Wastewater Treatment for Pollution Control and Reuse [3 ed.] 0070620997, 9780070620995

Table of contents :
Cover
Preface to the Third Edition
Preface to the First Edition
Contents
Chapter 1: Ecosystem Approach to Pollution Control
Chapter 2: Planning for WastewaterCollection and Treatment
Chapter 3: Principles of AerobicBio-Reactor Design
Chapter 4: Principles of Aerobic Biological Treatment
Chapter 5: Principles of Aeration
Chapter 6: Some Aerobic Biological Treatment Methods
Chapter 7: The "UASB" and Other Anaerobic Processes
Chapter 8: Mechanically Aerated Lagoons
Chapter 9: Natural Systems – 1 Classical Algal Pondsfor Treatment and Resource Recovery
Chapter 10: Natural Systems – 2 Hyacinth and Duckweed Ponds, Fish Ponds, Natural and Constructed Wetlands, and Vermiculture
Chapter 11: Natural Systems – 3 Wastewater Irrigation
Chapter 12: Some Physico-Chemical Methods and Membrane Technologies
Chapter 13: Solids Settling and Sludge Management
Chapter 14: Maximize output Water Conservation andReuse in Industry and Agriculture
Chapter 15: Wastewater Reuse to Augment Public Water Supplies
Chapter 16: Guidelines for Planningand Designing Treatment Plants and CETPs
Chapter 17: The Many Facets of Sustainable Waste Management
Index

Citation preview

Wastewater Treatment for Pollution Control and Reuse Third Edition

Wastewater Treatment for Pollution Control and Reuse Third Edition

Soli J Arceivala Formerly Chief, Environmental Health, WHO SE Asia Region and Chairman & Managing Director AIC Watson Consultants Mumbai

Shyam R Asolekar Professor Center for Environmental Science and Engineering Indian Institute of Technology Powai, Mumbai

Tata McGraw-Hill Publishing Company Limited NEW DELHI McGraw-Hill Offices New Delhi New York St Louis San Francisco Auckland Bogotá Caracas Kuala Lumpur Lisbon London Madrid Mexico City Milan Montreal San Juan Santiago Singapore Sydney Tokyo Toronto

Published by the Tata McGraw-Hill Publishing Company Limited, 7 West Patel Nagar, New Delhi 110 008. Fourth reprint 2009 RXCYCRDFDDXAZ Copyright © 2007, by Tata McGraw-Hill Publishing Company Limited. No part of this publication may be reproduced or distributed in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise or stored in a database or retrieval system without the prior written permission of the publishers. The program listings (if any) may be entered, stored and executed in a computer system, but they may not be reproduced for publication. This edition can be exported from India only by the publishers, Tata McGraw-Hill Publishing Company Limited. ISBN-13: 978-0-07-062099-5 ISBN-10: 0-07-062099-7 Head - Professional and Healthcare: Roystan La’ Porte Publisher Manager: R. Chandra Sekhar Asst. Sponsoring Editor: Vibhor Kataria Sr. Copy Editor: Sandhya Iyer Sr. Product Specialist: A. Rehman Khan Asst. General Manager - Production: B L Dogra Junior Executive: Anjali Razdan

Information contained in this work has been obtained by Tata McGraw-Hill, from sources believed to be reliable. However, neither Tata McGraw-Hill nor its authors guarantee the accuracy or completeness of any information published herein, and neither Tata McGraw-Hill nor its authors shall be responsible for any errors, omissions, or damages arising out of use of this information. This work is published with the understanding that Tata McGraw-Hill and its authors are supplying information but are not attempting to render professional services. If such services are required, the assistance of an appropriate professional should be sought. Typeset at Script Makers, 19, A1-B, DDA Market, Paschim Vihar, New Delhi 110 063, and printed at Sai Printo Pack Pvt. Ltd, A-102/4 Phase II, Okhla Industrial Area, New Delhi - 110 020 Cover: Sai Printo Pack

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Preface to the Third Edition

