Biology of wastewater treatment

For, example new separation technologiesand water reuse at the household level is reducing wastewater loadings.New advances with in-sewer treatment have been very successful in reduc-ing

Imperial College Press ICP

University of Dublin, Ireland

N F Gray Second Edition

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Copyright © 2004 by Imperial College Press

BIOLOGY OF WASTEWATER TREATMENT (2nd Edition)

Acknowledgement is gratefully made to the authors and publishers of terial that has been redrawn, reset in tables, reproduced directly or repro-duced with minor modifications The exact sources can be derived from thereferences For permission to reproduce copyright material thanks are due

ma-to the following copyright holders:

Chapter 1

International Water Supply Association: Table 1.4a

Office of Water Services (Ofwat): Tables 1.10a, 1.10b

Water Services Association of England and Wales: Table 1.4b

Chapter 2

Anglian Water: Table 2.1

Blackwell Science Publishers: Fig 2.23

Cambridge University Press: Table 2.6

Chartered Institution of Water and Environmental Management:

Tables 2.4, 2.5; Figs 2.15, 2.22, 2.26, 2.27

Edward Arnold (Publishers) Ltd.: Figs 2.14, 2.16

Ellis Horwood Ltd.: Figs 2.19, 2.20, 2.21

IWA Publishing: Fig 2.6

John Wiley and Sons Ltd: Fig 2.17

Dr H.J Kiuru: Fig 2.6

Mr J Lynch, County Engineer, Kildare County Council: Fig 2.10

McGraw Hill Inc.: Figs 2.3, 2.4

WRc plc: Table 2.7; Fig 2.28

v

Chapter 3

Academic Press: Tables 3.9, 3.10, 3.12

Applied Science Publishers: Tables 3.13, 3.14

Blackwell Science Publishers: Figs 3.21, 3.25

Chartered Institution of Water and Environmental Management:

Table 3.5

Controller of Her Britannic Majesty’s Stationary Office: Fig 3.14

CRC Press: Table 3.18

Elsevier: Table 3.11

John Wiley and Sons Inc.: Table 3.6

McGraw-Hill Inc.: Table 3.17; Figs 3.2, 3.17, 3.19

Prentice Hall Inc.: Figs 3.3, 3.5, 3.16, 3.18, 3.20, 3.27

Dr T Stones: Table 3.4

D Reidal Publishers: Table 3.16

Water Environment Federation: Table 3.2; Fig 3.24

Chapter 4

Academic Press: Table 4.18; Figs 4.1, 4.21

Blackwell Science Publishers: Fig 4.13

BritishEcological Society: Table 4.17; Figs 4.27, 4.28

BritishStandard Institution: Tables 4.5, 4.7

Chartered Institution of Water and Environmental Management:

Table 4.21; Figs 4.22, 4.25, 4.26, 4.31, 4.34, 4.40, 4.43, 4.49

Ellis Horwood Ltd.: Figs 4.46, 4.47

Elsevier: Tables 4.14, 4.15, 4.19; Fig 4.36

IWA Publishing: Figs 4.41, 4.44, 4.48

John Wiley and Sons Inc.: Fig 4.42

Dr M.A Learner: Tables 4.3, 4.4

Open University Press: Table 4.9

Dr I.L Williams: Fig 4.15

WRc plc: Figs 4.37, 4.39

Chapter 5

Academic Press: Tables 5.2, 5.26, 5.28, 5.29, 5.30; Figs 5.1, 5.12, 5.18b,

5.60, 5.79, 5.80, 5.83, 5.90, 5.91, 5.96

Biwater Treatment Ltd.: Fig 5.18a

Blackwell Science Publishers: Figs 5.92, 5.93

Carborundum Abrasives GB Ltd.: Fig 5.23

C.E.P Consultants, Edinburgh: Figs 5.30, 5.31, 5.48

Chartered Institution of Water and Environmental Management:

Tables 5.11, 5.25, 5.31; Figs 5.14, 5.24, 5.25, 5.28, 5.88, 5.98

Ellis-Horwood Ltd.: Figs 5.16, 5.33, 5.51, 5.52, 5.62, 5.63, 5.73, 5.77

Elsevier: Tables 5.17, 5.24; Figs 5.89, 5.112

IWA Publishing: Tables 5.12, 5.32; Figs 5.15, 5.34, 5.53

John Wiley and Sons Inc.: Fig 5.11

Mr G O’Leary: Figs 5.21, 5.22

Editor of Oikos: Fig 5.90

Rosewater Engineering Ltd.: Figs 5.26, 5.27

Dr J.P Salanitro, Shell Development Co: Figs 5.4, 5.81

Simon Hartley Ltd.: Table 5.8; Figs 5.17, 5.20

TNO ResearchInstitute for Environmental Hygiene, Delft: Fig 5.60

Water Environment Federation: Table 5.21; Figs 5.2, 5.5, 5.66,

5.72, 5.76, 5.103

Water ResearchCommission, SouthAfrica: Tables 5.13, 5.15, 5.16, 5.19,

5.20; Figs 5.9, 5.67, 5.71, 5.74Chapter 6

Academic Press: Figs 6.9, 6.21, 6.22

Editor, American Journal of Botany: Fig 6.14

BritishStandards Institution: F ig 6.3

Carl Bro Consultants, Leeds (Lagoon Technology International, Leeds):

IWA Publishing, London: Table 6.20; Fig 6.16

McGraw-Hill Inc.: Fig 6.2

National Standards Agency of Ireland: F ig 6.4

Pergamon Press (Elsevier): Fig 6.17

Springer, Wien: Table 6.3

University of Pennsylvania Press: Tables 6.5, 6.8; Figs 6.11, 6.13

US Department of Agriculture: Fig 6.1

US Environmental Protection Agency: Table 6.2

Dr J Vymazal: Table 6.10

Water Environment Federation: Tables 6.4, 6.7

World Health Organization, Geneva: Table 6.18

WRc plc: Tables 6.12, 6.14; Fig 6.15

Chapter 7

Academic Press: Table 7.3

Chartered Institution of Water and Environmental Management:

Figs 7.1, 7.2, 7.12, 7.16; Table 7.2, 7.4, 7.5, 7.10

Editor of Food Technology: Tables 7.11, 7.14

Ellis Horwood Ltd.: Table 7.9

Elsevier: Table 7.13

Institution of Engineers in Ireland: Tables 7.6, 7.8

IWA Publishing: Figs 7.3, 7.17; Table 7.12

Oklahoma State University, Stillwater: F ig 7.7

Texas State department of Health, Austin: F ig 7.8

WRc plc: F ig 7.6

Chapter 8

Cambridge University Press: Tables 8.23, 8.25

Chartered Institution of Water and Environmental Management:

Figs 8.1, 8.2, 8.5, 8.6, 8.7, 8.12, 8.13, 8.13, 8.16;

