Table of Contents 1 Introduction ...............................................................................................................................................................................3

2 Literature Review ....................................................................................................................................................................4

2.1 General: ...............................................................................................................................................................................4

2.2 Importance: ......................................................................................................................................................................4

2.3 Basic Design considerations ....................................................................................................................................4

2.3.1 Engineering Considerations ..........................................................................................................................4

2.3.2 Environmental Considerations ....................................................................................................................5

2.3.3 Process Considerations ....................................................................................................................................5

2.3.4 Cost Considerations ...........................................................................................................................................5

2.4 Pakistan Environmental Scenario.........................................................................................................................5

2.5 Design Considerations ................................................................................................................................................6

2.5.1 General Considerations ....................................................................................................................................6

2.5.2 Investigation on site: .........................................................................................................................................6

2.6 Public Health Engineering department design criteria for Sewerage System ..............................7

2.6.1 Design flow calculations ..................................................................................................................................8

2.6.2 Velocity at design Flow ....................................................................................................................................8

2.6.3 Septicity ....................................................................................................................................................................9

2.6.4 Large Deep Sewer ...............................................................................................................................................9

2.6.5 Local Losses............................................................................................................................................................9

2.7 Choice of Pipe Material ............................................................................................................................................ 10

2.7.1 Corrosion Protection ...................................................................................................................................... 10

2.8 Structural Design of Gravity Pipelines............................................................................................................. 10

2.8.1 Joints in RCC Sewer ......................................................................................................................................... 11

2.8.2 Load on Pipes ..................................................................................................................................................... 12

2.8.3 Design Procedures for Rigid Pipes .......................................................................................................... 13

2.9 Types of Sewers: ......................................................................................................................................................... 13

2.9.1 Sanitary Sewer:.................................................................................................................................................. 13

2.9.2 Storm Sewer: ...................................................................................................................................................... 14

2.9.3 Combined Sewer:.............................................................................................................................................. 14

2.9.4 Partially combined system: ......................................................................................................................... 14

2.9.5 Components of sewer system .................................................................................................................... 15

2.10 Sewer Appurtenances: ............................................................................................................................................. 15

2.10.1 Manholes ............................................................................................................................................................... 16

2.10.2 Backdrop Manholes......................................................................................................................................... 17

2.11 Sources of Wastewater: ........................................................................................................................................... 17

2.11.1 Domestic: .............................................................................................................................................................. 17

2.11.2 Industrial: ............................................................................................................................................................. 17

2.11.3 Storm water: ....................................................................................................................................................... 17

2.12 Sewer Beddings: .......................................................................................................................................................... 18

2.12.1 Load Factor: ........................................................................................................................................................ 18

2.13 Invert Level: .................................................................................................................................................................. 18

2.14 Pumping stations: ....................................................................................................................................................... 18

2.14.1 Components of Sewage Pumping Stations:......................................................................................... 18

2.14.2 General Design Consideration: .................................................................................................................. 18

3 Design Criteria ....................................................................................................................................................................... 20

3.1 Design equation ........................................................................................................................................................... 21

3.1.1 Manning’s equation ......................................................................................................................................... 21

3.1.2 Equation of continuity ................................................................................................................................... 21

3.2 Design Procedure: ...................................................................................................................................................... 21

3.3 Design of pumping station ..................................................................................................................................... 23

General Design Consideration ....................................................................................................................................... 23

3.3.1 OPERATING VOLUME OF WET WELL: .................................................................................................. 24

3.3.2 Design of Pumping Station .......................................................................................................................... 25

4 Longitudnal Profile .............................................................................................................................................................. 26

4.1 Profile No: 1 ................................................................................................................................................................... 26

4.2 Profile No: 2 ................................................................................................................................................................... 27

4.3 Profile No:3 .................................................................................................................................................................... 27

5 Comments ................................................................................................................................................................................. 28

6 Recommendations and Conclusion ............................................................................................................................. 28

7 Refrancess ................................................................................................................................................................................ 29

1 Introduction

The main Area of focus in this report is the design of sewerage system of NASHMAN IQBAL

HOUSING SOCIETY, its design criteria, design calculations, conclusion and

recommendations regarding the design.

Topography

Ground level of the housing society is FLAT, it is 252 m.

Land Use:

1. Area = 15.3 acre

2. No of plots = 937

3. Commercial plots = 4

4. Mosques = 2

5. Schools = 1

6. Open places = 6

7. Graveyard = 3

8. Unoccupied land = 1

9. Disposal station = 1

2 Literature Review

2.1 General:

We use a water supply system in order to provide easy availability of water for drinking ,

washing etc. when this water is used it becomes waste water called "sewage". A proper

system is required for the collection of wastewater and conveying it to the point of disposal

with or without treatment called as "sewerage system”. A sewerage system, like a river

system, consists of the main streams with many branches. Sewerage system appears on

plan a number of small trees rooted to the natural drainage system.

“A sewerage system should be designed to collect all wastewater generated within

a catchment area and to convey it to a sewage treatment works for treatment prior to

discharge into the receiving watercourse or the sea via the outfall facilities.”

