Trihalomethanes are primarily created during which process?

Trihalomethanes (THMs) are primarily formed during the disinfection of drinking water, particularly when chlorine is used as a disinfectant. The formation of THMs occurs when chlorine reacts with natural organic matter present in the water, such as humic and fulvic acids. This reaction tends to happen effectively during the pre-chlorination process, where chlorine is added to raw water before the treatment processes like coagulation or sedimentation take place. This stage allows for the disinfection of the water while also facilitating the removal of organic materials that can lead to THM formation.

By adding chlorine early in the treatment process, the opportunity for disinfection and consequently the generation of THMs is maximized, making this the critical phase where these compounds are primarily created. Recognizing the significance of the pre-chlorination stage allows water treatment operators to better manage the potential formation of these compounds, ensuring compliance with regulatory standards for safe drinking water.