Water Requirements

Environmental Engineering-I
Water Quantities Requirement
Unit-I

Syllabus
• Quantities of water: Types of demands, total
requirement of city,
• per capita demand and factors affecting it,
effect on design capacities of water treatment
and supply system, design period, population
data and forecasting, design calculation of
total water demand for various uses in a
city/areas

Water Quantity Estimation
• The quantity of water required for municipal uses for
which the water supply scheme has to be designed
requires following data:
• Water consumption rate (Per Capita Demand in litres
per day per head)
• Population to be served.
Quantity = Per capita demand x Population

Water Consumption Rate
• Very difficult to assess the quantity of water
demanded by the public, since there are many
variable factors affecting water consumption.
• There are various types of water demands in a city.

• Domestic water demand
• Industrial demand
• Institution and commercial demand
• Demand for public use
• Fire demand
• Loses and wastes
Water Demand

Domestic Water Demand
• water required in the houses for drinking,
bathing, cooking, washing etc.
• mainly depends upon the habits, social status,
climatic conditions and customs of the people.
• As per IS: 1172-1963, under normal
conditions, the domestic consumption of water
in India is about 135 litres/day/capita.

The details of the domestic consumption are
a) Drinking ------ 5 litres
b) Cooking ------ 5 litres
c) Bathing ------ 55 litres
d) Clothes washing ------ 20 litres
e) Utensils washing ------ 10 litres
f) House washing ------ 10 litres
--------------------------
135 litres/day/capita

Industrial Demand
• The water required in the industries mainly
depends on the type of industries, which are
existing in the city.
• The water required by factories, paper mills,
Cloth mills, Cotton mills, Breweries, Sugar
refineries etc. comes under industrial use.
• The quantity of water demand for industrial
purpose is around 20 to 25% of the total
demand of the city.

Institution and Commercial Demand
• Universities, Institution, commercial buildings
and commercial centres including office
buildings, warehouses, stores, hotels, shopping
centres, health centres, schools, temple,
cinema houses, railway and bus stations etc
comes under this category.

Institution and Commercial Demand

Demand for Public Use
• Quantity of water required for public utility
purposes such as for washing and sprinkling
on roads, cleaning of sewers, watering of
public parks, gardens, public fountains etc.
comes under public demand.
• To meet the water demand for public use,
provision of 5% of the total consumption is
made designing the water works for a city.

The requirements of water for public utility shall
be taken as…

Fire Demand
• During the fire breakdown large quantity of
water is required for throwing it over the fire
to extinguish it, therefore provision is made in
the water work to supply sufficient quantity of
water or keep as reserve in the water mains for
this purpose.

Fire Demand
• Buston’s formula
• Q= 5663 √𝑷
• Freeman’s Formula
• Q= 1136 (
𝑷
𝟓
+ 𝟏𝟎)
• National Board of Fire Underwriter’s formula
• Q= 4637 𝒑(𝟏 − 𝟎. 𝟎𝟏 𝒑)
• P= Population in thousands

Loses and Wastes
• Losses due to defective pipe joints, cracked
and broken pipes, faulty valves and fittings.
• Losses due to, continuous wastage of water.
• Losses due to unauthorised and illegal
connections.
• While estimating the total quantity of water of
a town; allowance of 15% of total quantity of
water is made to compensate for losses, thefts
and wastage of water.

Water Consumption for Various
Purposes

PER CAPITA DEMAND
• If ‘Q’ is the total quantity of water required by
various purposes by a town per year and ‘p’ is
population of town, then per capita demand
will be
• Q
• Per Capita Demand = ------------------ litres/day
• P x 365

 Per capita demand of the town depends on various
factors like standard of living, no. and type of
commercial places in a town etc.
 For an average Indian town, the requirement of water
in various uses is as under-
Domestic purpose -------- 135 litres/c/d
Industrial use -------- 40 litres/c/d
Public use -------- 25 litres/c/d
Fire Demand -------- 15 litres/c/d
Losses, Wastage and thefts -------- 55 litres/c/d
--------------------------
Total : 270 litres/capita/day

