Short-term disinfection by-product formation in a chlorine/chloramine approach: Natural waters and model compounds

The chlorine/chloramine disinfection approach is being utilized and evaluated by drinking water utilities as a method to achieve disinfection and control disinfection by-product (DBP) formation in the distribution system. However, in this strategy substantial levels of the regulated DBPs, trihalomethanes (THMs) and haloacetic acids (HAAs), can be formed within a short chlorine contact time (2 to 4 hours). This "short-term" behavior is not well understood. This study investigates the short-term (<4 hours) THM and HAA formation, in a chlorine/chloramine approach by performing bench-scale experiments using model compounds and natural waters. beta-diketone acid compounds were studied in synthetic waters to determine whether they are possible fast-reacting DBP precursors for chlorinated and brominated THMs and HAAs. The impact of time, temperature, pH, chlorine dose, and bromide on short-term THM and HAA formations were assessed in coagulated natural waters. Also, the impact of prechlorination on long-term (1 to 20 days) dihaloacetic acid (DXAA) formations by chloramine was assessed in coagulated waters.; Five aliphatic beta-diketone compounds formed significant amounts of chloroform and one aliphatic beta-diketone compound formed a significant amount of dichloracetic acid (DCAA) after 4 hours. Aliphatic beta-diketone acid moieties could contribute to the source of THM, and in some cases, DXAA precursors within NOM.; For coagulated waters, significant amount of THMs and HAAs were formed during the first 4 hours of chlorination relative to that formed after 5 days: 39% of THM formation, 50% of HAA9 formation, and 48% of HAA5 formation. Chlorine consumption was used to simulate DBP formations during the short-term period. Temperature had little impact on short-term THM, DXAA, and trihaloacetic acid (TXAA) formations, which suggests seasonal temperature effects will not have a great impact upon short-term DBP formations. pH impacted short-term DBP formations where chloroform formations were favored at pH 8, pH had an unsystematic effect on DCAA formations, and TCAA formations were favored at pH 5.5 and 8.0. Short-term TXAA formations were more affected by chlorine dose than THM and DXAA formations. Higher bromide:TOC ratios yielded faster short-term DBP kinetics. A shift from chlorinated to more brominated DBPs is a potential problem as some of the brominated DBPs (i.e., BCAA) are thought to be of more of a health concern. When utilizing a chlorine/chloramine approach, DXAA formation from chloramine becomes more of an issue for long travel times (10 to 20 days).; This study promotes the understanding of the short-term behavior of THMs and HAAs by chlorine and the long-term behavior of DXAA by chloramines, which is critical to the effective use of chloramines.