2
Go with a depth of 3 meters; depths of circular primary clarifiers range from 3 to 4.9 meters. An additional 0.45 m is added to the depth to account for freeboard. Freeboard is additional length added to the depth to provide for variations in flow. Solution part c Peripheral weirs are usually placed around the circumference of the clarifier. In this case, the weir is placed 0.25 m from the edge of the clarifier. Therefore, the diameter of the peripheral weir is 24.5 meters. Peripheral weir diameter is equal to 25 meters - 210.25 m2 = 24.5 meters. q =
10,000 m3>d Q = = 130 m3/(d # m) weir length p124.5 m2
Peak weir loading rates generally should not exceed 248 m3/(d # m) (Reynolds and Richards, 1996).
10.6 SECONDARY TREATMENT Secondary wastewater treatment implies that a biological process is being used for treating the wastewater. Microorganisms indigenous to the wastewater use organic carbon, along with nitrogen and phosphorus, to grow more microorganisms, primarily bacteria. Bacteria use the organic matter as measured by BOD or COD for their energy and carbon source. Oxidation of the organic matter produces energy that is captured in the microbe’s biochemical pathways, while a portion of the organic matter is used in the synthesis of biomass. Equation (10.6) indicates that the organic material is being oxidized for energy, whereas Equation (10.7) shows organic matter being synthesized into new microbial cells 1C 60H 87O23N12P12: organics + O2 : CO2 + H 2O + energy organics + O2 + N + P
Microorganisms
" C H O N P 60 87 23 12
(10.6) (10.7)
The overall growth rate in continuous flow systems with a heterogeneous culture of microorganisms is expressed as follows (Heukelekian et al., 1951): a
dX dS b = Ya b - kdX dt NG dt U
where: a
dX = net microorganism growth rate, mass/1volume # time2, b dt NG
(10.8)
Section 10.6
a
Secondary Treatment 253
dS b = substrate utilization rate, mass/1volume # time2, dt U X = microorganism concentration, mass/volume, Y = growth yield coefficient, mass of biomass produced per unit of substrate oxidized, and kd = endogenous decay coefficient, time-1.
10.6.1 Activated Sludge Ardern and Lockett (1914) developed the activated-sludge process in England in 1914. It is an aerobic, suspended-growth, biological process, characterized by two major steps: (1) substrate adsorption and utilization in the aeration basin, (2) solids/ liquid separation in the secondary clarifier.A schematic of the process is presented in Figure 10.5. Wastewater flows into the aeration basin, where it is brought into contact with a heterogeneous culture of microbes, consisting primarily of heterotrophic bacteria. The liquid inside the aeration basin is called “mixed liquor” or “activated sludge.” The concentration of microorganisms in the aeration basin is generally measured as mixed liquor suspended solids (MLSS). Microorganisms growing in the aeration basin are indigenous to the wastewater. Given proper environmental conditions, they grow and proliferate by binary fission approximately every 20 to 25 minutes. Microorganisms use organic matter (substrate) for synthesis and energy. Synthesis involves the production of carbohydrates, lipids, proteins, etc. for cell maintenance and for reproduction. Substrate is also oxidized through respiration for the production of energy to drive the biochemical reactions required for synthesizing biomass and for motility. Given the generic formula of a microorganism as C 60H 87O23N12P1 with a molecular weight of 1374, it is easy to see that the major constituents required for growth are carbon 1312 * 604/1374 = 52%2, nitrogen 1312 * 144/1374 = 12%2, and phosphorus 13314/1374 = 2.3%2. On entering the secondary clarifier, the microorganisms or biomass are separated from the liquid portion of the wastewater by gravity. The supernatant or clarified wastewater that flows over the effluent weir is called secondary effluent. The biomass that settles to the bottom of the secondary clarifier forms a layer of sludge called the “sludge blanket.” Thickened biomass pumped or returned back to the head of the aeration basin is called the recycle flow 1Qr2 or return activated sludge (RAS) flow. RAS pumps are typically designed to pump up to 100 to 120% of the design flow.
System boundary Air Influent Q Xi
Q Qr S0
Aeration basin
Si
Return activated sludge, RAS
Figure 10.5 Schematic of activated sludge process.
Alternative sludge wasting
X, V, Se
(Q Qr) X, Se
Qr, Xr, Se
Secondary clarifier
RAS/WAS Pumping station
Effluent (Q Qw) Xe, Se WAS Qw, Xr
254
Chapter 10
Domestic Wastewater Treatment
The quantity of biomass wasted from the system determines the mean cell residence time (MCRT), which sets the concentration of MLSS that can be maintained in the aeration basin. To maintain steady-state operating conditions, the quantity of biomass grown within the system is equal to the quantity of biomass that must be wasted from the system. The sludge wasted 1Qw2 from the system is called waste activated sludge (WAS). Sludge wasting is normally accomplished by pumping the thickened sludge from the RAS/WAS pumping station to the sludge thickening or sludge stabilization processes. The concentration of suspended solids in the thickened sludge generally ranges from 0.5% to 1.5% solids, where, 1% = 10,000 mg/L, assuming that the specific gravity of the sludge is approximately 1. The sludge settling rate depends on the MLSS concentration entering the clarifier, the characteristics of the wastewater, MCRT, and the sludge return rate. Design and Operational Parameters
The main design and operational parameter for the activated-sludge process is the mean cell residence time (MCRT). MCRT represents the average time that the microorganisms or biomass remain in the system. Mean cell residence time is also called sludge age, solids retention time (SRT), or uc . Mathematically, MCRT is expressed as follows, using the nomenclature presented in Figure 10.5. MCRT1d2 =
XV 1Q - Qw2 Xe + QwXr
(10.9)
where: X = biomass or microorganism concentration in aeration basin expressed as TSS or VSS, mg/L, Xe = secondary effluent TSS or VSS concentration, mg/L, Xr = TSS or VSS concentration in return activated sludge, mg/L, V = volume of the aeration basin, MG 1m32, Q = influent wastewater flow rate, MGD 1m3/d2, Qr = return activated sludge flow rate, MGD 1m3/d2, Qw = sludge wastage flow rate, MGD 1m3/day2, and Qe = 1Q - Qw2 = effluent wastewater flow rate, MGD 1m3/d2. MCRT typically varies from 5 to 30 days and determines the overall removal efficiency of the process. Generally, the longer the MCRT, the lower the effluent substrate concentration as measured by BOD or COD. MCRT is much longer than the hydraulic detention time 1u2. Typically, uc is 10 to 40 times u. Recall that detention time is defined as the volume of the reactor or basin divided by the volumetric flow rate with units of time. Equations (10.10) and (10.11) present the equations used for calculating detention time (Lawrence and McCarty, 1970). u =
V Q
(10.10)
uœ =
V 1Q + Qr2
(10.11)
where: u = detention time based on influent wastewater flow rate, d or h, and uœ = actual detention time for recycle systems, d or h.
Section 10.6
In most environmental engineering texts, u is based only on the influent flow rate and is often called the “nominal” detention time. Another design and operational parameter is the food-to-microorganism ratio (F:M) which is defined as: F:M = where:
QS0 XV
(10.12)
F:M = food-to-microorganism or food-to-mass ratio, mg/1mg # d2, and S0 = influent substrate concentration to aeration basin expressed as BOD, COD, or TOC, mg/L.
Biochemical Kinetics of Activated Sludge in Completely Mixed Systems
Kinetic equations based on the growth rate of microbes in pure cultures have been used to model heterogeneous microbial cultures such as activated-sludge systems. This section presents the kinetic equations used for designing completely mixed flow reactors (CMFRs). Derivation of these equations is beyond the scope of this book. If further information is needed, consult Metcalf and Eddy, 2003; Reynolds and Richards, 1996; Benefield and Randall, 1980, Sherrard and Schroeder, 1973; and Lawrence and McCarty, 1970. The relationship between MCRT and net microbial growth rate is: 1Q - Qw2 Xe + QwXr XV
=
1dS>dt2U 1 = Y - kd uc X
(10.13)
Substrate utilization can be modeled by a Michaelis-Minton type equation as proposed by Lawrence and McCarty (1970): a
kXSe dS b = dt U KS + Se
(10.14)
where k = maximum specific substrate utilization rate, d-1. Substituting Equation (10.14) into Equation (10.13) results in Equation (10.15), which is used for calculating the effluent soluble substrate concentration 1Se2 from the activated sludge process. Se =
Ks11 + kd uc2 uc1Yk - kd2 - 1
(10.15)
The microorganism concentration (X) inside the aeration basin is calculated as follows: X =
Y1S0 - Se2 uc 1 + kd uc u
(10.16)
where S0 and Se = influent and effluent soluble organic concentration, mass/volume. Normally the influent total organic concentration, expressed as total BOD or total COD, is substituted for the influent soluble organic concentration. The total BOD or total COD includes the soluble (dissolved) and particulate organic matter. Soluble BOD or COD only includes the soluble organic matter.
Secondary Treatment 255
256
Chapter 10
Domestic Wastewater Treatment
Equation (10.16) may be rearranged to calculate the volume of the aeration basin. V =
YQ1S0 - Se2 uc 1 + kd uc X
(10.17)
The total quantity of biomass produced daily 1Px2 is calculated as follows: Px =
YQ1S0 - Se2 1 + kd uc
(10.18)
Oxygen Requirements
Oxygen serves as an electron acceptor in the activated sludge process, so sufficient quantities of oxygen must be provided to ensure that microbial growth is not negatively affected. Under most operating conditions, a dissolved oxygen concentration of 2.0 mg/L should be maintained in the aeration basin to meet process requirements for both the carbonaceous oxygen demand and nitrogenous demand (total oxygen demand). Equation (10.19) is used for estimating the quantity of oxygen required to meet the total oxygen demand. The nitrogenous demand (NOD) is the amount of oxygen required for the nitrifying bacteria, Nitrosomonas and Nitrobacter. O2 = Q1S0 - Se211 - 1.42Y2 + 1.42kdXV + NOD NOD = Q1TKNo24.57
(10.19) (10.20)
where O2 = total oxygen required to meet the carbonaceous and nitrogenous oxygen demand, pounds per day, ppd (kg/d).
EXAMPLE 10.2 Activated sludge design A completely mixed activated sludge process is designed to treat 20,000 m3/day of domestic wastewater having a BOD5 concentration of 250 mg/L following primary treatment. The NPDES permit requires that the effluent BOD5 and TSS concentrations not exceed 20 mg/L on an annual basis. The following biokinetic coefficients are to be used in the design of the process: Y = 0.6 mg VSS/mg BOD5 , k = 5 d-1, KS = 60 mg/L BOD5 , and kd = 0.06 d-1. Assume that the MLVSS concentration in the aeration basin will be maintained at 3000 mg/L and the ratio of VSS:TSS is 0.75. Determine the following: a. Effluent soluble BOD5 (SBOD5) concentration necessary to meet the total BOD5 (TBOD5) requirement of 20 mg/L. b. Mean cell residence time necessary to meet the NPDES permit. c. Volume of the aeration basin in cubic meters. d. The quantity of waste sludge produced at the mean cell residence time determined in part b.
Section 10.6
Solution part a First, it is necessary to estimate the BOD5 of the effluent suspended solids (particulate BOD5-PBOD5) so that the soluble effluent BOD5 concentration may be determined. The particulate BOD5 can be estimated using the following equation: mg mg VSS 2 2 * 0.75 b particulate BOD5 = a b11.42 VSS2eff = a b a1.42 * 20 3 3 L mg TSS mg = 14.2 L The (SBOD5) or Se in the effluent is estimated by rearranging the following equation: TBOD5 = SBOD5 1Se2 + PBOD5 SBOD5 1Se2 = TBOD5 - PBOD5 SBOD5 1Se2 = 20 mg/L - 14.2 mg/L = 5.8 mg/L Solution part b Calculate the mean cell residence time using Equations (10.13) and (10.14). 1Q - Qw2 Xe + QwXr XV a
=
1dS>dt2U 1 = Y - kd uc X
kXSe dS b = dt U KS + Se
1dS>dt2U 1kXSe2>1KS + Se2 kSe 1 = Y - kd = Y - kd = Y - kd uc X X KS + Se 15d-1 * 5.8 mg>L2 mg VSS 1 = ¢ 0.6 - 0.06 d-1 = 0.204 d-1 ≤ uc mg BOD5 160 mg>L + 5.8 mg>L2 uc = 4.89 days Typically, MCRTs for completely mixed activated sludge processes range from 3 to 15 days. Solution part c Next, the volume of the aeration basin may be determined using Equation (10.17). 0.6 mg VSS 120,000 m3>d21250 - 5.8 mg>L214.89 d2 YQ1So - Se2 uc mg BOD5 V = = 1 + kd uc X 11 + 0.06 d-1 * 4.89 d2 13000 mg VSS>L2 = 3690 m3
Secondary Treatment 257
258
Chapter 10
Domestic Wastewater Treatment
Solution part d The quantity of waste sludge produced at a MCRT of 4.89 days is determined using Equation (10.18). First, change the units on the growth yield coefficient as follows: Y = ¢
Px =
0.6 mg VSS 1 mg TSS 0.8 mg TSS ≤¢ ≤ = mg BOD5 0.75 mg VSS mg BOD5
YQ1So - Se2 1 + kd uc 0.8
Px =
mg TSS mg m3 120,000 21250 - 5.8 2 mg BOD5 d L
Px = 3020
11 + 0.06 d
-1
* 4.89 d2
¢
1 kg 6
10 mg
≤¢
1000 L ≤ m3
kg d
The largest quantity of sludge produced occurs at short MCRTs. At long MCRTs, less sludge is produced.
10.6.2 Aerator Systems There are two major types of aeration systems: mechanical aerators and diffusers. Mechanical or surface aerators consist of mixers or brush rotors that transfer oxygen into the wastewater by spraying the wastewater into the atmosphere. Figure 10.6 is a photo of a mechanical aerator. Diffused aeration systems are similar to fish aquarium tanks in which oxygen diffuses through a diffuser stone or membrane, thereby transferring oxygen into the wastewater. Diffused aeration systems are categorized by the size of bubble they produce, ranging from coarse-bubble (large) to medium- to fine-bubble diffusers. Coarse-bubble diffusers are usually made of stainless steel pipes with orifices. Fine-bubble diffusers may consist of ceramic materials or membranes with small pore Figure 10.6 Mechanical aerator in operation.
Section 10.6
Secondary Treatment 259
Figure 10.7 Diffused aeration system in operation.
sizes. In general, diffused aeration systems are more efficient in transferring oxygen into the wastewater and are more widely used than mechanical aeration systems. Figure 10.7 is a photo showing a diffused aeration system in operation. Aeration manufacturers test aeration equipment using standard conditions, which include using clean tap water at 20°C containing zero mg/L of dissolved oxygen (initially) and at 1 atmosphere of pressure. Aeration equipment must transfer the desired quantity of oxygen to the microorganisms in the wastewater. Alpha 1a2 and beta 1b2 coefficients are used to translate the oxygen transfer rate under standard conditions to those under process conditions (i.e., in the wastewater). Each coefficient is defined by the following equations: a =
b =
1KLa2ww 1KLa2tap water 1DOsat2ww 1DOsat2tap water
(10.21)
(10.22)
where: 1KLa2ww = overall oxygen transfer coefficient under process conditions in wastewater, time1, (KLa)tap water overall oxygen transfer coefficient under standard conditions in water, time-1, 1DOsat2ww = dissolved oxygen (DO) saturation concentration at specified temperature and pressure in wastewater, mass/volume, 1DOsat2tap water = DO saturation concentration at standard conditions in water, mass/volume, and T = actual temperature of wastewater under process conditions,°C.
260 Chapter 10
Domestic Wastewater Treatment
The actual oxygen transfer rate equation under process conditions (transferring oxygen into wastewater) is: AOTR = 1SOTR2
a1bCs - Ct211.02421T°C - 20°C2 9.17
(10.23)
where: AOTR = actual oxygen transfer rate under process conditions, ppd (kg/d), SOTR = standard oxygen transfer rate at standard conditions, ppd (kg/d), Cs = DO saturation concentration in water at a specific temperature and pressure, mg/L, and Ct = desired DO concentration in aeration basin, mg/L. Sufficient oxygen must be supplied to the microorganisms so that biological processes are not limited.Also, mixing requirements must be maintained so that the microorganisms remain suspended in the aeration basin. If diffused aeration systems are used, then a specified amount of air is required for mixing.The mixing requirement for a mechanical aerator is based on the amount of energy supplied to the water in terms of power per unit volume. Aeration systems must be designed to meet the following conditions. Process air requirements: O2 = Q1S0 - Se211 - 1.42Y2 + 1.42kdXV + NOD Mixing requirements: diffused aeration: 20 to 30 scfm/1000 ft3 (20 to 30 m3/min # 1000 m3); mechanical aerators: 0.75 to 1.5 HP/1000 ft3 (20 to 40 kW/1000 m3) 10.6.3 Trickling Filters An alternative secondary wastewater treatment process uses trickling filters. A trickling filter is an attached growth biological process. With trickling filters, wastewater is applied to some type of filter medium on which a heterogeneous culture of microorganisms is growing. Rock or slag were the media of choice in the past, but they have been replaced with plastic media. Trickling filters do not filter the wastewater like a traditional sand filter; however, the biomass growing on the media uses the organic matter along with a portion of the nitrogen and phosphorus to grow new microorganisms. Trickling filters must be preceded by primary clarifiers to ensure that suspended solids do not clog the openings (orifices) on the distribution arms of the trickling filters. The pressure of the wastewater discharging from the distributor arms provides the driving force causing the arms to rotate. Figure 10.8 is a schematic Figure 10.8 Schematic of a trickling filter wastewater treatment plant.
Recirculation Primary
Pump Secondary
Trickling filter humus sludge
Primary & Secondary Sludge
Section 10.6
Secondary Treatment 261
Figure 10.9 Photo of conventional trickling filter with rock media.
Figure 10.10 Trickling filter with plastic media and biological growth.
diagram of a trickling filter system. Figure 10.9 shows a conventional trickling system with rock media. The media has no biological growth on it, since it has been taken out of service. Figure 10.10 shows a trickling filter consisting of plastic media with biological growth. The design of trickling filter processes has primarily been based on organic and hydraulic loading rates. Kinetic equations proposed by Eckenfelder have been used successfully when plastic media are used (Eckenfelder, 1989).
262 Chapter 10
Domestic Wastewater Treatment
10.7 SECONDARY CLARIFICATION The effectiveness of the activated sludge process is only as good as the design of the secondary clarifiers. It is essential for the secondary clarifiers to efficiently separate the suspended and biological solids from the liquid wastewater. Figure 10.11 is a photo of a circular secondary clarifier. Zone settling occurs during the settling of activated sludge. Secondary clarifier design protocol requires calculating the area of the clarifier based on clarification in addition to estimating the area necessary to achieve solids thickening. The largest area controls the design of the clarifier. The area of the clarifier based on clarification is determined by dividing the design flow rate by the overflow rate according to the following equation: AC =
Q Vo
(10.24)
where: A C = the area of clarifier based on clarification, ft2 1m22, Q = design flow rate, excluding sludge return flow applied to the secondary clarifier, MGD 1m3/d2, and Vo = overflow rate or surface loading rate, gpd/ft2 3m3/1d # m224 When considering thickening of the solids, the area of the secondary clarifier is estimated by dividing the total suspended solids loading to the secondary clarifier by the design solids loading rate (SLR). Routinely, a solids loading rate is assumed and the area of the clarifier based on thickening considerations is estimated. Equation (10.25) is used for determining the secondary clarifier area based on thickening considerations. AT =
1Q + QR2MLSS SLR
(10.25)
where: A T = secondary clarifier surface area based on thickening considerations, ft2 1m22, Q = wastewater design flow rate applied to the secondary clarifier, excluding the return activated sludge flow rate, MGD 1m3/d2, QR = return activated sludge flow rate, MGD 1m3/d2, MLSS = suspended solids concentration in the aeration basin, and SLR = solids loading rate design criteria, typically 25 ppd/ft2 3122 kg/1d # m224 at average daily flow. Figure 10.11 Photo of circular secondary clarifier.
Section 10.7
Secondary Clarification
263
Typical design criteria for secondary clarifiers (EPA, 1975) are presented as follows: average overflow rates ranging from 400–800 gpd/ft2 [16.3–32.6 m3/(d # m2)], solids loading rates of 20–30 ppd/ft2 [98–147 kg/(d # m224, and depths of 12–15 ft (3.7–4.6 m). Secondary settling basins are normally rectangular or circular in plan view. Standard sizes come in 5 ft (1.6 m) intervals. The design of a secondary clarifier is presented in Example 10.3.
EXAMPLE 10.3 Secondary clarifier design Design a secondary clarifier system for an activated sludge WWTP that treats 10.0 MGD of wastewater. The MLSS concentration is 3000 mg/L, return activated sludge (RAS) flow = 4.0 MGD, overflow rate is 600 gpd/ft2, and a solids loading rate of 30 ppd/ft2 will be used. Assume a minimum of two circular units. Determine the diameter of the clarifiers. Solution First, determine the surface area of the secondary clarifiers based on clarification considerations using Equation (10.24): AC =
Q 10 * 106 gpd = = 16,700 ft2 Vo 600 gpd>ft2
Next, determine the surface area of the secondary clarifiers based on thickening considerations using Equation (10.25).
AT =
1Q + QR2MLSS SLR
110 + 4 MGD213000 mg>L2a =
8.34 lb>MG mg>L
b
30 ppd>ft2
= 11,700 ft2 The surface area of the secondary clarifiers must equal to 16,700 ft2, since it is the larger of the two areas calculated. Since there are two circular clarifiers, the surface area must be divided by 2. The radius and diameter of the clarifiers can then be determined. 16,700 ft2 A = = pr2 clarifier 2 r = 28350 ft2>p = 51.5 ft
diameter = 103 ft
Therefore, use two 105-ft-diameter secondary clarifiers. Clarifier scraper mechanisms are designed in 5-foot increments. Circular secondary clarifier diameters range from 10 to 200 feet (3 to 60 meters).
264 Chapter 10
Domestic Wastewater Treatment
10.8 DISINFECTION OF WASTEWATER Before treated wastewater can be discharged to surface waters or reused for irrigation, it must be properly disinfected. Historically, chlorine has been the disinfectant of choice, because it effectively kills pathogens and is economical compared with other disinfection methods. If chlorine is used for disinfection, it must be neutralized or removed from the wastewater prior to discharge to surface waters, since it is toxic to fish and other aquatic life. Sodium thiosulfate, sodium bisulfite, and sulfur dioxide are some of the chemicals most widely used for removing chlorine from wastewater (Reynolds and Richards, 1996). 10.8.1 Disinfectants Used in Wastewater Treatment Chlorine 1Cl 22, ozone 1O32, and ultraviolet (UV) radiation are three major disinfectants that have been used successfully for treating wastewater prior to discharge. Although more costly from both a capital and an O & M viewpoint, the use of ozone and UV radiation is increasing. Both of these disinfectants offer the advantage of eliminating the production of trihalomethanes (THMs), which are chlorinated organic species and suspected carcinogens that result when chlorine is added to water containing organic compounds. Ozone is a powerful oxidizing agent and effectively kills bacteria, viruses, and cysts. The problem with ozone is that no residual is formed, and chlorine or other disinfectants must be added following ozonation to provide a disinfectant residual if the treated wastewater is to be distributed in a reuse system. Using UV radiation as the major disinfectant requires a low turbidity or suspended solids concentration in the wastewater, since these species absorb the UV light and shield microorganisms from exposure. UV radiation systems are expensive, and the light bulbs are housed in quartz glass sleeves that must be periodically cleaned to prevent the build-up of scale and biological growth that may occur. As with ozonation, an auxiliary disinfectant must be added if the wastewater is to be reused. An advantage of both ozonation and UV radiation is elimination of the need to remove chlorine if the effluent is discharged to surface waters. 10.8.2 Chlorination of Wastewater Chlorine is delivered to wastewater treatment facilities in pressurized containers, which range in size from 150-lb (68-kg) to 1-ton (908-kg) cylinders all the way up to 55-ton (49.9-Mg) railroad tank cars (Metcalf and Eddy, 1991). The chlorine is liquefied under high pressure and may be withdrawn as a gas or a liquid, depending on the withdrawal rate. At large WWTPs, chlorine is usually withdrawn as a liquid and must pass through an evaporator to be converted to a gas. Evaporators are recommended where the quantity of chlorine supplied approaches 2000 lb/d (908 kg/d). The chlorine gas is then dissolved into a side stream of water using a vacuum pressure system and regulator. Because of safety concerns about potential gas leaks (chlorine is toxic and corrosive), the dissolving of chlorine gas directly into the wastewater is seldom practiced. Chlorine dosage rates are dependent upon the type of treatment process used. Typical chlorine dosages range from 2–8 mg/L for activated sludge effluent and from 1–5 mg/L for filtered activated sludge effluent (Metcalf and Eddy, 2003). 10.8.3 Chlorine Contact Basin Once chlorine is added to the secondary effluent, it must have sufficient contact with the wastewater to kill potential pathogens. Long, rectangular serpentine channels simulating the plug flow regime are used in the design of chlorine contact basins
Section 10.8
Disinfection of Wastewater 265
(CCB). Length-to-width (L:W) ratios of 10:1 to 40:1 are used in design to prevent short circuiting (Metcalf and Eddy, 1991). The design of a CCB is based on detention time, which is calculated by dividing the volume of the basin by the design flow rate.
EXAMPLE 10.4 Chlorine contact basin design A chlorine contact basin is to be designed for maintaining a detention time of 15 minutes at peak hourly flow (PHF) and/or 30 minutes at the average daily flow (ADF). The activated sludge wastewater treatment plant treats an average daily flow of 60,000 m3/d, and the ratio of the estimated peak hour flow to average daily flow is 3:1. Design a three-pass, serpentine chlorine contact basin using a length-to-width ratio of 10:1. Solution Calculate the volumes for the chlorine contact basin for the two flow scenarios. First, rearrange the detention time equation so that the volume may be calculated. Volume of CCB at ADF: V = uQ = 30 min ¢
60,000 m3 1h 1d ≤¢ ≤¢ ≤ = 1250 m3 60 min 24 h d
Once the peak hourly flow is estimated, the volume of CCB at PHF can be determined. The PHF is calculated as: PHF = 3 ¢ 60,000
m3 m3 ≤ = 180,000 d d
V = uQ = 15 min ¢
180,000 m3 1h 1d ≤¢ ≤¢ ≤ = 1875 m3 60 min 24 h d
The volume of the CCB is controlled by the PHF and should be 1875 m3. Assume that we have two chlorine contact basins of equal size, 3 m deep. The volume per basin and the surface area are calculated below. volume 1875 m3 = = 938 m3 ⬵ 940 m3 basin 2 area 940 m3 = = 313 m2 basin 3m Assuming three passes per basin and a L:W ratio of 10:1 1L = 10W2, the area of the basin per pass is calculated as: A = L * W = 10W2 = 313 m2
W = 2313/10 = 5.6 m
266 Chapter 10
Domestic Wastewater Treatment
Use a width of 5.6 m and length of 313/5.6 = 56 m. Each CCB basin will have a plan view as shown below. Q 5.6 m 5.6 m Q 5.6 m
56 m
10.9 SLUDGE TREATMENT AND DISPOSAL Sludge or biosolids and other residuals are generated at wastewater treatment facilities. Screenings and grit from preliminary treatment unit operations are normally collected in dumpsters and then hauled off to sanitary landfills for ultimate disposal. The residuals produced from primary and secondary treatment produce large quantities of sludge that must be properly stabilized. The sludge volume must be reduced before it may be land applied or disposed of in a sanitary landfill. Primary sludge contains organic solids and pathogens that must be properly treated. Traditionally, anaerobic digestion has been used to stabilize primary sludge and to kill potential pathogens. Secondary sludge consists primarily of biological organisms, i.e., biomass, and typically is stabilized using aerobic digestion or anaerobic digestion. Lime stabilization is another widely used method for killing pathogens in sludge by raising the pH of the sludge above 10 for several hours. Unfortunately, the addition of lime to the sludge creates more solids to dispose of. Both anaerobic and aerobic digestion decreases the volume and quantity of sludge. Sludge treatment and disposal generally consists of four distinct steps. The first step involves reducing the sludge volume through some sort of thickening operation. The second step involves stabilization with anaerobic or aerobic digestion. Stabilization reduces the volatile solids content, kills pathogens, and reduces the volume of the sludge. The third step involves sludge dewatering operations to increase the concentration of solids. The final step is ultimate disposal. 10.9.1 Sludge Weight and Volume Relationships The designer of solids handling and treatment systems must be familiar with sludge weight and volume relationships. In addition to a discussion of the equations presented in this section, sources, characteristics, and quantities of waste sludges can be found in Viessman and Hammer (2005). The specific gravity of wet sludge is estimated using the following equation: S =
WW + WS 1WW>1.02 + 1WS>SS2
where: S = specific gravity of wet sludge, WW = weight of water, lb (kg),
(10.26)
Section 10.9
Sludge Treatment and Disposal
267
WS = weight of dry solids, lb (kg), and SS = specific gravity of dry solids. The volume (V) occupied by wet sludge 1ft3 or m32 is determined using the following equation: V =
WS 1s>1002g S
(10.27)
where: s = solids content, %, and g = specific weight of water, 62.4 lb/ft3 (1000 kg/m32. 10.9.2 Thickening Operations Thickening of sludge is very important, since it reduces the volume of the sludge to be handled, thereby reducing the size of subsequent solids handling processes such as sludge stabilization. Increasing the solids content of sludge from 1% to 2% results in a 50% reduction in the sludge volume. Typical sludge thickening operations and the approximate solids concentration associated with each include gravity thickening (2% to 10% solids), gravity belt thickeners (3% to 6% solids), dissolved air flotation (3% to 6% solids), and thickening centrifuges (4% to 8% solids). Recall that a 1% solids concentration is approximately equal to 10,000 mg/L, assuming that the specific gravity of the sludge is 1.0. Designing sludge thickening unit operations involves performing mass balances on the sludge flows and sludge quantities into and out of the unit operation. Example 10.5 illustrates the design of a gravity belt thickener.