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Wastewater Treatment for Pollution Control has seen success through three editions in one decade. Preparing a revised edition is an opportunity to start afresh and perhaps more intelligently! With that objective, in the current edition, Wastewater Treatment for Pollution Control and Reuse, we extend the discussion from treatment and disposal of wastewater for pollution control to its reuse. This makes the book more complete and meaningful to the present day problems. Reuse of wastewater can augment public water supplies, create new employment opportunities and help improve sanitation all round. With such profound benefits new chapters discussing methods that are critical to reuse of wastewater have been added. Physico-chemical treatment methods, including membrane technologies, are much needed along with biological methods for wastewater treatment and reuse—these are discussed in the new Chapter 12. Also, we take the opportunity to discuss decentralized systems for collection and treatment of wastewater and the natural treatment methods that go with themalgal, hyacinth and duckweed ponds, constructed wetlands. These methods are gaining worldwide acceptance, and we do hope this discussion will increase their application in India and other developing countries. To establish the discussion of wastewater reuse, Chapters 14 and 15 deal with the reuse of wastewater in agriculture, industry and for public water supplies. The information in these chapters is augmented by new sections on methods of so-called "wastewater harvesting." Wastewater harvesting requires a special mention because of the tremendous income opportunities—it can transform polluted water bodies into lakes that offer recreational facilities. An important issue that has been addressed is of sanitation in India and other developing countries of Asia and Middle East. We are aware that in these countries sanitation is largely confined to the metropolitan cities and towns. Even in these areas there are huge pockets of slums without any sanitation, which generate small, polluted water bodies. A special section on retrofitting sanitation in slums and shantytowns has been added, and methods outlined on how to clean up and fix the menace of these polluted water courses. Besides the important new topics, the existing content has been significantly updated and made more meaningful with the coverage of both mechanized and natural treatment systems, sludge management, overall design of treatment plants and a fuller discussion of factors that affect sustainable waste management. We hope that the design data and solved examples will prove appropriate for India and similar warm weather countries of Asia. We will consider our efforts well rewarded if the book, in any small measure, increases wastewater treatment for pollution control and reuse. Finally, we would like to thank everyone ever willing to help us in putting this book together and to our own families for their cheerful understanding at all times.

SOLI J ARCEIVALA SHYAM R ASOLEKAR

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Preface to the First Edition

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Simpler methods of waste treatment have come to be developed, particularly over the last two decades, capable of reducing the cost and complexities of treatment without sacrificing the requirements of pollution control. These methods have special relevance for the developing countries which would like to keep mechanisation and imports to a minimum and use methods which are reliable and appropriate to the local conditions. Most of the existing textbooks on wastewater treatment deal with a large variety of processes and, therefore, tend to deal with the simpler methods like oxidation ditches and aerated lagoons in less detail This book deals with the simpler methods only and adopts a unified approach based on the principles of aerobic biological treatment and flow kinetics. Since the design requirements for these methods are greatly affected by climatic conditions, the book includes not only data from colder climates but also from the warmer regions from which data has hitherto not much appeared in textbooks. The information contained should, therefore, have applicability to countries with widely varying climatic conditions to facilitate pollution control activities. This book is in part the result of field work as an UNDP/WHO expert, and in part the outgrowth of notes prepared for courses on “Wastewater Treatment” given by the author at the Environmental Engineering departments of universities in India and Turkey, and at the International Courses in Hydraulic and Environmental Engineering at Delft, Netherlands. The material presented in the book should be of use to both students and practising engineers alike. The designs of lagoons, ponds and ditches take into account modern concepts of mixing kinetics resulting in more realistic and economical units, thus reducing pollution control costs. Chapters on reactor design and on sludge disposal and irrigational use of wastewaters cover several practical aspects of design. Moreover, design methods have been illustrated throughout the book by a large number of solved examples.

SOLI J ARCEIVALA SHYAM R ASOLEKAR

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Contents

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Preface to the Third Edition Preface to the First Edition

v vii

1. Ecosystem Approach to Pollution Control 1.1 1.2 1.3 1.4 1.5 1.6

1—37

Interaction of Wastewater with the Ecosystem 2 Wastewater Characteristics 10 Wastewater Management in India 19 Some Issues Concerning Wastewater Disposal to the Environment Environmental Impact Assessment 35 Strategies for Control of Pollution 36 References 37

2. Planning for Wastewater Collection and Treatment 2.1 2.2 2.3 2.4 2.5

38—64

Initial Planning 38 Newer Approaches in Wastewater Collection System Design 44 Provision of Sanitation in Unsewered Slums and Shantytowns 49 Some Wastewater Treatment Methods and their Characteristics 55 Factors in Choice of Waste Treatment Methods 60 References 63

3. Principles of Aerobic Bio-Reactor Design 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9

24

Reaction Order 65 Flow Patterns of Reactors 67 Ideal Plug Flow 70 Ideal Completely-mixed Flow 70 Dispersed Flow 71 The Dispersion Number, D/UL, and Its Estimation 75 Multiple Cells with Series or Parallel Arrangements 81 Effect of Shock Loads 85 Estimation of Wastewater Temperature in Large Reactors

