Understanding DBP Formation During Chloramination
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Understanding DBP Formation during Chloramination Ying Hong, Suibing Liu, & Tanju Karanfil rihalomethanes (THM) and haloacetic formation has not been investigated. acids (HAA) are the two major disinfec- Since there are three possible reac- Ying Hong, Ph.D., is a civil engineer in the tion byproducts (DBPs) commonly tants—NH2Cl, HOCl, and dichloramine Orlando office of Black & Veatch. Suibing Tfound in waters disinfected with free chlorine. (NHCl2) can interact with DOM to form Because of the potential risk of such DBPs to HAA during chloramination (Figure 1)—the Liu, Ph.D., is an environmental engineer in public health, the United States Environmental first objective of this study was to examine the the Maitland office of CDM. Tanju Protection Agency (EPA) has imposed stringent relative significance of these reactions resulting Karanfil, Ph.D., is a professor in the regulations under its Disinfectants/Disinfection in HAA formation during chloramination. Environmental Engineering & Earth Byproducts Rule (D/DBPR). Today, there is significant discrepancy Science Department at Clemson University. Stage 1 of the D/DBPR set the maximum among the limited chloramination literature The article was presented as a technical contaminant levels (MCLs) of THM and that has been reported by various researchers paper at the Florida Section AWWA Conference in November 2007. HAA5 at 80 and 60 µg/L, respectively, for the (Figure 2). Singer et al. (1999) reported that 50 running annual average of a water system. percent of the 72-hour dihaloacetic acid Stage 2 of the D/DBPR requires utilities to (DXAA) (the major species of HAA during comply with the DBP regulations at individ- chloramination) formed within the first hour at ual locations in distribution system (i.e., pH 8.Speitel et al.(2004) and Hua and Reckhow Materials & Methods locational running annual averages). (2005) reported even faster HAA formation To comply with current and future DBP kinetics, showing about 70-75 percent of the 72- DOM Solutions regulations, drinking water utilities have been hour DXAA formed after only 0.5 hour at pH 8. Two surface waters with significantly considering various treatment and disinfec- By contrast, in some of the preliminary different DOM characteristics were obtained tion strategies. Chloramination has received experiments of the authors’ study, only 10 from the influents of two water treatment increased attention in recent years because it percent of 72-hour DXAA formation plants in South Carolina. Myrtle Beach water almost suppresses THM formation while occurred in the first hour at pH 7, and its for- has a high specific ultraviolet absorbance at reducing HAA concentrations to 3-30 per- mation gradually increased with time. 254 nm (SUVA254) (5 L/mg-m), whereas cent of the levels observed with chlorination. All these chloramination experiments Greenville water has relatively low SUVA254 It appears that during chloramination, were performed at similar pHs, and mono- value (2.5 L/mg-m). The DOMs in these formation and speciation of HAA, rather than chloramine were dosed to maintain approxi- waters were concentrated using a pilot-scale THM, will dictate the compliance of water mately 2 mg/L residual after 24-hour incuba- reverse osmosis (RO) system. utilities with more stringent DBP regulations. tion time as required by the uniform formation Before each experiment, RO isolate was To date, it is believed that HAA formation condition (UFC) protocol (Summers et al., diluted by distilled and deionized water during chloramination was attributed prima- 1996); therefore, research was needed to eluci- (DDW) to the target dissolved organic car- rily to the formation of chlorine (HOCl) dur- date the factors responsible for the observed bon (DOC) concentration of 7.5 mg/L. Solution pH was adjusted by addition of 4 ing monochloramine (NH2Cl) decomposi- differences. The second objective of this study tion (reaction 1 in Figure 1); however, the sig- was to investigate the potential causes of signifi- mM carbonate buffer and acid/base solutions at different strengths. nificance of direct reaction between NH2Cl cantly different patterns observed for DXAA and dissolved organic matter (DOM) on HAA (HAA) formation kinetics in the literature. Continued on page 52 Figure 1: Conceptual illustration of HAA formation Figure 2: DXAA formation kinetics pathways during chloramination. reported by different researchers. FLORIDA WATER RESOURCES JOURNAL • APRIL 2008 • 51 Continued from page 51 Table 1: Background ammonia effect formation (through production of HOCl on monochloramine decay and and/or formation of NHCl2) was significant- Chloramination Experiments ly suppressed. Finally, 20 mg/L ammonia was DBP formation at pH 6. Before each experiment, a fresh chlo- used for later kinetic study. ramine stock solution was prepared by