AWWA ACE59839

The Stage 1 Disinfectants/Disinfection Byproducts Rule (D/DBPR) and the forthcoming Stage 2 D/DBPR have caused many utilities to reevaluate their methods for maintaining compliance with the maximum contaminant levels (MCLs) for disinfection byproducts (DBPs). One such utility is the Charleston Commissioners of Public Works (CPW) in Charleston, South Carolina. Charleston CPW operates a conventional surface water treatment plant with a capacity of 118 million gallons per day (mgd). The raw water is characterized by high total organic carbon (TOC) concentration, with low to moderate alkalinity. Chlorine is currently used as the primary disinfectant, with chloramines utilized for secondary disinfection. Charleston CPW has traditionally accomplished control of DBPs through the use of chloramines in the distribution system, and through the periodic addition of chlorine dioxide downstream of filtration. In order to help further refine the strategy for control of DBPs, testing was performed during the summer in two separate years to assess the effect of pH during chlorine contact time prior to ammonia addition, and the effect of chlorine dioxide oxidation on the formation of trihalomethanes (THMs) and haloacetic acids (HAAs). Ongoing work includes evaluation of the use of chlorine dioxide in lieu of free chlorine for primary disinfection. This paper presents the key findings from the work at Charleston CPW, including the following: for many of the water quality conditions that occur, operation at a lower pH during chlorine contact will help assist Charleston CPW in meeting the Stage 1 D/DBPR for THMs, with no significant increase seen in HAA5 formation (while HAA5 formation did not increase at the lower pH, there was an increase in the formation of the unregulated HAAs); water quality conditions, in particular elevated levels of bromide, appear to influence the effectiveness of pH adjustment as a method for minimizing the formation of THMs (an increase in bromide concentration during the second test period affected the speciation of THMs formed and was associated with an increase in the yield of THMs per amount of TOC present; in addition, the elevated bromide levels coincided with a degree of continued THM formation following the addition of ammonia to form chloramines); and, chlorine dioxide addition prior to chlorine contact was effective in the reduction of THM formation during both test periods, a strategy that has worked effectively full-scale. The evaluations at Charleston CPW are illustrations of strategies that are available for addressing new and forthcoming issues for controlling disinfection byproducts, and the effects that varying water quality conditions can have on these strategies.