65—91

86

Contents

x

3.10 Guidelines for Choice of Reactors References 90

88

4. Principles of Aerobic Biological Treatment 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 4.10 4.11 4.12 4.13 4.14

Microbial Growth Rates 92 Treatment Kinetics 94 Handling of Solids 96 Sludge Age and Hydraulic Retention Time Food/Micro-organism Ratio 97 Build-up of Solids in the System 98 Substrate Removal Efficiency 99 Temperature Effects 100 Estimation of Final Effluent BOD 100 Oxygen Requirements 101 Nutrient Requirements 102 Phosphorus Removal 103 Nitrogen Removal 104 Choice of Sludge Age 104 References 107

97

5. Principles of Aeration 5.1 5.2 5.3 5.4 5.5

109—121

Theory of Aeration 109 Factors Affecting Oxygen Transfer 110 Oxygenation and Mixing Requirements 114 Types of Aerators 116 Use of Renewable Sources of Energy 120 References 120

6. Some Aerobic Biological Treatment Methods 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 6.9 6.10 6.11

92—108

Suspended and Attached Growth Systems Activated Sludge 123 Extended Aeration 131 Nitrification–Denitrification Systems 140 Phosphorus Precipitation 151 Sequencing Batch Reactors 152 Contact Stabilization Process 157 Package Plants 158 Trickling Filters 160 Submerged Media Beds 166 Rotating Bio-Disc Systems 167 References 171

122

122—172

Contents

7. The "UASB" and Other Anaerobic Processes 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.9 7.10 7.11 7.12 7.13 7.14

173—200

Background in India 173 Typical Arrangements 178 Approach to Design 178 Useful Parameters for Design 180 Sulphates and Sulphides 182 Gas Production and COD Balance 184 Gas Recovery 185 Nutrients 185 Toxicity 185 Plant or commissioning and Operation 186 Post-treatment Requirements 187 Some Pros and Cons in the Use of UASB 188 Other Types of Anaerobic Units 196 A Possible New System 199 References 200

8. Mechanically Aerated Lagoons 8.1 8.2 8.3 8.4 8.5 8.6 8.7 8.8 8.9

201—230

Types of Aerated Lagoons 202 Design of Facultative Aerated Lagoons 208 Design of Aerobic Flow-through Type Lagoons 215 Design of Dual-powered Aerated Lagoons 218 Design of Aerobic Lagoons with Recycling of Solids (Extended Aeration Lagoons) 222 Optimization of Lagoon and Pond Combinations 222 Combinations of Different Processes 224 A Possible New System: A Deep Lagoon With Small Solar-Powered Aerators 228 Construction Features 228 References 230

9. Natural Systems – 1 Classical Algal Ponds for Treatment and Resource Recovery 9.1 9.2 9.3 9.4 9.5 9.6 9.7 9.8 9.9

xi

Types of Ponds 231 Typical Flow-Sheets 233 Factors Affecting Pond Ecosystems 234 Algal Growth Dynamics 235 Oxygen Production in a Pond 235 Substrate Removal Rate in Pond 238 Choice of Flow Pattern and Pond Detention Time Pond Depth 241 Anaerobic Activity in Ponds 242

240

231—259

xii

Contents

9.10 9.11 9.12 9.13 9.14 9.15 9.16 9.17 9.18 9.19 9.20 9.21 9.22 9.23 9.24

Sulphide Production in Ponds 242 Micro-organism Removal in Ponds 243 Micro-organism Reduction Prior to Reuse in Irrigation 243 Sludge Accumulation 244 Phosphorus Removal in Ponds 244 Nitrogen Removal in Ponds 246 Example of a Pond Design 246 Pre-treatment in Anaerobic Units 248 Ponds for Animal Wastes 249 'Complete' Waste Treatment 250 Resource Recovery Based Ponds 252 Fish Ponds after Algal Ponds 253 Advanced Integrated Pond System 254 Pond-enhanced Treatment and Operation (PETRO) System 255 Construction, Operation and Maintenance of Ponds 256 References 258

10. Natural Systems – 2 Hyacinth and Duckweed Ponds, Fish Ponds, Natural and Constructed Wetlands, and Vermiculture 260—296 10.1 10.2 10.3 10.4 10.5 10.6

Hyacinth Ponds 260 Duckweed Ponds 264 Sewage-fed Fish Ponds 277 Natural Wetlands 277 Constructed Wetlands (Reed Beds) 278 Vermiculture, Earthworm Technology 289 References 294

