2. WATER DEMAND
While planning a water supply scheme, it is
necessary to find out not only the total yearly
water demand but also to assess the required
average rates of flow and the variations in
these rates. The following quantities are therefore
generally assessed and recorded.
1. Total annual volume in Litres or million litres.
(1MLD = 106L/d)
Annual Average rate of flow in litres per day , i.e.
V/365
Annual average rate of flow in litre per day per
person (l/c/d), called per capita demand.
6. PER CAPITA DEMAND
It is the annual average amount of daily water
required by one person and includes the
domestic use, industrial and commercial uses,
public use, wastes etc. It may therefore be
expressed as:
Per capita demand (q) in l/d/h or l/c/d =
Total yearly water requirement of the city in
litres (V) / 365 X Design population
7. FACTORS AFFECTING PER
CAPITA DEMAND
Size of the city
Climatic conditions
Types of habitat of people
Industrial and commercial activities
Quality of water supplies
Pressure in the distribution system
Development of sewage facilities
8. Why Design period is provided ?
Design period is provided because
It is very difficult or impossible to provide
frequent extension.
It is cheaper to provide a single large unit
rather to construct a number of small units.
9. 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:
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.
11. ARITHMETIC INCREASE
METHOD
This method is suitable for large and old city with
considerable development. If it is used for small,
average or comparatively new cities, it will give lower
population estimate than actual value. In this method
the average increase in population per decade is
calculated from the past census reports. This increase
is added to the present population to find out the
population of the next decade. Thus, it is assumed that
the population is increasing at constant rate.
Hence, dP/dt = C i.e., rate of change of population with
respect to time is constant.
17. 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.
21. Logistic Curve
Formula: Yc = 1 / Yc-1 whereYc-1 = c + abX
c = Upper limit
b = ratio of successive growth
a = constant
Identical to the Modified Exponential and Gompertz
curves, except:
observed values of the modified exponential curve and the
logarithms of observed values of the Gompertz curve are
replaced by the reciprocals of the observed values.
Result: the ratio of successive growth increments of the
reciprocals of the Yc values are equal to a constant
Appeal: Same as the Gompertz Curve
