USAGE & WATER DEMAND

TOPIC 2:
USAGE &DEMAND OF WATER SUPPLY

PREPARED BY :
TUAN NOOR LAILY TUAN HAMAT

CIVIL ENGINEERING @ PKB

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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 as big cities have sewered houses.

 Presence of industries.
 Climatic conditions.
 Habits of people and their economic status.
 Quality of water: If water is aesthetically & medically safe, the consumption will

increase as people will not resort to private wells, etc.

 Pressure in the distribution system.
 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.

Average Daily Per  Hourly variations are very important as they
Capita Demand have a wide range. During active household
working hours i.e. from six to ten in the
 Average Daily Per Capita Demand morning and four to eight in the evening, the
bulk of the daily requirement is taken. During
q (litres) = Quantity Required in 12 Months other hours the requirement is negligible.
(365 x population) Moreover, if a fire breaks out, a huge quantity
of water is required to be supplied during short
If this average demand is supplied at all the times, it will duration, necessitating the need for a maximum
rate of hourly supply.
 not be sufficient to meet the fluctuations.
Seasonal variation: The demand peaks during summer.  Maximum daily demand = 1.8 x average daily demand
Firebreak outs are generally more in summer, Maximum hourly demand of maximum day i.e. Peak demand
= 1.5 x average hourly demand
 increasing demand. So, there is seasonal variation . = 1.5 x Maximum daily demand/24
Daily variation depends on the activity. People draw out = 1.5 x (1.8 x average daily demand)/24
more water on Sundays and Festival days, thus = 2.7 x average daily demand/24
increasing demand on these days. = 2.7 x annual average hourly demand

Water Quantity Estimation

 Water demand is based on population served, per

capita consumption, service factor, industrial
and other special demands. In estimating water
demand, various other factors should be taken
into account directly or indirectly. These other
factors include unaccounted-for-water,
unsatisfied improvement in living standards,
increase in service factor over time and
maximum day demand.

 In general 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

Basic formula for water demand Estimation

WDn = (Pn x q x F1 x F2) + Dm
,Where

WDn = water demand at the end of year “n”
Pn = population of “n” year
q = per capita demand
F1 = services factor

F2 = design factor
Dm = additional demand

Population Forecasting Methods

The various methods adopted for ❑ Arithmetic Increase Method
estimating future populations ❑ Geometric Increase Method

are given below. The particular ❑ Incremental Increase Method
method to be adopted for a ❑ Decreasing Rate of Growth Method
particular case or for a ❑ Simple Graphical Method
particular city depends largely ❑ Comparative Graphical Method
on the factors discussed in the ❑ Ratio Method
methods, and the selection is
❑ Logistic Curve Method
left to the discretion and

intelligence of the designer.

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Population Forecasting Methods

Arithmetic Increase Method Geometric Increase Method

This method is based on the This method is based on the assumption that
assumption that the population percentage growth rate is constant i.e.
increases at a constant rate; i.e. dP/dt=kP; lnP= lnPo+kt.
dP/dt=constant=k; Pt= Po+kt. This
method is most applicable to large This method must be used with caution, for
and established cities when applied it may produce too large results
for rapidly grown cities in comparatively
short time. This would apply to cities with
unlimited scope of expansion. As cities grow
large, there is a tendency to decrease in the
rate of growth

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Population Forecasting Methods

Incremental Increase Method Decreasing Rate of Growth Method

Growth rate is assumed to be In this method, the average decrease in
progressively increasing or decreasing, the percentage increase is worked out,
depending upon whether the average of and is then subtracted from the latest
the incremental increases in the past is percentage increase to get the
positive or negative. The population for percentage increase of next decade.
a future decade is worked out by adding
the mean arithmetic increase to the last
known population as in the arithmetic
increase method, and to this is added the
average of incremental increases, once
for first decade, twice for second and so
on.

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Population Forecasting Methods

Simple Graphical Method Comparative Graphical Method

 In this method, a graph is  In this method, the cities

plotted from the available having conditions and
data, between time and characteristics similar to the
population. The curve is city whose future population is
then smoothly extended up to be estimated are selected. It
to the desired year. This is then assumed that the city
method gives very under consideration will
approximate results and develop, as the selected similar
should be used along with cities have developed in the
other forecasting methods past.

Population Forecasting Methods

 Ratio Method
 In this method, the local population and the country's population for the last four

to five decades is obtained from the census records. The ratios of the local
population to national population are then worked out for these decades. A graph
is then plotted between time and these ratios, and extended upto the design
period to extrapolate the ratio corresponding to future design year. This ratio is
then multiplied by the expected national population at the end of the design
period, so as to obtain the required city's future population.

 Drawbacks:
 Depends on accuracy of national population estimate.
 Does not consider the abnormal or special conditions which can lead to population

shifts from one city to another.

Population Forecasting Methods

Logistic Curve Method

 The three factors responsible for changes in
population are :

(i) Births, (ii) Deaths and
(iii) Migrations.

Logistic curve method is based on the
hypothesis that when these varying influences
do not produce extraordinary changes, the
population would probably follow the growth
curve characteristics of living things within
limited space and with limited economic
opportunity. The curve is S-shaped and is
known as logistic curve.

