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Why does ultraviolet disinfection not leave a residual in drinking water?

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Diagram of a water treatment plant showing the process from water inlet through mixing, coagulation, flocculation, settling, filtering, chemical treatment with chlorine, storage, and high-lift pumping to the distribution system. — water purification Water from inlets located in the water supply, such as a lake, is sent to be mixed, coagulated, and flocculated and is then sent to the waterworks for purification by filtering and chemical treatment. After being treated, the water is pumped into water mains for storage or distribution.

Ultraviolet (UV) radiation functions as a physical disinfection process rather than a chemical one. Unlike chlorination, which introduces chemical agents like chlorine or chloramines into the water supply to provide ongoing protection, UV radiation works by exposing water to electromagnetic radiation to destroy pathogens.1234

Because UV light is a form of energy that does not leave behind a chemical substance, it does not provide a residual concentration that can persist throughout a distribution system to protect water quality after the initial treatment. While this lack of residual means UV radiation does not cause taste or odor issues, it also means that the water is not protected against potential recontamination once it leaves the treatment facility. Consequently, while UV is an effective tool for destroying pathogens, it is often used in conjunction with other methods when a lasting residual is required for public health safety.23