Design Of Sewer System

We now know that sanitary sewers are designed to carry domestic wastes originating from the sanitary conveniences of dwellings, business buildings, factories or institutions including industrial wastes produced in the area that these serve. Whereas storm sewers carry surface runoff developed during or following the period of rainfall over concerned area including street wash. In sanitary sewer system, lateral sewer collects discharges from houses and carries them to another branch sewer, and has no tributary sewer lines. Branches or sub-main lines receive waste-water from laterals and convey it to large mains. A main sewer, also called trunk or outfall sewer, carries the discharge from large areas to the treatment plant. Manholes are provided at intersection of sewer lines and also at regular intervals to facilitate regular inspection and cleaning. Surface waters enter a storm drainage system through inlets located in street gutters or depressed areas that collect natural drainage. Catch basins under street inlets are connected to the main storm sewer located in the street right-of-way, often along the centre line, by short pipelines. Since no house connections are required, the storm sewers may not depend upon the individual lots, and this may permit them to be run by shorter routes than that of sanitary sewers. Pipelines gradients follow the general slope of the ground surface such that water entering can flow downhill to a convenient point for discharge. Storm sewer pipes are set shallower as compared to sanitary sewers as far as possible.

A major difference in design philosophy between sanitary and storm sewers are that the latter are assumed to surcharge and overflow periodically. For example, a storm drain sized on the basis of a ten-year rainfall frequency presumes that one storm every ten years will exceed the capacity of the sewer. However, sanitary sewers are designed and constructed to prevent surcharging. Where backup of sanitary sewers does occur, it is more frequently attributable to excess infiltration of ground water through open pipe joints and unauthorized drain connections. A second easily recognizable difference between sanitary and storm sewers is the pipe sizes that are needed to serve a given area. Storm drains are larger than the pipes collecting domestic wastewater. Consequently, only a small amount of infiltrating rain water results in overloading domestic sewers.