2.2 Importance:

History reveal that the health of the people remained top priority. An efficient sewerage

system helps to protect public health and the environment. Sewerage system conveys

wastewater from homes, industries, hospitals and commercial institutes to treatment

facilities for safe disposal. Any interruption of sewerage system can have dramatic

consequences for public health and health of the environment. The state of the

environment is a major concern to the people of the region. Improving the drainage and

sewerage system to result in environmental protection, which has been identified as a

matter of priority.

2.3 Basic Design considerations

In designing wastewater collection, treatment and disposal systems, planning generally begin from the final disposal point going backwards to give an integrated and optimized design Co suit the topography and the available hydraulic head, supplemented by pumping if essential. Once the disposal points are tentatively selected, further design is guided by the following basic design considerations:

a. Engineering b. Environmental c. Process d. Cost

These considerations are discussed below in detail:

2.3.1 Engineering Considerations

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

Design period, stage wise population to be served and expected sewage flow and fluctuations

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

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

Ground water depth and its seasonal fluctuation affecting construction, sewer Infiltration, structural design. Soil bearing capacity and type of strata expected to be met in construction On site disposal facilities, including the possibilities of segregating the sludge water

and sewage and reuse or recycle sullage water within the households

2.3.2 Environmental Considerations

The environmental and socioeconomic impacts of a sewage treatment plant may prove adverse during the operation stage. Therefore, the following aspects should be considered during design.

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

2.3.3 Process Considerations

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

Waste water flow and characteristics Degree of treatment required Performance characteristics Other process requirements such as land, power operating equipment’s, Skilled staff, nature of maintenance problems, extent of sludge production And its disposal requirements, loss of heat through plant in relation to Available head etc.

2.3.4 Cost Considerations

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

2.4 Pakistan Environmental Scenario

The main water source in Pakistan is rivers, rainfall and ground water. Considering

Pakistan’s environmental scenario it's become increasingly obvious that water issues are

the most pressing. Human health, agriculture, forest, water bodies, and aquatic life, in fact

the whole ecosystem is affected by problems associated with water. Not only is there a

scarcity of drinking water but also polluting of water bodies by effluents from industries

and the sewerage system have compounded the problems. The discharge of sewage and

contaminated water in rivers and water bodies not only affected marine production, use of

such water for agriculture, results in the contaminations of food chain. (Memon, 1998)

In Pakistan, sewage water is re-channeled to irrigate crops, which contaminates them with

pathogens. As a result, 50% of the crops are contaminated by untreated sewage. Water

borne diseases are the largest killers in the country and health problems from polluted

water cost a large amount of money.

2.5 Design Considerations

In the ancient ages, sewers were constructed without considering any design criteria due to

lack of technology and research. With the development of civilization and the tremendous

increase in population, it becomes essential to formulate some logical basis for design of

sewerage system, this led to evaluation of previous studies in the light of knowledge gained

through experience. The modern research has formulated following design basis. (WASA,

1998)

2.5.1 General Considerations

Provide adequate sewerage for areas require careful engineering. The sewers must be

adequate in size otherwise; they overflow will result in property damage and danger

health. Adequacy in size calls for estimation of the amount of the sewage and use of

hydraulics to determine proper size and grades of the sewers (WASA). Another important

consideration is the velocity of the flow in the sewers. If not great enough, deposits of solids

will occur with accompanying odors and stoppages. After the sewage is collected, it

becomes a liability to the city because of its potential danger health and possible

production of nuisances in streams. This necessitates a careful study of bodies of water that

will receive the sewage; and evaluation of the need for any treatment needed before

disposal. (WASA, 1998).

2.5.2 Investigation on site:

The first stage of investigation consists of a visitor visits to the site, examination of the

terrain and collection of the existing data in the form of maps, drawing and statistics. The

investigator, once on the site, might find that considerations other than those mentioned by

the authority were worth.

2.5.2.1 Population

Particulars of the total population, a tendency to increase, seasonal variation, housing

scheme, densities of buildings, town planning proposals and restrictions, type of house in

different areas etc. must be taken into count.

2.5.2.2 Water supply

Quantity consumed per capita of population , type of supply and extend, trade

consumption, name address and telephone number of the principal offices of the water

authority, source of supply, positions of large mains liable to affected by the scheme.

2.5.2.3 Rivers and streams

Particulars of rivers and streams liable to be affected by daily scheme, a rough estimation

of quantity flowing in them, information regarding any water supply taken from them,

name, address and telephone number of river board, slandered of sewage effluent required

by river board.

2.5.2.4 Site for works

Notes on suitable for pumping stations, outfall works etc., with particulars of subsoil,

subsoil water level, normal water level in the stream for outfall.

2.5.2.5 Existing work:

Positions, levels and sizes of existing sewers, whether combined, separate or partially

separate, dry weather flow, storm flow, infiltration, strength of sewage. Details of existing

pumping machinery and sewage treatment works, particulars of overflows, their number,

position and rate at which they commence to discharge, particulars of existing sea outfall,

normal tide and maximum recorded tides (WASA,1998.)

