Septic tank provide a sanitary disposal of sewage from residential building, hotel, institution, camps etc. they are suitable for isolated area where sewer are not laid for carrying sewage.

Principle

Septic tank is combine sedimentation cum digestion tank where sewage is held for some period when suspended solid settle down at bottom.

The settle solid are anaerobically digested resulting in reduction of volume sludge. And release of gases such as CO2, H2S etc.

Construction

The material to be used for construction should be resistance to corrosion.

It is generally in rectangular shape.

It should be made water tight.

Construction of septic tank should be such that direct currents are not established between the inlet and outlet. This is done by providing pipe tees completely submerged.

Scum board provided near the inlet and outlet end to prevent the escape of scum.

The level of outlet is about 15cm lower than that of inlet level.

Septic tank should be properly ventilated by the provision of air vent pipe.

Manhole is provided at top for the inspection and cleaning of tank.

If necessary the cast iron step may be provided in the tank.

The sludge is accumulated at the bottom of the tank and it removal at intervals by manual labour or by pumping.

For large tanks sludge removal pipe is provided

Sludge is disposal off in sanitary manure after drying.

The tank is required to be filled in with water.

The effluent of tank should be properly disposed off.

Soak pit

A soak pit is a hallow circular pit.

The total depth of soak pit various from 1.2m to 1.8m.

The effluent falls in the pit and it is allowed to be soaked or absorb into the surrounding soil.

The pit may be kept empty or filled with brick bat stones. Detail of empty soak pit in this case line with brick, stone, or concrete as shown in fig below.

Brick lining is supported below the inlet level by at least 75mm thick back course aggregate.