Sludge Digestion Tanks
A sludge digestion tank is an R.C.C. tank of cylindrical shape with a hopper bottom and is covered with a fixed or floating type of roof. The latter makes the digestion of sludge much more effective. The weight of the cover is supported by sludge, and the liquid forced between the tank wall and the side of the cover provides a good seal. The raw sludge is pumped into the tank where it is seeded with digested sludge.
On undergoing anaerobic digestion, gases of decomposition (chiefly Ch4 , CO2) are given out by the anaerobic digester. Sludge gas rises out of the digesting sludge. It moves along the ceiling of the cover and collects in the gas dome. The cover can float on the surface of the sludge between the landing brackets and the overflow pipe. Rollers around the circumference of the cover keep it from binding against the tank wall.
The digested sludge, which settles down to the bottom of the tank is removed under hydrostatic pressure periodically, say, once a week. To maintain optimum temperature, the tank is generally provided with heating coils through which hot water is circulated.
The supernatant liquor i.e., the part of the tank content lying between the scum and the sludge is withdrawn at the optimum level through a number of withdrawal points located at different elevations of the tank. As it is high in BOD and suspended matter contents, it is sent back to the incoming raw sewage for undergoing re-treatment. During the digestion of sludge, the scum formed at the surface gets broken up by the recirculating flow or through mechanical rackers called scum-breakers.
The amount of sludge gas produced during digestion of sludge varies from 0.014 to 0.028 m3 per capita with 0.017 m3 being quite common. The gas produced contains 65 per cent of methane with a calorific value 5400 -- 5850 kcal. m3, 30 per cent of carbon dioxide and balance 5 per cent of nitrogen and other inert gases. It resembles natural gas and may be used as a fuel for cooking. Principal uses however, are for driving gas engines, and for heating sludge to promote quick digestion of sludge.
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