Sewage treatment (or domestic wastewater treatment, municipal wastewater treatment) is a type of wastewater treatment
which aims to remove contaminants from sewage to produce an effluent that is suitable for discharge to the surrounding
environment or an intended reuse application, thereby preventing water pollution from raw sewage discharges.
Sewage contains wastewater from households and businesses and possibly pre-treated industrial wastewater.
There are a high number of sewage treatment processes to choose from. These can range from decentralized systems
(including on-site treatment systems) to large centralized systems involving a network of pipes and pump stations
(called sewerage) which convey the sewage to a treatment plant. For cities that have a combined sewer, the sewers
will also carry urban runoff (stormwater) to the sewage treatment plant. Sewage treatment often involves two main stages,
called primary and secondary treatment, while advanced treatment also incorporates a tertiary treatment stage with
polishing processes and nutrient removal. Secondary treatment can reduce organic matter
(measured as biological oxygen demand) from sewage,? using aerobic or anaerobic biological processes.