Working Of Septic Tank
In the grit chamber, sand, grit, settleable inorganic matter, etc. settle before sewage enters the anaerobic chamber. In the anaerobic chamber, organic solids settle at the bottom of the tank where anaerobic bacteria act on it and convert complex unstable compounds into simple stable compounds. Nitrogen is converted into ammonia and the mixture becomes alkaline. The colloidal matter is flocculated and then liquefied and finally digested. Discharged organic matters are also digested. Due to these processes, there is a considerable reduction in the volume of sludge and it settles at the bottom of the tank. The clarified sewage discharges through the outlet. Cases are liberated due to sludge digestion which rises to the surface constantly in the form of bubbles. These bubbles carry small particles of decomposed sludge and a floating layer of scum (black in color) is formed on the surface. The scum contains anaerobic bacteria that attacks the undissolved components of the incoming sewage sludge particles freed from gas bubbles at the surface lose their buoyancy and settle back at the bottom of the tank. Sludge in a septic tank maybe 25% to 30% less in weight and 75% to 80% less in volume than the sludge from a sedimentation tank.
Sludge removed is disposed of in a safe manner after drying before the tank is put to use. Effluent should be properly disposed of. A septic tank may be constructed in series to act like two stage sludge digestion tanks. But single stage septic tanks are very common.
The accumulation of sludge at the bottom of the tank decreases its storage capacity and hence septic tanks should be cleaned every 6 to 12 months. In rare situations, it should be cleaned at least once in two years.