Design Example

Septic Tank

Septic tanks are horizontal continuous flow, small sedimentation tanks through which sewage is allowed to flow slowly to enable the sewage solids to settle to the bottom of the tank, where they are digested anaerobically. The tank is de-sludged at regular intervals usually once every 1-5 years.


It is a pit excavated in soil with water tight lining and loose lining by stone or brick to provide for leaching of wastewater by sides and the pit is covered. The leaching type is suitable for porous soils. The capacity should not be less than one day's flow into the pit. If all the water in a test pit of one meter diameter and 2 m deep, disappears in 24 hours, such soil is best suitable for cesspools. The bottom of the cesspool must be well above the ground water level. After sometime the sides of pit get clogged by the sewage solids, reducing the leaching capacity. At overflow level, an outlet is provided to take-off unleached liquid into a seepage pit. The settled matter is removed at intervals. Water tight cesspools are cleaned every 6 months and their capacity must not be less than 70 l/person/month.

Seepage Pit

The seepage pit is needed to discharge the effluent of cesspool, aquaprivy, septic tank or sullage from bathrooms and kitchens. The difference between seepage pit and cesspool is that the seepage pit is completely filled up with stones. The fine suspended solids adhere to the surface of stones and get decomposed by the zoogleal film, which are on the stones and the effluent is leached into the side walls.