2.5.2.6 Surveying and Mapping

The second part of the investigation is usually carried on paper with the aid of Ordinance

maps, together with the information that has been collected. It may often be possible to

frame a preliminary report obtained by his means prior to the prosecution of a detailed

level survey of the ground. However, once the proposals have been made lines of sewer

decided by a topographical survey of the site have to be made. The map may be on the scale

of 1 inch to 200ft. Unless there are details to be shown, in which case 1 in 100ft., or even

large scale may be used. Contours should be shown, with interval depending upon the

topography. For a very flat area, the contour interval may be as small as 1 ft., usually five or

10ft., will satisfy. (E.W Steel, 1979).

2.6 Public Health Engineering department design criteria for Sewerage

System

Following design criteria has been proposed by Punjab Public Health Engineering

Department in 1998.

Sewerage

i. The disposal station should be located at a place where from sewage system can be

disposed of safely.

ii. The sewers must be designed as a partially combined system allowing a surcharging

of the system for some time.

iii. By-pass arrangement at the disposal station must be planed where level permits.

iv. Outfall sewer in the village to be provided if otherwise economical and safer as

compared to Punjab Standard Drain Type 1/11.(PHED, 1998)

v. Design Periods:

Civil works 20 years

Machinery 10 years

vi. Sufficient area for the disposal station should be acquired to accommodate further

extension for next 40 years.

vii. Master plan for sewerage scheme should be prepared and phasing out to be done

according to priority of work/area. (PHED, 1998).

2.6.1 Design flow calculations

i. The sewage contribution of the water consumed will be as follows:

For semi urban/town committee 70% to 80%

For urban area 80% to 85%

ii. Infiltration rate should never be greater than 15 L/Km of the sewer / day/mm DIA.

iii. Multiply the average daily by the peak factor to calculate maximum dry weather

flow.

iv. Add allowance for industrial waste as per actual assessment. On treated industrial

waste as per National Environmental

2.6.2 Velocity at design Flow

2.6.2.1 Minimum Velocity

A minimum velocity enables the sewage flow to self-cleanse the nominal amount of silt

carried through the sewers, and helps to minimize sewer chokage because of situations and

grease accumulation. Subsequent maintenance costs and environmental nuisance are

reduced. The self-cleansing can also relieve the problems of septicity due to siltation. As it

is designed as a partially combined sewerage system, therefore the velocity has been kept

as 0.7 m/s as per the criteria of WASA.

It has been established that self-cleansing velocities vary with the particle sizes in

sediments and sizes of sewers. It is not easy to determine the particle sizes of silt in the

sewerage system because both the range of sizes and the variety of sizes are great.

2.6.2.2 Maximum Velocity

The maximum velocity of sewerage flow is not taken more than 2.4 m/sec as per the WASA

criteria

Very fast flow is not desirable because:

Very fast flow is not stable and will give rise to scouring and cavitation especially

when the pipe surface is not smooth, and if the sewer contains junctions, bends, and

manholes. The usual hydraulic equations for flow prediction may not be applicable.

More importantly, severe erosion causes damage to the sewerage system;

Very fast flow occurs when the sewer is laid at steep gradient and the flow becomes

supercritical. When the gradient eventually flattens, the flow may become

subcritical and a hydraulic jump will occur. The potential damage associated with

the uncontrolled energy dissipation is substantial; and

Inspection and maintenance of sewers with fast flowing sewage are unsafe, usually

difficult and sometimes impossible.

2.6.2.3 Minimum Pipes Size

To facilitate inspection and cleaning, pipes of diameter less than 225mm (WASA, PHED)

should normally not be used as sewers unless agreed by the operation and maintenance

agents.

2.6.3 Septicity

Septicity occurs when the residence time of sewage is long, the temperature is high and

there is a lack of air exchange. The deposition of sediments in the sewers also facilitates the

formation of sulfide. This frequently occurs in gentle gravity sewers in which the flow is too

slow. In addition, sulphide formation will be more serious with saline sewage as seawater

contains a high level of sulphate.

The adverse effects of septicity in gravity sewers can be mitigated by suitable design to

shorten residence time, minimize sediments deposition and adopt corrosion resistant

construction materials.

2.6.4 Large Deep Sewer

Difficulties are frequently encountered in the inspection, maintenance and repair of large deep gravity sewers, which is defined as gravity sewers of diameters not less than 675mm and invert levels exceeding 6m below ground level. Laying of large deep gravity sewers should be avoided as far as possible. However, if it cannot be avoided, the designer should consider the following:

In the design of sewerage facilities, evaluation of various alternatives, including the

adoption of shallower sewers by the provision of intermediate pumping stations

and localized sewage treatment plants if necessary, should be carried out.

To facilitate inspection and maintenance of surcharged large deep sewer, over

pumping is usually required to draw down the sewage level in the sewer.

If over pumping is not practically feasible, the designer should consider a twin line

design.

(DSD Practice Note no. 3/2010)

2.6.5 Local Losses

In order to minimize the head losses in pipe flow, the selection of the pipe materials and

the joint details are very important. The resistance in pipes will be influenced by the pipe

material but will be primarily dependent on the slime that grows on the pipe surface. Other

factors such as the discontinuities at the pipe joints, the number of manholes, the number

of branch pipes at manholes and their directions of inflow will all affect the head losses.