Tables 8.16, 8.24, 8.28

Ellis Horwood Ltd.: Tables 8.6, 8.7

European Commission: Table 8.15

IWA Publishing: Figs 8.14, 8.15; Tables 8.3, 8.10, 8.11, 8.12, 8.26; 8.27,

8.29, 8.30

John Wiley and Sons Inc.: Fig 8.4

McGraw-Hill Inc.: Table 8.5

National Board for Science and Technology, Dublin:

Figs 8.9, 8.10, 8.11; Table 8.2, 8.21

National water Council: Tables 8.13, 8.32, 8.33

Open University Press: Table 8.1

Oslo and Paris Commissions: Fig 8.17; Table 8.35, 8.36

Water ResearchCommission, SouthAfrica: Table 8.31

WRc plc: Fig 8.3; Tables 8.17, 8.18, 8.19, 8.20, 8.22, 8.34, 8.40, 8.43

Chapter 9

Academic Press: F ig 9.17

American Public HealthAssociation: Fig 9.3

American Society of Civil Engineers: F ig 9.22

American Society for Microbiology: Table 9.43; Figs 9.4, 9.5, 9.12

Americam Water Works Association: Figs 9.1, 9.11

Blackie & Co.: Figs 9.24, 9.27

Blackwell Science Publishers: Tables 9.6, 9.23

Carl Bro Consultants, Leeds (Lagoon Technology International, Leeds):

Fig 9.18

Chartered Institution of Water and Environmental Management:

Tables 9.4, 9.5, 9.57

Controller of Her Britannic Majesty’s Stationary Office: Table 9.20.

DEFRA, London: Table 9.49

Ellis Horwood Ltd.: Table 9.52

Elsevier: Tables 9.3, 9.29; Figs 9.15, 9.21

Editor, Environmental HealthPerspectives: Table 9.54

Environmental Sanitation Information Centre, Bangkok: Table 9.33

European Commission: Tables 9.16, 9.17, 9.30, 9.31, 9.32, 9.34

IWA Publishing: Tables 9.11, 9.26, 9.46, 9.47, 9.50, 9.51, 9.53; Fig 9.6 John Wiley and Sons Inc.: Tables 9.9, 9.10, 9.27, 9.58;

Figs 9.7, 9.13, 9.23

John Wiley and Sons Ltd.: Tables 9.22, 9.44; Figs 9.26

Editor, Journal of Hygiene, Cambridge: Tables 9.35, 9.48

Kluwer Academic Publishers: Table 9.45

McGraw-Hill Inc.: Fig 9.10

Pergamon Press (Elsevier): Fig 9.20

Van Nostrand Reinhold, New York: Tables 9.55, 9.56

Water Environment Federation: Tables 9.15, 9.24; Fig 9.19

World Health Organization, Geneva: Table 9.18

WRc plc: Tables 9.2, 9.19,9.36, 9.37, 9.38, 9.39, 9.42 ; Fig 9.16

US Environmental Protection Agency: Table 9.7, 9.8

Chapter 10

Academic Press: Tables 10.4, 10.13, 10.15; Figs 10.2, 10.19

American Society for Microbiology: Tables 10.23, 10.25

Applied Science Publishers: Table 10.7

BritishSugar Corporation Ltd.: Fig 10.1

Editor of Biotechnology Bulletin: Table 10.1

Blackwell Science Publishers: Table 10.3; Fig 10.13

Cambridge University Press: Fig 10.3

Centre Europen d’Etudes des Polyphosphates: Table 10.5

Chartered Institution of Water and Environmental Management:

Tables 10.19, 10.26; Figs 10.8, 10.9, 10.10, 10.11, 10.29, 10.38

Dr A.D Wheatley: Table 10.6

Ellis Horwood Ltd.: Table 10.8; Figs 10.27, 10.28

Elsevier: Tables 10.20, 10.24, 10.29; Figs 10.6, 10.22, 10.23, 10.24, 10.25,

10.26, 10.30, 10.35, 10.36

Professor Isumi Hirasawa: Fig 10.5

IWA Publishing: Figs 10.32, 10.37

John Wiley and Sons Ltd: Table 10.12; Fig 10.15

Marcel Dekker Inc: Tables 10.21, 10.22

Nature Press: Tables 10.9, 10.10; Fig 10.4

National Institute of Agricultural Engineering, Silso: Tables 10.17, 10.18 Pergamon Press (Elsevier): Tables 10.12, 10.14; Fig 10.14

Purdue University: Fig 10.33

Surveyor Magazine: Fig 10.12

Chapter 11

Dr Annelies Balkema: Table 11.1

Chartered Institution of Water and Environmental Management:

Fig 11.1

Elsevier: Tables 11.2, 11.4, 11.5; Fig 11.5

IWA Publishing: Tables 11.4, 11.6, 11.7, 11.8, 11.9, 11.10, 11.11, 11.12,

11.13; Figs 11.2, 11.3, 11.4, 11.6

I would also like to thank Dr Anne Kilroy for permission to reproducejointly published material in chapter 3

