When are trihalomethanes primarily created?

Trihalomethanes (THMs) are primarily formed during the chlorination process, specifically when chlorine reacts with organic matter present in water supplies. This reaction occurs most significantly during disinfection processes where chlorine is introduced to treated water. While pre-chlorination, which takes place before the primary treatments, can contribute to THM formation, the main production typically occurs when chlorine is actively used to disinfect water after it has undergone treatment.

The context of THM generation highlights that the presence of organic compounds, mainly natural organic matter, is a key factor. These compounds, when chlorinated, lead to the formation of various THMs, which can be a concern for water quality and public health. Therefore, understanding the timing of chlorination and its connection to THM formation is crucial in water treatment practices.