MUNICIPAL WASTEWATER ENGINEERING (18CV55)

MODULE 1

NEED FOR SANITATION

Every community produces both liquid and solid wastes .The liquid portion –waste water–
is essentially the water supply of the community after it has been fouled by a variety of uses such
as spent water from bathroom kitchen, lavatory basins, house and street washings, from various
industrial processes semi solid wastes of human and animal excreta, dry refuse of house and
street sweepings, broken furniture, wastes from industries etc are produced daily.

If proper arrangements for the collection, treatment and disposal are not made, they will go on
accumulating and create foul condition. If untreated water is accumulating, the decomposition of
the organic materials it contains can lead to the production of large quantity of mal odorous
gases. It also contains nutrients, which can stimulate the growth of aquatic plants and it may
contain toxic compounds. Therefore in the interest of community of the city or town, it is most
essential to collect, treat and dispose of all the waste products of the city in such a way that it
may not cause any hazardous effects on people residing in town and environment.

Waste water engineering is defined as the branch of the environmental engineering where
the basic principles of the science and engineering for the problems of the water pollution
problems. The ultimate goal of the waste water management is the protection of the
environmental in manner commensurate with the economic, social and political concerns.

Although the collection of stream water and drainage dates from ancient times the collection
of waste water can be treated only to the early 1800s. The systematic treatment of waste water
followed in the 1800s and 1900s.

Importance of sewerage system

One of the fundamental principles of sanitation of the community is to remove all

decomposable matter, solid waste, liquid or gaseous away from the premises of dwellings as fast
as possible after it is produced, to a safe place , without causing any nuisance and dispose it in a
suitable manner so as to make it permanently harmless.

Sanitation though motivated primarily for meeting the ends of preventive health has come to be
recognized as a way of life. In this context, development of the sanitation infrastructure of any
country could possibly serve as a sensitive index of its level of prosperity. It is needless to
emphasize that for attaining the goals of good sanitation, sewerage system is very essential.
While provision of potable drinking water takes precedence in the order of provision of
Environmental Engineering Services, the importance of sewerage system cannot be last sight
and cannot be allowed to lag behind, as all the water used by the community has to flow back as
the sewage loaded with the wastes of community living , unless properly collected , treated and
disposed off , this would create a serious water pollution problems.

Definitions of some common terms used in the sanitary engineering.

REFUSE:

This is the most general term to indicate the wastes which include all the rejects left as
worthless, sewage, sullage – all these terms are included in this term.

GARBAGE:

It is a dry refuse which includes, waste papers, sweepings from streets and markets, vegetable
peelings etc. The quantity of garbage per head per day amounts to be about .14 to .24 kg for
Indian conditions. Garbage contains large amount of organic and putrifying matter and therefore
should be removed as quickly as possible.

RUBBISH:

It consists of sundry solid wastes from the residencies, offices and other buildings. Broken
furniture, paper, rags etc are included in this term. It is generally dry and combustible.

SULLAGE:

It is the discharge from the bath rooms, kitchens, wash basins etc., it does not include discharge
from the lavatories , hospitals , operation theaters , slaughter houses which has a high organic
matter .

SEWAGE :

It is a dilute mixture of the wastes of various types from the residential, public and industrial
places . It includes sullage water and foul discharge from the water closets, urinals, hospitals,
stables ,etc.

STORM WATER :

It is the surface runoff obtained during and after the rainfall which enters sewers through inlet.
Storm water is not foul as sewage and hence it can be carried in the open drains and can be
disposed off in the natural rivers without any difficulty.

SANITARY SEWAGE :

It is the sewage obtained from the residential buildings & industrial effluents establishments‘.
Being extremely foul it should be carried through underground conduits.
DOMESTIC SEWAGE:

It is the sewage obtained from the lavatory basins, urinals &water closets of houses, offices &
institutions. It is highly foul on account of night soil and urine contained in it. Night soil starts
putrefying & gives offensive smell. It may contain large amount of bacteria due to the
excremental wastes of patients. This sewage requires great handling &disposal.

INDUSTRIAL SEWAGE:

It consists of spent water from industries and commercial areas. The degree of foulness
depends on the nature of the industry concerned and processes involved.

SEWERS:

Ewers are underground pipes which carry the sewage to a point of disposal.

SEWERAGE:

The entire system of collecting, carrying &disposal of sewage through sewers is known as
sewerage.

DRY WEATHER FLOW (DWF):

Domestic sewage and industrial sewage collectively, is called as DWF. It does not contain
storm water. It indicates the normal flow during dry season.

BACTERIA:

These are the microscopic organisms. The following are the groups of bacteria:

-Aerobic bacteria: they require oxygen &light for their survival.

-Anaerobic bacteria: they do not require free oxygen and light for survival.

- Facultative bacteria: they can exist in the presence or absence of oxygen. They grow more in
absence of air.

Invert:

It is the lowest point of the interior of the sewer at any c/s.

SLUDGE:

It is the organic matter deposited in the sedimentation tank during treatment.

Methods of domestic waste water disposal

After the waste water is treated it is disposed in the nature in the following two principal
methods

a. Disposal by Dilution where large receiving water bodies area available

b. Land disposal where sufficient land is available

The choice of method of disposal depends on many factors and is discussed later.

Sanitary engg starts at the point where water supply engg ends.

It can be classified as

- Collection works
- Treatment works
- Disposal works

The collection consists of collecting tall types of waste products of town. Refuse is collected
separately. The collection works should be such that waste matters can be transported quickly
and steadily to the treatment works. The system employed should be self cleaning and
economical.

Treatment is required to treat the sewage before disposal so that it may not pollute the
atmosphere & the water body in which it will be disposed of .The type of treatment processes
depend on the nature of the waste water characteristics and hygiene, aesthetics and economical
aspects.

The treated water is disposed of in various ways by irrigating fields or discharging in to natural
water courses.

Different Methods of domestic waste water disposal include (Systems of Sanitation)

1) CONSERVENCY SYSTEM
2) WATER CARRIAGE SYSTEM

CONSERVENCY SYSTEM

Sometimes the system is also called as dry system. This is out of date system but is prevailing in
small towns and villages. Various types of refuse and storm water are collected conveyed and
disposed of separately. Garbage is collected in dustbins placed along the roads from where it is
conveyed by trucks ones or twice a day to the point of disposal. all the non combustible portion
of garbage such as sand dust clay etc are used for filling the low level areas to reclaim land for
the future development of the town. The combustible portion of the garbage is burnt. The
decaying matters are dried and disposed of by burning or the manufacture of manure.
Human excreta are collected separately in conservancy latrines.The liquid and semi liquid wastes
are collected separately after removal of night soil it is taken outside the town in trucks and
buried in trenches. After 2-3 years the buried night soil is converted into excellent manure. In
conservancy system sullage and storm water are carried separately in closed drains to the point
of disposal where they are allowed to mix with river water without treatment.

WATER CARRIAGE SYSTEM

With development and advancement of the cities urgent need was felt to replace conservancy
system with some more improved type of system in which human agency should not be used for
the collection and conveyance of sewage .After large number of experiments it was found that
the water is the only cheapest substance which can be easily used for the collection and
conveyance of sewage. As in this system water is the main substance therefore it is called as
WATER CARRIAGE SYSTEM.

