Sewerage

Works Audit – Sewerage and Sewage Treatment

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HANDBOOK ON SEWERAGE AND SEWAGE TREATMENT

Office of the Principal Accountant General (Civil Audit) Chennai


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SEWERAGE AND SEWAGE TREATMENT 1. PREAMBLE The standard specification prescribed in the Manual on Sewerage and Sewage Treatment issued by Central Public Health and Environmental Engineering Organisation, Ministry of Urban Development, in December 1993, guidelines issued by Ministry of Environment and Forest Department, National River Conservation and guidelines prescribed by Chennai Metropolitan and Water Supply and Sewerage Board and TWAD Board on Sewerage 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 that sewage or excreta and sullage discharged from communities is properly collected, transported, treated to the required degree and finally disposed of without causing any health or 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 water disposal systems is different in each case, they still have to function within the priorities fixed by the national and state governments governments and to keep in view view overall requirements of the area. The waste water disposal projects formulated by the various State sponsoring Authorities at present present do not always contain all the essential essential elements for appraisal. When projects are assessed for their cost benefit be nefit ratio and for institutional or funding purposes, they are not amenable for comparative study and appraisal. appraisal. Also at times different standards are adopted by the Central and State agencies regarding regarding various design parameters. It is necessary therefore to specify appropriate appropriate standards and design criteria criteria and to avoid different approaches

CPHEEO Manual) 3. Basic Design considerations ( Para 1.3 of CPHEEO In designing waste water collection, treatment and disposal systems, planning generally begins from the final disposal point going backwards to give an integrated and optimum design Co suit the topography and the available hydraulic head, supplemented by pumping if essential.



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SEWERAGE AND SEWAGE TREATMENT 1. PREAMBLE The standard specification prescribed in the Manual on Sewerage and Sewage Treatment issued by Central Public Health and Environmental Engineering Organisation, Ministry of Urban Development, in December 1993, guidelines issued by Ministry of Environment and Forest Department, National River Conservation and guidelines prescribed by Chennai Metropolitan and Water Supply and Sewerage Board and TWAD Board on Sewerage 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 that sewage or excreta and sullage discharged from communities is properly collected, transported, treated to the required degree and finally disposed of without causing any health or 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 water disposal systems is different in each case, they still have to function within the priorities fixed by the national and state governments governments and to keep in view view overall requirements of the area. The waste water disposal projects formulated by the various State sponsoring Authorities at present present do not always contain all the essential essential elements for appraisal. When projects are assessed for their cost benefit be nefit ratio and for institutional or funding purposes, they are not amenable for comparative study and appraisal. appraisal. Also at times different standards are adopted by the Central and State agencies regarding regarding various design parameters. It is necessary therefore to specify appropriate appropriate standards and design criteria criteria and to avoid different approaches

CPHEEO Manual) 3. Basic Design considerations ( Para 1.3 of CPHEEO In designing waste water collection, treatment and disposal systems, planning generally begins from the final disposal point going backwards to give an integrated and optimum design Co 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 basic design considerations:

a. b. c. d.

Engineering Environmental Process Cost

These considerations are discussed below in detail: a)

Engineering Considerations ( Para 1.3.1 of CPHEEO Manual)

Topographical, engineering and other considerations which figure prominently in project design are noted noted below: 1. Design period, stage wise population to be served and expected sewage flow and fluctuations 2. Topography of general general area to be served, served, its slope and terrain. Tentative sites available for treatment plant, pumping stations and disposal works 3. Available hydraulic head in the system up to high flood level in case of disposal to a nearby river or high tide level in case of coastal discharge or the level of the irrigation are to be commanded in case of land disposal 4. Ground water depth and its seasonal fluctuation affecting construction, sewer infiltration, structural design (uplift considerations) 5. Soil bearing capacity and type of strata expected to be met in construction 6. On site disposal facilities, including the possibilities of segregating the sullage water and sewage and reuse or recycle sullage water w ater within the households b)

Environmental Considerations: (Para 1.3.2 of CPHEEO Manual)

The environmental and socio-economic impacts of a sewage treatment plant may prove adverse during the operation stage. Therefore the following aspects should be considered during design. i) ii) iii) iv) v)

Surface water Hydrology and Quality Ground water quality Coastal water quality Odour and Mosquito nuisance Public Health and

vi)

Land scaping

c)

Process Considerations: ( Para 1.3.3 of CPHEEO Manual)

Process considerations involve factors which affect the choice of treatment method, its design criteria and related requirements such as the following: i) ii) iii)

Waste water flow and characteristics Degree of treatment required Performance characteristics

iv)

Other process requirements such as land, power operating equipments, skilled staff, nature of maintenance problems, extent of sludge production and its disposal requirements, loss of head through plant in relation to available head etc



d)

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Cost Considerations: ( Para 1.3.4 of CPHEEO Manual)

The overall costs (Capital and operating) have to be determined in order to arrive at the most optimum solution 4 Design Period ( Para 1.4 of CPHEEO CPHEEO Manual)

Sewerage projects may be designed normally to meet the requirements over a thirty year period after their completion. completion. The period between design and and completion should also be taken into account which should be between three to six years depending on the type and size of the project. The thirty year period may however be modified modified in regard to certain components of the project depending on their useful life or the facility for carrying out extensions when required 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 the beginning itself. Where expensive tunnels and large aqueducts are involved entailing large capital 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 30

1

Collection System i.e. Sewer network

2.

Pumping stations (Civil Works)

30

3.

Pumping Machinery

15

4.

Sewage Treatment Plant

30

5.

