manual sewage treatment

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MANUAL ONSEWERAGE AND SEWAGETREATMENT(FIRST EDITION)

PreparedbyTHE EXPERr COMMITTEEConstitutedbyTHE GOVERNMENT OF INDIA

CENTRAL PUBLIC HEALTHAND ENVIRONMENTAL ENGINEERING ORGANISATION

M I N I S T R Y O F W O R K S A N D H O U S I N G ,N E W D E L H I

1 9 8 0

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M A N U A L O NS E W E R A G E A N D S E W A G ETREATMENT

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CENTRAL PUBLIC HEALTHANDENVIRONMENTAL ENGINEERING ORGANISATION

MINISTRY OF WORKS AND HOUSINGNEW DELHI1980


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ioftfasfa ark 3WIH,M inister ofW orks and H ous ing ,India

New Delhi, August 18, 1980Q-16011 /2 /79 -CPHEEO

F O R E W O R DSanitation though m otivated primarily for meeting the ends of preventivehealth has come to be recognised as a way of life. In this context, developm entof the sanitation infra-structure of any cou ntry could p ossibly serve as a sen sitiveindex of its level of prosperity. It is needless to emph asise that for attaining thegoals of good sanitation, sewerage and sewage treatment is a necessary concom i

tant. While provision of potable drinking water takes precedence in the orderof provision of environmen tal engineering services, the importance of sewerageand sewag e treatment canno t be lost sight of and can not be allowed to lag behind,as all the water used by the community has to flow back a s sewage loaded withall the wastes of comm unity living. Unless properly co llected, treated and disposed of, this wou ld create serious water pollution prob lems. More prosperouscomm unities have attempted to look a t water supply a nd sewerage as an integralwho le but such an outlook has not been possible in this country, primarilybecause of the paucity of resources.2. Nevertheless, there has been a steady effort in the recent years in ourcountry to provide sewerage and sewage treatment facilities to cover at least the

major town s particularly in the context of growing population. To facilitate thepractising engineer in the work of adequ ate design, implementation and man agement of these facilities, a man ual is absolutely necessary.3 . It is gratifying to note that the Expert Com mittee appo inted by theGovernm ent for preparing a Ma nual on this subject have b een able to bring outthis useful volume. Its usefulness has been all the more enhan ced since thisMan ual has been discussed at length and reviewed critically at a Wo rkshop ofChief Engineers and Senior Engineers, specially convened for this purpose. I amhapp y that this volume is being brought out at an approp riate time since the nationis becom ing more and more conscious of protection of the environmen t and amajor thrust is being given by the Governmen t towards meeting this end as evidenced by the keen interest evinced by our beloved Prime Minister on this subject.We are on the threshold o f the International Decad e for Drinking Wa ter Supp lyand Sanitation and I am sure that this Ma nual wou ld meet the professional needsof all the Public Health Engineers and Scientists in this field to discharge their taskthat would be demanded of them in the Decade.

w ? r t a T O ^

( P . C . SE T HI)


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PREFACESince the inception of the National Water Supply and Sanitation Programmeand the streamlining of the Public Health Engineering Departments inthe Slatesto keep pace withthedevelopmentsin this field of specialisation, there

has been a consistent demand from theEngineersin theprofession for a Manualon Sewerage and Sewage treatment. The Manual on Water Supply and Treatmenthadnecessarily to precede this Manual becauseof the greater emphasisandresource allocations that were given to community water supplies. With theincreasing importance bestowed to the sanitation aspect under theprogramm einrecent years, the need for a Manual on the subject has been keenly felt. Toachieve this objective, a Committee was set up by the Government of India inthe then Union Ministry of Health and Family Planning in August, 1971,withthe following composition:1. ShriJ. M.Dave, ChairmanAdviser (PHE),MinistryofHealth& FP,(Deptt.ofH ealth),N;wD elhi.2. Prof. S. J.Arceivala, MemberDirector,Central Public Health EngineeringResearch Institute,Nehru Marg, Nagpur.3 . Prof. N. Majumdar,Prof, ofSanitary Engineering,All India Institute of Hygieneand Public Health, Calcutta-12.4. Shri A.C.Chaturvedi, ,,Superintending Engineer,L.S.G.E.D., Govt ,of

Uttar Pradesh, Lucknow.

1. Adviser (PH EE ), ChairmanCentral Public Health&Environmental Engineering Organisation,Ministryof Works and Housing,New Delhi.2. Dr. T. R. Bhaskaran, MemberTechnical Director,Geo-Miller&Co.,New Delhi.

3 . ShriA. C. Chaturvedi, Chief Engineer, LSGED(UP),Lucknow;presently, Director,Department of Ecology,Lucknow.4 . ShriD. R.Singal,Chief Engineer, P.W.D.,Public Health Branch, Patiala;presently, Chairman,

Punjab State Board for Prevention & Control of Water Pollution,Patiala.

5. ShriV. D.Desai, MemberCity Engineer,Bombay Municipal Corpn.6. ShriD. R.Singal,Chief Engineer, P.W.D.,Public Health Branch, Punjab,Patiala.

7. ShriS.Chatterjee, Chief Engineer,Calcutta Metropolitan PlanningOrganisation, Calcutta.8. ShriT.D urairaj, MemberDeputy Adviser (PHE), SecretaryMinistryofHealth& FP,

New Delhi.

5. Dr. R. H. Siddique, MemberScientist, N ational Env. Engg.Research Institute, Nagpur,presently, Associate Prof, ofCivil Engineering, University ofPetroleum & Chemicals,Saudi Arabia.6. Shri S. Chatterjee, Chief Engineer (Retd.),Calcutta Metropolitan PlanningOrganisation, Calcutta.7. Shri Ajitha Simha,Director (CED), IndianStandards Institution,New Delhi.8. ShriV. D.Desai,Special Commissioner (Engg.)Municipal Corporation, Bombay.9. ShriT. Durairaj,Deputy Adviser (PHE) (Retd.),

presently, Consultant,Shah Construction Company,Madras.

The Committee was reconstituted by the Union Ministry of Works andHousing (towhich the subject sanitation was transferred) in June, 1973,withthefollowing:


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(iv)10. Prof. S. Subba Rao , Member 11. Shri B. B. Rau, Member-Mi India Institu te of Hygiene Deputy Adviser (PH E), Secretaryand Public Hea lth Ministry of Works and Hou sing,Calcutta. New Delh i.

Seven meetings were held under the Chairmanship of Shri J. M. Dave andeight meetings were held when Shri B. B. Rau officiated as Adviser (PHEE). ShriT. S. Swamy took over as Adviser (PHE E) on 26-3-74 and the m aterial was reviewed for finalisation. The Manual has been finalised after Dr. Nilay Chau-dhuri took over the charge of Adviser (PHEE) in December, 1977.

In the preparation of this Manual the Committee has made extensive useof all the available literature on the subject and wishes to place on record itsgratitude to the sources. The Com mittee thanks the Ministries of Health &Fam ily Welfare and W orks & Ho using, Gov ernm ent of India, for affording allfacilities. The Com mittee wishes to place on record its deep appreciat ion of allthe assistance provided and arrangemen ts made by the All India Institute ofHygiene and Public Health, Indian Standards Institution and National Environmen tal Engineering Research Inst itute for the meetings held outside Delhi. Special thanks are due to Sarvashri V. Raman, Head, Sewage Division, NationalEnvironmental Engineering Research Institute, Nagpur; A. Raman, ScientistIncharge, D elhi Zonal Centre, Natio nal Env ironmental Engineering ResearchInstitute; Dr. R. N. Chakravarthy, Managing Director, Universal EnvirosciencePvt. Ltd ., D elh i; Dr. A. M. M ichael, Dr. K. V. Paliwal and Dr. S. L. Pandey ofthe Water Technology Centre. Indian Agriculture Research Institute, Delhi; andDr. S. S. Ram aswamy , Deputy Director Gene ral, Central Labou r Institu te, Bombay, for their valuable contributions.

The Com mittee expresses its app reciation to Shri B. B. Rau for his untiringefforts in making possible the completion of the Manual in spite of his arduousnorm al duties. Special mention is made of the services of Shri M. R. Par tha -sarathy, Asstt. Adviser (PHE) and Dr. I. Radhakrishnan, Scientific Officer, whounstintingly devoted their time even after office hours in all phases of this work.The valuable contributions of Asstt. Advisers (PHE), Shri M. M. Datta, Dr. S. R.Shukla and Shri J. D . Sheth (until he rejoined his pare nt departm ent) are gratefully acknowledged. Th e Comm ittee also desires to record their appreciation ofthe services rendered by the different officers and staff members of the DrawingSection and Secretariat of the Central Pub lic Health & Environmental EngineeringOrganisation.The type-script of the Manual was circulated to all the State Public HealthChief Engineers and Institutions imparting public health engineering training toelicit their comm ents and views. Th e contents of the Manu al were again discussed in greater detail, topic by topic, at a Special Workshop of all the Chiefand Senior Public Health Engineers of the States convened at Bhubaneswar(Orissa) between 17th and 19th February, 1979, under the Chairmanship of ShriS. T. K hare, the then Adviser (PHEE), M inistry of Works & Housing, New D elhi.The Seminar was also attended by special invitees from the Educational and Research Inst itut ions . Valuable suggestions tha t emerged during the discussionshave been incorporated to make this Manual useful from the practising Engineers'point of view.


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(v)The following attended the deli

1. Shri N. Ramachandra Rao ,Chief Engineer (Rural Water Supply),Panshayati Raj, Andhra Pradesh. j2. Shri S.K. Shah ,Chief Engineer (Public Health),Gujarat.

3 . Shri A.T . Upadhyay,Superintending Engineer (Public Health),Gujarat.4. Shri L. M. Chaudhuri,Engineer-in-ChiefPublic Health Branch, Haryana.5. Shri A. N . Mehendale,Chief Engineer,Haryana Urabn DevelopmentAuthority.6. Shri Gu rdia Singh,Superintending Engineer,Public Health,Haryana.7. Shri I. D. Mirchan dani,Chief Engineer,Public Works Department,Himachal Pradesh.8. Shri B. Subb aiah,Chief Engineer,Karnataka.9. Shri D.R. Jagannatha Rao,Chief Engineer,Public Health Engineering Deptt.Madhya Pradesh.