EXAMPLE 10.5 Gravity belt thickener design Design a gravity belt thickener (GBT) to be used for thickening waste activated sludge (WAS) from 1% solids to 5% solids. Ten thousand pounds of WAS are produced daily. The WAS is pumped from the RAS/WAS pumping station to a sludge holding tank. Positive displacement pumps are used to pump the WAS from the holding tank to the GBT. The GBT will operate 8 hours a day, 5 days a week. The TSS concentration in the filtrate is 1000 mg/L. Use the following design criteria in designing the GBT: hydraulic loading rate = 140 to 450 gpm/m, solids loading rate = 370 to 1200 lb/(m # hr), and washwater rate = 25 gpm/meter of belt width. Solution First, calculate the daily solids loading rate to the GBT. 10,000 ppda
7 d 1 wk 1 d lb lb ba ba b = 1750 or 14,000 wk 5d 8h hr d
Next, determine the sludge volume entering the GBT using Equation (10.27) and assuming the specific gravity of the wet sludge (S) is equal to 1.0. V =
WS 14,000 ppd = = 168,000 gpd or 350 gpm 1s>1002g S 11%>100218.34 lb>gal211.02
350 gpm is the volume of sludge treated in an 8 hour operating period
268 Chapter 10
Domestic Wastewater Treatment
Next, draw a schematic diagram so that a flow and materials balance can be performed. Washwater 0% solids Sludge feed
Sludge cake
GBT
1% Solids 14,000 ppd
5% Solids 1,000 mg/L
Filtrate
Determine the belt width of the GBT. Belt widths range from 0.5 to 3 meters in increments of 0.5 meter. Typical GBTs in service use 2.0-meter belts. Pick a hydraulic loading rate of 175 gpm/m from the design criteria given: belt width =
350 gpm = 2.00 m 175 gpm>m
Check the actual solids loading rate (SLR) to see if it meets the design criteria: solids loading rate =
1750 pph of solids = 875 lb>1m # h2 2m
This is acceptable, since the SLR design criteria specify 370 to 1200 lb/1m # h2. Next, determine the sludge cake and filtrate flows by performing a flow and materials balance around the GBT. Cake and filtrate flows are denoted as QC and QF , respectively. Qsludge + Qwashwater = QF + QC 350 gpm + 12 * 25 gpm2 = QF + QC 400 gpm = QF + QC 400
gal 60 min 8 h 1 MG a ba b¢ 6 ≤ = 0.192 MGD min h d 10 gal 0.192 MGD = QF + QC QC = 0.192 MGD - QF
Perform a materials balance on solids. Recall that 1% solids is equal to 10,000 mg/L. Msludge = MC + MF lb lb mg mg MG MG 14,000 ppd = QC a8.34 2 + QF a8.34 2 b15 * 10,000 b 11000 mg mg L L L L
Section 10.9
Sludge Treatment and Disposal
14,000 ppd = 417,000QC + 8,340QF 14,000 ppd = 417,00010.192 - QF2 + 8,340QF 408,660QF = 66,064
QF = 0.162 MGD
QC = 0.192 MGD - QF = 0.192 - 0.162 = 0.030 MGD Now, calculate the quantity of solids in the filtrate. 10.162 MGD2a8.34
lb>MG mg>L
b11000 mg>L2 = 1351 ppd
The percent capture through the GBT can be calculated using the following equation: % capture = % capture =
3solids in feed - solids in filtrate4 100 solids in feed 314,000 ppd - 1351 ppd4 100 14,000 ppd
= 90.4%
The accompanying schematic diagram shows the complete materials balance:
Washwater
0% Solids 50 gpm
Sludge feed
Sludge cake
GBT 1% Solids 14,000 ppd 350 gpm
5% Solids 12,649 ppd 63 gpm 1,000 mg/L 1351 ppd 337 gpm Filtrate
10.9.3 Stabilization The objectives of sludge stabilization are to reduce the liquid volume and quantity of sludge solids, prevent nuisance odors, and reduce the pathogen content. Two primary means of stabilizing wastewater sludge or biosolids include anaerobic and aerobic digestion. Presented next is a brief description of anaerobic digestion, followed by a discussion of aerobic digestion. Anaerobic Digestion
Anaerobic digestion is a biological process that takes place in an enclosed reactor with no oxygen present. It is modeled as a three-step process: hydrolysis, acidogenesis, and methanogenesis. During hydrolysis complex organic solids are hydrolyzed
269
270 Chapter 10
Domestic Wastewater Treatment
by bacteria through secretion of extracellular enzymes. Carbohydrates, proteins, and fats are converted to simple carbohydrates, amino acids, and fatty acids. Acidogenesis involves the conversion of soluble carbon formed during hydrolysis into organic acids and H 2 by facultative and anaerobic bacteria. The pH drops, and other fermenting bacteria partially oxidize organic acids to acetic acid and hydrogen gas. Methanogenesis is the bacterial conversion of fatty acids and hydrogen into CH 4 and CO2 by strict anaerobes. This gasification of organics is accomplished by acidsplitting, methane-forming bacteria known as methanogens. Anaerobic digestion produces a stable sludge and a valuable by-product, methane gas, which can be combusted to provide heat for the digesters or used for generating electricity. Only small quantities of organic matter and cellular protoplasm remain in the digested sludge. Anaerobic digesters are used extensively at WWTPs that have primary clarifiers. A disadvantage of using anaerobic digestion is the required use of large reactors due to long detention times. Anaerobic digestion is sensitive to operate and prone to biological upsets. Anaerobically digested sludge is usually difficult to dewater by mechanical means. Normally, two-stage anaerobic digestion is practiced. Undigested sludge is fed first to a mixed, heated, anaerobic digester, then to an unmixed and unheated second digester. Figure 10.12 is a schematic of a two-stage anaerobic digester system. Design criteria for conventional or standard-rate anaerobic digesters prescribe (Viessman and Hammer, 2005) volatile solids loading rates of 0.02–0.05 lb VSS/1d # ft 32 [0.32–0.80 kg/1d # m324 and detention times of 30–90 days. High-rate anaerobic digesters are designed for volatile solids loading rates of 0.1–0.2 lb VSS/d # ft3 [1.6–3.2 kg/(d # m3)] and detention times of 10-20 days. Low- or standard-rate anaerobic digester volume can be estimated as follows: V =
Q1 + Q2 * T1 + Q2 T2 2
(10.28)
where: V Q1 T1 Q2 T2
= = = = =
total digester capacity, ft3 1m32, volume of raw sludge fed daily, cfd 1m3/d2, period required for digestion, approximately 25 d, volume of daily digested sludge accumulation in tank, cfd 1m3/d2, and period of digested storage, 20 to 120 d. Gas removal
Figure 10.12 Schematic of two-stage anaerobic digestion.
Gas transfer
Gas
Gas storage
storage
Scum layer Sludge in
Sludge heater
Mixer
Supernatant withdrawal
Supernatant layer Actively digesting sludge
Digested sludge Stabilized sludge thickened
Section 10.9
Sludge Treatment and Disposal
271
Equation (10.29) is used for determining the volume of a high-rate anaerobic digester. VI = Q1 * T
(10.29)
where: VI = digester capacity required for first-stage or high-rate digestion, ft3 1m32, and T = period required for digestion, d. The volume of a second-stage, low-rate anaerobic digester is determined as: VII =
Q1 + Q2 * T1 + Q2 T2 2
(10.30)
where: VII = digester capacity required for second stage, ft3 1m32, Q1 = volume of digested sludge feed = volume of average daily raw sludge feed cfd 1m3/d2, Q2 = volume of daily digested sludge accumulation in tank, cfd 1m3/d2, T1 = period required for thickening, days, and T2 = period required for digested sludge storage, days. Example 10.6 shows how to design a single-stage conventional anaerobic digester.
EXAMPLE 10.6 Conventional anaerobic digester design Calculate the anaerobic digester capacity 1m32 required for conventional single-stage anaerobic digestion, given the following data: raw sludge production = 630 kg/d; VS in raw sludge = 70%; moisture content of raw sludge = 95%; percent solids content of raw sludge = 5% solids; digestion period = 30 days; VS reduction = 50%; moisture content of digested sludge = 93%; storage time required = 90 days; and percent solids in thickened digested sludge = 7%. Solution First, calculate the volume of raw sludge fed to the digester using Equation (10.27) assuming S is 1.0. V =
630 kg/d Ws = = 12.6 m3>d 1s>1002g S 15%>100211000 kg>m3211.02
Perform a materials balance on the mass of solids or sludge entering and exiting the digester. First, determine the volatile and fixed solids in the raw sludge fed to the digester. By definition, total solids (TS) ⴝ volatile solids (VS) ⴙ fixed solids (FS). Mass of VS in raw sludge = 0.70 1630 kg/d2 = 441 kg/d Mass of FS in raw sludge = TS - VS = 630 - 441 = 189 kg/d
272 Chapter 10
Domestic Wastewater Treatment
Next, calculate the volatile and total solids quantities remaining after digestion. Digestion of the solids causes a 50% reduction in the volatile solids, and the fixed solids are assumed to remain unchanged. Mass of volatile solids remaining after digestion = 0.50 * 441 kg/d = 220.5 kg/d Mass of total solids remaining after digestion = VS + FS = 220.5 + 189 = 409.5 kg/d Now, calculate the volume of digested sludge that accumulates daily in the tank, assuming that the sludge thickens to 7% solids, using Equation (10.27) and taking the specific gravity of the digested sludge to be 1.0. V =
WS 409.5 kg/d = = 5.85 m3>d 1s>1002g S 17%>100211000 kg>m3211.02
Finally, the volume of the conventional, single-stage anaerobic digester is determined from Equation (10.28). 112.6 m3>d + 5.85 m3>d2 Q1 + Q2 5.85 m3 * T1 + Q2 T2 = * 30 d + 190 d2 2 2 d 3 = 803 m
V =
Aerobic Digestion
Aerobic digestion is similar to the activated sludge process and is actually a continuation of the process operating in the endogenous phase. Organic sludge is aerated for extended periods so that the organic material is oxidized biologically and the microorganisms oxidize their own protoplasm, once the exogenous (external) carbon has been exhausted. Equation (10.31) is a qualitative biochemical reaction, showing what happens during aerobic digestion: organic matter + O2
aerobic microbes
" new cells + energy + CO + H O (10.31) 2 2 + end products
The next equation is a quantitative biochemical reaction for aerobic digestion, where the microbial solids are represented as C 5H 7O2N: C 5H 7O2N + 7 O2 : 5 CO2 + 3 H 2O + H + + NO3-
(10.32)
Equation (10.32) indicates that the pH will drop due to the production of hydrogen ions 1H +2 and that digestion requires approximately 1.98 lb of oxygen per lb of cells 17 * 32/1132 or 1.98 kg of oxygen per kg of cells. The process may utilize continuous flow or batch operation. Aerobic digestion works best on waste activated sludge, since exogenous carbon has been depleted, and therefore synthesis of new biomass does not occur. Aerobic digestion is called a process for significant reduction of pathogens (PSRP). Equation (10.33) is used for calculating the required detention time for completely mixed aerobic digesters:
Section 10.9
u = td =
31X02 - 1Xe24 V = Q Kd 31Xe2 - 1Xn24
Sludge Treatment and Disposal
273
(10.33)
where: u Kd X0 Xe Xn
= = = = =
td = detention time or digestion time, d, degradation constant or aerobic digestion rate constant, d-1, total VSS concentration in influent sludge to digester, mg/L, total VSS concentration in effluent digested sludge, mg/L, and nondegradable VSS in effluent digested sludge, mg/L.
The detention time 20°C in an aerobic digester ranges from 12 to 22 days (Reynolds and Richards, 1996). The detention time may be corrected for temperatures other than 20°C as follows: tT°C = t20°C11.0652120 - T°C2
(10.34)
Sufficient oxygen must be supplied to the aerobic digester so that oxygen does not limit the biological reactions that are occurring. For design purposes, 2.0 lb of oxygen per lb of solids destroyed (2.0 kg/kg) must be supplied to meet process oxygen requirements. This value is used for both waste activated sludge and trickling filter humus. Approximately 1.9 lb of oxygen per lb of BOD (1.9 kg/kg) is used for computing the oxygen required for stabilizing primary sludge (Reynolds and Richards, 1996). Also, air or mechanical energy must be input to the digester to keep the microorganisms in suspension. Mechanical mixers need 0.5 to 1.25 HP/1000 ft3 (13 to 40 kW/1000 m3) to meet the mixing requirement. Diffused aeration systems must supply 20 to 35 cfm/1000 ft3 [25 to 35 m3/(min # 1000 m3)] for digesters treating WAS and 60 cfm/1000 ft3 360 m3/(min # 1000 m324 for aerobic digesters treating a combination of primary sludge and WAS. The design of aerobic digesters is usually based on hydraulic detention time or solids loading rate.The hydraulic detention time at 20°C varies from 12–22 days depending on the type of sludge being treated (Reynolds and Richards, 1996). Reynolds and Richards (1996) indicate that the volatile-solids loading rate will vary from 0.04–0.20 lb VS/1d # ft 32 30.64–3.20 kg VS/1d # m324. The operating temperature should be greater than 15°C. The design of an aerobic digestion system is presented in Example 10.7.
EXAMPLE 10.7 Aerobic digester design An aerobic digester is to be designed to stabilize 3266 lb of combined primary and wasteactivated sludge. The volume of the combined sludge that will be fed to the digester is 1036 cfd. The volatile solids concentration of the combined sludge is 70%, and the minimum design operating temperature is 16°C.The primary sludge contains 940 lb of BOD with an oxygen demand of 1.9 lb of oxygen per lb of BOD, whereas the WAS consists of 865 lb of VSS with an oxygen demand of 2.0 lb of oxygen per lb of VSS destroyed. Determine the following: a. b. c. d. e.
Design hydraulic detention time or digestion time in days. The aerobic digester volume. The volatile solids loading rate to the aerobic digester. The quantity of oxygen required to stabilize the primary sludge. The quantity of oxygen required to stabilize the secondary sludge (WAS).
274 Chapter 10
Domestic Wastewater Treatment
f. The total quantity of oxygen required to stabilize the combined sludge. g. The total air required in cfm if air contains 0.0175 lb of oxygen per cubic foot of air and the diffusers have a transfer efficiency of 5.0%. Solution part a Use a hydraulic detention time of 20 days and correct it for the minimum design operating temperature of 16°C, using Equation (10.34). tT°C = t20°C 11.0652120 - T°C2 = 20 d11.0652120 - 16°2 = 25.7 days Solution part b The aerobic digester volume is calculated by multiplying the hydraulic detention time by the volume of sludge fed to the digester: V = tQsludge = 25.7 days ¢ 1036
ft3 ≤ = 26,600 ft3 d
Solution part c Next, calculate the volatile solids loading to the digester. VS loading = 3266 ppd10.702 = 2286 ppd The volatile solids loading rate to the digester is determined as: VS loading to digester =
2286 ppd VS 26,600 ft3
= 0.086
lb VS ft3 # d
Solution part d Oxygen required to stabilize the BOD in the primary sludge is estimated as: O2 to stabilize primary BOD = a940
lb O2 lb O2 lb BOD b ¢ 1.9 ≤ = 1786 d lb BOD d
Solution part e Oxygen required to stabilize the VS in the secondary sludge (WAS) is estimated as: O2 to stabilize VS in WAS = a2286
lb O2 lb O2 lb VS b ¢ 2.0 ≤ = 4572 d lb BOD d
Solution part f The total oxygen required to stabilize the combined primary and secondary sludge is determined below: total O2 required = 1786 + 4572 = 6358
lb O2 d
Solution part g Finally, the total air required (cfm) to stabilize the combined sludge is estimated as follows, assuming that air contains 0.0175 lb O2/ft3 and the diffuser transfer efficiency is 5%: air required =
6358 lb O2>d
1d 1h ba b = 5050 cfm 310.0175 lb O2>ft 210.0524 24 h 60 min 3
a
Summary
Check to see if the mixing requirements have been met air required 1000 ft
3
=
5050 cfm 190 cfm 60 cfm 11000 ft32 = 7 3 3 26,600 ft 1000 ft 1000 ft3
This exceeds mixing requirements, but the process oxygen requirements dictate that this amount of air must be supplied so that aerobic digestion can be accomplished.
10.9.4 Dewatering Dewatering operations consist of reducing the sludge volume and increasing the solids content, primarily using mechanical means. Typical dewatering operations and their anticipated solids concentration include: centrifuges (5% to 40% solids), belt filter presses (12% to 50% solids), vacuum filtration (20% to 30% solids), plate and frame presses (34% to 60% solids), and sand drying beds (up to 50% solids). In most instances the sludge must be conditioned by adding a polymer or coagulant aid such as lime or ferric chloride to increase the effectiveness of the dewatering operations. Material balances on flow and solids must be performed when designing any type of solids-handling equipment and process. 10.9.5 Sludge Disposal Once sludge has been properly treated, several options are available for disposal.The U.S. Environmental Protection Agency encourages the beneficial reuse of biosolids through land application. The type of sludge category, whether or not it is Class A or B, will determine if the biosolids can be land applied without any site restrictions. A discussion of the various options, along with explanations of the Standards for the Use or Disposal of Sewage Sludge, can be found in “A Plain English Guide to the EPA Part 503 Biosolids Rule” (EPA, 1994). Biosolids may also be placed in “sludge only” landfills or monofills, but this really is not a beneficial reuse option. Marketing and distribution of biosolids that have been stabilized, composted, or heat treated have been successful for many municipalities. The city of Tampa, Florida, pelletizes its biosolids and then sells them to fertilizer manufacturers as an amendment to increase the organic content of the fertilizer. The city of Milwaukee, Wisconsin, is successful in selling its biosolids product, Milorganite. Another beneficial reuse of biosolids is called dedicated disposal. This involves the application of biosolids to strip mines to grow vegetation and try to recover these types of areas for other uses. Incineration of sludge is an option that is becoming less attractive because of the high cost of building and operating incinerators, plus the creation of air pollution and need to landfill the ash. Ocean dumping has been banned in the United States, so that is no longer an option.
S U M M A RY This chapter presented the objectives of treating wastewater along with a classification system for identifying the types of wastewater treatment plants (WWTPs).Various unit operations and unit processes used for treating wastewater were discussed, and examples were presented showing how to design primary clarifiers, activated sludge systems, secondary clarifiers, and chlorine contact chambers.You learned that unit operations involve physical treatment of the wastewater,whereas unit processes involve a biological or chemical
275
276 Chapter 10
Domestic Wastewater Treatment
reaction. Biokinetic equations were presented that are used for designing activated sludge processes. Unit operations and unit processes used in handling, treating, and disposing of wastewater sludge or biosolids were introduced. Design of these systems requires that material balances on flow and solids be performed around the specific system. Example problems were presented to illustrate the design of thickening, and destabilzation systems.
KEY WORDS
activated sludge actual oxygen transfer rate aeration basin aerobic digestion anaerobic digestion AWT bar screens biokinetic equations biomass centrifuge completely mixed detention time domestic wastewater
endogenous decay force main gravity belt thickener gravity sewer grit removal industrial wastewater land application mass balances mean cell residence time microbial growth microorganisms nitrogenous oxygen demand nonpoint source
overflow rate plug flow primary clarifier secondary clarifier solids loading rate standard oxygen transfer rate substrate utilization weir loading rate yield coefficient
REFERENCES Ardern, E., and Lockett, W.T. (1914). Experiments on the Oxidation of Sewage Without the Aid of Filters, Journal Society Chemical Industry, 22, 523–539, 1122–1124. Benefield, Larry D., and Randall, C.W. (1980). Biological Process Design for Wastewater Treatment, Prentice Hall, Englewood Cliffs, NJ. Eckenfelder, W.W. (1989). Industrial Water Pollution Control, McGraw Hill, New York. EPA (1994). A Plain English Guide to the EPA, Part 503 Biosolids Rule, EPA/832/R-93/003. EPA (1993). Manual Nitrogen Control, EPA/625/R-93/010, Office of Research and Development, Office of Water, Washington, DC. EPA (1975). Suspended Solids Removal, Process Design Manual, Office of Research and Development, Office of Water, Washington, DC. Heukelekian, H., Oxford, H.E., and Manganelli, R. (1951). Factors Affecting the Quantity of Sludge Production in the Activated Sludge Process, Sewage and Industrial Wastes, 23, 945. Lawrence, A.W., and McCarty, P.L. (1970). Unified Basis for Biological Treatment and Design and Operation, Journal of Sanitary Engineering Division, ASCE, 96, 757–778. Metcalf and Eddy, Inc. (2003). Wastewater Engineering: Treatment and Reuse, McGraw Hill, New York. Metcalf and Eddy, Inc. (1991). Wastewater Engineering: Treatment, Disposal, and Reuse, McGraw Hill, New York Reynolds, T.D., and Richards, P.A. (1996). Unit Operations and Unit Processes in Environmental Engineering, PWS Publishing Company, Boston. Sherrard, J.H., and Schroeder, E.W. (1973). Cell Yield and Growth Rate in Activated Sludge, Journal Water Pollution Control Federation, 5, 1189. Thabaraj, G.J. (1993). AWT Processes in Florida, Florida Department of Environmental Regulation, Tampa, FL, 1–19. Viessman, Warren, and Hammer, Mark J. (2005). Water Supply and Pollution Control, Pearson/ Prentice Hall, Upper Saddle River, NJ. Water Environment Federation (1998). Biological and Chemical Systems for Nutrient Removal, 601 Wythe Street, Alexandria, VA. Water Environment Federation (1998). MOP #8, Design of Wastewater Treatment Facilities, 601 Wythe Street, Alexandria, VA.
Exercises 277
EXERCISES 10.1
10.2
10.3
10.4
10.5
Two circular primary clarifiers are to be designed to treat domestic wastewater following preliminary treatment. State regulatory requirements stipulate that the overflow rate must be 600 gpd/ft2 at the average daily flow and 1000 gpd/ft2 at the peak hourly flow. The detention time should be between 1 and 2 hours for all flows. The minimum weir loading rate at peak hourly flow cannot exceed 30,000 gpd/ft. The average daily flow to the primary clarifiers is 9 MGD. The peaking factor for the PHF:ADF is 2.5:1. Determine the minimum-diameter clarifier to the nearest 5-ft interval for two clarifiers operating in parallel to meet the criteria. Determine the side water depth (SWD) to meet the detention time requirements. Calculate the weir length necessary to meet the weir loading condition established by the regulatory agency. Two rectangular primary settling basins operating in parallel are to be designed to treat domestic wastewater following preliminary treatment. The average daily flow coming into the treatment plant is 10,000 m3/d and the PHF:ADF ratio is 3:1. Each basin is expected to treat 75% of the peak hourly flow with one unit out of service. State regulatory requirements stipulate that the maximum overflow rate must not exceed 100 m3/1d # m22. The detention time should be between 1 and 2 hours at the average daily flow. The maximum weir loading rate at peak flow cannot exceed 350 m3/1d # m2. Determine the dimensions (L, W, D) of the settling basins using a L:W ratio of 4:1 to meet the design criteria. Determine the side water depth (SWD) to meet the detention time requirements. Calculate the weir length necessary to meet the weir loading condition established by the regulatory agency. Design a completely-mixed activated sludge process using three different design approaches (i.e., calculate the volume of the aeration basin). Base your design for the first approach on detention time, for the second one on BOD loading, and for the third one on kinetics. The influent design flow rate to the activated sludge process is 10,000 m3/day with a BOD5 concentration of 200 mg/L. Completely-mixed activated sludge processes typically have a detention time ranging from 3 to 6 hours and BOD5 loadings ranging from 50 to 120 lb BOD5/1d # 1000 ft32. Typical biokinetic coefficients (Reynolds and Richards, 1996) used in design are: Y = 0.6 mg VSS/mg BOD5 , k = 3 d-1, KS = 60 mg/L, and kd = 0.06 d-1. Use a MCRT = 10 days and X = 2500 mg/L VSS. A completely-mixed activated sludge process is to be designed to treat 10 MGD of domestic wastewater containing 200 mg/L of COD. Use the following design parameters in the design process: X = 2500 mg/L MLVSS, influent TKN = 20 mg/L. The following biokinetic coefficients should be used: Y = 0.40 mg VSS/mg COD, k = 7 d-1, KS = 75 mg/L, and kd = 0.06 d-1 . Determine the volume and the oxygen required at a MCRT = 15 days. A circular secondary is to be designed for a pure oxygen activated sludge process. The state’s regulatory agency design criteria are as follows: peak overflow rate = 50 m3/1d # m22, average overflow rate = 20 m3/1d # m22, peak solids loading rate = 245 kg/1d # m22, peak weir loading rate = 375 m3/1d # m2, and depth = 3.5 to 4.5 meters. The average daily flow to the aeration basin prior to the junction with the recycle flow is 6800 m3/day. The maximum recycled sludge flow is 100% of the average daily flow. The design mixed liquor suspended solids concentration is 5000 mg/L, and the ratio of the peak hourly
278 Chapter 10
Domestic Wastewater Treatment
to the average daily flow is 2.8. Determine the diameter, detention time at peak hourly flow, and peak weir loading, assuming a peripheral weir is used. 10.6 A rectangular settling basin is to be designed for a conventional, plug flow activated sludge process. The flow is 10 MGD, the overflow rate 1Vo2 is 700 gpd/ft2, the detention time 1u2 is 5 hours, and the weir loading rate is 22,000 gpd/ft. The settling basin sludge rake mechanism that will be used is for square basins; therefore, two will be used in tandem to provide a length-to-width ratio of 2:1. Determine the dimensions of the settling basin (L, W, and D) and the total length of the effluent weir. 10.7 Compute the volume 1m32 of 1000 kg of waste activated sludge if the moisture content is 96%. What volume will it occupy if the solids content is increased to 8% by gravity thickening? 10.8 A gravity belt thickener (GBT) is proposed for thickening 7000 kg/day of waste activated sludge containing 1% solids. The hydraulic loading rate to the GBT will be 200 gpm/meter and the solids loading rate will be between 200 to 600 kg/1m # h2. The anticipated concentration of solids in the thickened sludge is 6%. Assume that 5 kg of dry polymer are required per Mg (1 Mg = 106 g) of dry solids fed to the GBT. Determine the effective width of the belt, assuming that 25 gpm of wash water is required per meter of belt and determine the suspended solids concentration in the centrate. Assume that the solids capture rate is 95%. 10.9 Design a conventional, single-stage anaerobic digester based on the following parameters: daily raw sludge production = 650 kg; volatile solids in undigested sludge = 70%; solids content in raw sludge = 5%; digestion period = 30 days; storage volume for digested sludge = 90 days; volatile solids destruction during digestion = 50%; and solids content of thickened, digested sludge = 7%. 10.10 An aerobic digester is to be designed to treat the waste activated sludge (WAS) from a completely-mixed activated sludge process that does not have primary clarifiers. Approximately 1300 kg of dry solids (WAS) are produced daily containing 0.6% solids and 70% volatile solids and having a wet specific gravity of 1.10. The minimum design temperature is 60°F and a temperature correction factor of 1.065 will be used in design. The detention time in the digester should be between 18 and 22 days at a temperature of 20°C. Determine the following: (a) design detention time (days) at 60°F, (b) volume of the aerobic digester 1m32 if the sludge is thickened to 2.5% solids prior to digestion, (c) the quantity of oxygen (kg) required daily if 2.0 kg of O2 are required per kg of volatile solids destroyed (assume that 60% of the volatile solids are destroyed during aerobic digestion), (d) the air flow rate 1m3/min2 if air contains 0.281 kg of oxygen per m3 and the efficiency of the diffused aeration system is 5%, (e) the volatile solids loading rate on the digester 31.6–3.2 kg/1d # m324, and (f) the volume of air required for mixing 3m3/1min # 1000 m324. 10.11 A belt filter press (BFP) with an effective belt width of 2.0 meters is used for dewatering 100 gpm of anaerobically digested sludge with a solids content of 7.0%. The polymer dosage is 6.5 gpm, containing 0.20% powdered polymer by weight. The wash water consumption is 30 gpm per meter of effective belt width. The cake solids content is 30%, and the suspended solids concentration in the filtrate is 1800 mg/L. Calculate the hydraulic loading rate, solids loading rate, and polymer dosage, and estimate the solids recovery.
CHAPTER
11
Air Pollution Objectives In this chapter, you will learn about: Sources and effects of air pollutants Air pollution regulations Global environmental issues Applications and design of air pollution control equipment
11.1 INTRODUCTION Air pollution can be categorized into segments including personal, occupational, indoor, and ambient pollution. Personal air pollution is usually associated with the contamination of respired air from cigarette, pipe, or cigar smoking. Other activities that result in personal air pollution include glue sniffing and other volatile-substance abuse practices. Occupational air pollution from the contamination of ambient breathing air in industrial settings where gases, vapors, and respirable particles are generated can result in potentially high levels of exposure to workers. The Occupational Safety and Health Administration (OSHA) is tasked to ensure that the work environment is safe such that workers are protected from exposure to air toxics and irritants. During the late 1980s, the United States Congress empowered the Environmental Protection Agency (EPA) to assess the extent of the indoor radon problem and to educate the public on hazards associated with radon exposure. Regulations associated with indoor air pollution and secondary, or passive, cigarette smoke were promulgated in the 1990s. This chapter focuses on ambient or community air pollution, which can be defined as the introduction of pollutants into the ambient air for durations which adversely affect human health, the public welfare, and the ecological balance. It introduces ambient-air pollution sources, associated environmental and health effects, and current regulatory practices. End-of-pipe unit operations and processes for treating air streams laden with gases and particulates are explored. Humans are totally dependent upon air for survival. Each day the average male ingests approximately 1.4 lb (0.64 kg) of food on a dry-weight basis (NASA, 1991), drinks 4.4 lb (2.0 kg) of water (U.S. EPA, 2000), and inhales 40 lb (18.1 kg) of air (U.S. EPA, 1997). Principal factors causing air pollution to increase include: (1) overdependence on the burning of fossil fuels for transportation and energy needs, (2) population growth—according to U.S. Census Bureau (2004 data), the world’s population will exceed 9 billion by the year 2050, (3) technological changes that result in new products, (4) a rising standard of living, and (5) societal changes such as urbanization, which is characterized by the population shift from rural areas to the urban environment.
280 Chapter 11
Air Pollution
Table 11.1 Most Common Chemical Species Found in Air Substance
Volume or mole %
N2
78%
79% N2
O2
20.9%
21% O2
Argon
' 1%
Typically used in Engineering Calculations
CO2
0.033%
The major constituents of air are nitrogen, oxygen, argon, and carbon dioxide. Table 11.1 provides the volumetric percent concentration of each gas. For most engineering calculations, nitrogen and oxygen concentrations are assumed to be 79% and 21% by volume, respectively.
11.2 HISTORY Air pollution is not a new phenomenon. Historical evidence indicates that ambient air pollution from copper smelting occurred in Turkey as early as the sixth millennium B.C.E. Similarly, pollution from iron smelting is believed to have occurred during the late third millennium B.C.E., and pollution from lead smelting during the rule of the Roman Empire has been traced all over Europe. One could argue that air pollution has been an issue since human species have controlled fire. Although somewhat controversial, evidence found in Kenya indicates that the Homo erectus species was intentionally using fire up to 1.5 million years ago. Tending of continuous fire has been dated back 460,000 years in China. In 61 C.E. the Roman Philosopher Seneca wrote: “As soon as I had gotten out of the heavy air of Rome, and from the stink of the smoky chimneys thereof, which, being stirred, poured forth whatever pestilential vapours and soot they had enclosed in them, I felt an alteration of my disposition.” Modern-era air pollution issues date to the early 13th century, when England’s King Edward I recognized that nuisance fumes were produced from the burning of sea coal. This awareness prompted the introduction of a law prohibiting the burning of sea coal while Parliament was in session, a violation punishable by the torturing and hanging of those who sold or burned the coal. The air pollution generated as a result of the Industrial Revolution, the invention of Watt’s steam engine, and the burning of fossil fuels for power generation motivated many smoke-control ordinances enacted in England and the United States during the late 1800s and early 1900s. An era of air pollution episodes also began in the early 1900s. An air pollution episode is defined as a major air pollution disaster ensuing from high levels of pollution that results in a large number of people dying. The first reported modern-day air pollution episode was recorded in the Meuse Valley, Belgium, where stable atmospheric conditions caused by a temperature inversion and a high-pressure system caused a buildup of sulfur-containing compounds, metallic oxides, and inorganic acids in the air of the densely populated, highly industrialized town. Reportedly, 63 people died, as well as a number of cattle. Many people became ill, with symptoms including sore throats, shortness of breath, cough, phlegm, nausea, and vomiting. Other notorious air pollution episodes occurred in Donora, Pennsylvania (1948), London, England (1952), Bhopal, India (1984), and New York City (September 11, 2001). Each incident deserves to be recognized and remembered. Most of the disasters occurred in densely populated
Section 11.3
Regulatory Overview 281
or heavily industrialized areas under winter weather conditions characterized by stagnant air and temperature inversion.
11.3 REGULATORY OVERVIEW The Clean Air Act (CAA) promulgated in 1970 and the associated 1990 Amendments empower the EPA to establish guidelines for controlling air pollution. Table 11.2 outlines the nine titles associated with the CAA, and an electronic version of the CAA can be found at the EPA’s Office of Air and Radiation website (www.epa.gov/oar/ caa/), which also provides a useful overview of the CAA. As a result of Title I, criteria pollutants were identified and National Ambient Air Quality Standards (NAAQS) were set.The EPA identified six common air pollutants— carbon monoxide (CO), nitrogen dioxide 1NO22, sulfur dioxide 1SO22, lead (Pb), particulate matter (PM), and ozone 1O32—as criteria pollutants. Primary and secondary time-dependent concentration standards were set and are summarized in Table 11.3. Primary standards are set to protect the general population, including children, asthmatics, and the elderly, from health risks associated with exposure to criteria pollutants in ambient air. The secondary standard is public welfare based and is intended to prevent unacceptable effects on buildings and property, crops, and the ecosystem. Primary and secondary standards are the same for all criteria pollutants with exception to CO, for which no secondary standard exists, and SO2 , where the secondary standard is 0.50 parts per million volume (ppmv) for a 3-hr time average. Title III of the CAA defines National Emission Standards for Hazardous Air Pollutants (NESHAPS). Standards are set both to protect public health and to prevent undesirable effects on the environment, including aquatic life and wildlife. Currently the list includes 188 compounds, with the majority being classified as volatile organic compounds (VOCs). VOCs listed include those commonly associated with gasoline, such as benzene and toluene, and with the dry cleaning industry, such as tetrachloroethylene. Metals containing lead, cadmium, and mercury are also listed. The current listing of EPA-regulated hazardous air pollutants (HAPs) can be found online at http://www.epa.gov/ttn/atw/188polls.html. HAP emission regulations are determined differently for new and existing sources and are controlled by standards dictated by maximum achievable control technologies (MACT) or generally available control technologies (GACTs). If, after attainment of the MACT or GACT standards, Table 11.2 Principal Initiatives of the 1990 Clean Air Act Title I
Focused on attaining national air quality standards for criteria pollutants
Title II
Establish more stringent fuel standards for and control of tailpipe emissions from vehicles
Title III
Aspires to reduce emissions of hazardous air pollutants from major sources
Title IV
Control emissions of pollutants responsible for acid rain
Title V
Outlines a comprehensive permitting program for stationary sources
Title VI
Focused on global climate issues, such as protecting the stratospheric ozone layer by regulating the production of substances with an ozone-depleting potential (CFCs, HCFCs) and monitoring greenhouse gases
Title VII
Provisions regarding enforcement
Title VIII
Miscellaneous provisions
Title IX
Clean air research
Source: Available on the EPA Office of Air and Radiation website at http://www.epa.gov/oar/caa/.