11. Natural Systems – 3 Wastewater Irrigation 11.1 11.2 11.3 11.4 11.5 11.6 11.7 11.8

297—335

Wastewater Irrigation 297 Aspects in Planning an Irrigation System 300 The Irrigation System and Its Layout 301 Soil-Crop-Water System Design and Sustainability 310 Operational and Monitoring Aspects 325 Price Charged to Farmers for Use of Treated Wastewaters 326 Design of a Municipal Wastewater Irrigation System 327 Design of an Industrial Wastewater Irrigation Scheme 333 References 335

12. Some Physico-Chemical Methods and Membrane Technologies 12.1 Removal of Oil and Grease 336 12.2 Removal of Fines Using Flotation Technology 338 12.3 Multimedia Filtration 339

336—365

Contents

12.4 12.5 12.6 12.7 12.8 12.9 12.10 12.11

Lime-Soda and Zeolite Softening 343 Ion Exchange for Removal of Trace Ionic Pollutants 345 Activated Carbon Adsorption Systems 346 Advance Oxidation Processes 348 Disinfection 349 Membrane Processes 352 Combined Biological and Physico-Chemical Methods 359 The “CAACO” Process 364 References 364

13. Solids Setting and Sludge Management 13.1 13.2 13.3 13.4 13.5 13.6 13.7 13.8 13.9 13.11 13.12 13.13 13.14 13.15

366—395

Solids Removal by Screening 366 Grit Removal 367 Primary Sedimentation 369 Chemically-aided Sedimentation 373 Secondary or Final Settling Tanks 374 Sludge Thickeners 376 Anaerobic Digestion 377 Aerobic Digestion 380 Sludge Dewatering 382 Rational Method of Estimating Drying Time 386 Sludge Lagoons 390 Use of Evaporation/Evapotranspiration Ponds 392 Mechanized Methods of Sludge Dewatering 393 Sludge Disposal 394 References 395

14. Water Conservation and Reuse in Industry and Agriculture 14.1 14.2 14.3 14.4 14.5 14.6 14.7

xiii

396—425

Types of Reuse 398 Reuses of Urban Wastewater in Agriculture and Horticulture from Sewered Areas 398 Reuse of Urban Wastewater from Polluted “Nallahs” Draining Unsewered Areas 401 Reuse in Industry and Commerce to Meet Water Shortages 403 Reuse in Commercial Buildings Using On-site Sources of Wastewater 407 Reuse in Large Industries Using Off-site Sources of Wastewater 410 Reuse in Industries to Meet Various Objectives in Addition to Water Shortages 418 References 424

15. Wastewater Reuse to Augment Public Water Supplies 15.1 Reuse of Treated Wastewaters for Augmenting Public Water Supplies 427 15.2 Supply of Reclaimed Water through a Dual Distribution System 429 15.3 Supply of Reclaimed Water Using Membrane Bio-Reactors 431

426—454

xiv

Contents

15.4 15.5 15.6 15.7 15.8 15.9 15.10 15.11 15.12 15.13

Groundwater Recharge for Reuse 432 Reuse of Wastewater to Raise Aquifer Levels 438 Reuse of Wastewater to Prevent Sea Water Intrusion in Coastal Areas 439 Use of Reclaimed Wastewater for Recreational Purposes 440 Reuse of Wastewater for Supplementing Evaporation Losses from Non-potable Lakes in India 444 Reuse of Wastewater for Supplementing Potable Water Lakes 446 Direct Reuse of Reclaimed Water in South Africa 447 The Indian Situation of Unwitting Direct Reuse 449 Indirect Reuse of Reclaimed Water for Potable Purposes 449 Need for Environmental Impact Assessment and Follow-up Monitoring 452 References 453

16. Guidelines for Planning and Designing Treatment Plants and CETPs 16.1 16.2 16.3 16.4 16.5 16.6 16.7

455—493

Various Considerations in Site and Process Selection 457 Overall Planning for a Plant 466 The Treatment System 471 Other Important Requirements of Plant Design 479 Commissioning and Operation of Plants 482 Cost Considerations and Funding of Projects 485 Special Requirements of Common Effluent Treatment Plants (CETPs) 489 References 493

17. The Many Facets of Sustainable Waste Management

494—509

17.1 Improving Sustainability At the Industry Level 495 17.2 Improving Sustainability at the Community Level 501 References 509

Index

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Chapter 1 uce/reuse/recycle

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Ecosystem Approach to Pollution Control

