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The Effect of Ozonation and Biological Activated Carbon Treatment of Wastewater Effluents on Formation of Nnitrosamines and Halogenated Disinfection Byproducts Yi-Hsueh Chuang, and William A. Mitch Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b04693 • Publication Date (Web): 17 Jan 2017 Downloaded from http://pubs.acs.org on January 19, 2017
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The Effect of Ozonation and Biological Activated Carbon Treatment of Wastewater Effluents on Formation of N-nitrosamines and Halogenated Disinfection Byproducts
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Yi-Hsueh Chuang1 and William A. Mitch1, *
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Department of Civil and Environmental Engineering, Stanford University, 473 Via Ortega, Stanford, California 94305, United States
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*Corresponding author: email:
[email protected], Phone: 650-725-9298, Fax: 650-723-7058
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Abstract
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Ozonation followed by biological activated carbon (O3/BAC) is being considered as a key
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component of reverse osmosis-free advanced treatment trains for potable wastewater reuse. Using a
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laboratory-scale O3/BAC system treating two nitrified wastewater effluents, this study characterized
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the effect of different ozone dosages (0-1.0 mg O3/mg dissolved organic carbon) and BAC empty bed
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contact times (EBCT; 15-60 minutes) on the formation after chlorination or chloramination of 35
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regulated and unregulated halogenated disinfection byproducts (DBPs), 8 N-nitrosamines, and
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bromate. DBP concentrations were remarkably similar between the two wastewaters across O3/BAC
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conditions. Ozonation increased bromate, TCNM and N-nitrosodimethylamine, but ozonation was
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less significant for other DBPs.
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treatment at 15 minute EBCT, but little further reduction was observed at higher EBCT where low
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dissolved oxygen concentrations may have limited biological activity. The O3/BAC-treated
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wastewaters met regulatory levels for trihalomethanes (THMs), haloacetic acids (HAAs), and
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bromate, although N-nitrosodimethylamine exceeded the California Notification Level in one case.
DBP formation generally decreased significantly with BAC
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Regulated THMs and HAAs dominated by mass. When DBP concentrations were weighted by
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measures of their toxic potencies, unregulated haloacetonitriles, haloacetaldehydes and
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haloacetamides dominated.
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of the O3/BAC-treated chloraminated effluents were comparable or slightly higher than those
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calculated in a recent evaluation of Full Advanced Treatment trains incorporating reverse osmosis.
Assuming toxicity is additive, the calculated DBP-associated toxicity
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Introduction
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Highly-treated municipal wastewater effluents are increasingly considered as a local, reliable
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supply of potable water.1,2 Advanced treatment trains for potable reuse frequently employ Full
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Advanced Treatment (FAT), most often consisting of microfiltration, reverse osmosis (RO), and the
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UV/hydrogen peroxide (H2O2) advanced oxidation process (AOP). However, potable reuse facilities
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are evaluating RO-free alternatives due to the high energy consumption of RO, and, for inland
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utilities, issues associated with brine discharges.3
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Ozonation followed by biological activated carbon filtration (O3/BAC), wherein even exhausted
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activated carbon can achieve organic compound removal via biodegradation due to attached biofilms,
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has received attention as a component of RO-free potable reuse trains.3-7 Previous work on O3/BAC
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treatment of municipal wastewater has focused on removal of pharmaceuticals and pesticides or bulk
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parameters such as dissolved organic carbon (DOC). Contaminant removal increased with the ozone
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to DOC ratio (O3:DOC).4-8 While removal was 90% of many contaminants, particularly when coupled with BAC treatment.3,4,7
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However, some compounds (e.g., diuron7, benzophenone5) were more resistant. Activated carbon