In this system the excremental matter is mixed up in large quantity of water their ars taken out
from the city through properly designed sewerage systems, where they are disposed of after
necessary treatment in a satisfactory manner.

The sewages so formed in water carriage system consist of 99.9% of water and .1% solids .All
these solids remain in suspension and do not changes the specific gravity of water therefore all
the hydraulic formulae can be directly used in the design of sewerage system and treatment
plants.

CONSERVENCY SYSTEM WATER CARRIAGE SYSTEM

1) Very cheap in initial cost. 1) It involves high initial cost.
2) Due to foul smells from the latrines, they 2) As there is no foul smell latrines remain
are to be constructed away from living room so clean and neat and hence are constructed with
building cannot be constructed as compact rooms, therefore buildings may be compact.
units.
3)The aesthetic appearance of the city cannot 3) Good aesthetic appearance of city can be
be improved obtained.
4)For burial of excremental matter large area 4) Less area is required as compared to
is required. conservancy system.
5) Excreta is not removed immediately hence 5) Excreta are removed immediately with
its decomposition starts before removal, water, no problem of foul smell or hygienic
causing nuisance smell. trouble.
6) This system is fully depended on human 6)As no human agency is involved in this
agency .In case of strike by the sweepers; there system ,there is no such problem as in case of
is danger of insanitary conditions in city. conservancy system
SEWERAGE SYSTEMS:

1) SEPARATE SYSTEM OF SEWAGE

2) COMBINED SYSTEM OF SEWAGE
3) PARTIALLY COMINED OR PARTIALLY SEPARATE SYSTEM

1. SEPARATE SYSTEM OF SEWERAGE

In this system two sets of sewers are laid .The sanitary sewage is carried through sanitary sewers
while the storm sewage is carried through storm sewers. The sewage is carried to the treatment
plant and storm water is disposed of to the river.

ADVANTAGES:

1) Size of the sewers are small

2) Sewage load on treatment unit is less

3) Rivers are not polluted

4) Storm water can be discharged to rivers without treatment.

DISADVANTAGE

1) Sewerage being small, difficulty in cleaning them

2) Frequent choking problem will be their

3) System proves costly as it involves two sets of sewers

4) the use of storm sewer is only partial because in dry season the will be converted in to
dumping places and may get clogged.

2. COMBINED SYSTEM OF SEWAGE

When only one set of sewers are used to carry both sanitary sewage and surface water. This
system is called combined system.

Sewage and storm water both are carried to the treatment plant through combined sewers

ADVANTAGES:

1) Size of the sewers being large, chocking problems are less and easy to clean.

2) It proves economical as 1 set of sewers are laid.

3) Because of dilution of sanitary sewage with storm water nuisance potential is reduced
DIS ADVANTAGES:

1) Size of the sewers being large, difficulty in handling and transportation.

2) Load on treatment plant is unnecessarily increased
3) It is uneconomical if pumping is needed because of large amount of combined flow.
4) Unnecessarily storm water is polluted.

3. PARTIALLY COMINED OR PARTIALLY SEPARATE SYSTEM

A portion of storm water during rain is allowed to enter sanitary sewer to treatment plants while
the remaining storm water is carried through open drains to the point of disposal.

Advantages:-

1. The sizes of sewers are not very large as some portion of storm water is carried through
open drains.
2. Combines the advantages of both the previous systems.
3. Silting problem is completely eliminated.

Disadvantages:-

1. During dry weather, the velocity of flow may be low.

2. The storm water is unnecessary put load on to the treatment plants to extend.
3. Pumping of storm water in unnecessary over-load on the pumps.

Suitable conditions for separate sewerage systems:-

A separate system would be suitable for use under the following situations:

1. Where rainfall is uneven.

2. Where sanitary sewage is to be pumped.
3. The drainage area is steep, allowing to runoff quickly.
4. Sewers are to be constructed in rocky strata. The large combined sewers would be more
expensive.

Suitable conditions for combined system:-

1. Rainfall in even throughout the year.

2. Both the sanitary sewage and the storm water have to be pumped.
3. The area to be sewered is heavily built up and space for laying two sets of pipes is not
enough.
4. Effective or quicker flows have to be provided.
After studying the advantages and disadvantages of both the systems, present day construction of
sewers is largely confined to the separate systems except in those cities where combined system
is already existing. In places where rainfall is confined to one season of the year, like India and
even in temperate regions, separate system are most suitable.

Table -2.2:- Comparison of Separate and Combined systems

Sl. Separate system Combined system

no.
1. The quantity of sewage to be treated is less, As the treatments of both are done,
because no treatment of storm water is done. the treatment is costly.
2. In the cities of more rainfall this system is In the cities of less rainfall this
more suitable. system is suitable.
3. As two sets of sewer lines are to laid, this Overall construction cost is higher
system is cheaper because sewage is carried than separate system.
in underground sewers and storm water in
open drains.
4. In narrow streets, it is difficult to use this It is more suitable in narrow streets.
system.
5. Less degree of sanitation is achieved in this High degree of sanitation is achieved
system, as storm water is disposed without in this system.
any treatment.

Sources of Sewage:-

Sanitary sewage is produced from the following sources:

1. When the water is supplied by water works authorities or provided from private
sources, it is used for various purposes like bathing, utensil cleaning, for flushing
water closets and urinals or washing clothes or any other domestic use. The spent
water for all the above needs forms the sewage.
2. Industries use the water for manufacturing various products and thus develop the
sewage.
3. Water supplied to schools, cinemas, hotels, railway stations, etc., when gets used
develops sewage.
4. Ground water infiltration into sewers through loose joints.
5. Unauthorized entrance of rain water in sewer lines.

Nature of Sewage:-

Sewage is a dilute mixture of the various types of wastes from the residential, public and
industrial places. The characteristics and composition i.e. the nature of sewage mainly
depends on this source. Sewage contains organic and inorganic matters which may be
dissolved, suspension and colloidal state. Sewage also contains various types of bacteria,
 virus, protozoa, etc. sewage may also contain toxic or other similar materials which might
have got entry from industrial discharges. Before the design of any sewage treatment plant
the knowledge of the nature of sewage is essential.

Quantity of Sanitary Sewage and Storm Water:-

The determination of sanitary sewage is necessary because of the following factors

which depend on this:

1. To design the sewerage schemes as well as to dispose a treated sewage efficiently.

2. The size, shape and depth of sewers depend on quantity of sewage.
3. The size of pumping unit depends on the quantity of sewage.

Estimate of Sanitary Sewage:-

Sanitary sewage is mostly the spent water of the community into sewer system with
some groundwater and a fraction of the storm runoff from the area, draining into it. Before
designing the sewerage system, it is essential to know the quantity of sewage that will flow
through the sewer.

The sewage may be classified under two heads:

1. The sanitary sewage, and

2. Storm water

Sanitary sewage is also called as the Dry Weather Flow (D.W.F), which includes the
domestic sewage obtained from residential and residential and industrials etc., and the industrial
sewage or trade waste coming from manufacturing units and other concerns.

Storm water consists of runoff available from roots, yards and open spaces during
rainfall.

Quantity of Sewage:-

It is usual to assume that the rate of sewage flow, including a moderate allowance for
infiltration equals to average rate of water consumption which is 135 litre/ head /day according
to Indian Standards. It varies widely depending on size of the town etc. this quantity is known as
Dry Weather Flow (D.W.F). It is the quantity of water that flows through sewer in dry weather
when no storm water is in the sewer.