10. Shri W. D. Bhide,Addl. Chief Engineer,Urban Development andPublic Health D eptt. andMember Secretary,Water Research Management Board,Maharashtra.11. Shri G.F . Kham bati,Chief Engineer, Bombay,Municipal Corporation, Bombay.12. Shri Y. V. Dam le,Deputy Chief Engineer (Sewerage)Planning and Design, Municipal

Corporation of Greater Bombay.13 . Shri B .K Singh,Chief Engineer,Public Health Engineering,Manipur, Imphal.14. Shri P. Arunachalam ,Chief Public Health Engineer,Meghalaya.15. Shri Akangam eron Ao ,Superintending Engineer,Nagaland.16. Shri D. N . Singh Deo,Chief Engineer,

Public Health, Orissa.

ations of the Workshop :17. Shri M. S. Hyder,Superintending Engineer,Public Health, Orissa.18. Shri A. K. Patnaik,

Superintending Engineer,Public Health, Orissa.19. Shri N . Mishra,Superintending Engineer,Public Hea lth, Orissa.20. Shri G. N . Patra,Superintending Engineer,Public Health, Orissa.21 . Shri R. N . Pattn aik,Superintending Engineer,Public Health, Orissa.22. Shri H. S. Man ocha,Man aging Di reel or,

Punjab Water Supply andSewerage Board.23 . Shri M. S. Sandhu,Chief Engineer,Punjab Water Supply andSewerage Board.24. Shri H. S. Puri,Chief Engineer (Rural Water Supply),Punjab Public HealthDeptt.25. Shri R. Krishnaswamy, Chief Engineer,Tamilnadu Water supply andDrainage B oard.26 . Shri George Mathew,Project Engineer,Tamilnadu Water supply andDrainage Board.27 . Shri V. Varad arajan,Engineering Director, MadrasMetropolitan Water Supply andSewerage Board.28. Shri K. N . Dwivedi,Managing Director,U. P. Jal Nigam, Lucknow.29. Shri B. P. Varma,General Manager,Uttar Pradesh Jal Nigam.30. Shri P. K. Chatterjee,Director, Water Supplyand Environmental Hygiene,Calcutta Metropolitan DevelopmentAuthority.31. Shri S.K . Das Gupta ,Director, Sewerage and Drainage,Calcutta Metropolitan DevelopmentAuthority.32. Shri A. K. Poddar,Chief Engineer,Public Health Engineering Directorate,

West Bengal.


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(vi)33. Shri Satyabrata Kundu ,Superintending Engineer,Public Health Engineering Directorate,West Bengal.34. ShriS. K. Roy,Chief Engineer,IrrigationandFlood Control Deptt. ,Tripura.35. Shri Mallinalh Jain,Chief Engineer (Water),Dilhi Municipal Corportation.36. ShriJ. Deivasigamany,Director,Public Works Department,Pondicherry-1.

37. ShriD. Guin,Executive Engineer,PanchayatandRural Works Deptt. ,Sikkim.38. Prof. S.SubbaRao,Professor of Sanitary Engineering,All India InstituteofHygieneandPublic Health, Calcutta.39. Prof. S. V. Patwardhan,Prof, of Environmental Engineering,UniversityofRoorkee.40. ShriV.Raman,Head,Sewage Treatment Division,National Environmental EngineeringResearch Institute,Nagpur.


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CONTENTS

INTRODUCTION PAGEXIXCHAPTER PART ISEWERAGE1 SURVEY AND INVESTIGATION . . . ' . .1.1 BASIC INFO RMA TION1.1.1 Physical Aspects1.1.2 Developm ental Aspects1.1.3 Fiscal Aspects1.1.4 Othe r Aspects1.2 PROJECT SURVEYS1.2.1 Prelimina ry Project Surveys1.2.2 Detailed Project Surveys1.3 CONS TRUC TION SURVEYS .1.3.1 Preliminary Layou ts1.3.2 Setting Line and Gra de1.4 INVESTIGATIONS2 PROJECT PREPARATION2.1 PROJECT REPORTS2.1.1 Preliminary Project Re port2.1.2 Detailed Project Report2.1.2.1 Capital cost "2.1.2.2 Recurring costs2.1.3 Plans3 DESIGN O F SEW ERS . . . .3.1 ' ESTIMATE OF SANITARY SEWAGE3.1.1 Design Period3 .1.2 Population Estim ate . . . .3.1.3 Area3.1.4 Per capita Sewage Flow3.1.5 Storm Runoff3.1.6 Ground Water Infiltration .3.2 ESTIMATE OF STORM RUN OFF3 .2.1 Ra ti onal Method . . . . .3.2.1.1 Runoffrainfall intensity relations hip3 .2 .1.2 Storm frequency . . . .3.2.1.3 Intensity of precipitation3.2.1.4 Time of concentration3.2 .1.5 Co-efficient of runoff(a) Imperviousness(b) Tributary area(c) Duration of storm .(d) Computation of runoff coefficient3.3 SEWER DESIGN .3.3.1 Flow Assumptions3.3.2 Self Cleansing Velocity3.3 .3 Velocity at Minimum Flow3.3 .4 Erosion and Maximum Velocity3.3 .5 Minimum Size3.3.6 Hydraulic Formu lae .

222222233344447777778

99999910101010101011


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6 STRUCTURAL DESIGN OF BURIED SEWE RS 236.1 TYPES OF LOADS 236.2 LOADS ON COND UITS DUE TO BACKFILL 236.2.1 Types of Installation or Construction Conditions 236.2.2 Load s for Different Con dition s 256.2.2.1 Trench conditions 25(a) Load producing forces 25(b) Com putation of loads 25(c) Influence of width of trench 256.2.2.2 Emban kmen t or projecting conduit condition 26(i) Positive projecting conduit 26(a) Load producing forces 26(b) Com putat ion of loads 28(ii) Negative projecting conduit 29(a) Compu tation of loads 29(iii) Imperfect trench conduits 316.2.2.3 Tunnel condition 32

(a) Load producing forces 32(b) Load computations 326.2.2.4 Effect of submergence 326.3 LOAD ON CONDU IT DUE TO SUPERIMPOSED LOADS 326.3.1 Concentrated Load 326.3.2 Distribute d Load 346.3.3 Cond uits Under Railway Track 356.4 SUPPORTING STRENG TH OF RIG ID CON DUIT 356.4.1 Labo ratory Test Strength 356.4.2 Field Supporting Strength 356.4.3 Supporting Strength in Trench Conditions 35(a) Classes of bedding 35(b) Load factors 356.4.4 Supporting Strength in Emban kment Conditions 37(a) Classes of bedding 37Class A 37Class B 37Class C 37Class D 37(b) Load factors 38(i) Positive projecting conduits 39(ii) Negative projecting conduits 39(iii) Imperfect trench conditions 396.4.5 Conduits Under Simultaneous Internal Pressure and External Loading 396.5 RELATIONSHIP BETWEEN THE DIFFER ENT ELEMENTS IN STRUCTURA L DESIGN 396.6 SAMPLE CALC ULATIO NS 396.7 RECOM MENDATIONS 417 CONSTRUCTION OF SEWERS 427.1 CONSTRUCTION METHODS 427.1.1 Trench 427.1.1.1 Dimensions 427.1.1.2 Excavation 427.1.1.3 Shoring 427.1.1.4 Undergrou nd services 427.1.1.5 Dewatering 427.1.1.6 Foundation 437.1.2 Tunnelling 437.1.2.1 Shafts 437.1.2.2 Ventilation 43


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.1.2.3 Construction7.1.2.4 Methods . . . .(a) Auger or boring' .(b) Jacking(c) Mining7.1.3 Laying of Pipe Sewers7.1.3.1 Stoneware pipes7.1.3.2 R.C.C. pipes .7.1.3.3 Cast-in-situ concrete sections7.1.3.4 Construction of brick Sewers7.1.3.5 Cast iron pipes7.1.4 Jointing of Sewers7.1.4.1 Stoneware pipes7.1.4.2 Concrete pipes .7.1.4.3 C.I. Pipes7.1.5 Hydraulic Testing of Pipe Sewers7.1.5.1 Water test7.1.5.2 Air testing7.1.6 Check for Obstruction7.1.7 Construction of Manholes .7.1.8 Sewer Connections7.1.9 Backfilling of the trenches .7.1.10 Removal of sheeting .8 MAINTENANCE OF SEWERAGE SYSTEM8.1 PROVISIONS IN DESIGN .8.2 SEWER MAINTENANCE8.2.1 Problems8.2.1.1 Clogging of sewers . . . .8.2.1.2 Hazards . . . . .8.2.2 Precautions . .8.2.2.1 Precautions against gas hazards .8.2.2.2 Precautions against infections,8.2.3 Safety Equipment8.2.3.1 Gas masks . . . . .8.2.3.2 Oxygen breathing apparatus-(a) Air hose respirator

(b) Pure oxygen respirator .8.2.3.3 Portable lighting equipment8.2.3.4 Nonsparking tools8.2.3.5 Portable air blowers . . . .8.2.3.6 Safety belt '8.2.3.7 Inhalators8.2.4 Sewer Cleaning Equipment and Devices8.2.4.1 Portable pump set8.2.4.2 Sectional sewer rods . . .8.2.4.3 Flexible sewer rod . . . .8.2.4.4 Ferret used in conjunction with a fire hose8.2.4.5 Sewer cleaning bucket machine . 8.2.4.6 Dredger8.2.4.7 Rodding machine with flexible sewer rods8.2.4.8 Scraper8.2.4.9 Automatic flushing tanks8.2.4.10 Hydraulically propelled devices .,

(a) Flush bags(b) Sewer balls . . . .(c) Sewer scooter

8.2.5 ProceduresPreventive maintenance


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8.2.5.2 Corrective maintenan ce 538.2.6 Chemical Treatm ent 549 SEWAGE ANDS T O RM W A T ER P UM P I NG S T A T I ONS 559.1 GENER AL CONSIDER ATIONS 559.1.1 Location 559.1.2 Capacity 559.1.3 Types of pump ing stations 559.2 PUMP HOUSE STRUCT URE 559.3 WET WELL DESIGN 559.4 PUMPS 569.4.1 Capacity 569.4.2 Protection against Clogging 569.4.3 Types 569.4.3.1 Centrifugal pum ps 56

(a) Axial flow pum ps 56(b) Mixed flow pum ps 56(c) Rad ial flow or centrifugal pum ps 569.4.3 .2 Pneum atic ejectors 579.4.4 Head of Pumping 589.4.4.1 Standard terms 58(a) Datum 58(b) Suction lift 58(c) Suction head 58(d) Tot al discharge head 58(e) Total head 58(f) Net positive suction head (NPS H) 589.4.4.2 Compu tation of total head and selection of head 589.4.5 Miscellaneous Con siderations 589 .5 PRIM E M OVERS 599 .6 ELECTRICAL EQU IPME NT 599.6.1 Switch G ears 599 .6.2 Motor StartingEquipment 599.6.3 Cables 599.7 CONTROLS 599.8 FLOW MEASU RING DEVICES 599 .9 FUNCTIONAL REQUIREM ENTS 599.9.1 Ventilation 59, 9.9.2 Safety Measures 599.9.3 Other Facilities 60

PART IISEWAGE TREATMENT10 BASIC DESIGN CONSIDERA TIONS 6310.1 DEGREE OF TREATM ENT 6310.2 PERIOD OF DESIGN 6310.3 POPULATION SERVED 6310.4 SEWAGE FLOW S 6310.4.1 Population Equivalent 6310.5 SEWAGE CHA RAC TERISTICS 6410.5.1 Temperature 6410.5.2 Hydrogen Ion Concentration 64jO.5.3 Colour and Odour 6410.5.4 Solids 6410.5.5 Nitrogen 64


