Chlorination of water containing natural organic matter can lead to the formation of THMs. Which practice helps minimize THM formation?

• A. Add chlorine as early as possible in the treatment process
• B. Disinfect after all contact time in the clear well
• C. Limit chlorine dose and remove organics upstream to minimize THMs
• D. Use high concentrations of chlorine to quickly kill organisms

Answer: (C)

Chlorination forms THMs when chlorine reacts with natural organic matter already present in the water. The amount of THMs produced depends on how much organic material is left before chlorine is added and how much chlorine is used, along with factors like contact time, pH, and temperature. The best way to minimize THMs is to reduce the organic precursors upstream and keep the chlorine dose as low as practicable for effective disinfection. Coagulation, flocculation, sedimentation, and filtration remove a lot of natural organic matter, so when chlorine is finally added, there is much less material to form THMs. Using only the necessary chlorine residual after those treatment steps limits the reaction opportunities with any remaining NOM. In contrast, adding chlorine early, waiting to disinfect after contact time, or using very high chlorine concentrations can all increase THM formation.