Wastewater treatment is a process used to convert wastewater into an effluent (outflowing of water) that can be returned to the environment with minimal impact or directly reused. The latter is called water reclamation because treated wastewater can then be used for other purposes. The treatment process generally takes place in a wastewater treatment plant (WWTP, also called a sewage treatment plant STP). Pollutants and pathogens in municipal wastewater (households and small industries) are removed, reduced and broken down with the goal of reducing pollutants and pathogens to levels that will not produce adverse effects to humans or the environment.

Wastewater treatment plants that discharge to waters bodies are regulated under the Clean Water Act and require an operating permit issued by the EPA delegated authority usually the state environmental regulatory authority. In the state of Oregon, this is the DEQ. This permit is called an NPDES (national pollutant discharge elimination system) permit. Treatment plants not directly discharging into a water of the United States are regulated under a different permitting paradigm. In Oregon, these permits are called WPCF (Water pollution control facility) permits.