Proper benching with smooth curves should be provided to accommodate the changes in

pipe direction.

2.7 Choice of Pipe Material

The following factors should be taken into account in selecting the type of pipe for a

project:

Hydraulic design: gravity or pressure flows

Structural design: crushing test strengths (and pressure ratings in the case of

pressure pipelines) that are available

The nature of the fluid to be conveyed

Nature of ground water and external environment

Cost considerations: capital and maintenance costs

Pipe jointing system: ease of installation, past performance

Durability: resistance to corrosion and abrasion

Availability of pipe sizes, fittings and lengths in the market for construction and

subsequent maintenance

Ease of cutting and branch connections

Length and weight of individual pipes in relation to transportation and handling

Future operating procedure and system development

2.7.1 Corrosion Protection

In general, most of the pipes and fittings are susceptible to both internal and external

attacks by corrosion unless appropriate protective measures are adopted. The degree of

attack depends upon the nature of the soils, the characteristics of the fluid being conveyed

and the type of pipe protection used.

To protect the pipe from corrosion, pipes made of corrosion resistant material or coated

with inert protective materials should be considered. For concrete pipes with protective

liners, the pipe joints should also be covered by linings and the lining must be subsequently

jointed after installation if the pipe diameter is large enough for man access. If the diameter

is too small, the pipes should be supplied with joint surfaces already safeguarded with

lining.

2.8 Structural Design of Gravity Pipelines

Pipes can be categorized into rigid, flexible and intermediate pipes as follows:

a. Rigid pipes support loads in the ground by virtue of the resistance of the pipe wall

as a ring in bending.

b. Flexible pipes rely on the horizontal thrust from the surrounding soil to enable them

to resist vertical load without excessive deformation.

c. Intermediate pipes are those pipes, which exhibit behavior between those in (a) and

(b). They are also called semi-rigid pipes.

Concrete pipes and vitrified clay pipes are examples of rigid pipes while steel, ductile iron,

UPVC, MDPE and HDPE pipes may be classified as flexible or intermediate pipes, depending

on their wall thickness and stiffness of pipe material.

Types of Pipes based on material:

PVC

AC

PCC

RCC

Steel

Clay

RCC pipes are commonly used.

2.8.1 Joints in RCC Sewer

Bell and Spigot Joint

Tongue and Groove Joint

Sewer diameter

Type of joint

310-760mm Either joint >760 mm Tongue and groove

2.8.2 Load on Pipes

The load on rigid pipes is concentrated at the top and bottom of the pipe, thus creating

bending moments. Flexible pipes may change shape by deflection and transfer part of the

vertical load into horizontal or radial thrusts, which are resisted by passive pressure of the

surrounding soil. The load on flexible pipes is mainly compressive force, which is resisted

by arch action rather than ring bending.

2.8.2.1 Load Estimation

The loads on buried gravity pipelines are as follows:

The first type comprises loading due to the fill in which the pipeline is buried, static

and moving traffic loads superimposed on the surface of the fill, and water load in

the pipeline.

The second type of load includes those loads due to the relative movements of pipes

and soil caused by seasonal ground water variations, ground subsidence,

temperature change and differential settlement along the pipeline.

Loads of the first type should be considered in the design of both the longitudinal section

and cross section of the pipeline. Provided the longitudinal support is continuous and of

uniform quality, and the pipes are properly laid and jointed, it is sufficient to design for the

cross section of the pipeline.

In general, loads of the second type are not readily calculated able and they only affect the

longitudinal integrity of the pipeline. Differential settlement is of primary concern

especially for pipelines to be laid in newly reclaimed areas. The effect of differential

settlement can be catered for by using either flexible joints (which permit angular

deflection and telescopic movement) or piled foundations (which are very expensive). If

the pipeline is partly or wholly submerged, there is also a need to check the effect of

flotation of the empty pipeline.

The design criteria for the structural design of rigid pipes is the maximum load at which

failure occurs while those for flexible pipes are the maximum acceptable deformation

and/or the buckling load. The approach to designing rigid pipes as mentioned in this

chapter is not applicable to flexible pipes, deeply laid pipes or pipes laid by tunneling

methods. For the structural design of flexible pipes, deeply laid pipes or pipes laid by

tunneling methods, it is necessary to refer to relevant literature such as manufacturers'

catalogue and/or technical information on material properties and allowable deformations

for different types of coatings, details of joints etc.

2.8.3 Design Procedures for Rigid Pipes

The design procedures for rigid pipes are outlined as follows:

i. Determine the total design load due to:

The full load, which is influenced by the conditions under which the pipe

is installed, i.e. narrows trench or embankment conditions;

The superimposed load which can be uniformly distributed or

concentrated traffic loads; and

The water load in the pipe.

ii. Choose the type of bedding (whether granular, plain or reinforced concrete) on

which the pipe will rest. Apply the appropriate bedding factor and determine the

minimum ultimate strength of the pipe to take the total design load.

iii. Select a pipe of appropriate grade or strength.