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“ To protect is the voice of the present. To prevent is the divine whisper of the future’’. —Swami Vivekananda The environment has two major components: (i) living, and (ii) non-living. Ecology is the study of the complex inter-relationships between these two components and the effect that any interference or disturbance has on the natural equilibrium or balance between them. An ecological system (ecosystem) approach to pollution control and reuse is, therefore, necessary if this balance has to be maintained. The various pollutants entering an ecosystem through wastewater may belong to one of the following three groups: 1. Degradable 2. Non-degradable (conservative) 3. Biologically accumulative (persistent). Degradable pollutants include complex organic substances (e.g. sewage or industrial wastes) and dead organisms which can undergo gradual microbial decomposition. Degradable pollutants also include substances which undergo physical degradation or decay (e.g. radioactive isotopes), though some of them may have such a long half-life that they may be considered as practically non-degradable. Degradation is a time-rate process, with the rate generally following first order kinetics (see Chapter 3). Some organic substances may be readily degradable while others may be more difficult (slow) to degrade. Microbial degradation of organic matter results in oxygen consumption, which can become an indirect method of estimating the amount of organic matter present. This is done in the biochemical oxygen demand (BOD) test. Non-degradable (conservative) substances are those which are inert to biological action and do not degrade with time. These include inorganic chemicals (e.g. chlorides), heavy metals (e.g. mercury, lead), and certain refractory organics [e.g. polychloride biphenyles (PCB), DDT etc.] Biologically accumulative (persistent) substances are those that tend to accumulate in the food chain. They include mercury, cadmium, arsenic, lead, manganese, pesticides, radioactive isotopes, and others.

2

1.1

Wastewater Treatment for Pollution Control and Reuse

INTERACTION OF WASTEWATER WITH THE ECOSYSTEM

The living component of an ecosystem consists of three principal groups: (i) primary producers (plants, vegetation, algae); (ii) consumers (worms, fish, animals, man); and (iii) decomposers (microorganisms like bacteria, some of which are aerobic as they depend on oxygen, some are anaerobic as they grow only in the absence of free oxygen, and some are ‘facultative’ as they can grow under either of the two conditions. The non-living components include water, gases, minerals, soil, chemicals, etc. Photosynthesis is one of the most important processes in nature since it yields the life-giving organic compounds for the primary producers which constitute the essential source of food for the entire population of consumers and decomposers. Figure 1.1 shows a typical food chain in a natural ecosystem. At each step in the chain, the conversion efficiency of the potential food energy is roughly only about 10 per cent. Thus, for example, 40 units of food energy at the primary producer level can sustain about 4.0 units at the primary consumer level, 0.4 units at the secondary consumer level, and so on, diminishing rapidly. Primary producers are also responsible, through photosynthesis, for the co-production of oxygen, which is equally essential for many forms of life. Thus, any factor, which eventually affects photosynthesis has a critical effect on primary productivity and on the whole food chain. Some of the factors that affect photosynthesis and its rate are as follows: 1. 2. 3. 4. 5. 6.

Light energy received at growth surface Availability of water Availability of macro-nutrients (carbon, nitrogen, phosphorus, etc.) Availability of micro-nutrients (growth factors) Temperature Other factors affecting metabolic activity (e.g., toxic substances).

Most of the photosynthetic production is carried out in water by algae. Algal production can be denoted by an equation, such as the following, to show how water and its nutrients (carbon, nitrogen and phosphorus) combine through photosynthesis to give algae plus oxygen: light

® C106H2633O110N16P1 + 138O2 106CO2 + 122H2O + 16NO3 + HPO4 + 18H ¾¾¾ (2427)

(3800)

algae

oxygen

On a stoichiometric basis, the weight of oxygen produced is 1.3 times the weight of algae produced by photosynthesis. Up to about 14 per cent of this oxygen is used up in meeting the respirational requirement of the algae itself while it is living. When algae die, however, the dead organic matter is biochemically broken down by decomposers and Equation (1) now occurs in reverse. In an aerobic environment, decomposition thus consumes the same amount of oxygen that photosynthesis produces and gives the same nutrients back in an inorganic form. In an anaerobic environment, decomposition occurs but the end-products are different, though they can be oxidized if the conditions again become aerobic.

Ecosystem Approach to Pollution Control

3

Photosynthesis

Solar energy (say) 4000 kcal / m2-day + Water + Nutrients Vegetation, algae, etc.

