A Sewage Treatment Plant (STP) is the process of eliminating contaminants from municipal wastewater, primarily consisting of household sewage and some industrial wastewater. Physical, chemical, and biological methods are employed to remove contaminants and generate treated wastewater, also known as treated effluent, which meets safety standards for discharge into the environment. A byproduct of sewage treatment is a semi-solid waste or slurry known as sewage sludge. This sludge undergoes further treatment before it is suitable for disposal or application to land.

While sewage treatment is often referred to as wastewater treatment, the latter is a broader term that can also encompass industrial wastewater. In most urban areas, the sewer system transports a portion of industrial effluent to the sewage treatment plant, which has usually undergone pre-treatment at the factories to reduce the pollution load.

In the case of a combined sewer system, it transports urban runoff (stormwater) to the sewage treatment plant. Sewage water reaches treatment plants through piping and gravitational flow, assisted by pumps. The initial stage of sewage filtration typically involves a bar screen to filter solids and large objects, which are then collected in dumpsters and disposed of in landfills. Additionally, the removal of fat and oil occurs before the primary treatment of sewage.