HYDRAULIC DESIGN OF SEWER
General approach for the design of sewer is the same as that of water mains. But the following are the two main differences between the basic principles of design of sewers and those of water mains.
Presence of particles :
The water carried by water mains is practically free from particles of any solid matter – organic and inorganic. Sewage, on the other hand, contains such particles in suspension and the heavy particles settle down at the bottom of sewers which may ultimately result in the clogging of sewers. The sewers are, therefore, to be laid down at gradient and they should be capable of resisting the wear and tear due to abrasion of these particles.
Pressure :
The water mains normally carry water under pressure and hence, within certain limits, they may be carried up and down the hill.
The sewers, on the other hand, are treated as open channels and they must, therefore, be laid at continuous gradient in downward direction. If sewage has to be carried under pressure, it will require elaborate equipment at each house which is to be connected to the drainage system.
Minimum and Maximum velocities :
The silting or deposition of particles of solid matter is undesirable in sewers and hence, the sewers should be laid at such a gradient that a minimum velocity which will prevent the silting of particles in sewers is developed over a wide variation in discharge of sewage.
Such a minimum velocity is known as self-cleansing velocity and for keeping the sewers free from any trouble, this velocity should be developed at least once in a day, preferably twice a day.
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