handbook on sewage and swerage treatment plants

7/22/2019 Handbook on Sewage and Swerage Treatment Plants

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Works Audit Sewerage and Sewage Treatment

Office of the Principal Accountant General (Civil Audit) Chennai

1

HANDBOOK

ON

SEWERAGE AND SEWAGE

TREATMENT


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SEWERAGE AND SEWAGE TREATMENT

1. PREAMBLE

The standard specification prescribed in the Manual on Sewerage and SewageTreatment issued by Central Public Health and Environmental Engineering Organisation,Ministry of Urban Development, in December 1993, guidelines issued by Ministry ofEnvironment and Forest Department, National River Conservation and guidelines prescribed

by Chennai Metropolitan and Water Supply and Sewerage Board and TWAD Board onSewerage and Sewage Treatment which are more useful for audit are given below:

The Sewerage system consists mainly of :-

i) Collection system (sewer, sewer appurtenances)ii) Conveyance system (pumping station, pumping main etc.)iii) Treatment plant

2. PLANNING(Chapter 1 of CPHEEO Manual)

1. Objective

The objective of a public waste water collection and disposal system is to ensure thatsewage or excreta and sullage discharged from communities is properly collected,

transported, treated to the required degree and finally disposed of without causing any healthor environmental problems.

2. Need for planning

Planning is required at different levels; national, state, regional and community.Though the responsibility of various organizations in charge of planning public waste waterdisposal systems is different in each case, they still have to function within the priorities fixed

by the national and state governments and to keep in view overall requirements of the area.

The waste water disposal projects formulated by the various State sponsoringAuthorities at present do not always contain all the essential elements for appraisal. When

projects are assessed for their cost benefit ratio and for institutional or funding purposes, theyare not amenable for comparative study and appraisal. Also at times different standards are

adopted by the Central and State agencies regarding various design parameters. It isnecessary therefore to specify appropriate standards and design criteria and to avoid differentapproaches

3.Basic Design considerations( Para 1.3 of CPHEEO Manual )

In designing waste water collection, treatment and disposal systems, planning generallybegins from the final disposal point going backwards to give an integrated and optimum designCo suit the topography and the available hydraulic head, supplemented by pumping if essential.


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Once the disposal points are tentatively selected, further design is guided by the following basicdesign considerations:

a. Engineeringb. Environmentalc. Processd. CostThese considerations are discussed below in detail:

a) Engineering Considerations ( Para 1.3.1 of CPHEEO Manual)

Topographical, engineering and other considerations which figure prominently inproject design are noted below:

1. Design period, stage wise population to be served and expected sewage flowand fluctuations

2. Topography of general area to be served, its slope and terrain. Tentative sitesavailable for treatment plant, pumping stations and disposal works

3. Available hydraulic head in the system up to high flood level in case ofdisposal to a nearby river or high tide level in case of coastal discharge or the level of theirrigation are to be commanded in case of land disposal

4. Ground water depth and its seasonal fluctuation affecting construction, sewerinfiltration, structural design (uplift considerations)

5. Soil bearing capacity and type of strata expected to be met in construction6. On site disposal facilities, including the possibilities of segregating the sullage

water and sewage and reuse or recycle sullage water within the households

b) Environmental Considerations: (Para 1.3.2 of CPHEEO Manual)

The environmental and socio-economic impacts of a sewage treatment plant mayprove adverse during the operation stage. Therefore the following aspects should be

considered during design.i) Surface water Hydrology and Qualityii) Ground water qualityiii) Coastal water qualityiv) Odour and Mosquito nuisancev) Public Health andvi) Land scapingc) Process Considerations: ( Para 1.3.3 of CPHEEO Manual)

Process considerations involve factors which affect the choice of treatment method, itsdesign criteria and related requirements such as the following:

i) Waste water flow and characteristicsii) Degree of treatment requirediii) Performance characteristicsiv) Other process requirements such as land, power operating equipments,

skilled staff, nature of maintenance problems, extent of sludge productionand its disposal requirements, loss of head through plant in relation toavailable head etc


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d) Cost Considerations: ( Para 1.3.4 of CPHEEO Manual)

The overall costs (Capital and operating) have to be determined in order to arrive atthe most optimum solution

4 Design Period ( Para 1.4 of CPHEEO Manual)

Sewerage projects may be designed normally to meet the requirements over a thirtyyear period after their completion. The period between design and completion should also betaken into account which should be between three to six years depending on the type and sizeof the project.

The thirty year period may however be modified in regard to certain components ofthe project depending on their useful life or the facility for carrying out extensions whenrequired and rate of interest so that expenditure far ahead of its utilization is avoided.

Necessary land for future expansion /duplication of components should be acquired in thebeginning itself. Where expensive tunnels and large aqueducts are involved entailing largecapital outlay for duplication, they may be designed for ultimate project requirements.

The project components may be designed to meet the periods mentioned below:

Design Periods For Components Of Sewerage System And Sewage Treatment

(Table 1.1 of CPHEEO Manual)Sl. No. Component Recommended Design

period in years

Clarification

1 Collection System i.e.Sewer network

30 The system should be designed for theprospective population of 30 years, as itsreplacement is not possible during its use.

2. Pumping stations (CivilWorks)

30 Duplicating machinery within the pumpingstation would be easier/cost of civil workswill be economical for full design period.

3. Pumping Machinery 15 Life of pumping machinery is generally 15years.

4. Sewage Treatment Plant 30 The construction may be in a phased manneras initial the flows may not reach the

designed levels and it will be uneconomicalto build the full capacity plant initially.(Refer Chapter 10.2).

5. Effluent disposal andutilization

30 Provision of design capacities in the initialstages itself is economical.

5. Population Forecast (Para 1.5 of CPHEEO Manual)

The design population will have to be estimated with due regard to all the factorsgoverning the future growth and development of the project area in the industrial,commercial, educational, social and administrative spheres. Special factors causing suddenimmigration or influx of population should also be foreseen to the extent possible.

A judgement based on these factors would help in selecting the most suitable method

of deriving the probable trend of the population growth in the area or areas of the project fromout of the following mathematical methods, graphically interpolated where necessary. Thefollowing are some of the methods prescribed by the CPHEEO for working out the projected

population.

a) Demographic method of Population Projectionb) Arithmetical increase Methodc) Incremental increase Method


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d) Geometrical increase Methode) Decreasing rate of Growthf) Graphical method

i) Graphical method based on single city

ii) Graphical method based on cities with similar growth pattern

g) Logistic Methodh) Method of Density

Note: Detailed procedure for estimating the population forecast given in Manual on WaterSupply & Treatment may be referred to.

Final Forecast: (Para 1.5.2 of CPHEEO Manual)

While the forecast of the prospective population of a projected area at any given timeduring the period of design can be derived by any one of the aforesaid methods appropriate toeach case, the density and distribution of such population within the several areas, zones ordistricts will again have to be made with a discerning judgement on the relative probabilitiesof expansion within each zone or district, according to its nature of development and based on

existing and contemplated town planning regulations.

Wherever population growth forecast or Master plans prepared by town planning orother appropriate authorities are available, the decision regarding the design populationshould take their figures into account.

The population estimate is guided by the anticipated growth rates of each community.These differ in different zones of the same town. A design period of 30 years (excludingconstruction period) is recommended for all types of sewers. (Para 3.2.1 of CPHEEOManual)

Where a Master Plan containing land use pattern and zoning regulations is availablefor the town, the anticipated population can be based on the ultimate densities and permitted

floor space index provided for in the Master Plan. In the absence of such information onpopulation the following densities as suggested for adoption. (Para 3.2.2 of CPHEEOManual)

Size of town

(population)

Density of population

per hectare

Up to 5,000 75-150

5,000 to 20,000 150 250

20,000 to 50,000 250 300

50,000 to 1,00,000 300 350

Above 1,00,000 350 1000


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In cities where Floor Space Index (FSI) or Floor Area Ratio (FAR) limits are fixed bythe local authority this approach may be used for working out the population density. FSI orFAR is the ratio of total floor area (of all the floors) to the plot area. The densities of

population on this concept may be worked out as in the following example:

Assume that a particular development plan rules provide for the following reservationsfor different land uses.