1.1 Introduction 1

1.1.1 The wastewater problem 1

1.1.2 Legislation 4

1.2 Nature of Wastewater 14

1.2.1 Sources and variation in sewage flow 15

1.2.2 Composition of sewage 26

1.2.3 Other wastewaters 47

1.3 Micro-organisms and Pollution Control 55

1.3.1 Nutritional classification 56

1.4 Microbial Oxygen Demand 63

1.4.1 Self purification 63

1.4.2 Biochemical oxygen demand 93

1.4.2.1 The test 93

1.4.2.2 Methodology 101

1.4.2.3 Factors affecting the test 112

1.4.2.4 Sources of error 124

2 How Man Deals with Waste 133 2.1 Basic Treatment Processes 133

2.1.1 Preliminary treatment 138

2.1.2 Primary treatment 146

xi

2.1.3 Secondary treatment 147

2.1.4 Tertiary treatment 148

2.1.5 Examples of treatment plants 148

2.2 Sedimentation 151

2.2.1 The settlement process 151

2.2.2 Design of sedimentation tanks 157

2.2.3 Performance evaluation 163

2.3 Secondary (Biological) Treatment 173

2.4 Tertiary and Advanced Treatment 178

2.4.1 Tertiary treatment 179

2.4.2 Advanced wastewater treatment 190

3 The Role of Organisms 191 3.1 Stoichiometry and Kinetics 191

3.1.1 Stoichiometry 195

3.1.2 Bacterial kinetics 204

3.1.3 The BOD test 217

3.2 Energy Metabolism 223

3.3 Aerobic Heterotrophic Micro-organisms 230

3.3.1 The organisms 230

3.3.2 Nutrition 245

3.3.3 Environmental factors 253

3.3.4 Inhibition 257

3.4 Anaerobic Heterotrophic Micro-organisms 259

3.4.1 Introduction 259

3.4.2 Presence in the treatment plant 260

3.4.3 Anaerobic digestion 262

3.4.4 Sulphide production 271

3.4.5 Denitrification 272

3.4.6 Redox potential 275

3.5 Autotrophic Micro-organisms 277

3.5.1 Introduction 277

3.5.2 Nitrification 282

3.6 Assessing Treatability, Toxicity, and Biodegradability 290

3.6.1 Introduction 290

3.6.2 Biochemical tests 291

3.6.3 Bacterial tests 297

3.6.4 Other approaches 317

3.6.5 Continuous simulation tests 320

3.6.6 Conclusion 324

4 Fixed-Film Reactors 325

4.1 Percolating Filters 326

4.1.1 Design and operation 330

4.1.2 Process modifications 356

4.1.3 The organisms and their ecology 364

4.1.4 Factors affecting performance 417

4.1.5 Nitrifying filters 440

4.2 Rotating Biological Contactors 441

4.3 Submerged Fixed Film Systems 450

4.3.1 Introduction 450

4.3.2 Fluidised bed reactors 451

4.3.3 Biological aerated flooded filters 455

4.3.4 Submerged aerated filters 460

4.3.5 Moving bed biofilm reactor 462

5 Activated Sludge 465 5.1 F locculation 469

5.2 Operating Factors 477

5.2.1 Process control 477

5.2.1.1 Mixed liquor suspended solids 477

5.2.1.2 Sludge residence time or sludge age 478

5.2.1.3 Plant loading 479

5.2.1.4 Sludge settleability 483

5.2.1.5 Sludge activity 484

5.2.1.6 Recirculation of sludge 487

5.2.2 Factors affecting the process 488

5.2.3 Aeration methods 496

5.2.3.1 Surface aeration 497

5.2.3.2 Air diffusion 504

5.2.3.3 Testing aerators 511

5.3 Modes of Operation 516

5.3.1 Conventional activated sludge processes 517

5.3.1.1 Plug-flow systems 519

5.3.1.2 Completely mixed systems 528

5.3.1.3 Sequencing batch reactor technology 530

5.3.2 Extended aeration 532

5.3.2.1 Oxidation ditches 532

5.3.2.2 Packaged plants 539

5.3.3 High-rate activated sludge processes 541

5.3.3.1 A–B process 543

5.3.4 Advanced activated sludge systems 544

5.3.4.1 ICI Deep Shaft
process 545

5.3.4.2 Pure oxygen systems 548

5.4 Sludge Problems 556

5.4.1 Deflocculation 558

5.4.2 Pin-point floc 560

5.4.3 F oaming 561

5.4.4 F ilamentous bulking 569

5.4.5 Identifying problems 583

5.4.6 Non-filamentous bulking 592

5.4.7 Denitrification 592

5.5 Ecology 593

5.5.1 Bacteria 596

5.5.2 F ungi 599

5.5.3 Protozoa 599

5.5.4 Other groups 615

5.6 Nutrient Removal 618

5.6.1 Denitrification 622

5.6.2 Phosphorus removal 628

6 Natural Treatment Systems 641 6.1 Land Treatment 643

6.1.1 Purification process 644

6.1.2 On-site subsurface infiltration 646

6.1.3 Slow rate land application 651

6.1.4 Rapid infiltration land treatment systems 654

6.1.5 Overland flow 656

6.2 Macrophyte-Based Systems 658

6.2.1 Algae and submerged macrophytes 660

6.2.2 Floating macrophytes 663

6.2.3 Emergent macrophytes 673

6.3 Stabilisation Ponds 697

6.3.1 Anaerobic ponds and lagoons 700

6.3.2 Oxidation ponds 704

6.3.3 Aeration lagoons 731

7 Anaerobic Unit Processes 735 7.1 Introduction 735

7.2 F low-Through Systems (Digestion) 743

7.2.1 Combined systems 744

7.2.2 Digestion 754

7.3 Contact Anaerobic Systems 777

7.3.1 Anaerobic activated sludge process 779

7.3.2 Sludge blanket process 781

7.3.3 Static media filter process 783

7.3.4 Fluidised and expanded media 790

8 Sludge Treatment and Disposal 793 8.1 Sludge Characteristics and Treatment 793

8.1.1 Treatment options 798

8.1.2 Disposal options 819

8.2 Land Disposal 829

8.2.1 Sludge disposal to land sites 829

8.2.2 Sludge utilisation to farmland 834

8.3 Sea Disposal 864

8.3.1 Introduction 864

8.3.2 Legislative control 866

8.3.3 Dumping sites 871

8.3.4 Environmental impact 872

9 Public Health 885 9.1 Disease and Water 885

9.2 Water-Borne Diseases 888

9.2.1 Introduction 888

9.2.2 Bacteria 889

9.2.3 Viruses 906

9.2.4 Protozoa 914

9.2.5 Parasitic worms 929

9.3 Indicator Organisms 931

9.3.1 Escherichia coli and coliforms 941

9.3.2 F aecal streptococci 953

9.3.3 Faecal coliform/faecal streptococci (FC/FS) ratio 959

9.3.4 Clostridium perfringens 962

9.3.5 Bacteriophage 964

9.3.6 Bifidobacteria 967

9.3.7 Rhodococcus spp 968

9.3.8 Heterotrophic plate count bacteria 969

9.3.9 Other indicator organisms 971

9.3.10 Chemical indicators 974

9.4 Hazards Associated with Wastewater and Sludge 976

9.4.1 Water pollution 976

9.4.2 Land Pollution 996

9.4.3 Atmospheric pollution 1008

9.4.4 Antibiotic resistance in enteric bacteria 1011

9.5 Removal of Pathogenic Organisms 1013

9.5.1 Environmental factors and survival 1013

9.5.2 Treatment processes 1021

9.5.3 Sterilization and disinfection methods 1040

10 Biotechnology and Wastewater Treatment 1057 10.1 The Role of Biotechnology 1057

10.2 Resource Reuse 1060

10.2.1 Fertiliser value 1060

10.2.2 Reuse of effluents 1061

10.2.3 Metal recovery 1067

10.2.4 Phosphorus recovery 1078

10.3 Biological Conversion 1083

10.3.1 Bio-energy 1083

10.3.2 Single-cell protein and biomass 1099

10.3.3 Composting 1124

10.4 Environmental Protection 1154

10.4.1 Breakdown of recalcitrants 1155

10.4.2 Bioscrubbing 1160

10.4.3 Bioaugmentation 1164

10.4.4 Immobilised cells and biosensors 1169

11 Sustainable Sanitation 1179 11.1 Introduction 1179

11.2 The Problems 1180

11.3 Sustainable Options 1190

11.3.1 Source contamination 1190

11.3.2 Treatment 1196

11.3.3 Final disposal 1203

11.4 Implementation 1212

Preface to the Second Edition

Since writing the first edition of Biology of Wastewater Treatment the

wastewater industry has changed quite dramatically While the basic cepts remain the same, the processes and the industry that design, buildand operate treatment systems have all radically altered So why has waste-water technology changed so much since 1990? In Europe the introductionand rapid implementation of the Urban Wastewater Treatment Directivehas to be a major factor Nutrient removal, especially biological phosphorusremoval, is now commonplace This in turn has forced us back to the use ofthe original batch reactor designs for activated sludge The large increase

con-in sludge production has required the development of con-integrated disposalstrategies linked with better recovery and reuse technologies The rapidexpansion of wastewater treatment is allowing manufacturers to experimentwith new innovative designs and processes, and for the first time in nearlyhalf a century new sewage treatment plants are being built rather thanexisting plants merely being upgraded or retrofitted Privatisation in the

