Drinking Water Treatment Operator Class 1 Practice Exam 2026 - Free Drinking Water Treatment Practice Questions and Study Guide

Trihalomethanes (THMs) are indeed an example of a disinfectant byproduct. They are formed when chlorine, a common disinfectant used in water treatment, reacts with organic matter present in the water. This reaction occurs during the chlorination process, where chlorine is introduced to disinfect the water and eliminate pathogens. Unfortunately, while chlorine is effective at disinfection, the unintended reaction with organic compounds leads to the formation of these byproducts, which can pose health risks if they are present at elevated levels in drinking water.

In contrast, the other substances listed—sodium hypochlorite, chlorine dioxide, and carbon dioxide—serve different roles in water treatment. Sodium hypochlorite is a chlorine compound used as a disinfectant, and chlorine dioxide is another disinfectant used for its effectiveness against bacteria and viruses without forming THMs. Carbon dioxide, on the other hand, does not function as a disinfectant and is often involved in pH stabilization and remineralization processes in treated water. Thus, THMs are specifically recognized as byproducts associated with chlorine disinfection, making them the correct answer in this context.