Roads 20 %

Gardens 15%

Schools (including play grounds) 5%

Markets 2%

Hospital and Dispensary 2%

Total 44%

Area available for Residential Development (100 44) = 56 %

Actual total floor area = Area for residential development x FSI

Assuming an FSI of 0.5 and floor area of 9 m2/person

Number of persons or density per hectare = 0.56x10,000x0.5 = 3119

6.Estimation of Waste Water Flow (Para 1.6 of CPHEEO Manual)

There are two types of sewerage systems viz.

i) Sanitary Sewer system, designed to receive domestic sewage andindustrial wastes excluding storm water. Storm water sewers designed to carry 5 carry offstorm water and ground water but excluding domestic sewage and industrial waste.

ii) Combined sewer system is designed to receive domestic sewage,industrial wastes and storm water. The combined sewer system though economical initiallysuffers from several disadvantages and is normally not recommended.

1..Estimation of Sanitary Sewage: (Para 3.2 of CPHEEO Manual)

The Sewer capacity to be provided must be determined from the analysis of thepresent and probable quantities expected at the end of design period. The estimation of flowis based upon the contributory population and the per capita flow of sewage both the factors

being guided by design period as discussed below:

a) Per capita Sewage flow : (Para 3.2.4 of CPHEEO Manual)

The entire spent water of a community should normally contribute to the total flow ina sanitary sewer. However, the observed Dry Weather Flow quantities (DWF) usually areslightly less than the per capita water consumption, since some water is lost in evaporation,seepage into ground, leakage etc. In arid regions, mean sewage flows may be as little as 40

percent of water consumption. In well developed areas, flows may be as high as 90 % due toindustrial wastes, changed water use habits etc. Generally, 80 % of the water supply may beexpected to reach the sewers unless there is data available to the contrary. However, the


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sewers should be designed for a minimum waste water flow of 100 litres per cpaita per day.Industries commercial buildings often use water other than the municipal supply and maydischarge their liquid wastes into the sanitary sewers. Estimates of such flows have to bemade separately. The details of requirements of water for institutions and industries arediscussed in Chapter 2 of Manual on Water Supply and Treatment. Industrial wastes have to

be treated to the standards prescribed by the regulatory authorities before being dischargedinto sewers. For some areas, it is safe to assume that the future density of population for

design purpose to be equal to the saturation density. It is desirable that all sewers serving asmall area be designed on the basis of saturation density.

Infiltration into sewer may occur through pipes, pipe joints and structures. Theprobable amount has to be evaluated carefully.

b. Flow Assumptions : (Para 3.2.5 of CPHEEO Manual)

The flow in sewers varies considerably from hour to hour and alsoseasonally, but for the purposes of hydraulic design it is the estimated peak flow that isadopted. The peak factor or the ratio of maximum to average flow depends upon contributory

population and the following values are recommended. These peak factors will be applied tothe projected population for the design year considering an average wastewater flow based on

allocation

Contributory population Peak factor

Up to 20,000 3.0

20,000 to 50,000 2.50

50,000 to 7,50,000 2.25

Above 7,50,000 2.00

The peak factors also depend upon the density of population, topography of the site,hours of water supply and therefore it is desirable to estimate the same in individual cases, ifrequired. The minimum flow may vary from 1/3 to 1/2 of average flow.

c Ground water infiltration : (Para 3.2.7 of CPHEEO Manual)

Estimate of flow in sanitary sewers may include certain flows due to infiltration ofground water through joints. The quantity will depend on workmanship in laying of sewersand level of the ground water table and permeability of the surrounding soil. Since sewers aredesigned for peak discharges, allowance for ground water infiltration for the worst conditionin the area should be made. Suggested estimates for ground water infiltration for sewers laid

below ground water table are as follows:

Units Minimum Maximum

Litres/Ha.d 5,000 50,000

Litres/Km.d 500 5,000

Lpd/manhole 250 500


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With improved standards of workmanship and availability of various constructionaids, these values should tend to the minimum, rather than the maximum. These valuesshould not mean any relaxation on the water tightness test requirements.

d. Effect of Industrial Waste

Waste from industries can form an important component of sewage flow both in volumeand composition. It is therefore necessary to collect detailed data about nature of industries,quantity and character of the waste and their variations, which may affect the sewerage system orthe sewage treatment process. Quality and character of waste water are to be based on flowmeasurements and laboratory analysis of the composite samples.

Estimation of Combined Sewer: : (Para 3.3 of CPHEEO Manual)

Estimation of combined sewer includes flow of sanitary sewage and storm water runoff

Estimation of Storm water Run off

Sanitary sewers are not expected to receive storm water. Strict inspection andvigilance and proper design and construction of sewers and manholes should eliminate thisflow or bring it down to a very insignificant quantity.

Storm runoff is that portion of the precipitation, which drains over the ground surface.Estimation of such runoff reaching the storm sewers therefore is dependent on intensity andduration of precipitation, characteristics of the tributary area and the time required for suchflow to reach the sewer. The storm water flow for this purpose may be determined by usingthe rational method, hydrograph method, rainfall-runoff correlation studies, digital computermodels, inlet method or empirical formulae. Of all these methods, the rational method ismore commonly used.

Rational Method (Para 3.3.1 of CPHEEO Manual)

The entire precipitation over the drainage district does not reach the sewer. The

characteristics of the drainage district, such as, imperviousness, topography includingdepressions and water pockets, shape of the drainage basin and duration of the precipitationdetermine the fraction of the total precipitation which will reach the sewer. This fractionknown as the coefficient of runoff needs to be determined for each drainage district. Therunoff reaching the sewer is given by the expression,

Q = 10 C I A

Where Q is the runoff in m3/hr

C is the coefficient of runoff

I is the intensity of rainfall in mm/hr

A is the area of drainage district in hectares

7 Survey and Investigation(Para 1.8 of CPHEEO Manual)

Survey and investigation are pre-requisites both for framing of the preliminary report andthe preparation of a detailed sewerage project. The engineering and policy decisions taken aredependent on the correctness of the data collected and its proper evaluation. It includescollection of basic information, project surveys and preparation of project report.

1. Basic information


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It includes collection of datas relating to physical aspects (viz., topography, selection ofsites for various components including disposal sites, subsoil conditions etc.,) developmentalaspects (viz. type of land used, density of population, growth of population industries etc,existing drainage and sewerage facilities, flow characteristics, disposal rate etc) fiscal aspects(viz source of land, factors affecting the repayment of loan such as revenue etc) and otheraspects likely to influence the project.

2. Project Surveysi) Preliminary project surveys

This is concerned with the broad aspects of the project. Data on aspects such ascapacity required, basic arrangement and size, physical features affecting general layoutand design, availability of affluent disposal facilities, probable cost and possible methodsof financing, shall be collected to prepare an engineering report describing the scope andcost of the project with reasonable accuracy. In framing such estimates, dueconsideration must be given to the escalation of prices of basic materials and theiravailability. While extreme precision and detail are not required in this phase all the basicdata obtained must be reliable.

ii) Detailed project surveys

Surveys for this phase form the basis for the engineering design as well as for thepreparation of plans and specifications for incorporation in the detailed project report. Incontrast to preliminary survey this survey must be precise and contain contours of all theareas to be served giving all the details that will facilitate the designer to prepare designand construction of plans suiting the field conditions. It should include, interalia,network of benchmarks and traverse surveys to identify the nature as well as extent of theexisting underground structures requiring displacement, negotiation or clearance. Suchdetailed surveys are necessary to establish rights of way, minimize utility relocationcosts, obtain better bids and prevent changing and rerouting of lines.

iii) Construction SurveysAll control points such as base lines and bench marks for sewer alignment and

grade should be established by the engineer along the route of the proposed construction.All these points should be referred adequately to permanent objects.

a) Preliminary Layouts

Before starting the work, rights-of-way, work areas, clearing limits and pavementcuts should be laid out clearly to ensure that the work proceeds smoothly. Approachroads, detours, by-passes and protective fencing should also be laid out and constructed

prior to undertaking sewer construction work. All layout work must be completed andchecked before construction begins.

b) Setting Line and GradeThe transfer of line and grade from control points, established by the engineers, to

the construction work should be the responsibility of the executing agency till work iscompleted.

3) Project Report (1.9 of CPHEEO Manual)

All projects have to follow distinct stages between the period they are conceivedand completed. The various stages are:


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a) Pre-investment Planning

- Identification of a project

- Preparation of project report

b) Appraisal and Sanction

c) Construction of facilities and carrying out support activitiesd) Operation and maintenancee) Monitoring and feed back

Since project preparation is quite expensive and time consuming, all projectsshould normally proceed through three stages and at the end of each stage a decision should betaken whether to proceed to the next planning stage and commit the necessary manpower andfinancial resources for the next stage. Report at the end of each stage should include a timetable and cost estimate for undertaking the next stage activity and a realistic schedule for allfuture stages of project development, taking into consideration time required for review andapproval of the report, providing funding for the next stage, mobilizing personnel or fixingagency (for the next stage of project preparation) data gathering, physical surveys, siteinvestigations etc.

