Sodium Hypochlorite Pre-Oxidation as a Key Contributor to Trihalomethane Formation and Carcinogenic Risk: Evidence from Adjacent Water Treatment Plants

An on-site investigation was conducted to analyze the causes of excessive trihalomethane (THM) formation in Plant A and to mitigate the health risks associated with THM exposure in drinking water. Adjacent Plant B was used as a reference plant. Both water treatment plants used the same source water but employed different pre-oxidants. Systematic stage-specific sampling and analysis of historical monitoring data were conducted to identify the key contributing stage for THM formation. The investigation revealed that 85% of the trichloromethane in Plant A’s finished water originated from the sodium hypochlorite pre-oxidation step, identifying this stage as the key contributing stage. THM concentrations were generally higher at Plant A than at Plant B. A multi-pathway health risk assessment of THM in drinking water indicates that non-carcinogenic risks are negligible, but lifetime carcinogenic risks warrant attention. The findings demonstrate that pre-oxidant selection is a significant governing factor of finished water disinfection by-product (DBP) levels. Following these findings, Plant A implemented measures, including enhanced chlorination management, reduced pre-chlorination, and upgraded sedimentation and V-shaped filters, which substantially reduced chlorinated DBPs in the finished water.