2.9 Types of Sewers:

Different types of the sewer are as follows;

2.9.1 Sanitary Sewer:

Sewer which carries sanitary sewage i.e. wastewater originating from a municipality

including domestic and industrial wastewater.

2.9.1.1 Conditions of sanitary sewer

In flat areas

If sufficient funds are not available currently

If annual precipitations very small

Nearness of a natural river or drain

If pumping is a must

If an existing sewerage system can be used only for sanitary sewage

Is rocky areas

2.9.1.2 Advantages

The size of the sewers is small

Sewage load on treatment units is small

River or stream waters are not polluted

Storm water can be discharged into streams or rivers without any treatment

Economical for sewage pumping since the quantity is small

2.9.1.3 Disadvantages

Small sewer easily gets choked and are difficult to clean

Laying two sets of sewer is costly

Storm water sewers are only used during the rainy season

2.9.2 Storm Sewer:

It carries storm sewage including surface runoff and street washes.

2.9.3 Combined Sewer:

It carries domestic, industrial and storm sewage.

2.9.3.1 Conditions for combine sewer

If sufficient annual rainfall is present

If pumping is required for both

If space is limited

If diversion of excess flow can be provided

If existing system can carry both sewages

2.9.3.2 Advantages

Large sewer size doesn't clog easily and are easy to clean

Laying one set of sewer is economical

The strength of sewage is reduced by dilution

Maintenance cost is reasonable

2.9.3.3 Disadvantages

Large sewers are difficult for handling and transport

Due to storm water load the treatment plant is high

During heavy rains sewers may overflow causing nuisance

Pumping is uneconomical

Storm water is unnecessarily polluted

2.9.4 Partially combined system:

In this system, some portion of storm water or surface runoff is allowed to be carried along

with sanitary sewage. It is the best economical option for our design of sewerage system.

2.9.4.1 Advantages

Small sewers sizes are required

Has the advantages of both systems

Silting problem is eliminated

The problem of disposing off storm water from homes is eliminated

2.9.4.2 Disadvantages

The velocity of flow may be low during dry weather

The storm water increases the load on pumps and treatment units

Components of sewer system

House sewer:

Conveying an individual structure to a common sewer or other point of disposal.

Lateral sewer:

A common sewer collects flow from house sewers.

Sub main sewer:

Collects sewage from one or more laterals as well as house sewers.

Main or trunk sewer:

Collects flow from several sub-mains as well as laterals and house sewers.

Force mains:

Pressurized sewer lines, which convey sewage from a pumping station to another man or to a

point of treatment or disposal.

Interceptor sewer:

Separates dry weather flow and conveys it to a wastewater treatment plant

Relief sewer:

Built to carry a portion of the flow in a system with inadequate capacity.

Outfall sewer:

Carries the collected waste to a point of treatment or disposal.

Infiltration and Inflow:

The water enters sewer through poor joints, cracked pipes and walls and covers of manholes.

Infiltration and inflow are almost nonexistent in dry weather but it will increase during the rainy

season.

According to WASA criteria rate of infiltration w.r.t. Sewer diameter is given in the table. (This

amount is to be taken by the sewer)

Sewer Appurtenances:

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 outfall structures.

Sewer diameter Infiltration

225mm to 600mm 5% of average sewage flow

>600mm 10% of average sewage flow

Manholes

A manhole is the top opening to an underground utility vault used for making connections or

performing maintenance on underground and buried public utility and other services including

sewers, telephone, electricity, storm drains and gas. It is protected by a manhole cover. They are

vertical openings provided in the sewerage system.

Purpose of Manhole

Cleaning.

Inspection.

House connection.

Need of Manhole

Manhole provided when we want to

Change in sewer direction.

Change in diameter.

Change in slope.

At the junction.

Access Openings

Access openings are generally of two types, one for man access and the other for desilting

purposes. Desilting openings should not be smaller than 750 mm by 900 mm, and should be

placed along the centerline of the sewer to facilitate desilting. Man access opening should not be

smaller than 675 mm by 675 mm. If ladders are installed in the manhole, minimum clear opening

should be 750 mm by 900 mm. Man access openings should be placed off the centerline of the

sewer for deep manholes and along the centerline of the sewer for manholes shallower than 1.2

m.

Access Shafts

Access shafts should be sufficiently large for persons to be able to go down in comfort and yet

small enough that one can reach the shaft walls by hand while climbing down for feeling a sense

of security. Minimum size of access shaft should be 750 mm by 900 mm. The access shaft

should be orientated such that the step irons are provided on the side with the smaller dimension.

The access openings should be confined to one traffic lane.

Covers

Manhole covers should be sufficiently strong to take the live load of the heavy vehicle likely to

pass over them, and should remain durable in a damp atmosphere. Heavy-duty manhole covers

should be used when traffic or heavy loading is anticipated; otherwise medium duty covers can

be used.

Manhole covers should not rock when initially placed in position, or develop a rock with wear.

Split triangular manhole covers supported at three corners are commonly used to reduce rocking.

The two pieces of triangular cover should be bolted together to avoid a single piece of the cover

being accidentally dropped into a manhole.

