Environmental Engineering: Design of a simple sewerage system

DESIGN OF A SEWERAGE SYSTEM (Theory)

Sewers are always designed for partial flow conditions. This is done to ensure open channel flow condition and provision of open space above surface of sewage for ventilation
The sewers should be designed such that the depth of flow (d) should be at-least half the diameter (D) but not more than 0.8D during peak flow conditions
The minimum velocity at which no solids get deposited in the sewer is known as "self cleansing velocity"
This velocity should be maintained at-least once a day during peak flow at all sections of the sewerage system
Generally, the self-cleansing velocity is a little more than 0.75 m/s
The velocity in the sewer should not be too high as suspended solids in the sewage might erode the inner surface of the pipe
The maximum permissible velocity to prevent erosion of the inner material of the pipe is called non-scouring velocity and is limited to 3.0 m/s
The slope of the sewer should be such that the velocity of flow is maintained between self-cleansing velocity and scour velocity
The sewer sizes and slopes should be such that velocity of flow increases steadily from head section to the entire length of sewer.
The minimum size of public sewers should not be less than 150mm
For major towns and cities where topography is flat the minimum size of sewer should be 200 mm
The following equations are used in design of a sewerage system:

Environmental Engineering