Effluent disposal and utilization

30

Clarification

The system should be designed for the prospective population of 30 years, as its replacement is not possible during its use. Duplicating machinery within the pumping station would be easier/cost of civil works will be economical for full design period. Life of pumping machinery is generally 15 years. The construction may be in a phased manner as initial the flows may not reach the designed levels and it will be uneconomical to build the full capacity plant initially. (Refer Chapter 10.2). Provision of design capacities in the initial stages 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 factors governing the future growth and development of the project area in the industrial, commercial, educational, social social and administrative spheres. Special factors causing causing sudden immigration 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 from out of the following following mathematical methods, methods, graphically interpolated where necessary. The following are some of the methods prescribed by the CPHEEO for working out the projected population. a)

Demographic method of Population Projection

b)

Arithmetical increase Method

c)

Incremental increase Method



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d)

Geometrical increase Method

e)

Decreasing rate of Growth

f)

Graphical method i) Graphical method based on single city ii) Graphical method based on cities with similar growth pattern

g)

Logistic Method

h)

Method of Density

Note: Detailed procedure for estimating the population forecast given in Manual on Water Supply & 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 time during the period of design can be derived by any one of the aforesaid methods appropriate to each case, the density and distribution of such population within the several areas, zones or districts will again have to be made with a discerning judgement on the relative probabilities of 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 or other appropriate authorities are available, the decision regarding the design population should 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 (excluding construction period) is recommended for all types of sewers. (Para 3.2.1 of CPHEEO Manual) Where a Master Plan containing land use pattern and zoning regulations is available for 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 on population the following densities as suggested for adoption. (Para 3.2.2 of CPHEEO Manual)

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 by the local authority this approach may be used for working out the population density. FSI or FAR 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 reservations for 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 m 2 /person Number of persons or density per hectare = 0.56x10,000x0.5 = 311 9 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 and industrial wastes excluding storm water. Storm water sewers designed to carry 5 carry off storm 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 initially suffers 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 the present and probable quantities expected at the end of design period. The estimation of flow is 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 in a sanitary sewer. However, the observed Dry Weather Flow quantities (DWF) usually are slightly 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 to industrial wastes, changed water use habits etc. Generally, 80 % of the water supply may be expected 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 may discharge their liquid wastes into the sanitary sewers. Estimates of such flows have to be made separately. The details of requirements of water for institutions and industries are discussed 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 discharged into 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 a small area be designed on the basis of saturation density. Infiltration into sewer may occur through pipes, pipe joints and structures. probable amount has to be evaluated carefully. b. Flow Assumptions

:

The


The flow in sewers varies considerably from hour to hour and also seasonally, but for the purposes of hydraulic design it is the estimated peak flow that is adopted. 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 to the 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, if required. The minimum flow may vary from 1/3 to 1/2 of average flow. c Ground water infiltration :


Estimate of flow in sanitary sewers may include certain flows due to infiltration of ground water through joints. The quantity will depend on workmanship in laying of sewers and level of the ground water table and permeability of the surrounding soil. Since sewers are designed for peak discharges, allowance for ground water infiltration for the worst condition in 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 construction aids, these values should tend to the minimum, rather than the maximum. These values should 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 volume and 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 or the sewage treatment process. Quality and character of waste water are to be based on flow measurements 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 run off Estimation of Storm water Run off

Sanitary sewers are not expected to receive storm water. Strict inspection and vigilance and proper design and construction of sewers and manholes should eliminate this flow 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 and duration of precipitation, characteristics of the tributary area and the time required for such flow to reach the sewer. The storm water flow for this purpose may be determined by using the rational method, hydrograph method, rainfall-runoff correlation studies, digital computer models, inlet method or empirical formulae. Of all these methods, the rational method is more 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 including depressions and water pockets, shape of the drainage basin and duration of the precipitation determine the fraction of the total precipitation which will reach the sewer. This fraction known as the coefficient of runoff needs to be determined for each drainage district. The runoff reaching the sewer is given by the expression, Q = 10 C I A Where Q

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is the runoff in m /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 and the preparation of a detailed sewerage project. The engineering and policy decisions taken are dependent on the correctness of the data collected and its proper evaluation. It includes collection 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 of sites for various components including disposal sites, subsoil conditions etc.,) developmental aspects (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 other aspects likely to influence the project. 2. Project Surveys i)

Preliminary project surveys

This is concerned with the broad aspects of the project. Data on aspects such as capacity required, basic arrangement and size, physical features affecting general layout and design, availability of affluent disposal facilities, probable cost and possible methods of financing, shall be collected to prepare an engineering report describing the scope and cost of the project with reasonable accuracy. In framing such estimates, due consideration must be given to the escalation of prices of basic materials and their availability. While extreme precision and detail are not required in this phase all the basic data obtained must be reliable. ii) Detailed project surveys Surveys for this phase form the basis for the engineering design as well as for the preparation of plans and specifications for incorporation in the detailed project report. In contrast to preliminary survey this survey must be precise and contain contours of all the areas to be served giving all the details that will facilitate the designer to prepare design and 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 the existing underground structures requiring displacement, negotiation or clearance. Such detailed surveys are necessary to establish rights of way, minimize utility relocation costs, obtain better bids and prevent changing and rerouting of lines. iii)

Construction Surveys

All 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 per manent objects. a)

Preliminary Layouts

Before starting the work, rights-of-way, work areas, clearing limits and pavement cuts should be laid out clearly to ensure that the work proceeds smoothly. Approach roads, 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 and checked before construction begins. b)

Setting Line and Grade

The 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 is completed. 3)

Project Report (1.9 of CPHEEO Manual)

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



a)

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Pre-investment Planning - Identification of a project - Preparation of project report

b)

Appraisal and Sanction

c)

Construction of facilities and carrying out support activities

d)

Operation and maintenance

e)

Monitoring and feed back

Since project preparation is quite expensive and time consuming, all projects should normally proceed through three stages and at the end of each stage a decision should be taken whether to proceed to the next planning stage and commit the necessary manpower and financial resources for the next stage. Report at the end of each stage should include a time table and cost estimate for undertaking the next stage activity and a realistic schedule for all future stages of project development, taking into consideration time required for review and approval of the report, providing funding for the next stage, mobilizing personnel or fixing agency (for the next stage of project preparation) data gathering, physical surveys, site investigations etc. The basic design of a project is influenced by the authorities/organizations who are involved in approving, implementing, operating and maintaining the project. Therefore the institutional arrangements, through which a project will be brought into operation, must be considered at the project preparation stage. Similarly responsibility for project preparation may change at various stages. Arrangements in this respect should be finalized for each stage of project preparation. Some times more than one organization may have a role to play in the various 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 waste water 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 cost involved 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 for the stipulated designed period if not financial implication may be commented.

4.