Step-irons and Ladders

Step-irons should be securely fixed in position in manholes, and should be equally spaced and

staggered about a vertical line at 300 mm centers. Ladders should be used in manholes deeper

than 4.25 m or which are entered frequently. It is safer and easier to go down a ladder when

carrying tools or equipment. Step-irons and ladders should start within 600 mm of the cover level

and continue to the platform or benching.

.

Backdrop Manholes

Backdrop manholes are used to connect sewers at significantly different levels, and should be

used where the level difference is greater than 600 mm.

The backdrop can be provided by means of:

A vertical drop in the form of a downpipe constructed inside/outside the wall of a manhole

A gradual drop in the form of cascade.

Flushing Tank:

Located at the head of a sewer. They are designed for 10 minutes flow as a self-cleansing

velocity of 0.6 m/sec. The capacity of these tanks is usually 1/10 of the cubic capacity of sewer

length to be flushed.

2.10 Sources of Wastewater:

2.10.1 Domestic:

It is wastewater from residential buildings, offices, other buildings etc.

2.10.2 Industrial:

It is liquid waste from industrial processes like dying, papermaking, fertilizers, chemicals,

leather etc.

2.10.3 Storm water:

It includes surface runoff generated by rainfalls and street wash.

2.11 Sewer Beddings:

Provision of proper bedding is very important;

In developing the strength of the pipe

Assuring it is laid to the proper grade

Preventing subsequent settlement

2.11.1 Load Factor:

Load factor expresses the increase in strength of sewer by provision of proper bedding.

2.12 Invert Level:

The lowest inside level at any cross section of a sewer section pipe is known as the invert

level at that cross section. Sewer must be designed and laid at a specific slope to attain self-

cleansing velocities. The required slopes are achieved through calculation of invert levels of

the sewer at various manholes

2.13 Pumping stations:

A pumping station is, by definition, an integral part of a Pumped-storage hydroelectricity

installation. Pumping stations are facilities including pumps and equipment for pumping

fluids from one place to another.

They are used to elevate and transport wastewater when:

Continuation of gravity flow is no longer feasible.

Basements are deeper.

Any obstacle lies in the path of sewer.

Receiving stream is higher than the sewer.

Sewage is to be delivered to an above ground treatment plant.

2.13.1 Components of Sewage Pumping Stations:

Screens: used to screen out large floating matter that can damage the pump

Wet Well: to receive wastewater

Dry Well: Used to house the pump

2.13.2 General Design Consideration:

a. More than 1 pump should be provided to cope with available discharge, two pumps

for small pumping stations and more than two for large pumping stations should be

used out of which one is for min. flow, one is for avg. flow and one for max. Flow.

b. Total pumping capacity of pumping station must be equal to the peak sewage flow.

c. Stand by pump must be provided at the pumping station. Its capacity should be at

least 50% of peak sewage flow.

d. Alternate sources of power must be there at pumping station. (Either power from

two different feeders or a diesel operated pumps).

e. Pumps should be of self-priming type, should be of self-priming type, and should

operate under positive suction head.

f. Each pump should have individual intake.

g. Screens with 50mm opening should be provided at pump station to avoid entrance

of big particles in pumps.

h. Size of a dry well should be sufficient to house pumping machinery and for working.

i. Dry wells are provided with sump pump, which are usually reciprocating pumps to

pump out sewage leaks in dry wells.

j. Sluice valve must be provided at suction and delivery side of pump and non-return

valve at the delivery side (to reduce back hammer effect)

k. Detention time in the wet well should not be more than 30min to avoid septic

conditions.

3 Design Criteria

Serial No

Component Value/Specification

1 Sewerage System Partially Separated

2 Design Area Nasheman-Iqbal Housing

Scheme

3 Total Plots 937

4 Population Density 10 person per plot

5 Design Population 9370 people

6 Peak Factor 3

7 Average Flow 254.25 lpcd

8 Infiltration 10% of avg flow

9 Peak Sewage Flow 762.75 lpcd

10 Storm Flow equal to peak flow

11 Design Flow 0.1681 m3/sec

12 Minimum Dia Of Sewer 225 mm

13 Minimum Velocity 0.7 m/sec

14 Maximum Velocity 2.4 m/sec

15 Minimum Cover 1 m

16 Elevation Of Ground 252 m

17 Pipe Material RCC

18 Type Of Joint Bell and Spigot Joint

19 Ground Surface Flat

20 Maximum Dia Of Sewer 610 mm

21 Design Period For Collection

Work 75 years

22 Design Period For Treatment

Work 15 years

23 Design Period For Pumping

Station 15 years

24 Load Factor Of Sewers 1.5

25 Type Of Sewers Class B

26 Type Of Pump Centrifugal,non-clogging

27 Minimum Trench Size 650mm

28 Maximum Trench Size 1250 mm

3.1 Design equation

3.1.1 Manning’s equation

Manning's formula is used for finding the slope of sewer flowing under gravity.

