Boron Can Be Used to Predict Trace Organic Rejection through

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Article Cite This: Environ. Sci. Technol. 2018, 52, 13871−13878

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Boron Can Be Used to Predict Trace Organic Rejection through Reverse Osmosis Membranes for Potable Reuse Lauren N. Breitner,†,§ Kerry J. Howe,*,† and Daisuke Minakata‡ †

Department of Civil, Construction, and Environmental Engineering, University of New Mexico, Albuquerque, New Mexico 87131, United States ‡ Michigan Technological University, Houghton, Michigan 49931, United States

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S Supporting Information *

ABSTRACT: Potable water reuse is a viable option for communities with extreme water scarcity. Improvements in measurement capabilities and greater occurrence of contaminants of emerging concern (CECs) have made the investigation of the removal of CECs through advanced treatment facilities essential for further reuse considerations. Reverse osmosis (RO) has been demonstrated to remove many CECs, but poor removal has been observed for many low molecular weight (MW), neutral organic compounds. With the availability of many RO membrane products on the market, it is increasingly important to be able to predict organics rejection through different products without detailed information about the RO membrane’s properties or structure. This laboratory-scale study investigated the rejection of lowMW, neutral organics, boron, and sodium chloride by six RO membrane products. The experimental results were used to develop a correlation between the removal of organics and boron. If the rejection of boron and a neutral organic through one reference membrane is available, then the rejection of that organic through any other membrane product can be estimated using the rejection of boron through that membrane.

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INTRODUCTION Factors such as increasing population, urbanization, and climate change have made the management of water resources a challenge for many municipalities. Wastewater reclamation, consisting of either direct potable reuse (DPR) or indirect potable reuse (IPR), and desalination of seawater are increasingly practiced and considered viable options to supplement water sources for these communities.1−3 Wastewater contains a wide variety of constituents that can have detrimental health effects to humans. These constituents include viruses and other pathogens that cause acute health concerns, as well as trace organics whose long-term toxicological impacts to humans are unknown. Wastewater reclamation systems provide multiple barriers to remove these constituents, and potable reuse treatment facilities must be designed and operated with redundant barriers to ensure that the potential hazardous constituents are removed.4 Reverse osmosis (RO) is a membrane-based treatment process that separates dissolved contaminants from water by forcing water through the membrane under pressure. RO is commonly included in the treatment train for potable reuse facilities because of its well-established excellent performance for removing a wide variety of constituents.5−7 While RO achieves excellent removal of many dissolved organics, studies have shown poorer rejection of neutral, low molecular weight © 2018 American Chemical Society

(less than 150 to 250 Da, depending on the membrane product) organics.7−10 A large variety of RO membrane products are available, ranging from nanofiltration or ultralow pressure (ULP) membranes to high pressure seawater (SW) membranes. The RO membranes most commonly used in potable reuse applications are brackish water (BW) membranes. RO products have a range of removal capabilities and are not equally applicable to potable reuse applications, so criteria for selecting appropriate RO products are necessary. Regulations in California require that potable reuse systems use membranes that achieve at least 99.2% sodium chloride (NaCl) rejection.11 However, NaCl rejection may not adequately reflect the ability of RO membranes to remove neutral trace organics due to differences in the rejection mechanisms for charged versus neutral species. This suggests a need to develop alternate criteria for selecting potable reuse membranes and monitoring the rejection of organics in potable reuse applications. Boron may be an attractive and promising surrogate indicator of RO performance. Boron is present as neutral Received: Revised: Accepted: Published: 13871

June 20, 2018 October 27, 2018 November 5, 2018 November 16, 2018 DOI: 10.1021/acs.est.8b03390 Environ. Sci. Technol. 2018, 52, 13871−13878