282 Chapter 11
Air Pollution
Table 11.3 Summary of Primary National Ambient Air Quality Standards for Criteria Pollutants Primary standard Criteria pollutant
Time average
CO
8-hr (not to be exceeded more than once/year)
9 ppmv
1-hr (not to be exceeded more than once/year)
35 ppmv
Concentration
NO2
Annual arithmetic mean
0.053 ppmv
SO2
Annual arithmetic mean
0.03 ppmv
24-hr average
0.14 ppmv
Pb
Maximum quarterly average
1.5 mg>m3
PM2.5
Special and annual
15 mg>m3
98th percentile of the 24-hr average
35 mg>m3
PM10
24-hr average (not to be exceeded more than once/yr on average over 3 years)
150 mg>m3
O3
Maximum daily 1-hr average (to be exceeded no more than once/yr averaged over 3 consecutive years)
0.12 ppmv
3-year average of the annual fourth highest daily 8-hr average
0.08 ppmv
Source: Table adapted from www.epa.gov/air/criteria.html.
the EPA considers there is still health or environmental risk associated with contaminants emitted, more stringent emissions standards may be imposed.
11.4 SOURCES AND EFFECTS Sources of air pollution can be categorized as either natural or anthropogenic (manmade). Natural sources include particulates from erosion and emissions from fires, volcanoes, and decaying vegetation. During the onset of each spring flowering season, many allergy sufferers would agree that pollen is a pollutant of major concern. Major anthropogenic emission sources include motor vehicles, domestic heating from stoves burning wood, oil, and coal, and agricultural processes including tilling and applications of pesticide and herbicide. Major industrial pollutant contributors include power plants, the pulp and paper industry, and coking and metal-processing facilities including steel mills. Air pollutants are defined as either primary or secondary, depending on their origin. Primary pollutants are emitted directly into the atmosphere from a specific source, while secondary pollutants are produced from chemical reactions between both naturally occurring and anthropogenic gases and particles found in the atmosphere. Examples of primary pollutants include SO2 and CO. Examples of secondary pollutants include O3 and acidic aerosols such as sulfuric and nitric acids. 11.4.1 Particulates Particulate matter (PM) is classified as any dispersed airborne solid or liquid particle that is larger than a single molecule and has an aerodynamic size of 2 * 10-10 m up to 5 * 10-4 m. (Refer to Section 11.5 for a definition of aerodynamic size.) Particulates are generated by both anthropogenic and natural processes. Anthropogenic particulate generation can occur from burning wood, gasoline, and diesel fuel. Many
Section 11.4
industrial sites produce particles through crushing and grinding operations. A variety of agricultural practices, including plowing and burning off fields, produce particulates. Both biotic and abiotic sources contribute to natural PM production. For example, the decomposition of plants, radiological decomposition, dust storms, sea spray, forest fires, and volcanic eruptions produce significant quantities of respirable PM. Routinely, particles having an aerodynamic size less than 2.5 mm 11 mm = 10-6 m2 are classified as the fines fraction, while those with diameters greater than 2.5 mm and less than 10 mm are considered coarse.The term PM 2.5 refers to particles that are … 2.5 mm in diameter. PM 10 particles are defined as having a diameter … 10 mm. Fumes are particles formed by either a chemical reaction or condensation having an aerodynamic diameter between 0.001 μm and 1 mm. Mists, large compared to fumes, have an aerodynamic size between 0.1 μm and 10 mm and are formed through condensation. The term aerosol embodies a variety of particulates, including fumes, mist, smoke, and fog. Reduced visibility is a common effect of PM. Particulate emissions also have an effect on materials, vegetation, and animals. For example, particulates cause soiling of painted surfaces, clothing, and curtains. Acidic aerosols are corrosive to metal and cement surfaces and are potentially harmful to buildings, monuments, and statues. Particulates containing fluorides, which are emitted naturally from rock weathering and volcanoes as well as generated anthropogenically through mining processes and coal combustion, can cause damage to plants. Animals that graze on these plants may exhibit dental fluorosis. Particulates from mining, smelting, and coal-fired power plants may also contain arsenic, which has been shown to cause arsenic poisoning in cattle and sheep. Human health concerns associated with particulates are focused primarily on particles of respirable size. It has been shown that particles ranging in size from 2.5 to 10 mm are most likely to be inhaled and trapped within the respiratory system. Serendipitously, these particles are inherently difficult to remove with end-of-pipe air pollution-control technology. In humans, atmospheric particulates can interfere with the mechanisms which normally clear the respiratory tract. Particles may also be intrinsically toxic because of their chemical or physical properties, or they may serve as carriers of sorbed toxic substances. Factors that determine the degree of damage include the exposure time, concentration of particulates, synergistic effects, and particle size. Common medical problems resulting from exposure to particles include respiratory infections, cardiac diseases, bronchitis, asthma, pneumonia, lung cancer, cough, and pulmonary emphysema. Additional effects include a decreased respiratory efficiency, diminished pulmonary circulation, and the enlargement of the heart and blood vessels. Allergic skin and eye irritation may also occur. Health and environmental concerns associated with particulate emissions and other criteria pollutants are summarized in Table 11.4. 11.4.2 Nitrogen Oxides (NOx ) Nitrogen oxides, including nitric oxide (NO) and nitrogen dioxide 1NO22, are formed during any high-temperature combustion process, as exemplified by the burning of gasoline within automotive internal-combustion engines, and during the combustion of fossil fuels, natural gas, coal, and oil for generating electricity. NOx compounds are important precursors to ozone and acid rain formation. NO2 is a reddish-brown, highly reactive gas that has been shown to suppress growth of some crops and affect the lung function of children with preexisting respiratory illness, such as asthma. In the atmosphere, NO2 reacts with water vapor to form nitric acid 1HNO32, a form of acid rain. The reaction is as follows: 3 NO21g2 + H2O : 2 HNO31aq2 + NO1g2 . Since NO2 also absorbs visible light, it contributes to a reduction in visibility.
Sources and Effects
283
284 Chapter 11
Air Pollution
Table 11.4 Health and Environmental Effects from Exposure to Criteria Pollutants Criteria pollutant
Affected body system
Health effects
Carbon monoxide
Circulatory—pollutant interferes with oxygen transfer to the body
• • • •
Lead
Organs and soft tissue (blood, nervous and renal systems)— pollutant accumulates in bones and soft tissue
• • • •
Anemia Hypertension Cancer Neurological disorder (cerebral palsy, seizure disorders) • Digestive problems
• Harmful to wildlife
Nitrogen dioxide
Respiratory—toxic at high levels
• Asthma • Compromised immunity
• Acid rain precursor • Reduced visibility • Acidic aerosols can damage
CO poisoning Angina pectoris Brain damage Asphyxiation
to infectious disease
Environmental/property effects
• None
buildings and monuments Particulate matter
Lower respiratory system
• • • •
Asthma Bronchitis Cancer Nose and throat irritant
Ozone
Respiratory
• Lung and throat irritant • Eye irritant • Reduced resistance to
• Reduced visibility • Soil surfaces
• Damages plants and trees • Reduce visibility • Damages rubber
infection Sulfur dioxide
Respiratory
• Asthma • Bronchitis • Heart attack
• • • •
Precursor to acid rain Reduced visibility Damages trees and lakes Damages statues and buildings
Source: Adapted from Franek and DeRose, 2003, Chapter 2, Human Health and Environmental Effects of Air Pollution, Figure 2.1.
11.4.3 Carbon Monoxide (CO) Carbon monoxide is a colorless, odorless gas that is commonly emitted as a product of incomplete combustion of gasoline, natural gas, coal, oil, and other hydrocarbons.There is no known detrimental effect of CO on material surfaces and no harmful effect on higher plant life when the concentration is below 100 ppmv. The toxic effect of CO on humans is well understood.When inhaled, CO is absorbed by the lungs and reacts with hemoglobin to form carboxyhemoglobin (COHb), thus reducing the carrying capacity of oxygen to the body. Hemoglobin has an affinity for CO approximately 240 times its affinity for O2 . The concentration of COHb in the body is directly proportional to the concentration of CO inhaled. At a CO concentration of 100 ppmv, most people experience dizziness and headaches. Concentrations above 750 ppmv will cause death. 11.4.4 Sulfur Oxides (SOx ) Sulfur oxides are formed during the burning of coal and oil to generate power and are also emitted from industrial processes, such as paper and metal production. The
Section 11.4
dominant SOx species formed are sulfur dioxide 1SO22 and sulfur trioxide 1SO32, such that the ratio of SO2 :SO3 typically observed during the combustion of fossil fuels ranges from 40:1 to 80:1 (Wark, et al. 1998). Acidic aerosols, such as sulfuric acid 1H 2SO42, are formed when SO2 reacts with water. The reaction series can be generalized as follows: SO21g2 +
1 O : SO31g2 2 21g2
SO31g2 + H 2O : H 2SO4
(11.1)
Sulfuric acid aerosols significantly contribute to a decrease in visibility and, along with NOx , are a major contributor to the acid rain phenomenon. With the passage of the 1990 CAA Amendments (Title IV) and the expectation of reductions in NOx and SOx emissions as a consequence, the EPA predicted that the average annual visibility would increase by 6 miles (9.7 km) in the southeastern United States by the year 2010. Metal corrosion is increased in the presence of sulfuric acid. Sulfur dioxide gas has also been shown to increase the drying time of paint and reduce the durability of the painted surface. While active outdoors, the inhalation of SO2 can cause respiratory distress for asthmatic children and adults. Epidemiological studies have shown that the presence of SO2 in an environment containing particulate matter and water vapor is conducive to the formation of sulfuric acid and increases the irritant response associated with the pollutant. 11.4.5 Lead Historically, the burning of leaded gasoline was the primary source for lead emission into the atmosphere.As a result of the 1970 CAA, lead was phased out of gas, and now stationary sources associated with lead storage battery manufacturing, iron and steel production, lead smelting, and the painting of cars and houses are the primary emitters of lead. Lead compounds tend both to persist and to bioaccumulate in the environment. In humans, low-level lead poisoning results in kidney and neurological cell damage. Lead may also contribute to hypertension and resulting heart disease. Children are particularly sensitive to the effects of lead exposure, and in animal studies, lead has been shown to reduce fertility and cause birth defects. 11.4.6 Ozone 1O32 Ozone, a photochemical oxidant, is a secondary pollutant formed by a series of reactions involving volatile organic hydrocarbons (VOCs) and NOx . Photochemical smog formation is initiated by the production of nitric oxide (NO) through the photolytic decomposition of NO2 into NO and O. In general NO2 , O2 , and VOCs, combined with energy from the sun, produce photochemical smog; a mixture of O3 , HNO3 , organics, and free radicals. Ozone exposure has been linked to increased hospital visits resulting from respiratory distress. Ozone exposure aggravates preexisting conditions, such as asthma, and has been shown to increase coughing, chest pain, and susceptibility to respiratory infection. Ozone also affects the growth and yield of vegetation and may increase a plant’s susceptibility to disease. Visible injury to leaves may also be apparent. Both natural and synthetic rubber polymers are susceptible to cracking and hardening from contact with ozone. Ozone has been shown to reduce the life of tires and outdoor electrical conduit covering. Ozone exposure damages nylon and acrylic fibers and causes dye colors to fade in fabrics.
Sources and Effects
285
286 Chapter 11
Air Pollution
11.4.7 Volatile Organic Compounds (VOCs) VOCs contribute to the formation of ozone and other smog-related photochemical oxidants. VOCs are emitted from the combustion of fuels and from a variety of industrial settings. For example, styrene, methyl ethyl ketone, and acetone are emitted during the manufacture of fiberglass boats or bathtubs. Chlorofluorocarbons and chlorotoluene are emitted during the manufacture of electronics and computers. During the painting of a car or an airplane, a variety of VOCs are emitted, including xylene, butanol, and acetone. The realization that VOCs and NOx are emitted simultaneously from internal-combustion engines (passenger vehicles) helps us understand the diurnal ozone concentrations observed in the troposphere during summer months. Some VOCs, including benzene and ethylbenzene (gasoline constituents), trichloroethylene, formaldehyde, methylene chloride, vinyl chloride, and chloroform (all on the EPA priority hazardous air pollutant list - 40 CFR Part 63), are well established or suspected as human carcinogens, and there is evidence that formaldehyde and ethylene are harmful to plant life. Title VI of the CAA is focused on global atmospheric issues, including depletion of stratospheric ozone and global warming. Ratification of the Montreal Protocol by 27 countries in 1987 created an international consortium committed to limiting and eliminating the production of ozone-depleting chemicals, primarily chlorofluorocarbons (CFCs). In the United States, the 1990 CAA amendments required that CFC production be limited and by 1996 production of many ozonedepleting substances ended. The EPA reports (U.S. EPA, 2007) that since 1998, the ozone layer has not grown thinner over most of the world and appears to be recovering because of the reductions in emissions of ozone-depleting substances. Although controversial, results from atmospheric modeling efforts suggest that the stratospheric ozone layer over the Antarctic should return to pre-1980 levels by about 2075. 11.4.8 The Greenhouse Effect and Global Climate Change Global warming theory refers to an average increase in temperature near the earth’s surface that potentially contributes to changes in global climate patterns. Although climate fluctuations are well documented throughout earth’s history, many scientists believe that recent additions of anthropogenic greenhouse gases are increasing the average troposphere temperature by altering the steady-state conditions achieved through the greenhouse effect. Key greenhouse gases include carbon dioxide 1CO22, water vapor 1H 2O2, methane 1CH 42, nitrous oxides 1N2O2, and several anthropogenic gases (i.e., gases that do not occur naturally), such as chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFC), hydrofluorocarbons (HFCs), sulfur hexafluoride 1SF62, bromo- and chlorocarbons, and perfluorocarbons (PFCs). These greenhouse-gas emissions are thought to be altering the natural carbon cycle in our atmosphere, causing a net increase of carbon from solid to the gas phase and consequently a net increase in greenhouse gas concentration in our atmosphere. The greenhouse effect is a naturally occurring process. It causes the average global temperature to be approximately 60°F and thereby makes the earth’s surface inhabitable. To understand the greenhouse effect, consider the macro scale energy balance shown in Figure 11.1. Energy from solar radiation has a mean wavelength of 0.5 mm and includes ultraviolet radiation 1UV, 0.001–0.4 mm2, visible radiation (light, 0.4–0.7 mm), and infrared radiation 1IR, 0.7–100 mm2. A significant portion of incoming solar radiation, approximately 31 percent, is backscattered by air molecules, reflected by clouds and the earth’s surface (reflective surfaces such as ice,
Section 11.4
Sources and Effects
287
Space SUN
Incoming short wave solar radiation ⬃343W/m2
Outgoing longwave radiation
Atmosphere (Location of greenhouse gases that absorb IR energy) Back-scatter by air
Absorption by clouds, water vapor and gases
Reflected by clouds and earth’s surface
Energy emissions by clouds, water vapor and gases
Absorbed by water vapor, clouds, dust, ozone Longwave radiation ⬃49% absorbed by Earth’s land and water (⬃168 W/m2)
Earth
Figure 11.1 Summary of the greenhouse gas effect and potential for global warming.
snow, sand, and water). This reflective property, known as the albedo, can change as earth’s surface becomes more or less reflective. Plausible destinations for the 69% that does not get reflected include absorption by the stratospheric ozone layer 1 ' 19%2, tropospheric clouds 1 ' 4%2, or the earth’s surface 1 ' 47%2. The average energy that reaches a specified area of the earth’s surface per unit time is defined as insolation. Because the majority of the sun’s UV energy is absorbed by the stratospheric ozone layer, the majority of the insolation that reaches the earth is in the visible region. For equilibrium to occur, the incoming solar radiation must be balanced by the outgoing radiation (terrestrial radiation) from the earth. The average temperature of the earth is about 300 K, and thus it primarily emits infrared radiation having an average wavelength of 10 mm. The temperature of the earth’s surface predicted from this steady-state radiative equilibrium is considerably lower than the actual average temperature on earth by about 30ºC (Kellogg, 1996; Peixoto and Oort, 1992; Henderson-Seller and Robinson, 1986). This difference is caused by the presence of IR energy-absorbing compounds in our atmosphere. Trace atmospheric compounds (as compared to N2 and O2), such as water vapor, nitrous oxide, methane, and carbon dioxide, are almost transparent to solar UV energy but effectively absorb IR radiation and then reradiate in many wavelengths in all directions. Some of this energy is redirected downward, allowing
Surface gains more heat (and IR is emitted again) from IR reemitted by greenhouse gases
288 Chapter 11
Air Pollution
800 700 600 500 400 300
CH4 - ppbv
300
CO2 - ppmv
250 200 150
5
Temp diff ºC (WRT mean recent time value)
0 5 10
400
350
300 250 200 150 100 Thousands of years before present (Kyr BP)
50
0
CO2 - ppmv
Temp diff ºC (WRT mean recent value)
Figure 11.2 Climate records from the Vostok ice core spanning the previous 420,000 years (Source: Petit et al., 1999).
CH4 - ppbv
heating of the earth’s atmosphere and surface. This is the greenhouse effect. The upward shift in earth’s temperature caused by the greenhouse effect is analogous to covering one’s body with a blanket on a cold winter night. The blanketing greenhouse gases do not completely prevent heat emission into space, but they make it more difficult, causing the covered layer’s surface equilibrium temperature to rise. To understand the potential problem of global warming, consider both natural and anthropogenic sources as well as sinks for CO2 and other greenhouse gases. CO2 is released into the environment by a variety of natural events, including (1) volcanic eruptions, (2) forest fires, (3) decomposition of vegetative matter, and (4) mammalian respiration. Removal of atmospheric CO2 also occurs naturally through the photosynthesis process and by absorption of CO2 by water bodies. Such natural processes help keep atmospheric CO2 concentrations reasonably balanced. Some scientists argue that, since the invention of Watt’s steam engine and the resulting Industrial Revolution that began in the late 1700s, the CO2 balance has been disturbed and atmospheric greenhouse gas concentrations have increased as a result of human interference. In fact, a variety of researchers have documented an increase in the equilibrium atmospheric CO2 concentration from approximately 228 ppmv in the preindustrial era to about 364 ppmv in 1997 (Hansen et al., 1998; Keeling and Whorf, 1998). Similar increases have been observed for atmospheric concentrations of both CH 4 and nitrous oxide 1N2O2. Methane concentrations in the preindustrial era averaged around 700 parts per billion volume (ppbv), and in 1994 they were about 1721 ppbv. During the same period, nitrous oxide increased from about 275 to 312 ppbv (Houghton et al., 1996). In effect, this would allow earth’s atmosphere to behave as a “thicker blanket,” causing the earth’s mean temperature to rise. Figure 11.2. provides a historical perspective of the previous 420,000 years of global CO2 and CH 4 concentrations, as well as the temperature difference as compared to the mean recent time average. These palaeoclimate data, derived from ice cores drilled at the Russian Vostok station in East Antarctica, provide documentation of the climate variability on Earth, punctuated by a series of glacial–interglacial climate changes with cycles that last about 100,000 years. These data confirm a strong correlation between greenhouse gases, such as CO2 and CH 4 , and temperatures in the Antarctic (Petit et al., 1999).
Section 11.5
Control of Particulate Matter from Stationary Sources
11.5 CONTROL OF PARTICULATE MATTER FROM STATIONARY SOURCES Stationary sources for particulate emissions can be categorized as either point or fugitive (nonpoint). Sources that are unconfined in a duct or stack prior to their release are considered fugitive. Examples of fugitive sources include pressure-relief valves, flanges, open-ended lines, pump seals, and valves. Point sources include those typically found in industrial processes, such as combustion stacks and distillation systems. Additional point-source emissions are derived from wastewater treatment facilities, landfills, and the handling and loading of materials. Stationary-source emissions can be controlled through pollution-prevention activities or application of appropriate “end-of-pipe” control technology. Pollution-prevention law was included in the 1996 RCRA legislation and the Toxic Substance Control Act of 1976 and highlighted in the 1990 Pollution Prevention Act. The EPA encourages industries to implement pollution-prevention activities by promoting the EPA 33/50 program (reduction of toxic chemicals), the Green Lights Program (use of efficient lighting), and the National Industrial Competitiveness through Energy, Environment and Economics 1NICE32 program (conducting large-scale demonstration projects). Shen (1995), defined five categories of pollution-prevention activities: (1) improvement of plant operations, characterized by regular preventive maintenance and employee training programs; (2) in-process recycling, focused on the direct reuse of accumulated wastes and the reclamation and reuse of solvents; (3) process modifications, including improved control over critical process variables such as temperature and pressure; (4) materials and products substitutions, exemplified by the use of less toxic solvents and the production of recyclable products; and (5) use of materials separation processes, such as supercritical fluid extraction, ultrafiltration, and reverse osmosis, during the production of chemicals and the recovery of waste products for recycling. Anthropogenic generation of aerosols includes industrial and motor-vehicle emissions, electric power generation, and municipal solid waste generation. Natural emissions include ocean salt, volcanic ash, erosion, and forest fires. Particle size, shape, and density influence their aerodynamic properties, effect on human health, and visibility. Particulate control technology is focused on removing particles having a diameter of 10 μm or less. Regulation and thus removal are focused on this size range because these particles are considered respirable, as they can avoid entrapment by the body’s respiratory filters and penetrate deeply into the lungs. Commonly used particulate control devices include settling chambers, venturi scrubbers, cyclones, fabric filters, and electrostatic precipitators (ESPs). Key exhaust-gas characteristics that influence both the type of control process implemented and its design include exhaust-gas temperature, volumetric flow rate, corrosivity, composition, moisture content, particle-size distribution, and the regulation-driven required efficiency of removal. For particulate removal devices, the size of the particle affects the efficiency of removal. As most particles are not spherical, their diameters are often described by their aerodynamic behavior or size.The aerodynamic size of a particle is defined as the diameter of a spherical particle with settling velocity equal to that of the particle in question, but having a density of 1 g/cm3. Therefore particles that have similar size and shape, yet differ in densities, will be characterized by differing aerodynamic diameters. Collection efficiency increases with particle size. For one device it is given as: h1%2 =
Cin - Cout * 100 Cin
(11.2)
where h is the collection efficiency and Cin and Cout represent the inlet and effluent contaminant concentrations associated with the control technology. Typically, C is expressed in mg/m3 or ppmv.
289
290 Chapter 11
Air Pollution
Among the various types of particulate control equipment are mechanical separators, typified by settling chambers and cyclones, electrostatic precipitators, fabric filters such as baghouses, and wet scrubbers.The remainder of this section will briefly describe how settling chambers and electrostatic precipitators work and provide general design parameters and associated equations that can be used to predict removal efficiencies. 11.5.1 Mechanical Separators—Settling Chambers Gravity settling chambers are used to remove the large-particle fraction from gas streams. Collection in a settling chamber depends on the settling velocity (also referred to as Stokes’ velocity) of the particles being removed. The terminal settling velocity for particles can be estimated as: vs = B
g1rp - r2d2 18 m
R
(11.3)
where: vs g rp r
= = = =
terminal settling velocity or Stokes’ settling velocity, m/s, gravitational acceleration, 9.81 m/s2, particle density, kg/m3, density of the displaced fluid (in environmental applications, this will frequently be air), kg/m3, d = particle diameter, m, and m = fluid viscosity, kg/1m # s2.
EXAMPLE 11.1 Stokes’ law application Assume a point-source particulate emission having an effective stack height of 200 feet. The plume contains particles having a density of 2.3 g/cm3, the stack-gas temperature is 80ºF, and the wind speed is constant at 5 mph. Assuming the terrain is flat, calculate the distance downwind that a 5-, 10-, and 100-mm particle will travel. Solution Determine the Stokes’ settling velocity for each size particle using Equation (11.3). At 80ºF, the viscosity and density for air are assumed as 6.66 * 10-2 kg/1m # hr2 and 0.0012 g/cm3, respectively. For the 5-mm particle, Stokes’ equation predicts: vs = B
g1rp - r2d2
9.81 vs 15 mm2 =
18 m
R
g g 1 kg 1003 cm3 m -6 2 2.3 0.0012 15 * 10 m2 * * ¢ ≤ 1000 g s2 cm3 cm3 1 m3
= 1.69 * 10-3
18a6.66 * 10-2 m s
kg 1 hr b # m hr 3600 s
Section 11.5
Control of Particulate Matter from Stationary Sources
Similarly, Stokes’ velocity can be determined for the 10- and 100-mm particles: vs 110 mm2 = 6.67 * 10-3 m/s vs 1100 mm2 = 6.67 * 10-1 m/s Converting the average wind speed to m/s yields 5 mph *
1 hr 5280 ft 1m m * * = 2.235 s 3600 s 1 mi 3.281 ft
The height each particle must drop is 200 ft *
1m L 61 m 3.281 ft
The time for each particle to drop is given by t = H/vs and the distance traveled is equal to t times the average wind speed. Results are tabulated for each particle size below.
Particle diameter ( m) 5 10 100
Travel time, t =
H vs
(sec)
Distance traveled, D = t * wind speed (m)
36,100 9,150 91.5
8.07 * 104 2.05 * 104 205
Settling chambers are used to remove the relatively large particles in a gas stream and are often used as a precleaner. Particles with diameters greater than 50 μm to 100 mm are effectively removed, and settling chambers are typically designed having a velocity less than or equal to 3 m/s (10 fps), best results being obtained with velocities less than 0.3 m/s (1 fps). The simple schematic of a settling chamber shown in Figure 11.3 can be used to consider the fate of a particle in a settling chamber and to determine the minimum particle size that can theoretically be removed with 100% efficiency. For a particle that enters the top of the collector and is removed, the time for the particle to fall the collector height, H, must be less than or equal to the time of the horizontal movement within the collector such that: time for particle to fall H distance = time of horizontal movement t = vs =
H L = vs V VH L
where L is the length of the hopper and V is the horizontal flow- through velocity. Substituting this value for Stokes’ velocity into Equation (11.3), neglecting the density of
291
292 Chapter 11
Air Pollution
air (since it is very small compared to the density of the solid), and solving for the particle diameter that is 100% removed yields: D = B
18 mHV 0.5 R gLrp
(11.4)
Notice that reducing the H/L ratio will result in removing smaller particles. For particles sizes below that predicted by Equation (11.4), the removal efficiency, h, for laminar-flow situations is determined by: h = 100
vsLW Q
(11.5)
where Q = volumetric flow rate; m3/min 1ft3/s2. Figure 11.3 Schematic of particle motion in a settling chamber.
L V V Air inlet
H vs Air outlet
Dust hopper
EXAMPLE 11.2 Settling chamber design Design a settling chamber to collect particles 50 mm in diameter and 2000 kg/m3 in density from an air stream with a volumetric flow of 1.5 m3/s. The chamber is to be 2.0 m in width and 2.0 m in height. a. How long must the chamber be to give theoretical perfect collection efficiency? b. Determine the collection efficiency for particles of the same density that are 25 mm in diameter. Solution part a Using Stokes’ law, determine the terminal settling velocity for the 50-mm particle. Assume that the density and viscosity of air are 1.2 kg/m3 and 6.66 * 10-2 kg/1m # hr2, respectively. vs = B
g1rp - r2d2
9.81 vs 150 mm2 =
18 m
R
kg kg m ¢ 2000 3 - 1.2 3 ≤ 150 * 10-6 m22 2 s m m 18a6.66 * 10-2
kg 1 hr ≤ m # hr 3600 s
= 0.147
m s
Section 11.5
Control of Particulate Matter from Stationary Sources
Solve Equation (11.5) for the length of chamber required for theoretically removing 100% of the 50-mm particles. h = 100
vsLW Q
Qh = 100vsW
L =
¢ 1.5
m3 ≤ 11002 s
m 11002 ¢ 0.147 ≤ 12 m2 s
= 5.1 m
Solution part b For the 25-mm particle, similarly, use Stokes’ law to determine the terminal settling velocity. vs = B
g1rp - r2d2
9.81 vs 125 mm2 =
18 m
R
kg kg m ¢ 2000 3 - 1.2 3 ≤ 125 * 10-6 m22 2 s m m 18a6.66 * 10-2
kg 1 hr b # m hr 3600 s
= 0.037
m s
Use Equation (11.5) to determine the removal efficiency for the 25-mm particles.
h = 100
vsLW = 100 Q
a0.037
m b15.1 m212 m2 s m3 1.5 s
= 25%
So, only 25% of the particles having a diameter of 25 mm are removed by the settling chamber.
11.5.2 Electrostatic Precipitators (ESPs) Electrostatic attraction is used to separate the particles from the gas stream in an electrostatic precipitator (ESP). ESPs establish a field (or corona) to charge particles. The corona generates electrons, which are readily adsorbed by electronegative gases such as oxygen or sulfur dioxide. The ions produced are adsorbed onto the PM to be removed. The charged PM is passed through an electric field of grounded collecting electrodes. The charged particles migrate toward the oppositely charged electrode at right angles (perpendicular to the bulk gas flow) and are collected on the electrodes (plates). Liquid particles are removed from the electrode by gravity flow. Solid particles are removed by rapping or mechanically striking the plates with a sharp, hammerlike blow. In some instances, water is used to flush solid and liquid PM from the electrodes. Spacing between collecting electrodes is typically 8 to 10 inches (20 to 25 cm), typical collecting electrode length is between 20 and
293
294 Chapter 11
Air Pollution
Clean gas out
Negatively charged wire Negative electrode connected to electrical power source
Figure 11.4 Schematic of an electrostatic precipitator. (Source: U.S. EPA, Principles and Practices of Air Pollution Control, 2003).
Dry gas in
Grounded collecting plate with positive charge
Hopper to discharge
30 ft (6.1 and 9.1 m), and collecting electrode height commonly ranges between 30 and 40 ft (9.1 and 12.1 m). A schematic of an ESP is shown in Figure 11.4. Advantages associated with ESP technology that make it popular in industry (especially coal-fired electric-power generators) include: 1. Ability to handle large gas volumes with small pressure drops, up to 4 * 106 scfm 11.1 * 105 m3/min2. 2. Ability to handle small gas volumetric flow rates, exemplified by household air conditioning systems. 3. High removal of both wet and dry small-diameter particles (removal efficiencies can be 99% to 99.9%). 4. Ability to operate within a wide range of temperature. 5. Low operation and maintenance (O&M) costs, a result of low power requirement because of relatively low pressure drop (0.1 to 0.5 in. of H 2O). 6. Acts only on the particle, not on the gas stream. Disadvantages for ESPs include the following: 1. Certain types of dust cannot be removed efficiently. Particles with electrical resistivity less than 104 ohm-cm or greater than 1010 ohm-cm are not effectively removed. Particles with low resistivity, such as carbon dust, too readily give up their negative charge to the collecting electrode, assuming a positive charge which repels them back into the main gas stream (reentrainment). Particles with high resistivity, such as powdered sulfur, will not give up their negative charge, and a layer of dust builds up on the collecting electrode, acting as an insulator. The potential drop across the layer builds up, causing a “back discharge” or “back corona.” In these instances, removal may occur only if conditioning agents such as water vapor, ammonia, or sulfur trioxide are added to change resistivity.
