INTRODUCTION:
Necessity 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.
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.
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 economic condition, changes in
habit of people, and enhanced demand for public utilities.
Factors affecting the quantity of sewage
flow:-
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
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.
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.
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 tc.
In fact, just about every hydrologist or engineer has a favorite method. All
methods for estimating tc 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 tc be less than 300 minutes
(5 hours) and greater than 10 minutes. Other agencies require that tc
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),
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),
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
|
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
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