V=R2/3S1/2/n

Where;

V = Velocity of flow, m/sec

R = Hydraulic mean depth = D/4 (When pipe is flowing or half full)

S = Slope of the sewer

n = coefficient of roughness for pipes. (n=0.013 for RCC pipes)

3.1.2 Equation of continuity

Q= AV

Where;

A= Cross-section area of pipe

V = Velocity of flow

Steps for Design of Sewer

Preliminary Investigations

Design consideration/Formulation of design criteria

Actual Design

Preparation of drawings and BOQ

Subsequent modifications

3.2 Design Procedure:

It includes following things;

Preparation of Hydraulic Statement:

1. To find the present population of the project area. Then find the design population

for the given design period. Afterwards find average sewage flow for the design

population, a select peak factor for your project area from WASA PEAK FACTOR

TABLE.

PEAK FACTOR:

1+

M = peak factor

P = population in 1000

Avg. flow in sewer m3/d

Peak Factor to obtain Qmax.

2500 4.0 2500-5000 3.4 5000-10,000 3.1 10,000-25000 2.7 25000-50,000 2.5 50,000-100,000 2.3 100,000-250,000 2.15 250,000-500,00 2.08 > 500,000 2.0

2. Draw the layout of the sewer system keeping in view the layout of the roads and

streets (represent each sewer with a line and manhole with a dot) and assign a

particular direction to sewer towards the disposal area with an arrow sign.

3. Number the manholes and identify each sewer line.

4. Allocate plots or area to each sewer line.

5. Measure the length of each sewer line as per scale of map

6. By considering per capita sewage flow as 80% of water consumption, calculate

average sewage flow and infiltration for each sewer line. For this design, take

infiltration as 10% of average sewage flow.

7. Calculate peak sewage flow and finally the design flow of the sewer lines.

8. Using the Continuity equation and Manning’s formula, find appropriate dia. And

slope for the sewer assuming that the sewer is flowing full. Take a self-cleansing

velocity as 0.7 m/Sec.

9. If actual velocity and depth of flow are satisfactory then the dia. And the slope of the

pipe is considered as final. If the velocity is less than self-cleansing velocity then

increases the slope of the sewer.

10. At the end find the invert levels for the all the sewers and complete the table of

calculations called “hydraulic statement”.

11. Draw the longitudinal profile of the main sewer.

3.3 Design of pumping station

Centrifugal, single suction non-clogging type pumps are normally used. These have

impeller, having 2 vanes. Pump suction pipe is usually larger than the discharge pipe by

about 25%.

Smallest discharge pipe=75mm (3”)

Smallest suction pipe =100mm (4”)

General Design Consideration

Total pumping capacity of pumping station must be equal to the peak sewage flow.

Standby pump must be provided at the pumping station. Its capacity should be at

least 50% of peak sewage flow.

Alternate sources of power must be there at pumping station. (Either power from

two different feeders or a diesel operated pumps).

Pumps should be of self-priming type and should operate under the positive suction

head.

Each pump should have individual intake.

Screens with 50mm opening should be provided at pump station to avoid the

entrance of big particles in pumps.

Size of a dry well should be sufficient to house pumping machinery for working.

Dry wells are provided with sump pump, which are usually reciprocating pumps to

pump out sewage leaks in dry wells.

Sluice valve must be provided at suction and delivery side of pump and non-return

valve at the delivery side (to reduce back hammer effect)

Detention time in the wet well should not be more than 30min to avoid septic

conditions.

Size of a dry well should be adequate to accommodate all pumps.

Pumps should not be started and stopped frequently. A minimum cycle time should

be as given below.

o For small pump = 10 min.

o For large pump = 20-30 min.

In addition, the pump should run at least 2min.

Cycle time is defined, as the time required filling up and emptying the well.

o Cycle time = to empty +time to fill.

The minimum level of sewage in wet well should always be above the casing of the

pump, so that there is always a positive suction head.

3.3.1 OPERATING VOLUME OF WET WELL:

Two important considerations for sizing of wet well.

a. Pumps should not be stopped and started frequently i-e; size of wet well should be

large enough.

Pumps should run at least for 2 minutes.

The time b/w successive starts called the cycle time, should be more than minimum

time specified by the manufacturers.

Generally, for small and large pimps cycle time is 10 minutes and 20-30 min

respectively.

b. Sewage should not stay in the wet well for larger time, otherwise it is putrefied.

Therefore, detention time should be less than 30 min.

Formula 24derivation

The formula to find out the operating volume of the wet well.

Cycle time = Time to empty + Time to fill

Where

Time to empty = V/(P-Q)

Time to fill= V/Q

V = Operating Volume of Wet Wells.

P= Pumping rate (pumping capacity)

= Peak sewage flow

Q= Waste water flow

Cycle Time = T= V/ (P – Q) + V/Q……………………. (A)

Differentiating W.R.T Q

dt/dQ =V/ (P – Q)2 – V/Q2

Equating dt/dQ = 0

V/(P – Q)2 –V/Q2 = 0

V/(P – Q)2 = V/Q2

(P – Q)2 – Q2 = 0

P2 – Q2 – 2PQ – Q2 = 0

P2 = 2PQ

Q = P/2

Therefore, cycle time will be minimum when wastewater flow is half of pumping rate.

