Environ. Sci. Technol. 2009, 43, 6242–6247
Applying Surrogates and Indicators to Assess Removal Efficiency of Trace Organic Chemicals during Chemical Oxidation of Wastewaters ERIC R. V. DICKENSON,† ¨ R G E . D R E W E S , * ,† D A V I D L . S E D L A K , ‡ JO ERIC C. WERT,§ AND SHANE A. SNYDER§ Advanced Water Technology Center (AQWATEC), Environmental Science and Engineering Division, Colorado School of Mines, Golden, Colorado 80401, Department of Civil and Environmental Engineering, University of California, Berkeley, California 94720, and Applied Research and Development Center (ARDC), Water Quality Research and Development Division, Southern Nevada Water Authority, Henderson, Nevada 89015
Received December 29, 2008. Revised manuscript received June 3, 2009. Accepted June 4, 2009.
To respond to concerns associated with wastewater-derived contaminants water utilities are looking for new approaches for monitoring trace organic chemicals in conventional and advanced water treatment processes. This study examines the use of a combination of surrogate parameters and indicator compounds tailored to monitor the removal efficiency of advanced oxidation processes employed by treatment plants engaged in indirect potable water reuse programs. Potential surrogate parameters and indicator compounds, identified by reviewing previous publications and classified by their structural properties, were tested in pilot- and full-scale treatment systems. Dilantin, DEET, meprobamate, and iopromide are good indicators to assess optimized oxidation conditions while ozonating tertiary-treated wastewaters. UVA reduction, ozone byproduct formation, such as simple organic acids, and ozone exposure correlated with “sweet spot” compounds, where ozone exposure correlated with trace organic removal across five tertiary-treated wastewaters. Findings indicate that the proposed framework can serve as a conservative monitoring approach for advanced oxidation processes as well as other indirect potable reuse processes to ensure proper removal of identified and unidentified wastewater-derived organic contaminants, to detect failures in system performance, and is protective of public health.
Introduction Wastewater-derived chemical contaminants recently have received considerable attention from the research community, regulatory authorities, and the general public despite the fact that few compounds have been detected at concentrations that pose potential risks to drinking water supplies (1) or aquatic ecosystems (2, 3). For the majority of the * Corresponding author phone: (303) 273-3401; fax: (303) 2733413; e-mail:
[email protected]. † Colorado School of Mines. ‡ University of California, Berkeley. § Southern Nevada Water Authority. 6242
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compounds, it is difficult to assess human health or ecological risks because little information is available on their potential impacts at the relatively low concentrations encountered in wastewater effluents. In response to uncertainties associated with risk posed by these compounds, some scientists and regulators support the adoption of treatment technologies to minimize exposure of humans and aquatic ecosystems to wastewater-derived chemical contaminants until more data on potential risks are collected. Because a consensus on the risks posed by wastewater-derived contaminants is unlikely to be reached in the near future and it is difficult to quantify more than a few of the compounds at a time, water utilities and regulators need monitoring approaches to assess the ability of conventional and advanced treatment systems to remove hundreds of different contaminants potentially present using data on a limited subset of readily measurable compounds and general water quality parameters. This study focuses on chemical oxidation treatment of wastewaterderived contaminants. Chemical oxidation treatment systems can be used to control wastewater-derived chemical contaminants by removing or transforming compounds. Published research on the mechanisms of removal indicates that it should be possible to predict the extent of compound removal for compounds exhibiting similar properties provided that those properties determine the behavior of the compound during oxidation treatment (4–8). Furthermore, the removal of specific compounds or families of compounds with closely related properties may be correlated with the removal of other routinely measured compounds or operational parameters that can be monitored continuously. Although the idea of using other constituents (i.e., coliform bacteria) and water quality parameters (i.e., turbidity) as a means of predicting the behavior of difficult-to-measure contaminants is well established among researchers concerned with waterborne pathogens (9), it has not been adopted widely by researchers studying chemical contaminants because in the past most of the chemical contaminants of concern were relatively easy to quantify. To address the need for simplifying the analysis of wastewater-derived contaminant removal by engineered treatment systems, a suite of readily measurable surrogate parameters and indicator compounds can be used for assessing the removal of wastewater-derived contaminants during oxidation processes.