Rate of flow varies throughout 24 hours and is usually the greatest in the fore-noon and
very small from midnight to early morning. For determining the size of sewer, the maximum
flow should be taken as three times the D.W.F.
Design Discharge of Sanitary Sewage
The total quantity of sewage generated per day is estimated as product of forecasted population
at the end of design period considering per capita sewage generation and appropriate peak factor.
The per capita sewage generation can be considered as 75 to 80% of the per capita water
supplied per day. The increase in population also result in increase in per capita water demand
and hence, per capita production of sewage. This increase in water demand occurs due to
increase in living standards, betterment in economical condition, changes in habit of people,
and enhanced demand for public utilities.

Factors affecting the quantity of Dry Weather Flow (DWF):-

The quantity of sanitary sewage is mainly affected by the following factors:

1. Population
2. Type of area
3. Rate of water supply
4. Infiltration and exfiltration

In addition to above, it may also be affected by habits of people, number of industries and
water pressure etc.

Population:-

The quantity of sanitary sewage directly depends on the population. As the population
increases the quantity of sanitary sewage also increases. The quantity of water supply is equal to
the rate of water supply multiplied by the population. There are several methods used for
forecasting the population of a community.

Type of area covered:-

The quantity of sanitary sewage also depends on the type of area as residential, industrial
or commercial. The quantity of sewage developed from residential areas depend on the rate of
water supply to that area, which is expressed a litres/ capita/ day and this quantity is obtained by
multiplying the population with this factor.

The quantity of sewage produced by various industries depends on their various industrial
processes, which is different for each industry.

Similarly the quantity of sewage obtained from commercial and public places can be
determined by studying the development of other such places.

Rate of water supply:-

Truly speaking the quantity of used water discharged into a sewer system should be a
little less than the amount of water originally supplied to the community. This is because of the
fact that all the water supplied does not reach sewers owing to such losses as leakage in pipes or
such deductions as lawn sprinkling, manufacturing processes etc. However, these losses may be
largely be made up by such additions as surface drainage, groundwater infiltration, water supply
from private wells etc. On an average, therefore, the quantity of sewage maybe considered to be
nearly equal to the quantity of water supplied.

Groundwater infiltration and exfiltration:-

The quantity of sanitary sewage is also affected by groundwater infiltration through

joints. The quantity will depend on, the nature of soil, materials of sewers, type of joints in sewer
line, workmanship in laying sewers and position of underground water table.

Infiltration causes increase to the ―legitimate‖ flows in urban sewerage systems. Infiltration
represents a slow response process resulting in increased flows mainly due to seasonally-elevated
groundwater entering the drainage system, and primarily occurring through defects in the pipe
network.

Exfiltration represents losses from the sewer pipe, resulting in reduced conveyance flows and is
due to leaks from defects in the sewer pipe walls as well as overflow discharge into manholes,
chambers and connecting surface water pipes. The physical defects are due to a combination of
factors including poor construction and pipe joint fittings, root penetration, illicit connections,
biochemical corrosion, soil conditions and traffic loadings as well as aggressive groundwater.

It is clear that Infiltration and Exfiltration involve flows passing through physical defects in the
sewer fabric and they will often occur concurrently during fluctuations in groundwater levels,
and particularly in association with wet weather events; both of which can generate locally high
hydraulic gradients. Exfiltration losses are much less obvious and modest than infiltration gains,
and are therefore much more difficult to identify and quantify. However, being dispersed in
terms of their spatial distribution in the sewer pipe, exfiltration losses can have potentially
significant risks for groundwater quality. The episodic but persistent reverse ―pumping‖ effect
of hydraulic gain and loss will inevitably lead to long term scouring of pipe surrounds and
foundations resulting in pipe collapse and even surface subsidence.

Suggested estimates for groundwater infiltration for sewers laid below ground water table are as
follows:

Minimum Maximum
Litre/ day/ hectare 5,000 50,000
Lpd/ km of sewer/cm dia. 500 5,000
Design Period
The future period for which the provision is made in designing the capacities of the various
components of the sewerage scheme is known as the design period. The design period depends
upon the following:
Ease and difficulty in expansion,
Amount and availability of investment,
Anticipated rate of population growth, including shifts in communities, industries and
commercial investments,
Hydraulic constraints of the systems designed, and
Life of the material and equipment.
Following design period can be considered for different components of sewerage scheme.
1. Laterals less than 15 cm diameter : Full development
2. Trunk or main sewers : 40 to 50 years
3. Treatment Units : 15 to 20 years
4. Pumping plant : 5 to 10 years

Variations in sewage flow:-

The sewage flow, like the water supply flow, is not constant in practice but varies. The
fluctuation may, in a similar way, be seasonal or monthly, daily and hourly.

Variation occurs in the flow of sewage over annual average daily flow. Fluctuation in flow
occurs from hour to hour and from season to season. The typical hourly variation in the sewage
flow is shown in the Figure . If the flow is gauged near its origin, the peak flow will be quite
pronounced. The peak will defer if the sewage has to travel long distance. This is because of the
time required in collecting sufficient quantity of sewage required to fill the sewers and time
required in travelling. As sewage flow in sewer lines, more and more sewage is mixed in it due
to continuous increase in the area being served by the sewer line. This leads to reduction in the
fluctuations in the sewage flow and the lag period goes on increasing. The magnitude of
variation in the sewage quantity varies from place to place and it is very difficult to predict.
For smaller township this variation will be more pronounced due to lower length and travel
time before sewage reach to the main sewer and for large cities this variation will be less.
The seasonal variations are due to climatic effect, more water being used in summer than in
winter. The daily fluctuations are the outcome of certain local conditions, involving habits and
customs of people. Thus, in U.S.A. and other European countries, Monday is the washing day, as
such, amount of sewage flow would be much greater than on any other day. In India, however,
Sundays or other holidays involve activities which permit greater use of water. Hourly variations
are because of varying rates of water consumption in different hours of the day.

The first peak flow generally occurs in the late morning it is usually about 200 percent of the
average flow while the second peak flow generally occurs in the early evening between 6 and 9
and the minimum flow occurring during the night after twelve or early hours of the
morning is generally about half of the average flow.
Importance:- the maximum and minimum rates of sewage flow are controlling factors in the
design of sewers. The sewer must have ample capacity to carry the maximum flow and also to
ensure sufficient velocity to produce the self cleaning which would be available in case of
minimum flow.

Typical hourly variations in sewage flow

Effects of Flow Variation on Velocity in a Sewer

Due to variation in discharge, the depth of flow varies, and hence the hydraulic mean depth (r)
varies. Due to the change in the hydraulic mean depth, the flow velocity (which depends directly
on r2/3) gets affected from time to time. It is necessary to check the sewer for maintaining a
minimum velocity of about 0.45 m/s at the time of minimum flow (assumed to be 1/3rd of
average flow). The designer should also ensure that a velocity of 0.9 m/s is developed atleast at
the time of maximum flow and preferably during the average flow periods also. Moreover, care
should be taken to see that at the time of maximum flow, the velocity generated does not exceed
the scouring value.