UK has also been hugely influential bringing into play the often-conflictingfactors of cost, especially operational cost, and accountability Better regu-lation and control in all countries, coupled with better process managementhas resulted in better treatment overall The concept of sustainability hasalso become an important factor, although it is still to have any real in-fluence on long-term design or planning Growing urbanization, climatechange, and new analytical techniques that are constantly allowing us toidentify new pollutants and understand the fate of others during treatmentand subsequently in receiving waters, have all significantly influenced thewastewater industry However, many fear that wastewater treatment will

xvii

eventually reach crisis point where existing technologies will prove to betoo expensive and energy dependent to be able to satisfy all the needs of

a modern society Also, long-term planning is difficult with legislation andregulation constantly changing So now is the time to stand back and take

a new look at the whole concept of the wastewater cycle from production

at the household level through to treatment Our highly diluted waters, heavily contaminated with metals, pharmaceutical drugs, oestro-gen mimicking compounds, more varied and dangerous pathogens, and analarmingly wide range of trace organic compounds is simply too difficult

waste-to treat effectively in a manner that is going waste-to be sustainable Ratherthan developing better and more efficient process designs we need to start

by looking at the basic concepts of treatment and redesign the system asthough starting from scratch For, example new separation technologiesand water reuse at the household level is reducing wastewater loadings.New advances with in-sewer treatment have been very successful in reduc-ing organic loads to treatment plants and at the same time creating a moretreatable wastewater entering the wastewater treatment plant Localisedtreatment plants rather than centralized systems are now thought to bemore efficient Removing pollutants at source rather than at the treatmentplant is making effluents and sludges in particular less hazardous What

is clear is that wastewater treatment will have to become a joint venturebetween all the stakeholders, with every person having to take some re-sponsibility for their waste

I have tried to retain as much of the original text as possible, but due tothe rapid changes that have occurred over the past decade then considerablerevision was necessary All sections have been updated with many expanded

to reflect the new importance or popularity of processes There is also a newchapter on sustainability

It is often forgotten by environmentalists, and the public in general,what an important role wastewater treatment plays in protecting both theenvironment and the health of the public Without it there would be nodevelopment and growth, without it our environment and our very liveswould be at risk It is a huge credit to all those involved in the industrythat this vital service is carried out in such a discreet and professionalmanner For all those of you who have made it your career, thank you Forthose who would like to, then welcome and I hope that you will also find itequally as rewarding and exciting as I have

Nick Gray

TCD

January 2004

‘To you it’s just crap, to me it’s bread and butter.’

Spike Milligan

Recollections of the latrine orderly

How Nature Deals with Waste

Each day, approximately 1× 106

m3of domestic and 7× 106

m3of industrialwastewater is produced in the UK This, along with surface runoff frompaved areas and roads, and infiltration water, produces over 20× 106m3

of wastewater requiring treatment each day To cope with this immensevolume of wastewater there were, in 1999, some 9260 sewage treatmentworks serving about 95% of the population (Water UK 2001) The size

of these plants varies from those serving small communities of < 100, to

plants like the Crossness Sewage Treatment Works operated by ThamesWater which treats the wastewater from over 1.7 million people living in a

240 km2area of London

In terms of volume or weight, the quantity of wastewater treated ally in the UK far exceeds any other product (Table 1.1) including milk,steel or even beer (Wheatley 1985), with vast quantities of wastewater gen-erated in the manufacture of most industrial products (Fig 1.1) The cost

annu-of wastewater treatment and pollution control is high, and rising annually,not only due to inflation but to the continuous increase in environmentalquality that is expected During the period 1994–1999, the ten main watercompanies in England and Wales invested£16.55bn into its services Overhalf of this was on wastewater provision In the year 1998/1999, £1.9bnwas spent on new wastewater treatment plants alone as compliance withthe European Union Urban Wastewater Treatment Directive continues Theindustry is extremely large, with the income for these water companies for

1

Table 1.1 The quantity of sewage treated in the UK far exceeds the quantity of other industrial products processed Comparative values are based on 1984 sterling values (Wheatley 1985).

Product Tonnes/annum (×106 ) Price ( £/tonne)

The safe disposal of human excreta is a pre-requisite for the supply ofsafe drinking water, as water supplies can only become contaminated wheredisposal is inadequate There are many infectious diseases transmitted in

excreta, the most important being the diarrhoeal diseases cholera, typhoid,and schistosomiasis The faeces are the major source of such diseases withfew infections, apart from schistsosomiasis, associated with urine Amongthe most common infectious water-borne diseases are bacterial infectionssuch as typhoid, cholera, bacillary dysentery, and gastro-enteritis; viral in-fections such as infectious hepatitis, poliomyelitis, and various diarrhoealinfections; the protozoal infections cryptosporidiosis, giardiasis, and amoe-bic dysentery, and the various helminth infections such as ascariasis, hook-worm, and schistosomiasis (bilharzia) Although the provision of clean wa-ter supplies will reduce the levels of infection in the short term, in thelong term it is vital that the environment is protected from faecal pollution(Feachem and Cairncross 1993; Mara 1996) Adequate wastewater treat-ment and the disinfection of water supplies has effectively eliminated thesewater-borne diseases from developed countries, but they remain endemic inmany parts of the world, especially those regions where sanitation is poor ornon-existent (Chap 9) In developed countries where there are high popula-tion densities, such as the major European cities, vast quantities of treatedwater are required for a wide variety of purposes All the water suppliedneeds to be of the highest quality possible, although only a small propor-tion is actually consumed To meet this demand, it has become necessary toutilise lowland rivers and groundwaters to supplement the more traditionalsources of potable water such as upland reservoirs (Gray 1997) Where thewater is reused on numerous occasions, as is the case in the River Severnand the River Thames Sec 10.2.2, adequate wastewater treatment is vital

to ensure that the outbreaks of waterborne diseases that were so prevalent

in the eighteenth and nineteenth centuries do not reoccur (Chap 9)

In terms of environmental protection, rivers are receiving large tities of treated effluent while estuaries and coastal waters have vastquantities of partially or completely untreated effluents discharged intothem Although in Europe, the Urban Wastewater Treatment Directivehas caused the discharge of untreated wastewater to estuarine and coastalwaters to be largely phased out Apart from organic enrichment endan-gering the flora and fauna due to deoxygenation, treated effluents rich inoxidised nitrogen and phosphorus can result in eutrophication problems.Where this is a particular problem, advanced or tertiary wastewater treat-ment is required to remove these inorganic nutrients to protect rivers andlakes (Sec 2.4) Environmental protection of surface waters is therefore amajor function of wastewater treatment In 1998, 30% of all rivers surveyed

quan-in England and Wales (12,241 km) were classified as havquan-ing doubtful, orworse, quality (i.e class D, E and Fusing the Environment Agency General

Table 1.2 The river quality in England and Wales based on the Environment

Agency GQA systems.