Herbivores

Primary producers

Primary consumers

Potential food energy at each level assumed conversion efficiency (kcal / m2-day)

40

Other Carnivores

Carnivores

Secondary

4.0

0.4

consumers

0.04

Fig. 1.1 Typical food chain in a natural ecosystem

The Global Ecosystem: The whole world, as well as its individual components like lakes, rivers, soils, estuaries, and bays are all good examples of ‘ecosystems’ and ‘eco-subsystems’. An ecosystem is one in which producers, consumers and decomposers all exist in balance as shown in Fig. 1.2 (Arceivala, 1981). Assuming that Fig. 1.2 represents the typical global ecosystem, one can see how the flow of energy and recycling of nutrients takes place in the ecosystem and a balance is maintained between the different components. In fact, the figure helps one understand how the whole global ecosystem works. It shows, firstly, how the natural resources of the earth are consumed in industrial and transport activity. These activities generate liquid, solid and gaseous wastes. Among the man-made wastes, some are degradable while others are non-degradable. The decomposers break down the degradable organics into stable inorganic end-products such as nitrates and phosphates, which happen to be nutrients that again stimulate organic growths in the presence of water and solar energy. Figure 1.2 shows how nutrients (NO3 , PO4, etc.) are conserved and recycled while energy is lost and must be continually supplied with solar energy to keep the system running. The nutrients undergo photosynthesis in the presence of water and solar energy to give primary producers like algae, grass, trees, etc. and ensure the simultaneous production of the all-important oxygen. The primary producers are consumed by the primary consumers (cattle, fish, insects) which, in turn, constitute the food supply for various carnivores and finally for human beings.

4

Wastewater Treatment for Pollution Control and Reuse

The unconsumed fraction of producers as well as the organic wastes generated by the succession of consumers and the dead consumers themselves all end up as dead organic matter, which are decomposed by either aerobic or anaerobic decomposers to give stable inorganic end-products (the nutrients), which are recycled to give more producers and consumers, provided solar energy is available once again. Man-made activities also produce non-degradable substances as shown in Fig. 1.2. During the process of recycling, some of the non-degradable substances which are accumulative in nature tend to accumulate in the food chain with time. This accumulation can eventually affect the health of the consumers when the concentrations of the concerned substances are excessive. Gas exchange also occurs as shown in the figure and can cause some problems. Most of the man-made industries and transport activities consume various natural resources like air, water, soil, fuels, minerals and chemicals, and yield gases like carbon dioxide, methane, etc. The various interactions lead to the production of what are called ‘greenhouse gases’ (water vapour, CO2, methane, nitrous oxide, chloroflurocarbons (CFCs), etc.), which lead to global warming. In fact, the originally cold and snow-bound earth became more and more livable as the greenhouse gases accumulated over thousands of years and made the earth warmer for life to exist. However, over the last century or so, the emission of greenhouse gases has increased to such an extent that the earth’s climate is undergoing a slow change. The burning of fossil fuels like coal and other activities has increased the emissions of CO2 and methane to such an extent that it has started affecting the earth’s climate adversely. Some of these gases are also called ‘ozone depleters’ (e.g. CFCs, Hydrochlorofluorocarbons (HCFCs)) which tend to reduce the ozone layer and increase the ultraviolet (UV) radiation reaching the earth’s surface. CO2 emission inventories carried out in recent time have shown that CO2 emitted in tons per US $ 1 million of Gross Domestic Product (GDP) approximate 363 tons/year as an average per country. Russia was high on the emission scale at 914 T/year whereas China and India were at 731 and 621, respectively. The US was at 171 and the UK at 118 while France, Japan and Germany averaged 56 to 80 T/year. The CO2 emission rate per million dollar GDP is a better measure than CO2 emission alone and shows the long way that most countries have to go if climate change is to be arrested. In a sense, ozone depletion and global warming are separate phenomena. Ozone depletion leads to over-exposure to UV light and consequent health problems. Global warming is the principal cause of climate change which, in turn, leads to a series of other problems. For example, warming will eventually melt the polar ice-caps, which, in turn, will raise sea levels, cause the flooding of coastal areas and the affect the drainage of low-lying areas. This will, in turn, lead to the undue growth of mosquitoes and insects that damage food crops and consequently affect public health. The world is presently facing an uncertain future owing to the excessive release of solid, liquid and gaseous pollutants far in excess of the earth’s capacity to assimilate them. Thus, Fig. 1.2 is indeed a very valuable diagram for understanding how the natural ecosystem works and how recycling and accumulation inevitably take place in nature with time.