Section 11.5
Control of Particulate Matter from Stationary Sources
295
2. Gas pollutants are not collected. 3. Inadaptibility to varying process conditions, such as PM load and volumetric flow. 4. High capital cost. 5. Large space requirements. 6. Generation of ozone, which is undesirable for household air conditioning systems. Information required for ESP design include characterization of emission source, including particle distribution, gas composition, temperature and pressure, particle-size distribution, dust concentration and resistivity, corrosive properties of gas and particulates, and the volumetric flow rate. The Deutsch-Anderson equation (Deutsch, 1922; Anderson, 1924) provides an estimate of collection efficiency of an ESP under ideal conditions in relation to various operating parameters. h = 1 - expa -
Av b Q
(11.6)
where: A = area of collection electrodes, m2 v = drift velocity of particles towards the plates as estimated by Equation (11.7) below, m/s Q = volumetric flow rate, m3/s Any internally consistent set of units must be used in the Deutsch-Anderson equation. The speed at which a charged particle migrates towards the collection electrode is referred to as the particle-migration velocity or the drift velocity. Variables influencing the magnitude of the drift velocity include particle size, the strength of the electric field, and the viscosity of the gas. For standard air at 60ºF, the drift velocity can be estimated as: v = 8.42 * 10-3E 2dP
(11.7)
where: v = drift velocity, fps, d = particle diameter, mm, E = potential applied to discharge electrodes, kV/in., and 3k P = , where k = dielectric constant for the particle k + 2
EXAMPLE 11.3 Electrostatic precipitator design Process exhaust gas with a volumetric flow rate of 500,000 cfm that contains 2 gr/ft3 (recall that gr is the abbreviation for grain; 1 lb = 7000 gr) of cement PM is to be treated with an ESP. Regulatory requirements dictate that the outlet PM concentration must be … 0.02 gr/ft3. Assume a plate-to-plate spacing of 8 inches and an applied voltage of 12,000 volts. The smallest particles in the gas stream are 2 mm in diameter, and all particles have a dielectric constant of 5. Estimate (a) the particle
296 Chapter 11
Air Pollution
drift velocity and (b) the required collection plate area to meet regulations. Assume a system temperature of 60ºF. Solution part a Knowing the dielectric constant for the dust, calculate the constant P. P =
3152 3k = = 2.14 k + 2 5 + 2
Use Equation (11.7) to estimate the drift velocity for 2-mm particles. v = 8.42 * 10-3 E 2 dP = 8.42 * 10 -3 a12,000 V * v = 0.32
1 kV 1 2 * b 12 mm212.142 1000 V 4 in.
ft s
Solution part b Use the Deutsch-Anderson equation [Equation (11.6)] to determine the required collector-plate surface area. First, determine the required removal efficiency using Equation (11.2):
h =
PM in - PM out = PM in
12 - 0.022 2
gr
gr ft3
= 0.99 or 99%
ft3
Convert units for the volumetric flow rate: Q = 500,000
ft3 1 min ft3 * = 8,333 s min 60 s
Solve the Deutsch-Anderson Equation for collector area, A: h = 1 - expa 1 - h = expa ln11 - h2 = -
Av b Q
Av b Q
Av Q
-Q A = 3ln11 - h24 = v
ft s 3ln11 - 0.9924 ft 0.32 s
- 8,333
A = 119,922 ft2 of collection area required
Section 11.6
Gas and Vapor Control Technology 297
If a plate length of 30 ft with height of 40 ft is used, the plate area can be calculated. Recognize that both sides of the plates are utilized as collectors: plate collection area = A plate = 130 ft * 40 ft2 * 2 = 2400 ft2 To determine the number of plates required: number of plates required - 1 =
number of plates required - 1 =
A A plate 119,922 ft2 2400
ft2 plate
number of plates needed = 51
11.6 GAS AND VAPOR CONTROL TECHNOLOGY A number of industries are regulated by the EPA to control hazardous air pollutant point-source emissions. For example, the National Emission Standards for Hazardous Air Pollutants for Source Categories listed in 40 CFR Part 63 include provisions for dry cleaning facilities emitting perchloroethylene (Subpart M) and for air pollutants from petroleum refineries (Subpart CC), pharmaceuticals production (Subpart GGG), and pesticide active ingredient production (Subpart MMM). Incineration and absorption are two common mitigation technologies used to remove hazardous gases from air streams. Both technologies are described briefly in the sections that follow. 11.6.1 Incineration Incineration is a high-efficiency process, occurring in the presence of oxygen, that removes combustible pollutants (gases, vapors, or odors) from contaminated air and other gas streams by raising the system temperature sufficiently high to autoignite targeted pollutants, converting them primarily into carbon dioxide and water. It is effective in streams containing either high or low concentration of contaminants. Odorous pollutants (mercaptans, cyanide gases) and organic aerosol plumes, gases, and vapors are effectively destroyed. Thermal and catalytic are two common incinerator classes. Thermal incinerators are also referred to as thermal oxidizers or direct-flame incinerators. If incinerators are properly designed, destruction efficiencies can be as high as 99.99%. Critical design variables include combustion-chamber temperature, turbulence (or degree of mixing), residence time, and concentration and type of contaminants (U.S. EPA, 1992). Commonly, the factors affecting incinerator performance are referred to as the 3 Ts (Time, Temperature, Turbulence). The EPA (1992) recommends a residence time of 0.75 seconds combined with a chamber temperature of 1600ºF to effectively control a stream contaminated with VOCs. If halogenated VOCs are present, the combustion temperature should be 2000ºF at a residence time of 1 second. Use of a scrubber downstream of the incinerator to control the acid gas effluent is desirable.
298 Chapter 11
Air Pollution
Advantages of thermal incinerators include: 1. When designed properly, removal of combustible pollutants is essentially complete. 2. Equipment is moderately adaptable to changes in flow and contaminant concentrations. 3. Performance is constant and predictable. Disadvantages of incineration include: 1. High operating and capital costs. 2. Formation of secondary products during incineration of compounds containing elements other than carbon, hydrogen, and oxygen (such as chlorine and sulfur), causing production of acidic gases that result in corrosion problems. Potential emissions include CO, NOx , SOx , HCl, Cl 2 , heavy metals, aldehydes, and furans. 3. Public opposition due to concern about emissions. Catalytic and thermal incineration processes are very similar. The primary difference is that, in the catalytic process, once the process gases pass through the flame, they pass over a catalyst, which increases the rate of reaction and allows the combustion chamber to operate at lower temperatures than those associated with thermal incineration and with potentially less auxiliary fuel. Common catalyst materials used for VOC oxidation include noble metals such as platinum, palladium, and rhodium, which are frequently deposited on an aluminum oxide (alumina) honeycomb support. Gas streams containing particulate matter are often difficult to treat by catalytic incineration, because PM of any size may coat the reactive sites on the catalyst, rendering it ineffective. This undesirable effect from PM is referred to as blinding. Catalysts can also be deactivated by certain gases and by high temperatures. Typical emission-stream volumetric flow rates treated by catalytic incinerators are similar to those treated by the thermal incineration process. As expected, operation temperatures are lower in catalytic than thermal incinerators. Typical incineration temperatures when a catalyst is used range from 320º to 430ºC (600º to 800ºF). Capital costs for catalytic units are higher than for thermal systems, while operating costs are lower because less supplemental fuel is needed (EPA-452/F-03-018). Cooper and Alley (1994) and AWMA (1992) summarize the advantages and disadvantages of catalytic incinerators versus other types of incinerators. Advantages include a comparative reduction in required fuel and operating temperature, a reduced risk of fire and flashbacks, and a reduction in combustion-chamber size. Disadvantages include a relatively high capital cost and an inability of the catalytic system to deal with emissions laden with PM, so that PM must be removed before vapors are treated. 11.6.2 Absorption—Packed-Bed Scrubbers and Flue Gas Desulfurization Absorption process equipment is used to control vapors in gas streams. When used to control emissions of inorganic gases such as SOx or HCl, it is often referred to as a “wet scrubber” or an “acid gas scrubber.” Absorption occurs when a soluble gas or a mixture of soluble gases is contacted by and dissolved in a liquid solvent. Physical absorption occurs when the soluble gas is dissolved into the liquid phase, and chemical absorption when the absorbed compound and solvent react. The driving force for this mass-transfer process is governed by the difference between the actual solute concentration in the solvent and the equilibrium
Section 11.6
Gas and Vapor Control Technology 299
concentration. A variety of unit operation configurations take advantage of the absorption process, including packed beds/packed towers, plate (or tray) columns, venturi scrubbers, and spray towers. Packed towers are commonly used to control VOC emissions, and spray towers (wet and dry) are often used to control acid-gas emissions. The extent of mass transfer, or absorption, depends on a properly designed system that provides adequate surface area and contact time. Removal efficiencies also depend on the solubility of the solute in the chosen scrubbing liquid. For control of inorganic contaminant-gas emissions and VOCs with relatively high water solubilities, water is commonly used as the solvent. Hydrocarbon oils are frequently used as the scrubbing liquid for hydrophobic VOCs. Other variables that control the absorption rate include the concentration of contaminant and the liquid-to-gas ratio. The rate of absorption is increased with an increase in surface area, contact time, contaminant concentration, and liquid-to-gas ratio. Packed towers are a commonly used device for control of gas emissions. These units are filled with packing material to provide a large wetted surface area, resulting in good contact between the gas and the liquid. Common types of random packing materials, shown in Figure 11.5, include Berl and Intalox saddles, Pall rings, and Raschig rings. As shown in Figure 11.6, the packing is contained in a column shell equipped with a mist eliminator, liquid distributors, and packing support structures. Mist eliminators, often just corrugated sheets, are installed at the top of the column and cause any mist exiting with the high-velocity exiting gas stream to coalesce and fall back into the column. The liquid distributor is designed to collect the scrubbing fluid off the column walls and redistribute it toward the center of the column in order to increase the contact between the solvent and gas. Many packed towers are operated in countercurrent fashion, with the effected gas stream entering the bottom of the device with the treated stream exiting the top. The solvent enters at the top of the column, passes over the packing material, and exits near the bottom of the column. If the solvent is costly, it may be processed to remove either the reactive product or absorbed pollutant, and then be reused. Parameters needed for design include: the waste-gas flow rate, contaminant composition and concentration, physical properties of pollutant and solvent, including the equilibrium relationship between them, and the desired pollutant-removal efficiency. The inlet-gas temperature also affects performance. In general, an increase in temperature results in a decrease in absorbance.
Figure 11.5 Typical packing material for scrubbers. (Source: U.S. EPA Air Pollution Control Cost Manual, 2002.) Pall ring
Tellerette
Berl saddle
Intalox saddle
Raschig ring
300 Chapter 11
Air Pollution
Gas out Mist eliminator
Liquid in
Liquid distributor Spray nozzle Packing restrainer
Shell Random Packing
Liquid re-distributor
Packing support
Gas in
Figure 11.6 Schematic of packed tower scrubber used for absorption of gas contaminants. (Source: U.S. EPA Air Pollution Control Cost Manual, 2002.)
Liquid out
Advantages and disadvantages of packed-bed towers follow (EPA-CICA Fact Sheet, Packed-Bed/Packed-Tower Wet Scrubber; AWMA, 1992): 1. Capital cost, pressure drop, and space requirement are relatively low. 2. Highly corrosive environments can be treated if towers are constructed with fiberglass-reinforced plastic (FRP). 3. High mass-transfer efficiencies can be achieved. 4. PM can be removed from gas streams as well as gases. Disadvantages: 1. Possible disposal problem with liquid effluent. 2. Relatively high maintenance costs. 3. PM in gas stream may cause plugging problems.
Section 11.6
Gas and Vapor Control Technology 301
Spray-tower absorption systems are often used for flue-gas desulfurization (FGD) to control sulfur-dioxide emissions. FGD is commonly utilized on coal- and oil-combustion units and can also be found in petroleum refineries and metal smelting industries. This type of scrubbing process typically uses a calcium- or sodiumbased alkaline reagent as the solvent. Typical reagents include CaCO3 , Ca1OH22 , NaOH, and MgSO3 . The alkaline liquid is introduced to the spray tower and reacts with the acid gas, producing either sodium sulfite, magnesium bisulfite, or calcium sulfate as a waste product. An FGD system is categorized as either a wet or a dry process, depending on whether or not the active agent is contained in a liquid solution. Within these two categories, FGD processes can either be considered as throwaway or regenerative processes. In throwaway processes, the products are disposed of in a landfill, and fresh alkaline reagents are continuously added to the scrubber makeup water. In regenerative processes, the reagents can be regenerated in a closed-loop system. One of the most common types of FGD processes combines either CaO or CaCO3 with wet scrubbing and the throwaway process. What follows is an overview of the primary reactions occurring with each alkali substance. The typical particle diameter of quick pebble lime (CaO) is Ú 75 mm. For the FGD process, CaO is either ground into smaller particles or slaked, forming 2 – 5 mm particles of Ca1OH22 . The exothermic slaking reaction is shown in Equation (11.8). This decrease in particle size provides an increase in surface area, which increases the dissolution rate of calcium hydroxide into the liquid slurry serving as the solvent and increases the rate of reaction. CaO + H 2O : Ca1OH22 + heat
(11.8)
The following generalized reaction between SO2 and Ca1OH22, results in the formation of calcium sulfite hemihydrate and occurs within the spray tower during the removal of the acidic gas. SO21g2 + Ca1OH221s2 + H 2O112 : CaSO3 #
1 3 H 2O1s2 + H 2O112 2 2
(11.9)
When limestone is used, the following generalized reaction occurs: SO21g2 + CaCO31s2 +
1 1 H 2O112 : CaSO3 # H 2O1s2 + CO21g2 2 2
(11.10)
The calcium sulfite hemihydrate produced by both reactions is often further oxidized to calcium sulfate dihydrate, as follows: 1 3 1 CaSO3 # H 2O1s2 + H 2O112 + O21g2 : CaSO4 # 2H 2O1s2 2 2 2 Advantages of FGD systems include (EPA-452/F-03-034): 1. Excellent removal of SO2 . 2. Reactant products may have resale potential. 3. Alkali materials are inexpensive and readily available.
(11.11)
302 Chapter 11
Air Pollution
Disadvantages include: 1. High capital and O&M costs. 2. Wet wastes are generated. 3. Reactant product disposal is a significant cost.
S U M M A RY Air pollution concerns related to anthropogenic emissions predate the modern era. In the United States, the passage of the 1970 Clean Air Act provided the EPA with statutory authority to establish and enforce guidelines for controlling air pollution. Since the promulgation of the Act, national attention has been given to the establishment of criteria pollutant guidelines as well as to global issues such as acid rain, stratospheric ozone, and the greenhouse effect. The chapter introduced end-of-pipe pollution-control technologies used to mitigate gases containing particulates and gas and vapors, and it provided preliminary operational and design criteria.
KEY WORDS
ambient or community air pollution absorption aerodynamic size aerosol air pollution episode albedo anthropogenic emission sources carbon monoxide chlorofluorocarbons criteria pollutants electrostatic precipitators fumes
generally available control technologies (GACTs) global warming greenhouse effect greenhouse gases hazardous air pollutants (HAPs) incineration insolation lead maximum achievable control technologies (MACT) mists
National Ambient Air Quality Standards (NAAQS) natural sources nitrogen oxides ozone particulate matter (PM) primary pollutants primary standards secondary pollutants secondary standards settling velocity sulfur oxides volatile organic compounds (VOCs)
REFERENCES Anderson, E. (1924). Report, Western Precipitator Co., Los Angeles, CA, 1919, Transactions of the American Institute of Chemical Engineers, 16, 69. AWMA (1992). Air & Waste Management Association, Air Pollution Engineering Manual, Van Nostrand Reinhold, New York. Cooper, C. David, and Alley, F.C. (1994). Air Pollution Control: A Design Approach, 2nd ed., Waveland Press, Inc., Prospect Heights, Illinois. Deutsch, W. (1922). Annals of Physics (Leipzig), 68, 335. EPA-CICA Fact Sheet, Catalytic Incinerator, EPA-452/F-03-018. EPA-CICA Fact Sheet, Flue Gas Desulfurization (FGD), EPA-452/F-03-034. EPA-CICA Fact Sheet, Packed-Bed/Packed-Tower Wet Scrubber, EPA-452/F-03-015. EPA-CICA Fact Sheet, Thermal Incinerator, EPA-452/F-03-022. Franek, W., and DeRose, L. (2003). Principles and Practices of Air Pollution Control Student Manual, APTI Course 452, 2nd ed., Air Pollution Training Institute, Environmental Research Center, MD, Research Triangle Park, NC. Hansen, J., Sato Mki., Glascoe, J., and Ruedy, R. (1998). A Common Sense Climate Index: Is Climate Changing Noticeably? Proc. Natl. Acad. Sci., 95, 4113–4120.
Exercises 303
Henderson-Sellers, A., and Robinson, P.J (1986). Contemporary Climatology, John Wiley & Sons, New York, 439 pp. Houghton, J.T., Filho, L.G.M., Callander, B.A., Harris, N., Kattenberg, A., and Maske, K. (1996). Climate Change 1995: The Science of Climate Change, Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge, GA, 1996, 572 pp. Keeling, C.D., and Whorf, T.P. (1998). Atmospheric CO2 concentrations—Mauna Loa Observatory, Hawaii, 1958–1997 (revised August 1998). NDP-001. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, Oak Ridge, TN. Kellogg, W.K. (1996). Greenhouse Effect, in Encyclopedia of Climate and Weather, Ed. S.H. Schneider, pp. 368–371, Oxford University Press, New York. NASA (1991). Environmental Control and Life Support System Architectural Control Document, SSP-30262, Revision D, National Aeronautics and Space Administration, Space Station Freedom Program Office, Reston, Virginia, July 1991. Peixoto, J.P., and Oort, A.H. (1992). Physics of Climate, American Institute of Physics, New York, 530 pp. Petit, J.R., Jouzel, J., Raynaud, D., Barkov, N.I., Barnola, J.-M., Basile, I., Benders, M., Chappellaz, J., David, M., Delaygue, G., Delmotte, M., Kotlyakov,V.M., Legrand, M., Lipenkov,V.Y., Lorius, C., Pépin, L., Ritz, C., Saltzman, E., and Stievenard, M. (1999). Climate and Atmospheric History of the Past 420,000 Years from the Vostok Ice Core,Antartica, Nature, 399, 429–436. Shen, T.T. (1995). Industrial Pollution Prevention, Springer-Verlag, Berlin. U.S. EPA (1992). Control Techniques for Volatile Organic Emissions from Stationary Sources, U.S. EPA Research Triangle Park, NC, EPA-453/R-92-018. U.S. EPA (1997). Exposure Factors Handbook, Volume 1, General Factors, August 1997, U. S. Environmental Protection Agency, National Center for Environmental Assessment, Washington DC, EPA/600/P-95/002Fa. U.S. EPA (2000). Estimated Per Capita Water Ingestion in the United States Based on Data Collected by the United States Department of Agriculture’s 1994–96 Continuing Survey of Food Intakes by Individuals, April 2007, U.S. Environmental Protection Agency, Office of Water, Washington DC, EPA-822-R-00-008. U.S. EPA (2002). Air Pollution Control Cost Manual, Office of Air Quality Planning and Standards, U.S. EPA Research Triangle Park, NC, EPA/452/B-02-001. U.S. EPA (2003). Principles and Practices of Air Pollution Control, Student Manual, APTI Course 452,Air Pollution Training Institute, Environmental Research Center, MD E142-01. U.S. EPA (2007). Achievements in Stratospheric Ozone Protection, U.S. EPA Office of Air and Radiation, Washington, DC, EPA-430-R-07-001. Wark, K., Warner, C.F., and Davis, W.T. (1998). Air Pollution: Its Origin and Control, 3rd ed., Addison Wesley Longman, Inc.
EXERCISES 11.1
11.2
11.3
11.4
Browse the portion of the U.S. EPA website focused on the Office of Air and Radiation (http://www.epa.gov/air/) and provide (a) a list of the current criteria pollutants listed in the National Ambient Air Quality Standards, (b) a distinction between primary and secondary standards, (c) a definition of nonattainment and attainment areas, and (d) a list of current nonattainment areas near your home town, and (e) a summary of the EPA required plan for areas deemed nonattainment. Identify a locally operated industrial facility that is regulated under Title V of the Clean Air Act. Find their permit online and briefly summarize their permitting requirements. Summarize the chemical equations primarily responsible for acid rain. Use online resources to determine the average pH of rainfall in proximity to your university. Has the pH of rainfall changed during the last several decades? Using only reputable online resources, describe the “hole in the ozone layer.” Items to consider as you conduct your research include: the chemical reactions
304 Chapter 11
Air Pollution
that occur, how ozone is measured, units for quantifying ozone, historical evidence of ozone depletion, where the ozone layer is most impacted, associated environmental and human health effects, and what predictions are made regarding the future of stratospheric ozone recovery. 11.5 Using your own words, describe the greenhouse effect and global warming. 11.6 Air pollution can influence the limit of visibility by both absorbing and scattering light. Using the library and internet resources, determine how the 1970 Clean Air Act impacted visibility within the United States. What are the primary pollutants that contribute to this phenomenon? 11.7 Consider particles characterized with a density of 1.5 g/cm3. At 25ºC and 1 atm, determine the terminal settling velocity of particles falling in air that are (a) 15, (b) 25, and (c) 150 mm in diameter. 11.8 Consider a settling chamber to treat a particulate ladened air stream having a volumetric flow of 1.0 m3/s. Design a settling chamber to collect particles 10 mm in diameter having a density of 1700 kg/m3. The chamber is to be 1.5 m in width and 1.5 m in height. (a) How long must the settling chamber be to give theoretical perfect collection efficiency? (b) Using the length determined in part (a), determine the collection efficiency for 5-mm particles with the same density. 11.9 Design a settling chamber to collect particles 50 mm in diameter and 111 lb/ft3 in density from an air stream with a volumetric flow of 8 ft3/s. The cross-sectional area of the square inlet is 9 ft2. (a) How long must the chamber be to give theoretical perfect collection efficiency? (b) Determine the collection efficiency for 10-mm particles with the same density. Assume the gas-stream temperature is 70ºF, such that the gas density and viscosity are 0.075 lb/ft3 and 1.23 * 10-5 lb/1ft # s2, respectively. 11.10 Process exhaust gas with a volumetric flow rate of 100,000 cfm that contains 5 gr/ft3 of cement PM is to be treated with an ESP. Regulatory requirements dictate that the outlet PM concentration must be … 0.02 gr/ft3. Assume a plate-to-plate spacing of 10 inches and an applied voltage of 15,000 volts. The smallest particles in the gas stream are 2 mm in diameter, and all particles have a dielectric constant of 5.5. Estimate (a) the particle drift velocity and (b) the required collection-plate area to meet regulations. (c) Assume that plates with length and height of 30 ft are used. Estimate the number of plates required. Assume standard temperature and pressure.
CHAPTER
12
Fundamentals of Hazardous Waste Site Remediation Objectives In this chapter, you will learn about: The magnitude of the hazardous waste problem Contaminant characteristics Microbiological considerations Basic features for designing engineered solutions to hazardous waste sites
12.1 THE PROBLEM Environmental remediation is focused on removing hazardous pollutants from environmental media, including soil, sediment, and surface and groundwater. Cleanup efforts are typically regulatory driven, and cleanup standards are set to protect human health and the environment. In the United States, the legal authority to prompt restoration of contaminated land toward productive use is managed by the Environmental Protection Agency (EPA) through the implementation of the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA, or Superfund) and the Resource Conservation and Recovery Act (RCRA). For the European Union, the main initiative of the European Commission’s Environment Directorate-General (DG) is to initiate and define new environmental legislation and to assume responsibility for implementation in the Member States. The European Environmental Agency (EEA) and the Contaminated Land Rehabilitation Network for Environmental Technologies in Europe (CLARINET) provide reliable technical support to regulatory agencies and the public regarding environmental issues. In much of Europe, the “Dutch List” is used to provide guidance for soil cleanup standards. The Environment Congress for Asia and the Pacific (ECO ASIA) was created by governments in Asia and the Pacific Region to support long-term activities for environmental protection. The Environmental Information Network for Asia and the Pacific (ACO ASIA NET) resulted from this effort and provides a databank of environmental information created to support sustainable development for the 22 countries in the region. 12.1.1 Highlights in Hazardous Waste History In the 1940s, chemical production expanded in the United States, and by 1962 Rachel Carson had published a socially critical book, Silent Spring, describing the negative effects of the pesticide DDT. Silent Spring is often credited with spurring the environmental movement and prompting the United States to revisit its pesticide policies. A hazardous waste disposal facility was operated
306 Chapter 12
Fundamentals of Hazardous Waste Site Remediation
from 1956 until 1972 in Stringfellow, California, where it accepted over 34 million gallons (129 million liters) of industrial waste, primarily from metal finishing, electroplating, and pesticide production. Aqueous-based wastes sent to the facility were disposed of in evaporating ponds. Because of excessive rainfall in 1969, the ponds overflowed into the town of Glen Avon and resulted in the contamination of Pyrite Creek. In 1970, the EPA was created, and by 1972, DDT had been banned and the Federal Insecticide, Fungicide, and Rodenticide Act enacted. Additional historical highlights of hazardous waste history are given in Table 12.1. CERCLA was created in 1980 in response to discoveries in the late 70s of numerous abandoned, leaking, and hazardous waste dumps that threatened human health and the environment. Possibly the most noted hazardous waste site associated with this time period was “Love Canal.” Located in Niagara Falls, New York, the original intent of the canal, owned by Mr. William T. Love, was to provide hydroelectricity to the region and to join the two sides of the Niagara River separated by the Niagara Falls with a navigable waterway. When the project proved infeasible, the land was acquired by Hooker Chemical and Plastics Corporation and was subsequently used as a hazardous waste repository. During a five-year span beginning in 1947, nearly 21,000 tons of hazardous material were buried in the area. Once the canal was full, the land was sold to the City of Niagara Falls Board of Education. By the late 70s, the land was surrounded by single-family and low-income housing as well as three nearby schools. Public health concerns were realized in 1978 as a result of reports published in the local newspaper and by the New York State Department of Health. The publicity resulted in a school closure and demolition. Most residential dwellings in the impacted area were also demolished. CERCLA, initially established as a five-year program, was created to identify and take action to remedy hazardous waste sites, and to force responsible parties to pay for the cleanup. In addition, CERCLA established a $1.6 billion “Superfund” to support government cleanup actions and established guidelines for inclusion of sites on the National Priorities List (NPL) based on the Hazardous waste site Ranking System (HRS). The HRS ranks the hazardous character of sites based on risk. Risks from the migration of toxic compounds through the air, groundwater, or surface water, from direct contact with contaminated soils at the site, and from hazards associated with potential fires or explosions are considered. In 1986, the Superfund Reauthorization and Amendments (SARA) extended CERCLA five more years, increased the fund to $8.5 billion, established the Leaky Underground Storage Tank (LUST) trust fund, initiated the Emergency Planning and Community Right-toKnow Act (EPCRA) and associated Toxic Release Inventory (TRI). EPCRA and TRI were created in response to the 1984 release of methyl isocyanate (MIC) at the Union Carbide pesticide production facility located in Bhopal, India. The MIC gas, a lung irritant, caused up to 2,000 deaths and 350,000 injuries, leaving 100,000 people with permanent disabilities. As a consequence of EPCRA and the TRI, companies which process more than 25,000 lb/year (11,340 kg/year) or use 10,000 lb/year (4536 kg/year) of any of the 654 EPA listed hazardous chemicals must report all waste generated and all releases and transfers of hazardous chemicals. Companies are also required to develop an emergency response plan that allows the surrounding communities to protect themselves from catastrophic accidents associated with toxic chemical release. Appropriations for SARA continue today. Additional provisions of CERCLA include the National Contingency Plan (NCP), developed as the federal government’s blueprint for responding to both oil spills and the unintended release of hazardous substances. CERCLA also allows the EPA to conduct or require both “Removal” and “Remedial” Actions. A CERCLA
Section 12.1
The Problem
Table 12.1 Historical Events Relevant to the History of Hazardous Wastes Management Year
Significant Event
1940s
Increase in chemical production
1962
Rachel Carson published Silent Spring
1970
EPA Created
1972
FIFRA Created and DDT banned Iraq—grain coated with a mercury fungicide was ground into bread, exposing over 50,000 people; 459 were reported dead and over 6000 hospitalized. Bangladesh—Arsenic-contaminated drinking-water wells affected millions
1976
Toxic Substance Control Act (TSCA) and RCRA created
1977
End of 30 years of PCB contamination in the Hudson River, during which the General Electric Company (GE) discharged as much as 1.3 million pounds of PCBs directly into the river from facilities located in Hudson Falls and Fort Edward, NY
1978
Love Canal
1980
CERCLA (Superfund)—allowing the EPA to clean up, order abatement, and recover the cost of cleanup
1982
Times Beach, MO—largest civilian exposure to dioxin in the U.S. resulting from spraying oil to control dust. The town was evacuated and registered as a Superfund site.
1984
Bhopal, India—MIC release
1986
Emergency Planning and Community Right-to-Know Act (EPCRA) Toxic Release Inventory (TRI) Chernobyl nuclear disaster produced a plume of radioactive debris that spread to Ukraine, eastern Europe, Scandinavia, UK, and eastern United States Superfund Amendments (SARA) Because of the magma lying beneath it, Lake Nyos in Cameroon is saturated with carbon dioxide. The lake released an extreme amount of carbon dioxide suffocating 1800 people in addition to livestock in surrounding villages.
1988
Indoor Radon Abatement Act passed Ocean Dumping Ban Act passed
1989
Exxon Valdez oil spill
1990
Oil Pollution Act Pollution Prevention Act passed
1990s
Continued public awareness highlighted by film makers • A Civil Action • Erin Brokovich Radioactive waste disposal problems • New Mexico, Waste Isolation Pilot Plant (WIPP) • Hanford, Washington • Rocky Flats, Colorado
1994
Chemical Industries Air Toxic Reduction Rule passed
2002
Small Business Liability Relief and Brownfields Revitalization Act—provides relief from Superfund liability for small businesses, and promotes the revitalization of ”brownfields,” properties where redevelopment is hindered by the presence of contamination.
307
308 Chapter 12
Fundamentals of Hazardous Waste Site Remediation
Removal Action is triggered by a release of a dangerous substance into the environment by an incident such as a tanker truck accident or damage to a drum filled with hazardous waste. Removal Actions are designed to address emergencies and are typically short-term endeavors designed to remove the immediate threat. CERCLA Remedial Actions are designed to provide a permanent solution and are usually longer-term, higher-dollar projects. In 1987, the quantities of hazardous chemicals released to streams, disposed of in landfills, and discharged to wastewater treatment plants were 4 million, 1.2 million, and 0.87 million tons, respectively (Alvarez and Illman, 2006). According
Table 12.2 Top 20 Environmental Contaminants Associated with the 2005 CERCLA Priority List of Hazardous Substances 2005 Rank
Substance
Common current and historical uses and sources
1
Arsenic
Wood preservative, pesticide
2
Lead
Burning gasoline, lead-based paint and solder, batteries, ammunition
3
Mercury
Production of chlorine gas and caustic soda, burning coal
4
Vinyl chloride
Used in the production of PVC, wire and cable coating, packaging materials
5
Polychlorinated biphenyls
Insulator, coolant, and lubricant in transformers and capacitors
6
Benzene
Gasoline, plastics, resins, synthetic fibers, rubber, lubricants, dyes
7
Polycyclic aromatic hydrocarbons
Incomplete combustion of fossil fuels and other organics
8
Cadmium
Batteries, pigments, metal coating, plastics
9
Benzo(a)pyrene
Is not manufactured and has no industrial use; formed during combustion of organic matter
10
Benzo(b)fluoranthene
Leaches from water-distribution system tanks lined with coal tar or asphalt; from combustion of organic matter; no industrial use
11
Chloroform
Inhaled anesthetic, chemical manufacturing
12
p,p¿-DDT (dichlorodiphenyltrichlorethane)
Pesticide
13
Aroclor 1254
14
Aroclor 1260
Lubricant, hydraulic fluid, insulator, plasticizer, dedusting agent, transformer fluid
15
Dibenzo(a,h)anthracene
No known use or application; found in vehicle exhaust, cigarette smoke condensate, soot, coal tar
16
Trichloroethylene
Degreaser, paint remover, typewriter correction fluid
17
Dieldrin
Insecticide; breakdown product of aldrin
18
Chromium, hexavalent
Chrome plating, dyes, pigments, leather tanning, wood preserving
19
Phosphorus, white
Ammunition, fertilizers, food additives, pesticides
20
p,p¿-DDE (dichlorodiphenyldichloroethane)
Microbial and industrial processing byproduct of DDT; has no commercial use
Section 12.2
Contaminant Characteristics and Phase Distribution
309
to the U.S. EPA RCRAInfo National Database, the quantity of RCRA hazardous waste generated in the United States decreased by 29% between 1999 and 2003. The National Biennial RCRA Hazardous Waste Report estimated that the United States produced approximately 38.3 million tons of hazardous waste in 2005 (EPA National Analysis, 2005). In that same year, 21.8 million tons of RCRA hazardous wastes were disposed of by deep well or underground injection. The EPA has projected that approximately 294,000 contaminated sites need to be remediated within the United States and have an estimated cleanup cost of $209 billion. Most of this cost will be assumed by the property owners and those deemed potentially responsible for the contamination. Most sites are contaminated with solvents and other organics, metals, and petroleum products (U.S. EPA, 2004). Table 12.2 lists the top 20 hazardous substances on the CERCLA Priority List of Hazardous Substances for 2005. The complete list can be obtained online at www.atsdr.cdc. gov/cercla/05list.html.
