A Pilot Plant Study of the Anaerobic Digestion of a Mixture of Municipal Sewage and Industrial Sludges

With up to 100 tonnes/day of municipal sewage sludge, the municipality of Cork County Council had a problem of disposal. Spreading on agricultural land was the most common method of disposal until the 1986 EC Directive 86/278/EEC imposed severe restrictions on such a method. For industry in the county, the method of disposal was sometimes landfilling, but more often disposal at sea. The Helsinki Agreement terminates sea disposal. An evaluation of treatment and disposal means identified anaerobic digestion as an alternative to land and sea disposal. After anaerobic digestion, the sludge is sufficiently sti ilised to spread on land. A pilot project using a 20m³ reactor was studied for one year, using a fixture of municipal sewage sludge, yeast from a citric acid industry and abattoir waste (including blood, paunch contents and sheep's offal). The results were impressive; input sludge of 10% dry matter was reduced to 2%; the COD reduction approached 90%; in the mesophylic temperature range, the sludge was stabilised after an HRT of 29 days. The sludge after anaerobic digestion satisfies the EC limits for landspreading.