Article

Environmental Science & Technology boric acid (H3BO3) (pKa: 9.2 and molecular weight (MW): 61.8 g/mol)12 at the typical pH in reuse facilities. Boron is present in seawater at about 4.5 mg/L and, as a low-MW neutral compound, is poorly rejected by RO membranes. Because of the potential human health impacts and toxicity to plants,13 poor rejection of boron by RO is an important consideration during the design of seawater desalination facilities. Accordingly, membrane manufacturers report the rejection of boron on the specification sheets of SW RO membrane products. However, boron rejection data are not currently present on BW RO specification sheets. Although the rejection of boron and organics through RO membranes have been studied separately in recent years,9,10,14−18 a few studies have investigated the possible correlation between these two. For instance, the rejection of nitroso compounds19−21 such as N-nitrosodimethylamine (NDMA) showed a correlation with that of boron through six different membrane products at various operating conditions (i.e., pH and temperature)21 in both bench-scale and pilot-scale studies.20 Although some evidence of a relationship between the rejection of boron and neutral organics exists, no previous literature has proposed boron as a surrogate for the selection of RO products and design of RO systems for potable reuse applications. The objective of this research was to develop a correlation that can predict the rejection of low-MW, neutral compounds through one RO membrane product if its rejection through a different membrane product is known. The research was conducted at the bench scale and evaluated the rejection of 73 neutral organic compounds through six membrane products. This study provides, for the first time, a method to evaluate new RO membrane products for their potential use in potable reuse systems using information that could be readily included on membrane specification sheets provided by membrane manufacturers. This study could assist in the design and selection of membrane products for potable reuse systems.

Figure 1. Process flow diagram for laboratory experimental system.

during the remaining tests provided additional validation of repeatable performance of membranes positioned in series. A schematic of the experimental setup is presented in Figure 1. A Wanner Hydra-Cell D03 positive displacement pump circulated water from a 20 L stainless steel feed tank through each membrane cell. All tubing and hoses for the system were stainless steel. The membrane coupons had an area of 0.016 m2. The permeate and concentrate were recycled to the feed tank except during sampling. The water was maintained at 20 °C by a Thermo Neslab temperature controller. Feed, concentrate, and permeate pressure; feed temperature; and permeate and concentrate flow rate were measured continuously with online instruments and a DATAQ DI-808 data acquisition system. The permeate samples were collected one at a time by switching permeate flow from a combined stream to a single permeate flow stream, where a flow meter and sample valve was located. All online instruments and sensors were calibrated before experiments began. The rejection of solutes through a RO membrane depends on the relative rates of mass transfer of the solutes and water through the membrane, as expressed in the following equations:

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MATERIALS AND METHODS Experimental Setup. A custom-designed polypropylene flat-sheet membrane cell that can contain five separate sheets of membrane material was used as the RO membrane testing system, as described previously.22 The use of this unique cell allows five individual membrane coupons to be tested in series with the same feed solution simultaneously. In this study, different membrane products were placed in each position in the system with a duplicate of the same product in the first and last positions. This strategy increased the number of membrane products that could be evaluated. A mass balance on each membrane cell was used to determine the feed concentration to the next cell. The recovery from each individual membrane is small, thus, the feed concentration changes very little from membrane to membrane, and feed concentrations determined by mass balance coupled with measured permeate concentrations from each membrane provide accurate calculations of rejection. To verify that rejection could be accurately determined for membranes positioned in series, preliminary tests were performed with the ESPA2-LD membrane product in all 5 positions, with 5 measurements of acetone rejection over a 118 h period. No systematic error was introduced by the coupon position or operating time; the 25 measurements of acetone rejection averaged 66% with a standard deviation of 2.7%. In addition, the duplicate membrane products in the first and last position

Rej = 1 − CP =

CP CF

(1)

JS JW

(2)

JS = k S(βC F − C P)

(3)

JW = k W[ΔP − (βπF − πP)]

(4)

where Rej = rejection, CP = permeate concentration (mg/L), CF = feed concentration (mg/L), JS = solute mass flux (mg/ m2-h), JW = water volumetric flux (L/m2-h), kS = solute mass transfer coefficient (L/m2-h), β = concentration polarization factor (dimensionless), kW = water mass transfer coefficient (L/m2-h-bar), ΔP = transmembrane pressure (bar), πF = feed osmotic pressure (bar), and πP = permeate osmotic pressure (bar). The concentration polarization factor was calculated as described in Crittenden et al. (2012)7 using the Mariñas and Urama correlation.23 Membrane Products. Six membrane products were tested using membrane material provided by four membrane manufacturers. The membranes included four BW products and two SW products. Performance data available on the 13872