mixing Parallel kinetics experiments were per- sodium hypochlorite (5 to 6 percent available NH3-N* TCM DCAA NH2Cl NHCl2 formed using the same DOM solution under free chlorine) and ammonium sulfate solu- (mg/L) (µg/L) (µg/L) (mg/L) (mg/L) two different ammonia conditions, i.e., in the tions in water at a Cl2/N mass ratio of 3.5:1 at absence and presence of 20 mg/L background 0 3 21 1.7 0.4 pH 9.0. Preliminary experiments were con- ammonia. The results of experiments in the ducted for each water sample to determine 10 3 18 2.1 <MRL presence of ammonia (where NH2Cl hydroly- the ratio of NH2Cl/DOC that would provide a 20 2 18 2.2 <MRL sis and NHCl2 formation were significantly total combined residual of about 2 mg/L chlo- suppressed) were compared with those in the ramine as Cl2 after 24 hours of contact time 30 2 18 2.2 <MRL absence of ammonia (where NH2Cl decom- following the UFC protocol. 40 2 16 2.2 <MRL position can occur) to assess the relative Kinetic experiments were performed for importance of the direct reaction of NH2Cl 0.5, 1, 2, 4, 8, 24, 48, and 72 hours. At the end 50 2 16 2.3 <MRL with DOM in the HAA formation. of each target contact time, samples were col- 100 2 17 2.3 <MRL HAA formation under two different lected for pH, free and combined chlorine ammonia conditions is shown in Figure 3. measurements. Then samples were quenched *: Additional amount of NH3-N added in each DCAA was the only HAA species formed dur- bottle at the beginning of the experiment. MRL: immediately with a stoichiometric amount of ing these experiments. Considering the slow Minimum Reporting Level. Initial NH2Cl =3.5 sodium sulfite (Na2SO3) and stored in a mg/L. Greenville water DOC=7.5 mg/L. Free nature of HOCl and NHCl2 production from refrigerator (4-6°C). All the samples of a chlorine after 24-hour reaction period was NH2Cl decay, they were probably not signifi- kinetic experiment were analyzed together always below the MRL. cant enough to impact NH2Cl decay and HAA for DBPs after the last sample (i.e., 72-hour) formation during the early reaction period. of the kinetic experiment. direct reaction of NH2Cl with DOM in the HAA After the first four-hour period, HAA for- formation during chloramination. mation trends start to separate for the two test Results & Discussions Initial experiments were conducted conditions. A higher degree of HAA formation using Greenville water RO isolate and pre- was observed with increasing reaction time in HAA Formation Pathway Study formed chloramine at pH 6 at several back- the absence than in the presence of back- It is evident from the reactions in Figure 1 ground ammonia concentrations. Dichloro- ground ammonia. At the end of 24 hours, the that by increasing background ammonia con- acetic acid (DCAA) was the only HAA species HAA concentrations were 26 and 20 ppb in the centration, autodecomposition of NH2Cl, thus observed in these experiments. absence and presence of background ammonia HOCl production (reaction 1) and formation of Table 1 shows the concentrations of samples, respectively (Figure 3), indicating that NHCl2 (reactions 2 and 3), can be significantly chloroform (TCM), DCAA, NH2Cl, and direct reaction of NH2Cl with DOM is the suppressed. When HOCl and NHCl2 formations NHCl2 measured after 24-hour contact time major pathway, accounting for about 80 per- are minimized, the occurrence of reactions during these experiments. Constant concen- cent of HAA formation during chloramina- involving these species will also be reduced, so trations of NH2Cl, TCM, and DCAA meas- tion. An experiment performed with only pre- by performing experiments in the absence and ured and the absence of NHCl2 formation formed NHCl2 and DOM at pH 6 showed no presence of high background ammonia, it is above 10 mg/L background NH3-N indicated HAA formation; therefore, the HOCl pathway possible to assess the relative significance of that decomposition of NH2Cl leading to HAA accounts for the other 20 percent. Figure 3: Formation of HAA in the absence (NH3=0 mg/L) Figure 4: Formation of HAA in the absence (NH3=0 mg/L) and presence (NH3=20 mg/L) of additional background and presence (NH3=20 mg/L) of background ammonia ammonia at pH 6. addition at pH 9. 52 • APRIL 2008 • FLORIDA WATER RESOURCES JOURNAL A similar experiment was also per- formed at pH 9. Because monochlormaine decomposition and HOCl formation was suppressed at high pH, the difference in HAA formation was very small (within 2 ppb) for the two ammonia level experiments (Figure 4). The 24-hour DCAA concentration at pH 9 in the presence of background ammonia (where direct reaction of NH2Cl with DOM is dominant) was 28 percent of DCAA formed at pH 6, indicating that DCAA formation from direct reaction strongly depends on pH. The Impact of Quenching Agent on HAA Kinetic Experiments It is evident from the kinetic experiment results presented in Figures 3 and 4 that no fast formation kinetics were observed in this study.