The basic design of a project is influenced by the authorities/organizations who areinvolved in approving, implementing, operating and maintaining the project. Therefore theinstitutional arrangements, through which a project will be brought into operation, must beconsidered at the project preparation stage. Similarly responsibility for project preparationmay change at various stages. Arrangements in this respect should be finalized for each stageof project preparation. Some times more than one organization may have a role to play in thevarious stages of preparation of a project. It is therefore necessary to identify a single entity to

be responsible for overall management and coordination of each stage of project preparation.It is desirable that implementing authority is identified and those responsible for operation of a

project are consulted at the project preparation stage.

Audit Approach

Inter-alia the following points could arise:

1. Whether population forecast was worked out correctly and the estimate of wastewater assessed correctly for the design period. Over estimation of population would

lead to creation of infrastructure in excess of the actual requirement involving extra

cost. Under assessment lead to creating additional infrastructure to meet the

requirement of the full design life involving extra cost.

2. Cases where pump set designed for ultimate stage may be verified and extra costinvolved on erection of pumpset and motor for ultimate stage instead of intermediate

stage and also power consumption on higher capacity of motor may be worked out

and commented.

3. Whether various components of sewerage system were designed and constructed forthe stipulated designed period if not financial implication may be commented.

4. Whether detailed survey and investigation carried out and alignment for pumpingmain, sewer main fixed correctly taking into account topography of the ground and

level difference needed for laying the sewers and location of outfall and disposal

works. The following point could emerge

(i) Cases where the sewage could not reach the collection well due to leveldifferences


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(ii)Cases of shifting the alignment due to various reasons(iii)Cases where the pumping station and disposal site could not be located due to

public objection or other reasons

(iv)Whether investigation report specified the details of land required to be acquiredor transferred to for the scheme. Cases where the schemes held up due to non

assessment of the land required initially and incorporated in the Project report and

subsequent delay thereof may be commented.5. Whether funds for implementation of Project was identified before taking up the

Schemes, cases where projects held up due to want of funds could be commented.

3. DESIGN OF SEWER AND APPURTENANTS

1 Design of Sewer(Chapter 3 of CPHEEO Manual)

Sewerage system may be classified as sanitary sewers designed to receive domesticsewage and industrial waste excluding storm water. Storm sewers designed to carry off stormwater and ground water but excluding domestic sewage and industrial wastes and Combinedsewers designed to receive sewage, industrial waste and storm water. The combined system of

sewerage though may be economical initially, suffer from several disadvantages such as sluggishflow during non-stormy days, leading to deposition of sewage, solids causing foul odours andincreased cost of eventual sewage treatment or pumping cost, associated with disposal ofsewage. In view of this, the combined system is normally not recommended.

The design of sewer interalia included estimation of sanitary sewage, estimation of stormwater runoff and hydraulic, of sewer; design of sewer system etc. The method for estimation ofsewage and storm water runoff is discussed in the previous chapter.

Hydraulics of Sewers (Para 3.4 of CPHEEO Manual)

Flow in sewers is said to be steady if the rate of discharge at a point in a conduit remainsconstant with time and if the discharge varies with time it is unsteady. If the velocity and

depth of flow are the same from point to point along the conduit, the steady open channelflow is said to be uniform flow and non-uniform if either the velocity, depth or both arechanging.

A properly functioning sewer has to carry the peak flow for which it is designed andtransport suspended solids in such a manner that deposits in a sewer are kept to aminimum. The design for wastewater collection system presumes flow to be steady anduniform. The unsteady and non uniform waste water flow characteristics are accountedin the design by proper sizing of manholes

Flow friction: (Para 3.4.2 of CPHEEO Manual ) - The available head in waste waterlines is utilized in overcoming surface resistance and in small part, in attaining kineticenergy for flow. For design purpose, Mannings formula for open channel flow and

Hazen William and Darcy-Wcisback formula for closed conduit or pressure flow is usedfor working out the head loss due to friction

Design criteria:- It is better practice to design sewers with partial full condition toprovide ventilation and keeping sewage in fresh condition. Hence peak factor for designsewer shall range between 2 to 3.5. From consideration of ventilation in waste waterflow, sewers should not be designed to run full. All sewers are designed to flow 80

percent of full ultimate flow. For design of sewer net work the slope and diameter ofsewers should be decided to meet the following two conditions:


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i. A self cleansing velocity is maintained at present peak flowii. A sewer runs at 0.80 full at ultimate peak flow.

Self cleansing velocity:- To ensure that deposition of suspended solids does not takeplace, minimum self cleansing velocities are required to be considered in the design.

Hydraulic elements of circular sewers possess equal self cleansing properties at alldepths. The self cleansing properties for different conduit are given below:

i) Sanitary Sewer: For design peak flow 0.8 metre/secFor present peak flow 0.6 metre/sec

ii) Open drain: - 0.75 to 0.9 metre/sec

iii) Inverted siphon: - 1.00 metre/sec

iv) Minimum velocity for force main: - 0.3 metre/sec

Maximum permitted depth of flow: The pipes will be designed to flow at depth indicatedbelow where the maximum permissible depth of flow in sewers for established velocitycriteria:

Diameter in

mm (d)

Depth of flow which will convey

designed quantity

Up to 400 0.50 d

400 to 900 0.67 d

Above 900 0.75 d

Velocity: (Para 3.4.3 of CPHEEO Manual) The flow in sewer varies from hour to hourand also seasonally. But for the purpose of hydraulic design, estimated peak flow isadopted. The size of Sewer is to have adequate capacity for the peak flow to be achieved atthe end of design period so as to avoid steeper gradient and deeper excavation. It isdesirable to design sewers for higher velocity wherever possible. The sanitary sewer is

designed to obtain adequate scouring velocities at the average or at least at the maximumflow at the beginning of the design period for a given flow and slope. Velocity is littleinfluenced by pipe diameter. The recommended slope for minimum velocity is given

below which ensure minimum velocity of 0.60 metre/sec.

Table 3.7 of CPHEEO Manual)Present peak flow (lps) Slope per 1000 m

2 6.0

3 4.0

5 3.1

10 2.0

15 1.30

20 1.2030 1.00

After arriving at slopes for present peak flows, the pipe size should be decided on the basisof ultimate design peak low and the permissible depth of flow. The minimum diameter of

public sewer may be 150 mm. In hilly areas, where extreme slope are prevalent, the size ofsewer may be 100 mm.


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Maximum Permissible Velocity:Description Maximum permissible velocity

Stoneware pipe 1.4 metre/sec

Brick drain 1.8 to 2.1 metre/sec

Concrete drain 2.5 metre/sec

Cemented drain 3.0 metre/secCast Iron pipe 3.0 metre/sec

Depth of cover: One meter cover on pipeline is normally sufficient to protect thepipelines from external damage.

2 Sewer Appurtenances(Chapter 4 of CPHEEO Manual)

Sewer appurtenances are devices necessary in addition to pipes and conduits for the pipesfunctioning of any complete system of sanitary, storm or combined sewers. They includestructures and devices such as various types of manholes, lamp holes, gully traps, interceptingchambers, flush tanks, ventilation shafts, catch basins, street inlets, regulators, siphons, greasetraps, side float weir, leaping weir, venture-flumes and out fall structures.

1. Manhole: (Para 4.2 of CPHEEO Manual)

A manhole is an opening constructed in the alignment of a sewer for facilitating aperson to access the sewer for the purpose of inspection, testing, cleaning andremoval of obstruction of the sewer line.

Spacing : Manhole should be built at every change of alignment, gradient ordiameter at the head of all sewers and branches and at every junction of two ormore sewers.

The maximum distance between manholes should be 30 m. Spacing of manhole in large sewers above 900 mm diameter to 1500mm may be

of above 90 to 150 m in straight run sewer and spacing of manholes at 150 to 200m may be allowed in straight run sewer of 1.5 to 2.0 m dia., which may further beincreased up to 300 m for sewer of over 2 m diameter. A spacing allowance of100 m per 1 m dia of sewer is a general rule in case of very large sewer.

Manholes are of rectangular, arch type and circular type Circular manholes are stronger than rectangular and arch type manhole and hence

circular manhole is preferred over other two types. The circular manholes can beprovided for all depths, starting from 0.9 metres. Depending on the depth of

manhole, diameter of manhole changes. The internal diameter of the manholesmay be kept as follows for varying depths: (para 4.2.1.2 of CPHEEO Manual)

i) For depth above 0.90 m up to 1.65 m - 900 mm dia.

ii) For depth above 1.65 m up to 2.30 m - 1200 mm dia

iii) For depth above 2.30 m up to 9.0 m - 1500 mm dia

iv) For depth above 9.0 m up to 14.0 m - 1800 mm dia


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The width/diameter of the manhole should not be less than the internal diameterof the sewer plus 150 mm benching on both sides (150 mm + 150 mm).

