Trihalomethanes in finished drinking water in relation to dissolved organic carbon and treatment process for Alberta surface waters

Abstract

Disinfection by‐products are formed when the disinfectant (often a strong oxidizer) reacts with bulk dissolved organics in the water during treatment and distribution. The relationship between trihalomethanes (THMs) and bulk dissolved organics (DOC) in the finished drinking water for municipal treatment plants in Alberta using surface waters has been summarized. Treatment processes considered included chlorination with filtration alone, chlorination with complete treatment (coagulation, sedimentation and filtration [CSF], with or without additional treatment such as aeration and activated carbon), and chloramination with CSF. Trihalomethane levels were significantly correlated with the content of DOC as would be expected. For complete treatment systems using chlorination, the average THM yield was approximately 11 μg THM per mg of DOC. These data suggest that if the DOC level of the treated water exceeds 4 mg L^‐1, the average THM will likely exceed 50 ug L‘¹ if chlorination is used. For chloramination, the THM yield was 14‐fold lower and THM values were consistently below 50 ug L^‐1. The THMs produced by chlorination represent less than 15% of the THM formation potential. Maintaining a free chlorine residual throughout the distribution system will likely be difficult for these conditions.

Keywords:

• Trihalomethanes (THM)
• dissolved organic carbon (DOC)
• drinking water treatment process
• chlorination