Whether detailed survey and investigation carried out and alignment for pumping main, 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 level differences



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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 acquired or 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 f unds 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 domestic sewage and industrial waste excluding storm water. Storm sewers designed to carry off storm water and ground water but excluding domestic sewage and industrial wastes and Combined sewers 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 sluggish flow during non-stormy days, leading to deposition of sewage, solids causing foul odours and increased cost of eventual sewage treatment or pumping cost, associated with disposal of sewage. In view of this, the combined system is normally not recommended. The design of sewer interalia included estimation of sanitary sewage, estimation of storm water runoff and hydraulic, of sewer; design of sewer system etc. The method for estimation of sewage 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 remains constant 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 channel flow is said to be uniform flow and non-uniform if either the velocity, depth or both are changing. A properly functioning sewer has to carry the peak flow for which it is designed and transport suspended solids in such a manner that deposits in a sewer are kept to a minimum. The design for wastewater collection system presumes flow to be steady and uniform. The unsteady and non uniform waste water flow characteristics are accounted in the design by proper sizing of manholes Flow friction: (Para 3.4.2 of CPHEEO Manual ) - The available head in waste water lines is utilized in overcoming surface resistance and in small part, in attaining kinetic energy 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 used for working out the head loss due to friction Design criteria:- It is better practice to design sewers with partial full condition to provide ventilation and keeping sewage in fresh condition. Hence peak factor for design sewer shall range between 2 to 3.5. From consideration of ventilation in waste water flow, 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 of sewers should be decided to meet the following two conditions:



•

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i.

A self cleansing velocity is maintained at present peak flow

ii.

A sewer runs at 0.80 full at ultimate peak flow.

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

Hydraulic elements of circular sewers possess equal self cleansing properties at all depths. The self cleansing properties for different conduit are given below: i) Sanitary Sewer:

For design peak flow – 0.8 metre/sec For 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: •

Maximum permitted depth of flow: The pipes will be designed to flow at depth indicated below where the maximum permissible depth of flow in sewers for established velocity criteria: Diameter in mm (d) Up to 400 400 to 900 Above 900

•

•

0.3 metre/sec

Depth of flow which will convey designed quantity 0.50 d 0.67 d 0.75 d

Velocity: (Para 3.4.3 of CPHEEO Manual) The flow in sewer varies from hour to hour and also seasonally. But for the purpose of hydraulic design, estimated peak flow is adopted. The size of Sewer is to have adequate capacity for the peak flow to be achieved at the end of design period so as to avoid steeper gradient and deeper excavation. It is desirable 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 maximum flow at the beginning of the design period for a given flow and slope. Velocity is little influenced 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) 2 3 5 10 15 20 30

Slope per 1000 m 6.0 4.0 3.1 2.0 1.30 1.20 1.00

After arriving at slopes for present peak flows, the pipe size should be decided on the basis of 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 of sewer may be 100 mm.



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•

Maximum Permissible Velocity: Description Stoneware pipe Brick drain Concrete drain Cemented drain Cast Iron pipe

•

14

Maximum permissible velocity 1.4 metre/sec 1.8 to 2.1 metre/sec 2.5 metre/sec 3.0 metre/sec 3.0 metre/sec

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

2 Sewer Appurtenances (Chapter 4 of CPHEEO Manual) Sewer appurtenances are devices necessary in addition to pipes and conduits for the pipes functioning of any complete system of sanitary, storm or combined sewers. They include structures and devices such as various types of manholes, lamp holes, gully traps, intercepting chambers, flush tanks, ventilation shafts, catch basins, street inlets, regulators, siphons, grease traps, 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 a person to access the sewer for the purpose of inspection, testing, cleaning and removal of obstruction of the sewer line. Spacing : Manhole should be built at every change of alignment, gradient or diameter at the head of all sewers and branches and at every junction of two or more 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 200 m may be allowed in straight run sewer of 1.5 to 2.0 m dia., which may further be increased up to 300 m for sewer of over 2 m diameter. A spacing allowance of 100 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 be provided for all depths, starting from 0.9 metres. Depending on the depth of manhole, diameter of manhole changes. The internal diameter of the manholes may 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


Works Audit – Sewerage and Sewage Treatment •

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The width/diameter of the manhole should not be less than the internal diameter of 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 selfcleansing 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 left for new connection insertion of new Y or T is not prescribed. -- House service connection should be minimum size of 150 mm diameter sewer with minimum slope 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 be joined with sewer pipe already embedded within the wall of the manhole while constructing the 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 House service 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 flow characteristics, 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 flow characteristics , availability in the sizes required including fitting and case of handling and installation, water fighters and simplicity in assembling, physical strength, resistance to acids, alkalies, gases solvents etc. resistance to scour, durability and cost including handling and installation. No single material will meet all the conditions that may be encountered in sewer design. Selection should be made for the particular application and different materials may be selected for parts of a single project. •

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

(i). Brick work is used for construction of sewer particularly for large diameters. Brick sewers shall have cement concrete or stone for invert and 12.5 mm thick cement plaster with neat finish. 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 the Rising main will jeopardize the whole system since the Rising main is the most vital part of the 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 used for 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 to corrosion and erosion due to grit and high velocities. Though a minimum crushing strength of 1600 kg/m is usually adopted for all sizes manufactured presently, vitrified clay pipes of crushing strength 2800 kg/m and over are manufactured in other countries. The strength of vitrified clay pipes often necessitates special bedding or concrete cradling to improve field supporting 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).



17

Size of Pipe internal diameter in mm 100 150 200 250 300 350 •

Wall thickness of stoneware pipe 12 mm 15mm 16mm 20mm 25mm 30mm

Jointing of Sewer pipes: From structural considerations of structural requirements joints may be classified as rigid and flexible joints. Joints such as cement mortar, lead, flanged and welded joints are under the category of rigid joints. All types of mechanical joints such 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 1. Upto an average depth of 120 cm 2. Above 120 cm

Width of Trench Diameter of pipe + 30 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.



18

4. STRUCTURAL DESIGN OF BURIED SEWERS (Chapter 6 of CPHEEO Manual)

The structural design of a sewer is based on the relationship that the supporting strength of the sewer as installed divided by a suitable factor of safety must equal or exceed the 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 to provide safe installations are therefore: (i)

Determination of the maximum load that will be applied to the pipe based on the trench and backfill conditions and the live loads to be encountered.

(ii)

Computation of the safe load carrying capacity of the pipe when installed and bedded in the manner to be specified using a suitable factor of safety and making certain the design supporting strength thus obtained is greater than the maximum load to be applied. Specifying the maximum trench widths to be permitted, the type of pipe bedding to be obtained and the manner in which the backfill is to be made in accordance with the conditions used for the design. Checking each pipe for structural defects before installation and making sure that only sound pipes are installed and Ensuring by adequate inspection and engineering supervision that all trench widths, sub grade work, bedding, pipe laying and backfilling are in accordance with design assumptions as set forth in the project specifications.