Quantity of storm water flow:-

When rain falls over the ground surface, a part of it percolates into the ground, a part is
evaporated in the atmosphere and the remaining part overflows as storm water. This quantity of
storm water is very large as compared with sanitary sewage.

Factors affecting storm water:-

The following are factors which affect the quantity of storm water:

1. Rainfall intensity and duration.

2. Area of the catchment.
3. Slope and shape of the catchment area.
4. Nature of the soil and the degree of porosity.
5. Initial state of the catchment.
 If rainfall intensity and duration is more, large will be the quantity of storm water
available. If the rainfall takes place very slowly even though it continues for the whole day, the
quantity of storm water available will be less.

Harder surface yield more runoff than soft, rough surfaces. Greater the catchment area
greater will be the amount of storm water. Fan shaped and steep areas contribute more quantity
of storm water. In addition to the above it also depends on the temperature, humidity, wind etc.

Estimate of quantity of storm water:-

Generally there are two methods by which the quantity of storm water is calculated:

1. Rational method
2. Empirical formulae method

In both the above methods, the quantity of storm water is a function of the area, the
intensity of rainfall and the co-efficient of runoff.

Rational method:-

Runoff from an area can be determined by the Rational Method. The method gives a
reasonable estimate up to a maximum area of 50 ha (0.5 Km2).
Assumptions and Limitations
Use of the rational method includes the following assumptions and limitations:
Precipitation is uniform over the entire basin.
Precipitation does not vary with time or space.
Storm duration is equal to the time of concentration.
A design storm of a specified frequency produces a design flood of the same
frequency.
The basin area increases roughly in proportion to increases in length.
The time of concentration is relatively short and independent of storm intensity.
The runoff coefficient does not vary with storm intensity or antecedent soil
moisture.
Runoff is dominated by overland flow.
Basin storage effects are negligible.
The minimum duration to be used for computation of rainfall intensity is 10 minutes. If
the time of concentration computed for the drainage area is less than 10 minutes, then 10
minutes should be adopted for rainfall intensity computations.

This method is mostly used in determining the quantity of storm water. The storm water
quantity is determined by the rational formula:

Q=
Where,

Q= quantity of storm water in m3/sec

C= coefficient of runoff

i= intensity of rainfall
A=area of drainage in hecta
Runoff coefficient:-

In rational method, the value of runoff coefficient, C is required. The whole quantity of
rain water that fall over the ground does not reach the sewer line. A portion of it percolates in the
ground, a portion evaporates, a portion is stored in ponds and ditches and only remaining portion
of rainwater reaches the sewer line. The runoff coefficient depends mainly on characteristics of
ground surface as porosity, wetness, ground cover etc., which varies from 0.01 for forest or
wooded area to 0.95 for a water tight roof surfaces.

As every locality consists of different types of surface area, therefore for calculating the
overall runoff coefficient the following formula is used:

unoff coefficient (overall) C =

Where:

 A1, A2 ,A3….are different types of area and

 C1, C2, C3…..are their runoff coefficient respectively.
Empirical formula method:-

For determining the runoff from very large areas, generally empirical formulae are used.
All the empirical formulae are applicable only under certain specific conditions such as slope of
land, imperviousness, rate of rainfall etc.

1. Mc maths formula:

Q =

2. Burki-Zeiglar formula:

Q=

3. Fuller‘s formula:
Q=

4. Talbot‘s formula:
Q=

5. Fanning‘s
formula: Q =

Where:

Q= runoff in m3/sec

C= coefficient of runoff

i = intensity of rainfall in cm/hour

S = slope of area in metre per thousand metre

A = area of drainage in hectares

M = area of drainage in square km

Empirical formula for rainfall intensities:-

The empirical formula given by British Ministry of Health is given by:

i= (for storm durations of 5-20 min)

 i= (for storm durations of 20-100 min)

where;

i = intensity of rainfall in mm/hour

t = duration of storm in minutes

Time of concentration:-

The time taken for the maximum runoff rate to develop, is known as the time of
concentration, and is equal to the time required for a drop of water to run from the farthest point
of the watershed to the point for which the runoff is to be calculated.

The time of concentration, t c, of a watershed is often defined to be the time required for a parcel
of runoff to travel from the most hydraulically distant part of a watershed to the outlet. It is not
possible to point to a particular point on a watershed and say, ―The time of concent ration is
measured from this point.‖ Neither is it possible to measure the time of concentration. Instead,
the concept of t c is useful for describing the time response of a watershed to a driving impulse,
namely that of watershed runoff.
In the context of the rational method then, t c represents the time at which all areas of the
watershed that will contribute runoff are just contributing runoff to the outlet. That is, at t c, the
watershed is fully contributing. We choose to use this time to select the rainfall intensity for
application of the rational method. If the chosen storm duration is larger than t c, then the rainfall
intensity will be less than that at t c. Therefore, the peak discharge estimated using the rational
method will be less than the optimal value. If the chosen storm duration is less than t c, then the
watershed is not fully contributing runoff to the outlet for that storm length, and the optimal
value will not be realized. Therefore, we choose the storm length to be equal to t c for use in
estimating peak discharges using the rational method.

The time of concentration refers to the time at which the whole area just contributes runoff
to a point.
tc te tf

Where,
tc = time of concentration
te = time of entry to the inlet (usually taken as 5 – 10 min)
tf = time of flow in the sewer
Time of concentration is made up of inlet time (over land flow) and channel flow time.
Time of entry (inlet time or overland flow): is the time required for water to reach a defined
channel such as a street gutter, plus the gutter flow time to the inlet.
Channel flow time: is the time of flow through the sewers to the point at which rate of flow is
being assessed.The channel flow time can be estimated with reasonable accuracy from the
hydraulic characteristics of the sewer. The channel flow time is then determined as the flow
length divided by the average velocity.
The inlet time is affected by numerous factors, such as rainfall intensity, surface slope, surface
roughness, flow distance, infiltration capacity, and depression storage. Because of this, accurate
values are difficult to obtain. Design inlet flow times of from 5 to 30 min are used in practice.

Estimating Time of Concentration

There are many methods for estimating t c. In fact, just about every hydrologist or engineer has a
favorite method. All methods for estimating t c are empirical, that is, each is based on the analysis
of one or more datasets. The methods are not, in general, based on theoretical fluid mechanics.
For application of the rational method, TxDOT recommends that t c be less than 300 minutes
(5 hours) and greater than 10 minutes. Other agencies require that t c be greater than 5 minutes.
The concept is that estimates of i become unacceptably large for durations less than 5 or 10
minutes. For long durations (such as longer than 300 minutes), the assumption of a relatively
steady rainfall rate is less valid.

Morgali and Linsley Method

For small urban areas with drainage areas less than ten or twenty acres, and for which the
drainage is basically planar, the method developed by Morgali and Linsley (1965) is useful. It is
expressed as

tc =0.94(nL)0.6
i0.4 S0.3
where:
tc = time of concentration (min),
i = design rainfall intensity (in/hr),
n = Manning surface roughness (dimensionless),
L = length of flow (ft), and
S = slope of flow (dimensionless).

Kirpich Method

For small drainage basins that are dominated by channel flow, the Kirpich (1940) equation
can be used. The Kirpich equation is
tc = 0.0078(L3/h)0.385
where:
tc = time of concentration (min),
L = length of main channel (ft), and
h = relief along main channel (ft).
The Kirpich method is limited to watershed with a drainage area of about 200 acres.