Manhole covers: A minimum clear opening of 60 cm is recommended. Floor slab of manhole: RCC 150 mm thick to withstand uplift. Drop manholes: Required when the maximum difference in inverts between the

shallowest incoming and the outgoing sewer of a manhole is more than 60 cm.2. Flushing Tank:

Located at the head of a sewer. They are designed for 10 minutes flow as a self-cleansing velocity of 0.6 m/sec.

Capacities:150 mm sewer - 6400 litres

200 mm sewer - 11000 litres

250 mm sewer - 18000 litres

The capacity of these tanks is usually 1/10 of the cubic capacity of sewer length to be

flushed.

House Service Connection (Para 4.4 of CPHEEO Manual)

-- For large diameter of sewers, house service connections may be given through rider sewers,which should be connected through manhole or drop manhole. Where there is no Y or T leftfor new connection insertion of new Y or T is not prescribed.

-- House service connection should be minimum size of 150 mm diameter sewer with minimumslope of 1:60 laid as far as possible to a straight line and grade.

-- The House service connection sewer line has to be connected to the manhole and will bejoined with sewer pipe already embedded within the wall of the manhole while constructingthe manhole. The House service connection will be taken up to the property boundary. The

property owner shall connect the sewer line laid up to the property boundary with Houseservice connection.


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3 Materials for Sewer Construction (Chapter 5 of CPHEEO Manual)

Factors influencing the selection of materials for sewer construction are flowcharacteristics, availability size required including fittings and ease of handling and installations,water tightness and simplicity of assembly, physical strength, resistance to acids, alkalies, gases,solvents etc., resistance to scour, durability and cost including handling and installation.

Type of materials (Para 5.1 & 5.2 of CPHEEO Manual)

Factors influencing the selection of Material for sewer construction are flowcharacteristics , availability in the sizes required including fitting and case of handling andinstallation, water fighters and simplicity in assembling, physical strength, resistance to acids,alkalies, gases solvents etc. resistance to scour, durability and cost including handling andinstallation. No single material will meet all the conditions that may be encountered in sewerdesign. Selection should be made for the particular application and different materials may beselected for parts of a single project.

According to CPHEEO Manual the following type of materials may be used for sewerconstruction.

(i). Brick work is used for construction of sewer particularly for large diameters. Brick sewersshall have cement concrete or stone for invert and 12.5 mm thick cement plaster with neatfinish. To prevent ground water infiltration, it is desirable to plaster the outer surface.

(ii)In sewerage pumping system or Rising Main, the internal pressure is very high sometimes.There may be pressure fluctuations and occasional surge. Any failure or breakage in theRising main will jeopardize the whole system since the Rising main is the most vital part ofthe sewerage system. At present for pressure mains Pre- stressed concrete (PSC), Cast Iron(CI) and Ductile Iron (DI) pipes are used. Use of MS pipes should be avoided since MS

pipes are very much prone to chemical and septic corrosion. MS. pipe should not be usedfor partially full sewage. But for higher diameters in the range of 1200 to 1800 mm MS

pipes /PSC pipes with Sulphate Resistant Cement (SRC) lining can be used.

(iii)In case of gravity sewer system, Reinforced Cement Concrete (RCC) pipes, Stoneware pipes,CI pipes and DI pipes with SRC lining are usually adopted

Stoneware or Vitrified clay (Para 5.2.3 of CPHEEO Manual)

The Vitrified clay pipes is advantageous over other pipe material on high resistance tocorrosion and erosion due to grit and high velocities. Though a minimum crushing strength of1600 kg/m is usually adopted for all sizes manufactured presently, vitrified clay pipes ofcrushing strength 2800 kg/m and over are manufactured in other countries. The strength ofvitrified clay pipes often necessitates special bedding or concrete cradling to improve fieldsupporting strength. The stoneware pipes and fittings shall withstand internal hydraulic test

pressure of 0.3 Mpa and 0.15 Mpa respectively without showing sign of injury or leakage. The

pressure shall be applied at a rate not exceeding 0.075 Mpa in 5 seconds (IS 3006:1979).


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Size of Pipe internal diameter

in mm

Wall thickness of

stoneware pipe

100 12 mm

150 15mm

200 16mm250 20mm

300 25mm

350 30mm

Jointing of Sewer pipes: From structural considerations of structural requirements jointsmay be classified as rigid and flexible joints. Joints such as cement mortar, lead, flangedand welded joints are under the category of rigid joints. All types of mechanical jointssuch as rubber gasket joints are flexible. Flexible joints are preferable to rigid joints

particularly with granular bed.

Width of Trenches : (Cause 3.2 of IS 4127:1967)

The width of the trench corresponds to the depth of the trench is given.Depth of Trench Width of Trench

1. Upto an average depth of 120 cm Diameter of pipe + 30 cm

2. Above 120 cm Diameter of pipe + 40 cm

Note: Width should not be less than 75 cm for depth exceeding 90 cm

Back filling: Trench shall be divided into 3 zones

Zone A: From bottom of trench to the level of center line of the pipe

Zone B : From the level of the center line of the pipe to a level 30 cm above top of the pipe

Zone C: From top of Zone B to the top of the trench

Zone A shall be refilled with sand, fine gravel or other approved materials

Zone B and Zone C shall be refilled with materials as prescribed by department.


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4. STRUCTURAL DESIGN OF BURIED SEWERS

(Chapter 6 of CPHEEO Manual)

The structural design of a sewer is based on the relationship that the supportingstrength of the sewer as installed divided by a suitable factor of safety must equal or exceedthe load imposed on it by the weight of earth and any superimposed loads.

The essential steps in the design and construction of buried sewers or conduits toprovide safe installations are therefore:

(i) Determination of the maximum load that will be applied to the pipe basedon the trench and backfill conditions and the live loads to be encountered.

(ii) Computation of the safe load carrying capacity of the pipe when installedand bedded in the manner to be specified using a suitable factor of safetyand making certain the design supporting strength thus obtained is greaterthan the maximum load to be applied.

(iii) Specifying the maximum trench widths to be permitted, the type of pipebedding to be obtained and the manner in which the backfill is to be made

in accordance with the conditions used for the design.(iv) Checking each pipe for structural defects before installation and making

sure that only sound pipes are installed and

(v) Ensuring by adequate inspection and engineering supervision that alltrench widths, sub grade work, bedding, pipe laying and backfilling are inaccordance with design assumptions as set forth in the projectspecifications.

Proper design and adequate specifications alone are not enough to ensure protection fromdangerous overloading of pipe. Effective value of these depends on the degree to which thedesign assumptions are realized in actual construction. For this reason thorough andcompetent inspection is necessary to ensure that the installation conforms to the design

requirements. There are three type of construction of Sewer (a) embankment condition (b)trench condition and (c) tunnel condition. (Para 6.1 & 6.31 of CPHEEO Manual)

Generally Sewers are laid in trenches by excavation of earth and refilling to the originalground level. Hence type of loads in trench condition are discussed below:

Type of loads (Para 6.2 CPHEEO Manual)

In a buried sewer, stresses are induced by external loads and also by internal pressure incase of a pressure main. The external loads are of two categories viz. load due to

backfill material known as backfill load and superimposed load which again is of twotypes viz. concentrated load and distributed load. Moving loads may be considered asequivalent to uniformly distributed load. Sewer lines are mostly constructed of stoneware,concrete or cast iron which are considered as rigid pipes (while steel pipes, if used are not

considered as rigid pipes). The flexibility affects the load imposed on the pipe and the stressesinduced in it.

Loads on conduits due to backfill: (Para 6.3 of CPHEEO Manual)


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The vertical dead load to which a conduit is subjected under trench conditions is theresultant of two major forces. The first component is the weight of the prism of soil within thetrench and above the top of the pipe and the second is due to the friction or shearing forcesgenerated between the prism of soil in the trench and the sides of the trench produced bysettlement of backfill. The resultant load on the horizontal plane at the top of the pipe within thetrench is equal to the weight of the backfill minus these upward shearing forces.

Computation of loads: The load on rigid conduits in trench condition is given by theMarstons formula in the form

Wc= Cdw B2

d

Wc= the load on the pipe in kg per linear metre

.w = the unit weight of backfill soil in kg/m3

Bd= the width of trench at the top of the pipe in m and

Cd= the load coefficient which is a function of a ratio of height of fill to width of trench(H/Bd)

H = Depth of refilling of soil from top of pipe to the ground level in metres.Weights of common filling materials (w) and values of Cdfor common soil conditions

encountered are given in Table 1 and 2 respectively.