River length (%) in each quality grade

of a century to raise to a standard suitable for recreation (Department ofthe Environment 1984)

or environmental quality standards Table 1.3 lists the key Directivesconcerning the aquatic environment that govern legislation in countries(Member States) comprising the European Union The principal Direc-tives are those dealing with Surface Water (75/440/EEC), Bathing Waters(76/160/EEC), Dangerous Substances (76/464/EEC; 86/280/EEC), Fresh-water Fish (78/659/EEC), Ground Water (80/68/EEC), Drinking Water

Table 1.3 EUDirectives concerning inland waters by year of introduction.

Council Directive concerning the quality of bathing waters (76/160/EEC)

Concil Directive on pollution caused by certain dangerous substances discharged into the aquatic environment (76/464/EEC)

1977

Council decision establishing a common procedure for the exchange of information on the quality of surface in the Community (77/795/EEC)

1978

Council Directive on titanium oxide waste (78/178/EEC)

Council Directive on quality of fresh waters needing protecting or improvement in order

to support fish life (78/659/EEC)

1979

Council Directive concerning the methods of measurement and frequencies of sampling and analysis of surface water intended for the abstraction of drinking water in the Mem- ber States (79/869/EEC)

Council Directive in the quality required for shellfish wates (79/923/EEC)

Council Directive on the assessment of the effects of certain public and private projects

on the environment (85/337/EEC)

Table 1.3. (Continued )

1986

Council Directive on the limit values and quality objectives for discharge of tain dangerous substances included in List I of the Annex to Directive 76/464/EEC (86/280/EEC)

1990

Council Directive amending Annex II to the Directive 86/280/EEC on limit values and quality objectives for discharges of certain dangerous substances included in List I of the Annex to Directive 76/464/EEC (90/415/EEC)

1991

Council Directive concerning urban waste water treatment (91/271/EEC)

Council Directive concerning the protection of waters against pollution caused by nitrates from agricultural sources (91/676/EEC)

The Dangerous Substances Directive (76/464/EEC) requires licensing,monitoring and control of a wide range of listed substances discharged

to the aquatic environment List I (Black List) substances have been lected mainly on the basis of their toxicity, persistence and potential forbioaccumulation Those that are rapidly converted into substances that arebiologically harmless are excluded List II (Grey List) substances are consid-ered to be less toxic, or the effects of which are confined to a limited area

se-Table 1.4 List I and List II substances defined by the EUDangerous Substances rective (76/464/EEC).

Di-List no 1 (‘black list’)

Organohalogen compounds and substances which may form such compounds in the aquatic environment

Organophosphorus compounds

Organotin compounds

Substances, the carcinogenic activity of which is exhibited in or by the equatic ment (substances in List 2 which are carcinogenic are included here)

environ-Mercury and its compounds

Cadmium and its compounds

Persistent mineral oils and hydrocarbons of petroleum

Persistent synthetic substances

List no 2 (‘greylist’)

The following metalloids/metals and their compounds:

Zinc, copper, nickel, chromium, lead, selenium, arsenic, antimony, molybdenum, nium, tin, barium, beryllium, boron, uranium, vanadium, cobalt, thalium, tellurium, silver

tita-Biocides and their derivatives not appearing in List 1

Substances which have a deleterious effect on the taste and/or smell of products for human consumption derived from the aquatic environment and compounds liable to give rise to such substances in water

Toxic or persistent organic compounds of silicon and substances which may give rise to such compounds in water, excluding those which are biologically harmless or are rapidly converted in water to harmless substances

Inorganic compounds of phosphorus and elemental phosphorus

Non-persistent mineral oils and hydrocarbons of petroleum origin

Water policy in the EU has recently been rationalized into three keyDirectives: Drinking Water (80/778/EEC), Urban Waste Water Treatment(91/271/EEC), and the Water Framework Directive (2000/60/EEC)

The Water Framework Directive (2000/60/EEC) brings together the isting Directives on water quality of surface fresh water, estuaries, coastalwaters and ground water It covers all aspects of aquatic ecology and wa-ter quality, including the protection of unique and valuable habitats, theprotection of drinking water resources and the protection of bathing wa-ters It achieves this by managing all water resources within River Basin

ex-Table 1.5 Environmental quality standards for List I and List II substances in England and Wales (Environment Agency 1998).

List I substances Statutory EQS a (µg/l) Number of discharges

Hexachlorocyclohexane (all isomers) 0.1 123

a Standards are all annual mean concentrations

List II substances Operational EQS a(µg/l) Measured as

A = annual average, P = 95% of samples, D = dissolved, T = total, M = maximum.

a Standards quoted for metals are for the protection of sensitive aquatic life at hardness 100–150 mg/l CaCO 3 , alternative standards may be found in DoE circular 7/89.

b Standards for these substances are from the Surface Waters (Dangerous Substances) (Classification) Regulations 1997, Sl 2560 in which case these are now statutory.

Districts for which management plans will be drawn up using environmentalquality standards (EQSs) (Table 1.5) The Directive sets clear monitoringprocedures and lists specific biological, hydromorphological and physico-chemical parameters to be used for rivers, lakes, estuaries and coastal wa-ters For each of these resource groups, definitions of high, good and fairecological quality are given for each specified parameter

The Urban Waste Water Treatment Directive (91/271/EEC) makes ondary treatment mandatory for sewered domestic waste waters and alsoall biodegradable industrial (e.g food processing) waste waters Minimumeffluent standards have been set at BOD 25 mg l−1, COD 125 mg l−1 andsuspended solids 35 mg l−1 Those receiving waters that are considered

sec-to be at risk from eutrophication are classified as sensitive so that charges require more stringent treatment to bring nutrient concentrations

dis-of final effluents down to a maximum total phosphorus concentration dis-of

2 mg l−1 for P and a total nitrogen concentration of 10–15 mg l−1 for N(Table 1.6) Due to the cost of nutrient removal, the designation of receiv-ing waters as sensitive has significant cost implications for Member States

Table 1.6 The Urban Wastewater Treatment Directive (91/271/EEC) sets

dis-charge limits for wastewater treatment plants Values for total phosphorus and

nitrogen only apply to discharges > 10, 000 population equivalents (PE)

discharg-ing to surface waters classed as sensitive (e.g those subject to eutrophication).