Kerby-Hatheway Method

For small watersheds where overland flow is an important component, but the assumptions
inherent in the Morgali and Linsley approach are not appropriate, then the Kerby (1959) method
can be used. The Kerby-Hatheway equation is

tc = (0.67NL/√S)0.467

where:
tc = time of concentration (min),
N = Kerby roughness parameter (dimensionless), and
S = overland flow slope (dimensionless).

Problem:

Calculate the quantity of sewage for separate and partially separate systems for a town, given the
following data:
i. Area of the town – 250 hectares
ii. Intensity of rainfall – 50 mm/hr
iii. Population density – 300 persons/hectare
iv. Rate of water supply – 250 ltrs/capita/day
v. Peak factor - 2.0
vi. Surface classification:
----------------------------------------------------------------------------
Type of surface % Area Run-off co-efficient
----------------------------------------------------------------------------
Roofs 50% 0.9
Paved surfaces 20% 0.85
Non paved surfaces 30% 0.30
----------------------------------------------------------------------------
Assume 80% of the water supplied reaches the sewer.

Answer:

Quantity of sewage for separate system, Q1 = 0.4166 m3/sec

Co-efficient of run-off, C = 0.8857
Quantity of storm water partially separate system,
Q2 = 17.222 m3/sec
Quantity of sewage for separate system, Q=Q1+ Q2= 17.639 m3/sec

Problem
A city has a projected population of 60,000 spread over area of 50 hectare. Find the design
discharge for the separate sewer line by assuming rate of water supply of 250 LPCD and out of
this total supply only 75 % reaches in sewer as wastewater. Make necessary assumption
whenever necessary.

Answer:
Given data
Q = 250 lit/capita/day
Sewage flow = 75% of water supply
= 0.75* 250 = 187.5 LPCD
Total sewage generated = 187.5*60000/(24*3600) = 130.21 lit/sec
= 0.13 m3/s
Assume peak factor = 2
Total design discharge = 0.26 m3/s.

Problem:

A population of 40,000 is residing in a town having an area of 60 hectares, if the average

coefficient of runoff for this area is 0.50 and the time of concentration of the design rain is 30
minutes. Calculate the discharge for which the sewer of a proposed combined system will be
designed for the town in question.

Answer:

Storm discharge – 1.7 m3/sec

Sewage discharge – 0.0625 m3/sec
Combined discharge – 1.7625 m3/sec

Problem:

Calculate the quantity of sewage for combined system for a town, given the following data:
1. Area of the town = 500 hectares, 2. Time of concentration = 30 mins, 3. Population density =
300 persons / hectare, 4. Rate of water supply = 135 l / capita / day, 5. Peak factor = 2.0,

Type of surface % Area Run off coefficient

Roofs 50 0.95
Paved surfaces 30 0.80
Non paved surfaces 20 0.25
Assume 80% of the water supplied reaches the sewer.

Answer:

Population P = 1,50,000
Quantity of sewage flow, Q1 = 0.375 m3/sec
Co-efficient of run-off, C = 0.765
Intensity of rainfall, I = 20.32 mm/hr
Quantity of storm water flow, Q2 = 21.59 m3/sec
Total combined flow, Q = 21.965 m3/sec

Problem:

Design a circular stone - ware sewer with N value 0.012, running half - full to serve a town with
the following data:
Estimated population = 1, 00,000
Rate of water supply = 135 lpcd
Average sewage discharge = 85% of water supply
Peak flow factor =3
Slope of sewer = 1:300
Is the velocity developed in the sewer in self - cleansing.

Answer:

Quantity of sewage flow, Q = 0.398 m3/sec -

Diameter of sewer, d = 0.7885 m
Velocity of flow, v = 1.63 m/sec
Velocity developed in the sewer is self cleansing

SEWER APPURTENANCES
Sewage flowing in the sewer line contains a large number of impurities in the form of silt,
fats, oils, rags etc. Under normal flows they are not likely to settle and choke the sewers, but
during small flows self-cleansing velocity is not likely to develop and the chances of choking of
the sewers are increased. Chokings have to be removed time to time, and facilities should be
provided on the sewer lines for this purpose. Therefore, for proper functioning and to facilitate
maintenance of the sewage system, various additional structures have to be constructed on the
sewer lines. These structures are known as sewer appurtenances. Following are the important
appurtenances,

1. manholes
2. inlet chamber
3. flushing device
4. inverted siphons
5. grease and oil traps
6. lamp holes etc
The manholes are R.C.C or masonry chambers constructed on the sewer line to facilitate a man
to enter the sewer line and make the necessary inspection and repairs. These are fitted with
suitable cast iron covers. The manholes should be installed at every points where there is a
change in direction, change in pipe size, or considerable change in gradient. As far as possible
sewer line between two subsequent man holes should be straight. The centre distance between
manholes is less for sewers of smaller size while it may behave such a size that man can easily
enter in the working chamber. The minimum size is 50cm diameter.
 Drop Manhole

It is a measure of connecting high level branch sewer to low level main sewer. They are
connected through a vertical pipe. The installation of a drop manhole becomes necessary when
there is difference in levels is more than 60cm between branch sewer and the main sewer, which
can be avoided by increasing the sewer grade.

Drop Manhole

Components parts of a Deep Manhole are:

i)Access shaft ii) Working chamber iii) Bottom or Invert iv) Side walls v) Steps or ladder vi) Top cover

Inlets

These are meant to admit the surface runoff to the sewers and form a very important part of
the system. Their location and design should therefore be given careful consideration.

Storm water inlets may be categorised under three major groups viz, curb inlets, gutter
inlets, and combination inlets, each being either depressed or flush depending upon their
elevation with reference to the pavement surface.

The actual structure of an inlet is usually made of brick work. Normally cast iron gratings
gratings confirming to IS:961 shall be used. The clear opening shall not be more than 25mm. The
connecting pipe from the street inlet to the main street sewer should not be less than 200mm dia.
and should have sufficient slope.

Maximum spacing of inlets would depend upon various conditions of road surface, size and
type of inlet and rainfall. A maximum spacing of 30m is recommended.
Curb Inlets

Curb inlets are vertical openings in the road curbs through which the storm water flows and
are preferred where heavy traffic is anticipated.

Gutter Inlets

Gutter inlet is sometimes called horizontal inlet also. This inlet is constructed in road gutter
and storm water enters directly into it through horizontal grating provided at the top of the inlet.
Such inlets are suitable for roads having steep slope, because its capacity to handle storm water
is quite large. Fig.4.1d

Combination Inlets

These are composed of a curb and gutter inlet acting as a single unit. Normally, the gutter
inlet is placed right in front of the curb inlet but it may be displaced in an overlapping end-to-end
position.
 Catch Basins

Catch basins are the structures of pucca chamber and a stout cover. They are meant for the
retention of suspended grit, sludge and other heavy debris and floating rubbish from rain water
which otherwise might have entered and cause choking problems. The outlet pipe from the catch
basin may be submerged in order to prevent the escape of odours from the sewer and provision
that also causes retention of floating matter. Their use is not recommended since they are more
of a nuisance and a source of mosquito breeding apart from posing substantial maintenance
problems.