Problem

Population Forecast by Different Methods

Problem:
Predict the population for the years 1981, 1991, 1994, and 2001 from the following
census figures of a town by different methods.

Year 1901 1911 1921 1931 1941 1951 1961 1971

Population:
(thousands) 60 65 63 72 79 89 97 120

Solution:

Year Population: Increment per Percentage Increment per

1901 (thousands) Decade Decade
1911
1921 60 - -
1931
1941 65 +5 (5/60) x100=+8.33
1951
1961 63 -2 (-2/65) x100=-3.07
1971
Net values 72 +9 (9/63) x100=+14.28
Averages
79 +7 (7/72) x100=+9.72

89 +10 (10/79) x100=+12.66

97 +8 (8/89) x100=8.98

120 +23 (23/97) x100=+23.71

1 +60 +74.61

- 8.57 10.66

+=increase; - = decrease

Solution:

Arithmetical Progression Method:
Pn = P + k∆t
Average increases per decade = i = 8.57
Population for the years,
1981= population 1971 + k∆t here ∆t =1 decade

= 120 + 8.57 = 128.57
1991= population 1971 + k∆t, here ∆t =2 decade

= 120 + 2 x 8.57 = 137.14
2001= population 1971 + k∆t, here ∆t =3 decade

= 120 + 3 x 8.57 = 145.71
1994= population 1991 + (population 2001 - 1991) x 3/10

= 137.14 + (8.57) x 3/10 = 139.71

Solution:

Geometric Progression Method:
Average percentage increase per decade = 10.66
P n = P (1+i/100) ∆t
Population for 1981 = Population 1971 x (1+i/100) n
= 120 x (1+10.66/100), i = 10.66, n = 1
= 120 x 110.66/100 = 132.8
Population for 1991 = Population 1971 x (1+i/100) n
= 120 x (1+10.66/100) 2 , i = 10.66, n = 2
= 120 x 1.2245 = 146.95
Population for 2001 = Population 1971 x (1+i/100) n
= 120 x (1+10.66/100) 3 , i = 10.66, n = 3
= 120 x 1.355 = 162.60
Population for 1994 = 146.95 + (15.84 x 3/10) = 151.70

Contoh

Anggaran penduduk Daerah Kinta pada tahun 2010 adalah 562,500 orang , kirakan

keperluan air harian pada tahun tersebut di mana liputan bekalan air adalah 95%. Data-data

di bawah telah diperolehi dari Lembaga Air Perak (LAP)

i. Permintaan air penduduk = 275 liter/kapita/hari

ii. Keperluan air industri = 1/3 daripada keperluan penduduk

iii. Faktor rekabentuk = 1.5

iv. Peratusan NRW = 15%

•Rumus bagi permintaan Harian, WDn = (Pn x q x F1 x F2) + Dm

Penyelesaian

Daripada data-data yang diberi :
Permintaan air harian, WDn = (Pn x q x F1 x F2) + Dm
WD2010 = ( 562,500 x 275 x 0.95 x 1.5 ) + 1/3 (562,500 x 275 x1x 1.5)

+ 0.15[ (562,500 X 275 X 0.95 X 1.5) + 1/3 (562,500 x 275 x 1.5)]
= 220,429,688 + 77,343,750 + 44,660,016
= 342,439,454 liter/hari
= 342.44 x 106 liter/hari

= 342.44 MLD

Problems

If the population (P) is 32003 and
the total daily water per capita is
230 liter/day (q), calculate the
water demand estimation ( WDn) if
the service factor ( F1) is 0.98 and
the design factor (F2) is 2.5.
Assume there is no additional
demand.

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PROBLEM

The table below shows the additional population for Ketereh, Kelantan for 1970 to
2010 :

(i) Predict the population of the city in year 2020 and 2030 by using Arithmetic
Method and Geometric Method.

(ii) If the capita water consumption (q) is 180 people/ liter/ day, calculate the water
demand quantity to be supplied in 2020 and 2030 if the service factor is 0.8 and
the design factor is 2.5. Assuming that there is no additional demand in the two
years.

Year 1970 1980 1990 2000 2010
Population 12,550 14,569 17,770 22,271 28,112

Question 1 SELF ASSESSMENT

Table 1 shows the population data for an area in Penang. The population data for the year 1971 is missing

from the bank data storage. However, calculation of record for the year 2021 population estimations

made by using arithmetic increase method is 30,666 people. This calculation is based on records of the last

population in 2001. Base from this information, calculate the population in 1971 and then estimate the

population in 2021 by using geometric method.

Table 1

Year 1971 1981 1991 2001
Populations ? 21,120 22,950 25,600

Question 2
•The following data was obtained from Taman Seri Cemerlang in 2000. Calculate the volume of water

demand (WD) in 2008.
Total household = 1200 households

Average household members = 6 people

Per capita water consumption = 200 liters/person/day

Population growth = 2.7 % per year

Institutional water needs for higher education = 1/3 of the population needs

Design factor = 2.5

NRW percentage = 20 %

Water supply coverage = 90 %
( Pn = Po (1 + r )∆t , WDn = Pn x q x F1 x F2 + Dm )