FACTORS AFFECTING PER CAPITA
DEMAND
• Size Of The City: Per Capita Demand For Big
Cities Is Generally Large As Compared To
That For Smaller Towns .
• Presence Of Industries-
• Climatic Conditions-
• Habits Of People And Their Economic Status-
• Pressure In The Distribution System-

• Quality of Water: If water is aesthetically &
medically safe, the consumption will increase .
• Efficiency of water works administration: Leaks
in water mains and services; and unauthorised use
of water can be kept to a minimum by surveys.
• Cost of Water-
• Policy of metering and charging method: Water tax is
charged in two different ways: on the basis of meter
reading and on the basis of certain fixed monthly
rate.
FACTORS AFFECTING PER CAPITA
DEMAND

FLUCTUATIONS IN RATE OF DEMAND
• Average Daily Per Capita Demand
• = Quantity Required in 12 Months
• (365 x Population)
• If this average demand is supplied at all the times, it
will not be sufficient to meet the fluctuations.
• Maximum daily demand = 1.8 x average daily
demand

• Maximum hourly demand of maximum day i.e. Peak
demand
= 1.5 x average hourly demand
= 1.5 x Maximum daily demand/24
= 1.5 x (1.8 x average daily demand)/24
= 2.7 x average daily demand/24
= 2.7 x annual average hourly demand

• Seasonal Variation: The demand peaks during
summer. Firebreak outs are generally more in
summer, increasing demand. So, there is seasonal
variation .
• Daily Variation depends on the activity. People draw
out more water on Sundays and Festival days, thus
increasing demand on these days.

• Hourly Variations are very important as they have a
wide range. During active household working hours
i.e. from six to ten in the morning and four to eight in
the evening, the bulk of the daily requirement is
taken. During other hours the requirement is
negligible.
• Moreover, if a fire breaks out, a huge quantity of
water is required to be supplied during short duration,
necessitating the need for a maximum rate of hourly
supply.

• So, an adequate quantity of water must be available to
meet the peak demand.
• To meet all the fluctuations, the supply pipes, service
reservoirs and distribution pipes must be properly
proportioned.
• The water is supplied by pumping directly and the
pumps and distribution system must be designed to
meet the peak demand.

• The effect of monthly variation influences the
design of storage reservoirs and the hourly
variations influences the design of pumps and
service reservoirs.
• As the population decreases, the fluctuation rate
increases.

DESIGN PERIODS
• The future period for which a provision is made in the
water supply scheme is known as the design period.
• Design period is estimated based on the following:
• Useful life of the component, considering
obsolescence, wear, tear, etc.
• Expandability aspect.

• Anticipated rate of growth of population, including
industrial, commercial developments & migration-
immigration.
• Available resources.
• Performance of the system during initial period.
DESIGN PERIODS

POPULATION FORECASTING METHODS
• The various methods adopted for estimating future
populations are.
• Arithmetic Increase Method
• Geometric Increase Method
• Incremental Increase Method
• Decreasing Rate of Growth Method
• Simple Graphical Method
• Comparative Graphical Method
• The Master Plan method

FACTORS AFFECTING CHANGES IN
POPULATION ARE
• Increase due to births
• Decrease due to deaths
• Increase/ decrease due to migration
• Increase due to annexation.
• The present and past population record for the city can be
obtained from the census population records. After collecting
these population figures, the population at the end of design
period is predicted using various methods as suitable for that
city considering the growth pattern followed by the city.

POPULATION FORECASTING METHODS
• The particular method to be adopted for a particular
case or for a particular city depends largely on the
factors discussed in the methods, and the selection is
left to the discretion and intelligence of the designer.

