In a water treatment plant using coagulation, sedimentation, and filtration, which chlorine application point minimizes THM formation?

• A. In the finished water clear well
• B. In the raw water intake prior to treatment
• C. Immediately after coagulation during rapid mixing
• D. Within the distribution system after a booster pump

Answer: (A)

THMs form when chlorine reacts with natural organic matter in water, so reducing contact between chlorine and organic matter minimizes their formation. After coagulation, sedimentation, and filtration, most NOM has been removed, leaving water with a much lower THM formation potential. Adding chlorine at the finished-water stage—just before distribution—provides the needed disinfectant residual while minimizing the opportunity for THMs to form, since there’s little NOM left to react with.

Chlorinating raw water introduces chlorine when NOM is high, leading to more THM formation. Chlorinating immediately after coagulation or in the distribution system keeps more NOM present or allows THMs to form during transit, respectively. Hence, dosing chlorine in the finished water in the clear well best minimizes THM formation.