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Module 1:
Water Quantity Requirements for Design of Water
Supply Scheme
Ø Objectives of Public Water Supply System
Ø Planning of Water Supply Scheme
Ø Components of Public Water Supply Scheme
Ø Estimating the Water Demand
Ø Types of Water Demand
Ø Factors Affecting Rate of Demand of Water
Ø Variations in Rate of Demand of Water and its Effect on the Design Capacities of the
Components of Water Supply Scheme
Ø Methods of Population Forecasts

Objectives of Public Water Supply System

The main objectives of any public water supply system are

ü To Supply safe and wholesome water to the consumers

ü To supply water in adequate quantity
ü To make water available within easy reach of the consumers so as to encourage the
general cleanliness

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Planning of Water Supply Scheme

In planning a water supply scheme for a town or city the following points need to be
considered

ü Source of water
ü Quantity of water
ü Quality of water
ü Sanitary survey
ü Population
ü Rate of consumption
ü Topography of area
ü Financial aspects
ü Trends of future development

Components of Public Water Supply Scheme

The various components of a public water supply scheme are

ü Source of water supply – surface and sub-surface sources

ü Intakes for collecting surface water
ü Water treatment plant having screening, sedimentation, filtration, disinfection units,
etc.
ü Service reservoirs
ü Control valves
ü Distribution system comprising mains, sub-mains branch pipelines, etc.
ü Hydrants for fire fighting, flushing streets, etc.

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Fig: Flow diagram of water supply scheme

Importance of a safe water supply system

The city or town should be given the benefit of water supply scheme, wherever
possible. Any water supply project grants the following advantages:

ü The growth of new industries for various pipe appurtenances such as air valves, etc.
takes place in the locality granting employment opportunities.
ü The industries which require pure water for their working are saved from the
expenditure of installing their own water purification plant.
ü The installation and maintenance of the water supply scheme grant opportunities of
employment to the local people.
ü The public in general gets treated reliable water for consumption and other uses.
ü The sanitation of the area is considerably improved by the adequate water supply
ü There are less chances of water borne diseases to occur resulting in saving of human
lives and working hours.
ü The available water in the locality is used in the best possible manner and its misuse
and wastage are avoided to a considerable extent.

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Estimating the Water Demand

The first step in the design of a water supply project is the determination of the water
demand or the quantity of water that will be required for various purposes with the
provision for the estimated requirements of the future.

For estimating the water demand or the quantity of water required for a town or city
three factors are required to be known
ü Rate of demand
ü Design period
ü Population

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Design Period

A water supply should be so planned that is has a sufficient capacity to meet the demand
not only for the present times but also for a reasonable future period or number of years

The future period or the number of years for which a provision is made while planning
and designing a water supply project is know as design period

The consideration of a design period is generally affected by the factors:

ü Useful life of the component structures
ü Ease and difficulty in future expansion
ü Availability of funds
ü Rate of interest on the borrowings
ü Anticipated rate of population growth, including possible shifts in communities,
industrial and commercial establishments

Design period for project components

S. No Components Design period
(years)
1 Storage by dams 50
2 Infiltration works 30
3 Pumping
i. Pump house (civil works) 30
ii. Electric motors and pumps 15
4 Water treatment units 15
5 Pipe connection to sveral treatment units and other 30
small appurtenances
6 Raw water and clear water conveying mains 30
7 Raw water reservoirs at the head works, balancing tanks 15
and service reservoirs (overhead or ground level)
8 Distribution system 30

Source: CPHEEO Manual on water supply and Treatment, 1999

In practice a design period of 20 to 30 years is generally considered sufficient for the
design of water supply project

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Population

It represents the total number of persons residing in a town or city at any particular time
who are to be served by the water supply project.

A water supply project is planned to meet not only the present requirements but also the
future requirements. As such it is essential to know the present population of the town or
city and also to estimate the future population.

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Types of Water Demand

The various types of water demands for a town or city may be divided into five
categories
ü Domestic water demand
ü Industrial water demand
ü Commercial and Institutional demand
ü Demand for civic or public use
ü Fire demand
ü Loss and waste of water

In order to estimate the total water demand of a town or city it is necessary to consider
the quantity of water required to fulfil each of these demands

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Domestic water Demand

Recommended per capita water supply levels for designing schemes

Source: CPHEEO Manual on water supply and Treatment, 1999

IS:1172-1993 mentioned that the minimum value of water supply given as 200
litres per head per day, which may be reduced to 135 litres per head per day for
Low Income Groups (LIG) and Economically Weaker Section (EWS) of society,
depending upon prevailing conditions

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The approximately break up of percapita per head per day may be taken as shown
below

Average domestic water consumption for Indian towns or cities

The total domestic water demand generally amounts to about 40 to 60% of the total
water

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Industrial Water Demand

Water requirements for factories or industries

In Industrial cities, the per capita water requirements may be computed to be

as high as 450 lphd as compared to the normal industrial requirements of 50
lphd

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Institutional and Commercial Water Demand

Water requirements for various Institutional activities

On an average a per capita demand of 20 lphd is usually considered to be enough

an this demand may be as high as 50 lphd for highly commercialized cities

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Demand for Civic or Public Use

This include the quantity of water required for civic or public utility purposes
such as
ü watering of public parks or gardens,
ü road washing,
ü sprinkling of water on dusty roads,
ü cleaning public sanitary blocks, large markets, etc.,
ü use in decorative features such as public fountains, etc.

