Poor water treatment lets water interact with the surfaces of pipes and vessels which contain it. Water treatment is used to optimize most water-based industrial processes, such as heating, cooling, processing, cleaning, and rinsing so that operating costs and risks are reduced.

The term 'wastewater treatment' is often used to mean 'sewage treatment'. However, before it is treated, the options for disposal or reuse must be considered so the correct treatment process is used on the wastewater. The main purpose of wastewater treatment is for the treated wastewater to be able to be disposed or reused safely. Treated wastewater can be reused as reclaimed water. : Ch.14 Biogas can be another by-product if anaerobic treatment processes are used. The main by-product from wastewater treatment plants is a type of sludge that is usually treated in the same or another wastewater treatment plant. Processes commonly used in wastewater treatment include phase separation (such as sedimentation), biological and chemical processes (such as oxidation) or polishing. Further types of wastewater treatment plants include Agricultural wastewater treatment and leachate treatment plants. For industrial wastewater, treatment either takes place in a separate Industrial wastewater treatment, or in a sewage treatment plant (usually after some form of pre-treatment). For domestic wastewater (also called municipal wastewater or sewage), the treatment plant is called a Sewage Treatment. There are several kinds of wastewater which are treated at the appropriate type of wastewater treatment plant. The treatment process takes place in a wastewater treatment plant.

Once returned to the water cycle, the effluent creates an acceptable impact on the environment or is reused for various purposes (called water reclamation). Wastewater treatment is a process which removes and eliminates contaminants from wastewater and converts this into an effluent that can be returned to the water cycle.

Sewage treatment plant (a type of wastewater treatment plant) in Cuxhaven, Germany. Many proprietary systems also claim to remove residual disinfectants and heavy metal ions. Such treatments can include water softening or ion exchange. Water supplied to domestic properties such as for tap water or other uses, may be further treated before use, often using an in-line treatment process. It is therefore common practice to keep residual disinfectants in the treated water to kill bacteriological contamination during distribution and to keep the pipes clean. Measures taken to ensure water quality not only relate to the treatment of the water, but to its conveyance and distribution after treatment. These substances continue to cause great harm to several less developed countries who do not have access to effective water purification systems. Research including Professor Linda Lawton's group at Robert Gordon University, Aberdeen is working to improve detection of cyanobacteria. The removal or destruction of microbial pathogens is essential, and commonly involves the use of reactive chemical agents such as suspended solids, to remove bacteria, algae, viruses, fungi, and minerals including iron and manganese.

Faeces can be a source of pathogenic bacteria, viruses, protozoa and helminths. In general terms, the greatest microbial risks are associated with ingestion of water that is contaminated with human or animal (including bird) faeces. Treatment for drinking water production involves the removal of contaminants and/or inactivation of any potentially harmful microbes from raw water to produce water that is pure enough for human consumption without any short term or long term risk of any adverse health effect. Typical drinking water treatment processes

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