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filters were more effective than sand filters, but contaminant removal was comparable across high
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BAC empty bed contact times (i.e., 30-120 min EBCTs).6 O3/BAC treatment removes ~20-70% of
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DOC, mostly via the BAC treatment.4,6,7,9 This removal percentage is higher than the ~10% removal
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observed for BAC treatment of surface waters.10 DOC removal, like pharmaceuticals, was lower
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when sand was used in place of activated carbon for biofiltration.6 DOC removal increased when the
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EBCT increased from 18 to 30 minutes, but leveled off thereafter.6 In addition to recalcitrant DOC,
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oxygen consumption within the biofilters may limit DOC removal at higher EBCT.6,11 O3/BAC
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treatment reduced non-specific toxicity by 55-75%,7 estrogenicity by >99%, and genotoxicity by
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>95%.4
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However, the chemicals targeted by most previous research (e.g., pharmaceuticals) accounted
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for haloacetonitriles >
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haloacetamides ~ haloketones > I-THMs is also consistent with previous work investigating DBP
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occurrence in authentic disinfected nitrified effluents, although lower concentrations were measured
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in those effluents due to lower disinfectant exposures (i.e., 60%. The efficient reduction in I-THM formation concurs with the rapid
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rate constant for I- oxidation to IO3- by O3 (1.2×109 M-1s-1).52 While TCNM did not form upon SDS
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chlorination of WWTP A regardless of ozone exposure, ozonation of WWTP B at 0.7 mg O3/mg
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DOC doubled TCNM formation, and ozonation at 1.0 mg O3/mg DOC increased TCNM formation
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five-fold relative to no ozone treatment. Ozonation increased bromate formation, particularly in
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WWTP A, which featured the higher bromide concentration. For WWTP A, 3.1 µg/L bromate formed
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at 0.35 mg O3/mg DOC. Formation increased linearly with ozone dose thereafter (Figure S3),
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reaching 85 µg/L at 1.0 mg O3/mg DOC (a 16% molar yield relative to bromide). For WWTP B,
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bromate reached 37 µg/L at 1.0 mg O3/mg DOC (10% molar yield), but was not detectable at lower
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ozone doses.
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The reduction in calculated toxicity was less significant than the reduction in DBP
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concentrations (Figures 1B, 2B and S5). Indeed, for WWTP B, the calculated toxicity did not
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significantly change for ozone treatment up through 1.0 mg O3/mg DOC. Haloacetonitriles and
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haloacetaldehydes dominated the calculated toxicity, with lower contributions from HAAs. Even at 16
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1.0 mg O3/mg DOC, the 85 µg/L bromate that formed contributed 90% at 15 min EBCT, while bromate was non-detectable at higher EBCT. Bromate
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removal likely was fostered by the low DO concentrations resulting from BAC treatment.17 The
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potential for bromate reduction under low DO conditions suggests a potential advantage to
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permitting low DO concentrations to occur within the BAC unit.
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Like for ozonation alone, the reduction in DBP formation resulting from BAC treatment was
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greater than the reduction in calculated toxicity due to an increase in BSF (Figure S4). The 61-74%
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removal of DOC by BAC treatment increased the Br- to DOC ratio, which promotes bromine
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incorporation during chlorination.53 Accordingly, DBP speciation was dominated by the fully
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brominated species. While haloacetonitriles, haloacetaldehydes and HAAs remained the dominant
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contributors to calculated toxicity, the importance of haloacetaldehydes declined with increasing
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EBCT due to preferential removal of their precursors. Overall, BAC filtration with or without
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pre-ozonation reduced halogenated DBP formation by 88% (±5.3% standard deviation across all
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ozone dose and EBCT combinations, n = 22), slightly higher than the 80.5(±12)% removal of
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calculated toxicity.
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Effect of O3/BAC treatment during SDS chloramination.
