A suspension of solids in liquid, usually in the form of a liquid or a slurry. It is the residue that results from wastewater treatment operations, and typical concentrations range from 0.25 to 12% solids by weight. An estimated 8.5 million dry tons of municipal sludge is produced in the United States each year.

The volume of sludge produced is small compared with the volume of wastewater treated. The cost of sludge treatment, however, is estimated to be from 25 to 40% of the total cost of operating a wastewater-treatment plant. In the design of sludge treatment and disposal facilities, the term sludge refers to primary, biological, and chemical sludges, and excludes grit and screenings. Primary sludge results from primary sedimentation, while the sources of biological and chemical sludges are secondary biological and chemical settling and the processes used for thickening, digesting, conditioning, and dewatering the sludge from the primary and secondary settling...