12.2 CONTAMINANT CHARACTERISTICS AND PHASE DISTRIBUTION This section describes the structures of organic compounds commonly found at hazardous waste sites, and the properties that influence their fate in the environment. The concept of contaminant partitioning within the soil:water matrix is also introduced. 12.2.1 Contaminants of Concern Soil and groundwater can be contaminated by a single organic compound, or a mixture of such compounds, that are either pure product or dissolved in the aqueous phase. This section describes the structures of organic compounds commonly found at hazardous waste sites and provides examples of potential sources. Table 12.3 lists the names and associated properties of some hazardous organic chemicals commonly found at waste sites. The behavior of the carbon atom is the focus of organic chemistry. The most elementary organic compounds contain just carbon and hydrogen and are called hydrocarbons (HC), but many organics of environmental concern contain additional elements such as oxygen, nitrogen, sulfur, and halogens such as chlorine, fluorine, and/or bromine. Hydrocarbons can be classified as either aliphatic, aromatic, or polycyclic aromatic, depending on the nature of the carbon-carbon bonds. Aliphatic Hydrocarbons
Aliphatic hydrocarbons contain either straight or branched chains of single-, double-, or triple-bonded carbon atoms. If the backbone in the aliphatic contains only singlebonded carbon atoms, the hydrocarbon is termed saturated and is referred to as an alkane or paraffin. Alkanes can generically be described with the chemical formula C nH 2n+2 , where n represents the number of carbon atoms in the chain. Figure 12.1a
H
H
H
H
H
H
C
C
C
C
C
H
H
H
H
H
H
Figure 12.1a Pentane is an example of a straight-chain aliphatic hydrocarbon with chemical formula C5H12 .
310 Chapter 12
Fundamentals of Hazardous Waste Site Remediation
shows pentane, a five-carbon aliphatic, as an example of a straight-chain alkane. Hydrocarbons that contain a carbon-carbon double bond are called alkenes and have the general formula C nH 2n . Alkynes are characterized by having a carbon-carbon triple bond. As a result of high use and historically questionable disposal practices, chlorinated methanes, ethanes, and ethenes, such as trichloroethylene (TCE), tetrachloroethylene (PCE or Perc), dichloroethylene (DCE), and vinyl chloride (VC), are now among the most prevalent contaminants in groundwater. Chlorinated aliphatics are used in a variety of industrial settings. For example, VC is used in the manufacturing process of poly vinyl chloride (PVC), and both TCE and PCE have been widely used in the dry cleaning industry and for metal degreasing. The NPL lists VC, TCE, and chloroform as three of the most frequently reported organic contaminants at hazardous waste sites. Many of these chlorinated compounds are known or suspected carcinogens, and regulatory cleanup requirements approach analytically detectable limits. Chlorinated aliphatics have variable vapor pressures, are minimally soluble in aqueous systems, and often are more dense than water. A variety of chlorinated aliphatics of environmental concern are shown in Table 12.3a with associated chemical characteristics important to determining their fate in the environment. Aromatic and Polycyclic Aromatic Hydrocarbons (PAHs)
As shown in Figure 12.1b, aromatic compounds are characterized by the benzene ring. The benzene ring contains six carbon atoms, arranged in a hexagonal form, joined by Table 12.3a Names, Structures, and Properties of Short-Chain Aliphatic Hydrocarbons Commonly Found at Contaminated Sites (Chlorinated Alkanes and Alkenes are Emphasized) Compound name (Acronym)
Structure
Vinyl chloride (VC)
Cl
H C
Henry’s law constant (unitless)
Log K OW
T (°C)
62.5
3.35
1.38
25
131.4
0.372
2.38
25
119.4
0.141
1.97
20
153.8
0.957
2.64
20
165.8
1.08
2.6
20
C
H Trichloroethylene (TCE)
Molecular weight
H
Cl
H C
C
Cl
Cl
Chloroform (CF)
H Cl
C
Cl
Cl Carbon tetrachloride (CT)
Cl Cl
C
Cl
Cl Tetrachloroethylene (PCE)
Cl
Cl C
Cl
C Cl
Sources (for Tables 12.3a–d): U.S. EPA, 1990, Kuo, J., 1999, LaGrega, et al., 1994., Mackey, et al., 1997.
Section 12.2
Contaminant Characteristics and Phase Distribution
311
Figure 12.1b Both ring structures shown are commonly used to represent benzene, the base for all aromatic and polycyclic aromatic hydrocarbons. The benzene ring contains six carbon atoms joined by alternating single and double bonds and the molecule contains six hydrogen atoms. The positions of the bonds are known to change, resulting in benzene often being depicted as a hexagon with an internal ring that represents the shifting bond positions, causing all the bonds to have an equal length.
alternating single and double bonds. Other aromatic compounds are formed by the addition of functional groups onto the aromatic ring. Table 12.3b shows several aromatic compounds that are commonly found at contaminated sites. Benzene, Toluene, Ethyl benzene, and Xylene (BTEX) are often associated with petroleum products and typically enter the environment as a result of LUSTs. Many states use the concentration of BTEX compounds for cleanup standards of petroleum-contaminated sites. In addition to its prominence in petroleum-based products, benzene is used by a variety of other industries to manufacture plastics, resins, detergents, pesticides, and synthetic fibers; it is ranked sixth on the 2005 CERCLA NPL list and is believed to be a human carcinogen. Aromatics are less dense than water, have a high vapor pressure and are thus considered volatile, and are sparingly soluble in water. Table 12.3b Names, Structures, and Properties of Aromatic Hydrocarbons Commonly Found at Contaminated Sites Compound name
Structure
Benzene
Molecular weight
Henry’s law constant (unitless)
Log K OW
T (°C)
78.1
0.227
2.13
25
Toluene
CH3
92.1
0.279
2.73
20
Ethyl benzene
C2H5
106.2
0.268
3.15
20
106.2
0.212
3.0
20
m-xylene
CH3
CH3
312 Chapter 12
Fundamentals of Hazardous Waste Site Remediation
Polycyclic aromatic hydrocarbons (PAH) contain two or more fused benzene rings, and adjacent rings share two or more carbons. For example, when two benzene rings are joined, naphthalene is formed (see Figure 12.1c). PAHs are formed through the incomplete combustion of fossil fuels and are associated with industrial processes such as petroleum refining and coal gasification. Characteristics common among PAHs include high molecular weight and low volatility and solubility, and they readily sorb to soil matrices. Table 12.3c highlights several PAH compounds commonly found at hazardous waste sites.
Figure 12.1C Napthalene and phenanthrene are common twoand three-ring polycyclic aromatic hydrocarbons.
Naphthalene Phenanthrene
Table 12.3c Names, Structures, and Properties of Polycyclic Aromatic Hydrocarbons Commonly Found at Contaminated Sites
Compound name
Structure
Molecular weight
Henry’s law constant (unitless)
Log K OW
T (°C)
Naphthalene
128.2
1.977 * 10-2
3.36
25
Phenanthrene
178.2
1.15 * 10-2
4.46
25
Benzo(a)pyrene (BaP)
252.3
4.624 * 10-5
6.2
25
Dibenzo(a,h)anthracene
278.4
6.016 * 10-7
6.5
25
Section 12.2
Contaminant Characteristics and Phase Distribution
Polychlorinated Biphenyls (PCBs)
The biphenyl structure consists of two aromatic rings connected at one carbon site in each ring, as shown in Figure 12.1d. Ten sites are available on the biphenyl molecule for potential inclusion of one or more chlorine atoms. The term polychlorinated biphenyl (PCB) is used to refer to the biphenyl molecule with one to ten chlorine substitutions. The acronym PCB is often used generically to describe a mixture of the 209 possible congeners (or isomers) of substituted biphenyl molecules. In the United States, Monsanto began production of PCBs in 1929 under the trade name Aroclor® combined with a numerical designation that specified the amount of chlorine present in a particular mixture. For example Aroclor® 1260 contains 60% chlorine, while Aroclor® 1254 is 54% by weight chlorine.The number 12 in the Aroclor® designation indicates that there are twelve carbon atoms per molecule. PCBs are oily liquids or solids that appear colorless to light yellow and are chemically stable (even at high temperatures), do not burn easily, are sparingly soluble in water, viscous, adsorb strongly to soil and organics, and have a high dielectric constant. These characteristics make them ideal for use as coolants and lubricants in transformers, capacitors, and other electrical equipment. As a result of their chemical stability, PCBs tend to be recalcitrant in the environment and are also known to bioaccumulate and biomagnify in the food chain. As a result, PCB production in the United States was suspended in 1977. Worldwide, more than 1.5 million tons of PCBs have been produced, and it is unknown what quantity has been released to the environment (UNEP, 1999). The EPA has stated that PCBs are probably carcinogenic to humans. Additional Hydrocarbons of Concern: Explosives, Pesticides, Fuel Additives
Pesticides are a group of agents that destroy or control pest organisms and include insecticides, fungicides, and herbicides. Approximately 500,000 tons are applied each year in the United States to prevent the spread of disease (malaria, typhus, yellow fever) and to increase agriculture production. DDT (dichlorodiphenyltrichloroethane) is an organochlorine insecticide that was commonly used throughout World War II and saved lives through preventing malaria by controlling the mosquito population that carried and transmitted the disease. Use was banned in the United States in 1972 as a result of its bioaccumulation in the food chain. DDT has been found in marine sediments, wildlife, and in humans. It is known to cause the thinning of eggshells in birds, significantly affecting the population of apex predators, and has been cited as a major cause of the decline of the bald eagle population during the 1950s and 1960s. It is estimated that approximately 2 million tons of DDT have been produced worldwide and it is twelfth on the 2005 CERCLA NPL listing. Dichlorodiphenyldichloroethane (DDE), a commercial byproduct and microbial metabolite of DDT, is listed twentieth on the NPL list.
Biphenyl X represents possible Cl sites
Figure 12.1d Possible sites for chlorine atoms to be bonded to a biphenyl molecule are shown. From 1 to 10 chlorine atoms can be bonded to a biphenyl molecule for a total of 209 possible congeners.
313
314 Chapter 12
Fundamentals of Hazardous Waste Site Remediation
Explosives are usually made of aliphatic or aromatic ring compounds that contain a nitro functional group 1-NO22. Environmental contamination from this group of compounds is associated primarily with military production and storage, but it is also widely used by the mining and construction industries. Explosives tend to be environmentally persistent and they are considered toxic. One of the most common bulk explosives is 2,4,6-trinitrotoluene (TNT). TNT has been an important military explosive since World War I because of its relative safety during manufacturing, loading, transportation, and storage. It is often mixed with other explosives, such as Royal Demolition Explosive (RDX) and High Melting Explosive (HMX). Wastewaters from munitions programs, often referred to as “pink water,” may be difficult to remediate and are often colored pink as a result of the presence of TNT and RDX. TNT is known to be poisonous and is listed as a possible human carcinogen. Prolonged exposure may result in anemia and abnormal liver functions, and it is thought to be harmful to the immune system if ingested or breathed. Skin contact causes irritation that turns the skin a bright yellow-orange color. As a result of the promulgation of the 1970 Clean Air Act (CAA), additives were blended with gasoline in an effort to reduce pollution from vehicles by burning cleaner fuels. Additives included octane enhancers, antiknock compounds, corrosion inhibitors, detergents, and oxygenates. Reformulated gasoline (RFG) also contained a higher concentration of benzene, a carcinogenic compound. RFG is required in cities exhibiting the worst smog problems and is now used in 17 states and the District of Columbia. Two of the most common fuel oxygenates are acetone and methyl tertiary butyl ether (MTBE). It was anticipated that with the reduction of HC emissions caused by the addition of oxygenates to the mixture, the production of the secondary air pollutant, ozone, would be diminished. The anticipated drop in tropospheric ozone concentrations was not realized in most regions. Also, as a result of MTBE’s high aqueous solubility, it travels farther and faster in water than other gasoline constituents and it now contaminates many subsurface waters and drinking water supplies. Representative chemical structures and characteristics for pesticides, explosives, and fuel additives are shown in Table 12.3d. Inorganics of Concern
Metal contamination is commonly found in both solid and aqueous environments. The 2005 CERCLA NPL ranks arsenic, lead, and mercury as the top three contaminants most frequently found at hazardous waste sites. Metals are used in a variety of industrial activities and therefore have varied pathways into the environment. Organic arsenic is used to make herbicides and fungicides and is used in semiconductor production, while inorganic arsenic in the form of copper chromated arsenic (CCA) is used during the preservation process of pressure-treated wood. Lead is ubiquitous to the environment. Much lead contamination is a result of the burning of gasoline in vehicles before the forced phase-out by the CAA of the organometallic compound tetraethyl lead (TEL). Water pipes in some older houses may contain lead solder, and deteriorating lead-based paints are of concern. Lead is also commonly used in the production of auto batteries, ammunition, electronics, and plastics. Mercury is found in three forms, including elemental, inorganic, and organic. Metallic and inorganic mercury compounds enter the environment as a result of mining, burning coal, and waste disposal from manufacturing facilities. Metallic mercury is used to produce chlorine gas and caustic soda and is also used in thermometers and batteries.
Section 12.2
Contaminant Characteristics and Phase Distribution
315
Table 12.3d Names, Structures, and Properties of Additional Organic Compounds Commonly Found at Contaminated Sites. Structures Include PCBs, Explosives, Gasoline Additives, and Insecticides
Compound name (acronym) Aroclor® 1254 Dichlorodiphenyltrichlorethane 1p,p¿-DDT2
Structure
Molecular weight
Henry’s law constant (unitless)
n.a.*
n.a.
0.155
6.03
20
H
354.5
3.315 * 10-4
6.19
25
72.1
1.81 * 10-3
0.256
20
227.13
8.6 * 10-7
1.6
20
Cl
Log K OW
T (°C)
Cl CCl3
Methyl ethyl ketone (MEK)
CH2CH3 O
C CH3
2,4,6-Trinitrotoluene (TNT)
O2N
NO2
NO2 …
n.a.—not applicable, as Aroclors contain a mixture of PCBs, not just a single compound.
Metals such as arsenic, lead, and mercury are potentially toxic to the kidneys, decrease mental capabilities, and can cause weakness, headaches, abdominal cramps, diarrhea, and anemia (U.S. EPA, 2004). While these compounds cannot be destroyed through a bioremediation process in the environment, they can be transformed by methylation, complexation, sorption, and change of valence state to affect their solubility (and therefore their mobility and bioavailability) or toxicity. 12.2.2 Contaminant Characteristics As shown in Figure 12.2, the fate of contaminants released into the environment is varied and depends on a variety of factors. Once released to the soil or aqueous matrix, they can volatilize, dissolve into solution and potentially enter the groundwater, adsorb to the soil matrix, or be biologically altered. The parameters described below help engineers project the fate of contamination in the environment. Solubility and Nonaqueous Phase Liquids
The solubility of a compound describes its tendency to dissolve into the liquid phase (for environmental concerns, the solvent is usually water) and is important when determining its distribution within the environment. Solubility is defined as the maximum amount of compound dissolved in a unit volume of solvent when the system is at equilibrium. Sparingly soluble chemicals, when released, may form a layer of nonaqueousphase liquid (NAPL) either on the water table or at an impervious layer within an aquifer. A NAPL phase persists when the mass of contaminant exceeds its solubility limit in groundwater. If the density of the spilled organic is less dense than water (e.g., gasoline and oil products), then a light nonaqueous-phase liquid (LNAPL) will
316 Chapter 12
Fundamentals of Hazardous Waste Site Remediation
Exposure point
Prevailling wind direction Transport medium (air) Release mechanism (volatilization) Exposure point
Inhalation exposure route
Exposure point
Release mechanism (spill) Exposure medium (soil)
Ingestion exposure route
Water pile (source) Release mechanism (site leaching)
Water table
Transport medium (ground water) Ground-water flow
Figure 12.2 Cartoon depicting a leaky underground storage tank and resulting contaminant fate in the environment. (Source: U.S. EPA. 1989.)
form on the water table and provide a continual source of contamination to the aquifer if not removed. Similarly, compounds that have a specific gravity greater than one, such as TCE and PCE, will tend to sink to a nonpervious layer within the subsurface and form a dense nonaqueous-phase liquid (DNAPL) pool. Partitioning Coefficients
Partition coefficients are used to describe how organic compounds distribute between the solid, liquid, and gas phases of the environment. Many compounds released into the environment are bound or sorbed in the soil and sediment present. The octanol-water partition coefficient, KOW , describes a compound’s hydrophobicity and how it partitions between the organic and aqueous phase. The KOW is determined by quantifying the ratio of the compound in both phases of an immiscible mixture of octanol and water. KOW =
Concentration in octanol phase Concentration in aqueous phase
(12.1)
Values of KOW are unitless, vary widely from 10-3 to 107, and tend to increase with the size of the molecule. Large KOW values indicate that the compound is relatively hydrophobic and tends to partition to the organic phase. Organic compounds with low KOW values are hydrophilic and thus tend to stay in the aqueous phase, showing minimal soil adsorption. Soil-Distribution Coefficient
The sorption, or the distribution of nonpolar hydrophobic organic compounds to a particular soil matrix can often be well characterized by a linear relationship (recognize
Section 12.2
Contaminant Characteristics and Phase Distribution
317
this relationship as the linear Freundlich isotherm) described by the soil-distribution coefficient 1KSD2. KSD =
s CL
(12.2)
where: KSD = soil-distribution coefficient (often referred to as the partition coefficient), L/mg or m3/g, s = mass of solute sorbed per unit mass of dry soil, unitless, and CL = liquid-phase equilibrium concentration of the targeted compound, mg/L or g/m3. The soil-distribution coefficient is strongly dependent on the hydrophobicity of the solute as well as the organic content of the soil, such that the partition coefficient is not the same for every soil. Eweis et al., (1998), provide a relationship for the partition coefficient that requires knowledge of the KOW and the mass fraction of organic carbon in the soil, fOC . KSD = 6.3 * 10-7 focKOW where fOC = fraction of organic carbon in solids, a
(12.3)
mass of carbon b mass of solid
EXAMPLE 12.1 Estimation of sorption to soil Groundwater is contaminated with 150 mg/L (150 ppb) TCE. Assume the aquifer solids contain 1% organic carbon (i.e., fOC = 0.01) and that sorption is adequately described by a linear model. Estimate the mass of TCE sorbed to the solid material. Solution From Table 12.3a, the log KOW for TCE is 2.38. Use Equation (12.3) to estimate the soil-distribution coefficient, KSD . KSD = 6.3 * 10-7 fOCKOW = 6.3 * 10-7 10.0121102.382 = 1.51 * 10-6 KSD = ¢ 1.51 * 10-6
L mg
106 mg L L ≤¢ ≤ = 1.51 mg kg kg
Using Equation (12.2), which describes linear sorption, and the KSD value obtained from Equation (12.3), determine the equilibrium TCE concentration sorbed to the solid phase in the aquifer. s = KSDCL = a1.51
mg mg mg TCE sorbed L b a 150 b a b = 0.23 kg L 1000 mg kg soil
318 Chapter 12
Fundamentals of Hazardous Waste Site Remediation
Henry’s Law Constant
Henry’s law can be used to determine how a contaminant will distribute between the liquid and gas phases. The law states that the concentration of the solute in the liquid phase is proportional to the partial pressure or the concentration of the gas in which it is in equilibrium, such that: Cg = kH,A CL
(12.4)
where: kH,A = dimensionless Henry’s constant, Cg = equilibrium gas-phase concentration, mg/L, and CL = equilibrium liquid-phase concentration, mg/L. Care should be taken when using Henry’s law constants, as their values depend on the concentration units for the gas and liquid phases.
EXAMPLE 12.2 U s i n g H e n r y ’s l a w t o e s t i m a t e e q u i l i b r i u m c o n c e n t r a t i o n s Results from a soil-vapor study showed a soil-vapor concentration for toluene of 1000 ppmv. Use Henry’s law to estimate the surrounding liquid-phase toluene concentration. The dimensionless kH,A for toluene is 0.279. Solution The molecular formula for toluene is C 7H 8 . Determine the molecular weight for toluene as shown. MW = 71122 + 8112 = 92 Use the equation shown below to convert the volume-based vapor-phase concentration to a mass-per-unit volume basis when the gas temperature is 20°C. mg 3
m
mg m3
=
ppmv * MW * 103 24
=
ppmv * MW * 103 mg mg 1000 * 92 * 103 = = 3.83 * 106 3 = 3.83 24 24 L m
1at 20°C2
Use Henry’s law to calculate the aqueous-phase concentration, Cg = kH,A CL
CL =
Cg kH,A
mg mg L = 13.7 toluene in the aqueous phase 0.279 L
3.83 =
Important Physical Properties of Soil
To adequately describe the fate and transport of contaminants in the subsurface, several physical properties of soil must be defined. Refer to Figure 12.3 for a simplistic representation of a soil sediment and associated variables that will be used to mathematically describe the soil matrix. Table 12.4 provides a brief definition of
Section 12.2
Contaminant Characteristics and Phase Distribution
Volume relationships
319
Mass relationships
Va
Air
Vw
Water rw 1.0 g/cm3
Ww Vv
Wt
Vt Vs
Solids rs 2.65 g/cm3
Ws
Figure 12.3 Phase diagram for a partially saturated soil. Volume and mass relationships are identified.
Table 12.4 Volume-Mass Relationships of Soil Constituents Property
Mathematical Expressions
Symbol
Definition
Porosity
h
volume of voids total volume
h =
Vv Vt
Volumetric water content (at saturation ® = h)
®
volume of water total volume
® =
Vw Vt
Bulk density
rw
total weight
rw =
Wt Vt
rb =
Ms Vt
total volume Dry bulk density
rb
mass of solids total volume
each property, its Greek symbolic reference, and a mathematical description. From the figure, notice that the total weight of the soil is equal to the weight of the water plus the weight of the solids. The weight of air is neglected, such that the total weight is given mathematically as Wt = Ww + Ws . The void volume in the soil is equal to the sum of the volumes of air and water, such that Vv = Va + Vw . The total soil volume can be represented as Vt = Vv + Vs = Va + Vw + Vs .
EXAMPLE 12.3 Determine mass and volume relationships in soil A sample of moist soil having a wet weight of 1000 g and volume of 650 cm3 was oven dried and found to have a dry mass of 800 g. Determine the soil’s dry bulk density and porosity and the volumetric water content. Solution From the problem statement, recognize that: Mt = 1000 g, Ms = 800 g, and Vt = 650 cm3
320 Chapter 12
Fundamentals of Hazardous Waste Site Remediation
Assume a soil density of rs = 2.65 g/cm3. Given that the mass of the solid is 800 g, the volume of the soil is Vs = 1800 g2/12.65 g/cm32 = 301.9 cm3. Assume that the mass of the air is negligible, so Ma = 0. The mass of the water present in the sample can be determined as Mw = 11000 - 8002g = 200 g. Assuming the density of water is 1 g/cm3, the volume of water in the sample is Vw = 200 cm3. Recognize that the total sample volume is Vt = 650 cm3 = Va + Vw + Vs . Knowing the volumes of the water and solid phases, we can algebraically determine Va = 148.1 cm3. With Vt = 650 cm3, Vw = 200 cm3, Vs = 301.9 cm3, and Va = 148.1 cm3, we can determine the unknown mass and volume soil relationships. Use Figure 12.3 and Table 12.4 to determine appropriate definitions and equations. Ms 800 g g = = 1.23 3 3 Vt 650 cm cm 1148.1 + 2002cm3 Vv Va + Vw porosity = h = = = = 0.54 Vt Vt 650 cm3 dry bulk density = rb =
volumetric water content = ® =
Vw 200 cm3 = = 0.31 Vt 650 cm3
12.2.3 Darcy’s Law Fate and transport models simulate the movement and chemical/biological alterations of contaminants as they move with groundwater through the subsurface. Most models consider three main mechanisms associated with contaminant transport: advection, mechanical dispersion, and molecular diffusion. Advective transport, or convection, is the process where contaminants are carried along with the bulk movement of the flowing groundwater. Mechanical dispersion describes the mixing of the groundwater containing contaminant(s) as it flows around the solids in the soil matrix. Without dispersion, flow through an aquifer would behave like a plug flow reactor. But the variations in velocity caused by dispersion actually allow the contaminant to reach some distance x from the source sooner than by seepage velocity alone. Molecular diffusion is controlled by a concentration gradient, such that the movement of the contaminant is from an area of greater concentration toward an area where it is less concentrated. Darcy’s law is often used to describe flux (discharge per unit area with typical units in m/s) in porous media. q = KC
dh dL
(12.5)
where: q = Darcy flux along the flow path (often called the superficial velocity, discharge velocity, specific discharge, or Darcy’s velocity), m/s, KC = hydraulic conductivity, m/s, and dh = hydraulic gradient, dimensionless. dL
Section 12.2
Contaminant Characteristics and Phase Distribution
321
The porosity 1h2 relates the actual pore velocity (v) to the Darcy flux (q). The flux is divided by the porosity in recognition that the flow through the subsurface is limited by the available pore space in the matrix, such that the average velocity through the porous media is larger than the Darcy flux. The pore velocity, v (also referred to as the average linear velocity and the seepage velocity), is equal to Darcy’s velocity (q) divided by porosity 1h2, shown as: v =
q h
(12.6)
Sorption to sediments is mathematically characterized by the concept of retardation and is fundamental to understanding contaminant transport. The retardation factor, R, relates the groundwater seepage velocity to the velocity of the contaminant plume 1vp2. R =
v vp
(12.7)
Flow of a conservative, inert material that travels at the speed of the seepage velocity has an R value equal to 1. In a given environment, the plume of a hydrophobic compound with R = 2 is estimated to travel at half the groundwater seepage velocity. If the flow system is assumed to be at equilibrium and sorption can be characterized as linear and reversible, the retardation coefficient can be estimated as: R = 1 +
rb K h SD
(12.8)
EXAMPLE 12.4 G ro u n d w a t e r m i g r a t i o n a n d r e t a r d a t i o n o f c o n t a m i n a n t s Assume a leak in a 200-L underground storage tank containing water contaminated with methyl tertiary-butyl ether, benzene, and benzo(b)fluoranthene. Estimate how far each contaminant will travel in one year. Assume the following hydrogeologic characteristics: Soil porosity = 0.5
Specific gravity of soil = 2.5
Hydraulic gradient = 0.05 ft/ft
Aquifer conductivity = 0.0001 cm/s
Fraction of organic content in the soil = 1%
Dry bulk density = 1.25 g/mL
Benzene KOW = 102.13
MTBE KOW = 101.24
Benzo(b)fluoranthene KOW = 106.12
Solution Use Equation (12.5) to calculate the Darcy velocity: q = KC
dh cm ft cm = a 0.0001 b a0.05 b = 5 * 10-6 s s dL ft
322 Chapter 12
Fundamentals of Hazardous Waste Site Remediation
From Equation (12.6), the pore velocity can be estimated as:
q = h
v =
5 * 10-6
cm s
0.5
= 1 * 10-5
cm s
Develop a summary table for the compounds spilled. Compound
KOW
fOC
KSD(L/mg)
R
-7
vp (cm/s)
Distance (cm)
1.27
7.9 * 10
-6
250
8.5 * 10-7
3.13
3.2 * 10-6
100
8.31 * 10-3
20,776
4.8 * 10-10
MTBE
1.24
10
0.01
1.09 * 10
Benzene
102.13
0.01
benzo(b)fluoranthene
106.12
0.01
0.015
Calculate the KSD for each compound using Equation (12.3). For example, the KSD for MTBE is found as follows: KSD = 6.3 * 10-7 fOCKOW = 16.3 * 10-7210.0121101.242 = 1.09 * 107
L mg
Use Equation (12.8) to determine the retardation factor for each compound. The calculation is demonstrated with MTBE.
R = 1 +
rb K = 1 + h SD
a1.25
1000 mg g 1000 ml L ba b ba b a1.09 * 10-7 mg g ml L 0.5
= 1.27 Relying on Equation (12.7), calculate the velocity of each contaminant. For MTBE:
vp =
v = R
1 * 10-5 1.27
cm s
= 7.9 * 10-6
cm s
Now, determine an estimate of the distance traveled in one year. Again, MTBE is used as an example: distance = vp * time distance = a7.9 * 10-6
365 d 24 hr 60 min 60 s cm b11 yr2a ba ba ba b = 250 cm s yr d hr min
Notice the role sorption plays for benzo(b)fluoranthene! Recognize that due to subsurface heterogeneities, assumptions of uniform fOC , differences in sorption of different soil organics, and the linear sorption assumption in the retardation-factor formula, these results are simply estimates. Therefore, do not report too many significant figures when providing your answers.
Section 12.3
Overview of Microbial Processes
12.3 OVERVIEW OF MICROBIAL PROCESSES Biological processes are used in conjunction with many remediation schemes to treat contaminated soil and groundwater. Bioremediation processes use living organisms to clean up or prevent the introduction of hazardous chemicals in the environment. To this end, an understanding of some microbiological systems is required to appropriately design and analyze remediation processes. 12.3.1 An Overview of Bacteria A variety of types of organisms are used in bioremediation processes, including: plants, fungi, algae, actinomycetes (filamentous bacteria), and—possibly the most important group—bacteria. Bacteria are single-cell prokaryotic (lacking internal membrane structure) microorganisms that reproduce primarily by binary fusion, such that daughter cells are identical to the parent cell. Bacteria are classified by shapes that include rods (bacillus), spheres (coccus), and spirals (spirillum), and they range in size from 0.5 to 3.0 mm. Nutritional requirements for microorganisms can be divided into two general, often overlapping categories: those that supply energy and those that supply carbon. Nutrients may be supplied to organisms as an element or as a specific combination of elements (compound) required for growth and repair. Organisms are often classified according to their nutritional choice of energy and carbon sources. The energy source provides a supply of high-energy electrons used to make adenosine triphosphate (ATP), which is required for the biosynthesis of cell material. Common organic energy sources include glucose, lipids, amino acids, and other carbohydrates, while inorganic sources include sulfur, ammonia, and hydrogen. A chemotroph is an organism whose energy sources are electron-donating organic and inorganic compounds such as those listed in the previous sentence. A phototroph is an organism whose energy source is light. The carbon source is used by the microbe as a building block for the organic compounds found in the organism. An autotroph is an organism whose principle carbon source is carbon dioxide, which it uses to make non-CO2 carbon compounds. A heterotroph is an organism which uses organic compounds as its principle source of carbon. As a result, many organisms used in bioremediation are heterotrophs. 12.3.2 Environmental Factors Affecting Microbial Metabolism Microorganisms transform organic contaminants during their growth and reproduction processes. Organics provide carbon used to build cell material, and electrons are used as a source of energy. The cells act as a catalyst in the oxidation of organic chemicals (electron donors), causing transfer of electrons from the organic chemical to some electron acceptor. In aerobic oxidation, the acceptor is oxygen. In anaerobic environments, the possible acceptors include (with decreasing efficiency): nitrate, sulfate, and carbon dioxide. Microorganims also require essential nutrients such as nitrogen and phosphorus to sustain viability. Chemical, physical, and biological factors influence an organism’s viability, the target compound bioavailability, and therefore the degradation capacity of microorganisms. This section provides a synopsis of the most influential of these environmental factors. Important variables that effect microbial activity include the system temperature and pH, soil moisture content, availability of a suitable electron acceptor and required nutrients, and an appropriate and adequate bacterial population. The site must also exhibit nontoxic conditions for the organisms, and there should be an absence of competitive organisms.
