Ultraviolet (UV) treatment and chlorination are two commonly used water disinfection methods. While both methods are effective in killing bacteria and other microorganisms, they can also produce unintended disinfection byproducts (DBPs). Trihalomethanes (THMs) are a group of DBPs that have been linked to several adverse health effects, including cancer and birth defects.

Chlorination is the more traditional method of water disinfection. Chlorine is a strong oxidizing agent that reacts with organic matter in water to produce a variety of DBPs, including THMs. UV treatment is a newer disinfection method that uses ultraviolet radiation to kill bacteria and other microorganisms. UV treatment does not produce DBPs directly, but it can actually increase the production of THMs when used in combination with chlorination.

Here's how it happens: UV treatment can damage organic matter in water, making it more susceptible to reaction with chlorine. This reaction produces higher levels of THMs. The increase in THM production is most pronounced when UV treatment is used at high doses.

The potential for increased THM production is a concern when using UV treatment in combination with chlorination. It's important to carefully monitor the levels of THMs and other DBPs in water treated with UV and chlorine to ensure that they do not exceed health standards.

One way to reduce the formation of THMs is to use lower doses of UV radiation. Another option is to use alternative disinfectants, such as ozone or chloramines, which produce fewer THMs.

In summary, adding UV treatment to water chlorination can increase the production of toxic trihalomethane byproducts. This occurs because UV treatment can damage organic matter in water, making it more susceptible to reaction with chlorine, which in turn, leads to higher levels of THMs. It is important to carefully monitor the levels of THMs and other DBPs in water treated with UV and chlorine to ensure that they do not exceed health standards.