For most of the water supply projects in India, depending upon the availability
of water, a provision of usually 5 to 10% of the total consumption of water is
made to meet these demands. A figure of 10 lphd is usually added while
computing total water requirement.

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Fire Demand
Generally for a moderate fire break out three jet streams are simultaneously thrown
from each hydrant-one on the burning property and one each on the adjacent property
on either side of the burning property. The discharge of each stream should be about
1100 liters per minute

In a town or city having a population of 30 lakhs, if it is assumed that four fires may break
out in a day and each fire may last for 3 hours, then the total quantity of water required
to meet the fire demand

= 3*1100*60*3*4 = 2376000 liters per day

Further the quantity of water required per head

= 2376000/3000000 = 1 liter per head per day

The above example indicates that to meet the fire demand the requirement of water per
head per day is usually quite low as compared to the requirement of water for other
purposes

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There are many methods available for determining the fire demand and they are unique
in their own way.

In the design of a water supply project the quantity of water required to meet the fire
demand is usually determined by using various empirical formulae. In these empirical
formulae the quantity of water required to meet the fire demand has been expressed as
a function of population.

The empirical formulae, which are commonly used for determining the quantity of water,
are

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Water requirement to compensate Losses in thefts and Wastes

ü Leakage in mains, valves and other fittings,

ü Worn or damaged meters,
ü Meter slippage,
ü Theft of water through unauthorized water connections,
ü Loss and waste of water due to other miscellaneous reasons

In the design of water supply project in order to account for the loss and waste
of water a provision of about 30 to 40% of the total water consumption is
usually made.

If the distribution of water is entirely through meters and if the distribution is

well maintained, it is possible to bring down the loss and waste of water to
about 10 to 15% of the total water consumption

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Factors affecting loss and waste

ü Water tightness of joints

ü Pressure in the distribution system
ü System of supply of water
ü Metered or unmetered water supply
ü Unauthorized water connections

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Total per Head (or per Capita) per Day or Rate of Demand of water
For an average Indian town or city as per IS recommendations the per head per day
demand of water for various purposes may be considered as indicated below

Per head per day demand

The rate of demand of water is thus obtained as 270/335 liters per head per day

Total quantity of water required for a town or city per year

The rate of demand of water when multiplied by the prospective population at the end
of the design period will give the average requirement of water per day and this when
multiplied by 365 days will give the total yearly requirement of water.

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Factors Affecting Rate of Demand of Water

The variation in the rate of demand of water is due to several factors, which must be
carefully studied and analyzed before fixing the rate of demand of water for a particular
town or city.

ü Size of town or city

ü Climate conditions
ü Cost of water
ü Pressure in the distribution system
ü Standard of living
ü Commercial and industries activities
ü Quality of water
ü Development of Sewerage system
ü System of supply of water
ü Policy of metering & method of charging

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Variations in Rate of Demand of Water

The variations in the rate of demand of water are classified as
ü Seasonal or monthly variations
ü Daily variations
ü Hourly variations

Seasonal or monthly Variations

The rate of demand of water varies considerably from season to season (or month to
month).
ü In summer season the average rate of demand of water is usually 30 to 40% above the
annual average rate of demand of water
ü In winter season the average rate of demand is about 20% lower than the annual
average rate of demand of water
ü Similarly during rainy season the rate of demand of water will be much less

Maximum seasonal/monthly demand = Seasonal/monthly peak factor * Annual average

demand

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Daily Variations

The rate of demand of water varies day to day due to

ü Change in the day-to-day climate conditions
ü Festival day
ü Day being holiday

Maximum daily demand = Daily peak factor *Annual average demand

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Hourly Variations

The demand of water varies from hour to hour of the day.

Maximum hourly demand = Hourly peak factor *Annual average demand

A typical graph showing the hourly variation in the rate of demand of water (expressed in
liters per head per hour) is shown below

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Assessment of Maximum Demands of Water

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Coincident draft or Coincident demand of water

during fire fighting
In the design of the distribution system of a water supply scheme a provision should be
made to meet the fire demand in addition to the maximum demand of water for the
consumers. However, it is highly improbable that the demand of water for fire fighting
will occur at the same time when there is maximum hourly draft or demand of water for
the consumers.

For general community purposes, the maximum requirement of water during fire is
determined by adding the fire demand to the maximum daily demand of water and not
to the maximum hourly demand.

The maximum daily demand when added to fire draft for working out the total draft is
known as coincident draft.

