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

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.

The design population will have to be estimated with due regard to all the factors governing the future growth and development of the project area in the industrial, commercial, educational, social and administration spheres. Special factors causing sudden immigration or influx of  population should also be foreseen to the extent possible.

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 low result 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.

Geometric Increase Method

In this method the percentage increase in population from decade to decade is assumed to remain constant. Geometric mean increase is used to find out the future increment in population. Since this method gives higher values and hence should be applied for a new
industrial town at the beginning of development for only few decades.

Incremental Increase Method

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.

Decreasing rate of Growth

In this method it is assumed that rate of percentage increase decreases and the average decrease in the rate of growth is calculated. Then the percentage increase is modified by deducting the decrease in rate of growth. This method is applicable only in such cases
where the rate of growth of population shows a downward trend.

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 (Density)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.

Projected population is calculated by;

P=P_s/{1+m{log_e}^{-1}(n.t)}

 Where,

P = Population at any time t

Ps = Saturation Population

t = time in years

For only three pairs of characteristic values P0, P1, P2 at times t = t0 = 0, t1and t2 = 2t1 extending over the past record

P_s={2P_0P_1P_2-{P_1}^2(P_0+P_2)}/{P_0P_2-{P_1}^2}

m={P_s-P_0}/P_0

n=2.3/t_1log_10({P_0(P_s-P_1)}/{P_1(P_s-P_0)})

Component Method

Main reasons of the population change are birth, death and migration. Where information regarding births and deaths is available, the natural increase can be easily estimated. When calculating, net migration should be calculated first, otherwise it will not affect the number of births and deaths recorded.
It is a useful method when migration is not the main factor in population change.