ARITHMETIC INCREASE METHOD
• This method is based on the assumption that the
population is increasing at a constant rate.
• The rate of change of population with time is
constant. The population after ‘n’ decades can be
determined by the formula
• Pn = P + n.c where
• P → population at present
• n → No. of decades
• c → Constant determined by the average of increase
of ‘n’ decades

Example
• The following data have been noted from
census department
Year Population
1940 8000
1950 12000
1960 17000
1970 22500

Example
• Calculate the probable population in the year 1980,
1990, 2000
Year Population Increase in Population
1940 8000
1950 12000 4000
1960 17000 5000
1970 22500 5500
Total 14500
Average Increase 4833

Example
Year Population
(Pn =P + n C)
1980 22500 + 1 x 4833 = 27333
1990 27333 + 1 x 4833= 32166
2000 32166 + 1 x 4833 = 36999

GEOMETRIC INCREASE METHOD
• This method is based on the assumption that the
percentage increase in population from decade to
decade remains constant.
• In this method the average percentage of growth of
last few decades is determined.
• The population at the end of ‘n’ decades is calculated
by- Pn = P (1+ IG/100) n
•where
• P → population at present
• IG = geometric mean (%)

Example
Year Population Increase in population Percentage Increase in
Population
1940 8000
1950 12000 4000 4000 x 100= 50 %
8000
1960 17000 5000 5000 x 100 = 41.7 %
12000
1970 22500 5500 5500 x 100 = 32.4 %
17000
Total 14500 124.1
Average 4833 41.37 %

Example
• The Population at the end of various decades shall be as
• Pn = P (1+ IG/100) n
• Where, IG = geometric mean (%)
• P = Present population
• N = no. of decades.
Year Population Increase
1980 22500 + 41.37 x 22500 = 31808
100
1990 31808 + 41.37 x 31808 = 44967
100
2000 44967 + 41.37 x 44967= 63570
100

INCREMENTAL INCREASE METHOD
• This method is improvement over the above
two methods.
• The average increase in the population is
determined by the arithmetical method and to
this is added the average of the net incremental
increase once for each future decade.

• This method is modification of arithmetical increase
method and it is suitable for an average size town
under normal condition where the growth rate is
found to be in increasing order. While adopting this
method the increase in increment is considered for
calculating future population.

• The incremental increase is determined for each decade from
the past population and the average value is added to the
present population along with the average rate of increase.
Hence, population after nth decade is
• Pn = P+ n.X + {n (n+1) }.Y
2
• Where,
• Pn = Population after nth decade
• X = Average increase
• Y = Incremental increase

Example
• Forecast the population by means of Incremental
Increase method
Year Population Increase in
Population
Incremental
Increase
1940 8000
1950 12000 4000
1960 17000 5000 +1000
1970 22500 5500 + 500
Total 14500 1500
Average 4833 (+750)

Example
• P1980 = P+ n. X + {n (n+1) }.Y
2
• P1980 = 22500 + 1 x 4833 + 1 (1+1) x 750
2
= 22500 + 4833 +750
P 1980= 28083

Example
P1990 = 28083 + 1 x 4833 + 1(1+1) x 750
2
P1990= 33666
P2000 = 33666 + 1 x 4833 + 1 (1+1) x 750
2
P2000= 39249

Decrease Rate of Growth Method
• In this method, the average decrease in the
percentage increase is worked out, as is then
subtracted from the latest percentage increase
for each successive decades.

Example
Year Population Increase Percentage Increase Decrease in
percentage
increase
1940 8000
1950 12000 4000 4000 x 100= 50
8000
1960 17000 5000 5000 x 100= 41.7
12000
+8.3
1970 22500 5500 5500 x 100= 32.4
17000
+9.3
Total 14500 17.6
4833 8.8

Example
Year Net Percentage Increase Population
1980 32.4- 8.8 = 23.6 22500 + 23.6 x 22500=27810
100
1990 23.6-8.8= 14.8 27810 + 14.8 x 27810= 31926
100
2000 14.8-8.8= 6.0 31926 + 6 x 31926 = 33842
100

GRAPHICAL METHOD
• In this method, the populations of last few decades are
correctly plotted to a suitable scale on graph. The population
curve is smoothly extended for getting future population. This
extension should be done carefully and it requires proper
experience and judgment. The best way of applying this
method is to extend the curve by comparing with population
curve of some other similar cities having the similar growth
condition.