Parameter Minimum concentration Minimum percentage reduction

Strict completion dates have been set by the Commission for the provision

of minimum treatment for waste waters entering freshwater, estuaries andcoastal waters For example, full secondary treatment (Sec 2.1) includingnutrient removal for all discharges to sensitive waters with a population

equivalent (PE) >10,000 must be completed by the end of 1998 By 31 December 2005 all waste waters from population centres <2,000 PE dis- charged to freshwaters, and <10,000 PE to coastal waters must have suf-

ficient treatment to allow receiving waters to meet environmental qualitystandards, while populations centres larger than these require secondarytreatment (Fig 1.2) The Directive also requires significant changes in thedisposal of sewage sludge including:

(i) That sludge arising from waste water treatment shall be reused ever possible and that disposal routes shall minimise adverse effects

when-on the envirwhen-onment

(ii) Competent authorities shall ensure that before 31 December 1998, thedisposal of sludge from waste water treatment plants is subject togeneral rules (i.e Codes of Practice) or legislation

(iii) The disposal of sludge to surface waters by dumping from ships ordischarge from pipelines or other means shall be phased out by 31December 1998

(iv) That the total amount of toxic, persistent or bioaccumable material insewage sludge is progressively reduced

This wide scoping legislation is considered in more detail in Chap 8.The disposal options for sewage sludge are further limited if it contains

Fig 1.2 The implementation of the EUUrban Wastewater Directive, with dates for compliance by Member States.

metals or listed substances which may categorise it as a hazardous wasteunder the EU Directive on Hazardous Waste (91/689/EEC)

Industrial effluents have in the past been a major cause of pollution.The discharge of industrial effluents is generally governed by two objec-tives: (1) the protection of environmental water quality, and (2) the need

to protect sewers and wastewater treatment plants (Table 1.7) To meetthese objectives, discharge standards are required that are a compromisebetween what is needed to protect and improve the environment and thedemands of industrial development Most industrialists accept that the ap-

plication of the best practical technology (i.e effluent treatment using the

best of current technology to meet local environmental requirements atthe lowest financial cost) is a reasonable way to comply with the effluentdischarge standards set However, where discharges contain dangerous ortoxic pollutants which need to be minimised, then the application of the

best available technology is required (i.e effluent treatment using the best

of current technology to minimise local environmental change, especiallythe accumulation of toxic materials, where financial implications are sec-ondary considerations) Where effluent standards are necessary that areeven unobtainable using the best available technology, then of course in-dustries can no longer continue at that location

Table 1.7 Typical effluent standards for discharges to sewers (Gledhill 1986).

pH 6 to 10 Protection of sewer and sewage works fabric

from corrosion.

Suspended solids 200–400 mg l−1 Protection from sewer blockages and extra

load on sludge disposal system.

BOD 5 No general limit Local authorities would be concerned with

large loads on small sewage works and ancing of flows may be required in order not to overload treatment units.

bal-Oils/fats/grease 100 mg l−1 Prevention of fouling of working equipment

and safety of men Soluble fats, etc can be allowed at ambient temperature.

Inflammables,

hy-drocarbons, etc.

Prohibited Prevention of hazards from vapours in sewers.

Temperature 43◦C Various reasons — promotes corrosion,

in-creases solubility of other pollutants, etc.

Toxic metals 10 mg l−1 Prevention of treatment inhibition The

solu-ble metal is more toxic and different als can be troublesome Total loads with a limit on soluble metals more realistic.

met-Sulphate 500–1000 mg l−1 Protection of sewer from sulphate corrosion.

Cyanides 0–1 mg l−1 Prevention of treatment inhibition Much

higher levels can also cause hazardous working conditions due to HCN gas accu- mulation in sewer.

The integrated pollution prevention and control (IPPC) Directive(96/61/EEC) was adopted in September 1996 Integrated pollutionprevention and control is a major advance in pollution control in that alldischarges and environmental effects to water, air and land are considered,

together with the Best Practicable Environmental Option (BPEO) selected

for disposal In this way, pollution problems are solved rather than ferred from one part of the environment to another In the past, licensing

trans-of one environmental media (i.e air, water or land) created an incentive torelease emissions to another Integrated pollution prevention and controlalso minimises the risk of emissions crossing over into other environmentalmedia after discharge (e.g acid rain, landfill leachate) There is only one li-cence issued under IPPC covering all aspects of gaseous, liquid, solid wasteand noise emissions, so that the operator only has to make one application

as well as ensuring consistency between conditions attached to the licence inrelation to the different environmental media In Europe, IPPC applies tothe most complex and polluting industries and substances (e.g large chem-ical works, power stations, etc.) In England and Wales, the Environment

Agency issues guidance for such processes to ensure that the BPEO is ried out The aim of IPPC is to minimise the release of listed substances

car-and to render substances that are released harmless using Best Available Techniques Not Entailing Excessive Cost (BATNEEC) The objective of the

guidance notes is to identify the types of techniques that will be used bythe Agency to define BATNEEC for a particular process The BATNEECidentified is then used as a base for setting emission limit values (ELVs).Unlike previous practice in the identification of BATNEEC, emphasis isplaced on pollution prevention techniques such as cleaner technologies andwaste minimisation rather than end-of-pipe treatment Other factors forimproving emission quality include in-plant changes, raw material substi-tution, process recycling, improved material handling and storage practices.Apart from the installation of equipment and new operational procedures

to reduce emissions, BATNEEC also necessitates the adoption of an going programme of environmental management and control which shouldfocus on continuing improvements aimed at prevention, elimination andprogressive reduction of emissions

on-The selection of BATNEEC for a particular process takes into account(i) the current state of technical knowledge, (ii) the requirements of environ-mental protection, and (iii) the application of measures for these purposeswhich do not entail excessive costs, having regard to the risk of significantenvironmental pollution For existing facilities, the Agency considers (i) thenature, extent and effect of the emissions concerned, (ii) the nature and age

of the existing facilities connected with the activity and the period duringwhich the facilities are likely to be used or to continue in operation, and(iii) the costs, which would be incurred in improving or replacing these ex-isting facilities in relation to the economic situation of the industrial sector

of the process considered Thus, while BATNEEC guidelines are the sis for setting licence emission standards, other factors such as site-specificenvironmental and technical data as well as plant financial data are alsotaken into account In Ireland, similar IPPC licensing procedures are op-erated by the Environmental Protection Agency (EPA 1994), and like theEnvironment Agency in England and Wales, public registers of all licencesare maintained

ba-The introduction of the polluter which pays charging system out Europe and the USA is an attempt to achieve such environmentalobjectives, at least in terms of the cost to the community, by reinforcingthe philosophy that the polluter is responsible for all aspects of pollutioncontrol in relation to its own effluent (Deering and Gray 1987) Two distinct

