Texas Commission on Environmental Quality (TCEQ). Class C Water License Exam

Which substance reacts with organic chemicals to form trihalomethanes?

1. Chlorine dioxide

2. Chloramine

3. Free available chlorine

4. Ozone

The correct answer is: Free available chlorine

Trihalomethanes are a group of chemical compounds that can be formed when chlorinated disinfectants react with organic matter present in water. Free available chlorine, which includes forms such as hypochlorous acid and hypochlorite ions, is a common disinfectant used in water treatment processes. When free available chlorine is introduced to water containing organic substances, it reacts with those organics to create trihalomethanes as a byproduct. This formation occurs particularly when chlorine reacts with natural organic matter, such as decomposed plant material, leading to the synthesis of various trihalomethanes. The presence of these byproducts is significant because trihalomethanes are subject to regulatory limits due to their potential health risks when consumed in drinking water. Other substances listed, while also used in water treatment, do not produce trihalomethanes in the same manner. Chlorine dioxide serves as a disinfectant but does not lead to the formation of trihalomethanes. Chloramine, a compound created by combining chlorine with ammonia, generally leads to lower levels of trihalomethanes compared to free chlorine. Ozone, while an effective oxidizer and disinfectant, typically breaks down organic compounds rather than forming chlorinated byproducts