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10.5.6 Phosphorus . . . . . . 6410.5.7 Chlor ides 6410.5.8 Biochemical Oxygen Demand 6410.5.9 Chemical Oxygen Deman d 6510.5.10 Toxic Met als and Compounds 6510.6 EFF ECT OF INDU STRIAL WASTES 6510.7 DUM PING CHUTES FOR NIGHTSOIL 6510.8 EFF LU EN T DISPOSAL AND UTILISATION 6510.9 CHO ICE OF PROCESSES 6511 PRETREATMENTSCREENING AND GRIT REMOVAL 6711.1 SCR EFN ING 6711.1.1 Coarse Screens 6711.1.2 Medium Screens 6711.1.3 Fine Screens 6711.1.4 Com minuting Devices 6811.1.5 Locat ion of Screens 6811.1.6 Housing of Screens 6811.1.7 Hydraulics 6811.1.8 Velocity 6811.1.9 Head Loss 6811.1.10 Quan tity of Screenings 6811.1.11 Dispo sal of Screenings 6811.2 GR IT REM OVAL 6811.2.1 Com position of Grit 6911.2.2 Types 6911.2.2.1 Mechanically cleaned grit chambers 6911.2.2.2 Manually cleaned grit chambers 6911.2.3 Aerated Grit Chambers 6911.2.4 Design Dat a 6911.2.5 Design of Grit Chamb er 6911.2.5.1 Settling velocity or hydraulic subsidence value 6911.2.5 .2 Overflow rate 7011.2.5.3 Detention period 7011.2.5.4 Botto m Scour and flow through velocity 7011.2.5.5 Velocity contro l devices 71(a) Prop ortion al flow weir 71(b) Sutro weir 71(c) Parshall flume 7211.2.5.6 Num ber of units 7311.2.5.7 Dimensions of each unit 7311.2.6 Loss of Head 7311.2.7 Disposal of Grit 7312 SEDIMEN TATION AND CHEM ICAL TREATMENT 7412.1 CHAR ACTER ISTICS OF SETTLEABLE SOLIDS 7412.2 DESIG N CONS IDERAT ION 7412.2.1 Facto rs Influencing Design 7412.2.2 Design Criteria 7412.2 .2.1 Overflow rate or surface loading rate 7412.2.2.2 Deten tion period 7412.2.2.3 Weir loading 7412.2.2.4 Depth 7512.2.2 .5 Slugde removal 7512.2.2.6 Inlets and outlet s 7612.2.2.7 Scum removal 7612.2.2.8 Types and shapes 76


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(xiii)PAOB12.3 PERFORMANCE 7 7

12.4 CHEMICAL PRECIPITATION 7 712.4 .1 Chemicals Used 7 712.4.1 .1 Iron salts 7 712.4.1.2 Aluminium salts 7 712.4.1.3 Lime and sodium carbonate 7712.4.2 Unit operations 7 712.4.2 .1 Mixing '_ 7 712.4.2.2 Flocculation 7 812.4.2.3 Sedimentation 7 813 ACTIVATED SLUDGE PROC ESS 7 913.1 PROCESS MECH ANISM 7 913.2 PROCESS VARIABLES 7913.2.1 Loading Rat e 7 913.2.2 Mixing Regime 7 913.2.3 Flow Scheme g13.3 CONVENTIONAL SYSTEM AND MODIFICATIONS 8 013.3.1 Conventional System 8 013.3.2 Tapered Aeration 813.3.3 Step Aeration ' g 013.3.4 Cont act Stabilisation 8 Q13.3.5 Com plete Mix 8213.3.6 Modified Aera tion 8 213.3.7 Extended Aeration g213.4 DESIGN CONSIDERA TION 8 313.4.1 Aeration Tank 8 ,13.4.2 Oxygen Requirements g ,13.4.3 Aeration Facili ties g>13.4.3 .1 Diffused air aerat ion g 413.4.3.2 Mechanical aerators g*13.4.3.3 Mixing requirements 8 513 4.4 Measuring Devices g -13.4.5 Secondary Settling g 513.4.6 Sludge Recycle g 513.4.7 Excess Sludge Wasting g ,13 .5 DESIGN OF OXIDATION DITCH g 613.6 NITRIFICATION g 613.7 OPERATION g 713.8 ROTATING BIOLOGICAL CONTACTOR g 714 TRICKLING FILTERS g g14.1 TYPES OF FILTE RS g g14.2 PROCESS DESIG N 8 914.2.1 Rankin's Equations 8 914.2.2 NR C Equations 9 114.2.3 Other Equations 9 114.2.3 .1 Velz equat ion 9 114.2.3 .2 Eckenfelder equat ion 9 ,14.2.3.3 Galler and Gotaas equation 9214.2.4 Applicability of the Different Equat ions 9214.3 CONSTRUCTIONAL FEATURES 9 214.3.1 Shape of Filter 9 214.3.2 Provision for Filter Flood ing 9214.3.3 Fil ter Walls 9 214.3.4 Fil ter Floor 9214.3.5 Und erdrainage System 92


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14.3.614.3.714.3.814.3.8.114.3 .914.3.1014.3.1114.3.1214.3.1314.4IS15.115.215.2.115.2.215.2.315.315.3.115.3.215.3.315.3.415.415.4.115.4.215.4.315.4.415.4.515.4.615.4.715.4.815.4.915.4.1015.4.1115.4.1215.515.615.715.81616.116.216.2.116.2.216.2.316.2.416.2.516.316.3.116.3.216.3.316.3.416.3.516.3.616.3.716.3.8

Main Collecting Chan nel .VentilationFilter MediaPlastic mediaFilter DosingFlow Distribution .Multiple UnitsPlant HydraulicsPumping Arrangements

OPERATIONAL PROBLEMSSTABILISATION POND S .CLASSIFICATIONMECHANISM OF PURIFICATION

Diurnal Variation . . . .Odour ControlAlgae

DESIGN CONSIDERATIONSSurface Area .Deten tion Period .DepthSludge AccumulationCONSTRUCTION DETAILS

Site SelectionPretreatmentConstruction in StagesMultiple UnitsPond ShapeBalanced Cut ting . . . .EmbankmentPond BottomInfluent LinesPond OutletsPond InterconnectionsOther A spects

OPERATION AND MAINTENANCE .PERFO RM ANC E . . . .APPLICA TIONS . . . .AERATED LAGOONS . . . .SLUDGE DIGEST ION .ANAEROBIC DIGESTION .DIGESTER CAPACITY

Volume of Sludge . . . .Temperature .Storage for Digested SludgeConventional DigestionHigh Rate Digestion . . . .

DESIGN OF DIGESTER ELEMEN TS .Number of Units . . . .Tank Shape and SizeWater Depth and Free BoardBottom CharacteristicsRoofingDigester Control RoomMixing of Digester ContentsPiping

929393939394949495959696969696969797979797979798989898989898989999999999100100101101101101102103103104104104104104105105105105105

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(Xv)PAGE16.3.9 Sampling Sinks and Valves 10616.3.1 0 Liquid Level Indicator 10 616.3.11 G as Collection 10616.3.12 G as H older 107

16.3.13 Performance of Digesters 10716 .4 AEROBIC DIGE STION 1 0 71 7 S L U DG E T H I CK E N I N G , DE W AT E R I N G AN D DI S PO S AL 1 0 81 7.1 S L U D G E T H I C K E N I N G 1 0 817.1.1 G ravity T hickening I 0 817.1.2 Air Floatation 10g17.1.3 Centrifugatiou 10g1 7.2 S L U DG E DE W AT E R I N G . i 0 917.2.1 Sludge Drying Beds IO917.2.1.1 Design criteria 1 0 9(a) Area of beds . . . . . ^ IO9

(b) Bed Specifications 1 0 9(i) G ravel ,' 1 0 9(i i) Sand J 0 9(iii) underdrains 1 Q 9(iv) W alls . ' . ' . ' 109(v) Dimensions j 0 9(vi) N umber of beds 109ivii) Sludge inlet 109(viii) Drainage 1 0 9(c) Preparation of beds 109(d) W ithdrawal o f sludge H 0(e) Depth of sludge H 0(f) R emoval of sludge cake H 0(g) H auling and storage of sludge cakes JJQ17 2.2 M echanical M ethods U 017.2.2.1 Sludge cond itio ning 1 1 0(i) Chemical conditioning 110(a) Elutriation U Q(ii) H eat treatment (Porteus process) U l17.2.2.2 Equipment H I(a) Vacuum filters m(b) Filterpresses j U(c) Centrifugation U j17.2.3 H eat Drying H17.2.4 Incineration j j21 7.3 S L U DG E DI S PO S AL . n 217.3.1 Sludge as Fertiliser 1 1 217.3.2 Sludge L agooning j 1 217.3.3 Dispo sal in W ater or Sea j j 21 8 S L U D G E P U M P IN G 1 1 31 8.1 S L U D G E P U M P S , , 318.2 APPLICATION OF PUM PS n 318.2.1 Pumps for Primary Sludge 1 1 318 .2.2 Pumps for Seco ndary Sludge U 313 2.3 Pumps for R ecirculation and Transfer of Sludge 1 1 313 .2.4 Pumps for E lutriation of Sludge U 318.2.5 Pumps for T hickened and Concentrated Sludge 1 1 418.2.6 Scum Pumping 1141 8.3 S T AN DBY U N IT S R E QU IR E D . . . . . . . . . . . . m18 .4 S LU DG E CON VE YIN G PIPIN G . . . , 1 41 8 .5 P U M P APP U R T E N AN CE S 1 1 418 .5.1 Air Chamber H 42-^180 M . o f W & H / N D / 79


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(xvi)PAOB18 .5 .2 Revo lut i on Counter . . . . . . . . . 11418.5.3 G land Seals 11418 .5.4 Valves 11 418.5.5 G auges . 114

18 .5.6 S ampling Devices . . . . > . . . , 11418.5.7 W ashouts and Drains 11518.5 .8 T imeClocks 11518 .5.9 M easuring Devices 1151 8 .6 P U M P D R I V E E Q U I P M E N T H 51 9 C H L O R I N A T I O N O P S E W A G E . 1 1 619 .1 PURPOSE OF CHL ORIN ATION H 619 .2 DOSAG E n 61 9 .3 C H L O R I N B C O N T AC T C H A M B E R S H 61 9 .4 C H L OR I N E S T O R A G E , H A N D L I N G A N D F B E D I N G . . H 62 0 E F FL U E N T D I S P O S A L A N D U T I L I S A T I O N 1 1 720 .1 DISPOSAL ON LA N D 11720 .2 DISPOSAL BY DILU TION , 1 720.2.1 Basic Inform ation J J 720.2.2 Standards . . H 7

2 0 .3 R E C L A M A T I O N O F W A S T E W A T E R S l l 720.4 PISCICU LT UR E . . . . . 1 1 72 0 .5 A R T I FI CI A L R E C H A R G E O F A Q U I F E R S 1 1 72 1 S E W A G E F AR M I N G 1 1 92 i. i W AT ER QU AL IT Y CON SIDE RAT ION S FOR IR R IG AT ION W ATE RS . . . . 11921.1.1 Osmotic Effects U


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(xvii)PAOB23.1.3 Sedimentation Tanks j 2 g23.1.3.1 Sludge 12923.1 .3.2 Skimmings . 2o23.1.3 .3 Structures .5 9