through-types of charges exist: effluent charges are levied by local authorities for

discharges directly to surface waters, whereas user charges are levied for the

use of the authority’s collective treatment system (Table 1.16) By ing industry for treating their effluents in terms of strength and volume, itencourages them to optimise production efficiency by reducing the volumeand strength of their effluent Most important of all, such charging systemsensure that effluent disposal and treatment costs are taken into account bymanufacturers in the overall production costs, so that the cost of the finalproduct reflects the true cost of production (Deering and Gray 1986)

charg-Wastewater treatment is not solely a physical phenomena controlled byengineers, it also involves a complex series of biochemical reactions involv-ing a wide range of micro-organisms The same micro-organisms that occurnaturally in rivers and streams are utilised, under controlled conditions, torapidly oxidise the organic matter in wastewater to innocuous end productsthat can be safely discharged to surface waters Compared with other indus-tries which also use micro-organisms, such as brewing or baking, wastewa-ter treatment is by far the largest industrial use of micro-organisms usingspecially constructed reactors As treatment plants that were constructedduring the early expansion of wastewater treatment in the late nineteenthand early twentieth centuries now near the end of their useful lives, it isclear that the opportunities for the biotechnologists to apply new technolo-gies, such as genetic manipulation combined with new reactor designs, topollution control are enormous (Chap 10) In the future, cheaper, more ef-ficient, and more compact processes will be developed, with the traditionalaims of removing organic matter and pathogens to prevent water pollu-tion and protect public health replaced with a philosophy of environmentalprotection linked with conservation of resources and by-product recovery(Chap 11)

Natural scientists, whether they are trained as microbiologists, chemists, biologists, biotechnologists, environmental scientists or any otherallied discipline, have an important role in all aspects of public health en-gineering They already have a significant function in the operation andmonitoring of treatment plants, but their expertise is also needed in theoptimisation of existing plants and in the design of the next generation ofwastewater treatment systems

Although there has been a steady increase in the discharge of toxic organic and organic materials, it is still the biodegradable organic wastes

in-that are the major cause of pollution of receiving waters in Britain andIreland (Gray and Hunter 1985; DETR 1998; Environment Agency 1998,1999; EPA 2000) Organic waste originates from domestic and commercialpremises as sewage, from urban runoff, various industrial processes and agri-cultural wastes Not all industrial wastes have a high organic content that

is amenable to biological treatment, and those with a low organic content,insufficient nutrients, and which contain toxic compounds, require specificchemical treatment, such as neutralisation, chemical precipitation, chemi-cal coagulation, reverse osmosis, ion-exchange, or adsorption onto activatedcarbon (Table 1.8) (Casey 1997)

This book concentrates on non-toxic wastewaters It is these that are

of particular interest to the biologist and biotechnologist in terms ofreuse, conversion, and recovery of useful constituent materials Primar-ily sewage containing pathogenic micro-organisms is considered, althoughother wastewaters, such as agricultural wastes from intensive animal rearingand silage production, food processing wastes, and dairy industry wastesare also briefly reviewed

The absolute minimum quantity of wastewater produced per person (percapita), without any excess water, is 4 litres per day At this concentration,the wastewater has a dry solids content in excess of 10% However, in mostcommunities that have an adequate water supply this minimum quantity isgreatly increased In those countries where technology and an almost un-limited water supply has led to the widescale adoption of water-consumingdevices — many of which are now considered to be standard, if not basic,human requirements — the volume of wastewater produced has increased

by a factor of 100 or more Flush toilets, baths, showers, automatic washingmachines, dishwashers and waste disposal units all produce vast quantities

of diluted dirty (grey) water with a very low solids content and all requiringtreatment before being discharged to surface waters For example, a flush

toilet dilutes small volumes of waste matter (< 1 litre) to between 10 or

30 litres each time it is used Domestic sewage is diluted so much that it isessentially 99.9% water with a dry solids content of less than 0.1% Con-ventional sewage treatment aims to convert the solids into a manageablesludge (2% dry solids) while leaving only a small proportion in the finaleffluent (0.003% dry solids)

The total volume of wastewater produced per capita depends on thewater usage, the type of sewerage system used and the level of infiltration

Table 1.8 Main chemical and biological unit processes employed in wastewater treatment.

Chemical unit processes

Neutralisation Non-neutral waste waters are mixed either with an alkali (e.g.

NaOH) or an acid (e.g H 2 SO 4 ) to bring the pH as close to tral as possible to protect treatment processes Widely used in chemical, pharmaceutical and tanning industries

neu-Precipitation Dissolved inorganic components can be removed by adding an acid

or alkali, or by changing the temperature, by precipitation as a solid The precipitate can be removed by sedimentation, flotation

or any other solids removal process Ion-exchange Removal of dissolved inorganic ions by exchange with another ion

attached to a resin column For example Ca and Mg ions can replace Na ions in a resin, thereby reducing the hardness of the water

Oxidation reduction Inorganic and organic materials in industrial process waters can be

made less toxic or less volatile by subtracting or adding electrons between reactant (e.g aromatic hydrocarbons, cyanides, etc.)

Biological unit processes

Activated sludge Liquid waste water is aerated to allow micro-organisms to utilise

organic polluting matter (95% reduction) The microbial biomass and treated effluent are separated by sedimentation with a portion

of the biomass (sludge) returned to the aeration tank to seed the incoming waste water

Biological filtration Waste water is distributed over a bed of inert medium on which

micro-organisms develop and utilise the organic matter present Aeration occurs through natural ventilation and the solids are not returned to the filter

Stabilisation ponds Large lagoons where waste water is stored for long periods to

al-low a wide range of micro-organisms to break down organic ter Many different types and designs of ponds including aerated, non-aerated and anaerobic ponds Some designs rely on algae to provide oxygen for bacterial breakdown of organic matter Sludge

mat-is not returned Anaerobic digestion Used for high strength organic effluents (e.g pharmaceutical, food

and drink industries) Waste water is stored in a sealed tank which excludes oxygen Anaerobic bacteria breakdown organic matter into methane, carbon dioxide and organic acids Final effluent still requires further treatment as has a high BOD Also used for the stabilisation of sewage sludge at a concentration of 2–7% solids

The volume of wastewater varies from country to country depending onits standard of living and the availability of water supplies (Table 1.9).Generally, the volume and strength of the sewage discharged in a particularcountry can be predicted fairly accurately For example, the mean dailyvolume of wastewater, excluding industrial waste but including infiltration,

Table 1.9 Specific water consumption in Europe (IWSA 1995).

