Index >> Waste Water Microbiology >> Anerobic Sludge Digestors

Anerobic Sludge Digestors- The solids which accumulate from the sedimentation tanks and the primary and secondary sludge are treated further in anaerobic (sludge) digestion tank, since they cannot be degraded effectively through aerobic processes.

In cases, where the suspended solid content is very high, the sewage can directly be treated in an anaerobic digestion tank. The anaerobic biological reactions are carried out by microorganisms. The microbial action on the constituents of sludge is termed sludge digestion.

Description of the Tank :Anaerobic sludge digestion tank is a specially designed tank which is maintained free of oxygen. It may be a completely closed tank or may be provided with a lid at the top, floating over the contents of the tank.

The majority of the conventional digesters are operated in the mesophilic range, between 25 and 40°C. A neutral pH is required for the sludge digestion. The retention time for mesophilic and thermophilic digesters is in the range of 25-35 days and in some cases 12-15 days.

PROCESS DESCRIPTION:Anaerobic biochemical reactions take place in the anaerobic sludge digestion tank. Both anaerobic and facultative bacteria degrade the organic solids by converting them into soluble substances and gaseous products. The gases consist mainly of methane (60 to 70 percent) and carbon dioxide (20 to 30 percent).

In addition hydrogen and nitrogen are also produced during sludge digestion in small amounts. This gas mixture can be used for heating and also for operating power. The supernatant liquid portion, the effluent, can be sent to the aerobic treatment. The digested sludge is disposed off in a proper way.