Wastewater treatment plants produce organic sludge as wastewater is treated; this sludge must be further treated before ultimate disposal. Sludges are generated from primary settling tanks, which are used to remove settable, particulate solids, and from secondary clarifiers (settling basins), which are used to remove excess biomass production generated in secondary biological treatment units.

Disposal of sludges from wastewater treatment processes is a costly and difficult problem. The processes used in sludge disposal include: (1) reduction in sludge volume, primarily by removal of water, which constitutes 97–98% of the sludge; (2) reduction of the volatile (organic) content of the sludge, which eliminates nuisance conditions by reducing putrescibility and reduces threats to human health by reducing levels of microorganisms; and (3) ultimate disposal of the residues.

Aerobic sludge digestion is one process that may be used to reduce both the organic content and the volume of the sludge. Under...