Household and Industry small businesses and others Total

2 UK values not available in this format.

produced per capita in England is 180 l d−1, compared 230 l d−1in Irelandand 250 l d−1 in Scotland The equivalent volume of sewage produced inthe USA is on average 300 l per capita per day (100 US gallons d−1).The amount of wastewater produced per capita can be estimated quiteaccurately from the specific water consumption

The variation in volume depends on a number of variables includingthe amount of infiltration water entering the sewer The higher volume ofwastewater produced in Scotland is primarily due to the widescale use of

a larger flushing cistern, 13.6 l compared with 9.0 l in England and Wales,although other factors also contribute to this variation Guidelines fromthe Department of the Environment in England and Wales stipulate thatall new cisterns manufactured after 1993 should have a maximum flushingvolume of 7.5 l However, the reliance of water closets which function on asiphon rather than a valve to release water restricts the minimum opera-tional volume to between 4–5 l (Pearse 1987) The Building Research Es-tablishment (1987) highlights the potential water saving from the adoption

of new cistern designs and suggests the need for new British Standards

Comparative studies were carried out using a ‘standard turd’, which is a

43 mm diameter ball of non-absorbent material with a relative density of1.08, and with a cohesive shear strength, coefficient of friction, and adhesiveproperties very close to the real thing

In rural areas, where water is drawn from boreholes or from small munity water schemes, water may be at a premium, so the necessary con-servation of supplies results in reduced volumes of stronger sewage Occa-sionally, the water pressure from such rural supplies is too low to operateautomatic washing machines or dishwashers and results in an overall reduc-tion in water usage and subsequent wastewater discharge

com-In the home, wastewater comes from three main sources Approximately

a third of the volume comes from the toilet, a third from personal washingvia the wash basin, bath, and shower, and a third from other sources such

as washing up, laundry, food and drink preparation (Tables 1.10 and 1.11).Outside the home, the strength and volume of wastewater produced percapita per day will fluctuate according to source, and this variation must

be taken into account when designing a new treatment plant For example,the flow per capita can vary from 50 l d−1 at a camping site to 300 l

d−1 at a luxury hotel (Table 1.11) More detailed tables of the volume ofwastewater produced from non-industrial sources, including the strength

of such wastewater, are given by Hammer (1999) and also by Metcalf andEddy (1991)

The diluted nature of wastewater has led to the development of thepresent system of treatment found in nearly all the technically-developedcountries, which is based on treating large volumes of weak wastewater

In less developed communities, the high solids concentration of the waste

Table 1.10 Comparison of the percentage consumption of water for various

pur-poses in a home with an office; indicating the source and make-up of wastewater

from these types of premises (Mann 1979).

Home (sources) consumed (%) Office (sources) consumed (%)

WC flushing

Washing/bathing

35 25

WC flushing Urinal flushing

43 20

63

Food preparation/drinking 15 Washing 27

Canteen use 9

Car washing/garden use 5 a

a May not be disposed to sewer.

Table 1.11 Daily volume of wastewater produced per capita from various

non-industrial sources (Mann 1979).

Volume of sewage

Restaurants (toilet and kitchen wastes per customer) 30–40

Camping site with limited sanitary facilities 80–120

Day schools with meals service 50–60

Boarding schools: term time 150–200

Table 1.12 Comparison of the concentration of various compounds reported in urban runoff with precipitation, strictly surface runoff from roads and with combined sewer overflow (Pope 1980) All units are in mg l−1unless specified Those marked with† are

in mg kg−1and‡ in kg curb km −1.

Reported concentration range (mg l−1)

Parameter Precipitation Road/street Urban Combined

runoff runoff sewer overflow

Total dissolved solids — 66–33050 9–574 —

Conductance (µmho cm −1) 8–395 10000 5.5–20000 —

Total organic carbon 1–18 5.3–49 14–120 —

Table 1.12. (Continued )

Reported concentration range (mg l−1)

Parameter Precipitation Road/street Urban Combined

runoff runoff sewer overflow

makes it difficult to move to central collection and treatment sites, while themore diluted wastewater flows easily through pipes, and can be transportedeasily and efficiently via a network of sewers to a central treatment works.

In isolated areas or underdeveloped countries, human waste is normallytreated on-site, due to its smaller volume and less fluid properties (Feachemand Cairncross 1993; Mara 1996)

The collection and transport of sewage to the treatment plant is via anetwork of sewers Two main types of sewerage systems are used, combinedand separate Combined sewerage systems are common in most towns inBritain Surface drainage from roads, paved areas, and roofs are collected

in the same sewer as the foul wastewater and piped to the treatment works.This leads to fluctuations in both the volume and the strength of sewagedue to rainfall, and although the treatment works is designed to treat up tothree times the dry weather flow of wastewater (DWF), problems arise if therainfall is either heavy or continuous During such periods, the wastewaterbecomes relatively diluted and the volume too great to be dealt with bythe treatment works Excess flow is, therefore, either directly discharged to

a watercourse as storm water or stored at the treatment works in stormwater tanks The stored wastewater can be circulated back to the start ofthe treatment works once capacity is available However, once the tanksbecome full, and then the settled wastewater passes into the river withoutfurther treatment where the watercourse, already swollen with rainwater,can easily assimilate the diluted wastewater because of the extra dilutionnow available

A separate sewerage system overcomes the problem of fluctuations insewage strength and volume due to rain, by collecting and transportingonly the foul wastewater to the treatment works, and surface drainage isdischarged to the nearest water course Such systems are common in newtowns in Britain and are mandatory in Canada and the USA This type

of sewerage system allows more efficient and economic treatment works to

be designed as the variation in the volume and strength of the wastewater

is much smaller and can be more accurately predicted A major drawbackwith separate systems is that the surface drainage water often becomespolluted All stormwater is contaminated to some degree because of contactduring the drainage cycle: it passes over paved areas along roadside gullies

to enter the sewer via a drain with a gully pot, which catches and removessolids that might otherwise cause a blockage in the sewer pipe (Bartlett1981) The quality of urban runoff is extremely variable and biochemicaloxygen demand (BOD) values have been recorded in excess of 7,500 mg

l−1 (Mason 1991; Lee and Bang 2000) It is the first flush of storm water

that is particularly polluting as it displaces the anaerobic wastewater, rich

in bacteria, that has been standing in the gully pots of the roadside drainssince the last storm (Butler and Memon 1999) The runoff from roads isrich in grit, suspended solids, hydrocarbons including polycyclic aromatichydrocarbons (Krein and Schorer 2000), heavy metals, pesticides such asthe herbicide atrazine (Appel and Hudak 2001), and, during the winter,chloride from road-salting operations Surprisingly, it also contains organicmatter, not only in the form of plant debris such as leaves and twigs, butalso dog faeces (Table 1.12) It has been estimated that up to 17 g m−2

y−1 of dog faeces are deposited onto urban paved areas and that the dog

Table 1.13 Chemical characteristics of treated effluents from three UK

sewage treatment plants.

Source Constituent a Stevenage Letchworth Redbridge

Anionic (as Manoxol OT) 2.5 0.75 1.4

Coliform bacteria (no./ml) 1300 3500

a Results are given in mg l−1, unless otherwise indicated.

b Absorptiometric turbidy units