Trihalomethanes are primarily created during which process?

Trihalomethanes (THMs) are primarily formed during the chlorination process, particularly when chlorine reacts with natural organic matter (NOM) found in water sources. In the context of water treatment, this reaction is most significant during the pre-chlorination process. Pre-chlorination involves adding chlorine to the water before it undergoes further treatment steps, which can include coagulation and sedimentation.

During pre-chlorination, organic material present in the water, including plant material, decayed matter, and other naturally occurring substances, interacts with chlorine, leading to the formation of trihalomethanes. This is a critical point in water treatment because controlling the levels of THMs is important for public health, as these compounds are regulated due to their potential carcinogenic effects.

While other processes, such as post-chlorination, can also contribute to THM formation, the initial reaction that generates these compounds predominantly occurs in the pre-chlorination stage due to the higher concentration of organic matter and chlorine interaction at that point in the treatment process.