Wastewater treatment, also called sewage treatment, is a process used to remove contaminants from wastewater and convert it into an effluent that is suitable for discharge to the natural bodies of water such as rivers, lakes, estuaries, and oceans.

When the wastewater is treated and the purified water is returned to the environment the main by-product of the process is a type of sludge, that can either be used as feedstock to the biogas production to produce methane through anaerobic digestion or to produce heat through a waste incineration plant.

For industrial wastewater treatment, there are either processes within the industry itself or takes place in a separate industrial wastewater treatment plant, before the quality of the water is good enough to be discharged into the environment or in the municipal sewage system.