High Quality Content by WIKIPEDIA articles! Sludge refers to the residual, semi-solid material left from industrial wastewater, or sewage treatment processes. It can also refer to the settled suspension obtained from conventional drinking water treatment, and numerous other industrial processes. The term is also sometimes used as a generic term for solids separated from suspension in a liquid; this `soupy` material usually contains significant quantities of `interstitial` water (between the solid particles).