Impact of Bromide Ion Concentration and Molecular Weight Cutoff on

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Downloaded by MICHIGAN STATE UNIV on February 19, 2015 | http://pubs.acs.org Publication Date: November 19, 1996 | doi: 10.1021/bk-1996-0649.ch016

Impact of Bromide Ion Concentration and Molecular Weight Cutoff on Haloacetonitrile, Haloketone, and Trihalomethane Formation Potentials 1

Steve H. Via and Andrea M. Dietrich Department of Civil Engineering, Environmental Division, Virginia Polytechnic Institute and State University, 330 Norris Hall, Blacksburg, VA 24061-0246 This research examined HANFP and HKFP formation and specific formation potentials with time (1 - 168 hours) and speciation in the context of THΜ formation and speciation. Controlling DOC concentration, DOC AMW, bromide concentration and Cl :C in raw and treated waters, coagulation resulted in a decrease in HANFP while THMFP and specific THMFP increased, shifting toward more brominated THMs. DCANFP was the most reduced HAN species. Lower MWCO NOM fractions generated the greatest DCAN formation and maintained a higher formation potential with time. TCPFP appeared unaffected by coagulation. Changes in the Br :Cl ratio: (1) increased the magnitude of specific molar THANFP more than TTHMFP; (2) created a higher percentage of brominated HAN species compared to formation of brominated THM species; and (3) decreased DCANFP more than CHCl FP. THANFP increased with increasing Br :Cl ratio, and the increase in Br-THANFP occurred to a greater extent on a relative basis than Br-THMFP. 2

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In 1979, health risks associated with halogenated DBPs resulted in the promulgation of a National Primary Drinking Water Standard for total trihalomethanes. In July 1994, the U.S. Environmental Protection Agency (EPA) proposed the Disinfectant / Disinfection Byproduct (D/DBP) Rule (i) to set the stage for future inclusion of haloacetonitriles (HANs), haloketones (HKs), and other DBPs into the nation's drinking water standards. A parallel regulation, the Information Collection Rule (ICR) (2), establishes monitoring requirements for HANs and HKs, to assess the extent of their formation in U.S. drinking water. Conventional treatment (coagulation-flocculation-filtration-disinfection) dominates water treatment plant (WTP) design in the U.S.. The proposed D/DBP Rule establishes enhanced coagulation as a best available technology for WTPs that: (1) are of conventional design with sedimentation basins, (2) utilize a water source that is influenced by surface water, and (3) experience total organic carbon (TOC) levels greater than 2 mg/L prior to disinfection. Enhanced coagulation is defined as addition of a coagulant at a dose beyond which an additional 10 mg/L will remove less than 0.3 mg-TOC/L. TOC abundance is a primary driver behind DBP formation; in Alberta, Canada, average 168 hour TTHMFP yields (pH 7,5 mg-Cl^ 1

Current address: 3631 Ranchero Drive, #102, Ann Arbor, MI 48108 0097-6156/96/0649-0282$15.00/0 © 1996 American Chemical Society In Water Disinfection and Natural Organic Matter; Minear, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.