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Table 1. Membrane Products Used, Data from Manufacturer Specification Sheets, and Corresponding MTCs for Water and NaCl manufacturer

product

type

perm. flow (m3/d)a

stabilized salt rejection (%)

test feed conc. (mg/L NaCl)

test pressure (bar)

kW (L/m2-h-bar)

kS (NaCl) (L/m2-h)

Hydranautics Toray Dow Filmtec GE Dow Filmtec Toray

ESPA2-LD TMG(D) BW30XFRLE AG LF SW30XHR TM800M

BW BW BW BW SW SW

37.9 45.8 43 39.7 23 26.5

99.6 99.7 99.3 99.5 99.82 99.8

1500 2000 2000 2000 32 000 32 000

10.5 10.3 10.3 15.5 55 55.2

4.87 6.48 6.07 3.37 1.07 1.27

0.131 0.117 0.259 0.171 0.041 0.051

a Permeate flow is for 8-in. diameter and 40-in. long membrane elements. Test concentration and pressure are from specification sheets. Specification sheets also contain membrane area, spacer thickness, recovery, and other information necessary to calculate the mass transfer coefficients.

buffered deionized water containing 2000 mg/L NaCl and H3BO3 at a concentration of 5 mg/L as B, and was completed on the medium test pressure for each membrane group. Samples of feed, permeate, and concentrate were collected after 24 h of run time. The rejection of a suite of low molecular weight organic compounds was determined during Phase 3. All of the organic compounds were added to the tank with buffered DI water. The spike concentration of each organic compound depended on the expected rejection. At the end of the 72-h equilibration period, feed and permeate samples were taken at all three test pressures for each group of membranes, with a minimum of an hour of operation at each pressure. A nominal cross-flow velocity of 0.25 m/s was maintained throughout the experiments, which corresponds to typical operating conditions in full-scale RO elements. Phase 2 and 3 feed solutions were kept at a constant pH of 6.5 by adding 1.6 mM sodium phosphate and 0.4 mM disodium phosphate. The pH of 6.5 was chosen based on the pH generally used in reuse practices, as well as to ensure all organics tested were neutral. Organic Compound Selection and Analysis. The 73 organic compounds used in these tests can be categorized in eight different groups: alcohols, aromatics, esters, ethers, haloalkanes, haloalkenes, nitriles, and ketones. The MW of these compounds ranged from 41 to 260 g/mol and the octanol/water partition coefficient (log Kow) values ranged from −0.34 to 4.78. The complete list of compounds tested, with MW and log Kow values, feed concentrations, and detection limits, is included in the Supporting Information (SI). This study focused on simple neutral organics with distinct and well-defined functional groups because it was part a larger body of work that examined the effect of functional chemistry on RO rejection of neutral organics. More complex organics with multiple functional groups, such as pharmaceuticals, personal care products, pesticides, and industrial solvents are also of concern in potable reuse but were not included in this study. Analysis of the mass transfer of these organic compounds through all membranes tested was completed by calculating each compound’s MTC value from the experimental data using eqs 1−4. The ESPA2-LD and TM800M membranes were placed in the first and last positions of the membrane test system, providing duplicate measurements for the MTC values at each test pressure for both of these membranes. Samples were collected in triplicate at the lowest test pressure for the last membrane in the test system for each membrane group. This resulted in eight calculated MTC values for the ESPA2LD and TM800M membranes, and three calculated MTC