Materials and Methods Potential Indicators. Potential oxidation indicator compounds were previously identified in secondary/tertiary treated wastewater effluents, where the selection of indicators was based upon detectable and high occurrence compounds (10). Full-Scale Evaluation. Two full-scale water reclamation facilities located in the United States were selected to evaluate ozone alone (O3) and UV/H2O2 treatments, respectively. Plant #1 had a design capacity of 76 ML/day and employed nitrification with tertiary treatment followed by ultrafiltration, granular activated carbon filtration, and ozonation. An ozone dose of 1 mg/L with an estimated contact time of less than 1 min was applied at this facility. The preozonated water had a TOC of 5.6 mg/L and an alkalinity of 70 mg/L as CaCO3. Plant #2 had a design capacity of 23 ML/day providing nonnitrified secondary effluent. For this plant, an advanced oxidation process (AOP) utilizing low-pressure high-output UV radiation (300-400 mJ/cm2) and 3.0 mg/L H2O2 was employed after an integrated membrane system using 10.1021/es803696y CCC: $40.75
2009 American Chemical Society
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ultrafiltration and reverse osmosis. The pre-AOP treated water had a TOC of 0.2 mg/L and an alkalinity of 15 mg/L as CaCO3. For the treatment process of interest, samples were collected from the influent and effluent by existing facility autosamplers used for quality and compliance purposes or collected manually. Autosamplers were set up to collect timebased samples over a 2-4-h period. Samples were collected directly into a single 20-L glass container. Only glass collection containers were used during sample collection and if plastic tubing was employed, well-conditioned tubing was used. Amber 1-L sampling bottles were precleaned and contained 1 g of sodium azide for preservation and 1 mL of 50 µg ascorbic acid solution to quench any residual chlorine. Once the samples were collected, they were split onsite and shipped overnight to the participating laboratories in ice-packed coolers. Upon arrival, samples were logged and stored at 4 °C pending further analysis. Pilot-Scale Evaluation. Pilot-scale oxidation studies were conducted using wastewater samples collected after tertiary treatment with nitrification/denitrification. The tertiarytreated wastewater had a TOC of 6.8-7.1 mg/L and an alkalinity of 100 mg/L as CaCO3. Ambient concentrations of trace organic compounds were used to evaluate removal by O3 and O3/H2O2 treatments. Ozonation experiments were conducted using a 12-cell, continuous-flow ozone contactor at a design flow rate of 1 L/min. The applied ozone dosages were 2.1, 3.6, and 7 mg/L as O3, which resulted in O3/TOC ratios of 0.3, 0.5, and 1.0. O3/H2O2 experiments were performed using a 2.0 mass ratio of O3:H2O2. Trace contaminant samples were collected after contact times of 6 and 18 min (2 and 3.6 mg/L of O3) and 2, 6, and 18 min (7.0 mg/L of O3). O3 exposure, commonly referred to as CT, was calculated according to the Extended Integrated CT10 method using a T10/T ratio of 0.65 (11). Further information about the ozone-contactor operational conditions and some of the trace contaminant oxidation results from these tests are provided in Wert et al. (12) and Snyder et al. (7), respectively. Analytical Methods. Standard methods (13) or peerreviewed methods were employed to quantify surrogate parameters for chemical oxidation processes, namely UV absorbance and products formed during oxidation (e.g., formate, oxalate, aldehyde, AOC). Multiple methods were used to quantify wastewater-derived contaminants. For GC/ MS, GC/MS-MS, and LC/MS-MS analyses samples were extracted within two weeks. The extracts were stored at 4 °C until analyses of extracts were completed. Field and laboratory blanks were processed like field samples. Pharmaceuticals, pesticides, and chlorinated flame retardants were extracted using C-18 solid-phase extraction (SPE) resin followed by derivatization and GC/MS as described by Reddersen and Heberer (14). Steroid hormones were extracted using C-18 SPE extraction disks followed by derivatization and GC/MS-MS as described by Kolodziej et al. (15). Acidic drugs were extracted using ENVI-18 SPE resin followed by derivatization and GC/MS-MS (16). A suite of analytes was also extracted using SPE followed by LC/MS-MS as described by Vanderford et al. (17). Findings from two round robin experiments and split samples analyzed during field monitoring efforts indicated that the methods employed during this study were comparable with relative standard deviations of less than 30%.