Fig. 1.2 The global ecosystem (Adapted from Arceivala (1981))

Ecosystem Approach to Pollution Control 5

6

Wastewater Treatment for Pollution Control and Reuse

If all organic matter produced in the world were to undergo aerobic decomposition, the net oxygen production would be zero, but on a global basis, all organic matter produced is not destroyed. Hence, a surplus of oxygen is observed in the earth’s general atmosphere. Oxygen may be depleted only in some localized pockets, which causes problems. When the natural rate of recycling and the ecological balance in a system are adversely affected by pollution, a correspondingly adverse effect may occur on the whole system. A river’s ‘self-purification’ capacity may become inadequate and fisheries may be ruined, a lake may become too rich in nutrients and undergo eutrophication, and polluted soil may eventually lose its porosity and moisture–retaining capacity, thus becoming useless for agriculture (see Section 1.4). 1.1.1

Food Chains and Webs

The new plant (organic) matter generated by photosynthesis provides food and energy for all other forms of life. The concept of trophic (nutritional) levels is based on the generalization that primary producers are consumed by the herbivores (vegetarians) who, in turn, are consumed by the carnivores (like man), who may consume all the previous forms including herbivores and primary producers. Thus, the lines of organic transfer may constitute quite complex food chains or networks of chains called ‘webs’. Figure 1.3 shows a typical food web which interconnects organisms across all three phases of the environment, viz. soil, water and air. As a result of such webs or networks some effects may be observed in ecosystems (see sub-sections 1.1.2 to 1.1.5). Man

Other herbivores (Birds)

Other herbivores (Animals)

Air Carnivore fish

Amphibious vertebrates

Plankton

Water

(Emergent vegetation)

Herbivore fish

(Algae)

Aquatic plants

Invertebrates (worms)

Soil

Bottom sediments

Fig. 1.3

A food web interconnecting the biota in air, water and soil

Ecosystem Approach to Pollution Control

1.1.2

7

Accumulation of Pollutants

A relatively non-degradable pollutant entering one species may eventually spread to all others in a food web. The fallout of a radioactive isotope from the atmosphere may enter a water body and eventually accumulate in algae, higher organisms, fish, and finally in man. As a large mass of food at one trophic level is needed to support life at the next higher trophic level (the efficiency of energy transfer being about 10 per cent or so), it is evident that the amount of pollutant originally contained in the large mass of food in one trophic state will be concentrated into a smaller mass in the next trophic state. Thus, the concentration of pollutant will increase at each step in the food chain. Within an organism, accumulation proceeds simultaneously with ‘elimination’ through natural processes (e.g. in the case of humans, elimination occurs through sweat, urine, faeces, nails, hair, etc.) Furthermore, during the time that a substance resides in an organism (or organ), physical decay or chemical breakdown may also occur. Depending on the nature of the substance, different mathematical models can be used to describe the net accumulation of a substance with time. Field measurements of accumulations give the integral effect of various factors which affect biological accumulation and elimination, and which may not be easy to separate individually in a theoretical study. Drawing upon the example of a relatively conservative substance like dichlorodiphenyltrichloroethane (DDT) in fresh water, Odum (1971) has given a striking instance of how build-up occurs along the food chain. A seemingly negligible concentration in water can be magnified into substantial values in the various consumers as listed below. Item Water Plankton Minnows (fish) Predator fish Scavenger bird Fish-eating duck

DDT, mg/l 0.00005 0.04 0.2–0.9 1.3–2.0 6.0 22.8

The ‘biological concentration factor’ for a substance is expressed as a ratio of its concentration in the organism (net weight) to its concentration in the medium (e.g. water). Thus, in the case of DDT just cited, the concentration factor for plankton is computed as 800 (0.04/0.00005). The concentration factors range from 1 to 10,000 depending on the organism and the pollutant substance. Donnier (1975) lists some concentration factors based on studies on the marine environment as shown in Table 1.1. Information on concentration factors is useful for setting realistic standards for water quality. For example, if the concentration of methyl mercury in fish flesh is not to exceed, say 0.2 mg/kg, and if the concentration factor is, say, 500 for the type of fish involved, then the permissible concentration of methyl mercury in water is 0.4 mg/l. Conversely, if the existing concentration in water is greater than 0.4 mg/l, the fishing industry in that area would have to be stopped, thereby resulting in economic loss.

