engineering

1. PROJECT REPORT ON
DESIGN OF SEWAGE SYSTEM
JAMIA MILLIA ISLAMIA
CIVIL ENGINEERING
7TH
SEMESTER
AAFREEN KHAN
MOHD. SOHRAB ALI
ABDUL BASIR
MOZAMMIL SARWAR

2. INTRODUCTION
Pollution in its broadest sense includes all charges that curtail natural utility
and exert deleterious effect on earth. The crises triggered by the rapidly
growing population and industrialisation with the resultant degradation of the
environment causes a great threat to the quality of life. Degradation of water
quality is the unfavourable alteration of the physical, chemical and biological
properties of water that prevents domestic, commercial, industrial,
agricultural, recreational and other beneficial uses of water. Sewage and
sewage effluents are the major sources of water pollution. Sewage treatment
is the process of removing contaminants from wastewater and household
sewage, both runoff (effluents) and domestic. It includes physical, chemical
and biological processes to remove physical, chemical and biological
contaminants. Its objective is to produce a treated effluent and a solid waste
or sludge suitable for discharge or reuse back into the environment. This
material is often inadvertently with many toxic organic and inorganic
compounds.
Sewage implies the collecting of wastewaters from occupied areas and
conveying them to some point of disposal. The liquid wastes will require
treatment before they are discharged into the water body or otherwise
disposed off without endangering the public health or causing offensive
conditions.
As the cities have grown, the more primitive method of excreta disposal has
gain place to the water-carried sewage system. Even in the small cities the
grate safety of sewage, its convenience and freedom from nuisance have
caused it to be adopted wherever finances permit.
SEWARAGE – GENERAL CONSIDERATION
Sewerage is the art of collecting, treating and finally disposing off the sewage.
Sewage is the liquid consisting of any one or a mixture of liquid waste origins
from urinals, latrines, bath rooms, kitchens of a dwelling, commercial building
or institutional buildings. Sewer should be design so as to transport the entire

3. sewage effectively and efficiently from the houses and up to the point 0f
disposal
In Sanitary Sewers, lateral sewer collects discharge from the houses and carry
them to branch sewer.
Strom Sewers carry surface runoff developed during the period of rainfall over
the concerned area including street wash.
Combined Sewers consists of a single sewer line of large diameter through
which the sewage and storm water are allowed to flow and are carried to the
treatment plant. As the single sewer line severs the double function, it
becomes economical.
The major role of sewer system is:
 Improvement in the environment by removing the sewage as it
originates.
 Preventing inundations of low-lying areas that may be otherwise caused
by not providing sewers.
 Prevention of vector propagation by sewage stagnations.
 Avoiding cross connections with freshwater sources of seepage.
PROCESS INVOLVED IN THE SEWAGE TREATMENT
The treatment of sewage consists of many complex functions. The degree of
treatment depends upon the characteristics of raw inlet sewage as well as the
required effluent characteristics.
Treatment process are often classified as:
1. Preliminary treatment
2. Primary treatment
3. Secondary treatment
4. Tertiary treatment
Preliminary Treatment
Preliminary treatment involves separating of floating materials like tree
branches, papers, pieces of rags, woods etc and heavy stable inorganic solids.
It helps in removal of oils and greases and reduces the BOD. The process
under this is:
 Screening - to remove floating papers, rags, clothes.

4.  Grit chambers - to remove grit and sand.
 Skimming tank – to remove oil and greases.
Primary Treatment
Primary treatment consists in removing large suspended organic solids. It is
usually accomplished by sedimentation in settling basins. The liquid effluent
from the primary treatment often contains a large amount of suspended
organic materials has a high BOD.
Secondary Treatment
Here the effluents from primary treatment are treated through biological
decomposition of organic matter carried out either aerobic or anaerobic
conditions
Aerobic Biological Units:
I. Filters (intermittent sand filters, trickling filters)
II. Activated Sludge Plant (feed of active sludge, secondary settling tank
and aeration tank)
III. Oxidation ponds and aerated lagoons
Anaerobic Biological Units:
I. Anaerobic lagoons
II. Septic tanks
III. Imhoff tanks.
The effluent from the secondary treatment contains a little BOD and may
contain several milligrams per litre of DO.
Tertiary Treatment
The purpose of tertiary treatment is to provide a final treatment stage to
raise the effluent quality before it is discharged to the receiving environment
(sea, rivers, lake, ground etc.). More than one tertiary process may be used at
any treatment plant. If disinfection is practised, it is always the final process.
It is known as “effluent polishing”.

