Chlorination is the process of adding the element chlorine to water as a method of water purification to reduce and prevent waterborne disease as it is effective against almost all bacteria and viruses, as well as amoeba and make it fit for human consumption as drinking water, especially when combined with filtration.

The advantage of chlorine in comparison to ozone is that the residual persists in the water for an extended period of time. This feature allows the chlorine to travel through the water supply system, effectively controlling pathogenic backflow contamination. In a large system this may not be adequate, and so chlorine levels may be boosted at points in the distribution system.

Chlorination is also used to sanitize the water in swimming pools and as a disinfection stage in sewage treatment.

Sigma provides an on-line automated system that continuously measures residual chlorine and pH without reagents.