Catch Basins

Flushing devices

Flushing tanks are provided to flush the sewers. They are seldom used. At such places
where self cleansing velocity is not developed or when the ground is flat and it is not possible to
lay the sewer lines at designed gradients, flushing tanks required to flush the sewer line. They are
installed at suitable intervals to clean the sewers of choking and obstructions.

It resembles a manhole but it is equipped with a siphon at the bottom. This is called the
automatic flushing tank in which the water is automatically released from the tank at suitable
intervals which may be water supply pipe tap.
 Grease and Oil traps

The sewage from kitchens of hotels and restaurants and industries contains oil and grease
and fats. If these oils and greases are not removed from the sewage they will stick to the interior
surface of the sewer and clogging. Sewage from garages, particularly from floor drains and wash
racks, contains oil, mud and sand.

The principle, on which oil and grease trap work, is since oil and grease being lighter
than water float on the surface of sewage, and the outlet is provided well below the surface so the
water is excluded from oil and grease.

If silt also has to be excluded, it is done by providing outlet at top. The silt settles at
bottom and silt free water can be drained through outlet.
Traps - Types and Uses
A trap is a device which is used to prevent sewer gases from entering the buildings. The traps
are located below or within a plumbing fixture and retains small amount of water. The retaining
water creates a water seal which stops foul gases going back to the building from drain pipes.
Therefore all plumbing fixtures such as sinks, washbasins, bathtubs and toilets etc. are equipped
with traps. This article tells you the features of traps, various types of traps and water seal.

A trap has following features.

It may be manufactured as an integral trap with the appliance as in some models of

European WC, or it may be a separate fitting called an attached trap, which is connected
to waste or foul water outlet of appliances.
The traps should be of a self-cleansing pattern.

Traps for use in domestic waste should be convenient for cleaning.

A good trap should maintain an efficient water seal under all conditions of flow.

Various Types of Traps

1. Gully Trap:
These traps are constructed outside the building to carry waste water discharge from washbasin,
sinks, bathroom etc. and are connected to the nearest building drain/sewer so that foul gases from
sewer do not come to the house. These are deep seal traps, the depth of water seal should be 50
mm minimum. It also prevents the entry of cockroach and other insects from sewer line to waste
pipes carrying waste water.
2. P. Trap:

This trap is used with Indian water closet (ORISSA Pattern). The traps are made from cast iron
or UPV sheet. This trap also has water seal and prevents entry of foul gases to the house.

3.S. Trap:
This trap is similar to P. trap and is used for fixing water closets in toilets. The only difference
between P trap and S trap is that P. trap is used for outlet through the wall whereas S. trap is used
for outlet through the floor.

Floor Trap or Nahini Trap:

This trap is provided in the floor to collect waste water from washbasin, shower, sink and
bathroom etc. These are available in cast iron or UPVC material and have removable grating
(JALI) on the top of the trap. The minimum depth of water seal should be 50 mm.
 Intercepting Trap:
This trap is provided at the last main hole of building sewerage to prevent entry of foul gases
from public sewer to building sewer. It has a deep-water seal of 100 mm.

1. Grease Trap:
This trap is a device to collect the grease contents of waste and can be cleaned from the surface.
This is generally used in food processing unit.

2. Bottle Trap:
This trap is used below washbasin and sinks to prevent entry of foul gases.
3. Q Trap:
This trap is used in toilet under water closet. It is almost similar to S trap and is used in upper
storey other than ground floor.

Water Seal
Water seal in a trap is the depth of water which should be removed from a fully charge trap
before gases at atmospheric pressure may pass from the waste pipe through trap into a building.
The tape is useless unless they retain their seals at all times. The seal may be broken due to air
compressor, momentum and evaporation. The trap in fittings in range is liable to siphonic action
and each trap should be ventilated.

The depth of water seal in various sanitary appliances is given below.

• Water closet 50 mm
• Floor trap 50 mm

Other fixtures are directly connected to the stack through branch waste pipe of diameter = 75 mm
and 40 mm

• Hand-pour flushed type 20 mm

 Basic principles of good drainage system

1.) Material have adequate strength and durability.

2.) Diameter of drain to be as small as possible.
3.) Accessibility- every part of drain within reach for inspection and maintenance.
4.) Laid in straight runs as far as possible.
5.) Drains laid to a gradient.
6.) Inlet be trapped. To prevent entry of foul air into building.
7.) Access fittings - Inspection chamber, rodding eye, manhole - placed at changes of direction
and gradient.
8.) Inspection chambers placed at junctions.
9.) Junctions between drains arranged so that incoming drain joints at oblique angle in direction
of flow.
Avoid drain under buildings if possible. Typical layout plan showing house drainage
connections
 Maintenance of house drainage

For efficient working of the house drainage system it should be properly maintained and cleaned
at regular intervals. Following points should be carefully looked at:

1. Entry of undesired elements - should take extreme precautions to avoid entry of

undesired elements in the system such as grit, sand, decayed fruits, pieces of cloths,
leaves, etc.
2. Flushing - advisable to flush the system once or twice in a day in order to maintain it in
proper working order.
3. Inspection - various unit should be inspected at regular intervals and the obstructions if
any should be removed. Damaged pipes should also be replaced.
4. Quality of materials - Better quality materials should be used

5. Use of disinfectants - Disinfectants should be freely used in the lavatory blocks,

bathrooms, etc., to maintain good sanitary conditions in the building.
6. Workmanship - Laying of drains and fixing of pipes should be carried out by licensed or
authorised plumbers only.

Sewage Pumping

Necessity of sewage pumping-

Although sewage flows in the sewers under gravity only, but still there sre occasions when it
may have to be lifted from lower level to higher level. Pumping becomes essential under
following situations,

1) Sewage of lavatory blocks located on basements of houses has to be lifted, because the level
of basement is lower than the invert level of municipal sewer or street sewer.

2) Sewage may have to be lifted in flat areas at certain intervals to avoid costly excavation and to
lay sewer at reasonable depths below the ground level. Reason for not taking sewers deep may
be sub soil water also.

3) To lift the sewage of the flow laying areas of the city and put it into street sewers at higher
level.

4) To lift the sewage from outfall sewers to treatment plant or to natural rivers or streams, if
outfall sewer lies at lower level than the level of the disposal system.

5) At treatment also, when it is required to make the sewage flow under gravity, if it is a low
level.

Some special problems in Sewage Pumping.

1) Sewage has lot of suspended, floating and solid matters. They cause very frequent problem of
clogging of pumps.

2) Sewage contains organic and inorganic wastes. They may act upon the pumping equipment
and may corrode then to reduce their life.

3) The flow of incoming sewage is not constant but keeps on fluctuating from time to time. Due
to this aspect pumping has to be adjusted accordingly by operating different units of pumps
having different pumping capacities.

4) Sewage pump has to be very reliable, otherwise it will cause flooding and lot of nuisance.
Since sewage also carries a number of disease producing bacteria, it may prove even worse for
public health if flooding nuisance is allowed to prevail. Therefore reliability of pumps has to be
of very importance.

Construction of sewers

Classification or types of sewers with respect to their material of construction:

The sewers may be made of:

(i) Asbestos cement

(ii) Bricks

(iii) Cast iron

(iv) Cement concrete plain or reinforced

(v) Corrugated iron sewers

(vi) Stoneware sewers

(vii) Steel sewers

(viii) Plastic sewers

(ix) Wooden sewers

Essential requirements of a good sewer

(a) Cost: - The cost of the material from which sewer is made, should not be high costly
materials, will result in costly sewer.