23.1.3.4 R ecords 2g23.1.4 Aeration Tanks ]2o23.1.4.1 Sewage flow I 2 923.1.4.2 Air supply j , Q23.1.4.3 M ixed liquor suspended solids I 3 Q23.1.4.4 R eturn Sludge * 1 3 023.1.4.5 Foaming 13023.1.4.6 M icroscopic examination j3g23.1.4.7 Records 1 3 o23 .1.5 Trickling Filters ' i 3 o23.1.5.1 Distributors I323.1.5.2 Ponding 1 3 123.1.5.3 U nderdrains J 3 J23.1.5.4 Odour 13123.1.5.5 Filter flies l 3 123.1.5.6 Records 1 3 123.1.6 Sludge Digestio n Tanks j 3 123.1.6.1 Digester operation j 3 J23.1.6.2 Records 1 3 223.1.7 Sludge Drying Beds j 3 223.1.8 Stabilization Ponds 1 3 223.1.8.1 Operation j 3 223J.8 .2 Records 1 3 32 3 .2 B U I L D IN G S A N D E Q U I PM E N T l 3 323.2.1 Building and Other Structures j 3 323.2.2 Equipment 1 3 32 3 .3 S A FE T Y I N T HE P L A NT l 3 42 3 .4 T RA I NI NG O F P E RS O NN E L , 3 423 .5 RE CORDING AN D REPORTING 13423 .6 CHECKLIST 1 3 424 PLANT CONTROL LABORATORY l 3 524.1 PLANN ING OF LABORATORY FACILITIES 1 3 524.1.1 Physical Facilities 1 3 524.1.1.1 Size of the laboratory l 3 524.1.1.2 L ocatio n 13524.1.1.3 Floor space I3 524.1.1.4 W alls 1 3 524.1.1.5 Lighting 1 3 524.1.1.6 Pow ersupply , i 3 524.1.1.7 Floo r . 1 3 524.1.1.8 W ork tables and benches l 3 524.1.1.9 Reagent Cabinets and cupboards 1 3 624.1.1.10 Sinks 1 3 624.1.1.11 Fume hoo ds and chambers j 3 624.1.1.12 G as supply l 3 624.1 .1 .13 Balanceroom I 3 624.1.1.14 Constant temperature roo m j 3 624.1.1.15 Sample preparation room j 3 6

24.1.1.16 M edia preparation and steril isation rooms 1 3 ,24.1.1.17 R ecord rooms j 3 g24.1 .2 E quipment and Chemicals , , . . . , , j 3 6


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(xviii)PAGE24.2 SAMPL ING OF SEWAGE AND WASTEWATER 137

24.2.1 Me thod s of Sam pling 13724.2.1.1 Gra bsam ples 13724.2.1.2 Com posite or iotegra ted samp les 13724.2.2 Samp le Volum e 13724.2.3 Selection of Samp ling Points 13724.3 TESTS PER FORM ED IN THE LABORATORY 13824.3.1 Raw Sewage 13824.3.2 Sedimentation Tanks 13824.3.3 Trickling Filters - 13824.3.4 Activated Sludge Aeration Ta nks . 13824.3.5 Seconda ry Settling Tan ks 13824.3.6 Septic Tan ks, Imhoff Tanks and Claridigesters 13824.3.7 Sludge Digest er 13824.3.8 Stabilization Pond s 13824.3.9 Digester Gas 13824.4 RESIDUA L CHL ORIN E 13824.5 SPEC IAL TESTS 13824.6 ANALYSIS REPORTS I3924.7 PERSONNE L 13 9

APPENDICES1. Abbrev iations and Symbo ls 1432 . Conversion Factors 1443 . List of Indian Stan dards relating to Sewerage and Sewage Treatment 1464 . Com putation of Storm Runoff and Design of Storm Sewers 1505. Mann ings Flow Chart Chart6. Design of Sanit ary Sewer System 1557. Thre e Edge Bearing Tests for pipe stren gth 1578. Characteristics of Com mon Gas es Causing Haz ards 1589. Equipm ent and Simple Tests for Det ection of Ga ses and Oxygen Deficiency 16010 . Sewer Cleaning Equipme nt and Devices 161

11 . Screens 16312 . Gri t Cham ber 16413 .Secondary Sedimentation Tank I6714 . Activated Sludge Plan t 16915 . Design of Trickling Filte rs 17016 . WasteS tabil ization Pond 17717 . Oxidation Ditch 18118 .Sludge Digestion 18319 .Sludge Thick ening 1852 0 .Sludge Drying Beds 1862 1 . Septic Ta nk 18722. Soil Perc olation Tests 19023 . Operation Troubles in Sewage Treatm ent Plants 1912 4 . Min iraim Equipme nt Needed for Tests 19325 . Tests 19426. Geora j tric Elements for Circular Chan nel Sections 19527.Bibliography 196


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I N TR ODU CTI ONWith the march of civilization and the growing socialconsciousness of the communities in the country, there has been anincessant demand for doing away with the existing dry latrines andreplacing the same with sanitary water seal latrines. Further, otherliquid wastes generated by the communities are now carried in opendrains leading to severe environmental problems. To dispose of theseliquid wastes in a proper and sanitary manner, it would be necessaryto collect them in a system of sewers.The sewage thus collected has perforce to be disposed of ina body of water, like stream, river, lake or sea. However, prope rcare has to be ensured that it does not interfere with the other usesof these water bodies, like water supply, recreation, pisciculture etc.Pollution caused by sewage is perhaps more important from the

point of view of the health of the community as it contains a numberof causative agents for many dangerous diseases, which can easilyspread through the water supply systems.The concern for crying a halt to the menace of pollution ofour water environment before it reaches unmanageable proportionshas already culminated in the enactment of the Central legislation"Water (Prevention and Cont rol of Pollution) Act, 1974". Themost logical solution to arrest the deterioration of the environmentlies in the provision of appropriate and adequate treatment facilitiesfor the sewage.The desirability of viewing water supply and sewerage asan integrated whole is quite obvious in Public Health Engineeringpractice. There has been a long felt need for forging uniformityin planning, design and construction of sewerage and sewagetreatment facilities. With this objective in view, this Manu al onSewerage and Sewage treatment lays emphasis on the variouspertinent aspects for execution of a comprehensive sewage collectionand treatment scheme to serve the interest of the practising PublicHealth Engineer.This Manual has considered the recent technical advancesand trends in the field of sewerage and sewage treatment and hasincorporated them as appropriate to Indian conditions.As adopted in the Manual on Water Supply and Treatment,all units of measurements, operational parameters and designcriteria have been furnished in the metric system. A table ofconversion factors has been included in th e appendices for easyverification wherever needed. Other appendices contain usefulinformation which will serve as a guide for solving the problemsencountered in the field.

(xix)


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PART ISEWERAGE


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CHAPTER 1S U R VEY AN D I N VES TI G ATI ON

Survey and investigation are prerequisitesb3 th for the framing of the preliminary repo rtand the preparat ion of a detailed sewerageproject. The engineering and policy decisions takenare dependent on the correctness of the dat acollected and its proper evaluation.1.1 BASIC INFORMATIONFor an effective investigation, a broad knowledge of the problems likely to be faced duringthe various phases of implementation of the project is essential. Information on physical, developmenta l, fiscal and other aspects has to becollected.1.1.1 Physical AspectsThese would necessitate the collection ofinformation relating to :(i) topography or elevation differences neededfor design of sewers and location ofoutfall and disposal wo rks;(ii) subsoil condition s, such as types ofstrata likely to be encountered, depthof ground w ater table and its fluctuation s.In the absence of any records, dat ashould be collected by pu ttin g at least3 trial bores or tria l pits per hecta re;(iii) underground structures like storm drainsand appurtenances, city survey stones and

utility services like house connections forwater supply and sewerage, electric andtelephone cables and gas lines; and(iv) location of streets and adjoining areaslikely to be merged or annexed.Possible sources of inform ation are existingmaps and plans showing streets from revenueor town surveys or Survey of India maps. Othersources are topograph ical maps from Surveyof India If available with existing spot levels,aerial photographs and photographs of complexsurfaces for supplementing the existing instrumen tal surveys by concerned autho rities likeMun icipalities and Road Departm ents.

1.1.2 Developmental AspectsThe following considerations should betaken into account :(i) types of land use, such as commercial,industrial, residential and recreational;

(ii) density of pop ulation , trends of population growth and demographic studies;(iii) type a nd number of ind ustries for de termining quan tity and nature of wastesand location of their discharge points;(iv) existing draina ge and sewerage faci

lities and data relating to them;(v) flow in ex istingsewersand sewers of similar areas to assess the flow characteristics;

(vi) historical and socio-economical d at a; and(vii) basis of design and information onthe maintenance of existing sewers.Possible sources of in form ation a re censusrecords, town and metrop olitan master plansregional planning records, land use plans, flowgauging records, stream flow records, meteorological da ta, industrial survey records and report s of the Water pollution prevention Boards.1.1.3 Fiscal AspectsThe various factors that will have an important bearing are :(i) existing policies or commitments of

obliga tions which may affect the finan cing of the projectj(ii) outstanding loan amounts and instalments of repayments;(iii) availability of Central and State loans,grants-in-aid, loans from other financingbodies such as Life Insurance Corp oratio n, Industrial Development Corporation , International Bank for R econstruction and Development and other Banks;(iv) present water rates and sewer-tax and revenue realised from them, size of propertyplots and land holdings, the economic

condition of the community with respect to their tax-paying capacity; and(v) factors affecting the cost of con struction,operation and maintenance.

Some of the information can be obtainedfrom the records relating to M unicipal and StateTax Levies, Acts and Rules governing loans,procedures for financing projects and registersand records of the auth orities, maintaining watersupply and sewerage systems.1.1.4 Other Aspects

The conside rations that are likely to influenceare: (i) changes in political boundaries by physicalacquisition or merger of adjacent communities o r by possible extension of lim its;(ii) feasibility of mu lti-regional or m ulti-mun icipal systems ;(iii) prevailing water pollution preventionstat utes , other rules and regulationsrelating to discharge of ind ustrial anddom estic wastes ;(iv) present status of the governmental,semi-governmental or municipal authority sponsoring the project, its cap acity ,adequacy and effectiveness ; and thedesirab ility of its modification or necessity of a new organ isation t o satisfactorilyimplement and maintain the project; and


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(v) the inconveniences likely t o be causedto the community during executionand the feasibility of minimising themby suitable alignment or location of thecomponents of the system.Possible sources of information are NationalActs, State and Municipal Laws and Byelaws,minutes of the past meetings of the municipalor other governing bodies and discussions withofficials, municipal councillors and other localleaders.