In the design of distribution system the total draft or demand is taken as maximum daily
demand and fire demand (or) the max hourly demand, whichever is more

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Effect of variations in demand of water on the design capacities of different

components of water supply scheme

Fig. Typical layouts of water

supply schemes

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Recommendations for designing the capacities of different

components of water supply scheme

Source of Water Supply:

The source of water supply should have sufficient capacity to meet the maximum daily
demand of water. Generally the impounding reservoirs have very large capacity, but in the
case of rivers and wells used for water supply it should be ensured that these have
sufficient capacity to meet the maximum daily demand of water.

Conduits or Pipe Mains Carrying Water from the Source to the Service Reservoir:
The conduits or pipe mains carrying water from the source to the service reservoir
(conduits I and II in Figure) should be designed for the maximum daily demand of water.

Conduits or Pipe Mains Carrying Water from the Service Reservoir to the Distribution
System:
The conduits or pipe mains carrying water from the service reservoir to the distribution
system (conduit III in Figure) should be designed for the sum of the fire demand and the
maximum daily demand of water (i.e., coincident draft with fire) or the maximum hourly
demand of water, whichever is more.

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Pumps:

Both the low lift and high lift pumps should be designed for the maximum daily demand
of water plus some reserve for break-downs and repairs. Thus the pumps may be
designed for 2 to 3 times the annual average daily demand of water instead of 1.8 times
the annual average daily demand of water.

Further if the pumps are not working for all the 24 hours, then the above indicated
design rates for the pumps should be multiplied by the ratio of 24 hours to the number
of hours for which the pumps are working, to arrive at the actual design rates for the
pumps.

Filters and other Units at Treatment Plant:

The filters and other units at treatment plant should be designed for the maximum daily
demand of water plus some reserve for break-downs and repairs. Thus these may be
designed for 2 times the annual average daily demand of water instead of 1.8 times the
annual average daily demand of water.

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Distribution System:

The distribution system should be designed for the sum of the fire demand and the
maximum daily demand of water (i.e., for the fire demand plus the coincident draft or
coincident demand of water) or the maximum hourly, demand of water whichever is
more.

Service Reservoir:

The service reservoir is designed to take care of the hourly fluctuations in water
consumption, fire demand, emergency reserve and the provision required when pumps
have to pump the entire day’s requirement of water in limited hours of the day.
Ordinarily the service reservoir is designed to hold or store the quantity of water, which
would be enough for a day’s consumption of the locality served by it.

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Methods of Population Forecasting

Design of water supply and sanitation scheme is based on the projected population of
a particular city, estimated for the design period.

Any underestimated value will make system inadequate for the purpose intended;
similarly overestimated value will make it costly.

Changes in the population of the city over the years occur, and the system should be
designed taking into account of the population at the end of the design period.

Factors affecting changes in population are:

ü Increase due to births
ü Decrease due to deaths
ü Increase/ decrease due to migration

Besides the above indicated three factors, some other factors such as wars, natural
havocs or disasters such as epidemics, earthquakes etc., may also bring about sharp
reductions in the population.

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Different Methods
ü Arithmetical Increase Method
ü Geometrical Increase Method
ü Incremental Increase Method
ü Decreasing Rate Method
ü Simple Graphical Method
ü Comparative Graphical Method
ü Master Plan Method (or) Zoning method
ü The Apportionment Method
ü The Logistic Curve Method

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Arithmetical increase method

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Geometrical Increase Method

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Incremental Increase Method

This method is improvement over the above two methods. In this method the per
decade growth rate is not assumed to be constant as in the arithmetic or geometric
progression methods, but is progressively increasing or decreasing, depending
upon whether the average of the incremental increase in the past data is positive or
negative.

The incremental increase may be positive or negative depending on the increase in

the population during a successive decade is more than or less than the increase in
the population during its preceding decade.

The incremental increase per decade is defined as the difference between the
increase in population during the successive decades.

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Decreasing Rate Method (or) Decrease Rate of Growth Method

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Simple graphical method

A graph is plotted from the available data, between time and population and the curve is
smoothly extended up to the desired year to get the desired value

The prospective population of each future decade is read from the extended curve

Fig. Population forecasting by graphical method

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Comparative graphical method

Cities of similar conditions and characteristics as that of city under consideration (X) are
selected and graph is plotted for population growth for all this selected city to find future
population of the city (X)

In this method it is
assumed that the city
whose future
population is to be
estimated will develop
in the same manner as
the other similar cities
have developed in the
past, and hence its
future population
growth will match with
the past population
growth of the similar
cities

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Master Plan method

Population Densities

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The Logistic Curve Method

Assumption

The curve is s-shaped and is

known as logistic curve which
gives complete trend of
growth of the city right from
beginning to saturation limit
of population of the city

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Logistic Differential Equation

Let ‘Ps’ represent the carrying capacity for a particular organism in a given environment,
and let ‘K’ be a real number that represents the growth rate. The function P(t)
represents the population of this organism as a function of time ‘t’, and the constant
‘Po’ represents the initial population (population of the organism at time t=0 ). Then the
logistic differential equation is

!" "
= %" &1 − )
!# "!

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