(b) Durability: - Sewer should be durable and should last long.

(c) Impervious: - This is very important and essential property of a good sewer. Infiltration
and exfiltration to sewer can be estimated to large extent by this property. The sewer should be
impervious enough to avoid these problems. Joints in sewer line should also be impervious.

(d) Resistance to corrosion: - Because of the corrosive quality of sewage, resistance to corrosion
is more important. Therefore the material used for sewer should be more corrosive resistant.

(e) Resistance to abrasion: - Since sewage contains sand and grit particles, when flowing in the
high velocity, the erosion of the sewer material may takes place. Therefore the material of the
sewer should be more resistant to abrasion.

(f) Weight: - To facilitate handling and transportation, the sewers should be light in weight.

Strength: - Sewers are mostly laid underground. They are therefore subjected to heavy external
loads. Also in soft soils there are chances of depressing the sewer at some points. This may
create beam action in the sewer line. To withstand all such effects, the sewer should be made
from strong material.

Types of sewers

Different types of sewers are discussed

1. Asbestos Cement Sewers

• These are manufactured from a mixture of asbestos fibers, silica and cement. Asbestos fibers
are thoroughly mixed with cement to act as reinforcement.
• These pipes are available in size 10 to 100 cm internal diameter and length up to 4.0 m.
• These pipes can be easily assembled without skilled labour with the help of special coupling,
called ‗Ring Tie Coupling‘ or Simplex joint.
• The pipe and joints are resistant to corrosion and the joints are flexible to permit 12o deflection
for curved laying.
• These pipes are used for vertical transport of water. For example, transport of rainwater from
roofs in multistoried buildings, for transport of sewage to grounds, and for transport of less foul
sullage i.e., wastewater from kitchen and bathroom.

Advantages
• These pipes are light in weight and hence, easy to carry and transport.
• Easy to cut and assemble without skilled labour.
• Interior is smooth (Manning n = 0.011) hence, can make excellent hydraulically efficient
sewer.

Disadvantages
• These pipes are structurally not very strong.
• These are susceptible to corrosion by sulphuric acid. When bacteria produce H2S, in presence
of water, H2SO4 can be formed.

2. Bricks sewers: - Brick sewers are made it site. They are used for construction of large size
sewers. Now a day‘s brick sewers are replaced by concrete sewers because lot of labour is
involved in the construction of brick sewers. This material is used for construction of large size
combined sewer or particularly for storm water drains. The pipes are plastered from outside to
avoid entry of tree roots and ground water through brick joints. These are lined from inside with
stone ware or ceramic block to make them smooth and hydraulically efficient. Lining also make
the pipe resistant to corrosion.

3. Cast Iron Sewers

These pipes are stronger and capable to withstand greater tensile, compressive, as well as
bending stresses. However, these are costly. Cast iron pipes are used for outfall sewers, rising
mains of pumping stations, and inverted siphons, where pipes are running under pressure. These
are also suitable for sewers under heavy traffic load, such as sewers below railways and
highways. They are used for carried over piers in case of low lying areas. They form 100% leak
proof sewer line to avoid ground water contamination. They are less resistant to corrosion;
hence, generally lined from inside with cement concrete, coal tar paint, epoxy, etc. These are
joined together by bell and spigot joint. IS:1536-1989 and IS:1537-1976 provides the
specifications for spun and vertically cast pipes, respectively.

4. Plain Cement Concrete or Reinforced Cement Concrete

Plain cement concrete (1: 1.5: 3) pipes are available up to 0.45 m diameter and reinforcement
cement pipes are available up to 1.8 m diameter. These pipes can be cast in situ or precast pipes.
Precast pipes are better in quality than the cast in situ pipes. The reinforcement in these pipes
can be different such as single cage reinforced pipes, used for internal pressure less than 0.8 m;
double cage reinforced pipes used for both internal and external pressure greater than 0.8 m;
elliptical cage reinforced pipes used for larger diameter sewers subjected to external pressure;
and hume pipes with steel shells coated with concrete from inside and outside. Nominal
longitudinal reinforcement of 0.25% is provided in these pipes.

Advantages of concrete pipes

• Strong in tension as well as compression.
• Resistant to erosion and abrasion.
• They can be made of any desired strength.
• Easily moulded, and can be in situ or precast pipes.
• Economical for medium and large sizes.
• These pipes are available in wide range of size and the trench can be opened and
backfilled rapidly during maintenance of sewers.

Disadvantages
• These pipes can get corroded and pitted by the action of H2SO4.
• The carrying capacity of the pipe reduces with time because of corrosion.
• The pipes are susceptible to erosion by sewage containing silt and grit.
The concrete sewers can be protected internally by vitrified clay linings. With protection lining
they are used for almost all the branch and main sewers. Only high alumina cement concrete
should be used when pipes are exposed to corrosive liquid like sewage.

5. Corrugated iron sewers: - Corrugated iron sewers are used for storm sewers. The sewers
should be protected from the effects of corrosion by galvanization or by bituminous coatings.
They are made in varying metal thickness and in diameters upto 450cm.

6. Plastic sewers: - (PVC pipes) Plastic is recent material used for sewer pipes. These are used
for internal drainage works in house. These are available in sizes 75 to 315 mm external diameter
and used in drainage works. They offer smooth internal surface. The additional advantages they
offer are resistant to corrosion, light weight of pipe, economical in laying, jointing and
maintenance, the pipe is tough and rigid, and ease in fabrication and transport of these pipes.
High Density Polythylene (HDPE) Pipes
Use of these pipes for sewers is recent development. They are not brittle like AC pipes and other
pipes and hence hard fall during loading, unloading and handling do not cause any damage to the
pipes. They can be joined by welding or can be jointed with detachable joints up to 630 mm
diameter (IS:4984-1987). These are commonly used for conveyance of industrial wastewater.
They offer all the advantages offered by PVC pipes.

7. Steel sewers: - There sewers are used where lightness, imperviousness and resistance to
high pressure are the prime requirements. There sewers are flexible and can absorb vibrations
and shocks efficiently. There are mainly used for trunk or outfall sewers. Riveting should, as far
as possible be avoided. These are used under the situations such as pressure main sewers, under
water crossing, bridge crossing, necessary connections for pumping stations, laying pipes over
self supporting spans, railway crossings, etc. They can withstand internal pressure, impact load
and vibrations much better than CI pipes. They are more ductile and can withstand water
hammer pressure better. These pipes cannot withstand high external load and these pipes may
collapse when negative pressure is developed in pipes. They are susceptible to corrosion and are
not generally used for partially flowing sewers. They are protected internally and externally
against the action of corrosion.

8. Vitrified Clay or Stoneware Sewers

These pipes are used for house connections as well as lateral sewers. The size of the pipe
available is 5 cm to 30 cm internal diameter with length 0.9 to 1.2 m. These pipes are rarely
manufactured for diameter greater than 90 cm. These are jointed by bell and spigot flexible
compression joints.

Advantages
• Resistant to corrosion, hence fit for carrying polluted water such as sewage.
• Interior surface is smooth and is hydraulically efficient.
• The pipes are highly impervious.
• Strong in compression.
• These pipes are durable and economical for small diameters.
• The pipe material does not absorb water more than 5% of their own weight, when
immersed in water for 24 h.