5. SEWAGE TREATMENT PROCESS
GENERAL
Sewage contains various types of impurities and disease bacteria. The sewage
is disposed of by dilution or on land after its collection and conveyance. If the
sewage is directly disposed of, it will be acted upon the natural forces, which
will convert into harmful substances. The natural forces of purification cannot
purify any amount of sewage within specified time. If the quantity of sewage
is more, then receiving water will become polluted or the land will become
sewage sick. Under such circumstances it become essential to do some
treatment of the sewage, so that it can be accepted by the land or receiving
water without any objections. These treatment processes directly depend on
the types of impurities present in the sewage and the standard up to which
treatment is required.
OBJECT OF TREATMENT
The main object of treatment units is to reduce the sewage contents from the
sewage and remove all the nuisance causing element and change the
character of the sewage in such a way that it can be safely discharged in
natural water course applied on the land.
In other words, the objective of sewage treatment plant is to produce a
disposable effluent without causing harm or trouble to the communities and
prevent pollution.
Practically the treatment of sewage is required in the big cities only where the
volume of the sewage is more as well as the quantity of various types of solid,
industrial sewage etc. is more and porous land or large quantity of water
bodies is not available for the proper disposal of sewage.
DEGREE OF TREATMENT
The degree of treatment is mostly be decided by regulatory agencies and the
extent to which the final product of treatment is to be utilized. The regulatory
bodies might have laid down standard for the effluent or might specify the
condition under which the effluent must be discharged into the natural
stream. The method of treatment adopted should not only meet the

6. requirement of the regulatory bodies, but also results in the maximum use of
the end product with economy.
ESTIMATION OF DESIGN FLOWS
DESIGN PERIOD
The length up to which the capacity of the sewer will be adequate is referred
to as the design period. In fixing a design period, consideration must be given
for the useful life of structures and equipment employed taking into account
obsolescence as well as wear and tear. The flow is largely a function of the
population served, population density, water consumption, lateral and sub
main sewers are usually designed for peak flows of the population at
saturation density as set forth in the master plan. The recommended design
period of various components shall be as in the following table.
SI. No Components Design Period,
Years (from base year)
1. Land Acquisition 30 years or more
2. Conventional Sewers 30
3. Non-Conventional Sewers 15
4. Pumping Mains 30
5. Pumping Stations-Civil Work 30
6. Pumping Machinery 15
7. Sewage Treatment Plants 15
8. Effluent Disposal 30
9. Effluent Utilization 15 or as the case may be
POPULATION FORECAST
The design population should be estimate by paying attention to all the factors
governing the future growth and development of the project area in the
industrial, commercial, educational, social and administration spheres. Special
factors causing sudden immigration or influx of population should also be
predicted as far as 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 areas or
areas of the project from various mathematical method, graphically
interpreted where necessary.

7. When the master plan containing land use patterns and zoning regulation is
available for the town, the anticipated population can be based on the
ultimate densities and permitted floor spaces index provided for in the master
plan.
TRIBUTARY AREA
The natural topography, layout of buildings, political boundaries, economic
factors etc., determine the tributary area. For larger drainage area, though it is
desirable that the sewer capacities be designed for the total tributary area,
sometimes, political boundaries and legal restrictions prevent the sewers to be
constructed beyond the limits of the local authority. The need to finance
projects within the available resources necessitates the design to be restricted
to political boundaries.
PER CAPITA SEWAGE FLOW
The observed dry weather flow quantities usually are slightly less than the per
capita water consumption of a community, since some water is lost in
evaporation, seepage into ground, leakage etc. The conventional sewers shall
be designed for a minimum sewage flow of 100 litres per capita per day or
higher as the case may be. Non-conventional sewers shall be designed as the
case may be. For new communities, design flow can be calculated based on the
design population and projected water consumption for domestic use,
commercial and industrial activity. In a sewer, for the purpose of hydraulic
design estimated peak flows are adopted which depends on contributory
population.
Contributory population Peak Factor
Up to 20,000 3.00
Above 20,001 to 50,000 2.50
Above 50,001 to 7,50,000 2.25
Above 7,50,001 2.00
Peak factor for Contributory Population
INFILTRATION
Estimate of flow in sanitary sewers may include certain flows due to infiltration
of ground water through joints. Since sewers are designed for peak discharges,

