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Septic
Tank - Septic tank is a kind of sewage settling tank. In rural areas, individual houses treat their sewage wastes in septic tanks built at ground levels near the house. Complex organic materials are anaerobically decomposed to simple organic molecules and fermentation gases.
They may be used with or without an initial primary settling tank. They are relatively small closed tanks vented for the escape of fermentation gases.
The tanks are designed to retain the solids of the sewage entering the tank for a longer period to permit adequate decomposition. Thus the unit accomplishes two processes sedimentation and biological degradation of the sludge. As sewage enters this type of tank, sedimentation occurs at the bottom leaving the liquid with fewer suspended solids to be discharged from the tank.
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The sedimented solids are subjected to degradation by anaerobic bacteria. As a result hydrogen sulfide and other organic acids are produced. These products along with the effluent water on the upper portion are discharged into the adjacent ground. When municipal wastes or other industrial wastes are processed the effluent water is subjected to further aerobic treatment.
Sedimented sludge accumulated at the bottom of septic tank is removed and may be dried, ground and spread on soil as a fertilizer. In a typical septic tank about 80% of the incoming suspended solids will be removed and the BOD will be reduced to about 45%. Septic tanks are sufficient for the treatment of house hold wastes.
When used for small scale industries the process will be economical. However this method cannot be relied upon to eliminate pathogenic microorganisms in sewage. The effluent will contain large number of bacteria and discharge from a septic tank may contain even 106 E. coli per 100ml.
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