(iii)

(iv) (v)

Proper design and adequate specifications alone are not enough to ensure protection from dangerous overloading of pipe. Effective value of these depends on the degree to which the design assumptions are realized in actual construction. For this reason thorough and competent 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 original ground 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 in case 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 two types viz. concentrated load and distributed load. Moving loads may be considered as equivalent 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 stresses induced in it.

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



19

The vertical dead load to which a conduit is subjected under trench conditions is the resultant of two major forces. The first component is the weight of the prism of soil within the trench and above the top of the pipe and the second is due to the friction or shearing forces generated between the prism of soil in the trench and the sides of the trench produced by settlement of backfill. The resultant load on the horizontal plane at the top of the pipe within the trench 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 the Marston’s formula in the form 2

Wc= Cd w B

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 C d for 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 3

Materials Dry sand

Weight (kg/m ) 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 2

Values of Cd for calculating loads on pipes in trenches (Wc=CdWB d) Ratio H/B

Safe working values of Cd



Minimum possible without cohesion

20

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



21

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 to super imposed concentrated load such as a truck wheel is given in the following form by Holl’s integration of Boussinesq’s 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 taxi ways, 1.75 for railways traffic) and Cs= the load coefficient which is a function of Bc ----2H

and

L ---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 Cs for various values of (B c/2H) and (L/2H) are obtained from Table 3 The effective length of the conduit is defined as the length over which the average load due to surface traffic units produces the same stress in the conduit wall as does the actual load which varies in intensity from point to point. This is generally taken as 1m or the actual length of the conduit if it is less than 1 m b) Distributed load : (Para 6.4.2 of CPHEEO Manual superimposed loads, the formula for load on conduit is given by

For the case of distributed

Wsd = Cs p F Bc Where Wsd = the load on the conduit in kg/m 2

.p = the intensity of the distributed load in kg/m 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 distributed load 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 under specified conditions of bedding and backfilling. The field supporting strength, however does not



22

include any factor of safety. The ratio of the strength of a pipe under any stated condition of loading 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 determined experimentally and analytically for the commonly used construction condition for both trench and 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 A bedding may be either concrete cradle or concrete arch. Class B is a bedding having a shaped bottom or compacted granular bedding with a carefully compacted backfill. Class C is ordinary bedding having a shaped bottom or compacted granular bedding but with a lightly compacted backfill. Class D is on with flat bottom trench with no care being taken to secure compaction of backfill at the sides and immediately over the pipe and hence is not recommended. Class B or C bedding with compacted granular bedding is generally recommended. Shaped bottom is impracticable and costly and hence is not recommended. The pipe bedding materials must remain 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 tendency to flow than uniformly graded materials. However, uniformly graded material is easier to place and 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 A A A A

B C D

CONDITION

a. concrete cradle plain concrete and lightly tamped backfill b. Concrete cradle plain concrete with carefully tampled backfill c. Concrete cradle – RCC with P-0.4 % d. Arch type plain concrete RCC with P-0.4% RCC with P-1.0% (P is the ratio of the area of steel to the area of concrete at the crown) Shaped bottom or compacted granular bedding with carefully compacted backfill Shaped bottom or compacted granular bedding with lightly compacted backfill Flat bottom trench

LOAD FACTOR 2.2 2.8 Upto 3.4 2.8 Upto 3.4 Upto 4.8 1.9 1.5 1.1

Note: C type of bedding is normally adopted.

The granular material used must stabilize the trench bottom in addition to providing a firm and uniform support for the pipe. Well graded crushed rock or gravel with the maximum size not exceeding 25 mm is recommended for the purpose. Where rock or other unyielding foundation material is encountered bedding may be according 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.



23

Class B or C: The hard unyielding material should be excavated below the bottom of the pipe and pipe 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 it should be refilled with granular material. Total encasement of non-reinforced rigid pipe in concrete may be necessary where the required safe supporting strength cannot be obtained by other bedding methods. The load factor for 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 are as 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 trench to a width of 0.8 m in depth of 1.30 metre in ordinary soil and wheel load of 6.25 tonnes.

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) 2

Wc=Cd W B d =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



24

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

=

22 2 2 1 ---- x --- x --- x----x 7 10 10 4

1000

x –0.6x

75 ----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 pipes are 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 reference to 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 to higher width of trenches may be commented.

4. SEWAGE AND STORM WATER PUMPING STATION (Chapter 9 of CPHEEO Manual)

Pumping stations handle Sewage/Storm water either for lifting the sewage so as to discharge 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 flow considering a design period of 15 years. The civil structures and pipelines of both dry sump and the wet well should be designed for a flow of 30 years. The needs of future expansion need special attention especially in respect of provision of additional space for replacing 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 initial flows 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 to avoid too infrequent pumping operation and long retention of sewage wet wells. (Para 9.3) of CPHEEO) Pumping stations traditionally have two wells, the wet well receiving the incoming sewage and dry well housing the pumps.



25

Dry well: (Para 9.6.1 of CPHEEO) The size of the dry well should be adequate to house number 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 sewage capacity to be provided. The capacity of the well is to be so kept that with any combination of inflow and pumping the cycle of operation for each pump will not be less than 5 minutes and the maximum 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 at each pump suction. Wherever possible grid removal ahead of pumping should be adopted to increase the life of 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 size of pump, the number of pumps, the capacity or flow rate of each pump range of throttling of each 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 at sewage pumping station. The size and number of unit for larger pumping station, shall be so selected that the variations of inflow can be handled by throttling of the delivery values or by varying the speed of the pump without starting and stopping the pump too frequently or 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) estimated for 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 DWF Capacity. For large capacity pumping station, 5 pumps are usually provided comprising of 2 of ½ DWF 2 for 1 DWF and 1 of 3 DWF capacity including standby. (Para 9.7.1 of CPHEEO Manual) The total head of pumping has to be calculated taking note of four factors; (Para 9.7.5 of CPHEEO Manual) (i)

the difference between the static level of the liquid in the suction sump in the wet well 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 of discharge (ie. Velocity head or kinetic head)

(iii) the difference in the pressure on the liquid in the suction sump and at the point of delivery 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)



26

Operation of the pumps—Dry running of pumps should be avoided—The delivery value should 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 is determined by the continuous current required for the station – load and the available short circuit characteristic 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 abnormal conditions. Safety involves the protection life and also the safe guarding of the equipment under all conditions of operation & maintenance. None of these three aspects shall be sacrificed for the sake of initial economy. The electrical system shall be designed with such flexibility as to permit one or more components to be taken out of service at any time without interrupting the continuous operation of the station. A proper selection of voltages in the electrical types is one of the most important 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 public safety protection etc., Duplicate transformer may be provided where installation so demands.



