POPULATION
FORECAST
General Considerations
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 addministration
spheres. Special factors causing sudden
immigration or influx of population should
also be foreseen to the extent possible.
A judgment
based on these factors would help in selection
the most suitable method of deriving the
probable trend of the population growth
in the area or areas of the project from
out of the following mathematical methods.
graphically interpreted where necessary.
Demographic
Method of population projection
Population
change can occur only in three ways. (I)
by births (population gain) (ii) by deaths
(population loss) or (iii) migration (population
loss or gain depending on whether movement
out or movement is occurs in excess). Annexation
of an area any be considered as special
form of migration. Population forecasts
are frequently obtained by preparing and
summing up of separate but related projections
of natural increases and of net migration
and is expressed as below:
The net effect
of births and deaths on population is termed
natural increase ( natural decrease if deaths
exceed births ).
Migration also
affects the number of births and deaths
in an area and so, projections of net migration
are prepared before projections for natural
increase.
TABLE 1.1
DESIGN PERIODS
FOR COMPONENTS OF SEWERAGE SYSTEM AND SEWAGE
TREATMENT
|
S.No
|
Component
|
Recommended Design Period in Years
|
Clarification
|
| 1. |
Collection
System i.e. Sewer Network |
30 |
The
system should be designed for the prospective
population of 30 years, as its replacement
is not possible during its use. |
| 2. |
Pumping
Stations (Civil Works) |
30 |
Duplicating
machinery with in the pumping station
would be easier/cost of civil works
will be economical for full design period. |
| 3. |
Pumping
Machinery |
15 |
Life
of pumping machinery is generally 15
years. |
| 4. |
Sewage
Treatment Plant |
30 |
The
construction may be in a phased manner
as initially the flows may not reach
the designed levels, and it will be
uneconomical to build the full capacity
plant initially. (Refer chapter 10.2). |
| 5. |
Effluent
disposal and utilisation |
30 |
Provision
of design capacities in the initial
stages itself is economical. |
The method
thus takes into account the prevailing and
anticipated birth rates and death rates
of the region or city for the period under
consideration. An estimate is also made
of the immigration from and immigration
to the community, its growth area wise and
the net increase of population is calculated
accordingly considering all these factors
by arithmetical balancing.