1.2 PROJECT SURVEYS1.2.1 Preliminary Project Surveys

This is concerned with the broad aspectsof the project. Dat a on aspects such as capacityrequired , basic arrangement and size, physicalfeatures affecting general layout and design,availability of effluent disposal facilities, probablecost and possible methods of financing shall becollected to prepare an engineering report describing the scope and the cost of the project withreasonable accuracy. In framing such estim ates ,due consideration must be given to the escalation of prices of basic materials and theiravailability. While extreme precision and detailsare not required in this phase, all the basic dataobtained must be reliable.1.2.2 Detailed Project Surveys

Surveys for this phase form the basis forthe engineering design as well as for the preparation of plans and specifications for incorporation in the detailed project report. In contrastto preliminary survey, this survey must be preciseand contain all the details that will facilitate thedesigner to prepare design and construction planssuiting the field conditions. It should include,inter-alia, net work of bench marks and traversesurveys to identify the nature as well as extentof the existing underground structures requiringdisplacement, negotiation or clearance. Suchdetailed surveys are necessary to establish rightsof way, minimise utility relocation costs, obt ainbetter bids and prevent changing and reroutingof lines.1.3 CONSTR UCTION SURVEYSAll control points such as base lines andbench marks for sewer alignment and grade shouldbe established by the engineer along the routeof the proposed construction. All these pointsshould be referenced adequately to permanen tobjects.1 .3 .1 Preliminary layouts

Before starting the work, rights-of-way, workareas, clearing limits and pavement cuts shouldbe laid out clearly to ensure tha t the work proceedssmoothly.. Approach roads, detours, bypassesand protective fencing should also be laid outand constructed prior to undertaking sewerconstruction w ork. All layout w ork must be completed and checked before construc tion begins.

1.3.2 Setting^Hne and GradeThe transfer of line and grade from controlpoint s, established by the engineer, to the construc tion work should be the responsibility ofthe executing agency till work is completed.The methods generally used for setting theline and grade of the sewers are discussed in7 . 1 . 3 .The procedure for establishing line andgrade where tunnels are to be employed in sewersystems are discussed in 7.1.2.3.

1.4 INVESTIGATIONSInvestigations may take many forms butgenerally are directed towards determining themost feasible and practical method of achievinga desired result. On small sewer projects, theymay involve not more than a n on the spot decisionto use conventional minimum standards for asimple gravity extension of an existing system.Larger projects, on the other hand, may haveseveral alternatives, all of which must be considered. Projects envisaging relief to existingsystems demand extensive studies for decidingthe design capacity and the most appropriatesolution.Proper investigation is a prerequisite forresolving, inter alia, the following :(i) extent of area to be served, pat tern ofpresent and future land use, zonal plansof the area with reference to regionalsewer plan;(ii) general arrangement of the system neededand easements required for this arrangement j/ (iii) prop ortion of combined flow to beintercepted for treatm ent from an existing combined system and possibilityof reducing the combined flow;(iv) estimated present and future flows;(v) storm frequency or pattern to be adoptedfor storm sewer design;(vi) multiplicity of discharge and treatmentpoin ts for the whole project j

(vii) requirements of other agencies (nation al,state or local highway departments andrailways) particularly with referenceto specific locations for crossing, right s-of-way, installation and details of ma terials of constructions;(viii) deep gravity sewers along circuitousrou tes versus shallow sewers withlarge number of lift stations;(ix) alternative materials to be used forsewer construction;(x) cost of construction and operation ofthe project and ways and means offinancing it liketaxes,levies and debentures j and(xi) necessity of establishing anewauthority, such as an autonomous board.


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CHAPTER 2PROJECT PREPARATION

Preparat ion of sewerage projects is normallydone bythe State public health engineering aut ho rity on behalf of the local bodies exceptingthemetropolitan towns and large industrial undertakingsthat have separate design organisations for thepurpose. The sewerage project needs approvalfrom the competent government authority eitherat the State or Central level before it is taken upfor execution. It is essential, for efficient andspeedy execution, that the project is preparedin detail after proper investigation and includesthe technical, financial and administrative aspectsgiving due consideration to economy, accuracyand soundness.

An accurate estimate of both capital andrunning costs of the project is particularly necessary as the sewerage projects are not directlyrevenue yielding and a re normally fundedby loans. At all stages of the preparation of theproject and its scrutiny , it Is very essential th attechnical and financial implications of possiblealternatives are borne In min d. This will entailexamination of alternative routes for sewers, thechoice of suitable mat erials, different diametersof conduits and their gradient s. The choicemay be between open cut (with or witho ut shoring) and tunnelling or deep gravity sewersand pumping. The design engineer should keephimself abreast of uptod ate costs of materialsand construction so that the alternatives canbe compared both fromthetechnical and economicconsiderations.2.1 PROJECT REPORTS

The project repor ts are prepared fn twostages, the preliminary and t he detailed ones.The former is unde rtake n when the local bodyconcerned decides to provide sewerage and sewagetreatment facilities, either as a new system oras an improvement to an existing system andmakes the necessary auth orisat ion to the publichealth engineering authority through the appropriat e channels. After the appro priate autho rityhas scrutinised and selected the best alternativefrom the preliminary report , the public healthengineering autho rity Is directed to prepare thedetailed project repo rt.2.1.1 Preliminary Project Report

This report, based on the survey and investigations referred to In Cha pter 1, should containbriefly :

(a) a historical retrospect leading to thejustification of the project;(b) description of existing facilities, ifany;(c) population studies, analysis and predictio n based on a critical appralsa]of available demographic data for atleast five preceding decades;(d) location of the water supply head-works for this area and the neighbouringcommunities;(e) availability of piped water supply inthe area, present and anticipated percapita water supply ;(f) total sewage flowpresent and prospective;(g) discussion of different possible disposalpoints and their comparative merits;(h) review of the different possible methodsof treatment;(j) engineering features and economicalaspects of the sewer system, an indexand a general layout plan with contours

at intervals of 2m alongwith a schematicdiagram;(k) basis for computation of surface runoff,drainage area and siting of storm sewageoverflow, wherever necessary;(1) number and location of pumping stationswith static and frictional heads on pum ps,pump duties and velocities in risingmains;

(m) a comparison of total costs of the alternativesinvolvedboth capital and maintenance, under major subheads;(n) the most satisfactory alternative for theproject as recommended;(p) probable stages of construction, procurement of proprietary materials for theproject and any special problems relatingthereto; and(q) fiscal aspects of the problem includingthe annual maintenance charges togetherwith the annuities on the capital loanwithin the assumed period of repayment,financial commitments to the community,repaying potential and methods of raisingthe capital for the project.


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42.1.2 Detailed Project Report

This report is drawn up after a detailedfield survey and investigationiscarried out (1.2.2and 1.4). In addition to the data included in thepreliminary project report, this would containdetailed zonalplanswith contours of 1mintervals,detailed design calculations and working drawingsfor various structures and other components,detailed hydraulic calculations for the sewers,longitudinal sections of all peripheral, main andtrunk sewers and layout plans for the differenttreatment units.2.1.2.1 Capital cost

The detailed estimates of capital costs wouldinclude the following:a detailed bill of quantities for the differ

ent components of the project; an abstractestim ate of cost showing priced schedule ofquantities with inclusive rates for the severalitems under each component; anda general abst ract of estimate for theentire project based on the individual estimates, based on an itemised schedule of costsdealing with each componen t of theproject, viz.,

(i) cost of sewerage system, zonewise;(ii) cost of pumping stations and pumpingequipments;

(iii) cost of pum ping m ains ;(iv) cost of treatment w orks, unitwise;(v) cost of power lines and telephone facilitiesto be laid;(vi) cost of approach roads;

(vii) cost of land acqu isition;(viii) provision for special tools and plan tand all ancillary items and equipmentscontingent on the proper executionof the project;(ix) contingencies and unforeseen w orksand(x) centage charges,

2.1.2.2 Recurring costsThe estimated cost of the annua l maintenanceof the project should also be worked out separatelyto include the cost of necessary technical andnon-technical staff for the ope ration and main

tenance, energy charges for the running of pum p-sets and other machinery, the cost of spares,consumable stores and replacements and thecost of chemicals envisaged in the treatment

of the sewage as also the an nuity on the capitalloans based on the assumed period of repaymentand the rate of interest. Con tribut ion towardsdepreciation of the plant and machinery shouldalso be indicate d.2.1.3 Plans

The following procedure is recommendedfor the nomenclature of sewers :The trun k sewer should be selected firstand drawn and other sewers should be considered as branches. The trunk sewer shouldbe the one with the largest dia that wouldextend farthest from the outfall works-Whenever two sewers meet at a point, themain sewer is the larger of the incomingsewers. The manholes of the trunk sewerare designated as 0,1,2, 3, etc., commencingat the lower end (outfall end) of the lineand finishing at the top end. Manholeon the mains or submalns are again numbered1, 2, 3, etc., prefixing the num ber of themanhole on trunk/m ain sewer where theyjoin (e.g. 3.2 represents the second manholeon the main sewer from the manhole no. 3onthe trun k sewer). W hen all the sewer linesconnected to the main line have thu s beencovered by giving distinctive numbers to themanholes, the manholes on the furtherbranches to the branch mains are similarlygivendistinctive numbers, again commencingwith the lower end. If there are two branches,one on each side meeting the main sewer orthe branch sewer, letter 'L ' (to represent left)or letter 'R ' (to represent right ) is againprefixed to the num bering system, reckoningagainst the direction of flow . If there ismore than one sewer either from the leftor right, they are suitably designated asLj, La, R15 R2, the subscript referring tothe line near to the sewer taking away thedischarge from the manhole.

Thus L2.R.4.2.3 (Figure 2-1) will pinpo inta particular manhole on the submain from whichthe flow reaches manhole number no. 4 on thetrunk sewer through a submain and a main.The first numeral (from the left) Is the numberof the manhole on the trunk sewer. The numeralson the right ofthis numeral, in order, representthe manhole numbers in the main, submain etc.respectively. The firs t letter immediately preceding the numeral denotes the main and that itis to the right of the trunk sewer. Letters to theleft in their order represent subm ain, branchrespectively. The same nomenclature Is used forrepresenting the sections e.g. Section L 2.R.4.2.3identifies the section between the manhole L a.R.4.2.3 and the adjoining downstream manhole.


