ABSTRACT
In this case the process, anaerobic digestion, has the primary aim of converting a highly polluting sludge into a stabilised, virtually odour-free, sludge which can be safely land-spread as fertilizer or dumped on land or at sea. The sludge can be dried leaving an innocuous, ‘gritty’ material which can be bagged for use as powder fertilizer. The biogas production is a secondary aspect of the process. The gas may be used by direct burning in boilers only to keep the digesters heated to mesophilic temperatures, but in the bigger installations it is used for powering gas engines or turbines; the waste heat from these is sufficient to heat the digesters.
