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Sludge is a semi-solid slurry and can be produced as sewage sludge from wastewater treatment processes or as a 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).
Industrial wastewater treatment plants produce solids that are also referred to as sludge, whether generated from biological or physical-chemical processes.
References
Sludge Wikipedia(Text) CC BY-SA