Septic tanks - Working principles and Design parameters

SEPTIC TANKS

Working principle of septic tanks

• Septic tank may be defined as a primary sedimentation tank with a longer detention period of almost 12 to 36 hours along with extra provisions for digestion of settled sludge. 
• In septic tanks, the digestion process is anaerobic in nature. 
• Anaerobic decomposition of waste organic matter results in formation of foul smelling gases which escape through a vent shaft .
• A septic tank is a horizontal continuous flow type sedimentation tank that directly admits raw sewage and removes 60 - 70% of suspended matter.
• The effluent from septic tanks is foul in nature and should be disposed off by sub-surface irrigation or cess-pools or soak-pits or be treated in trickling filters before final disposal in water courses.
• The sludge settled at the bottom of the tank and the oil and grease that rises to the top surface as scum remain in the tank for several months during which they are decomposed by anaerobic bacteria. 
• The quantity of sludge formed in septic tanks will be much less than that obtained from ordinary settling tanks since it is digested by bacteria.
• The digested sludge is removed periodically (not exceeding three years).
• The sludge is disposed hygienically after drying it.

Septic tanks are generally provided in areas where sewers have not been laid to cater to the sanitary disposal of sewage produced from isolated communities, schools, hospitals, hotels and other public institutions.

Design parameters of septic tanks

• Capacity of septic tanks
• Free-board
• Inlet and Outlet baffles
• Detention period
• Length to width ratio