27

Audit Approaches Interalia the following could be seen •

•

•

•

Cases where pumps and motor designed for ultimate stage instead of 15 years may be identified 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 of requirements. Unwarranted installation cost on extra pumpset may be commented Cases of failure of pumpset resulting in non pumping of Sewage loading for pollution may also 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 the same 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..



28

5. SEWAGE TREATEMENT (Chapter 10 of CPHEEO Manual )

The object of sewage treatment is to stabilise decomposable organic matter present in sewage and the effluent and sludge which can be disposed of in the environment without causing health hazards or nuisance. The processes commonly employed in domestic waste water treatment function and units used 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 the completion 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 period ranging 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 15 years or earlier should be examined to decide the most economical initial arrangements satisfactorily 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 variation and hence peak, average and minimum flows are important consideration. The process loading in the sewage treatment are based on the daily average flows and average characteristics as determined from a 24 hour weighted composite sample. In the absence of any data average flow of 150 lpcd may be adopted. The hydraulic design load varies from component to component of the treatment plant with all appurtenance conduits, channel, etc. being designed for the maximum 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 of both maximum and minimum flow is important in the design of screen and grit chamber.



3.

29

Unit operations/processes, their functions and devices used for domestic wastewater treatment (Table 10-1 of CPHEEO Manual) Unit operations and process

S.No

Functions

Treatment Devices

1

SCREENING

Removal of large floating, suspended and settleable solids

Bar racks and screens of various description

2

GRIT REMOVAL

Removal of suspended solids

Grit chamber

3

PRIMARY SEDIMENTATION

Removal of organic inorganic settable solids

4

a) AEROBIC BIOGICAL SUSPENDED GROWTH PROCESS

Conversion of colloidal, dissolved and residual suspended organic matter into settable biofloc and stable inorganics

Activated sludge process units and its modifications, waste stabilisation Ponds, Aerated Lagoons.

b) AEROBIC BIOGICAL ATTACHED GROWTH PROCESS

Same as above

Trickling Filter, Rotating Biological Contactor

ANAEROBIC BIOLOGICAL PROCESSES

Conversion of organic matter into CH4 & CO2 and organic relatively stable organic residue

Anaerobic Filter, Bed Submerged Media Anaerobic Reactor, Upflow Anaerobic Sludge Blanket Reactor; Anaerobic Rotating Biological Contactor

Same as above

Anaerobic Digester

5

6

GROWTH

ANAEROBIC STABILISATION ORGANIC SLUDGES

4.

inorganic and

Primary tank

sedimentation

OF

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 the degree of reduction of Biochemical Oxygen Demand (BOD), Suspended Solids (SS) and Total Coliforms. The efficiency of a treatment plant depends not only on proper design and construction but also on good operation and maintenance. Expected efficiencies of various treatment units are given below: Expected efficiencies of various treatment units (Table 10-3 of CPHEEO Manual ) S.No.

1 2 3

Percentage reduction Process

Primary Treatment (Sedimentation) Chemical Treatment Secondary Treatment (i) Standard trickling filters (ii)High rate trickling filters (a) single stage (b) Two stage (iii) Activated sludge plants (iv) (a) Stabilization ponds (Single cell) (b) Stabilization ponds (Two Cells)


SS

BOD

45-60 60-80 75-85

30-45 45-65 70-90

Total coliform 40-60 60-90 80-90

75-85 90-95 85-90 80-90 90-95

75-80 90-95 85-95 90-95 95-97

80-90 90-60 90-96 90-95 95-98


30

Tertiary treatment is adopted when reuse of effluent for industrial purposes is contemplated or when circumstances dictate the requirement of higher quality effluents. Cost is the prime consideration in the selection of the treatment method. It should include the cost of installation, capitalized cost of maintenance and operation taking into account interest charges and period of amortisation. An alternative will be to consider the annual cost covering 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 subsidy granted by the Government for the installation of the treatment works and the maintenance cost is borne entirely by the local body or the agency concerned. Both these will have to be taken into account for making realistic comparison of the alternatives. Other factors that may influence are ease of construction and maintenance, benefits that accrue from better environmental sanitation, location, availability of land and topographical conditions. 5.

Sedimentatio n (chapter 12 of CPHEEO manual)

The purpose of sedimentation of sewage is to separate the settleable solids so that the settled waste water, if discharged into water courses, does not form sludge banks and when used for land disposal does not lead to clogging of soil pores and excessive organic loading. Primary sedimentation 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 in grit chamber (ii) organic and residual inorganic solids, free oil and grease and other floating materials, etc. The settleable solids to be removed from waste water in primary or secondary settling 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, solids loading, 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 and consequent overloading for a short period. If hourly flow variations are wide as in the case of some industrial waste flows, it may be necessary to build an equalization tank, ahead of the treatment 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 nature and undergo flocculation. The major design parameters for secondary settling tanks designed to remove 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 greater of 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 time expressed as m3/d/m2 . Overflow rates must be checked both at average flows and peak flow. The smaller values in the ranges given are applicable to small plants of capacities less than 5 mld.



31

The solids loading rate or solid flux is an important decision variable for the design of secondary sedimentation tank receiving bioflocculated solids. The solid flux represents the solids loading per unit surface area of tank per unit time and is expressed as kg SS/m 2 d. Weir loading influences the removal of solids in sedimentation tank, particularly in secondary settling tanks where flocculated solids are settled. There is no positive evidence that weir 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 secondary tanks for activated sludge process, weir loading of the order of 125m 3/d.m. for average flows is recommended. For secondary settling tanks in activated sludge or its modifications, the weir 3 loading is around 185 m /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 existing sedimentation tanks can be improved by merely increasing their we ir length. The depth sets the detention time in the settling tank and also influences sludge thickening in secondary settling tanks of activated sludge plants. Design parameters for settling tanks or clarifiers (Table 12.1 of CPHEEO manual ) Type of settling

Overflow rate M3/d/m2

Average A. PRIMARY SETTLING 1) Primary settling only 25-30 2) Primary settling followed by 35-50 secondary treatment 3) Primary settling with activated 25 – 35 sludge return B. SECONDARY SETTLING 4) Secondary settling for trickling filter 15-25 5) Secondary settling for activated 15-35 sludge(excluding extended aeration) 6) Secondary settling for extended 8 –15 aeration

Note:

7.

