Former wastewater (sewage) is treated to remove solids and impurities and used in sustainable landscaping irrigation, to recharge groundwater aquifers, to meet commercial and industrial water needs, etc. Reused water is water used more than once or recycled.

Waste water cleaning includes physical, chemical, and biological processes to remove contaminants and produce environmentally safe treated wastewater (or treated effluent). Sewage treatment may also be referred to as wastewater treatment.

Industrial effluent usually receives at least pretreatment if not full treatment at the factories themselves to reduce the pollutant load, before discharge to the sewer. This process is called industrial wastewater treatment. Industrial wastewater may contain pollutants which cannot be removed by conventional sewage treatment.

The sludge accumulated in a wastewater treatment process must be treated and disposed of in a safe and effective manner. Sludge treatment depends on the amount of solids generated and other site-specific conditions. The sludge is passed through pre-thickener/centrifuges which de-waters the sludge.