AMS_ApplicationBrief_Benicia-WTP_THM_11.24

The City of Benicia, California, must address high levels of dissolved organics in their source water while servicing the water treatment needs of approximately 28,000 residents through a nominally rated 12 million-gallon-per-day (mgd) conventional treatment facility. With access to two water sources, the primary source water for the Benicia Water Treatment Plant (WTP) contains high levels of natural organic matter (NOM) with total organic carbon (TOC) levels ranging between 4-18 mg/L. While the Benicia WTP has successfully and continually reduced TOC levels by approximately 50% through pre-treatment efforts, residual TOC combines with the chlorine disinfectant used onsite to form moderate levels of trihalomethanes (THMs), leaving the WTP. The Benicia WTP has implemented a series of best management practices (BMPs) to help minimize the formation of THMs in its distribution system and ensure compliance with the Stage 2 Disinfectants and Disinfection Byproducts Rule (DBPR). In addition to minor pre-treatment and disinfection process changes, the Benicia WTP decided to monitor THM levels more frequently since real-time fluctuations in THM levels were difficult to characterize based on the frequency of quarterly grab samples and analysis timing. Monitoring the real-time correlation between operating conditions and THM levels was imperative. Some locations throughout the distribution system were experiencing high levels of THMs due to residence time and the age of water conditions. However, the turnaround time to receive results from external laboratory analysis proved too long to implement proactive management of THM mitigation strategies, limiting their efforts to reduce THMs leaving the plant and minimize the formation of THMs in the distribution system.