Disadvantages
• Heavy, bulky and brittle and hence, difficult to transport.
• These pipes cannot be used as pressure pipes, because they are weak in tension.
• These require large number of joints as the individual pipe length is small.

9. Wooden sewers: - In early stages these sewers were put into use. They are difficult to
construct and maintain. The life of sewers is short and they are now rarely in use.
Shapes of Sewers

Sewers are generally circular pipes laid below ground level, slopping continuously towards the
outfall. These are designed to flow under gravity. Mostly sewers of circular shape are used in all
the sewerage schemes, because of the following facts:

(i) It affords least perimeter and hence construction material required is minimum.

(ii) They are easy to construct and handle.

(iii) Since it has no corners, there are less chances of deposition of organic matters.

(iv) They possess excellent hydraulic properties.

However, sewers of non circular shapes are used for the following reasons.

(i) To develop self cleansing velocity in the sewer, when the flow is minimum.

(ii) To effect economy in the construction.

(iii) To increase the headway so that a man can enter easily for repairs, and cleaning.

Following are the non-circular shapes of sewers which are commonly used for sewers:

1. Box or rectangular sewers

2. Egg-shaped or avoid sewers

3. Basket-handle sections

4. Horse shoe sewers

5. Parabolic sewers

6. Semi-circular sewers

7. Semi-elliptical sewers

8. U-shaped sewers

1. Box or rectangular type sewers

In olden days these sewers were constructed by laying concrete at bottom and
constructing the sides with masonry. But now a day‘s masonry has been completely replaced by
concrete. These are mainly used for out fall sewers. They have got relatively high hydraulic
mean depth at large flows and therefore can have higher velocities when laid to the same slope as
that of a circular or egg-shared sewer. They are therefore most suitable for large size storm
sewers.
2. Egg-shared sewers
These are shown in figure. This share has got better hydraulic properties, but it is costly.
Firstly due to longer perimeter more material for construction is required and secondly because
of its odd shape it is difficult to construct. This sewer requires always a good foundation and
proper reinforcement to make structurally stable. In India they are rarely used. They are most
suitable in care of combined sewers.
The main advantage of this sewer is that it gives a slightly higher velocity during low
flow, than a circular sewer of the same size.

Standard Egg Shaped Sewer

New/ Modified Egg shaped Sewer

3. Basket-handle sewer.
The shape of this sewer resembles the shape of a basket handle. Small discharges flow
through the bottom narrower portion. During rainy days, the combined sewage flows in the full
section.

Basket-Handle Section

4. Horse-shoe sewers
This is as shown in fig. Its top is usually semi-circular with sides inclined or vertical. The
bottom may be flat, circular or paraboloid. Its height is more than width. It is mostly used for
sewers in tunnels. It is used for the construction of large sewers with heavy discharged such as
trunk sewers. This shape gives increased head room.

Horse shoe sewer section

5. Parabolic sewers
In this form of sewers, the upper arch takes the shape of parabola as shown in fig. The invert
of the sewer may be flat, parabolic or elliptical. They are used for the disposal of relatively
small quantities of sewage.
6. Semi-circular sewers
The semi-circular sewer gives a wider care at the bottom and hence, it becomes suitable
for constructing large sewers with less available headroom. Now a day there are replaced by
rectangular sewers.

Semi-circular Section

7. Semi-elliptical sewers
This shape of sewer is more suitable for soft soils as they are more stable. This shape is
not suitable for carrying low discharges and it is normally adopted for sewers having diameter
greater than 180cm or so.

Semi-elliptical section
8. U-shaped sewers
Two sections of U-shaped sewers are shown in fig. Trench provided at the bottom is
called cunnette. These are easy to construct. Their invert may be flat or semi-circular. The sides
are generally vertical and top may be flat or arched.

U-shaped section

Laying of sewers
After the sewer plan has been approved, the next step is to set out the work. The centre
line of the trench is first staked out on the ground. The centre line pegs are driven at a distance of
7.5m or 15m.

The sight rail and boning rod system is the accepted method for laying the drains
accurately to the gradients, indicated on the plans. Sight rails are set at all changes of gradients
and at intermediate positions, if the distance for sighting is large. The sight rails are set in such a
way that, the line sighted along the top edge of the rails represents, the true fall of the sewer, t his
gradient is shifted below the ground level by means of a Travellor of fixed length.

Sight rails are the horizontal cross rails placed on uprights. They are usually made up of a
good straight piece of timber of 15cm width and 5cm thick and length to extend over the width
of the trench. Travellor or boning rod contains of a rod and T-piece. It is most important that
boning rod should be cut to the exact length required; otherwise the pipes may not be laid
correctly to the required grade. The boning rod may be 8cm by 4cm timber piece of required
length. A T-piece of 9cm by 45cm is securely fixed by nails at top (Fig 3.3).

Since the work of laying pipes is generally started from the lower end, the sight rails will
therefore, be required to fix at this point. After fixing the first set of sight rails at the tower end, a
second set of sight rails is similarly set at some distance upstream side. Knowing the reduced
level of invert of the sewer at the lower end and the desired gradient of the sewer line, the
reduced level of invert at second set of sight rail is calculated. The depth of invert below both the
sight rails should be the same to obtain the desired correct gradient, because the top of sight rails
are adjusted to the correct reduced levels according to the gradient required.

Testing of sewer line

It is necessary to test the sewer after its laying for water tightness before backfilling of
the excaved earth.

Smoke test: - This test is performed for soil pipes, vent pipes laid above ground. The test is
conducted under a pressure of 2.5m of water and maintained for 15 minutes after all trap real

have been filled with water. The smoke is produced by burning oil waste or tar paper in
combustion chamber of a smoke machine.

Water test: - This test is performed for underground sewer pipes before back filling is done. The
test should be carried out by suitably plugging the lower end of the drain and filling the system
with water. A knuckle band shall be temporarily jointed at the top end and a sufficient length of
vertical pipe is jointed so as to provide the required test head.

Subsidence of test water may be due to

(a) Absorption by pipes and joints
(b) Leakages at joints etc.
Any leakage if visible and defective part of work if any should be made good.

Test for straightness and obstruction: - For this test, a mirror is placed in front of one end of
sewer and the image of the section is observed. If the sewer line is straight, the image should be
circular. If it is not a complete circle, then it is not straight.
For testing for obstruction, by inserting a steel call
at upper end and if there is no obstruction in the sewer line, the call will emerge out from the
lower end.

Ventilation of Sewer

Sewage flowing in sewer has got lot of organic and inorganic matters present in it. Some
of the matters decompose and produce gases. These gases are foul smelling, corrosive and
explosive in nature. If these gases are not disposed of properly they may create a number of
difficulties. They may cause air locks in sewers and affect the flow of sewage. They may prove
to be dangerous for the maintenance squad working in sewers. They may also cause explosions
and put the sewer line out of commission. For the disposal of these gases, ventilation of sewer
line is a must.

Methods of Ventilation

Following are some of the means or fittings which help in the ventilation of sewers,

1.Laying sewer line at proper gradient.

2.Running the sewer at half full or 2/3 depth.

3.Providing manhole with gratings.

4. Proper house drainage.

5. Providing the ventilating columns or shafts.