The accumulated, semiliquid material consisting of suspended, colloidal, and dissolved organic and inorganic matter separated from wastewater during treatment. Sludges are developed as contained pollutants, and contaminants are separated by mechanical, hydraulic, biological, or chemical processes. The various classes of solids that are removed and collected must be disposed of in a safe, nuisance-free manner without hazard to health or the environment. Collection, handling, transporting, and disposal of removed solids are difficult and costly, since they are offensive and putrescible, with 92–99.5% water content. Sewage solids must be treated by thickening, chemical conditioning, mechanical dewatering, thermal action, biological stabilization, or digestion to convert putrescible organic matter to relatively inert end products, remove water, and reduce weight and volume.