323
324 Chapter 12
Fundamentals of Hazardous Waste Site Remediation
Bacterial metabolic reactions are mediated by enzymes that serve as reaction catalysts. Enzyme activity increases linearly as substrate concentration increases, up to a value that represents saturation of enzyme activity. Above saturation, no additional increase in enzymatic activity is observed. Bacteria can grow over a wide range of temperatures; some can grow at 0ºC while others thrive in boiling water. Different classes of organisms show different temperature optima. For example, psychrophilic bacteria grow optimally around 20ºC, while mesophilic bacteria grow well between 20 to 45ºC, and thermophilic bacteria grow well at temperatures above 45ºC. In general, for each class of bacteria, enzyme activity and growth rate increase linearly with an increase in temperature, up to their optimal conditions. Above that temperature, activity drops abruptly as enzymes are destroyed/denatured. Favorable conditions for bioremediation typically occur when the temperature is from 15 to 45ºC. Environmental pH will also affect microbial activity. The most desirable pH will be site or process specific, but most systems have a pH optimum around neutrality, as enzymes often denature at pH extremes. As a rule of thumb, an acceptable pH range for most systems is 5.5 to 8.5. The availability of the contaminant to the microorganisms is complicated and is a function of compound solubility, polarity, soil pore structure, and the presence of water. Moisture content is important, because microbes obtain all of their nutrients and carbon requirements for growth from solution. So, nutritionally for the microbe, water is essential. In many remediation systems, oxygen is the electron acceptor of choice. Unfortunately, the solubility of oxygen in water is low ( ' 9.0 mg/L at 20ºC). So, a high soil moisture content may result in poor oxygen availability. Optimal soil moisture content ranges from 15% to 25% (mass basis, i.e., 150 to 250 g water per kg of dry soil). At higher soil water concentrations, airflow is inhibited, possibly resulting in limited oxygen availability. The molecular structure of the target compound is an additional chemical factor for consideration in bioremediation process design.
12.4 INTRODUCTION TO ENGINEERED REMEDIATION PROCESSES In any environmental remediation process, especially those that emphasize biological remediation, the behavior and fate of contaminants are site specific, making it difficult to generalize treatment efficacy from one case to another. To add complexity to the site-specific nature of remediation, recognize that the convolution of the hydrocarbon mixture, and perhaps its age, may affect the rate and extent (end point) of treatment; and that there are typically large variations in the biological removal of pollutants between large- and small-scale studies. In essence, determining the effectiveness of bioremediation, beyond mere compound removal, requires well-designed experiments that can be analyzed statistically. This section provides general application and design guidelines for selected widely used treatment technologies that combine bioremediation with a physical and/or chemical treatment option. 12.4.1 In situ Remediation Schemes In situ remediation schemes are introduced for treatment of both groundwater and the vadose zone. The vadose zone is the unsaturated zone (except in the capillary fringe) between the land surface and the water table. Air-sparging and pump-andtreat technologies are commonly used to remediate groundwater. Soil-vapor extraction and bioventing are popular treatment schemes used in the vadose zone.
Section 12.4
Introduction to Engineered Remediation Processes
325
Groundwater Remediation Technologies—Air Sparging and Pump and Treat
Pump-and-treat technology is the most common form of groundwater remediation. Between 1982 and 1992, it was used in almost 75% of the Superfund sites requiring groundwater treatment. Pump-and-treat systems remove the contaminated groundwater from the subsurface. Once at the surface, the groundwater is treated to remove the contaminants, and then the treated water is either recharged back into the ground or discharged into a receiving stream or municipal wastewater treatment plant. Once at the surface, a variety of physical, chemical, and biological processes have been used to treat the water. For example, organic compounds may be treated through air stripping, carbon adsorption, chemical oxidation, membrane filtration, or biodegradation. Inorganics may be removed from the groundwater through precipitation, ion exchange, adsorption, or reverse osmosis. Pump-and-treat systems can be designed to provide either hydraulic containment or treatment, and often a combination of both. When focused on hydraulic containment, the objective is to control the movement of contaminated groundwater, preventing expansion of the contaminated zone, and/or to redirect the contaminated plume to protect potential receptors. When designed for treatment, the overarching goal is to reduce the concentration of contaminants in the groundwater, such that cleanup objectives are realized. Figure 12.4 provides a generalized diagram of a pump-and-treat system. When designing such a system, important considerations include: selection of a pumping rate, estimation of the mass of contamination in the pumping zone, estimation of oxygen requirements and time for bioremediation if appropriate, and the number and placement of additional wells needed for sampling, monitoring, and containment. The pumping rate depends on the time available for remediation, the aquifer hydraulic conductivity, the size of the contaminated zone, and the soil-distribution coefficient of the contaminants. Air sparging is an in situ groundwater remediation technology that involves injecting pressurized air into the saturated zone to strip volatile contaminants from the dissolved and sorbed states, and to provide an electron acceptor for promotion of bioremediation. Once in the gas phase, contaminants migrate into the vadose zone, where they are extracted by a soil-vapor extraction system. Air sparging can
k
ee
Cr
Discharge pipe Extraction wells
Plume
Treatment building
Figure 12.4 Typical pump-and-treat system. (Source: U.S. EPA, 2001.)
326 Chapter 12
Fundamentals of Hazardous Waste Site Remediation
employ both vertical and horizontal wells to enhance contaminant extraction, with air bubbles traveling both horizontally and vertically through the subsurface. Systems should be designed such that the uncontrolled release of volatiles into the atmosphere is minimized. The treatment of offgas may be required. Sites contaminated with VOCs and fuels are often amenable to air sparging. The lower-molecular-weight constituents in gasoline, such as BTEX, are generally mitigated well with this technology, while less volatile, heavier mixtures such as diesel fuel do not respond adequately. Appropriate geologic and hydrogeologic site conditions are crucial to the application of an air-sparging system. Site considerations dictate the ability to effectively deliver air to the treatment area. In general, site conditions that favor air sparging include a moderate to high permeability, often correlating to relatively coarse-grained materials. Low-permeability (fine-grained) soils limit the migration of air into the subsurface and reduce the effectiveness of the treatment strategy. A typical air-sparging system is shown in Figure 12.5. Vadose Zone Treatment Schemes—Soil Vapor Extraction and Bioventing
Soil vapor extraction (SVE) is an effective and cost-efficient technology for removing VOCs from the vadose zone. In the SVE process, a vacuum is supplied to unsaturated soils, causing air movement that encourages NAPL, aqueous, or sorbed VOCs to volatilize and be removed. Once removed, the exit gas is treated (if necessary) and subsequently discharged. Recognize that SVE is a remediation, not a treatment process, as the goal is volatilization of the VOCs, although some bioremediation occurs indirectly. SVE systems consists of one or more extraction wells screened in the unsaturated zone, blowers or vacuum pumps, and often also a lowpermeability cap at the ground surface, an air/water separator, and an offgas treatment system. If the site is capped, ventilation wells may also be installed. Offgas treatment systems may include adsorption on granular activated carbon, catalytic oxidation, or incineration. If required, offgas treatment comprises typically over 50% of the SVE remediation cost. Figure 12.6 provides a typical schematic of an SVE system. Figure 12.5 Typical air-sparging treatment system. (Source: FRTR, 2002.)
Vent gas collection channels
Air treatment
Air blower
To further treatment or discharge
Groundwater extraction wells Vadose zone
Injection well
Contaminated groundwater Saturated zone Submersible pump
Section 12.4
Introduction to Engineered Remediation Processes
Clean air
Vacuum blower
Vapor treatment
(4)
Extracted vapor (3)
Process residual
Vaporliquid separator
Extraction well (1)
Air vent or injection well (2)
(5)
(6)
Liquid treatment
Clean water
Process residual
Air vent or injection well Impermeable cap (7)
Ground surface
Contaminated vadoss zone
Water table
Figure 12.6 Generic soil-vapor extraction system. (Source: U.S. EPA, 1991.)
Most SVE systems are installed to treat shallow contamination, where the site targeted for cleanup is less than 10 m deep. Typical vacuums of 0.1 to 0.2 atm (1.5 to 2.9 psi) are applied, resulting in extraction flow rates between 1 to 6 m3/min (35 to 212 ft3/min). In the vadose zone, recall that the contaminant is partitioned between three phases (possibly four phases, if pure product is available). Soils with high organic content tend to sorb VOCs, making SVE much more difficult, and desorption from the solid phase is considered rate limiting. SVE in the preferential pathways will be achieved primarily due to advection, while in the dead-end pores removal will be achieved primarily due to diffusion. Soil conditions should be dry enough so that air moves freely. However, the drier the soil, the more the compound will partition to the solid phase. Also recognize that many spills contain multiple compounds, each having a different vapor pressure. Typically, the more volatile compounds evaporate first, leaving the less volatile ones. This fractionalization process is sometimes called “weathering” the product. Bioventing (BV) treats contaminants in the unsaturated zone and, like SVE, forces air through the treatment area. But, as the focus in SVE is removal by volatilization followed by above-ground treatment, BV is designed to treat contaminants through bioremediation in situ. A BV system is designed to enhance the activity of indigenous bacteria, effectively stimulating the natural in situ biological degradation of hydrocarbons. To encourage biodegradation rather than removal through volatilization, bioventing systems are generally operated at vapor extraction or injection rates lower than those used for SVE systems, in an effort to provide only enough oxygen to sustain microbial activity. Bioventing can be used to treat all
327
328 Chapter 12
Fundamentals of Hazardous Waste Site Remediation
aerobically biodegradable constituents; however, it has proven to be particularly effective in remediating releases of petroleum products, including gasoline, jet fuels, kerosene, and diesel fuel. The technology is often applied to sites contaminated with midweight petroleum products, such as diesel and jet fuel. Lighter products such as gasoline tend to volatilize readily and are therefore more amenable to the SVE process. Heavier products such as lubricating oils generally take longer to biodegrade, making bioventing a less effective option. A BV system consists of one or more extraction or injection wells screened in the unsaturated zone, and blowers or vacuum pumps. Figure 12.7 shows a schematic of a typical bioventing system. Almost exclusively, air is injected. Extraction would be used only if there were a nearby subsurface structure or property boundary that an injected air source might seep into. Injection is preferred to prevent extracting volatile compounds from the well. 12.4.2 Ex situ Remediation Schemes – Composting and Landfarming Composting and landfarming are two common remediation schemes used to treat soils excavated from contaminated sites. In addition to treating contaminated soils, both technologies are extensively used for stabilization of wastewater sludges (biosolids). Composting is a controlled, biologically mediated process that transforms contaminants into innocuous, stable end products and has been shown effective at treating a wide range of contaminants, including PAHs, propellants, pharmaceutical wastes, and explosives. A wide range of soil and sediment types including sand, gravel, soils, clay, and lagoon and marine sediments, can be effectively remediated by composting. Soils to be composted are excavated and mixed with supplemental organics and amendments. The addition of the supplemental organics serves three main
Vapor treatment (if needed)
Nutrient tank
UST
Atomospheric discharge
Blower
Legend: Vapor phase Nutrients Air flow
Air flow
Adsorbed phase Dissolved phase Optional depending on the site conditions
Groundwater gradient
Figure 12.7 Typical bioventing system using vapor extraction. (Source: U.S. EPA, 2004.)
Section 12.4
Introduction to Engineered Remediation Processes
purposes: to serve as a simple food source to support a diverse microbial population, to provide a bulking agent useful for maintaining aerobic conditions, and to promote high bioactivity levels in the thermophilic range. Fresh manure, a supplement high in organic content, is often used to help increase the pile temperature. Additional amendments are typically applied to composting piles to serve as a bulking agent and as a nutrient and energy source. One of the most common bulking agents is wood chips; added at a volumetric ratio of 1 part wood chip to 2 to 10 parts soil, with a 1:3 ratio being most typical. Other frequently used bulking agents include sawdust, ground corncobs, straw, and vermiculite. Manure is often added as the nutrient and energy source. A C:N:P weight ratio of 100:10:1 is desired and is obtained by adding 1 part manure to 1 to 50 parts soil (by volume). Typically, manure is added at a ratio of 1:5. Other nutrient and energy amendments include dewatered municipal sludge, dry molasses, and grass clippings. Good mixing of all amendments with the contaminated soil is critical. Configurations for composting include piles, windrows, and in-vessel. Figure 12.8 provides a cartoon of a typical windrow composting facility. Windrows may span up to 100 yards (91 m) if space is available. Pile shape has an important effect on moisture content and oxygen availability. The top of the pile may be either concave or peaked. The concave pile traps rainfall and replenishes water lost from piles as steam. This type of configuration might be useful if anaerobic conditions are desirable. Peaked piles tend
Excavate and screen soils
Form windrows w/soil and amendments
Figure 12.8 Typical windrow composting process. (Source: FRTR, 2002.)
Periodic turning of windrow
Windrow monitoring
Compost analysis
Windrow disassembly and disposition
329
330 Chapter 12
Fundamentals of Hazardous Waste Site Remediation
to shed rainwater. To remain aerobic, the oxygen content needs to be approximately 10% in the large pores. Air can be supplied by either passive or active means. Passive oxygenation takes advantage of natural convection driven by the chimney effect. Natural air circulation through the pile is driven by warm, less dense air rising out of the top of the center of the pile, causing cool fresh air to be sucked in at the bottom and sides. Oxygen can also be added by mixing with a front end loader or by using a blower. Landfarming is a bioremediation activity that can be used for treatment of contaminated soil by spreading it on the ground surface, tilling the soil for aeration and/or for potential mixing with native surface soil. Landfarming is widely used to treat petroleum-impacted soils. Abiotic losses through volatilization and leaching are difficult to quantify. Although landfarming is a “low-tech” system, operational variables still significantly affect treatment time and total cost and as a consequence should not be ignored. Critical operational variables include: (1) soil application rates such that highly contaminated soils are appropriately diluted with native soil, bulking agents, or additional nutrients so that bioactivity can occur, (2) tilling frequency, since it is important to ensure aerobic conditions, (3) nutrient addition frequency, (4) control of moisture by irrigation or spraying, and (5) buffering of soil pH near neutral by addition of crushed limestone or dolomite. Figure 12.9 provides a schematic for a typical landfarming unit. Several factors may limit the applicability and effectiveness of the process. For instance, a large amount of space is required, and conditions affecting biological degradation of contaminants (e.g., temperature, rainfall) are largely uncontrolled, which increases the length of time to complete remediation. Inorganics are not treated in landfarming systems, and volatile organics, such as solvents, must be pretreated, because of their likeliness to volatilize into the atmosphere, causing air pollution. Dust control during tilling and other material-handling operations is an additional consideration.
Poly tunnel (optional)
Exiting ground surface
Microbes nutrients
Waste
Manhole
Concrete retaining wall footing
1% Slope
Compacted sand
To wastewater treatment plant
Leachate collection pipe
Figure 12.9 Typical landfarming treatment unit. (Source: FRTR, 2002.)
Compacted subgrade surface
Polyethylene geomembrane
References 331
S U M M A RY There are an estimated 294,000 CERCLA and RCRA contaminated sites in the United States. Cleanup costs are predicted to reach $209 billion. This chapter provided a brief historical perspective and presented regulatory requirements associated with hazardous waste. An introduction to organic chemistry was given to provide context for the significant compounds contributing to hazardous waste. Contaminant partitioning was introduced and its effect on the fate and transport of targeted compounds within the environment was discussed. An overview was given of microbial systems and environmental factors that affect metabolism. Both in situ and ex situ treatment schemes for soil and groundwater were presented.
air sparging alkane alkenes alkynes autotroph bioventing chemotroph composting Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA, or Superfund) Emergency Planning and Community
Right-to-Know Act (EPCRA) Hazardous waste site Ranking System (HRS) heterotroph hydrocarbons landfarming leaky underground storage tank (LUST) National Priorities List (NPL) octanol-water partition coefficient phototroph remedial action removal action
Resource Conservation and Recovery Act (RCRA) soil-vapor extraction Superfund Reauthorization and Amendments (SARA) Toxic Release Inventory (TRI)
REFERENCES Alvarez, P.J., and Illman, W.A. (2006). Bioremediation and Natural Attenuation Process Fundamental and Mathematical Models, Eds. J.L. Schnoor and A. Zehnder, Wiley-Interscience, Hoboken, NJ. EPA National Analysis (2005). The National Biennial RCRA Hazardous Waste Report: Based on 2005 Data. Available online at http://www.epa.gov/epaoswer/hazwaste/data. Eweis, J.B., Ergas, S.J., Chang, D.P.Y., and Schroeder, E.D. (1998). Bioremediation Principles, WCB/McGraw-Hill. Federal Remediation Technology Roundtable (FRTR) (2002). Remediation Technologies Screening Matrix and Reference Guide, 4th ed., prepared by J. Van Deuren, T. Lloyd, S. Chhetry, R. Liou, and J. Peck of Platinum International, Inc., for U.S. Army Environmental Center, SFIM-AEC-ET-CR-97053.Available online at www.frtr.gov. Kuo, J. (1999). Practical Design Calculations for Groundwater and Soil Remediation, Lewis Publishers, Boca Raton, FL. LaGrega, M.D., Buckingham, P.L., and Evans, J.C. (1994). Hazardous Waste Management, McGraw-Hill, New York. Mackay, D., Shiu, W. Y., and Ma, K. C. (1997). Illustrated Handbook of Physical and Chemical Properties and Environmental Fate for Organic Chemicals, Volumes I, II, and V, Lewis Publishers, New York. UNEP (United Nations Environment Programme): UNEP 1999 Report.
KEY WORDS
332 Chapter 12
Fundamentals of Hazardous Waste Site Remediation
U.S. EPA (1989). Risk Assessment Guidance for Superfund, Volume 1, Human Health Evaluation Manual (Part A), Interim Final, EPA/540/1-89/002, Office of Emergency and Remedial Response, Washington, DC, December, 1989. U.S. EPA (1990). CERCLA Site Discharges to POTWs Treatability Manual, EPA 540/2-90-007, Office of Water, U.S. EPA, Washington, DC. U.S. EPA (1991). Guide for Conducting Treatability Studies Under CERCLA: Soil Vapor Extraction, Office of Solid Waste and Emergency Response, Office of Emergency and Remedial Response Quick Reference Fact, Hazardous Site Control Division OS-220, September 1991, EPA/540/2-91/019B. U.S. EPA (2001). Groundwater Pump and Treat Systems: Summary of Selected Cost and Performance Information at Superfund-financed Sites, Office of Solid Waste and Emergency Response. EPA 542-R-01-021b. Available online at www.clu-in.org. U.S. EPA (2004). How to Evaluate Alternative Cleanup Technologies for Underground Storage Tank Sites. A Guide for Corrective Action Plan Reviewers. Solid Waste and Emergency Response, EPA 510-B-94-003; EPA 510-B-95-007; and EPA 510-R-04-002, May 2004, Available online at www.epa.gov/oust/pubs/tums.htm.
EXERCISES 12.1
12.2
12.3 12.4
12.5
12.6
12.7
12.8
Visit the Office of Solid Waste and Emergency Response EPA website (http:// www.epa.gov/superfund/) and research CERCAL NPL sites within your state of residence. Review the information provided online and write a summary paragraph describing your findings. From Table 12.2, select a metal, a chlorinated aliphatic, and a PAH compound of choice. Use the internet or library to write a paragraph summary describing potential uses, contamination sources, and health effects for each compound. Describe the difference between a CERCLA removal and remedial action. Research the regulations associated with deep-well injection of hazardous waste. Write a paragraph outlining these provisions that includes your opinion of this type of disposal practice. Groundwater is contaminated with 300 mg/L (300 ppb) of toluene. Assume that the aquifer solids contain 1.5% organic carbon and that sorption is adequately described by a linear model. Estimate the mass of toluene sorbed to the solid material. Results from a soil-vapor study showed a soil-vapor concentration for vinyl chloride of 1500 ppmv. Use Henry’s law to estimate the surrounding liquidphase VC concentration. A sample of moist soil having a wet weight of 1500 g and volume of 1000 cm3 was oven dried and found to have a dry mass of 1150 g. Determine the soil: dry bulk density, porosity, and volumetric water content. Assume a soil density of rs = 2.65 g/cm3. Assume a leak in a 500-gal underground storage tank containing water contaminated with Aroclor® 1254 and acetone. Estimate how far each contaminant will travel in one year. Assume the following hydrogeologic characteristics:
Soil porosity = 0.45
Specific gravity of soil = 2.5
Hydraulic gradient = 0.06 ft/ft
Aquifer conductivity = 0.0001 cm/s
Fraction of organic content in the soil = 0.5%
rb = 1.3 g/mL
Acetone log KO W = - 0.23
CHAPTER
13
Introduction to Solid Waste Management Objectives In this chapter, you will learn about: Municipal solid waste (MSW) management options The magnitude of MSW issues Federal regulations pertinent to solid waste management Quantification of MSW Components of a landfill
13.1 INTRODUCTION Western world civilizations realized as early as 500 B.C.E. the importance of solid waste disposal. The Greeks knew that garbage attracted rats and other vectors that contributed to the spread of disease, and in 320 B.C.E they passed the first known refuse-disposal decree, banning disposal in the streets of Athens. Athenian citizens were required to dispose of their waste at least one mile from the city walls. The Romans learned from the Greeks and followed a similar waste management plan. Interestingly, the fall of the Roman Empire coincided with an increased mismanagement of waste disposal. During the middle ages, septic wastes and their inherent disease threat were routed directly to a moat that surrounded the inhabitants of the castle. In colonial America, people pushing west had few possessions and therefore not much waste was produced. People in towns and cities of America and Europe, however, tossed their garbage out of windows and doors. The consequences of this practice were heightened by the industrial revolution, as a relatively large number of people moved to the city and for the first time in history were able to buy products made in mass quantity. Tossing trash from windows and doors continued through the mid 1800s, until piled garbage began to hinder access to cities, and citizens finally became concerned about unsanitary and malodorous conditions. By the early 1900s, several hundred solid waste incinerators were used in the United States. Industrial growth in the U.S. during WWII increased solid waste emissions. Plastics were developed and their many uses exploited, leading to new disposal problems. Postwar waste emissions became a national concern, resulting in a federal government response. In 1965, the first solid waste regulation directed at protecting public health in the United States was passed, the Solid Waste Disposal Act. In 1979, the Environmental Protection Agency (EPA) prohibited open dumping, and initial standards for landfills were mandated.Ten years later, the EPA initiated a goal to reduce waste nationally by 25% and to concentrate on recycling. This chapter provides an overview of municipal solid waste (MSW) management options, current regulations, and related issues.
Introduction to Solid Waste Management
Figure 13.1 Historical population and waste generation and handling trends. (Source for waste generation trends: Municipal Solid Waste in the United States: 2005 Facts and Figures; Source for historical population trends: www. census.gov; Source for population projections in years beyond 2000: Census Release-T1, 2000)
Waste generated U.S. population
Landfill disposal Recovered for recycle
350 Millions of tons of waste or population in millions
334 Chapter 13
300 250 200 150 100 50 0 1950
1960
1970
1980
1990
2000
2010
Time (year)
To develop a perspective on current solid waste management issues, consider Figure 13.1, which shows historical trends in waste generation, recovery, and disposal, along with population growth in the United States. Over 200 million tons of solid waste were generated in 2000, some 32 million tons more than a decade earlier. By 2005, the refuse generated amounted to 245.7 million tons. Of this, 32% was either recycled or composted—an increase of 16% from the 1990 observed rate. In 2005, Americans produced, on a per-capita basis, 4.54 lb (2.1 kg) of refuse per day. Figure 13.2 summarizes the most recent data available on how U.S.-produced waste is disposed. What are the major components of trash or municipal solid waste (MSW)? The EPA categorizes MSW by material and product categories. Materials are items such as paper and paperboard, yard trimmings, food waste, wood, rubber, leather, and others. In 2005, 34.2% (on a weight basis) of the MSW produced was materially categorized as paper and paperboard, 13.1% as yard trimmings, 11.8% as plastics, and 11.9% as food scraps. The products category includes include groupings of containers and packaging, durable and nondurable goods, and food scraps. Figure 13.3 shows the waste production by product category in 2005 and Table 13.1 gives an idea of items associated within each category. What does waste disposal cost? In 1982, the National Solid Waste Management Association (NSWMA) began investigating and tracking such costs. Historically, landfilling has cost less than incineration. In 2004, the national average tipping fee was reported as $34.29 per ton for landfill versus $61.64 a ton for incineration (Repa, 2005). This cost is for disposal only and does not include costs of recycling, collection, and transport. Consider the waste generation and waste management data presented in Figures 13.1 and 13.2. Using the tipping fees provided above; about how much does the United States spend on waste disposal every year? Figure 13.2 Management of municipal solid waste in the United States, 2005. (Source: Municipal Solid Waste in the United States: 2005 Facts and Figures.)
Combusted with energy recovery 13.6%
Recycled 32.1%
Discarded primarily through landfill 54.3%
Section 13.2
Durable goods 16.4%
Other wastes 1.5%
Yard trimmings 13.1% Food scraps 11.9%
Regulations and Solid Waste Management
Containers & packaging 31.2%
Nondurable goods 25.9%
Table 13.1 Examples of Products in Waste Categories Shown in Figure 13.3 Waste Category Durable goods
Nondurable goods
Containers and packaging
Steel
Paper
Steel
Aluminum
Plastics
Aluminum
Glass
Rubber and leather
Glass
Plastics
Textiles
Paper
Rubber and leather
Plastics
Wood
Wood
Textiles Source: Municipal Solid Waste in the United States: 2005 Facts and Figures.
13.2 REGULATIONS AND SOLID WASTE MANAGEMENT The American legislative system is based on common law, that is, based on court rulings. Theoretically, the law develops and evolves as needed. But it has a significant weakness in that it cannot act preemptively. As a remedy, the government (federal, state, or local) may pass laws to provide environmental guidelines or regulations before a situation arrives in a courtroom. When legislators determine that a law is required, it is developed, as shown in Figure 13.4. State regulatory requirements have to be at least as stringent as the federal requirements. In many cases, states have simply adopted the federal requirements. The Resource Conservation and Recovery Act (RCRA), promulgated in 1976, is the primary federal legislation governing solid and hazardous waste. RCRA regulations are outlined in 40 CFR Parts 240-99 and are easily accessed through an EPAmaintained website (http://www.epa.gov/epacfr40/chapt-I.info/). The pervasive goal of RCRA is to protect human health by providing an umbrella of safety from the hazards posed by waste disposal. RCRA is also focused on recycling, recovering, and reducing the amount of waste generated and ensuring that wastes are managed in an environmentally safe manner. Solid waste, hazardous waste, and underground storage tanks (USTs) are managed by RCRA. 13.2.1 RCRA Solid Waste Definition According to EPA regulations, solid waste means any garbage, or refuse, sludge from a wastewater treatment plant (biosolids), water supply treatment plant, or air pollution control facility, and any other discarded material, including solid, liquid,
335
Figure 13.3 Waste generated by product category in the United States, 2005. (Source: Municipal Solid Waste in the United States: 2005 Facts and Figures.)
336 Chapter 13
Introduction to Solid Waste Management
Federal house or senate bill
Federal act
Federal regulations
Federal rules or guidelines
Figure 13.4 Process overview for the United States environmental legislation.
State bill
Regional or local ordinance
State act
Local regulations
State rules or guidelines
Local rules or guidelines
semisolid, or contained gaseous material resulting from industrial, commercial, mining, and agricultural operations and community activities. But more practically, solid waste is defined as the waste generated by a community. 13.2.2 RCRA Hazardous Waste Definition Hazardous waste is regulated under the RCRA Subtitle C. By definition, RCRA defines hazardous waste as [RCRA §1004(5) definition]: A solid waste, or combination of solid waste, which because of its quantity, concentration, or physical, chemical, or infectious characteristics may (a) cause, or significantly contribute to, an increase in mortality or an increase in serious irreversible, or incapacitating reversible, illness; or (b) pose a substantial present or potential hazard to human health or the environment when improperly treated, stored, transported, or disposed of, or otherwise managed. To provide a more practical definition, regulations state that to be regulated under RCRA as a hazardous waste, a waste must first be a solid waste as defined by RCRA. A waste is an RCRA hazardous waste if it meets the solid waste stipulation, it is not specifically excluded from regulation under RCRA, and it is either: 1. A LISTED RCRA hazardous waste. (These are wastes from various industrial processes, wastes from specific sectors of industry, or wastes in the form of specific chemical formulations.) • F list—wastes from certain common industrial or manufacturing processes. Because the processes producing these wastes can occur in different sectors of industry, the F list wastes are known as wastes from nonspecific sources. • K list—wastestreams from certain specific industries. K list wastes are known as wastes from specific sources. • P list and U list—pure or commercial-grade formulations of certain specific unused chemicals.
Section 13.3
Waste Generation (Quantifying MSW Generation)
2. A waste with a CHARACTERISTIC property that poses a sufficient threat to deserve regulation as hazardous. The four (4) characteristics that identify a waste as hazardous are: • Toxicity (codes D004 to D043): leachate from the sample obtained using the Toxicity Characteristic Leaching Procedure (TCLP) contains any of 39 different toxic chemicals above specified regulatory levels. • Reactivity (code D003): can explode or violently react when exposed to water or under normal handling conditions, can create toxic fumes or gases when exposed to water or under common handling conditions, meets the criteria for classification as an explosive under Department of Transportation rules. • Ignitability (code D001): flashpoint 6 140°F, can spontaneously combust and sustain combustion, is an oxidizer. • Corrosivity (code D002): pH greater than or equal to 12.5 or less than or equal to 2 or the ability to corrode steel in an EPA-approved test. 3. A waste DECLARED to be hazardous by the generator. 13.2.3 RCRA Nonhazardous Waste RCRA’s nonhazardous solid waste management program, Subtitle D, is focused on source reduction, recycling, combustion, and landfilling. Sound environmental practices for solid waste management are encouraged. By definition, the term solid waste includes not only municipal garbage but also some hazardous waste (excluded from Subtitle C regulations) generated by households and conditionally exempt small-quantity generators (CESQGs). This solid waste management (SWM) program is responsible for the collection and disposal of municipal wastes generated by households and businesses that are typically disposed of in municipal solid waste landfills (MSWLFs). Subtitle D also contains technical criteria for the design of MSW landfills (40 CFR Part 258, Subchapter I). This chapter focuses on the management of municipal solid waste (nonhazardous, subtitle-D wastes).
13.3 WASTE GENERATION (QUANTIFYING MSW GENERATION) The quantification of MSW generation is important, as this knowledge facilitates compliance with federal and state diversion requirements, aids in equipment selection and with collection and management decisions, and is critical to proper facilities design. The objective of this section is to develop an appreciation for topics associated with MSW generation. It introduces two methodologies used to quantify MSW generation and shows differences among global production rates. Factors affecting waste generation are explored and per-capita generation rates highlighted. Two approaches are commonly used to quantify waste generation, (1) materialsflow methodology and, (2) load-count analysis. The application of either approach requires that wastes be categorized using the following mass balance: generated waste = disposed waste + recycled waste + diverted waste (13.1) where: generated waste = all solid waste materials generated, disposed waste = solid waste materials collected and taken to ultimate disposal (landfill or waste incinerator),
337
338 Chapter 13
Introduction to Solid Waste Management
recycled waste = solid waste materials separated for recycling, and diverted waste = solid waste materials not processed through the municipal waste-management channels. The materials-flow methodology for determining waste-generation rates is based on materials production or flow. Production data are gathered on a weight basis for each material and product category. For example, detailed material balances are performed for each generation source, such as an individual home or specific industry. The Franklin Study, a nickname for a 2001 Facts and Figures report on MSW in the United States, used this method. Several problems are associated with this methodology. For example, residue such as food materials or detergent remaining in a disposed container is not accounted for. Also, this method measures the amount of waste collected, not the actual amount generated and possibly disposed of at an alternate location. For load-count analysis, the number of vehicle loads hauled to a transfer center, materials recovery facility (MRF), incinerator, or landfill is noted along with corresponding vehicle size, types of waste, and estimated volume and weight recorded, if available. This calculation represents the quantity collected (delivered), not generated. Recycling, alternative disposal methods, and storage are not identified, making it difficult to discern whether the data collected properly represent waste generation.