The weights of common filling materials (w) are given in the table below

Table 1

Materials Weight (kg/m3)

Dry sand 1600

Ordinary (Damp sand) 1840

Wet sand 1920

Damp clay 1920

Saturated clay 2080

Saturated top soil 1840

Sand and Damp soil 1600

Table 2

Values of Cdfor calculating loads on pipes in trenches (Wc=CdWB2d)

Ratio H/B Safe working values of Cd


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Minimum

possible

without

cohesion

Maximum for

ordinary sand

Completely

saturated Top

Soil

Ordinary

maximum for

clay

Extreme

maximum for

clay

0.5 0.455 0.461 0.464 0.469 0.474

1.0 0.830 0.852 0.864 0.881 0.898

1.5 1.140 1.183 1.208 1.242 1.278

2.0 1.395 1.464 1.504 1.560 1.618

2.5 1.606 1.702 1.764 1.838 1.923

3.0 1.780 1.904 1.978 2.083 2.196

3.5 1.923 2.075 2.167 2.298 2.441

4.0 2.041 2.221 2.329 2.487 2.660

4.5 2.136 2.344 2.469 2.650 2.856

5.0 2.219 2.448 2.590 2.798 3.032

5.5 2.286 2.537 2.693 2.926 3.190

6.0 2.340 2.612 2.782 3.038 3.331

6.5 2.386 2.675 2.859 3.137 3.458

7.0 2.423 2.729 2.925 3.223 3.571

7.5 2.454 2.775 2.982 3.223 3.571

8.0 2.479 2.814 3.031 3.366 3.764

8.5 2.500 2.847 3.073 3.424 3.845

9.0 2.518 2.875 3.109 3.476 3.918

9.5 2.532 2.898 3.141 3.521 3.983

10.0 2.543 2.918 3.167 3.560 4.042

11.0 2.561 2.950 3.210 3.626 4.141

12.0 2.573 2.972 3.242 3.676 4.221

13.0 2.581 2.989 3.266 3.715 4.285

14.0 2.587 3.000 3.283 3.745 4.336

15.0 2.591 3.009 3.296 3.768 4.378

Very

Great

2.599 3.030 3.333 3.846 4.548

H- Depth of refill to top of pipe in metre

B- Trench width at top of pipe in metres


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2. Load on conduit due to super imposed loads: (Para 6.4 of CPHEEO Manuals)

The type of super imposed loads which generally encountered in buried conduits may be(a) concentrated load and (b) distributed load.

a) Concentrated Load: (Para 6.4.1 of CPHEEO Manual ) The formula for load due tosuper imposed concentrated load such as a truck wheel is given in the following form by Hollsintegration of Boussinesqs formula

Wsc= Cs(PF/L)

Wsc= the load on the conduit in kg/m

P = the concentrated load in kg acting on the surface

F = the impact factor (1.0 for air field runways, 1.5 for highway traffic and air field taxiways, 1.75 for railways traffic) and

Cs= the load coefficient which is a function of

Bc L----- and ----

2H 2H

Where

H = the height of the top of the conduit to ground surface in m

Bc= the outside width of conduit in m and

L=the effective length of the conduit to which the load is transmitted in m

Values of Csfor various values of (Bc/2H) and (L/2H) are obtained from Table 3

The effective length of the conduit is defined as the length over which the average loaddue to surface traffic units produces the same stress in the conduit wall as does the actual loadwhich varies in intensity from point to point. This is generally taken as 1m or the actual lengthof the conduit if it is less than 1 m

b) Distributed load : (Para 6.4.2 of CPHEEO Manual For the case of distributedsuperimposed loads, the formula for load on conduit is given by

Wsd= Csp F Bc

Where

Wsd= the load on the conduit in kg/m

.p = the intensity of the distributed load in kg/m2

f = the impact factor

Bc= The width of the conduit in m

Cs= the load coefficient, a function of D/2H and L/2H from Table 3

H = the height of the top of conduit to the ground surface in m and

D and L are width and length in m respectively of the area over which the distributedload

Field supporting Strength (Para 6.5.2 of CPHEEO Manual)

The field supporting strength of a rigid conduit is the maximum load per unit length,which the pipe will support while retaining complete serviceability when installed underspecified conditions of bedding and backfilling. The field supporting strength, however does not


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include any factor of safety. The ratio of the strength of a pipe under any stated condition ofloading and bedding to its strength measured by three edge bearing test is called the load factor.

The load factor does not contain a factor of safety. Load factors have been determinedexperimentally and analytically for the commonly used construction condition for both trenchand embankment conduits.

Supporting strength in Trench conditions (Para 6.5.3 of CPHEEO Manual)

Classes of bedding:

Four classes, A, B, C and D of bedding are used most often for pipes in trenches. Class Abedding may be either concrete cradle or concrete arch. Class B is a bedding having a shapedbottom or compacted granular bedding with a carefully compacted backfill. Class C is ordinarybedding having a shaped bottom or compacted granular bedding but with a lightly compactedbackfill. Class D is on with flat bottom trench with no care being taken to secure compaction ofbackfill at the sides and immediately over the pipe and hence is not recommended. Class B or Cbedding with compacted granular bedding is generally recommended. Shaped bottom isimpracticable and costly and hence is not recommended. The pipe bedding materials mustremain firm and not permit displacement of pipes which include Red gravel, coarse sand,crushed gravel etc. The material has to be uniformly graded or well graded.

Well graded material is most effective for stabilizing trench bottom and has a lesser tendencyto flow than uniformly graded materials. However, uniformly graded material is easier to placeand compact above sewer pipes.

Load factors (Para 6.5.3.2 of CPHEEO Manual)

LOAD FACTORS FOR DIFFERENT CLASSES OF BEDDING (Table 6.6 of CPHEEO Manual)

CLASS OF

BEDDING

CONDITION LOAD

FACTOR

A a. concrete cradle plain concrete and lightly tamped backfill 2.2

A b. Concrete cradle plain concrete with carefully tampled backfill 2.8A c. Concrete cradle RCC with P-0.4 % Upto 3.4

A d. Arch type plain concrete 2.8

RCC with P-0.4% Upto 3.4

RCC with P-1.0%(P is the ratio of the area of steel to the area of concrete at the crown)

Upto 4.8

B Shaped bottom or compacted granular bedding with carefully compactedbackfill

1.9

C Shaped bottom or compacted granular bedding with lightly compactedbackfill

1.5

D Flat bottom trench 1.1

Note: C type of bedding is normally adopted.

The granular material used must stabilize the trench bottom in addition to providing a firmand uniform support for the pipe. Well graded crushed rock or gravel with the maximum size notexceeding 25 mm is recommended for the purpose.

Where rock or other unyielding foundation material is encountered bedding may beaccording to one of the Class A,B or C but with the following additional requirements.Class A: The hard unyielding material should be excavated down to the bottom of the

concrete cradle.


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Class B or C: The hard unyielding material should be excavated below the bottom of the pipe andpipe bell to a depth of atleast 15 cm.

The width of the excavation should be atleast 1.25 times the outside dia of the pipe and itshould be refilled with granular material.

Total encasement of non-reinforced rigid pipe in concrete may be necessary where therequired safe supporting strength cannot be obtained by other bedding methods. The load factorfor concrete encasement varies with the thickness of concrete.

Relation ship between the different element in structural Design:

The basic design relationships between the different design elements for rigid pipes areas follows:

Safe working strength = Ultimate three edge bearing strength------------------------------------------Factor of safety

Safe field supporting strength = safe working strength x load factor

Note: The factor of safety recommended is `1.5

Problem: Determine the structural requirement of 200 mm dia stone ware pipe laid in a trenchto a width of 0.8 m in depth of 1.30 metre in ordinary soil and wheel load of 6.25tonnes.