Results and Discussion Indicators. In this study, an indicator compound is defined as an individual chemical that can be used to measure the effectiveness of a process for a family or group of compounds in the treatment process of interest. To develop a master list of indicator compounds for O3 only systems, indicator compounds for which observed removal was reported for similar conditions (i.e., operational and water quality) in
previously peer-reviewed published studies (5, 7, 18–22) were classified into four removal categories (Table 1). The operational conditions of the ozonation indicator master list are listed in Table 1, and detailed water quality and operational conditions are listed in Table S2. Wert et al. (22) determined the removal of trace organic compounds is impacted by the O3:TOC ratio where the ozone dose is larger than the initial ozone demand (IOD) in order to have sufficient • OH radicals available for oxidation of •OH selective contaminants. Therefore, at an O3:TOC ratio near 1 it is assumed the ozone dose is > IOD. For every candidate indicator compound the key structural moiety for an oxidation attack (6) is listed and compounds for which no previous treatment data was available were classified into similar subgroups of key moieties. Compounds that are in the good removal (>90%) category are compounds containing electron-rich aromatic systems, such as hydroxyl (e.g., estrone), amino (e.g., sulfamethoxazole), acylamino (e.g., triclocarban), alkoxy (e.g., naproxen), and alkyl (e.g., ibuprofen) aromatic compounds. In addition, good removal was observed for compounds containing deprotonated amine (e.g., diclofenac) and nonaromatic alkene (e.g., carbamazepine) groups. Note, most of the compounds in the good removal category have ozone second order reaction rate constants larger than 103 M-1s-1 (4, 5, 19, 20, 23-28). Some of the compounds contained multiple moieties of different types amenable to oxidation attack. In these cases the compound was categorized according to the structural moiety suspected to be predominately attacked. Though the density and number of amenable moieties was not considered in the categorization, the categorization provides a conservative assessment of these types of compounds, since these compounds will be more easily removed if they contain multiple reactive sites. Compounds that are in the partially removed category (90-50%) are aliphatic alkane, ketone, alcohol, acid, ether, and amide (e.g., meprobamate) or cycloalkane (e.g., methyl dihydrojasmonate) and nitro aromatic (e.g., musk ketone) type of compounds. A compound that is in the less removed category (50-25%) is a nitrosamine, NDMA. Compounds in the poorly removed category (90%) of many indicator compounds containing moieties that are highly amenable to oxidative attack (i.e., hydroxyl aromatic, amino aromatic, nonaromatic alkene, deprotonated amine, alkoxy aromatic) (Figure S1). Ozone exposures have been operationally defined as zero and correspond to the IOD occurring in the first ozone contact chamber. For carbamazepine, sulfamethoxazole, erythromycin, hydrocodone, triclocarban, and trimethoprim, greater than 2-log removal was observed (Table S3). These high removals also correspond with carbamazepine’s, sulfamethoxazole’s, and trimethoprim’s high second order ozone reaction rate constants, > 105 M-1s-1 (Table 1). For indicator compounds that were partially removed at low ozone exposures (9.9 mg-min/L) resulted in higher removals (>80%) for these compounds (Figure S1). VOL. 43, NO. 16, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Treatment Removal Categories for Indicators Compounds of Systems Using Ozone (Conditions: Secondary/Tertiary Treated Wastewater, O3:TOC = 0.6-1 mg/mg, Ozone Exposure CT10 = 4-11a mg min/L, pH 7-8, ∼20 °C, Alkalinity 90% removed, then other similar alkyl aromatic compounds, compounds containing moieties that are highly amenable to oxidative attack, will be removed. Meprobamate and iopromide are good indicators of other similar saturated aliphatic compounds, however to achieve >90% removal of these compounds the ozone exposure needs to be increased, which will lead to higher •OH exposures. During full-scale monitoring, samples were collected before and after post ozonation. For an O3:TOC ratio of 0.2 selected indicator compounds were classified by observed removal percentages in Table S3. Of the indicator compounds studied, most compounds exhibited removal significantly above 50% indicating a high degree of removal. These results show that at a low O3:TOC ratio of 0.2, only compounds that contain moieties that are highly amenable to oxidative attack (i.e., hydroxyl aromatic, amino aromatic, deprotonated amine, alkoxy aromatic) resulted in >90% removal. For carbamazepine and sulfamethoxazole a 2-log removal was observed. As observed with the pilot-scale experiments, partially removed (90-50%) compounds were dilantin and meprobamate. Note, DEET and iopromide were not analyzed in full-scale samples. These results confirm that dilantin 6244