Put Q = P/2 in equation (A)

tmin = V/ (P-P/2) + V/P/2

=V/P/2 + V/P/2

=2V/P/2

tmin = 4V/P

There are two extreme conditions.

For Q=0, t 0

It implies that for Q = 0, the well is not filled and we do not need to start the pump.

For Q=P, t ∞

In this Case, the level of sewage in the well remains constant and hence the pump cannot be

stopped.

3.3.2 Design of Pumping Station

Component Value

Q avg 0.027573177 m3/sec

Q avg 1.654390625 m3/sec

Q min 0.827195313 m3/min

Peak Factor 3

Q max 4.963171875 m3/min

P 4.963171875 m3/min

Cycle Time 20 min

Volume of wet well 14 m3

Time On 3.4 min

Time off 16.6 min

Depth of Wet Wells 2m

Diameter of wet well 2.96 m

4 Longitudnal Profile

4.1 Profile No: 1

249

249.2

249.4

249.6

249.8

250

250.2

250.4

250.6

250.8

251

0 5 10 15 20

Longitudnal Profile 1

Invert Level

4.2 Profile No: 2

4.3 Profile No:3

249

249.2

249.4

249.6

249.8

250

250.2

250.4

250.6

250.8

251

0 2 4 6 8 10 12

Series1

249

249.2

249.4

249.6

249.8

250

250.2

250.4

250.6

250.8

251

0 1 2 3 4 5 6 7 8 9 10

Longitudnal Profile 3

Longitudnal Profile 2

5 Comments

Designed sewerage system is partially separated sewerage system, which is

designed on the bases of maximum flow that includes peak sewage flow, infiltration

and storm flow.

Topography of the whole scheme is highly flat.

The system is designed to flow under gravity therefore it is difficult to maintain

minimum self-clearance velocity.

All the joints are bell and spigot because the diameter of sewer pipes less than

760mm.

In some sewer pipes, actual velocity in the system is less than designing minimum

self-clearing velocity because the actual calculated diameter was small.

Sewer type is class B.

The Ratio d/D and V/Vfull is calculated by using graph so there may be chances of

less accuracy.

6 Recommendations and Conclusion

The land slope, potential for flooding and surface water concentration, amount of

suitable area must be evaluated.

Soil texture and structure, stabilized percolation rate, groundwater and bedrock

conditions must be evaluated.

All liquid waste and wash water shall discharge into the septic tank.

The septic tank should be located where it is readily accessible for inspection and

maintenance.

A septic tank should be divided into two parts

A) For sludge

B) For waste water

Self-cleaning velocity must be achieved to avoid the settlement of suspended

particles.

Install flashing tank where the velocity of sewage is less than 0.5 m/Sec.

There is no need to install flash tank at every point where the velocity is low

because a flashing tank can cover at least three or four points where velocity is low.

Slope of bad checks before installing the sewer. This will be helpful if the slope is not

according to design criteria.

Minimum sewer size should be 225 mm

Where lateral sewers are to be laid at deeper slopes, it may be more economical to

lay them shallow and installing a Flushing tank there to achieve self-cleansing

velocity.

Drawings of the sewerage system should be made and kept carefully, because it

helps in future for its inspecting and maintenance.

Prior to design, the positions of all existing services should be ascertained as

accurately as possible.

Pipes must be joined crown to crown to avoid back flow.

No connection pipe should enter the manhole at an angle of greater than 90° in the

direction of the flow.

To avoid odour nuisance, ventilation of the manholes shall be considered.

Proper bedding must be provided to improve the load carrying capacity of the

sewer.

House or building sewers shall be of a sound, durable material of watertight

construction, have minimum diameter of four inches (10 cm).

There is only one pump available at disposal station but according to PHED

recommendation, there should be additional pump of 50% standby capacity.

Therefore, my report suggests that one more pump should be installed, so both the

pumps can work alternately.

The maintenance of the system should be carried out at regular intervals. To carry

out the job effectively there should be sufficient sewer men.

During heavy rainfalls, water accumulates around the manholes. Therefore, it is t

suggests that inlets should be provided at the corners of major streets and

walkways and at the midpoints of blocks so that crosswalks are not flooded.

7 References 1. Steel E.W. and Terence.J.McGhee; "WATER SUPPLY AND SEWERAGE" (5TH EDITION)

2. Babbit, Harold.E and E. Robert Bavmann; "SEWAGE AND SEWAGE TREATMENT"

(8TH EDITION).

3. Fair,G.M.,J.C.Geyer and D.A.OKUN; "WATER AND WASTE WATER ENGG" VOL.1 &

VOL .2

4. Bartlett, Ronald E; "PUMPING STATIONS FOR WATER AND SEWAGE"

5. " SUBMERSIBLE SEWAGE PUMPING SYSTEMS HAND BOOK" Lewis Publishers

6. DRAINAGE SERVICES DEPARTMENT Hong Kong; SEWERAGE MANUAL Key

Planning Issues and Gravity Collection System (Third Edition, May 2013).

7. HANDBOOK ON SEWERAGE AND SEWAGE TREATMENT Office of the Principal

Accountant General (Civil Audit) Chennai.