COMPARATIVE GRAPHICAL METHOD
• In this method the census populations of cities already
developed under similar conditions are plotted. The curve of
past population of the city under consideration is plotted on the
same graph. The curve is extended carefully by comparing
with the population curve of some similar cities having the
similar condition of growth. The advantage of this method is
that the future population can be predicted from the present
population even in the absent of some of the past census
report.

MASTER PLAN METHOD
• The big and metropolitan cities are generally not developed in
haphazard manner, but are planned and regulated by local
bodies according to master plan. The master plan is prepared
for next 25 to 30 years for the city. According to the master
plan the city is divided into various zones such as residence,
commerce and industry. The population densities are fixed for
various zones in the master plan. From this population density
total water demand and wastewater generation for that zone
can be worked out. So by this method it is very easy to access
precisely the design population.

LOGISTIC CURVE METHOD
• This method is used when the growth rate of population due to
births, deaths and migrations takes place under normal
situation and it is not subjected to any extraordinary changes
like epidemic, war, earth quake or any natural disaster etc. the
population follow the growth curve characteristics of living
things within limited space and economic opportunity. If the
population of a city is plotted with respect to time, the curve so
obtained under normal condition is look like S-shaped curve
and is known as logistic curve.

Example
• Predict the population for the year 2021, 2031,
and 2041 from the following population data.
Year 1961 1971 1981 1991 2001 2011
Population 8,58,545 10,15,672 12,01,553 16,91,538 20,77,820 25,85,862

Year Population Increment
1961 858545
1971 1015672 157127
1981 1201553 185881
1991 1691538 489985
2001 2077820 386282
2011 2585862 508042
Average increment 345463

• Population in year 2021 is,
• Population (Pn =P + n C)
• P2021 = 2585862 + 345463 x 1 = 2931325
Similarly,
• P2031 = 2585862 + 345463 x 2 = 3276788
• P2041 = 2585862 + 345463 x 3 = 3622251

Year Population Increment Geometrical Increase
Rate of Growth
1961 858545 -
1971 1015672 157127 157127 = 0.18
858545
1981 1201553 185881 185881 = 0.18
1015672
1991 1691538 489985 489985 = 0.40
1201553
2001 2077820 386282 386282 = 0.23
1691538
2011 2585862 508042 508042 = 0.24
2077820
Average increment 345463

Example
Geometric Mean
• IG = (0.18 x 0.18 x 0.40 x 0.23 x 0.24) 1/5
• = 0.235 i.e., 23.5%
• Population in year 2021 is, Pn = P {1+ IG/100}n
• P2021 = 2585862 x (1+ 0.235)1 = 3193540
• Similarly for year 2031 and 2041 can be calculated by,
• P2031 = 2585862 x (1+ 0.235)2 = 3944021
• P2041 = 2585862 x (1+ 0.235)3 = 4870866

Year Population Increment Incremental Increase
1961 858545 -
1971 1015672 157127
1981 1201553 185881 +28754
1991 1691538 489985 +304104
2001 2077820 386282 -103703
2011 2585862 508042 +121760
Average increment 1727317 350915
345463 87729

• P2021 = P+ n. X + {n (n+1) } Y
2
• P2021 = 2585862 + (345463 x 1) + {(1 (1+1) } x 87729
2
• P2021 = 3019054

• For year 2031
• P2031 = 2585862 + (345463 x 2) + {(2 (2+1) )}x 87729
• 2
• P2031 = 3539975
• P2041 = 2585862 + (345463 x 3) + { (3 (3+1) }x 87729
• 2
• P2041 = 4148625

