Environmental Engg II

Estimation of sewage quantities

Population characteristics Three characteristics of a population are geographic distribution, density, and growth rate. The density refers to the number of people per unit area. The growth rate is the ability of a population to increase. Statistical data shows that population of a city changes over time and follows some trends. There are numerous reasons of change of a population, however some main causes include i. New Births ii. Migration of people due to favourable socio economic conditions iii. City expansions

Design Periods & Population Forecast This quantity should be worked out with due provision for the estimated requirements of the future . The future period for which a provision is made in the water supply scheme is known as the design period. It is suggested that the construction of sewage treatment plant may be carried out in phases with an initial design period ranging from 5 to 10 years excluding the construction period. Design period is estimated based on the following: i. Useful life of the component, considering obsolescence, wear, tear, etc. ii. Expandability aspect. iii. Anticipated rate of growth of population, including industrial, commercial developments & migration- immigration. .

vi. Performance of the system during initial period iv. Anticipated trend change in wastewater generation v. Available resources. vi. Performance of the system during initial period

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: i. increase due to births ii. decrease due to deaths iii. increase/ decrease due to migration iv. 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.

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.

Arithmetic Increase Method This method is based on the assumption that the population increases at a constant rate; i.e. dP/dt=constant=k; Pt = P0+kt. t = time in years after base year P0 = Population at base year Pt = Population after t years k = rate of change of population with respect to time is constant = dP/dt

Example 1 Years Population (persons) 1981 200 1982 220 1983 241 1981 200 1982 220 1983 241 1984 266 With above data what will be population in 1990.

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.

. Geometric Increase Method This method is based on the assumption that percentage growth rate is constant i.e Pt = Po (1+i/100) t i= geometric mean (%)

Example 2 Recalculate the example 1 with geomertic increase method

This method must be used with caution, for 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.

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

Incremental Increase Method Population growth rate is given by Pt = P0 + (kavg + ki)t

Example 3 Recalculate example 1 with Incremental Increase method