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t * 4 t t

TRUNK SEWER

ifm39

R4.1

t2>

R 4.2

H.x R4.2.2

I, R4.2.1

(Ij SU B MAINo ooI . M . 2 . / I, R4.2.2 L, R 4.2.3R4.r

ftG2.f NOMENCLATUREOFSEWERS

119.00

IOO.OO260.0O 300.00 340.00

' 0 f 4 f

140.00 IO O .O O iso.oo*Q O SO 4Q 60

HOBOONTAt vemiCAL

FIG.2-2 A TYPICAL SEWER SECTION3480M. of W H/ND/79


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All longitudinal sections should be indicatedwith reference to the same datum line. The vertical scale of the longitudinal sections should bemagnified ten times the horizontal scale.Once the rough sections have been prepared ,the designer should go over the work for improving the spacing of manho les, the sizes andgradients of the sewers and so forth, economisingon materials and excavation to the extent possiblebut at the same time making sure that the sewerwill serve all users and that they can be actuallylaid according to the alignments shown in thedrawing and have sufficient gradients. Thesewers should have a minimum cover of 1m atthe start ing point or otherwise adequately protected with cement concrete encasing.The following scales may be adopted forthe various plans and drawings:

(a) Index plan . . . 1:100,000ov1:200,000(b) Keyplan & general layout 1:10,000or1:20,000plan.(c) Zonal plans . . 1:2,500or 1:5,000(d) Long itudinal sections of 1:600 or 1:1,250orsewers. 1:2,500(e) Struc tural drawings . . 1:20 or 1:50 or 1:100or 1:200.The sewers should be shown as thick linesand manholes as small circles in plan- In seetionthe sewer may be indicated by a line or two linesdepending upon the diameters and scales adopted.Grade , size and material of pipe, ground and

invert levels and extent of concrete protectionshould be indicated as shown in Figure 2.2.Standard vertical plan filing systems arenow available and are very convenient for storingof plans and taking them out quickly for reference. Normally, size A0 and Al (trimmed size841x1189 mm and 594x841 mm respectively)should be used while submitting the project drawings for approval.All documents including drawings, designcalculations, measurement sheets of estima tes,etc., should be in metric system. In draw ings,length should be indicated either entirely inmetres correct upto two decimals or entirely inmillimetres (for thickness etc.). If this practiceis followed, units would be obvious and in certain cases writing of m or mm with the figure canbe omitted. The flow should normally be indicated in litres per second (lps) or cubic metresper hour (m 3/hr) except for very largeflowswhichmay be indicated in cubic metres per second(cumec). For uniformity, lps for sewage flow sand cumec for storm flows is recommended.For all practical purposes one cubic metre maybe taken a s 1,000 litres. Similarly, areas in sewerplans and design calculations may be indicatedin hectares (ha ).While writing figures they should be groupedinto groups of three with a single space betweeneach group and without comma. In case of a

decimal number, this grouping may be on eitherside of the decimal (e.g. 47 342.294 31).


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CHAPTER 3DESIGN O F SEWERSSewerage systems can compose ofasystem of working out the population density. FSI orseparate sanitary sewers and storm sewers, a FAR is the ratio of total floor area (of all thesystem of combined sewers or a system cons isting floors) to the plot area. The densities of popula-of san itary sewers, part storm sewers and par t tion on this concept may be worked out as in thecom bined sewers. following example :Combined sewerage system invariably suffers Assume tha t a particular Development Planfrom the disadvant ages of sluggish flow du ring Rules prov ide for the following reservations formost part of the year leading to dep ositio n of different land uses,sewage solids and cre at ing foul and offensive Roads . . 20Vcon ditio ns. In view of this, the combined system Gardens 157Is normally not recommended in modern designs. schools(includingplaygrounds) '. '. 5%3.1 ESTIMATE OF SANITARY SEWAGE Markets 2%Sanita ry sewage is mostly the spent water of HospitalandDispensary . . . 2%

the community draining into th e sewer system withsome ground water and a fraction of the storm 44%run-off fromthearea,draining into it. The sewersshould be capable of receiving the maximum Area available for Rcsi- =56%discharge expected at the end of design period . (iaM 4) I J e v e l o p m c n tThe provision, however should n ot be much in A c t u a l t o t a l f l o o r a r e a = A r e a f o r r e s i d e n t i a I d e v e .excess of the actual discharge in the early years lopment x FSI.of its use to avoid deposition in sewers. The Assuming an FSI of 0-5estimate of flow, therefore, requires a very andfloorarea of 9 m*/careful consideration and is based upon the con tri- person,buto ry population and the per capita flow of Number of persons or 0- 56x 10000 x0- 5sewage, bo th the facto rs being guided by the density per hectare. = =311design period.3.1.1 Design Period 3 1 3 A r e aSince it is b) th difficult and uneconom ical to The tributary area for any section underaugment the capacity ofthesystem at a later date , consideration needs to be marked on a key plan,sewers are usually designed for the maximum The top og raphy , layout of buildings, legal limi-expected discharge to meet the requirements of tation s, etc., determine the tributa ry area drainingthe ultimate development of the area. Thu s, the to a sewer section. The area is to be measuredpopulation estimate is guided by the anticipated from the ma p.ultimate growth rates which may differ in thedifferent zones of the same tow n. 3.1.4 Per Capita SewageFlowA design period of 30 years for all type of Althou gh the entire spent water of a com -sewers is recommended. munity should contribute to the total flow in a3 1 2 Population Estimate sanitary sewer, it has been observed tha t a small' 'Th er e are several methods used for fore- portion is lost in evapo ration, seepage in ground ,casting the pop ulation of a comm unity. The leakage, etc. In some an d areas, the fractionS t suitable approach is to base the estimation reaching the sewers may be as low as 40% whileor on Floor space ma ex. may be expected to reach the sewers unless thereIn case the desired information on population j s d a t a available to the contrary,is no t available in the M aster Plan of the tow n, the , , . , , . . following densities are suggested for ado ption : The sewers should be designed for a minimumof 150 litres per capita per day.

Sizeof town (Popu.ation) Dcnsirj>aprf,ion Industries and comm ercial buildings often - use water other tha n from the municipal supplyUpto5,000 . . . . 75150 and m ore often d ischarge their liquid w astes i ntoM O K S O O O O ' " '. 250-300 th e sanitary sewers. In such cases, deviations50 000to l 60,000 . . . 300350 from th e general values may occur and est im ate sAbove 100 000 . . . 350-1000 of such flows have to be made separately. It is,

' ' however, desirable that when the industrial wasteIn cities where Floor Space Index (FSI) or is fairly large, it is segregated and disposed ofFloor Area Ratio (FAR) limits are fixed by the in a suitable mann er or treated suitably beforemunicipality, this approach may be used for discharge into sewers.


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83.1.5 Storm RunoffSanitary sewers are not expected to receivestorm water. Strict inspection and vigilance andproper design and construction of sewers andmanholes should eliminate this flow or bringit down to a very insignificant quantity.3.1.6 GroundW ater Infiltration

Estimate of flow in sanitary sewers mayinclude certain flow s due to infiltration of groundwater through joints. The quantity will dependon workmanship in laying of sewers and heightof the ground water tab le. Sincesewers are designed for peak discharges, allowance for groundwater infiltration for the worst condition in thearea should be made. Suggested estimates forground water infiltration for sewers laid belowground water table are as follows :Minimum Maximum

lpd/hectare . . . . 5,000 50,0001pd/km of sewer/cm dia . . 500 5,000lpd/m anhole . . . . 250 500

With improved standards of workmanshipand quality and availability of various constructionaids, these values should tend to the minimumrather than the maximum. These values shouldnot mean any relaxation on the water tightnesstest requirements in 7.1.5.3.2 ESTIMATE OF STORM RUNOFF

Storm runoff is tha t portion of the precipitation which drains over the ground surface. Estimation of such runoff reaching the storm sewerstherefore is dependent on intensity and durationof precipitation, characteristics of the tribu taryarea and the time required for such flow to reachthe sewer. The storm water flow for thispurpose may be determined by using therational method, hydrograph method, rainfall-runoff correlation studies, digital computermodels, inlet method or empirical formulae.The empirical formulae that are available forestimating the storm water runoff can be usedonly when comparable conditions to those for

which the equations are derived initially canbe assured.A rational approach, therefore, demands astudy of the existing precipitation data of thearea concerned to permit a suitable forecast.Stormsewersare not designed for the peak flow ofrare occurence such as once in 100 years o r morebut it is necessary to provide sufficient capacityto avoid too frequent a flooding of the drainagearea. There may be some flooding when theprecipitation exceeds the design value which hasto be permitted . The frequency of such permissibleflooding may vary from place to place, dependingon the importance of the area. Flooding at anytime, however, causes inconvenience to the citirzens but they m ay accept it once in a while considering the savings affected in storm drainage costs .

The maximum runoff which has to be carriedin a sewer section should be com puted for a condition when the entire basin draining at that pointbecomes contributory to the flow and the timeneeded for this is known as the time of concentration (tc) with reference to the concerned section.Thus for estimating the flow to be carried in thestorm sewer, the intensity of rainfall which lastsfor the period of time of concentration is theone to be considered contributing to the flow ofstorm w ater in the sewer. Ofthedifferent meth ods,the rational method is more commonly used.3.2.1 Rational Method3.2.1.1 Runoff-rainfall intensity relationship

The entire precipitation over, the drainagedistrict does not reach the sewer. The characteristics of the drainage district such as imperviousness,topography including depressions and water pockets, shape of the drainage basin and durationof the precipitation determine the fraction of thetotal precipitation which will reach the sewer.This fraction known as the coefficient of runoffneeds to be determined for each drainage district.The runoff reaching the sewer Is given by theexpression,

Q =1 0 CiA (31)where Q is the runoffin ms/hr;'C is the coefficient ofrunoff;*i' is the intensity of rainfall in mm/hr; and'A ' is the area of drainage district in hectares.3.2.1.2 Stormfrequency

The frequency of storm for which the sewersare to be designed depends on the importanceof the drainage area. Commercial and industrialareas should be subject to less frequent floodingthan the residential areas. In view of the presenteconomic conditions, the suggested frequency offlooding in the different aeas is as follows :(a) Residential areas

(i) Peripheral areas . . . twice a year;(ii) Central and comparatively . once a year,

high priced areas.(b) Comm ercial and high priced once in 2 years.areas.3.2.1.3 Intensity ofprecipitation

The intensity of rainfall decreases with duratio n Analysis of past records over a period ofyears of the observed data on intensity-duration ofrainfall in the area is necessary to arrive at a fairestimate of the intensity-duration for given frequencies. The longer the record available, themore dependable is the forecast.Table 3.1 gives the analysis of the frequency

ofstormsof stated intensities and durations during26 years for which rainfall data were availablefor a given town.


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9Table 3.1

DurationinM inutes51015203 040506090

30

8373341484

N o .of storms of stated intensity ormore for a period o f 26 years35 40 45 50 60 75 100 125 nun/hr

100 40 18 | 10 290 72 41 25 | 10 5 182 75 45 20 | 12 5 162 51 31 | 10 9 4 240 22 | 10 8 4 2161 8 4 2 1| 8 4 3 14 2 12The stepped line indicates the location of thestorm occurring once in 2 years, i.e. 13times in26 years. The time-intensity values for this frequency are obtained by interpolation and given

in Table 3.2 : Table 3.2i(mm/hr) t (min)30354045506075

51-6743-7536-4828-5718-5014-628 -12The relationship may be expressed by asuitable mathematical formula, several forms ofwhich are available. The following two equationsare commonly used :( i ) i =(ii) i =

[t n)t + b

(3 -2 )(3-3)

Wherei=intensity of rainfall (mm/hr);t=duration of storm (minutes); anda, b & n are constants.The available data on i and t are plotted onarithmetic paper (the second equation permits astraight line plot with the reciprocal of'i' plottedagainst t')- The values of the intensity, (i), canthen be determined for any given time of concentration, (Q.

3.2.1.4 Time of concentrationIt is the time required for the rain water toflow over the grou nd surface from the extremepoint of the drainage basin and reach the pointunder consideration. Time of concentration(t c) is equal to inlet lime (t ;) plus the time of flowin the sewer (tf). The inlet time is dependent on thedistance of the farthest point in the drainage

basin to the inlet manhole, the shape , characteristics and topography of the basin and may generallyvary from 5 to 30 minutes . In highly developedsections, the inlet tim e m ay be as low as 3 m inutes.

