Aquatic Pollution and Coastal Management

Unit 2 : Sewage and domestic wastes, treatment and reuse

2.5.4.2.Sludge Digestion
The sludge, which accumulates during primary and secondary sedimentation is very offensive and is piped for anaerobic digestion in the specially designed sludge digesters. In the sludge digesters, the sludge is microbially degraded in anaerobic conditions forming soluble substances and gaseous products. The gaseous products have a very high proportion of methane (60-70%) followed by CO₂ (20-30%) hydrogen and nitrogen. The gaseous mixture is highly combustible and is called as “biogas” which can serve as a source of energy and can be converted into power to run the entire treatment plant.

The process of sludge digestion is carried out by several bacteria anaerobes, facultative anaerobes and methanogens. Optimum temperature and pH for anaerobic digestion are 50-60⁰C and 6.8-7.2 respectively. Complete digestion requires a time of about 2-3 weeks. The sludge still remaining after digestion may be disposed off either in water or on land. The liquid effluent may be given a tertiary treatment before its final disposal.