EXAMPLE 13.1 Solid waste generation (Example adapted from Tchobanoglous et al., 1993.) Use the following information to estimate the unit waste generation rate per week and the recycling efficiency for a residential area consisting of 1200 homes. The average occupancy is 3.5 persons per home. Two observation locations were used, each for a period of one week. The local transfer station collected all of the waste for disposal, and a recycling facility was also monitored. According to estimates by the local wastewater treatment facility, 7,350 pounds of food waste is disposed of in home food grinders every week. Additional Data from the Transfer Station: • • • • • • • • •
Number of compactor truck loads = 9 Average size of compactor truck = 20 yd3 Estimated specific weight of waste on compactor truck = 500 lb/yd3 Number of flatbed loads = 7 Average flatbed volume = 2 yd3 Estimated specific weight of waste on flatbed truck = 225 lb/yd3 Number of loads from individual residents’ private vehicles = 20 Estimated average volume per domestic vehicle = 8 ft3 Estimated specific weight of waste from domestic vehicle = 150 lb/yd3 Data from the Recycling Station:
• Number of truck loads received = 12 • Average weight of each load = 1,420 lb
Section 13.3
Waste Generation (Quantifying MSW Generation)
Solution Develop a computation table to estimate the total weight of waste disposed during this week. Computation Table for Example 13.1 Waste source
Number of loads
Average volume, yd3
Waste specific weight, lb/yd3
Waste total weight, lb
Compactor Truck
9
20
500
90,000
Flatbed truck
7
2
225
3,150
Private vehicle
20
0.3
150 Total, lb/week =
900 94,050
Determine the weight of recycled waste produced during this week. Total recycled waste = 12 trucks * 1,420
lb = 17,040 lb recycled material truck
Remembering to account for ground-food disposal waste by including data from wastewater treatment facilities, estimate the unit waste generation:
unit or per-capita generation rate =
A 94,050 lb冫wk + 17,040 lb冫wk + 7350 lb冫wk B * a 1200 homes *
unit or per-capita generation rate = 4.03
wk 7d
3.5 persons b home
lb capita # d
Determine the recycling efficiency for the residential area: weight of recycled material total weight of waste collected 17,040 lb recycle rate = * 100 = 14.4% 194,050 + 17,040 + 7,3502 lb
total recycled waste =
Figure 13.5 provides information on global MSW production rates of major countries. The United States has the largest average production rate at 4.6 lb generated per capita per day. The average waste generation for the countries presented is 3.5 lb per capita per day. Figure 13.6 depicts how the rate of total and per-capita generation of waste increased in the United States between 1960 and 2005. In 1960, per capita waste generation was 2.7 pounds per person per day, and total waste generation was 88.1 million tons. By 2005, per-capita generation of waste was 4.5 pounds per person per day, and total waste generation was 245.7 million tons. Historically, increases in waste-generation rates correlate with increases of the Gross National Product, and population increase typically explains one-third of the increase in MSW generation. Other factors that affect generation rates include geographic location, source reduction and recycling, season, home food-waste grinders, frequency of collection, legislation, public attitudes, per-capita income, and size of household. Pay As You Throw (PAYT) programs that charge curbside residential
339
Introduction to Solid Waste Management
Country
340 Chapter 13
Figure 13.5 Municipal waste-generation rates of major countries. (Source: OECD, 2002.)
Average Japan Sweden Finland Italy France Germany Belgium United Kingdom Ireland Australia Netherlands Norway Canada Switzerland Denmark Austrailia United States 0.0
3.5 2.5 2.7 2.8 3.0 3.1 3.3 3.3 3.4 3.4 3.4 3.7 3.7 3.9 3.9 4.0 4.2 1.0 2.0 3.0 4.0 lb MSW generated per capita per day
4.6 5.0
Total waste generation 5
250
4
200
3
150
2
100 50 1950
1960
1970 1980 1990 Time (years)
2000
lb/(capita-day)
Figure 13.6 MSW generation in the United States. (Source: Municipal Solid Waste in the United States: 2005 Facts and Figures.)
Millions of tons
Per capita generation (lb/person/day) 300
1 2010
customers on a per-bag/can or weight basis have been shown to decrease production, mirrored by an increase in the recycle rate.
13.4 SOLID WASTE COLLECTION Solid waste (SW) collection is conducted primarily by individuals manning trucks that traverse a specified area, collecting refuse from homes or other generation sites. The collected refuse is transported to a transfer station, disposal site, or materials-recovery facility. In 1992, approximately 50% to 70% of the SW budget (including collection, transportation, and disposal) was spent on the collection.As fuel costs continue to escalate, the efficient collection of SW will become increasingly important. Solid waste collection can be considered a five-stage process that includes (1) transport of the waste inside the house to an outside can, (2) pickup, or loading the can contents onto the truck, (3) maneuvering the truck from house to house, (4) truck routing, and (5) maneuvering the truck and collected refuse to the disposal site.This section introduces the tasks and logistics of MSW collection and addresses the problems and concerns. 13.4.1 Types of Collection Systems Two general types of collection systems can be defined. The Hauled Container System (HCS) is often used at commercial, construction, industrial, institutional,
Section 13.4
and other sources characterized as having a high rate of waste generation. In the HCS, large containers are used to store waste at the generation site, and the container with its waste contents is then transported to the disposal site, emptied, and returned into service. In the Stationary Container System (SCS) operation, the containers used to store the waste remain at the generation site, except when moved to the curb or elsewhere to be emptied. Both residential and some commercial generators use the SCS process. 13.4.2 Equipment Vehicles and containers are required for both HCS and SCS collection systems. When the HCS is used, only one truck and a driver are required to accomplish collection. Each pickup, however, requires a trip to the disposal site underlying the importance of container size and utilization to the economic viability of this process. Three types of vehicles are used in HCS collection: 1. Hoist trucks are generally used to handle commercial wastes, in what is often characterized as a small operation with a few pickup locations that generate considerable amounts of waste. Scrap metal, construction debris, and other bulky items are also often picked up by hoist trucks. Containers used in conjunction with hoist trucks may vary in size from 2 to 12 yd3 (1.5 to 9.2 m3). 2. Vehicles equipped with a tilt frame are frequently used to collect solid waste from dumpsters, the large steel containers commonly used at apartment complexes and commercial sites. 3. Large dumpsters, often referred to as roll-off containers, are used to collect heavy rubbish and are often observed at construction and demolition sites. Roll-off container size can vary from 10 to 40 yd3 (7.6 to 30.6 m3). Trash-trailer systems equipped with a tilt frame are used for this type of collection. Packer trucks are commonly used in the SCS process for pickup of residential waste. Although packers can vary in design, most are covered vehicles having internal compactors. Packers vary in size from 16 to 20 yd3 (12.2 to 15.3 m3) and most are rear loaded, although some designs now incorporate side loading. Once loaded, the waste is compacted to approximately 600 to 700 lb/yd3 (357 to 416 kg/m3). On average, packers usually service approximately 200 customers before the truck is full and a trip is made to the landfill (Vesilind et al., 2002). 13.4.3 Problems and Concerns A variety of problems and concerns are commonly associated with solid waste collection. Waste-collection laborers are considered unskilled, yet the labor is intensive, and a high employee turnover can be expected. Injuries and poor working conditions are common, and there is limited opportunity for career advancement. And although the use of automated, mechanical equipment for curbside collection is becoming more prevalent, the prospect for the mechanical replacement of manual labor is dim. Customer-service issues related to refuse collection include the frequency of service, container and storage concerns, pickup location, and questions on how to deal with special wastes. Collection frequency has been declining since the 1950s. By the 1970s, once-a-week (1/wk) collection was common in half of the United States. This trend continued into the 1980s, but at a reduced rate. Some parts of the South still use twice-a-week collection (2/wk). Reasons for declining collection frequency include: • Decline in proportion of putrescible waste because of increased use of food grinders • Better design of collection vehicles to control odors and flies
Solid Waste Collection
341
342 Chapter 13
Introduction to Solid Waste Management
Table 13.2 Advantages and Disadvantages of Different Collection Frequencies Collection frequency
Potential advantages
Potential disadvantages
Favoring conditions
Once per week or less
Less expensive Requires less fuel
Improperly stored waste can create odor and vector problems
Cold to moderate climate
Twice per week
Reduces litter Reduces storage requirements
More expensive Requires more fuel
Warm climate
More than twice per week
Reduces litter Reduces storage requirements
More expensive Requires more fuel
Dense population
• Increased service costs • Decreased time between collection and disposal • Better management Advantages and disadvantages of different collection frequencies are summarized in Table 13.2. Management of solid waste disposal must deal with inflation and with changes associated with promulgation of new regulations. With the lack of consistent quality management, collection is often given a low priority, and collectors are often resistant to change and to implementing new technologies. Technical issues important to effectively managing the solid waste collection process include: • • • • • •
Collection of recyclables, yard waste, and special wastes Yard waste containers—bagged material must be debagged prior to composting Automated collection (still requires an operator) Development of efficient routes Vehicle weight restrictions Vehicle turning radius and clearance
The various factors affecting decisions and calculations associated with solid waste collection are characterized as either fixed or variable. Fixed factors include the climate, topography, and layout of the collection area influencing container access (alley, curbside, rear of house). The type of transportation systems, traffic, and roads available are also considered fixed. For instance, in Venice, Italy, boats are used, and in the Bahamas, barges are used to transport collection trucks from island to island to landfills. The population density and the types of waste collected are also considered fixed factors. Variable factors include the storage techniques employed, recycling, collection frequency, and crew size.
EXAMPLE 13.2 Solid waste collection Assume that each pickup location produces 75 lb of waste per week and the packer vehicle can compact the waste to a density of 750 lb/yd3. Estimate the number of customers that have their waste collected prior to making a trip to the landfill. Assume the truck capacity is 20 yd3.
Section 13.5
Landfill Containment and Monitoring Systems
343
Solution Determine the weight of waste on the truck when full: 20 yd3 * 750
lb = 15,000 lb yd3
Then the number of customers served can be calculated as: 15,000 lb = 200 customers lb 75 customer
EXAMPLE 13.3 Solid waste collection Assume a truck and crew are observed to service customers at a rate of 1.25 customers per minute, and the actual time spent per day collecting waste is 5 hours (recognizing that some time is spent each 8-hr day traveling to and from the garage and taking breaks). Estimate the number of customers that can be served per day. Solution If 1.25 customers are served per minute, then the number of customers served in 5 hours can be estimated as: a1.25
customers 60 min 5 collection hr customers b * a b * a b = 375 min hr d d
13.5 LANDFILL CONTAINMENT AND MONITORING SYSTEMS This section outlines the components of a landfill and the processes that take place in a landfill. You will gain a general familiarity with modern landfills and be able to describe their main components. By definition, a landfill is an engineered method of disposing of solid waste on land in a manner that protects the environment. Landfill operation includes the spreading of refuse in a thin layer, followed by compaction. A cover material such as soil, foam, geotextile, or tarp is placed over the waste at the end of each day. The EPA has developed a booklet, Criteria for Solid Waste Disposal Facilities: A Guide for Owners/Operators, available online at http://www.epa.gov/epaoswer/ non-hw/muncpl, that provides an excellent summary of provisions of the Agency’s Municipal Solid Waste Landfill (MSWLF) Criteria (for additional details, refer to 40 CFR, Part 258 and the Federal Register, October 9, 1991, 56FR50978). The booklet discusses the major regulatory requirements, how the rule is implemented and enforced, and where to obtain more information. Federal MSWLF standards include: • Location restrictions—ensure that landfills are built in suitable geological areas away from faults, wetlands, flood plains, or other restricted areas. • Composite liners requirements—to protect groundwater and the underlying soil from leachate releases, include a flexible membrane (geomembrane) overlaying two feet of compacted clay soil lining the bottom and sides of the landfill.
344 Chapter 13
Introduction to Solid Waste Management
• Leachate collection and removal systems—rests on top of the composite liner and removes leachate from the landfill for treatment and disposal. • Operating practices—that help protect public health, include compacting and covering waste frequently with several inches of soil to help reduce odors; control litter, insects, and rodents; and decrease leachate formation by increasing runoff. • Groundwater monitoring requirements—requires testing groundwater wells to determine whether waste materials have escaped from the landfill. • Closure and postclosure care requirements—include covering landfills and providing long-term care of closed landfills. • Corrective-action provisions—control and clean up landfill releases and achieve groundwater protection standards. • Financial assurance—provides funding for environmental protection during and after landfill closure (i.e., closure and postclosure care). Site selection for a landfill is paramount. The site should be large enough to accommodate the SW needs of the area it serves for a lifetime of approximately 10 years. It must be compatible with the local SW management programs, such that public health, safety, welfare, and the environment are protected. Every effort is made during site selection to minimize adverse impacts on the surrounding area, property value, and traffic flow. When selecting the site, the potential for fire, spills, and accidents must be realized. The site must be located outside of the 100-year flood plain and far from airports, because of the increased risk of birds in the landfill area and potential impact on planes. Landfills must be sited at least 10,000 ft (3050 m) from jetports and 5,000 ft (1524 m) from other airports. Other factors affecting site selection include land availability, haul distance, soil conditions, and topography. Geological restrictions on landfill sites require that no landfill should exist within 200 ft (61 m) of a fault or unstable area characterized by seismic activity. Leachate concerns dictate hydrological conditions associated with surface water and groundwater issues. Also considered are climate (rainfall and wind), environmental, and ecological conditions, public input and concerns, as well as the potential use for the site after closure are considered. Landfills can be classified as either wet or dry. A dry landfill is designed to keep waste dry in an effort to limit biological activity, while a wet landfill, often called a bioreactor, uses leachate recirculation to moisten and facilitate microbial degradation of the waste. Landfills are designed to accept either municipal (sanitary) or hazardous wastes. Controlled landfills may be used for co-disposal of both municipal and hazardous wastes. Three construction techniques used for landfills include excavation, above ground, and canyon/depression. The excavation method can be used only when the groundwater table (GWT) is distant from the surface. Trenches with typical dimensions of length = 100- 400 ft 130.5- 122 m2, width = 15- 25 ft 14.6 -7.6 m2, and depth = 10-15 ft 13-4.6 m2 are excavated, and the SW is dumped into the trenches for disposal. Tractors and compactors spread and compact the refuse by pushing it up the working face. Multiple passes are made with compaction equipment to maximize compaction density. Soil cover, obtained from excavation, is placed on the refuse. When the excavation method is not feasible, the above-ground method is used. SW is unloaded and spread in long narrow strips (15–30 inches or 38–76 cm thick). SW placement in strips is repeated multiple times until a predetermined cell thickness is reached (usually 5–20 ft or 1.5–6.1 m). Cover soil is applied (6–12 inches or 15–30 cm) at the end of each day. Geotextiles, foams, ground tires, compost, and other materials are also often used as cover. The embankment (working face) slope for a landfill is limited by the capabilities of the equipment used and is typically
References 345
3:1 or 4:1 (run:rise). The canyon/depression method is similar in operational technique to the above-ground method and is often used in areas where depressions exist (natural or artificial—canyons, valleys, ravines, borrow pits). Modern landfills may be equipped with a leachate recirculation system (LRS), which can be used to store leachate in the waste mass and/or enhance biological activity and improve decomposition of organic SW, resulting in a reduced time for landfill stabilization. Such landfills often accept biosolids to provide moisture, microorganisms, buffering capacity, and nutrients to further enhance the decomposition of wastes. Modern landfills have in situ storage and treatment facilities for leachate collection, show an increase in biogas production, and are equipped with facilities for gas/energy recovery. Such facilities also exhibit a reduction of liability, care, and costs associated with post-closure activities.
S U M M A RY This chapter provided an overview of solid waste management issues, options, and current governing regulations. Historical data were presented, providing insight concerning yearly increases in refuse production and the need for well-engineered solutions for disposal and reuse. RCRA characteristics for hazardous waste were presented and the concept of MSW generation (quantification) was introduced. The hauled container and stationary container systems were introduced as common collection schemes. Municipal solid waste landfill operations and basic design concepts were discussed. An overview was given of appropriate regulatory information regarding landfill siting, design, operation, and closure. hauled-container system hazardous waste hoist trucks landfill load-count analysis material and product categories
materials-flow methodology municipal solid waste nonhazardous waste packer trucks Resource Conservation and Recovery Act (RCRA)
roll-off container stationary container system underground storage tanks (USTs)
REFERENCES Census Release -T1 (2000). ”Annual Projections of the Total Resident Population as of July; Lowest, Middle, High Series: 1999 to 2100,” Population Estimates Program, Population Division, U.S. Census Bureau, Washington, D.C. 20233, February 14, 2000. Municipal Solid Waste in the United States: 2005 Facts and Figures. United States Environmental Protection Agency, Office of Solid Waste (5306P) EPA530-R-06-011 October 2006, www.epa.gov. OECD, 2002. OECD Environmental Data. Waste. Compendium 2002. Environmental Performance and Information Division, Environment Directorate, Organization for Economic Cooperation and Development (OECD), Working Group on Environmental Information and Outlooks. URL: http://www.oecd.org Repa, E.W. NSWMA’s 2005 Tip Fee Survey, NSWMA Research Bulletin 05-3 March 2005. National Solid Waste Management Association, Washington, DC, www.nswma.org. Tchobanoglous, G., Theisen, H., and Vigil, S. (1993). Integrated Solid Waste Management Engineering Principles and Management Issues, McGraw-Hill, New York. Vesilind, P.A., Worrell, W.A., and Reinhart, D.R. (2002). Solid Waste Engineering, Brooks/Cole, Pacific Grove, CA.
KEY WORDS
346 Chapter 13
Introduction to Solid Waste Management
EXERCISES 13.1
13.2
13.3
13.4
13.5
13.6 13.7
Use your text and online information available from the U.S. EPA Office of Solid Waste (http://www.epa.gov/epaoswer/osw/) to answer the following questions: (a) Define the acronym RCRA. Describe the difference between RCRAsubtitle C and RCRA-subtitle D. (b) Which federal agency is responsible for enforcing RCRA? (c) If an RCRA solid waste material is reused, is it still considered an RCRA solid waste? (d) Do RCRA regulations and verbiage address only hazardous and nonhazardous solid wastes? Provide examples to support your answer. (e) List the four physical characteristics which may result in a waste being categorized as hazardous. (f) Define the acronym CERCLA and briefly describe the main focus of this federal regulation. (g) In the early 2000s, the amount of hazardous waste generated in the United States was (greater than, the same as, or less than) the amount of municipal solid waste generated in the United States today. Please provide the data and source(s) used to support your response. (h) In 2005, approximately how many tons of waste did the United States generate? Please provide source(s) used. (i) How much waste is generated on a percapita basis in the United States? How does this generation rate correspond to the rates observed in other major countries? (j) What is the main component of waste generated in the United States? The terms generated, disposed, recycled, and diverted wastes are commonly used in relation to solid waste management. Briefly define these terms in your own words. The EPA recognizes a variety of waste management methods, including: (1) source reduction, (2) composting, (3) waste combustion, (4) landfilling, and (5) recycling. (a) Briefly define each of these methods and (b) identify the EPA’s preferred method for waste management. Consider the most recent waste-generation and waste-management data available to you. Using the 2004 national average landfill tipping fee, reported as $34.29 per ton, estimate the amount of money the United States spends on waste disposal by landfilling every year. For both your home state and the state in which you reside while in college, determine the name of the state agency that enforces the rules for solid waste management. What is the difference between a listed and a characteristic hazardous waste? The accompanying table contains information on the composition of the residual (disposed) waste stream generated by a community and the recycling efficiencies achieved for various components in the waste stream. Calculate the overall recycling efficiency achieved by the community to one decimal place.
Component
Composition of the residual (disposed) waste, % by wt.
Individual recycling efficiencies, %
Food waste
15
0
Mixed paper
30
30
Glass
7
20
Plastics
5
15
Exercises 347
Metal
3
60
Textiles
7
0
Wood
8
0
25
30
Yard waste Sum
100
[Hint: Use Equation (13.1) and assume 0% diverted.]
Assume that each pickup location produces 125 lb of waste per week, and the packer vehicle can compact the waste to a density of 800 lb/yd3. Estimate the number of customers that have their waste collected before the vehicle makes a trip to the landfill. Assume the truck capacity is 20 yd3. 13.9 Assume that a truck and crew are observed to service customers at a rate of 1.32 customers per minute, and the actual time spent per day collecting waste is 6 hours (recognizing that some time is spent each 8-hr day traveling to and from the garage and taking breaks). Estimate the number of customers that can be served per day. 13.10 A community needs to decrease the amount of waste being sent to the local landfill. A city engineer has proposed the installation of food grinders and the implementation of a recycling program. Using the following information, determine the reduction (as a percentage) in the waste being disposed (give your answer to one decimal place). Consider that food grinders will be installed in 70% of the homes. It is estimated that, through the food grinder, these homes will dispose of 45% of the food waste currently generated. [Note: Initially, the amount of waste disposed is the same as the amount of waste generated. Both the food grinders and recycling reduce the quantity of waste disposed.] 13.8
Component
Composition of the generated waste, % by wt.
Food waste
15
0
Mixed paper
30
60
Glass
11
70
8
40
Plastics Metal
Anticipated recycling efficiency, %
4
80
19
70
Misc.
13
0
Sum
100
Yard waste
This page intentionally left blank
Index
A ABET-accredited engineering program, 7, 13 Absolute system (cg), 15 Absorption, 298–302 Academia, projects/work for, 5 Acaryote, 60 Accuracy, 28–29 Acid, 42–46 Acid gas scrubber, 298 Acid mine drainage, 81 Acidity, classification of surface water susceptibility to, 48 Acidogenesis, 270 Activated sludge, 253–258 biochemical kinetics of, in completely mixed systems, 255–256 design and operational parameters, 254 design (example), 256–258 mean cell residence time (MCRT), 254 oxygen requirements, 256 waste activated sludge (WAS), 254 Actual oxygen transfer rate (AOTR), 260 Adenosine triphosphate (ATP), 323 Advanced Integrated Wastewater Pond System (AIWPS), 167–169 facultative pond, 168 high-rate pond (HRP), 168 Hollister (California) AIWPS, 169 maturation pond, 168 settling pond, 168 St. Helena (California), 169 type 2 AIWPS, 168
Advanced wastewater treatment (AWT): facilities, 245 systems, 246 Advective transport, 320 Aerator systems, 258–260 Aerobic digestion, 266, 272–275 Aerobic digestion design (example), 273–275 Aerosols, 283 anthropogenic generation of, 289 Agricultural exposure factors, 109 Agricultural runoff, 176, 183 Air and Waste Management Association (AWMA), 9 Air, land, and water environments/resources, 2, 12 Air pollution, 4–5, 7, 9, 279–304 anthropogenic emission sources, 282 carbon monoxide, 284 chlorofluorocarbons (CFCs), 286 community (ambient), 279 gas and vapor control technology, 297–302 generally available control technologies (GACTs), 281–282 global warming, 286–288 greenhouse effect, 286–287 hazardous air pollutants (HAPs), 281 health/environmental effects from exposure to criteria pollutants (table), 284 history, 280–281 lead, 285 maximum achievable control technologies (MACT), 281–282 natural sources, 282 nitrogen oxides, 283 349
350
Index
Air pollution, (Continued) ozone, 285 particulate matter (PM), 282–283 pollution-prevention activities, 289 primary national ambient air quality standards, summary of, 282 primary pollutants, 282 primary standards, 281 regulations/standards, 281–282 secondary pollutants, 282 secondary standards, 281 sulfur oxides, 284–285 volatile organic compounds (VOCs), 281, 286 Air pollution episodes, 280–281 Air sparging, 325–326 AIWPS, See Advanced Integrated Wastewater Pond System (AIWPS) Alachlor, 195 ALARA, 162 Albedo, 287 Algae, 59, 61–62, 70, 76 Aliphatic hydrocarbons, 309–310 Alkalinity, 209 and carbonate system, 47–48 defined, 47 Alkanes, 309–310 Alkenes, 310 Alkynes, 310 Alpha particles, 159–160 Alternative disinfectants, 237–238 chlorine dioxide, 238 high-pH treatment, 238 ozone, 237 UV radiation, 238 Aluminum, 209–210 American Academy of Environmental Engineers, 9 American Society of Civil Engineers (ASCE), 2–3, 9 Report Card on America’s Infrastructure, 2 and sustainability, 154 American Water Works Association (AWWA), 9, 203 Amino acids, 59 Ammonification, 79 Amoebic dysentery, 233 Amphoteric compounds, 44 Anaerobic digestion, 266, 269–271 design (example), 271–272 Analytical problem solving, 30 Animalia kingdom, 61 Anionic polymers, 210 Annelida, 64 Anthropogenic emission sources, of air pollution, 282
Apparent color, 181 Archimedes’ principle, 19 Aroclor®, 313 Aromatic hydrocarbons, 310–311 Arsenic, 195 ASCE, See American Society of Civil Engineers (ASCE) Association of Environmental Engineering and Science Professors (AEESP), 9 Atomic number (Z), 32, 159 determining (example), 160–161 Atomic shell, 32 Atomic weight (AW), 34 Atoms, 32 Autotroph, 70, 323 Average linear velocity, 321 Avogadro’s number, 21 AWT, See Advanced wastewater treatment (AWT) AWWA, See American Water Works Association (AWWA) B Bacillus, 62 Bacteria, 62, 233, 323 Bar screens, 248 Bases, 43 Batch reactors, 134–135 Beneficial use, of water, 172–175 Benzene, 195 Benzene, Toluene, Ethyl benzene, and Xylene (BTEX), 311, 326 Berl and Intalox saddles, 299 Beryllium-7 (Be-7), 161 Beta particles, 160 Bias, 28–29 Bioaccumulation, 74 Bioassays, 102 Biochemical oxygen demand (BOD), 3, 130, 177, 185, 245 calculating for a “seeded” sample (example), 190 calculating for an “unseeded” sample (example), 190–191 calculating given BODu (example), 187–188 chemical oxygen demand (COD), 177, 191 derivation of equation, 186–187 laboratory procedures, 188–189 pathogens and indicator organisms, 191–192 Biochemical reactions, 120–130 Biological denitrification, 201 Biological dissimilatory denitrification, 78 Biological nitrification, 201
Index
Biological water treatment technologies, 201 Biomagnification, 74 Biomass, 253 Bioreactors, 344 Biosolids, See Sludge Bioventing (BV), 327–328 BOD, See Biochemical oxygen demand (BOD) Bohr, Niels, 32 Boyle’s law, 52 Breakpoint chlorination, 236–237 Breeder reactors, 162 British thermal unit (BTU), 143 Brønsted, J. N., 43 Brønsted-Lowry acid, 43–44 BTEX, 311, 326 Buffering capacity, 47 C Cadmium, 195 Calcium hypochlorite, 235 Calorie (cal), 143 Carbon, 75–76 Carbon-14 (C-14), 161 Carbon cycle, 75–76 Carbon dioxide, 47, 76 Carbon monoxide, 284 Carbonate system, and alkalinity, 47 Carcinogenic effects, 103–104 Carcinogens, 74, 102 risk characterization for, 110 Carman-Kozeny equation, 228 Carson, Rachel, 305 Catalytic incineration, 297–298 Cationic polymers, 210 CDI, See Chronic daily intake (CDI) Cell structure, 60 Cell yield, microbes, 69 Centi-, 16 Centrifuge, 267, 275 CERCLA, See Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA/Superfund) Cesspools, 244 Cgs (centimeter-gram-second) system, 15 Charles’ law, 52 Chemical concepts, 32–58 atoms, 32 elements, 32 Chemical equations, 37 Chemical oxygen demand (COD), 177, 191 Chemical precipitation, 215–216
Chemical reactions, 37–42, 120–130 balancing, 38–40 first-order reactions, 123–125 rate law, 121 rates of reaction, 121 reaction order, 121–122 second-order reactions, 125–126 temperature corrections, 128–130 zero-order reactions, 122–123 Chemical water treatment technologies, 201 Chemoheterotrophic organisms, 71 Chemotroph, 323 Chernobyl nuclear disaster, 6, 7 Chick’s law, 234 Chloramines, 236 Chlorination, 264 chemistry of, 235 Chlorine, 14, 227, 264 chlorine species, ranking of, 237 problems in using, 237 Chlorine contact basin, 264–266 example, 265–266 Chlorine dioxide, 238 Chlorine residual, types of, 237 Chlorofluorocarbons (CFCs), 286 Chronic daily intake (CDI), 107–110, 116 calculating (example), 109–110 risk calculations using (example), 110–111 Chronic toxicity, 102 Clapeyron, Paul Émile, 51 Clays, 208, 210 Clean Air Act (CAA), 281, 314 Clean Water Act, 195 Coagulant aids, 210 Coagulants, 209–210 Coagulation, 207–210 colloidal particles, 208–209 defined, 207 Coal vs. nuclear energy, 167 Coarse-bubble diffusers, 258–259 COD, See Chemical oxygen demand (COD) Coliform bacteria, 192 Colloidal destabilization, 208–209 Colloidal particles, 208–209 Color, in water, 181–182, 208 Color unit (c.u.), 182 Combination reactions, 37 Combined gas law, 52 Combustion reactions, 37 Commercial/industrial exposure factors, 109 Common ion effect, 48
351
352
Index