The time offlows determined by the length of thesewer and the velocity of flow in the sewer. It isto be computed for eachlengl hof sewer as it isdesigned.3.2.1.5 Coe fficient ofrunoffThe portion of rainfall which finds its wayto the sewerisdependent on th e imperviousness andthe shape of tributary area apart from the durationof storm.

(a) IMPERVIOUSNESSThe percent imperviousness of the drainagearea can be obtained from the records of a particular district. In the absence of such dat a, thefollowing may serve as a guide :Type of area Percentage o fimperviousness. 70 to 90

. 60 to 75. 35 to 6010 to 20

Comm ercial and Industrial areasResidential Area :(i) High density(ii) Lo w densityParks & undeveloped areasThe weighted average imperviousnessdrainage basin for the flow concentratinga point may be estimated as

A x-I i+A 2 - Ta+1 =

ofat

A 7+A+where,A j , A s drainage areas tributary to thesection under consideration;

=imperviousne ss of the respectiveareasf andI =w eigh ted average imperviousnessof th et ot al drainage basin(b) TRIBUTARY AREAFor each length of storm sewer, the drainagearea should be indicated clearly on the map andmeasured. The boundaries of each tributaryare dependent on topography, land use, nature ofdevelopment and shape of the drainage basinsThe incremental area may be indicated separatelyon the compilation sheet and the total area computed.(c ) DURATION OF STORMContinuously long light rain saturates thesoil and produces higher coefficient than that dueto heavy but intermittent rain in the same areabecause of the lesser saturation in the latter caseRunoff from an area is significantly influenced bythe saturation of the surface nearest the point ofconcentration rath er than theflowfrom the distantare a. Th e runoff coefficient ofalarger area has tobe adjusted by dividing the area into zones ofconcentration and by suitably decreasing the coefficient with the distance of the zones.(d ) COMPUTATION OF RU NOFF COEFFICIENTThe weighted average runoff coefficients forrectangular arsas of length four times the widthas well as for sector shaped areas with varyingpercentages of impervious surface for different


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10times of concentration are given in Table 3 .3 .Although these are applicable to particular shapesof area, they also apply in a general way to theareas which are usually encountered in practice.Errors due to difference in shape of drainage arewithin the limits of accuracy of the rational method and of the assumptions on which it is based.

A typical example of the computation ofstorm runoff's given in Appendix-4.3.3 SEWER DESIGNSewers while carrying the waste water discharge for which they are designed have also to

transport suspended solids in such a manner thatdeposition and odour nuisance therefrom are keptto a minim um . Sewers are almost exclusivelydesigned for flows with free water surface and selfcleansing velocities. Pressure sewers, includingsiphons, should be avoided as far as prac ticable.3.3.1 Flow Assumptions

The flow in sewers* varies considerably fromhour to hour and also seasonally but for purposesof h ydraulic design it is the estimated peak flowthat is adopted.Table 3.3Runoff coefficients(After Horn er)

Duration, t , minutes 10 20 30 45 60 75 90 100 120 135 150 180Weighted average Coefficients

(1) Sector concentrating in stated time(a) Impervious . . . .(b) 60% Impervious(c) 40% Impervious(d) Pervious(2) Rectangle (le ng th =4 x width) concentrating in stated time(a) Impervious . . . .(b) 50% Impervious(c) 30% Impervious(d) Pervious

525365285125

550350269149

.588 642427 -477346-395185-230

648 -711442 499360 -414236 -287The peak factor or the ratio of maximum toaverage flows, depends upon the contributorypapulation and the following values are recommended :

Contributory Population Peak FactorUp to 20,000 3-520,000 to 50,000 . . . . 2 - 550,000 to 7 ,50,000 . . . . 2 - 2 5Above 7,50,000 2-0

3.3.2 Self-Cleansing VelocityIt is necessary to mainta in a minimum velocityor 'self-cleansing velocity' in a sewer to ensurethat suspended solids do not deposit and causenuisance . Self-cleansing velocity is determined byconsidering the particle size and the specificweight of the suspended solids in sewage (refer to

11.2.5.4. Grit Removal). A minimum velocity of0.8mps at design peak flow in the sanitary sewersis recommended subject to a minimum velocityof 0.6 mps for present peak flows as discussedi n 3 . 3 . 3 .3.3.3 Velocity at Minimum Flow

T o avoid steeper gradients which will requiredeeper excavations, it has been the practice todesign sewers for the self-cleansing velocity atultimate peak flows. T his is done on the assumption that although silting might occur at minimumflow, the silt would be flushed out during thepeak flows. However, the problem of siltingmay have to be faced in the early years, pa rticularly for smaller sewers which are designed toflow half-full, as the actual depth of flow then isonly a small fraction of the full depth. Similarly

700 -740 -771 -795 -813 -828 -840 -850 -865531 -569 -598 -622 -641 -656 -670 -682 -701446 -482 -512 -535 -554 -571 -585 -597 -618277 -312 -330 -362 -382 -399 -414 -429 -454

768 -808 -837 -856 -869 -879 -887 -892 -903551-590 -618 -639 -657 -671 -683 -694 -713464 -502 -530 -552 -572 -588 -601 -614 -636334 -371 -398 -422 -445 -463 -479 -495 -522upper reaches of laterals pose a problem as theyflow only partially full even at the ultimate designflow, because of the necessity for adopting theprescribed minimum size of sewer.

It has been shown that for sewers runningiartially full, for a given flow and slope, velocitys little influenced by pipe diam eter. It is, therefore, recommended tha t for present peak flowsupto 30 lps, the slopes given in Table 3.4 may beadopted, which would ensure a minimum velocityof 0.60 mps in the early years.Table 3.4nt peak flow In lps Slopes per 1,000

2 6-03 .5 .10 .15 .20 .30 .

4 03 12-01-31-21-0After arriving at slopes for present peakflows, the pipe size should be decided on the basisof ultimate design peak flow and the permissibledepth of flow.3.3.4 Erosion and Maximum VelocityErosion of sewers is caused by sand andother gritty material in the sewer and also byexcessive velocity. Velocity in a sewer is recom mended not to exceed 3.0. mps.3.3.5 Minimum Size

Minimum diameter for a public sewershallbe150mm. However, recommended practice is toprovide 200mm minimum size. Minimum size forhilly areas where extrem e slopes are prevalentmay be 100 mm.


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113.3.6 Hydraulic FormalaeFor open channel flow, Gutte r's or Manning'sformula may be used for designing slope anddiameter of a sewer line to carry the design flowat a stated velocity. Manning's formula which issimpler and used more commonly is1 Mr3 -s

For circular conduits-v = 3 . 9 6 8 x l 0 - 3 x d 3 x _ x ssi

. *a n d Q = 3 . U 8 x l O - x d 8 ' s x X snwhereQ=dischargeinlpss=slope of hydraulic gradientd=dia o fpipein mmr=hydraulic radius in mv=velocity in mpsand n=Manning's coefficient of roughness.A Chart for Manning's formula is given inAppendix 5.The values of Manning 's coefficient V fordifferent materials are given in Table 3.5 . Usually,the values corresponding to fair condition of theInterior surface are used in design.Table 3 .5Manning's Coefficients

Conduit Material" Condition of InteriorSurfaceGood

Salt glazed stonewareCement ConcreteCast IronBrick, unglazedAsbestos CementPlastic (Smooth)

0 0 1 20 0 1 30-0120 0 1 30 0 1 10 0 1 1

Fair0-0140 0 1 50-0130-0150 0 1 20-011

A reduction in the value of n has been reported with increase in dia. For cement concretepipes of dia 600mm and above, 0.0)3 may be used.3.3.7 Depth of FlowFrom considerations of ventilation in waste

water flow, sewers should not be designed t o runfull; upto 400mm dia sewers may be designed torun at half depth; 400 to 900 mm at two-thirdsdepth; and larger sewers at three-fourths deptn atultimate peak flows. The chart for Manning'sformula In the Appendix 5 gives the dischargesand the velocities when sewers are running full.These figures require io .be modified for partialflow conditions. The relation between flow ratioand velocity ratio with the depth ratio is givenin Fig. 3.1 and Table 3.6.Table 3.6

Hydraulic PropertiesofCircularSectionsConstant n

dJD1 00-90-80-70-60-50-40-30-20 1

v/V1 0 0 01 1 2 41-1401 1 2 01-0721-0000-9020-7760-6150-401

QlQ1 0 0 01-0660-9880-8380-6710-5000-3370 1 9 60-0880-021

ndjn1 0 01-071-141 1 81-211-241-271-281-271-22

Variable nv/v

1-0001-0561-0030-9520-8900-8100-7130-6050-4860-329

lIQ1 0 0 01 0 2 00-8900-7120-5570 -4050-2660 1 5 30 0 7 00 -017

where,D=Full depth of flow (Internal dia)d = Actual depth of flowV = Velocity at full depthv=Velocity at depth 'd 'Q=Discharge at full depthq=Discharge at depth 'd 'n=Manning's coefficient at full depthna=Manning's coefficient at depth d

I jOOS

S - 0 7

F'

rt1-"#'vr*'

,v

.\f 3?.

s

v

//7/s

t/

1//

\1

1

O OJ OJt 03 O 4 OJ O S 07 03 Ot W II It 1-3R A TIOS Or HYDRAULIC tLlMSHTS

'/y, i /O AMD M/t

FIG.3-1 BASIC HYDRAULIC ELEMENT S OF CIRCULARSEWERS FOR ALL VALUES OF ROUGHNESSAND SLOPE.

5-480 M. of W &H /ND /79
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1 23.3.8 Design Approach

For the present peak, discharge and velocityof 0.6 mps, the slope and dia are chosen. Forth is dia and velocity of 0 8 mps, the dischargeat full depth is found. Depending upon the dia,the depth ratiois fixed and the corresponding flowand velocity ratios are found:Qis then determinedand a check is applied to verify whether th isexceeds the actua l ultim ate peak discharge. Otherwise, the next higher dia or steeper slope isselected and adjusted to satisfy the velocity andflow requirements.

Appendix 6 gives a worked out example.3.3.9 Sewer Transitions

Sewer trans itions include change in sizeslope, alignment, volume of flow , free andsubmerged discharge at the end of sewer lines,passage through measuring and diversion devicesand sewer junctio ns. Allowance for the headlossthat occurs at these transitions has to be madein the design.Manholes should be located at all suchtransitions and a drop should be provided whereth e sewer is intercepted at a higher elevationfor streamlining the flow, taking care of theheadloss and also to help in maintenance.The vertical drop may be provided only, whenthe difference between the elevations is morethan 60 cm, below which it can be avoided byadjusting the slope in the channel in the m anhole connecting the two inverts.The following invert d rops are recommended:(a) Fo r sewers less Halfthe differ-than400mm. ence india .(b) 400 mm to 900 mm 2/3 the difference in dia .(c) Above 900 mm 4/5 the di ffer-enceindia .Trans ition from larger to smaller diametersshould not be made. The crowns of sewers arealways kept continuous. In no case, the hydraulicflow line in the larger sewers should be higher

than the incoming one.3.3.10 Backwater CurvesBackwater or drawdown curves resultingfrom abrupt changes in sewer slopes or whenthere is a free fall or an obstruction to the flowmay be calculated from the following formula:

(d+h y)

where d and h , are the changes in the waterdepth and velocity In a length L; Se and So beingthe slopes of energy grade line and the invertrespectively.