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residual) of 11 ug/mg-TOC were observed in a survey of water treatment systems employing conventional treatment (3). Although exact structural characterization of dissolved organic carbon (DOC) is critical to determining which DBPs will be generated upon chlorination, general trends can be attributable to characteristics like apparent molecular weight (AMW) (4,5,6,7). Venstra and Schnoor (4) found 88% of the TOC and 87% of TTHMFP to occur in the 500 molecular weight cutoff (MWCO). Pilot scale membrane filtration experiments and simulated distribution system (SDS) tests (72 hour, 0.1-0.55 CI2/L residual, 20°C, ambient pH) showed ultrafiltration (100,000 MWCO) did not change SDSFPs for THMs, but HANSDS response decreased 25% relative to raw water trials (11). Nanofiltration (400-600 MWCO) reduced both THMSDS and HANSDS greater than 70-80% respectively. DBPSDS was reduced further when the MWCO was reduced to 200 to 300 AMW. Coagulation preferentially removes larger (i.e., >30,000), typically more humic acid type DOC. The following hierarchy of coagulation effect on DBPFP removal was proposed by Reckhow and Singer (9) after a study of DBPFP associated with 10 southeastern U.S. raw water sources: "DCANFP > TCAAFP > DCAAFP > THMFP (~TOXFP) > TOC > TCPFP" (bench scale coagulation; formation potential tests at 72 hours, 20°C, pH7, 20 mg-Cl /L). DCANFP responsivity was attributed to removal of the hydrophobic humic fraction. Normalized THM, trichloroacetic acid (TCAA), TCP, and total organic halide (TOX) formation were found to vary less than 23% among the eight colored, raw waters; HAN and dichloroacetic acid (DCAA) formation were more variable, particularly DCANFP which had a coefficient of variance of 40% (9). More recently researchers observed a similar hierarchy of DBPFP response (THMFP > THAAFP > TOX > UV absorbance) to enhanced coagulation (12,13). Coagulation reduced CHFP and THANFP when TOC was greater (test conditions of 4 and 2 nig-TOOL) (13). Previous research has found THKFP insensitive to coagulation (75). Differences in THM speciation have been attributed to "chlorine acting preferentially as an oxidant, whereas bromine is a more effective halogensubstituting agent" (14). Previous authors have found that speciation of brominated DBPs is not just the product of stoichiometry; addition of bromide appears to enhance chlorine oxidation reaction rates and result in increased "total halogen consumption" (15). Increased THM yield and shifts in THM speciation occur with increasing bromide concentration (16). DBP speciation effects have been observed at 0.5-1.5 mg-Br/L (0.4-1 mg-residual Cl /L, pH 6.4-9.4, time 2-40 hours) (17). Specific effects on THMFP (168 hour, pH 7, 25°C, 11.2 mg-Cl /L dose, 2.8 mgNVTOC/L) include decreasing CHCI3FP, increasing CHBr FP, and passage of CHCl Br and CHClBr through a formation peak with increasing bromide (17). TTHM formation (24 hour, 20 C, pH 7,25 mg-Cl /L, 2.5 mg-DOC/L) has also been observed to increase and shift to more brominated THMs with increasing bromide concentration. Bromine incorporation as a percentage of TTHMFP at a range of temperatures (10°, 20°, and 30°C), chlorine doses (2.5, 5, 10 mg-Cl /L) and pH ranges (pH 6-9) varied with bromide (4,18). Symons et al. (19) suggested that the initial Br':average Cl molar ratio controls bromide substitution reactions in THM formation. The rate of THM-Br formation is faster than THM-C1 formation (19,20). Kinetics drive the substitution reaction to completion even in the presence of HOC1 which will only form CHC1 if it is present in excess when the bromide substitution reaction is complete. NOM precursors limit THM formation due to TTHM-Br 2

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In Water Disinfection and Natural Organic Matter; Minear, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

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consumption of available sites prior to TTHM-Cl formation and thus bromide concentration increases total DBP yield (19). Laine et al. (11) employed Bf :C1 weight ratios to illustrate the effect of bromide ion on THM and HAA speciation (1.5-1.7 Cl :TOC weightrweight ratios, 0.06-0.64 Bf :C1 dose weightrweight ratios, 3 day, 20°C, ambient pH SDS tests). The research suggested that total DBP production was constrained by DOC concentration even when bromide ion was present in abundance. Also, while water quality and ultrafiltration membrane type affected total THMSDS and HAASDS formation, the Br:Cl ratio was more significant than the type of precursor material in determining the distribution of THMSDS and HAASDS species. Peters et al. (21) investigated DC AN, BCAN, DBAN, CHCfe, CHClBr , CHClBr , and CHBr formation, at six Dutch WTPs applying chlorine (1.7-5.6 mgDOC/L, 0.34-186.7 DOC:Cl dose weight-weight ratio). HAN formation was 5% of THM formation on a weight basis. Brominated HANs were more abundant than chlorinated HANs, totaling 60% of the observed HAN concentration. Brominated HAN species concentrations were greatest when the DOC:Cl dose ratio was between 5.6 and 11.8 but insufficient data were developed to explain HAN speciation. A shift to brominated THM species and higher brominated THM formation was observed under low DOC concentrations (e.g., high Br":DOC ratio) 2

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(21).

