DESIGN DISCHARGE OF SEWER
Sewers need to be designed before commencing the actual laying work. Designing involves estimation of period or duration for the which the sewer will serve for an expected population and the discharge for which the sewer is to be designed.
Design Period
The length of time up to which the capacity of a sewer will be adequate is referred to as the design period. In fixing a period of design, consideration must be given for the useful life of structures and equipment employed, taking into account obsolescence as well as wear and tear. Because the flow is largely a function of population served, population density and water consumption, lateral and sub main sewers are usually designed for peak flows of the population at saturation density as set forth in the Master Plan.
Population Forecasting
There are several methods for estimation or forecasting of population which can predict or forecast population for a specific design period, usually three to four decades.
Tributary area
The natural topography, layout of buildings, political boundaries, economic factors, etc., determine the tributary area. For larger drainage areas, though it is desirable that the sewer capacities be designed for the total tributary area, sometimes, political boundaries and legal restrictions prevent the sewers to be constructed beyond the limits of the local authority.
Per capita sewage flow
The entire spent water of a community should normally contribute to the total flow in a sanitary sewer. However, the observed dry weather flow quantities usually are slightly less than the per capita water consumption, since some water is lost in evaporation, seepage into ground, leakage, etc. In arid regions, mean sewage flows may be as little as 40% of water consumption and in well developed areas, flows may be as high as 90%.
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