The computations are started from a pointwhere depth and velocity of flow are knownand L is worked out for different depths of flowupto the normal depth.3.3.11 Force Mains

Sewage may have to be carried to higherelevations through force mains. The size of themain should be determined bytakingintoaccountthe initial cost of pipeline and cost of operationof pumping for different sizes. Velocities mayrange from 0.8 to 3 mps. Hazen & Williams formula is generally used for computing the fric-tional losses which is expressed as:v =0.849 cr o-83so.s*where, v is velocity in mps;r is hydraulic radius in m;s is slope of hydraulic gradient; andc is Hazen and Williams coefficient forthe material of the pipe.The following values of c may be adopted fordesign purposes :

(i) Cast Iron . . . . 100(ii) S teeL - . . . . 100(iii) Asbestos Cement . 120(iv) Cement Concrete . HO(v) Plastic (sm ooth) . . . 120Losses in valves, fittings, etc ., are dependentupon the velocity head v2/2g. Loss in bends andelbows depend upon the rat io of absolute

friction factor to dia of pipe, besides velocityhead- Loss due to sudden enlargement dependsupon the ratio of diameters. The losses in bends,enlargements and tapers are given in 5.2.3of the. companion volum e. Manual on watersupply a n d ' treatment (second Edition). Inthe actual design of the force mains, it may notbe necessary to compute the losses individuallybut the same may be assumed arbitrarily as 5 to15% of the total frictional losses dependingupon the number of bends, tapers and other fittings. However, for shorte r mains with a largenumber of bends etc. , the actual loss may becomputed and expressed as equivalent lengthsof pipes.3.3.12 Inverted Siphon

When a sewer line dips below the hy drau lic grade line, it is called an inverted siphon .Thepurposeis tocarrythe sewerundertheobstruction and regain as much elevation as possibleafter the obstruction is passed. They should beresorted to only where other means of passingthe obstruction are not feasible as they requireconsiderable attention in maintenance. As thesiphons are depressed below the hydraulic gradeline, maintenance of self-cleansing velocity at allflow s is very important. It is necessary t o ascertain the minimum flows and the peak flows fordesign . To ensure self-cleansing velocities forthe wide variations in flows, generally, two or


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13more pipes not less than 200 mm dia are providedin parallel so that upto the average flows,one pipe is used and when theflowexceeds theaverage, the balance flow is tak en by the secondand subsequent pipes. The design criteria forinverted siphons are given inIS :4111 Part n i .So n;]of the important criteria are given below.

3.3.12.1 Hydraulic calculationsAsthe inverted siph onisa pipe under pressure,the difference in the water levels at the inlet andoutlet is the head under which the siphon operates.This head should be sufficient to cover the entry,exit and friction losses in pipes.

3.3.12.2 VelocityIt is necessary to have a self-cleansing velocity of 1.0 mps for the minimum flo w to avoiddeposition in the line.

* .sromtnow in txcrss or I AN Dt^ , ***i^Minmimo mwrnHtiTmow^

^MAXIMU M 0*r WtATHtHnow in uteris or iP L A N

mirr CHAMirn ovrterCHAHitn.VERTICAL SECTIONPI G. 3.2 INVERTED SIPHON OR SUPPRESSED SEWERFOR COMBINED SEWAGE.

3.3.12.3 Sizeand arrangemento fpipesIn the multiple pipe siphon, the inlet shouldbe such tha t the pipescomeinto actio n successivelyas the flow increases. This may be achieved byproviding lateral weirs with heights kept in accordance with the depth of flow at which one ormore siphon pipes function. Fig. 3.2 gives thegeneral arrangement for a three-way siphon.In the two-pipe siphon, the first pipe shouldtake 1.25 to 1.5 times the average flow and secondshould take the balance of the flow.3.3.12.4 Inletand outlet chambersThe design of inlet and outlet chambersshould allow sufficient room for entry for cleaningand m aintenance of siphons. The outlet chambersshould be so designed as to prevent the backflowof sewage into pipes which are~not being used atthe time of minimum flow.3.3.12.5 General requirements

Provision should be made for isolating theindividual pipes aswellas [the siphon to facilitatecleaning. This can be done by providing suitablepenstocks or stopboards at the inlet and outlet ofeach pipe and by providing stopvalve at its lowerpoin t if it is accessible. A manh ole at each end ofthe siphon shouldfbe provided with clearance forrodding- The rise, out of the siphon for smallpipes should be on a'moderate slope so that sandand other deposits may7 be moved out of thesipho n. It is desirable to provide a coarse screento prevent the^entry'ofragsetc., into the siphon.Proper bypass arrangements should be provided from the inlet chamber to a nearby streamif permitted by thefpollution control authority ;otherwise special arran gem ents should; be mad efor pumping the sewage to the^lower reach ofthe sewer line.

6^*80 M . o f W & H / N D / 7 9


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C H A P T E R 4SEWER APPURTENANCES

Sewer appurtenances are devices necessary, Inaddi t ion to pipes and condui ts , fo r the properfunctioning of any complete system of sani tary,storm or combined sewers. They include structuresan d devices such as various types of m anho les,lamp-holes, gul ly traps, intercepting chambers,flushing tan ks, v enti la t ion shafts , catch -basins,street inlets , regulat ors, siph ons, grease tr ap s,side flow weirs, leaping weirs, venturl-flumes andoutfal l s tructures.4 . 1 M A N H O L E S4.1.1 Ordinary M anholes

A manhole is an opening constructed on thealignm ent of a sewer for facilitatin g a pers onaccess to the sewer for the purpose of inspection,testing, cleaning and removal of obstructionsfrom the sewer line.4.1.1.1 Spacing

Manholes are general ly provided on straightreaches at convenient spacings which dependon the size of the sewers. The larger the d iam eterof the sewer, the greater m ay be th e spacing betweentwo manholes. The spacing between the mah noleswill also depend upon the nat ure of sewer cleaningdevices In use. The straigh t ru ns between m anholesare l imited in length to 30m for sewers upto300 mm India where manual roddlng is adopted.Fo r large sewers, they may go up to 100 m ormo re. These l imits can be considerably relaxedfor sewers sufficiently large permitting entry forinspection, cleaning or repair with access ma nholes placed qui te far apart ei ther symmetrical lyabove the sewer or tange ntial ly to one side.Ap art from these manho les on straight reaches,manh oles shal l also be prov ided at th e star t ofa sewer, at all junctions, at all points of change ofal ignmen t and at al l points of change of gra dient . When twin or multiple box sections are used,separate manholes for each condui t shal l be provided.4.1.1.2 Shape"and size 3

Manholes are general ly circular, square orrectangular in shape . The inside dimen sionshould be adequate to permit inspection andcleaning operations withou t difficul ty. A min imuminside dim ension of 120cmX90 cm for manh olesis usual ly recomm ended except for shal low man holes upto depth s of 1 .35 m, where minim umallowab le wid th may be red uced from 90cm to75cm.4.1.1.3 Construction'details

M anh oles are usual ly constru cted directlyover the centre l ine of the sewer. Fo r largersewers the manhole is preferably constructed at a

tangent to the side of the sewer for bet ter accessibi l i ty. Th e m anho les for very large sewersmay be located over the centre line of the sewerwith a suitable land ing platform offsetting froman opening in the seweritself.

The opening for entry into the manholeshould be of such m inimu m dimensions as toal low a workman with the cleaning equipmentsto get access into the inter ior of the man holewit hou t difficulty. A circular open ing is generallypreferred. A minim um clear open ing of 50 cmis recomm ended. Sui table steps usually of malleable cast iron shall be provided for entry.A slab, generally of plain cement concreteat least 150 ram thick should be provided atthe base to support the walls of the manholeand to prevent the entry of ground water.The thickness of the base slab shall be suitablyincreased upto 300 mm, for manholes onlarge dla sewers, with adequ ate reinforcementprovided to with stand excessive uplift pre ssu re s ,In the case of larger man ho les, the flow inthe sewer should be carried In U-shapedsmooth channel constructed integrally withthe concrete base of the manhole. Theside of th e ch annel shou ld be equal to thedia of the largest sewer pip e. The adjacentfloor should have a slope of1in10dra ining

to the channel . Where more than onesewer enters the manh ole the flow thro ughchannel should b : curved smoothly and shou ldhave sufficient capacity to carry the max imumflow.It is desirab le to place the first pipe join toutside the manhole as close as practicable.The pipe shall be built inside the wall of themanhole flush with the internal peripheryprotected with an arch of masonry or cementconcrete to prevent it from being crushed.The sidewalls of the man hole are usually

constructed of cement brickw ork 250 mmthick and corbel led sui tably to accommodatethe frame of the m anhole cover. Theinside and outside of the brickwork shal lbe plastered 20 mm thick with 1:2 cementmorta r .4.1.1.4 Cover and frame

The cover and frame may be of C I.or reinforced co ncrete. IS:172 6 should beadhered to when these are of C I. Themanhole frames shal l be 53 cm and not beless th an 160 mm th ick and be set conformingaccurately to the grade of the pavement.The frame shall rest on concrete band and


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15be set in appropriate concrete mix so thatthe space between the top of the manholemasonry and the bottom flange of the frameshall be completely filled and made watertight. A thick ring of mortar extending tothe ou ter edge of the masonry shall be placedall around the botto m flange. Heavyreinforced concrete covers with suitablelifting arrangements could be used insteadof C. I. manhole covers.4.1.2 Special Manholes4.1.2.1 Junction chambersWhere two sewers particularly of largedia intersect, the intersection is made bymeansofa brick or concrete structu re knownas a junction chamber. The junctionchamber provides access to the sewer andalso allows the flow from the sewers enteringthe chamberto be combined without excessive

turbulence and loss of head.The principal [objectivein the design of ajunction chamber It to provide a safe andeconomical structure which will combine theflow smoothly without decreasing the velocities appreciably and without causing backwater conditions in the sewers ^enteringthe chamber.4.1.2.2 Drop ManholesThese are provided when the differencein elevation of the invert levelsof the incomingand outgoing sewers of a manhole is morethan 60 cm. D:tails are discussed in 3.3.9and 7.1.8.4.1.2.3 Flushing manholesFlushing manholes are located generallyat the head of a sewer. The sewers areflushed once or twice a day. Sufficient velocity shall be imparted in the sewer to washaway the deposited solids. The flush isusually effective upto a distance of about300 mafter which the imparted velocitygets dissipated.Flushing operation rshould preferablybe automatic. In case of hard chokagesin the sewers, care should be exercised toensure tha t there is no possibility of backflowof sewage into the water supply (mains.The auto mat ic systems which are oper

manual sewage treatment

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