Waste water treatment is the job of rehabilitating the basic living water, which is getting polluted day by day, after it is used for domestic or industrial purposes.

This purification process can be done by physical, chemical or biological means:

• Chemical treatment consists of adding various chemicals in fast and slow mixing units of the waste water, and the precipitation of these chemicals by reacting with the pollutants in the waste water.
• Biological treatment is achieved by the biological decomposition of domestic or industrial wastewater with the help of bacteria with or without oxygen.
• Physical treatment is the physical removal of oil and coarse wastes in the wastewater by means of mechanical processes without using any chemicals or bacteria, with grates, oil scraper pallets and similar devices.

The main wastewater treatment processes are; screen (fine or coarse), pre-sedimentation basin, chemical or biological treatment units, final sedimentation basin and disinfection unit.

The selection of wastewater treatment processes or combinations of processes depends on the following criteria:

Wastewater character: This should include what form the pollutant is in, such as suspended, colloidal or dissolved, biodegradable, and its toxicity.

Leaving water quality: Any future discharge restrictions such as effluent toxicity (bioassay) test results should also be included in the planning.