The water treatment is a purposeful change in water quality. In addition to physical methods (aeration, filtration, UV-light, ..) and biological processes (sludge digestion, anaerobic wastwater treatment, ...) chemical treatment methods (oxidation, disinfection, ...) are used. Especially the use of ozone O3 and chlorine Cl2should be mentioned, but also chlorine dioxide ClO2.
In the chemical treatment process it is important to note that not too much be added, because a too abundant excess of chemical material is harmful and unwanted parallel reactions are caused. But a too small excess (residual concentration) is also not desirable, because the desired treatment success can not always be guaranteed.
Therefore it is important to measure the residual concentration of the chemicals in the treated water according to the guidelines of the water regulations.