Solution: Pipe thickness t= 16 mm for 200 mm dia

(i) Back fill load: BC = D + 2t = 200 + 2x16 = 232mm

Bd=0.8 m

H=1.30-0.232=1.068 m

H/Bd=1.068/0.8=1.335

Cd= 1.05 (From Table 2)

W= 1840 (From Table 1)

Wc=Cd W B2d =1.05 x 1840 x 0.8

2 = 1237 kg/m

(ii) Concentrated load

L = 0.60 (normal length of Stoneware pipe)

H= 1.068 m

L/2H=0.60/2 x 1.068 = 0.28

BC/2H= 0.232/2x1.068 = 0.11

From Table 3 of CPHEEO Manual for values L/2H = 0.28 and BC/2H = -11

Cs=0.0498

Wsc = C S P F/L= 0.0498 x 1.5 /0.60 = 778 kg/m


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(iii) Internal load ie Water Load at 75 % flow

22 2 2 1 75Water load = ---- x --- x --- x----x 1000 x 0.6x -----

7 10 10 4 100

= 14 kg/m

Total load WLo= 1237 + 778+14 = 2029 kg/m

Safe supporting strtength of 200 mm stone ware pipe

with `C clean bedding= 1650 x 1.5/1.5 = 1650 kg/m

Audit Approach

Interalia the following audit points could be seen

1. Cases where due to defective design and execution of sewer and sewer appurtenances,the designed quantity of sewer could not reach the collection well causing overflow or

leakages. This untreated sewage water due to leakage would pollute the river or lake

causing public ill health and pollution. This aspects may be analysed.

2. Though stoneware pipe were sufficient for collection sewer up to 350 mm dia, CI pipesare being used. The safety factor and design criteria for the sewer has to be examined

and the extra cost on use of pipes other than stone ware for collection systems upto 350

mm dia may be commented.

3. Even in case of use of other pipes, the class of pipe used may be analysed with referenceto designed pressure and extra cost on use of higher class of pipe may be commented.

4. Whether trenches were excavated to the specified width or not the extra cost due tohigher width of trenches may be commented.

4. SEWAGE AND STORM WATER PUMPING STATION


Pumping stations handle Sewage/Storm water either for lifting the sewage so as todischarge into another gravity sewer or for treatment/disposal of the Sewage/effluent.

The capacity of the pumping station has to be based on present and future sewage flowconsidering a design period of 15 years. The civil structures and pipelines of both drysump and the wet well should be designed for a flow of 30 years. The needs of futureexpansion need special attention especially in respect of provision of additional space forreplacing the smaller pumping units by larger area, increasing the capacity of the wet

well and constructing new pumping station to cope with the increased flow. The initialflows are generally too small and the effect of the minimum flow should be studied

before selecting the size of the pumps for the project to be commissioned in order toavoid too infrequent pumping operation and long retention of sewage wet wells. (Para9.3) of CPHEEO)

Pumping stations traditionally have two wells, the wet well receiving the incomingsewage and dry well housing the pumps.


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Dry well: (Para 9.6.1 of CPHEEO) The size of the dry well should be adequate to housenumber of pumps at the desired capacity of pumping.

Wet Well: (Para 9.6.2 of CPHEEO) The size of the wet well is influenced by the sewagecapacity to be provided. The capacity of the well is to be so kept that with any combination ofinflow and pumping the cycle of operation for each pump will not be less than 5 minutes and themaximum detention time in the wet well will not exceed 30 minutes of average flow.

In the wet well baffles should be provided at required places to ensure uniform flow ateach pump suction.

Wherever possible grid removal ahead of pumping should be adopted to increase the lifeof the pumps.

PUMPS: (Para 9.7 of CPHEEO Manual)

The selection of pump is based on many consideration such as the type of pump, the sizeof pump, the number of pumps, the capacity or flow rate of each pump range of throttling ofeach pump, the head of pumping and others.

Capacity of the pump shall be adequate to meet the peak rate of flow with 50% stand by.To obtain the least operating cost, the pumping equipments should be selected to perform

efficiently at all flow including the peak flow. Two or more pumps are always desirable atsewage pumping station. The size and number of unit for larger pumping station, shall beso selected that the variations of inflow can be handled by throttling of the delivery valuesor by varying the speed of the pump without starting and stopping the pump too frequentlyor necessitating excessive storage. (Para 9.7.1 of CPHEEO Manual)

The capacity of a pump is usually stated in terms of Dry Weather Flow (DWF) estimatedfor the pumping station. The general practices is to provide 3 pumps for small capacity

pumping station comprising of 1 pump of 1 DWF, 1 of 2 DWF and third of 3 DWFCapacity. For large capacity pumping station, 5 pumps are usually provided comprising of2 of DWF 2 for 1 DWF and 1 of 3 DWF capacity including standby. (Para 9.7.1 ofCPHEEO Manual)

The total head of pumping has to be calculated taking note of four factors; (Para 9.7.5 ofCPHEEO Manual)

(i) the difference between the static level of the liquid in the suction sump in the wetwell and the highest point on the discharge side ie. Static head.

(ii) the rate of flow and size of the discharge mouth determine the velocity at the point ofdischarge (ie. Velocity head or kinetic head)

(iii) the difference in the pressure on the liquid in the suction sump and at the point ofdelivery rate, delivery pressure will be higher than the atmospheric pressure (i.e.Pressure head)

(iv) the frictional losses across the pipes, values, bend and all such appurtenance (i.e.Frictional loss)


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Operation of the pumpsDry running of pumps should be avoidedThe delivery valueshould be operated gradually to avoid surges.

Electrical Equipment (Para 9-9 of CPHEEO Manual)

The electrical equipment selected shall be adequate, reliable and safe. The adequacy isdetermined by the continuous current required for the station load and the available short circuitcharacteristic of the power supply. The reliability depends upon the capacity of the electrical

system to deliver power, when and where it is required, under normal as well as abnormalconditions. Safety involves the protection life and also the safe guarding of the equipment underall conditions of operation & maintenance. None of these three aspects shall be sacrificed for thesake of initial economy. The electrical system shall be designed with such flexibility as to permitone or more components to be taken out of service at any time without interrupting the continuousoperation of the station. A proper selection of voltages in the electrical types is one of the mostimportant decisions that will affect the overall system characteristic and the plant performance.

Normally outdoor transformer sub station are provided and may be indoors also on publicsafety protection etc.,

Duplicate transformer may be provided where installation so demands.


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Audit Approaches

Interalia the following could be seen

Cases where pumps and motor designed for ultimate stage instead of 15 years may beidentified and comments on wasteful expenditure on installation of higher capacity of pump

set and recurring extra cost of power consumption due to higher capacity may be included.

Whether pumpsets are installed to the prescribed level for DWF or in excess ofrequirements. Unwarranted installation cost on extra pumpset may be commented

Cases of failure of pumpset resulting in non pumping of Sewage loading for pollution mayalso be commented after analyzing the causes for failure of pumpset.

Cases where transformer of the stand bye or one stand bye besides diesel Generator of thesame capacity provided especially by CMWSS (Metro Water) Chennai. Normally one

duplicate (Stand bye) is required for. The excess provision of transformer and the cost

there of may be commented..


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5. SEWAGE TREATEMENT

(Chapter 10 of CPHEEO Manual )

The object of sewage treatment is to stabilise decomposable organic matter present insewage and the effluent and sludge which can be disposed of in the environment without causinghealth hazards or nuisance.

The processes commonly employed in domestic waste water treatment function and unitsused to achieve these functions are listed below

1. Design Period:

Design period of 30 years after its completion. The time lag between the design and thecompletion could not oridinarily exceed 2 to 3 years and in exceptional circumstances 5 years.Construction of sewage treatment plant may be carried out in phases with an intial design periodranging from 5 to 10 years excluding the construction period so that expenditure far ahead of

utility is avoided. The comparative merits to cover the full 30 years period versus the first 15years or earlier should be examined to decide the most economical initial arrangementssatisfactorily to cover the first 15 years or lesser. (Para 10.2 of CPHEEO Manual)

2. Sewerage Flow:

The quantity of sewerage and its characteristic show a marked range of hourly variationand hence peak, average and minimum flows are important consideration. The process loadingin the sewage treatment are based on the daily average flows and average characteristics asdetermined from a 24 hour weighted composite sample. In the absence of any data average flowof 150 lpcd may be adopted. The hydraulic design load varies from component to component ofthe treatment plant with all appurtenance conduits, channel, etc. being designed for themaximum flow which may vary from 2.0 to 3.5 times the average flow.

Sedimentation tanks are designed on the basis of average flow while consideration ofboth maximum and minimum flow is important in the design of screen and grit chamber.


