Septic Tanks

Septic tank: Provided in areas where sewer have not been laid and for isolated communities, schools,hospitals, other public institution etc.

• It is a sedimentation tank with longer detention time  i.e. 12 to 36 hours
• Both sedimentation and sludge digestion takes place in this tank.
• Effluents should be disposed off either for sub surface irrigation or in cess pools or soak pits or treated inlet in trickling filter before disposed off in water course
• Sludge collected at the bottom gets digested anaerobically. The digested sludge is periodically removed.

Design Criteria:

• Capacity of tank = sewage stored in detention time + volume of sludge stored during period of cleaning
• Sludge: 30 lit/person/year
• Period of clearing 6 months to 3 years (generally 1 year)
• Detention time: 12 to 36 hours (generally 24 hours)

Disposal of effluents from septic tank: The effluent of septic tank will have BOD of 100 to 200 mg/I and hence it can not disposed of into water courses.

Method of Septic tank effluent disposal:

• Absorption trenches 
• Soak pit 
• Leaching cess Pool

Sub Surface irrigation  Using Absorption trenches: The suspended organic matter present in the effluent will be absorbed in the absorption trench   filled with gravel

Soak pit: The effluent is allowed to be soaked or absorbed into the surrounding soil. Pit is filled with xave1 brick bats etc

Cess pool: The top portion acts a absorption trench and bottom portion acts as a Septic

tank used when subsoil is porous and when there is no well nearby.

Imhoff tank:

• Incoming sludge is not allowed to get mixed up with sludge Suitable for small treatment plants where separate sedimentation tank and sludge digestion tank can not be constructed
• Depth of tank is more, costlier construction
• Obsolete these days
• Its is a anaerobic unit
• The upper chamber is used for sedimentation of solid-s and the lower chamber is meant for digestion and storage.