Solid loading Kg/m2/d

Depth M

Detention time Hour

50-60 80-120

2.5 – 3.5 2.5 – 3.5

2.0– 2.5

50 – 60

3.5 – 4.5

Peak

Average

Peak

40 – 50 40 – 50

70 –120 70 –140

190 210

2.5 – 3.5 3.5 – 4.5

1.5– 2.0 -

25 – 35

25– 120

170

3.5 – 4.5

-

Para 10.4 of CPHEEO manual stipulates the design of the clarifier for average flow.

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 2½ hours of settling and thereafter decreases appreciably. Hence, increase in the detention time beyond 2 to 2½ hours will not increase the percentage removal of BOD or SS proportionately. Longer detention period may affect the tank performance adversely due to setting in of septic conditions, particularly in tropical climate. Experience has shown that a detention period of 2 to 2½ hours for primary settling tanks and 1½ to 2 hours for secondary settling tanks will produce the optimum results. Longer detention periods in secondary settling tanks may result in denitrification which adversely affects the settling efficiency.



8.

32

Sludge removal (12.4.2.06 of CPHEEO Manual)

Sludge can be removed manually, hydrostatically or mechanically from the sedimentation tanks. 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, pathogenic contents and to improve its dewatering characteristics. This can be achieved through any of the following biological process (i) anaerobic digestion & (ii) aerobic digestion. During the process of 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 like hydrogen sulphide, hydrogen, nitrogen and oxygen. The combustible constituent in the gas is primarily the methane. Hydrogen sulphide in addition to its corrosive properties causes nuisance during the burning of the gas. In term of solids digested, the average gas production is about 0.9m3/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 of feeding of raw sludge into the digester. The minimum and maximum gas production rates may vary from 45% to more than 200%. In the continuous feeding system, the difference between the maximum and the minimum is considerably reduced. Intermittent mixing of digester contents is also responsible for wide fluctuations in gas production rates. Sludge gas should be collected under positive pressure in the gas holder from the primary and the secondary units besides from the sludge digester. A gas dome above the digester roof should 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 be located in the open for easy observation. Where the gas is to be used as domestic fuel or for power 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 reduce their emissions by implementing environment – friendly projects. In sewage treatment methane gas (bio gas) generates during the process of sludge digestion and the bio gas can be used for generating electricity there by avoiding the bio gas being let into air. Carbon credit is given for utilizing the bio gas by the United Nation body. Note : Chennai Metro Water is set to receive Rs.4 crore annually from United Nation body for using bio gas in sewage Treatment (The Hindu dated 3.6.2007)



33

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 resultant impact in public health, pollution etc. has to be analysed and commented.

3.

Table 12.1 of CPHEEO Manual prescribes design paramateres for settling tank or clarifier (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 in case 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 gas produced 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.



34

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 provide comparatively long detention periods extending from a few to several days. During this period organic matter in the waste is stabilized in the pond. Pond systems in which oxygen is provided through mechanical aeration are called aerated lagoons. Lightly loaded ponds used as tertiary step in waste treatment for polishing of secondary effluents and removal of bacteria are called maturation ponds. Under many situations in warm climate countries pond systems are cheaper to construct and operate compared to conventional methods. The only disadvantage of pond systems is the relatively large land that they require. Pond systems must be considered as an alternative when treatment of waste water or upgrading of existing facilities are planned and the life time costs of various other treatment system should be calculated and compared. CLASSIFICATION 1. Aerobic Aerobic 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 kept shallow 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 on experimental basis only. 2. Anaerobic Completely 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 5-50 day detention period would result in 50-85 percent BOD reduction. Such ponds are constructed with a depth of 2.5 – 5m to conserve heat and minimize land area requirement. Usually they have an odour problem. 3. Faculative The 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 the pond. The treatment effected by this type of pond is comparable to that of conventional secondary treatment processes. The facultative pond is hence best suited and most commonly used for treatment of sewage.



35

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 wastewater flows and from which after a retention time of several days (rather than several hours in conventional treatment processes), a well-treated effluent is discharged. WSP systems comprise a series of ponds – anaerobic, facultative and several maturation. The different functions and modes of operation of these three different types of pond are described in Section 3 of this manual. The WSP are low cost, high efficiency low energy, low maintenance and above all a sustainable method 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, or several such series in parallel. In essence, anaerobic and facultative ponds are designed for BOD removal and maturation ponds for pathogen removal, although some BOD removal occurs in maturation ponds and some pathogen removal in anaerobic and facultative pond. Designers are after 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 surface waters are given in the Environment (Protection) Rules 1986 (see CPCB. 1996). The more important of these for WSP design are as follows.

BOD (Bio oxygen Demand 30mg/I Suspended solids 100 mg/I Total N (Nitrogen) 100mg/N/I Total ammonia 50mg N/I Free ammonia 5 mg N/I Sulphide 2mg/I PH 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 effluent is to be used in agriculture or aquaculture. 1. Anaerobic Ponds

No advice is given on the design of anaerobic ponds in the Government of India’s Manual on Sewerage and Sewerage Treatment (Ministry of Urban Development 1995). However they can be satisfactorily designed on the basis of volumetric BOD loading 2. Facultative Pond

The 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 on latitude 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 remaining 30-35% to be used for a maturation pond. This increases the BOD surface loading on the facultative pond by 43-54% and resulting higher loading.



36

3. Maturation Ponds Maturation ponds may be single pond or series of ponds constructed for removal of faecal coliform, 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 BOD loading 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 of the BOD has been removed in the proceeding anaerobic and facultative ponds ( or 70% for temperature 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/day Om = Retention time in days D = Pond depth in metric usually 1.5m e = Net evaporation rate mm/day Note: In the past waste stabilisation pond technology was used for establishing sewage treatment plant 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 batch reactor and modified activated sludge process have emerged. They are cost effective. In 2006 Govt. of Tamil Nadu constituted Experts Committee with the task of identifying and recommending technology options on urban local bodies. Depending upon the requirement and suitability, the expert committee gives its advice to each local body (Source : The Hindu 27.4.2007) Thus the concept of WSP system may not be of much need in Tamilnadu.