8. allowances for ground water infiltration for the worst condition in the area
should be made as given in the table.
Minimum Maximum
Litres/ha/day 5,000 50,000
Litres/km/day 500 5,000
Litres/day/manhole 250 500
Once the flow is estimated as per the table, the design infiltration value shall
be limited to a maximum of 10% of the design value of sewage flow.
SEWAGE FROM COMMERCIAL INSTITUTIONS
Estimate of flows from industries and commercial buildings which use water
other than the municipal supply and may discharge their liquid waste into the
sanitary sewers, have to be made separately for their potable water supply.
INDUSTRIAL EFFLUENTS TO BE DISCOURAGED
Industrial areas having polluting industries are generally located such as to
avoid mixing with sewage as these are undesirable due to possible
determinantal of these effluents on the operation of biological sewage
treatment process.
In cities having polluting industries in pockets of mixed land use are required to
implement zero liquid discharge (ZLD), by reusing the effluents after
appropriate treatment in house.
STORM RUNOFF
The sanitary sewers are not expected to receive storm water. Strict inspection,
vigilance and proper design and construction of sewers and manholes should
eliminate this flow or bring it down to a very insignificant quantity.
In small habitation, having continuous rainfall it may be necessary to include
storm water in the design of sewer.
MEASUREMENT OF FLOW IN EXISTING DRAINS/SEWERS
Mostly, the measurement od flows in existing drains or sewers will provide
valuable data for a more realistic assessment of the design flow. In general,

9. non sewered areas will most certainly be having a set of drains where the
generated sewage will be flowing out.
The assessment of flows in drains can be done by a variety of methods right
from the rudimentary crude method to the most sophisticated dye tracer
method.
TYPES OF SEWERS
These are separate sewers, combined sewers, pressurized sewers and vacuum
sewers.
Separate Sewers
These sewers received domestic sewage and industrial waste pre-treated to
the discharge standards as per the Environment Protection Act 1986.
Combined Sewers
These sewers receive storm water in addition and have some advantage in
location of intermittent rainfall almost throughout the year and with a terrain
permitting gravitated collection and obviously being confined to a very small
region as a whole.
In regions of seasonal rainfall, the combined system will, have serious
problems in achieving self-cleansing velocities during dry seasons and
necessitating complicated egg-shaped sewers etc to sustain velocities as such
times.
Pressurized Sewers
These are for collecting sewage from multiple sources to deliver to an existing
collection sewer and to the STP are not dependent on gravity and topography
is not a challenge. The principal advantages are the ability to sewer areas with
undulating terrain, rocky soils conditions and high ground water tables as
these can be laid close to the ground and anchored well besides there cannot
be infiltration and exfiltration is quickly detected and set right.
This system is not suitable for continuous building area.
Vacuum Sewer System
These collects sewage from multiple sources and conveys it to the STP. These
sewers can take advantage of available slope in the terrain, but have limited
capacity to fill water.

10. SHAPE AND SIZE OF SEWERS
 In general, circular sewer sections are ideal as the hydraulic properties
are better for varying flows.
 For large flows, the egg-shaped sections are superior for both load
transmission and velocity at minimum flows plus ability to flush out
sediments in the bottom V portion when peak flow arises.
Minimum size of circular sewers
The minimum diameter may be adopted as 200mm for cities having
percent/base year population of over 1 lakh. However, depending on
growth potential in certain areas even 150mm diameter can also be
considered.
Flow in Circular Sewers
In the design of sanitary sewers an attempt shall be made to obtain
adequate scouring velocities at the average or at the beginning of the
design period.
The flow velocity in the sewers shall be such that the suspended materials
in sewage are not silted up i.e., the velocity shall be such as to cause
automatic self-cleaning effect.
Minimum velocity for preventing sedimentation
To ensure that deposition of suspended solids does not take place, self-
cleaning velocities using shield’s formula is considered in the design of
sewers.
V = 1/n (R1/6
√KS(SS - 1) dp)
Where,
n = Manning’s n
R = Hydraulic mean radius in m
KS = dimensionless constant with a value of about 0.04 to start motion of
granular particles and about 0.8 for adequate self-cleaning of sewers.
SS = Specific gravity of particles
dp = Particles size in mm

