Uses of Digested Sludge

Abstract

Whether the primary function of a digester is reduction of pollution or production of energy the process will produce a residue in the digested feedstock. Since, except for the ‘solid’ digestions previously mentioned, the feedstock is very largely water, decrease in volume due to gasification of solids will be small and the digested sludge will be of much the same volume as the feed. Digested sludge from some sewage works is just dumped on land or at sea and so a transport cost is involved in removing it. This is part of the overall cost of sewage purification for the city and is covered by city rates. But for the farmer or factory using a digester for pollution control and production of energy, dumping of digested sludge adds to the process an unwelcome cost. The same argument applies to digesters run primarily for large-scale gas production. If the sludge can replace something bought in to the farm or factory, or if it can be sold off-site, then this provides a monetary return which can decrease the cost of the pollution control or gas production. Finding a use for digested sludge is thus an integral part of the production of energy from wastes and it is also relevant to present-day thinking on recycling and maximum use of resources.