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3. Unit operations/processes, their functions and devices used for domestic wastewater treatment

(Table 10-1 of CPHEEO Manual)

S.NoUnit operations and

processFunctions Treatment Devices

1 SCREENING Removal of large floating,suspended and settleable solids

Bar racks and screens ofvarious description

2 GRIT REMOVAL Removal of inorganicsuspended solids

Grit chamber

3 PRIMARYSEDIMENTATION

Removal of organic andinorganic settable solids

Primary sedimentationtank

4 a) AEROBIC BIOGICALSUSPENDED GROWTHPROCESS

Conversion of colloidal,dissolved and residualsuspended organic matter intosettable biofloc and stableinorganics

Activated sludge processunits and itsmodifications, wastestabilisation Ponds,Aerated Lagoons.

b) AEROBIC BIOGICALATTACHED GROWTHPROCESS

Same as above Trickling Filter, RotatingBiological Contactor

5 ANAEROBICBIOLOGICAL GROWTHPROCESSES

Conversion of organic matterinto CH4 & CO2 and organicrelatively stable organicresidue

Anaerobic Filter, BedSubmerged MediaAnaerobic Reactor,Upflow AnaerobicSludge Blanket Reactor;Anaerobic RotatingBiological Contactor

6 ANAEROBICSTABILISATION OFORGANIC SLUDGES

Same as above Anaerobic Digester

4. Sewage treatment of processes (Para 10-10 of CPHEEO Manual )

Sewage treatment processes may be generally classified as primary, secondary and

tertiary. The general yardstick of evaluating the performance of sewage treatment plants is thedegree of reduction of Biochemical Oxygen Demand (BOD), Suspended Solids (SS) and TotalColiforms. The efficiency of a treatment plant depends not only on proper design andconstruction but also on good operation and maintenance. Expected efficiencies of varioustreatment units are given below:

Expected efficiencies of various treatment units (Table 10-3 of CPHEEO Manual )

S.No.Process

Percentage reduction

SS BODTotal

coliform

1 Primary Treatment (Sedimentation) 45-60 30-45 40-60

2 Chemical Treatment 60-80 45-65 60-90

3 Secondary Treatment(i) Standard trickling filters

75-85 70-90 80-90

(ii)High rate trickling filters(a) single stage(b) Two stage

75-8590-95

75-8090-95

80-9090-60

(iii) Activated sludge plants 85-90 85-95 90-96

(iv) (a) Stabilization ponds (Single cell)(b) Stabilization ponds (Two Cells)

80-9090-95

90-9595-97

90-9595-98


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Tertiary treatment is adopted when reuse of effluent for industrial purposes iscontemplated or when circumstances dictate the requirement of higher quality effluents.

Cost is the prime consideration in the selection of the treatment method. It shouldinclude the cost of installation, capitalized cost of maintenance and operation taking into accountinterest charges and period of amortisation. An alternative will be to consider the annual costcovering amortisation and interest charges for the loan obtained for the installation together with

the annual operating and maintenance costs. In some cases there is a component of subsidygranted by the Government for the installation of the treatment works and the maintenance costis borne entirely by the local body or the agency concerned. Both these will have to be takeninto account for making realistic comparison of the alternatives.

Other factors that may influence are ease of construction and maintenance, benefits thataccrue from better environmental sanitation, location, availability of land and topographicalconditions.

5. Sedimentation (chapter 12 of CPHEEO manual)

The purpose of sedimentation of sewage is to separate the settleable solids so that thesettled waste water, if discharged into water courses, does not form sludge banks and when usedfor land disposal does not lead to clogging of soil pores and excessive organic loading. Primarysedimentation of sewage also reduces the organics load on secondary treatment units.Sedimentation is used in waste water treatment to remove (i) inorganic suspended solids or grit ingrit chamber (ii) organic and residual inorganic solids, free oil and grease and other floatingmaterials, etc. The settleable solids to be removed from waste water in primary or secondarysettling tank after grit removal.

6. Design considerations: (for primary and secondary settling tank or clarifier)

(Para 12.4 of CPHEEO manual)

Several factors such as flow variations, density currents, solids concentration, solidsloading, area, detention time and overflow rate influence the design and performance of

sedimentation tanks. Sedimentation tanks are designed for average flow conditions. Hence,during peak flow periods, the detention period gets reduced with increase in the overflow rate andconsequent overloading for a short period. If hourly flow variations are wide as in the case ofsome industrial waste flows, it may be necessary to build an equalization tank, ahead of thetreatment units so that uniform loading is made possible in all treatment units.

For primary sedimentation tanks, both, surface overflow rate and detention period(hydraulic residence time) are important criteria as the solids to be settled are flocculent in natureand undergo flocculation. The major design parameters for secondary settling tanks designed toremove bioflocculated solids are solids loading rate or solid flux as well as surface over flow rate.The plan surface area of secondary settling tanks is determined using both criteria and the greaterof the two is adopted for design. In addition, other design parameters are depth, displacement

velocity (horizontal velocity of flow) and weir loading rate.

The overflow rate represents the hydraulic loading per unit surface area of tank in unit timeexpressed as m3/d/m2 . Overflow rates must be checked both at average flows and peak flow. Thesmaller values in the ranges given are applicable to small plants of capacities less than 5 mld.


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The solids loading rate or solid flux is an important decision variable for the design ofsecondary sedimentation tank receiving bioflocculated solids. The solid flux represents the solidsloading per unit surface area of tank per unit time and is expressed as kg SS/m2d.

Weir loading influences the removal of solids in sedimentation tank, particularly insecondary settling tanks where flocculated solids are settled. There is no positive evidence thatweir loading has any significant effect on removal of solids in primary settling tanks. However,certain loading rates based on practice are recommended both for primary as well as secondary

tanks. For all primary, intermediate and secondary settling tanks, except in the case of secondarytanks for activated sludge process, weir loading of the order of 125m3/d.m. for average flows isrecommended. For secondary settling tanks in activated sludge or its modifications, the weirloading is around 185 m

3/d.m. The loading should however ensure uniform withdrawal over the

entire periphery of the tank to avoid short circuiting or dead pockets. Performance of existingsedimentation tanks can be improved by merely increasing their weir length.

The depth sets the detention time in the settling tank and also influences sludge thickeningin secondary settling tanks of activated sludge plants.

Design parameters for settling tanks or clarifiers

(Table 12.1 of CPHEEO manual)

Type of settling Overflow rateM3/d/m2

Solid loadingKg/m2/d

DepthM

DetentiontimeHour

Average Peak Average Peak

A. PRIMARY SETTLING

1) Primary settling only 25-30 50-60 2.5 3.5 2.0 2.5

2) Primary settling followed bysecondary treatment

35-50 80-120 2.5 3.5

3) Primary settling with activatedsludge return

25 35 50 60 3.5 4.5

B. SECONDARY SETTLING

4) Secondary settling for trickling filter 15-25 40 50 70 120 190 2.5 3.5 1.5 2.0

5) Secondary settling for activatedsludge(excluding extended aeration)

15-35 40 50 70 140 210 3.5 4.5 -

6) Secondary settling for extended

aeration

8 15 25 35 25 120 170 3.5 4.5 -

Note: Para 10.4 of CPHEEO manual stipulates the design of the clarifier foraverage flow.

7. Detention period (Para 12.4.2.2 of CPHEEO Manual)

The rate of removal of BOD and SS is maximum during the first 2 to 2 hours of settlingand thereafter decreases appreciably. Hence, increase in the detention time beyond 2 to 2 hourswill not increase the percentage removal of BOD or SS proportionately. Longer detention periodmay affect the tank performance adversely due to setting in of septic conditions, particularly intropical climate. Experience has shown that a detention period of 2 to 2 hours for primarysettling tanks and 1 to 2 hours for secondary settling tanks will produce the optimum results.

Longer detention periods in secondary settling tanks may result in denitrification whichadversely affects the settling efficiency.


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8. Sludge removal (12.4.2.06 of CPHEEO Manual)

Sludge can be removed manually, hydrostatically or mechanically from the sedimentationtanks. Mechanical cleaning of sludge should be preferred to manual cleaning even in small plants,to prevent exposure of workers to health hazards.

9. Sludge digestion & production of gas (para 17.4 of CPHEEO manual)

The principal purposes of sludge digestion are to reduce its offensive odour, pathogeniccontents and to improve its dewatering characteristics. This can be achieved through any of thefollowing biological process (i) anaerobic digestion & (ii) aerobic digestion. During the processof sludge digestion, sludge gas is emanated. Sludge gas is normally composed of about 60 to 70%methane and 25 to 35% carbon dioxide by volume with smaller quantities of other gases likehydrogen sulphide, hydrogen, nitrogen and oxygen. The combustible constituent in the gas is

primarily the methane. Hydrogen sulphide in addition to its corrosive properties causes nuisanceduring the burning of the gas. In term of solids digested, the average gas production is about0.9m3/kg. of volatile solids destroyed at a normal operating pressure of 150 to 200mm of water.

