The rate of accumulation of sludge in septic tank is recommended as

Septic tank:

It works on the principle of anaerobic digestion. It is a primary sedimentation tank having a longer detention period of 12 to 36 hours, in which raw sewage is directly fed into it.

Construction Detail: It usually consists of a brick wall in cement not less than 20 cm thick and the foundation floor is of cement concrete having proportion in 1 : 3 : 6 or 1 : 2 : 4. Both inside and outside faces of wall and floor are plastered with a minimum thickness of 12 mm thick cement mortar 1 : 3 and all inside corners are rounded.

The following should be remembered about the septic tank.

1. The minimum capacity of the septic tank is 8 to 10 persons or 2250 liters and the maximum capacity of the septic tank is usually 300 persons.
2. BOD removal efficient – 25-65 %.
3.  Suspended solids removal efficiency – 60-70%.
4.  Rate of accumulation of sludge – 30 litres/person/year.
5.  Sewage flow without sullage- 40-70 lcd.
6. Sewage flow with sullage: 90-150 lcd
7. Cleaning period or removal of sludge: 6 to 12 months, mostly once in a year i.e. removal of sludge is less frequent.