Full Description

The Stage 1 Disinfectants/Disinfection Byproducts Rule (D/DBPR) and the forthcomingStage 2 D/DBPR have caused many utilities to reevaluate their methods for maintainingcompliance with the maximum contaminant levels (MCLs) for disinfection byproducts(DBPs). One such utility is the Charleston Commissioners of Public Works (CPW) inCharleston, South Carolina. Charleston CPW operates a conventional surface watertreatment plant with a capacity of 118 million gallons per day (mgd). The raw water ischaracterized by high total organic carbon (TOC) concentration, with low to moderatealkalinity. Chlorine is currently used as the primary disinfectant, with chloraminesutilized for secondary disinfection.Charleston CPW has traditionally accomplished control of DBPs through the use ofchloramines in the distribution system, and through the periodic addition of chlorinedioxide downstream of filtration. In order to help further refine the strategy for control ofDBPs, testing was performed during the summer in two separate years to assess the effectof pH during chlorine contact time prior to ammonia addition, and the effect of chlorinedioxide oxidation on the formation of trihalomethanes (THMs) and haloacetic acids(HAAs). Ongoing work includes evaluation of the use of chlorine dioxide in lieu of freechlorine for primary disinfection.This paper presents the key findings from the work at Charleston CPW, including thefollowing:for many of the water quality conditions that occur, operation at a lower pHduring chlorine contact will help assist Charleston CPW in meeting the Stage 1D/DBPR for THMs, with no significant increase seen in HAA5 formation (whileHAA5 formation did not increase at the lower pH, there was an increase in theformation of the unregulated HAAs);water quality conditions, in particular elevated levels of bromide, appear toinfluence the effectiveness of pH adjustment as a method for minimizing theformation of THMs (an increase in bromide concentration during the second testperiod affected the speciation of THMs formed and was associated with anincrease in the yield of THMs per amount of TOC present; in addition, theelevated bromide levels coincided with a degree of continued THM formationfollowing the addition of ammonia to form chloramines); and,chlorine dioxide addition prior to chlorine contact was effective in the reductionof THM formation during both test periods, a strategy that has worked effectivelyfull-scale.The evaluations at Charleston CPW are illustrations of strategies that are available foraddressing new and forthcoming issues for controlling disinfection byproducts, and theeffects that varying water quality conditions can have on these strategies.

Product Details

Edition: Vol. - No. Published: 06/17/2004 Number of Pages: 7File Size: 1 file , 240 KB