37

8.EFFLUENT DISPOSAL AND UTILISATION (Chapter 20 of CPHEEO Manual) •

•

•

•

•

Treatment effluent conforming to prescribed standards may be disposed into a stream course or into sea or a stagnant body of water. Since the treated waste water may still have a coliform density before disposal of water into the water body. Disposal of waste in a river causes organic chemical and microbial problem. Organic Pollution not only depletes the oxygen content in the river resulting in fish kill but also leads to heavy algal growth down stream. The waste water discharge into the river is to be regulated in such a manner that it does not exceed its waste assimilating capacity and the options in this respect include waste water treatment waste water reduction alternate waste disposal points and methods and increase of the quantity of the available dilution water where possible. Disposal into Ocean: To prevent backing up and spreading of waste water on the sea shore and consideration of marine echo system, the waste water should be taken to a distance of about 1 ½ km into the sea from shoreline and discharged in deep sea at a point 3 to 5 m below water level. Disposal to land: Sewage farming—The nutrients in sewage like nitrogen, phosphorus and potassium along with micro-nutrients as well as organic matter present in it could be advantageously employed for sewage farming to add to the fertility and improve the drainage characteristics of the soil along with the irrigation potential of the water extent. However, use of raw sewage or night soil or sullage is fraught with public health damages. Even application of treated effluent to land has to be carried out with certain precaution, as it is not completely free from risk. A good sewage farm should be run on scientific lines with efficient supervision with the primary objective of disposal of sewage combined with its utilisation to the disposal extent in a sanitary manner without polluting the soil, open water courses or artesian waters or contaminating crops raised on the sewage farm or imparting the productivity of soil. Effluent from properly designed waste stabilisation ponds is also suitable for application on land. Under no conditions, application of raw sewage on sewage farm should be permitted. Treated sewage water is being used in Industrial unit for various purpose (viz. Cooling purpose etc.,) Treated sewage water is sold to the Industries by Water Agency. Note : The Parameter prescribed by National Pollution Control Board and Tamil Nadu Pollution Control Board for letting the treated sewage water in the land, water course sea etc., has been prescribed by those agencies and treated effluent should conform to the prescribed standard parameter.

Audit Approaches 1. The utility of treated sewage water to industrial purpose and agreement entered with industrial unit concerned may be analysed to comment loss of revenue. 2. Sewage Farms are properly maintained and revenue on sale of grass etc. may be analysed.

9. PROCESS OF SEWAGE TREATMENT ( Chapter – Part –III of Data – Matrix for Public Health Engineers compiled by TWAD Board Engineers) i. Preliminary Treatment



38

a) Screening : Coarse Screens

: made of bars with opening of 7.5 to 15 cm

Bar screens : made of bars with openings of 5 to 10 cm Amount of materials removed by Screens : 3 to 180 cum/million cum of sewage screened

b) Grit Removal:

Velocity in grit channels Size of grit particles removed

: 30 cm/sec : 0.15 mm to 0.20 mm grains of sp.gr. 2.30 to 2.65 : 6 to 72 cum/million cum of sewage treated.

Amount of grit removed ii. Primary Treatment:

Plain settling of sewage to remove organic matter to the extent possible Usual efficiencies : 30% COD removal and 50% suspended solids removal iii. Secondary Treatment a) Intermittent sand filters b) Trickling filters or c) Activated sludge units Involves complete removal of organic matter. iv) Low Cost Treatment: By a) Waste stabilization ponds (aerobic, anaerobic and facultative) b) Aerated lagoons c) Oxidation ditches Design Features 1. Screens:

Velocity through screens

:

raked

Submerged area of the screen surface including bars and openings :

(i) 0.30 m/sec (at average rate of flow) for hand bar screen. ii) 0.75 m/sec. (max velocity during wet weather periods) for mechanically cleaned screens. 200 to 300 per cent of the cross sectional area of the approach sewer.

1) Net submerged screen area : notless than 0.04 to 0.06 sq.m/mld 2) Head loss through the screens : upto 0.75 m 3) Drop of floor level below the screen: a) For mechanically operated screens: 75 mm (min) b)For manually operated screens : 150 mm (min) 4) Top of screen : 300 mm (min) above highest flow level 5) Minimum free board : 300 mm



39

2. Grit Chambers:

Horizontal velocity of flow : 0.15 to 0.30 m/sec Detention time : 60 sec. (for 3 DWF) Surface loading rate : 40 m3/m2/hour Grit collection : 12 to 27 cum/million cum. 3. Settling Tanks: (Plain sedimentation) Detention time : 2.0 to 2.5 hours Overflow rate : 27 to 45 m3/day/m2 Depth of tank : 3.0 to 4.0 m Velocity of flow : 0.30 to 1.50 m/min Solids loading :30 kg/m2/day 4. Intermittent Sand Filters : Design loading on filters million litres/hectare/day 1) For raw sewage : 0.5 2) For presettled sewage (after screen and grit removal): 1.0 3) Primary treatment complete (after settling) : 3.0 5. Dosing Tanks: Enough flow to flood one unit of filter to a depth of 5 to 10 cm. Each filter bed should receive 1 or 2 doses per day . Average rate of dosage : 30 litres/sec for 500 m2( (with intermittent dosing and resting of sand beds) 6. Trickling filters : i) Hydraulic loading (in million litres/hectare metre/day) ii) Organic loading (in tonne BOD/hectare metre/day) iii) Depth in m Recirculation Filter volume iv) Power requirements (kw/million litres) Dosing interval Nature of dosing

Low rate

High rate

10 to 40

100 to 400

1.1 to 3.5 1.8 to 3.0 None 5 to 10 times

3.5 to 17.5 1.0 to 2.4 1:1 to 1:4 1

None 5 min. Intermittent

30 to 180 15 sec. Continuous

7. Activated Sludge Units Design Parameters for activated sludges aeration tank capacities and permissible loadings Process

Plant Design flow (mld)

Conventional

Upto 2.25 2.25 to 6.756.75 up All 2.50 to 6.75 6.75 up Upto 2.25 2.25to 6.75 6.75 up All