11. Criteria value
Minimum velocity at initial peak
flow
0.6 m/s
Minimum velocity at ultimate peak
flow
0.8m/s
Maximum velocity 3m/s
Maximum velocity
Erosion is caused by sand and other gritty material and is compounded by
high velocities and here the maximum velocity shall be limited to 3m/s.
Manning’s Formula for gravity flow
V = 1/n R 2/3
S1/2
For circular conduits
V = 1/n (3.98 x 10-3
) D2/3
S1/2
and Ɵ = 1/n (3.118 x 10-6
)D2.67
S1/2
where,
Ɵ = discharge in l/s
S = slope of hydraulic gradient
D = internal diameter of pipe line in mm.
R = Hydraulic radius in m
V = velocity in m/s
n = Manning’s coefficient of roughness
Design depth of flow
Sanitary sewers are design to run partially full (flow under gravity)
Pipe Size Design condition
D< 0.4m ½ full at max discharge
0.4 <= D <= 0.9m 2/3 full at max discharge
D>0.9m ¾ full at max discharge
Slope of sewers

12. Sewer Size(mm) Minimum Slope
As percent As 1 in
150 0.60 170
200 0.40 250
250 0.28 360
300 0.22 450
375 0.15 670
450 0.12 830
>=525 0.10 1000
LOCATION OF TREATMENT PLANT
The treatment plant should be located as near as near to the point of disposal
as possible. If the sewage as to be disposed finally into the river, the plant
should be located near the river bank. Care should be taken while locating the
site that it should be on the downstream side of the city and sufficiently away
from water intake works. If finally, the sewage as to be applied on the land, the
treatment plant should be located near the land at such a place from where
the treated sewage can directly flow under gravitational forces toward the
disposal point. The plant should not be much far away from the town to
reduce the length of the sewer line.
On the other hand, the site should not be close to the town, that it may cause
difficulties in the expansion of town and may pollute the general atmosphere
by smell and fly nuisance.
LAYOUT OF TREATMENT PLANT
The following points should be kept in mind while giving layout of any sewage
plant.
 All the plants should be located in the order of sequence, so that sewage
from one process should directly go to other process.
 If possible, all the plants should be located at such elevation that sewage
can flow from one plant into next under its forces of gravity only.
 All the treatment units should be arranged in such a way that minimum
area is required, it will also insure economy in its cost.
 Sufficient area should be occupied for future extension.
 Staff quarter and office also should be provided near the treatment
plant, so that operators can watch the plants easily.

13.  The site the treatment plant should be very neat and give very good
appearance.
 By pass and over flow weir should be provided to cut out of operation
any unit when required.
All channels conduits should be laid in such a way as to obtain flexibility,
convenience and economy in the operation.
DESIGN OF SEWER NETWORK
Before the sewer network can be designed, accurate information regarding the
site conditions is essential
 Site plan – A plan of the site to scale with topographical levels, road
formation levels, levels of the outfalls, location of wells, underground
stumps and other drinking water sources.
 The requirement of local bye- laws
 Subsoil conditions- subsoil condition governs the choice of design of
sewers and the method of excavation.
 Location of other services (such as position depth and size of all other
pipes).
 Topography.
i. Preliminary Investigation for Design of Sewer System
The anticipation of future growth in any community in terms of population or
commercial and industrial expansion forms the basis for preparation of plans
for proving amenities including installation of sewers in areas to be served.
ii. Detailed Survey
Besides the location of underground structure, detailed survey regarding the
pavement characteristics of the streets, location and the basement elevations
of all buildings, profile of all streets through which the sewer will run.
iii. Layout of system
The sewer system layout involves the following steps:
a) Selection of an outlet or disposal point.
b) Prescribing limits to the drainage valley or zonal boundaries.
c) Location of trunk and main sewers.
d) Location of pumping stations are found necessary

14. iv. Profile of sewer system
The vertical profile is drawn from the survey notes for each sewer line. The
profile shows ground surface, tentative manhole locations, grade, size and
material of pipe, ground and invert levels and extent of concrete protection
etc.
v. Available Head
Generally, the total available energy is utilized to maintain proper flow
velocities in the sewers with minimum head loss. However, in hilly terrain
excess energy may have to be dissipated using special devices. Hence, the
sewer system design is limited on one hand by hydraulic losses, which must be
within the available head and on the other hand to maintain self-cleansing
velocities.
vi. Precautions
Design of sewer systems for rocky strata especially in hilly terrain in walled
cities may have to invoke controlled blasting or chipping and chiselling both of
which can hindrance to traffic for long periods of time and may also cause
damages to heritage structures. It is necessary to consider the shallow sewer
option on both side of the road and if drains are already in position,
construction of the additional twin of the drain and manage the collection
system.