8

Wastewater Treatment for Pollution Control and Reuse Table 1.1: Some concentration factors in the marine environment (Donnier, 1975)

Food Chain

Item

Pelagic type (water–plankton–fish) Neritic type (water–plankton–mollusc)

Plankton Fish Plankton Molluscs

Cu

Zn

Cr 6

Pb

Hg

1000 20-25 1000 120

1000 200 7000 1000

10-20 2 1000 40

500 1 5000 150

1000 100 3000 400

Similar concentration factors for various Indian organisms, if available, would be useful in determining pollution control strategies. 1.1.3

Behaviour of Pollutants in Water Bodies

The fate of conservative, non-conservative, and persistent substances in a flowing body of water such as a river can be illustrated as in Fig. 1.4 which shows some indicators of pollution such as coliforms, nutrients, dissolved oxygen (DO), etc. Figure 1.4 (a) shows three towns situated on a river. All these three towns draw their water supply from the same river and discharge their wastewaters into it. Figure 1.4(b) shows how conservative and persistent pollutants tend to build up as the river flows past each town. Agricultural run-off from the lands in between the towns would also contribute to this build-up (not shown in the figure) as non-point sources of pollution. The effect of degradable organic matter on the DO concentration in the river water is shown in Fig. 1.4(c). The concentration of oxygen is the result of two different activities progressing simultaneously in the river: oxygen is used in bacterial activity while it is replenished in the flowing water by re-aeration from the turbulent surface of the river. The net result is that the oxygen concentration goes up and down as the river passes from town to town. As far as the fate of the organics themselves is concerned, their degradation leads to a gradual reduction in their concentration between the towns as shown by the curves in Fig. 1.4(d). The figure can also be used to show how bacteria, which are again non-conservative, die after passing each town. All these are time processes and sufficient time between two towns must elapse for ‘selfpurification’ to occur. Gradually, as self-purification proceeds, organic matter is degraded into inorganic nutrients, bacterial die-off becomes appreciable thereby rendering the bacteria harmless, dissolved oxygen is restored to the river’s original or any other desired value, and so on. As the river flows, it becomes ready to receive new pollution from the next town. Only the persistent and accumulative substances tend to remain in the water. In a river such as the Rhine, reuse is said to occur 52 times before it reaches the sea. 1.1.4

Biodiversity and Ecosystem Stability

The extent and type of an ecological community depends on the vast complex of physical, chemical and biological conditions present in its environment. The tolerance of an organism to the given conditions affects both its distribution and its relative dominance within an ecosystem (e.g. algae will flourish only where light is available, and aerobic decomposers will only be found where oxygen is available). The rate and amount of food production at a trophic level affects the number and variety of consumers at the next higher trophic level. The greater the variety of consumers (namely, the greater the diversity of species), the more complex and varied the food chains are likely to be, resulting in an increasing number of ‘channels’ through which energy can flow (Odum 1971). This increases the stability of the system since an organism may not be dependent on just one channel for receiving its food (a water distribution system in the form of a network gives a

Ecosystem Approach to Pollution Control

9

better assurance of water supply to an area than if the latter is served by a single pipeline). Thus it can be said that there is stability in diversity. The entry of organic wastes into a water body may create an unfavourable environment and reduce the number of fish species (air pollution may reduce the number of plant species and birds). Any unfavourable environment would tend to shift the population away from sensitive species to the more resistant ones, consequently reducing the number of species. If enough food is available but the environment is unfavourable, only the number of individuals in the resistant type of species may increase, thus resulting in less variety of species but more individuals per species. Town A Water supply

Town B

Town C

Drainage

(a) River

Initial concentration

Concentration of conservative pollutant increases after each outfall

(b)

River

Initial D.O.

The D.O. profile sags below each outfall as biodegradable organic matter exerts oxygen demand

Minimum D.O.

(c)

Minimum D.O.

River

Bacterial concentration increases below each outfall but rapidly diminishes as natural die off occurs

Initial bacterial concentration

(d) River

Fig. 1.4 Typical profiles of conservative and biodegradable pollutants discharged from three towns located on a river bank

10

Wastewater Treatment for Pollution Control and Reuse

The concept of ‘species diversity’ is, therefore, increasingly being used as an indicator of pollution. The species diversity index, Kd, can be found by noting the number of different species S, observed while counting a large enough number of individuals, I. Various equations are used for estimating the species diversity index. The higher the value of the index, the cleaner will be the water. For example, the expected value of the index is shown in Table 1.2 for different typical sites, by using one method. Table 1.2: Species diversity indices for some typical sites Site Rain forest water Tropical seas Bays Polluted water

Typical values of Kd 40–50 30–40 5–30