Modified or High rate Step Aeration Contact stabilization

Extended aeration

Aeration Retention Period (hours) 7.57.5 to 6.06.0 up

2.5 up 7.5 to 5.0 5.0 up 3.0* 3.0to 2.0* 1.5 to2.0* 24

Plant Design kg/BOD5 /day

Aerator loading 3 kg/BOD5 /m

Kg BOD5 /day Kg MLSS

Upto 450 450 to 1350 1350 up 900 up 450 to 1350 1350 up Upto 450 450 to 1350 1350 up All

0.50 0.50 to 0.65 0.65 up 1.60 0.50 to 0.80 0.80 0.50 0.50 to 0.80 0.80 up 0.20

0.50 to 0.25

* Detention time in contact zone which is 30 to 35% of total aeration capacity. Re-aeration zone comprises the balance of the aeration capacity. Office of the Principal Accountant General (Civil Audit) Chennai

1 or less 0.5 to 0.2 0.5 to 0.2

0.10 to 0.05


40

8. Design parameters for Secondary or Final Settling Tanks of Activated Slud ge Plants Type of Process

Average Design flow (mld)

Conventional, modified or High rate & Step aeration Contact stabilisation

Upto 2.25 2.25 to 6.75 6.75 up Upto 2.25 2.25to 6.75 6.75 up Upto 0.23 0.23 to 0.69 0.69 up

Extended aeration

Detention (hours) 3.0 2.5 2.0 3.6 3.0 2.5 4.0 3.6 3.0

time

Surface settling 3 2/ rates (m /m day 27 32 37 23 27 32 14 14 27

9. Relative Efficiencies of Sewage Treatment Processes Type of treatment or process

Coarse screens Fine screens Grit chambers Plain sedimentation Chemical precipitation Low-rate trickling filter, including presedimentation and final sedimentation High rate trickling filter including presedimentation and final sedimentation Conventional activated sludge, including presedimentation and final sedimentation High rate activated sludge, including pre-sedimentation and final sedimentation Contact aeration, including pre-sedimentation and final sedimentation Intermittent sand filtration, including pre-sedimentation Chlorination Raw or settled sewage Biologically treated sewage

Percent reduction Suspended BOD Bacteria matter 0-5 5-10 5-10 10-20 5-10 5-10 10-20 35-65 25-40 25-75 70-90 60-85 40-80 70-90 75-90 80-90

70-90

65-95

70-95

80-95

80-95

90-95

70-90

70-95

80-98

80-95

80-95

90-95

85-95

85-95

95-98

-

* *

90-95 98-99

*Reduction is dependent upon dosage 10. Digester Capacity requirements: A digester capacity of 1 cu.ft. per capita (including sludge storage) may be adopted, since the conditions in Tamil Nadu in respect of sludge digestion are more favourable. The following values may be adopted for the area of drying beds required for digested sludge. Type of Treatment Area of sludge drying beds in sq.m per capita (for open beds) Primary 0.10 Activated sludge 0.15 11.Disinfection of sewage Chlorine dosages required inmg/1 Raw sewage Primary settling tank effluent Trickling filter effluent Activated sludge effluent Sand filter effluent

: 20 to 25 : 20 : 15 :8 :6



41

12. Waste stabilization ponds

a.) For small schemes, a single facultative pond suffices and is designed as below: BOD loading = 220 kg/hectare/day Water depth =not more than 1.5 to 1.7 m Detention period = 18 to 24 days obtained by adjusting surface area BOD removal = 80 to 90% b) For large schemes where minimum clearance of 0.5 to 1 km from habitation is available, anaerobic pond followed by facultative pond is preferable to save land area and is designed as below: 13.Anaerobic Pond:

BOD loading = 880kg/hectare/day Water depth = not less than 2.5 m - not more than 3.0 m Detention period = 4 days BOD removal = 50 to 60% Facultative Pond Same design criteria as in (a) but influent BOD would be less. c) for hospitals, educational institutions, small colonies, etc. even chance odours must be absent and single facultative pond is designed as below: BOD loading = 110 kg/hectare/day Water depth = not more than 1.5 to 1.7 m Detention period = 35 to 40 day 14. Fish Ponds:

Where practicable fish ponds can be used for facultative pond effluents with the following design criteria: Water depth = 1.5 m Detention period = 5 days Cyprinus carpio (Carp) is the most profitable variety of fish with yields of 4 tonne/hectare/year. 15. Grass Farm

The facultative pond effluent or the fish farm effluent can be used for grass cultivation for cattle feed. The following criteria will apply: Application rate : 60 to 120 m3/hectare/day (clay to gravelly soils) For clayey soils, under drains system must not be recommended and only peripheral collection channels are to be provided. Grass variety : Para Grass Grass yield : 370-400 tonne/hectare/year in 11 cuttings 16. Aerated lagoon

Where land cost exceeds Rs.200,000/hectare, facultative waste stabilisation ponds would be uneconomical for a flat adoption and aerated lagoons can be used for pretreatment economically with the following design criteria: Detention time : 1 day Water depth : 2.5 m Oxygen required : 1 kg/each kg BOD removal BOD removal : 60% : Aerator sizing :0.8 kg 02/kw/h



42

HAND BOOK ON WORKS AUDIT-SEWERAGE AND SEWAGE TREATMENT

Office of the Principal Accountant General (Civil Audit) Tamil Nadu and Pondicherry Chennai



43

HAND BOOK ON WORKS AUDIT- SEWERAGE AND SEWAGE TREATMENT

Office of the Principal Accountant General (Civil Audit) Tamil Nadu and Pondicherry Chennai

FOR INTERNAL CIRCULATION AND USE ONLY The information given in this handbook has been gathered from various sources.For Reporting,the originals have to be referred And not provisions of this handbook. This handbook is only to educate the Officials engaged in works audit .


Shri.C.Sreedharan . SAO Shri.E.Jayaraman .AAO Shri.S.Ravichandran .AAO Smt.Chandra Seetharaman .AAO Smt.Sreelakshmi Sridhar .AAO


44

SEWERAGE AND SEWAGE TREATMENT

Contents

Page No.

Preamble

01

Planning, Design, Survey & Investigation

01

Design of Sewer and Sewer Appurtenant

11

Sewage and Storm Water Pumping Station

24

Sewage Treatment

27

Stabilisation Pond

33

Waste Water Stabilisation Pond

34

Effluent Disposal and Utilisation

36

Process of Sewage Treatment

37

Flow chart on Sewage Treatment Plan


Sewerage

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