• The populations of 5 decades from 1930 to 1970 are
given below. Find-out the population after one, two
and three decades beyond the last known decade by
arithmetic increase method.
Year 1930 1940 1950 1960 1970
Population 25000 28000 34000 42000 47000

Example
Year Population Increment
1930 25000
1940 28000 3000
1950 34000 6000
1960 42000 8000
1970 47000 5000
Total 22000
Average Increase per
Decade
22000= 5500
4

Example
• The future populations are now computed as
• Population (Pn =P + n C)
• P1980 = 47000+ 5500 x 1 = 52,500.
Similarly,
• P1990 = 47000 + 5500 x 2 = 58000
• P2041 = 47000 + 5500 x 3 = 63500

Year Population Increment Percentage Increase
in population
1930 25000
1940 28000 3000 3000 x 100= 12 %
25000
1950 34000 6000 6000 x 100= 21.4 %
28000
1960 42000 8000 8000 x 100 = 23.5 %
34000
1970 47000 5000 5000 x 100= 11.9 %
42000
Total 22000
Average Increase
per Decade
22000= 5500
4

• The geometric mean of the growth rates (r)
• = (12x 21.4x 23.5x 11.9)1/4
• = 16.37 % per decade
• Now assuming that the future population increases at this
constant rate.
• Pn = P {1+ IG/100}n
• Pn= Po (1+0.1637) n
• = Po (1.1637) n
• Using n= 1,2,3 decades
• P 1980= 47000 (1.1637) = 54694

• P1990 = Population after two decades
• = 47000 (1.1637)2= 63647.
• P2000= Population after 3 decades
• = 47000(1.1637) 3= 74066.

Year 1930 1940 1950 1960 1970
Population 25000 28000 34000 42000 47000

Year Population Increment Incremental Increase
1930 25000
1940 28000 3000
1950 34000 6000 (+) 3000
1960 42000 8000 (+) 2000
1970 47000 5000 (-) 3000
Total 22000 (+) 2000
Average Increase
per Decade
X=22000= 5500
4
Y= 2200 = (+) 667
3

• The future population Pn is now given as
• Pn = P+ n. X + {n (n+1) } Y
2
• P1980 = 47000+ 1. 5500 + {1 (1+1) } 667
2
= 47000 + 1 x 5500 + 1x 667
= 53167

• Pn = P+ n. X + {n (n+1) } Y
2
• P1990 = 47000+ 2. 5500 + {2 (2+1) } 667
2
= 47000 + 1 x 5500 + 3x 667
= 60001

• Pn = P+ n. X + {n (n+1) } Y
2
• P1980 = 47000+ 3. 5500 + {3 (3+1) } 667
2
= 47000 + 3 x 5500 + 6x 667
= 67502

DECREASE RATE OF GROWTH
METHOD
Year 1930 1940 1950 1960 1970
Population 25000 28000 34000 42000 47000

METHOD
Year Population Increment Percentage Increase in
population
Decrease in
Percentage Increase
1930 25000
1940 28000 3000 3000 x 100= 12 %
25000
1950 34000 6000 6000 x 100= 21.4 %
28000
(-) 9.4 %
1960 42000 8000 8000 x 100 = 23.5 %
34000
(-) 2.1 %
1970 47000 5000 5000 x 100= 11.9 %
42000
(+) 11.6 %
Total 22000 -11.5+11.6 = 0.1 %
Average
Increase
per Decade
22000= 5500
4
0. 1 = 0.03 %
3

METHOD
• The expected population at the end of year 1980
• P1980 = 47000 + 11.9 -0.03 x 47000
100
= 47000 + 11.87 x 47000
100
= 47000 + 5570
= 52570

METHOD
• P1990 = 52570 + 11.97 -0.03 x 52570
100
= 52570 + 11.84 x 52570
100
= 52570 + 6230
= 58800

METHOD
• P2000 = 58800 + 11.84 -0.03 x 58800
100
= 58800 + 11.81 x 58800
100
= 58800 + 6950
= 65750

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