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characterized DBP formation and calculated DBP-associated toxicity for 0.7 mg O3/mg DOC and
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BAC filtration with 15 min EBCT followed by SDS chloramination (Figure 3). This ozone and BAC
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treatment combination featured high removal of DBP-associated calculated toxicity (i.e., 67% and
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84% for WWTP A and B, respectively) upon SDS chlorination (Figures 1 and 2). N-nitrosamines, a
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class of DBPs associated with ozonation and chloramination of wastewater,54,55 were also
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incorporated in the calculated toxicity, although only NDMA and NMOR were detected. DBP
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formation was quite consistent upon SDS chloramination of the two WWTP effluents, averaging 10
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µg/L (±22% relative percent difference) for regulated THMs, 18.9 µg/L (±9%) for HAAs, 5.4 µg/L
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(±9.3%) for haloacetamides, 2.0 µg/L (±8.5%) for haloacetonitriles, 0.5 µg/L (±35%) for
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haloacetaldehydes, 1.0 µg/L (±15%) for TCNM, 1.6 µg/L (±29%) for haloketones, and 44 ng/L
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(±18%) for NDMA. However, the concentrations of NMOR (56 ng/L in WWTP A and 14 ng/L in
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WWTP B) and I-THMs (4.8 µg/L in WWTP A and 0.1 µg/L in WWTP B) were more variable.
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NDMA was present in WWTP B prior to chloramination at 4.7 ng/L, and NMOR was present at 56
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ng/L in WWTP A and 12 ng/L in WWTP B (Table 2). NDMA formed as a byproduct of
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chloramination, while NMOR did not, consistent with previous research.12
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While ozonation mildly increased the total DBP concentration formed upon chloramination, this
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increase was driven by increasing TCNM and haloacetamides (Figure 3). HAA formation declined.
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Direct ozone reactions formed 40 µg/L bromate in WWTP A (but none in WWTP B) (Figure 3), and
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significantly increased NDMA concentrations in both samples (Table 2). However, ozonation also
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decreased further NDMA formation upon chloramination, particularly in WWTP A. As observed
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during chlorination, BAC treatment effectively reduced DBP formation during chloramination
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(Figure 3). The bromate formed during ozonation of WWTP A was eliminated by BAC treatment.
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BAC treatment also significantly reduced the NDMA concentrations formed directly by ozonation,
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although NDMA concentrations remained higher than in the untreated samples (Table 2). BAC 19
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treatment reduced the NDMA concentration formed upon chloramination relative to the
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ozone-treated samples, and the untreated samples.
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As observed during chlorination (Figures 1 and 2), the DBP-associated calculated toxicity was
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similar for the two samples (Figure 3). However, compared to chlorination, chloramination produced
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lower calculated toxicity by ~3-80-fold. For samples treated with or without ozonation, DBP
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formation during chloramination decreased by 95-96% for THMs, 86-90% for HAAs, ~99% for
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haloacetaldehydes, ~95% for haloacetonitriles, and ~65% for haloketones. In addition to forming
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NDMA, chloramination formed 1.2-2.6 times more haloacetamides and 2.4-190 times more TCNM
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relative to SDS chlorination. The increase in haloacetamides was expected based on prior
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mechanistic work.24,56 Chloramination also promoted the formation of mono-iodinated THMs and
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iodoacetic acid in WWTP A where a higher total iodine concentration was observed (Table 1).
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Among iodinated DBPs, iodoacetic acid exhibits the highest toxic potency (lowest LC50 value; Table
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S4). While haloacetonitriles and haloacetaldehydes dominated the calculated toxicity of the
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chlorinated samples, haloacetamides, haloacetonitriles, and iodoacetic acid (in WWTP A) dominated
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the calculated toxicity of the chloraminated samples. In WWTP A, the 0.53 µg/L iodoacetic acid
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constituted ~0.9% of the total DBPs on a molar basis, but ~30% of the calculated toxicity. Ozonation
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prior to SDS chloramination increased the calculated toxicity, mainly because ozonation increased
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haloacetamide formation by 18-45% and the speciation of DBPs shifted towards the more
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brominated species (Table S9). BAC treatment reduced the post-chloramination calculated toxicity 20
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relative to both the ozone-treated and untreated samples. Note that the contribution of N-nitrosamines
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to calculated toxicity was relatively minor (