Several researchers have investigated the role oftimein DBP formation. A trend of increasing DBPFP withtimeis true of HAAs and THMs generally with THM formation essentially complete after four days (22). Two DBPs, chloral hydrate (CH) and DCAN, have been observed to achieve peak formation potentials and decline with time. DCAN was observed to decreased as a function of reaction time decaying completely over a seven day period at a pH of 7 (7,25). Water Characteristics and Treatment Train The water studied in this research has low alkalinity, moderate color, and low ammonia; it was obtained from a reservoir located in eastern Virginia (24,25). Storage of water in the reservoir is believed to alter raw water characteristics prior to the WTP treatment train, by promoting settling and algal growth. Comparison of DOC concentration in water samples prior to 0.45 umfilterand after filtration indicates that NOM > 0.45 um accounts for less than 15% of raw water TOC. Seasonal effects on water sample character were observed; such effects on TTHM and THAN are known to be small relative to other waters (25). WTP staff have optimized the treatment train, and at thetimeof this research, plant records indicated 50% reductions in THMFP and TOC and near complete color and turbidity removal. The WTP employs 30-45 mg-alum/L and 0.3 mg-cationic polymer/L for coagulation Pretreatment includes addition of potassium permanganate at 0.3 mg/L. Benchscale comparisons of the current treatment train parameters with enhanced coagulation as specified in EPA guidance reflected more positively on the current treatment train (13,25). NOM concentrations observed in this research varied with sample and MWCO. Full-scale treatment caused an apparent increase in [CHBr ], which is the same as reported by other researchers performing experiments on this water (75,25). As is typical for THMFP experiments, there was a distinct increase in TTHMFP and individual THMFPs over time (78). Raw water HANFP and TCPFP data (i.e., TCP was the only HK detected) demonstrated a different response pattern from that observed for the THM species (Figure 2). THAN and TCP concentrations peaked at about 24-48 hours in the formation potential test and then declined in concentration for the remainder of the seven day period. This pattern is consistent with that previously reported (7,23,25). The dominant HAN species was DCAN; its maximum concentration was about 5 ug/L at 24-48 hours, approximately 10% of the measured CHC1 concentration for this same water sample at the same time point. BCAN followed a formation potential pattern with time similar to DCAN but at reduced concentrations; DBAN was seldom observed. TCP was observed to form in the concentration range of 1-3 ug/L. The TCP concentrations and formation pattern with respect to time were comparable to that observed for DCAN and BCAN.


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Coagulated Water. Treatment of the raw water by full scale coagulation and upflow clarification resulted in similar formation potential patterns at slightly increased THM concentrations when compared with the THMFP of the raw water (Figure 1). The TTHMFP value at 7 days was about 160 ug/L (79 ug-TTHMP/mgDOC) for treated water. Formation of CHClBr was greater in the coagulated water than the raw water for all time points. Increased TTHMFP with treatment has also been reported by other authors (26). Increased THMFP after treatment may result from pretreatment with potassium permanganate or increased reaction of chlorine and bromine with THM-precursor material after coagulation. After treatment, the DCANFP remained the dominant HAN but its concentration was reduced to 65-80% of the raw water DCANFP values at all time points (Figure 2). Its maximum concentration was 3.8 ug/L at 48 hours, which was about 5% of the measured CHC1 concentration at the same time point. Treated water BCANFP values were similar in concentration to those observed for the raw water but at continuously declining concentrations with time. DBAN was more frequently detected in formation potential experiments for treated water samples compared to raw water. TCPFP results for the treated and raw water were very similar in concentration and pattern, with peak concentrations occurring at about 24 hours. In this water, haloketone formation appeared unaffected by coagulation; this trend was previously reported (75). A Matched Pair Wilcoxon Ranked Test was performed to compare specific formation potentials in raw and coagulated water across alltimeperiods; statistical results are presented in Table II. Differences (a = 0.2) were determined for CHC1 , 2

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In Water Disinfection and Natural Organic Matter; Minear, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

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Figure 1. Observed Raw and Coagulated Water THM Formation Potentials

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Impact of Bromide Ion Concentration and Molecular Weight Cutoff on

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