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would be a good indicator indicating that the applied operational conditions were not sufficient for >90% removal of similar alkyl aromatic compounds and the ozone exposure needs to be increased, resulting in higher •OH exposures, to observe >90% removal for these types of compounds. It is important to note that for some compounds the observable percent removal was limited by the detection limit of the compound (Table S3). For example, for the pilotscale experiment the quantifiable removals for ibuprofen and estrone were >82% and >77%, respectively, but the actual removal was likely higher for these alkyl and hydroxy aromatic compounds. Thus, when considering the percentage removal, it is critical to consider the detection limit of the compound and its concentration in the source water before ozonation. The indicator results for advanced oxidation pilot- and full-scale experiments (O3/H2O2 and UV/H2O2) are summarized in the Supporting Information. These results demonstrate that O3 alone and advanced oxidation processes (AOP) remove performance indicators similarly because ozonation of wastewater is essentially an AOP. Removal of wastewater-derived compounds observed during full- and pilot-scale monitoring confirmed the classification of indicator compounds into the proposed removal categories based upon previously reported studies as well as key structural moieties. The most sensitive compounds to assess the performance will be those that exhibit intermediate or good removal under normal operating conditions. Thus, a system failure will be indicated by poor removal of indicator compounds that are normally removed, while normal operating conditions will be indicated by partial or complete indicator compound removal. If a compound that is easily removed by the treatment system is chosen, the indicator will be less sensitive to partial failure whereas the selection of an indicator that is poorly removed under normal operating conditions will provide little insight into system performance
FIGURE 1. Correlation between ozone exposure (log scale) (top left), ultraviolet absorbance removal (top right), formate concentration (bottom left), and bromate concentration (bottom right), and removal of select indicator compounds during pilot-scale ozone treatment. under any conditions. For wastewater-derived organic compounds which are only moderately or poorly removed, an additional treatment barrier should be employed. Surrogates. A surrogate is a quantifiable parameter that can serve as a performance measure of treatment processes that relates to the removal of specific contaminants. Surrogate parameters provide a means of assessing water quality characteristics without conducting difficult trace contaminant analysis. Selected surrogate parameters (i.e., ozone exposure, UV absorbance (UVA) at 254, ozone formation byproducts) are summarized in Table S4 for both pilot-and full-scale operations. Changes in UVA with no change in TOC indicate significant changes of the organic matter structure during ozonation. Consequently, UVA reduction as well as formate and bromate formation correlated well with higher removal of selected indicator compounds for the pilot-scale study (Figure 1). This demonstrates that changes in UV absorbance and formate formation concentrations are potentially good surrogates to assess the removal of easily and more difficult to remove trace organic contaminants, such as those containing alkyl functional moieties (dilantin, DEET, meprobamate, iopromide). Note, the removal of these compounds also correlated well with oxalate, aldehyde, and AOC formation (data not shown). Similar to ozone, UV absorbance and organic oxidation byproduct changes could serve as viable surrogates for O3/H2O2 performance efficiency (Figure S5). Another promising surrogate for O3 only systems is the use of ozone exposure (22), where the ozone CT10 was also highly correlated with removal of the “sweet spot” indicators (Figure 1). Although the removals of these “sweet spot” compounds are likely governed by hydroxyl radical oxidation there seems to be a relationship between the total ozone exposure and hydroxyl radical exposure. For the more recalcitrant indicators,
