Hello V.V. Manoharan

Where there is no public sewerage system or where the houses are located in isolated places, there is a problem for disposal of waste. To solve this problem, septic tank or a soak pit is constructed for disposal of waste.  
Septic tank is a combined sedimentation cum digestion tank. Raw sewage of house flows down into septic tank. The solid settles down at bottom of the tank. Anaerobic bacteria convert the sewage into liquid and gases. By this process the volume of sewage reduces in semi solid known as sludge. The sludge settles down and lighter matters like grease and fats float on surface at the top. It is necessary that septic tank should be covered with water tight roof slab for its proper functioning.
Septic tanks are useful for limited population, not for more than 200 persons’ residences. It works satisfactorily where water supply is available. Detergents and disinfectants are not suited for septic tanks as they affect adversely on bacteria.      
The septic tank should have minimum 75 cm width and minimum depth one meter below water level. Length of the tank should be 2-4 times the width. The level of outlet pipe should be 7 CM below inlet pipe. Septic tanks generally constructed with brick masonry should be water tight. All inner surfaces of its wall, floor and slabs are plastered with rich mortar for its proper working to remove the sludge from the tank either half yearly or yearly.

Thanks.