Sewage (or domestic wastewater) treatment is the process of removing contaminants from sewage. It includes physical, chemical and biological processes to remove physical, chemical and biological contaminants. Its objective is to produce a wastestream (or treated effluent) and a solid waste or sludge also suitable for discharge or reuse back into the environment. This material is often inadvertently contaminated with toxic organic and inorganic compounds.

Sewage is created by residences, institutions, and commercial and industrial establishments. It can be treated close to where it is created (in septic tanks or onsite package plants), or collected and transported via a network of pipes and pump stations to a municipal treatment plant (see Sewerage and pipes and infrastructure). Sewage collection and treatment is typically subject to local, state and federal regulations and standards (regulation and controls). Industrial sources of wastewater often require specialized treatment processes (see Industrial wastewater treatment).

Typically, sewage treatment involves three stages, called primary, secondary and tertiary treatment. First, the solids are separated from the wastewater stream. Then dissolved biological matter is progressively converted into a solid mass by using indigenous, water-borne bacteria. Finally, the biological solids are neutralized then disposed of or re-used, and the treated water may be disinfected chemically or physically (for example by lagooning and micro-filtration).

The final effluent can be discharged into a natural surface water body (stream, river or bay), other environment (wetland, golf course, greenway, etc.) or reused for a number of different purposes.

According to European Norm smaller sewage is quantified as sewage from not more than 2,000 habitants if the final effluent is discharged into inland water bodies

or

sewage of not more than 10,000 habitants if the final effluent is discharged into costal waters.

How much waste water can be expected per habitant?
According to German norms 1 habitant produces between 150 - 200 liter per day.

How is waste water being defined?
It is assumend that every person produces a certain amount of organic waste within the household waste water. This is calculated in BOD (Biological Oxygen Deman) and assumed to be 60-65g (BOD5) per person and day in Germany. That figure may vary in other countries. More industrialised countries have usually higher BOD's in their waste water whereas less industrialised countries have lower BOD's.