Minimizing Trihalomethane Formation through Source Water Monitoring and Optimizing Treatment Practices

Trihalomethanes, a family of halogenated disinfection byproducts, are prevalent in finished drinking water. According to USEPA’s Disinfectants and Disinfection Byproducts Rules (DBPRs), the regulatory standard of the total trihalomethanes (TTHMs) is 80 μg/L, and water treatment plant operators are required to closely monitor TTHMs in the finished water on a quarterly basis. Although total organic carbon (TOC) has been identified as a precursor for the TTHMs, knowledge gaps still exist in how specific water and treatment related factors (e.g., intake water quality, seasonal variation, contact time, and treatment train) affect TTHMs formation. Focusing on water treatment plants in southwestern Pennsylvania, the objectives of this study include 1) developing a predictive statistical model for TTHMs level using source water quality data, and 2) analyzing the effects of water treatment and distribution factors on TTHMs formation. Water quality data were collected through monthly sampling at three water authorities and five distribution points from September 2018 to May 2019. These data were used to explore statistical relationships between intake water quality and TTHMs formation in the distribution systems. Two regression models were developed to predict TTHMs level using relevant water quality parameters including TOC, chloride, and temperature. Based on the analysis, the TOC level at the intake water greatly impacted the TTHMs formation, and the TTHMs level was higher during September and October when the temperature and TOC levels were elevated. With low bromide (mostly undetected) in the source water, the THM species in the finished water were dominated by the chlorinated species. A comparison between the predicted and measured TTHMs data showed that, in most cases, the error was greater than 10%. This is attributed to the effects of plant specific treatment operation on TTHMs formation. To achieve the second objective, analyses were conducted using the treatment related data collected from the Borough of Charleroi Water Authority. It was observed that installation of a sedimentation basin helped to reduce the TTHMs level. Based on these findings, recommendations are provided to reduce TTHMs including introducing alternative disinfectant, maintaining minimum retention time, and flushing low demand distribution networks.