iopromide and meprobamate, CT10 > 10 mg-min/L is required for >90% removal. Note, a CT10 > 10 mg-min/L is also necessary for this wastewater effluent if the most probable number (MPN) for total coliform is targeted to be less than 2/100 mL (12), which represents an upper margin of typical recycled water disinfection (31). Compared to ozone byproduct measurements, UVA differential and ozone exposure are much easier and faster to quantify, where UV absorbance and ozone exposure can be easily tracked or integrated into an online control system of an ozonation facility. This study only focused on differential UVA wavelength at 254 nm, however other UVA wavelengths may better capture oxidation effects on NOM. Though not examined in this study, differential fluorescence spectroscopy potentially could be another useful surrogate that can be readily measured online. UVA and ozone byproduct surrogate and indicator relationships are likely site- and water-quality-specific as they are dependent on the type of organic matter and inorganic constituents (i.e., bromide) initially present. However, the use of ozone exposure is potentially universally applicable. In Figure S2 the removal of nine selected “sweet spot” indicator compounds is plotted versus ozone exposure across five tertiary-treated wastewaters, which were ozonated by the same pilot-scale operation (this study, 6, 24). These five treated wastewaters originated from four different wastewater treatment plants located in Nevada, Colorado, and Florida. The wastewater qualities are described in Figure S2. These results illustrate the ozone exposure appears to be a good surrogate for trace organic chemical removal across these five tertiary-treated wastewater types. In addition, the required ozone exposure for >90% compound removal is specific to •OH selective compounds, due to differing •OH reaction rates. A summary of the correlations between the removal of “sweet spot” indicator compounds and ozone VOL. 43, NO. 16, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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The individual steps to apply the proposed surrogate/ indicator monitoring framework are summarized in Table S1. During commissioning of the full-scale operation, selected surrogates are monitored on a regular basis. Surrogate parameters, which are normally easier to measure than indicators, can be used to identify when a treatment process is not operating within predetermined conditions. While it is implied that proper performance of the full-scale treatment process will ensure appropriate removal of wastewaterderived organic contaminants, less frequent (i.e., monthly to annually) monitoring of indicators provides confidence for the end users and regulators in the quality of the water supplied. The proposed surrogate/indicator monitoring approach assures proper removal of identified and unidentified wastewater derived organic contaminants, detects failures in system performance, and is protective of public health.
Acknowledgments We thank the WateReuse Foundation (WRF) for its financial, technical, and administrative assistance in funding and managing the project through which this information was derived. We also gratefully acknowledge the financial contributions from the Water Environment Research Foundation (WERF). The comments and views detailed herein may not necessarily reflect the views of WRF or WERF, its officers, directors, affiliates, or agents. We are also grateful to the participating utilities for their technical and administrative support. We thank Brett Vanderford, Rebecca Trenholm, and Janie Zeigler at SNWA, Mong Hoo Lim and Edward Kolodziej at UC-Berkeley, and Christiane Hoppe, Gary Wang, and Terry Jennings at CSM for their assistance in sample logistics and analysis.
Supporting Information Available Discussion of indicator and surrogate results for O3/H2O2 and UV/H2O2 pilot- and full-scale experiments; additional figures and tables. This material is available free of charge via the Internet at http://pubs.acs.org.
FIGURE 2. CT10 and percent removal relationships for “sweet spot” indicators (a) ibuprofen and galaxolide, (b) DEET, dilantin, primidone, and benzophenone, and (c) iopromide, meprobamate, and musk ketone. The data are from five pilot-scale ozonated tertiary-treated wastewaters and data sources are shown in Figure S2. exposure across the five tertiary-treated wastewaters is illustrated in Figure 2. The source for each data point is shown in Figure S2. Within the alkyl-aromatic indicator group (Table 1), ibuprofen and galaxolide require a CT10 > 1.5 mg-min/L for >90% removal. However, other alkyl-aromatic indicators, dilantin, DEET, primidone, and benzophenone require CT10 > 4 mg-min/L. For the more recalcitrant indicators, iopromide, meprobamate and musk ketone, require CT10 > 10 mg-min/L. For a better quantitative assessment of the oxidation of an indicator, the RCT concept (32, 33) can be applied by using measured ozone and hydroxyl radical reaction rate constants, measured ozone exposure, and removal kinetics of one of the previously discussed •OH selective indicators to assess •OH exposure. For example, Wert et al. (22) observed that the indicators ibuprofen and iopromide can serve as good indicators of •OH exposure. The indicator/surrogate framework presented here does assess the oxidation of trace organic chemicals, however the framework cannot assess the formation, fate, and removal of trace organic metabolites. 6246
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