A sewage treatment plant is an industrial facility for the treatment of urban wastewater, largely from toilets and bathrooms, to produce a non-polluting effluent that can be used as part of another process or directly discharged into the environment (for example, rivers). They may also be known as "wastewater treatment works" or "wastewater treatment facilities", treated water as well as sludge are recycled. A WWTP is designed in accordance with the waste composition of the metropolitan area served.

In a sewage treatment plant, they treat the waste before it's discharged into the ocean.
The sewage treatment plant is divided into primary and secondary treatment stages. The primary stage takes place in the screening and grit removal system, where large solids are removed by grit chambers. The secondary stage takes place in several types of settlement tanks, where the activated sludge process is used to remove the organic matter from the wastewater. Treated effluent can then be discharged directly to surface water or further treated for reuse (biogas facilities for gas production or for irrigation as fertiliser, aquaculture facilities).

The sewage treatment plant is a handling plant for separating and reducing the polluting load of untreated municipal sewage brought into it by sewers. The main treatment process consists of sedimentation, where suspended matter falls to the bottom, followed by clarification and filtration. After treatment, the cleaned water, called effluent or secondary waste water, is discharged, either into streams via surface drains or into artificial bodies of water like lakes or reservoirs.