Community, 70 Completely mixed, batch reactor (CMBR), 135 design (example), 137 Completely mixed flow, 134 Completely mixed flow reactor (CMFR), 137–139 design (example), 139–140 Composting, 328–330 Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA/Superfund), 305–306 hazardous substances on Priority List of Hazardous Substances, 308–309 Remedial Actions, 308 Removal Actions, 308 Concentration, defined, 103 Concentrations, 20–26 Condensed phosphate, 184 Conditionally exempt small-quantity generators (CESQGs), 337 Conservative substance, 130 Consulting firms, 4 Contaminated Land Rehabilitation Network for Environmental Technologies in Europe (CLARINET), 305 Continuing education, 9 Continuous flow, 134 Continuously stirred tank reactors (CSTR), 138 Control rods, 162 Control volume, 120 Controlled landfills, 344 Convection, 320 Conversion factors, 17–18 Corrosivity, wastes, 337 Cosmogenic radionuclides, 161 Criteria pollutants, 281 Crustaceans, 63 Cryptosporidium parvum (parasite), 6, 66, 192, 195 Cultural eutrophication, 84 Curie (C), 161 Cyclops, 63 D Dalton’s law, 26, 53 Daphnia, 63 Darcy’s law, 320–321 Data collection and evaluation, 102 Deamination, 79 Death: leading causes of, 99 probability of, calculating, 98 Deci-, 16
Decomposers, 59, 66, 71 Decomposition reactions, 37 Dedicated disposal, 275 Deka-, 16 Denitrification, 78–79, 246 Denitrification stoichiometric coefficients (example), 79 Dense nonaqueous-phase liquid (DNAPL) pool, 316 Density, 18–20 of common substrates, 19 example, 19 problem solving with, 20 Deoxygenation, 86–88, 90 Deoxyribonucleic acid (DNA), 60, 66, 102 Detention time, 210–211, 250, See Residence time Detention time equation, 135–136 Dewatering, 275 Diaptomus, 63 Diarrhea, 233 Dichlorodiphenyldichloroethane (DDE), 313 Dichlorodiphenyltrichloroethane (DDT), 74, 208 Dichloroethylene (DCE), 310 Diffuse layer, 208 Diffused aeration systems, 258 Dimensions, 15 Direct-flame incinerators, 297 Discrete settling, 220–221 Disinfection, 233–238 alternative disinfectants, 237–238 breakpoint chlorination, 236–237 calcium hypochlorite, 235 Chick’s law, 234 chloramines, 236 chlorination, chemistry of, 235 chlorine, problems in using, 237 chlorine residual, types of, 237 mechanisms of disinfectants, 235 regulatory requirements, 234 sodium hypochlorite, 235 Surface Water Treatment Rule (SWTR), 234 of wastewater, 264–266 Disinfection and Disinfection By-Products Rule (DDBP), 234 Disinfection by-products (DDBPs), 207 Dispersed plug flow, 134 Dispersed plug flow reactors, 134 Disposal, of sludge, 275–276 Dissociation constant, 45 Dissolution, 48 Dissolved-oxygen depletion in streams, 85–88 Dissolved-oxygen (DO) concentration, and water quality, 185
Index
Domestic wastewater, 176, 183, 187, 198 characteristics of, 243–244 treatment, 243–278 Domestic water supply, 174 Dose, defined, 103 Dose-response assessment, 102–103 Dose-response curves, mathematical modeling of, 105–107 Double-layer electrical theory, 208 Double-replacement reactions, 37 Drift velocity, 295 Drinking water standards, 194–195, 202–203 primary, 194, 202–203 secondary, 195–196, 203 Dual-media filters, 227 E EBPR, See Enhanced biological phosphorus removal (EBPR) Ecological balance, 3 Ecological systems (ecosystems), 69–75 energy flow in, 70–71 Ecology, defined, 69 Edward I (king of England), 280 Effective size, filter media, 230 Einstein’s energy-mass-equivalence equation, 160 EIS, See Environmental Impact Statement (EIS) Electrical double layer, 208 Electrons, 32 Electrostatic precipitators (ESPs), 289, 293–295 design (example), 295–297 Elements, 32 periodic table of, 33 Emergency Planning and Community Right-to-Know Act (EPCRA), 306 Endocrine disruptors, 75 in sewage/water supply, 203 Endogenous decay, 68 Energy, 143 conversion factors, 18 Energy balance and efficiency (example), 73 Energy balances, 143–149 and coal-fired power plant mass (example), 146–149 Engineered remediation processes, 324–330 air sparging, 325–326 bioventing (BV), 327–328 composting, 328–330 ex situ remediation schemes, 328–330 groundwater remediation technologies, 325–328
353
landfarming, 330 pump-and-treat systems, 325 in situ remediation schemes, 324–328 soil vapor extraction (SVE), 326–327 vadose zone treatment schemes, 326–328 Engineering ethics, 8–9 Enhanced biological phosphorus removal (EBPR) systems, 246 wastewater treatment processes, 79 Entamoeba histolytica, 66 Enteroviruses, 192 Enthalpy, 144 Environment Congress for Asia and the Pacific (ECO ASIA), 305 Environmental calamities, 5–7 Bhopal, India, 6, 306 Chernobyl nuclear disaster, 6–7 Exxon Valdez, 6 Great Smokey Mountains smog, 6 Hurricane Katrina, 6 Love Canal, 5–6, 306 Milwaukee Cryptosporidium Outbreak, 6 Environmental engineering: academia, 5 continuing education, 9 defined, 12 definitions of, 1 educational requirements, 7–8 and engineering consulting firms, 4 government/municipality projects/work, 4 industries, 5 as lifelong journey, 7–9 matching your interests, 5 multidisciplinary teams, 4 outdoor work, 4 problem solving, 10 professional licensure, 7 regulatory agencies, 5 undergraduate curriculum, 8 Environmental engineers: characteristics of, 3–5 demand for, 2 duties of, 3–5 new challenges for, 3 Environmental impact analysis, 113–117 simplified environmental assessment problem (example), 114–115 Environmental impact statement (EIS), 12, 113–116 alternatives including the proposed action, 116 assessment, 113–114 defined, 113
354
Index
Environmental impact statement (EIS), (Continued) environmental consequences, 116 evaluation, 114 format, 115–116 inventory, 113 Environmental Information Network for Asia and the Pacific (ACO ASIA NET), 305 Environmental management, 12 Environmental Protection Agency (EPA), 12, 20, 98, 110, 116, 203, 279, 281, 306 Criteria for Solid Waste Disposal Facilities: A Guide for Owners/Operators (booklet), 343 hazard index (HI), 111–112 Environmental remediation, 305–332 Environmental systems: chemical and biochemical reactions, 120–130 design and modeling of, 120–153 energy balances, 143–149 flow regimes and reactors, 134–142 material balances, 120, 130–133 Environmentally intelligent design: Advanced Integrated Wastewater Pond System (AIWPS), 167–169 beverage container material selection, 166–167 coal vs. nuclear energy, 167 Haworth and Steelcase office chairs, 165 paper vs. plastic bags, 165–166 EPA, See Environmental Protection Agency (EPA) Epilimnion, 82–84 Equilibrium constants, 45 for acids/conjugate base in environment, 46 Equivalent, use of term, 35 Equivalent weight (EW), 34–35 calculating (example), 35 Escherichia coli (E. coli), 194, 234 ESPs, See Electrostatic precipitators (ESPs) Ethanes, 310 Ethenes, 310 Ethics, 8–9, 154 Eukaryotic cells, 60 Eutrophic lakes, 84 Eutrophication, 84 Explosives, 314 Exposure, 101 Exposure assessment, 107–109 External dose, 161 F F list (RCRA), 336 Fahrenheit (°F), 15 Fall turnover, 83, 85
Filter media, 230 specifications, 231 Filtration, 226–233 backwashing of filters, 230 defined, 227 filter dimensions and backwash quantity (example), 232–233 filter media, 230 gravity-type granular media filtration, 227–228 head loss through a clean filter, 228–229 number of filters, 230 rates of, 232 size of filters, 231 Fine-bubble diffusers, 258–259 First law of thermodynamics, 143–144 First-order reactions, 123–125 First-order removal reaction, 137–138 Fish, 63 Fission, 162, 164, 169 Fission fragments, 162–163, 169 “Flash” mixing, of coagulants, 210 Flocculants, 209 Flocculation, 227 defined, 207 Flocculation basin design (example), 214–215 Flocculation tanks, 213–215 Flocculent settling, 221 Flow rate, 26–27 calculating (example), 27 Flow regimes, 134 defined, 134 Flue-gas desulfurization (FGD) systems, advantages/disadvantages of, 301–302 Food chain, 59, 71–73, 93 energy use through (example), 73 Food-to-microorganism ratio (F:M), 255 Food web, 71–72 Force mains, 245 Franklin Study, 338 Free/discrete settling, 220–221 Free radicals, 34 Free settling, 220–221 Fuel additives, 314 Fumes, 283 Fundamentals of Engineering (FE) examination, 7 Fungi, 63 Fungi kingdom, 61 G Gamma (γ) radiation (gamma rays), 160 Gas and vapor control technology, 297–302
Index
absorption, 298–302 incineration, 297–298 Gas-phase chemistry, 51–56 Gas-phase concentration units, 23 conversion of, 25–26 Gastrointestinal problems, 233 Gay-Lussac law, 52 Genera, 61 Generally available control technologies (GACTs), 281–282 Giardia lamblia, 66, 192, 195, 234 Gibbs, Lois, 6 Giga-, 16 Global warming, 286–288 Godwin, Mills, 197 Government projects/work, 4 Grams per equivalent (g/eq), 35 Granular media filters, 230 Gravity belt thickener (GBT) design (example), 267–269 Gravity sewer, 245 Gravity-type granular media filtration, 227–228 Gray (Gy), 162 Green engineering, 154–156, 158 defined, 155 principles of, 155–156 Green Lights Program, 289 Greenhouse effect, 286–287 Greenhouse gases, 76, 286, 288 Green plants, 70 Grit removal, 248–249 GRIT SNAIL™ Quiescent Dewatering Escalator, 249 Groundwater migration and retardation of contaminants (example), 321–322 Groundwater, quality of, 205 Groundwater remediation technologies, 325–326 Groundwater treatment systems, 205–207 lime-soda ash treatment, 206 reverse osmosis treatment, 206–207 H Half-life, 125, 163 Hard water, 202, 215 Hauled Container System (HCS), 340–341 Haworth and Steelcase office chairs, 165 Hazard identification, 102 Hazard index (HI), 117 calculating (example), 111–112 Hazardous air pollutants (HAPs), 281 Hazardous waste site Ranking System (HRS), 306 Hazardous waste site remediation, 305–332
355
contaminant characteristics, 315–320 contaminants, 309–322 Darcy’s law, 320–321 engineered remediation processes, 324–330 estimation of sorption to soil (example), 317 groundwater migration and retardation of contaminants (example), 321–322 hazardous waste history, highlights in, 305–307 Henry’s law constant, 318 hydrocarbons (HC), 309–314 inorganics, 314–315 mass and volume relationships in soil (example), 319–320 microbial processes, 323–324 partition coefficients, 316 physical properties of soil, 318–319 soil-distribution coefficient (KSD), 316–317 solubility and nonaqueous phase liquids, 315–316 Hazards, 101 HC, See Hydrocarbons (HC) Heat, 144 Helminth parasites, 233 Helminths, 64, 192 Henry’s law, 47, 54–57, 76, 87 using to determine equilibrium and concentrations (example), 55–56 using to estimate equilibrium concentration (example), 318 Henry’s law constant, 318 Hepatitis, 233 Hepatitis A virus, 192 Herpes Simplex virus, 66 Heterogeneous culture, net growth rate of, 69 High Melting Explosive (HMX), 314 High-pH treatment, 238 High-rate pond (HRP), 168 Hoist trucks, 341 Hooker Chemical and Plastics Corporation, 6, 306 Hookworm, 233 Horticultural runoff, 174 Hydrocarbons (HC), 75, 309–314 aliphatic hydrocarbons, 309–310 aromatic hydrocarbons, 310–311 explosives, 314 fuel additives, 314 pesticides, 313 polychlorinated biphenyls (PCBs), 313 polycyclic aromatic hydrocarbons (PAHs), 312 Hydrogen-3 (H-3), 161 Hydrologic cycle, 175 Hydrology, 14
356
Index
Hydrolysis, 269–270 Hypolimnion, 83 I Ideal completely mixed flow, 134 Ideal gas law, 24, 51 Ideal gases, 51 Ideal settling basin, 221–224 Ignitability, wastes, 337 Imperial Units, 15 Incineration, 297–298 advantages of, 298 catalytic, 297–298 disadvantages of, 298 thermal, 297–298 Incremental lifetime cancer risk, 107 Indicator organisms, 191–192 Industrial water use, 174 Inorganic-P, 184 Insolation, 287 Integrated Risk Information System (IRIS), 106 Internal dose, 161 Internal energy (U), 143 International Organization for Standardization (ISO), 12 International System of Units (SI), 15, 162 Ions, associated with environmental chemistry, 35 IRIS, 110 Iron salts, 209 Irrigation, 174 Isotopes, 32, 159 J James River, kepone contamination of, 197–198 Joule (J), 16, 143 K K list (RCRA), 336 Kaolinite, 208 Kepone contamination of the James River, 197–198 Kilograms (kg), 15 Kinetic energy (KE), 143 Kinetics, 120 L Lag phase, 67 Lake stratification, 84–85 Land application, 266 reuse of biosolids through, 275 Landfarming, 330
Landfill containment and monitoring systems, 343–345 construction techniques, 344–345 controlled landfills, 344 dry landfill, 344 federal MSWLF standards, 343–344 leachate recirculation system (LRS), 345 site selection, 344 wet landfill, 344 Landfill, defined, 343 Law of the conservation of energy, 130 Law of the conservation of mass, 38, 130 Leachate recirculation system (LRS), 345 Lead, 285, 314 Leaky Underground Storage Tank (LUST), 4 trust fund, 306 Liebig’s law of the minimum, 85 Lifelong learning, 7–9 Light nonaqueous-phase liquid (LNAPL), 315–316 Lime-soda ash treatment, 206 softening equations, 216–218 Lime stabilization, 266 Limnology: defined, 81 stratification, 82–84 Linearized multistage model, 106 “Liquid” treatment train, 201 Livestock water, 174 Load-count analysis, 337–338 Love Canal, 5–6, 306 Love, William T., 5–6, 306 Lowest observed-adverse-effect level (LOAEL), 104 LUST, 306 M Macrophytes, 64–65, 82 Manure, and composts, 329 Mass, conversion factors, 17 Mass number (A), 32, 159 determining (example), 160–161 Mass percent, 21 Mass removal rates (example), calculating, 11–12 Master of Engineering (ME)/Master of Science (MS) in Environmental Engineering, 7 Material balances (mass balances), 120, 130–133 on air flow and particulates (example), 132–133 defined, 130 on flows and solids (example), 131–132 Material selection, 156–158 Materials-flow methodology, 338 Maturation pond, 168
Index
Maximum achievable control technologies (MACT), 281–282 Maximum contaminant level goals (MCLGs), 194 Maximum contaminant levels (MCLs), 194, 198 Mean cell residence time (MCRT), 254 Mechanical dispersion, 320 Median lethal dose, 103 Mega-, 16 Membrane-filter technique, 192–193 Membrane filtration, 205 Membrane softening, 206–207 Mercury, 314–315 Metalimnion, 83 Metathesis reactions, 37 Meters (m), 15 Methanes, 310 Methanogenesis, 270 Methanogens, 270 Methyl isocyanate (MIC), release of, in Bhopal, India, 6, 306 Methyl tertiary butyl ether (MTBE), 203, 314 in sewage/water supply, 203 Metric system, 15 Micro-, 16 Microbial metabolism, environmental factors affecting, 323–324 Microbial processes, 323–324 Micrococcus, 62 Microconstituents, 203 Microcrustaceans, 63 Micrograms per cubic meter (μg/m3), 20 Micrograms per liter (μg/L), 20 Microorganisms, 66–68, 76, 130, 252–253 activated sludge, 253–258 microbial growth equations, 67–68 specific growth rate, 68 Milli-, 16 Milligrams per liter (mg/L), 20–21 relationship between ppm and (example), 21 Milligrams per milliequivalent (mg/meq), 35 Millirem (mrem), 162 Milorganite, 275 Milwaukee Cryptosporidium Outbreak, 6 Mining water, 175 Mist eliminators, 299 Mists, 283 Mixed liquor suspended solids (MLSS), 253 MKS (meter-kilogram-second) system, 15 Moderators, 162 Molarity, 21 Molarity (M), 35
357
Mole (mol), 34 Molecular weight (MW), 34 Monera kingdom, 61 Monod, 68 Monomedia filters, 227 Montmorillonite, 208 Most probable number (MPN), 193 calculating (example), 193–194 MTBE, See Methyl tertiary butyl ether (MTBE) Multidisciplinary teams, 4 Multimedia filters, 227 Multiple “liquid” treatment trains, 201 Multiple-tube fermentation technique, 193 Municipal solid waste landfills (MSWLFs), 337 Municipal solid waste (MSW) management, 333–334 materials category, 334 products category, 334 Municipality projects/work, 4 Mushrooms, 63 Mutagenesis, 102 N N-nitrosodimethylamine (NDMA), 203 Nano-, 16 NASA Mars Climate Observer, 15 National Ambient Air Quality Standards (NAAQS), 281 National Biennial RCRA Hazardous Waste Report, 309 National Contingency Plan (NCP), 306 National Council of Examiners for Engineering and Surveying (NCEES), 7 National Emission Standards for Hazardous Air Pollutants (NESHAPS), 281 National Environmental Policy Act (NEPA), 12, 113 National Industrial Competitiveness through Energy, Environment and Economics (NICE3) program, 289 National Pollution Discharge Elimination System (NPDES) permit, 195–196, 198, 245 National Priorities List (NPL), 306 National Society of Professional Engineers (NSPE), fundamental canons promulgated by, 8–9 National Solid Waste Management Association (NSWMA), 334 Natural aging, 84 Naturally occurring radioactive material ( NORM), 161 Nematoda, 64 Nephelometric turbidity units (NTUs), 178 Neptunium-239 (Np-239), 162–163
358
Index
Nerst potential, 208 Neutrons, 32, 159 Newton (N), 16 Nitrification, 61, 77–78 Nitrification stoichiometric coefficients (example), 78 Nitrobacter, 77 Nitrogen (N): forms of, 183 in water, 183–184 Nitrogen cycle, 76–77 deamination, 79 denitrification, 78–79 nitrification, 77–78 Nitrogen oxides, 283 Nitrogenous oxygen demand, 256 Nitrosomonas, 77 No-observable-adverse-effect level (NOAEL), 104–105 Nominal detention time, 254–255 Nonaqueous-phase liquid (NAPL), 315–316 Noncarcinogenic effects, 104–105 Noncarcinogens, risk characterization for, 111 Nonionic polymers, 210 Nonpoint sources of pollution, 243 Nonreactive substance, 130 Normality (N), 35 calculation of concentration in (example), 36–37 Norris Lake (Tennessee), 82 NPDES permit, See National Pollution Discharge Elimination System (NPDES) permit Nuclear fission, 162–163 Nuclear physics, 158–165 alpha particles, 159–160 beta particles, 160 breeder reactors, 162 Einstein’s energy-mass-equivalence equation, 160 millirem (mrem), 162 nuclear fission, 162–163 nuclear reactors, 162–163 nuclear waste immobilization and storage, 164–165 primordial radionuclides, 161 radiation absorbed dose (rad), 162 radioactive elements, identifying (example), 159 radioactivity, sources of, 161 radioactivity, units of, 161–162 roentgen equivalent man (rem), 162 Nuclear reactors, 162–163 Nuclear waste immobilization and storage, 164–165 Nucleus, 32
Nutrient cycles, 75–81 carbon cycle, 75–76 nitrogen cycle, 76–77 phosphorus cycle, 79–80 sulfur cycle, 80–81 O Occupational Safety and Health Administration (OSHA), 279 Octanol-water partition coefficient (KOW), 316 Odor, water, 182 Oligotrophic lakes, 84 Organic-P, 184 Organic phosphate, 184–185 Organisms: algae, 61–62 bacteria, 62 classification of, 60–61, 70 crustaceans, 63 fish, 63 fungi, 63 helminths, 64 macrophytes, 64–65 major groups of, 61 microcrustaceans, 63 microorganisms, 66–69 protozoans, 66 rotifers, 66 viruses, 66 Orthophosphates, 184 Oswald, William, 167 Overflow rate, 222, 250 Ozone, 237, 285 P P list (RCRA), 336 Packed-bed towers, 299–300 advantages/disadvantages of, 300 Packer trucks, 341 Pall rings, 299 Paper vs. plastic bags, 165–166 Parallel treatment trains, 201 Paratyphoid, 233 Particulate matter (PM), 282–283 aerodynamic size of a particle, 289 electrostatic precipitators (ESPs), 293–295 settling chambers, 290–293 stationary sources, 289–292 Partition coefficients, 317 Parts per billion (ppb), 20 Parts per million on a volume basis (ppmv), 23
Index
Parts per million (ppm), 20 Pascal (Pa), 16 Pathogens, 191–192 and pH, 233 process for significant reduction of pathogens (PSRP), 272 Pay As You Throw (PAYT) programs, 339–340 PCBs, See Polychlorinated biphenyls (PCBs) Periodic table of the elements, 33 Personal care products, in sewage/water supply, 203 Pesticides, 313 pH, 42–45 calculating (example), 44–45 for a strong acid (example), 46 high-pH treatment, 238 of water, 194 Pharmaceuticals, in sewage/water supply, 203 Phosphorus cycle, 79–80 Phosphorus (P): phosphorus accumulating organisms (PAOs), 79 in water, 184 Photosynthesis, 59 Phototroph, 323 Phylogeny, 61 Physical forces, 201 Phytoplankton, 59, 71 Pico-, 16 picoCurie (pCi), 161 Pink water, 314 Pipe flow, See Plug flow; Plug flow reactor (PR) PISTA® Grit Removal System, 249 Plankton, 81–82 Plantae kingdom, 61 Plants, 59, 70, 76 Platyhelminthes, 64 Plug flow, 264 defined, 134 Plug flow reactor (PFR), 140–142 design (example), 142 second-order removal reaction, 140–142 PM, See Particulate matter (PM) pOH, calculating (example), 44–45 Poliomyelitis, 233 Pollution, 4–5, 7, 12, See also Air pollution nonpoint sources of, 243 water, 175–177 Pollution Prevention Act (1990), 289 Polyvinyl chloride (PVC), 310 Polychlorinated biphenyls (PCBs), 74, 208, 313 Polycyclic aromatic hydrocarbons (PAHs), 312 Polymers, as coagulant aids, 210
359
Population, 70 Pore velocity, 321 Potassium-40, 161 Potency factor (PF), 106 Potential energy (PE), 143 Powdered activated carbon (PAC), 210 Power, 143 conversion factors, 18 Precipitation, 37, 48 Precision, 28–29 Preliminary wastewater treatment systems, 247–249 grit removal, 248–249 screening, 248 Pressure, conversion factors, 17 Primary clarifiers, 250 design (example), 251–252 Primary consumers, 71 Primary drinking water standards, 194 Primary pollutants, 282 Primary producers, 70–71 Primary wastewater treatment, 249–252 Primordial radionuclides, 161 Probability of death, calculating, 98 Problem solving, 10 analytical, 30 Process for significant reduction of pathogens (PSRP), 272 Producers, 70 Professional engineer (PE), 7 societies and organizations, 9 Professional Engineering (PE) exam, 7 Prokaryotic, 60 Prokaryotic cells, 60 Proteins, 59 Protista kingdom, 61 Protons, 32, 159 Protozoan parasites, 233 Protozoans, 66 Pu-239, 163, 164 Public water supply, 173–174 Publicly owned treatment works (POTWs), 176 Pump-and-treat systems, 325 Pumping stations, 245 R Radiation absorbed dose (rad), 162 Radicals, 34 associated with environmental chemistry, 35 Radioactive atoms, 159 Radioactive decay, calculating (example), 164 Radioactive elements, identifying (example), 159
360 Index
Radioactivity: sources of, 161 units of, 161–162 Radionuclides, 159 half-lives of, 163 Radium-226 and radium-228, 195 Raoult’s law, 54–56 Rapid-mix basin design (example), 212–213 Rapid mixing, of coagulants, 210 Raschig rings, 299 RCRA, See Resource Conservation and Recovery Act (RCRA) Reaction order, 121–122 and k determination (example), 126–128 Reaction stoichiometry, 38–42 Reactive unit, 34 Reactivity, wastes, 337 Reactors, 134–142 batch reactors, 134–137 completely mixed batch-reactor design (example), 137 completely mixed flow reactor (CMFR), 137–139 completely mixed reactor design (example), 139–140 defined, 134 first-order removal reaction, 137–139 plug flow reactor (PFR), 140–142 second-order removal reaction, 137 zero-order removal reaction, 135–137 Reaeration, 87 coefficients, 87 Reasonable maximum exposure (RME), 107–109 Recreational exposure factors, 109 Recycle flow, 253 Reference dose (RfD), 104–105, 111 Regulatory agencies, 5 Rem, 162 Reproducibility, 28 Residence time, 27–28 calculating (example), 28 Residential exposure, 109 Residential exposure equations, for various pathways, 108 Resource Conservation and Recovery Act (RCRA), 305, 335–337 hazardous waste definition, 336–337 nonhazardous waste definition, 337 Retention time, See Residence time Return activated sludge (RAS) flow, 253 Reverse osmosis treatment, 206–207 Ribonucleic acid (RNA), 66
Risk: calculating (example), 99–100 defined, 98, 116 Risk assessment, 98–119 data collection and evaluation, 102 defined, 101, 116 dose-response assessment, 102–103 environmental impact statement (EIS), 12, 113–116 exposure, 101 exposure assessment, 107–109 hazard identification, 102 hazards, 101 and pollution prevention, 156 risk characteristics, 110–111 risk management, 112–113 toxicology, 102 Risk management, 101 Roentgen equivalent man (rem), 162 Roentgen (R), 161–162 Roll-off containers, 341 Rotating biological contactors (RBCs), 245 Rotifers, 66 Royal Demolition Explosive (RDX), 314 S Safe Drinking Water Act, 194 Safety, defined, 98 Saltwater, 202 Saprophytes, 63 Screening, 248 Second law of thermodynamics, 144 Second-order reactions, 125–126 Second-order removal reaction, 137, 140–142 Secondary clarifier design protocol, 262–263 example, 263 Secondary consumers, 71 Secondary drinking water standards, 195–196 Secondary effluent, 253 Secondary pollutants, 282 Secondary wastewater treatment, 245–246, 252–258 trickling filters, 260–261 Seconds (s), 15 Sedimentation, 220–226 flocculent settling, 221 free/discrete settling, 220–221 overflow rate, 222 settling basin, 221–222 settling-basin design criteria, 224–225 settling velocity, 222–224 weir loading rate, 224
Index
Seepage velocity, 321 Seneca, 280 Septic tanks, 244 Settling basin, 221–222 Settling-basin design criteria, 224–225 rectangular settling basin (example), 226 Settling chambers, 290–292 design (example), 292–293 Settling velocity, 222–224, 290 Sewerage system, 245 SI, See International System of Units (SI) SI system of units, 15–16 prefixes, 16 Sievert (Sv), 162 Significant figures, 29–30 Silent Spring (Carson), 305 Single-replacement reactions, 37 Slaking, 37 Slope factor (SF), 106–107 risk calculations using (example), 110–111 Sludge: aerobic digestion, 272–275 aerobic digestion design (example), 273–275 anaerobic digestion, 266, 269–271 anaerobic digestion design (example), 271–272 dewatering, 275 disposal, 275–276 gravity belt thickener (GBT) design (example), 267–269 sludge weight and volume relationships, 266–267 stabilization, 269–275 thickening of, 267–269 treatment and disposal, 266–275 Sludge age, 254 Sludge blanket, 253 Sludge-only landfills/monofills, 275 Sludge trains, 201 SLURRYCUP™ grit washing and classification unit, 249 Sodium hypochlorite, 235 Soft water, 202 Soil: estimation of sorption to (example), 317 physical properties of, 318–319 Soil-distribution coefficient (KSD), 316–317 Soil vapor extraction (SVE), 326–327 Solid-phase equilibrium reactions, 48 Solid Waste Association of North America, 9 Solid waste management, 333–347 collection equipment, 341 collection examples, 342–343
collection frequency, decline in, 341–342 collection systems, 340–341 customer-service issues, 341–342 Hauled Container System (HCS), 340–341 hazardous waste, 336 history of, 333 hoist trucks, 341 landfill containment and monitoring systems, 343–345 load-count analysis, 337–338 materials-flow methodology, 338 packer trucks, 341 problems/concerns, 341–342 regulations, 335–337 roll-off containers, 341 Stationary Container System (SCS), 341 underground storage tanks (USTs), 335 waste generation, 337–340 Solids: analyses (example), 180–181 solubility of (example), 49–50 in water, 179–180 Solids loading rate (SLR), 262–263 Solution chemistry (aqueous phase), 42 acid-base chemistry, 42–45 strong acids and bases vs. weak acids and bases, 45–46 Solution chemistry (gas phase), 51–56 general gas laws, 51–53 Solution concentration, determining (example), 21–23 Species, 61 Spray-tower absorption systems, 301 Spring turnover, 82–83, 85 Stabilization: lime, 266 sludge, 269–275 Standard Methods for the Examination of Water and Wastewater (APHA), 177 Standard Model (of the atom), 32 Starches, 59 Stationary Container System (SCS), 341 Sterilization, 233 Stern layer, 208 Stoichiometry, 38–42 reaction, 40–42 Stokes’ velocity, See also Settling velocity example, 290–291 Stratification, 82–84 Stream standards, 196
361
362 Index
Streams: calculating the critical deficit point in (example), 90–92 dissolved-oxygen depletion in, 85–87 Streeter-Phelps DO depletion model, 88–90, 93 Streptococcus, 62 Strong acids and bases vs. weak acids and bases, 45–46 Substrate adsorption and utilization, 253 Substrate utilization, 255 Sugars, 59 Sulfur cycle, 80–81 acid mine drainage, 81 sulfur dioxide emissions, 81 Sulfur dioxide emissions, 81 Sulfur oxides, 284–285 Summer stratification, 85 Superficial velocity, 320 Superfund Reauthorization and Amendments (SARA), 306 Surface water quality standards, 196 Surface water treatment processes, 207–238 coagulation, 207–210 filtration, 225–233 flocculation tanks, 213–215 mixing, 210–213 sedimentation, 220–226 water softening, 215–220 Surface Water Treatment Rule (SWTR), 234 Surface water treatment systems, 203–205 conventional, 203–204 membrane treatment, 205 Suspended solids (SS), 3, 179 removal, 221 Sustainability, 156 Sustainable development, 154–171 defined, 155 design, 155 material selection, 156–158 Sweep coagulation, 210 Synthesis reactions, 37 Synthetic organic compounds (SOCs), 5 Systems of units, 15
Tera-, 16 Teratogens, 102 Tetrachloroethylene (PCE/Perc), 310 Tetraethyl lead (TEL), 314 Theoretical oxygen demand, calculating (example), 185–186 Thermal discharges to river (example), 145–146 Thermal incinerators (oxidators), 297–298 Thermodynamics, 143 Thermoelectric power, 175 water for, 175 Thorium-232 (Th-232), 163 Thorium (Th), 161 Threshold number (FTN), 182 Threshold odor number (TON), 182 TNT, 314 Total dissolved solids (TDS), 179, 182 Total energy (E), 143 Total Kjeldahl nitrogen (TKN), 183 Total maximum daily loads (TMDLs), 176 Total nitrogen (TN), 245 Total phosphorus (TP), 245 Total reaction order, 122 Total solids (TS), 179 Total suspended solids (TSS), 179, 245 Total trihalomethanes (TTHMs), 195, 234 Toxic Release Inventory (TRI), 306 Toxic Substance Control Act (1976), 289 Toxicants, 102 Toxicity Characteristic Leaching Procedure (TCLP), 337 Toxicity, wastes, 337 Toxicology, 102 Trichloroethylene (TCE), 310 Trickling filters, 260–261 Trihalomethanes (THMs), 233, 237, 264 Trimedia filters, 227 Trophic level, 59 True color, 181 Turbidity, 178, 208 nephelometric turbidity units (NTUs), 178 Typhoid fever, 233
T Taste, water, 182 Teacup system (EUTEK Systems), 249 Temperature: conversion factors, 18 water, 183 Temperature corrections, 128–130 example, 129–130
U U-235, 164 U-238/U-239, 162–163, 164 U list (RCRA), 336 Ultimate BOD, 187–188 Uncertainty, in calculating risk, 98 Underground storage tanks (USTs), 335 Uniformity coefficient, 230
Index
Unit conversion, 24 Unit operation, 201, 220, 225–233, 245 Unit processes, 201, 215, 245 Universal Gas Constant (R), 23 Uranium (U), 161 Urban runoff, 176, 183 U.S. Customary units, 15 U.S. EPA RCRAInfo National Database, 309 U.S. Geological Survey, water-use report, 172–174 UV radiation, 238 V Vadose zone, 324 treatment schemes, 326–328 Valence, 34 Velocity gradient, 210–212 Velocity of flow in a pipe, computing (example), 10–11 Viruses, 60, 66, 233 VOCs, See Volatile organic compounds (VOCs) Volatile organic compounds (VOCs), 281, 286, 326 Volume, conversion factors, 17 Volume percent, 21 Vortex grit removal systems, 248–249 Vostok (Russia) ice core, 288 W Waste activated sludge (WAS), 254, 267 Waste generation, 337–340 example, 338–339 Pay As You Throw (PAYT) programs, 339–340 Wastewater: defined, 243 types of, 243 Wastewater collection system., 245 Wastewater effluent standards, 195–196 Wastewater treatment: advanced wastewater treatment (AWT) systems, 246 categorization, 245–246 chlorination, 264 chlorine contact basis, 264–266 disinfectants used in, 264 preliminary treatment systems, 247–249 primary treatment, 249–252 secondary wastewater treatment, 245–246, 252–258 systems, 246–247 Wastewater treatment plants (WWTPs), 79, 176, 245 design capacity of, 247 Wastewaters, 47
363
Water, 172–200 beneficial uses of, 172–175 biochemical oxygen demand (BOD), 185–191 chemical oxygen demand (COD), 191 classification based on best beneficial use for the state of Florida, 197 color, 181–182, 208 condensed phosphate, 184 conventional water quality assessment parameters, 177–194 dissolved-oxygen (DO) concentration, 185 domestic water supply, 174 drinking water standards, 194–195, 202–203 estimated world water supply and budget (table), 173 hydrologic cycle, 175 importance of, 172 industrial water use, 174 integrated watershed management, 176–177 irrigation, 174 kepone contamination of the James River, 197–198 livestock, 174 membrane-filter technique, 192–193 mining water, 175 multiple-tube fermentation technique, 193 nitrogen (N), 183–184 odor, 182 organic phosphate, 184–185 orthophosphates, 184 pathogens and indicator organisms, 191–192 pH of, 194 phosphorus (P), 184–185 physical, chemical, biological, and radiological characteristics of (table), 178 pollution, 175–177 public water supply, 173–174 quality, 202 quality parameters, 177–194 quality standards, 194–195 solids in, 179–181 surface water quality standards, 196 taste, 182 temperature, 183 thermoelectric power, 175 Total Coliform Rule in the Safe Drinking Water Act, 194 turbidity, 178 wastewater effluent standards, 195–196 wastewater treatment plant (WWTP), 176 water treatment plant (WTP), 176
364 Index
Water Environment Federation (WEF), 9 Water Infrastructure Network (WIN), 3 Water pollution, 175–177 types of, 175–176 Water softening, 215–220 chemical precipitation, 215–218 excess-lime and soda-ash softening problem (example), 218–219 ion exchange, 220 lime-soda ash softening equations, 216–218 semipermeable membrane, passage through, 220 Water treatment, 201–242 disinfection, 233–238 groundwater treatment systems, 205–207 microconstituents, 203 selecting appropriate technology, 202–203 surface water treatment systems, 203–205 water source and quality, 202 Water treatment plant residuals, treatment of, 238–239 Water treatment plant (WTP), 176
Weathering, 327 Weir loading rate, 224, 250 Wet scrubber, 298 Winkler method, 185 Winter stratification, 83, 85 Work, 143 Worms, 192 WWTPs, See Wastewater treatment plants (WWTPs) Y Yeasts, 63 Yield coefficient: growth, 253, 258 microbes, 69 Z Zero-order reactions, 122–123 Zero-order removal reaction, 135–137 Zeta potential, 208–209