Minimum or maximum rates of gas production will however depend upon the mode offeeding of raw sludge into the digester. The minimum and maximum gas production rates mayvary from 45% to more than 200%. In the continuous feeding system, the difference between themaximum and the minimum is considerably reduced. Intermittent mixing of digester contents isalso responsible for wide fluctuations in gas production rates.

Sludge gas should be collected under positive pressure in the gas holder from the primaryand the secondary units besides from the sludge digester. A gas dome above the digester roofshould be used for gas take off.

A distance of atleast 30m should be kept between a waste gas burner and a digestion tank

or gas holder to avoid the possibility of igniting the gas mixture. Waste gas burners should belocated in the open for easy observation. Where the gas is to be used as domestic fuel or forpower generation, additional equipments like compressor, etc. may have to be used.

Carbon credits :

The United Nations Frame Work Convention on Climate Change based in Bonn,Germany created the concepts of issuing certificates of Carbon credits to countries that reducetheir emissions by implementing environment friendly projects. In sewage treatment methanegas (bio gas) generates during the process of sludge digestion and the bio gas can be used forgenerating electricity there by avoiding the bio gas being let into air. Carbon credit is given forutilizing the bio gas by the United Nation body.

Note : Chennai Metro Water is set to receive Rs.4 crore annually from United Nationbody for using bio gas in sewage Treatment (The Hindu dated 3.6.2007)


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Audit Approaches

1. Whether the treatment plant was designed and constructed to the norms prescribed.Extra capacity of various components of treatment unit may be commented.

2. Cases of failure of treatment units leading for non treatment of sewage and resultantimpact in public health, pollution etc. has to be analysed and commented.

3. Table 12.1 of CPHEEO Manual prescribes design paramateres for settling tank orclarifier (Primary & Secondary) para 12.4.2.2 of CPHEEO Manual prescribes the

detection time. Longer detention time may affect adversely the performance of the tank

and setttling efficiency. Longer duration of detention time would if adopted in the

design parameters would increase the volume of the clarifier involving extra cost. The

correctness of design parameters adopted has to be analyzed and commented.

4. Para 10.4 of CPHEEO Manual stipulates the design of clarifier for average flow. But incase where design of clarifier for peak flow has to be analysed which involves increase

in capacity of clarifier. The excess capacity of clarifier has to be commented.

5. The guidelines of National River Conservation Project provided to utilize the gasproduced from sewage treatment for operation and maintenance of the sewage treatment

plant. Para 17.4 of CPHEEO Manual also prescribes various parameters on gas

generation and utility. Failure to design the sewage treatment for storing the gas and

utilizing it may be commented.


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6. STABILIZATION PONDS(Chapter 15 of CHPEEO manual)

Stabilization ponds are open, flow-through earthen basins specifically designed and

constructed to treat sewage and biodegradable industrial wastes. Stabilization ponds providecomparatively long detention periods extending from a few to several days. During this periodorganic matter in the waste is stabilized in the pond. Pond systems in which oxygen is providedthrough mechanical aeration are called aerated lagoons. Lightly loaded ponds used as tertiarystep in waste treatment for polishing of secondary effluents and removal of bacteria are calledmaturation ponds.

Under many situations in warm climate countries pond systems are cheaper to constructand operate compared to conventional methods. The only disadvantage of pond systems is therelatively large land that they require. Pond systems must be considered as an alternative whentreatment of waste water or upgrading of existing facilities are planned and the life time costs ofvarious other treatment system should be calculated and compared.

CLASSIFICATION

1. AerobicAerobic ponds are designed to maintain completely aerobic conditions. They are used for

primary effluent which allow penetration of light throughout the liquid depth. The ponds are keptshallow with depth less than 0.5m and BOD loadings of 40 120 kg/ha.d. The pond contents may

be periodically mixed. Such ponds develop intense algal growth and have been used onexperimental basis only.

2. AnaerobicCompletely anaerobic ponds are used as pretreatment for high strength industrial wastes

and sometimes for municipal wastewaters. They are also used for digestion of municipal sludges.Depending on temperature and waste characteristics, BOD load of 400 3000 kg/ha.d and 5-50day detention period would result in 50-85 percent BOD reduction. Such ponds are constructedwith a depth of 2.5 5m to conserve heat and minimize land area requirement. Usually they havean odour problem.

3. FaculativeThe facultative pond functions aerobically at the surface while anaerobic conditions

prevail at the bottom. The aerobic layer acts as a good check against odour evolution from thepond. The treatment effected by this type of pond is comparable to that of conventional secondarytreatment processes. The facultative pond is hence best suited and most commonly used fortreatment of sewage.


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7. WASTE STABILIZATION PONDS

(Design Manual for waste stabilization ponds in India by Duncan Mara)

Introduction

Waste stabilization ponds (WSP) are shallow man made basins into which wastewaterflows and from which after a retention time of several days (rather than several hours inconventional treatment processes), a well-treated effluent is discharged. WSP systems comprise aseries of ponds anaerobic, facultative and several maturation. The different functions and modesof operation of these three different types of pond are described in Section 3 of this manual. TheWSP are low cost, high efficiency low energy, low maintenance and above all a sustainablemethod of waste water treatment. They are highly appropriate under many conditions in India.

Type of WSP and their function (Chapter 3)

WSP systems comprise a single series of anaerobic, facultative nd maturation ponds, orseveral such series in parallel. In essence, anaerobic and facultative ponds are designed for BODremoval and maturation ponds for pathogen removal, although some BOD removal occurs in

maturation ponds and some pathogen removal in anaerobic and facultative pond. Designers areafter reluctant to use anaerobic ponds because of the fear of significant level of odous nuisance.

Effluent quality requirements (Chapter 4)

In India general standards for the discharge of treated waste waters into inland surfacewaters are given in the Environment (Protection) Rules 1986 (see CPCB. 1996). The moreimportant of these for WSP design are as follows.

BOD (Bio oxygen Demand 30mg/ISuspended solids 100 mg/ITotal N (Nitrogen) 100mg/N/ITotal ammonia 50mg N/I

Free ammonia 5 mg N/ISulphide 2mg/IPH 5.5. 9.0

Design Parameters (Chapter 4)

The four most important parameters for WSP design are temperature, net evaporation ,flow and BOD. Faecal coliform and helminth egg numbers are also important if the final effluentis to be used in agriculture or aquaculture.

1. Anaerobic Ponds

No advice is given on the design of anaerobic ponds in the Government of Indias Manualon Sewerage and Sewerage Treatment (Ministry of Urban Development 1995). However they can

be satisfactorily designed on the basis of volumetric BOD loading

2. Facultative PondThe Indian Manual on Sewerage and Sewage Treatment (Ministry of Urban Development

1995) gives two methods of selecting the permissible design value of BOD loading: one based onlatitude and one based on temperature.

The Manual on Sewerage and sewage Treatment (Ministry of Urban Development. 1995)permit only 65-70% of the calculated area to be used for the facultative pond with the remaining30-35% to be used for a maturation pond. This increases the BOD surface loading on thefacultative pond by 43-54% and resulting higher loading.


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3. Maturation Ponds

Maturation ponds may be single pond or series of ponds constructed for removal of faecalcoliform, Helminth egg ponds require careful design to ensure that their faecal coliform removal.BOD loading on the first maturation pond must not be higher than that on the preceding

facultative pond and it is preferable that is significantly lower. The maximum permissible BODloading or the first maturation pond is taken as 75 percent of that on the preceding facultative

pond. The loading on the first maturation pond is calculated on the assumption that 80 percent ofthe BOD has been removed in the proceeding anaerobic and facultative ponds ( or 70% fortemperature below 200C, The maturation pond area is calculated from the equation.

Am = 2 Qi Om/(2D+0.001e Om)Where Am = Area of maturation pond

Qi = mean flow m3/dayOm = Retention time in daysD = Pond depth in metric usually 1.5me = Net evaporation rate mm/day

Note:

In the past waste stabilisation pond technology was used for establishing sewage treatmentplant in some areas. For treating one million litres of sewage a day 2.5-3 acres land was required.Now new technologies requiring less land such as fluid aerobic bio-reactor, sequencing batchreactor and modified activated sludge process have emerged. They are cost effective. In 2006Govt. of Tamil Nadu constituted Experts Committee with the task of identifying andrecommending technology options on urban local bodies. Depending upon the requirement andsuitability, the expert committee gives its advice to each local body (Source : The Hindu27.4.2007)

Thus the concept of WSP system may not be of much need in Tamilnadu.


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8.EFFLUENT DISPOSAL AND UTILISATION


Treatment effluent conforming to prescribed standards may be disposed into a stream courseor into sea or a stagnant body of water. Since the treated waste water may still have acoliform density befo

handbook on sewage and swerage treatment plants

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