membrane manufacturer specification sheets is shown in Table 1. All of the membranes met the criteria of >99.2% NaCl rejection; thus, from the perspective of California’s potable use regulations, all are suitable for use in a potable reuse system. The SW membranes were included in the study to provide a wide range of solute and water mass transfer coefficients (MTCs) and expand the applicability of the results across a range of membrane products. Mass transfer coefficients can be calculated from specification sheet performance data using mass transfer principles that incorporate eqs 1−4. The MTCs for these products were calculated using a Microsoft Excel application that accounted for concentration polarization, recovery, and feed channel headloss and are presented in Table 1. The water MTC (kW) is analogous to the specific flux normalized to 20 °C as calculated in standard references such as the EPA Membrane Filtration Guidance Manual,24 but is based on the intrinsic performance of the membrane material, whereas the specific flux describes performance averaged over an entire pressure vessel. Experimental Procedures. The experimental program had two test series, one for the BW membranes and one for the SW membranes. For the BW test series, ESPA2-LD membranes were placed in positions 1 and 5 in the test system, and the TMD(D), BW30XFRLE, and AG LF were placed in positions 2, 3, and 4, respectively. For the SW test series, TM800M membranes were placed in the positions 1 and 3 and the SW30XHR was placed in position 2. The BW membranes were tested at 5.2, 10.3, and 15.5 bar (75, 150, and 225 psi), and the SW membranes were tested at 10.3, 15.5, and 24.1 bar (150, 225, and 350 psi) to provide a range of operating conditions. While the maximum test pressures (and maximum flux) were higher than might typically be used in a potable reuse application, they provided performance data across a wide variety of operating conditions. The test pressures for the SW membranes were selected to provide suitable flux values if these membrane products were used in low salinity applications (i.e., potable reuse). Across this range of operating conditions and membrane products, the water flux ranged from 11 to 74 L/m2-h for the BW membranes and 6 to 50 L/m2-h for the SW membranes. Each test series had three phases. The first phase was operation with deionized water (DI) for 24 h with the feed pressure set to the highest test pressure for each membrane group. The purpose of this phase was to compact the membrane material and clean the membrane products of any preservative. The feed tank water was changed to fresh DI water at 2, 4, and 6 h. NaCl and boron rejections were determined in the second phase. Phase 2 used a feedwater of 13873

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membrane products was generally the same for each compound: the AG LF membrane had the lowest rejection, the other three BW membranes were similar (although the TMG(D) was often the lowest of the three), and the two SW membranes had the highest rejection. This trend was true across most of the compounds in this study, with some exceptions due to experimental variability. This result is important because the compounds exhibit a range of functional chemistry: BCM is a haloalkane, acetone is a ketone, boron is an inorganic, PCE is an alkene, benzene is an aromatic, and IPE is an ether. The MW of these compounds ranges from 58 to 166 g/mol and the log(KOW) ranges from −0.24 to 3.4. Some systematic exceptions to this trend were observed. For instance, the AG LF had better rejection than the TMG(D) for all ethers. Between the SW membranes, the TM800M had better rejection for all the alcohols, ketones, and esters, but the SW30XHR had better rejection for most of the other compounds. Other than experimental variability and these few exceptions, however, the ordering of the rejection of the various membranes for each compound was remarkably similar. Empirical Correlation between Compounds and Membrane Products. On the basis of the observation that the rejection trend across membrane products was similar for nearly every compound, it was hypothesized that membrane products with similar chemistry (these were all polyamide thinfilm composite membranes) vary in their overall permeability, but the relative permeability of species stays the same from one membrane product to the next. Permeability is characterized by the MTC values, which can be calculated from experimental solute rejection and permeate flow data using eqs 1−4. Thus, the relationship between the MTC values of two species through a range of membrane products should be linear. It should be possible to develop an empirical correlation to predict the rejection of a target compound through a new membrane if the rejection of that compound through a base membrane is known, along with the rejection of a representative compound through the base and new membranes, according to the equation:

values for every other membrane. For model development, the average and relative standard deviation (RSD) of MTC values were calculated for the repeated measurements. If the RSD was calculated to be 0.5 or greater, then that compound was considered to have poor repeatability and thus was excluded from model development (see SI). Analytical Methods. The NaCl concentration was determined with an Oakton PC 2700 conductivity meter and a calibration curve that related NaCl concentration to conductivity. The boron concentration was analyzed using a PerkinElmer Optima 5300DV Inductively Coupled PlasmaOptical Emission Spectrometer (ICP/OES) with a detection limit of 0.01 mg/L. Trace organic concentrations were determined by a commercial laboratory using EPA Method 5030 (Purge and Trap) for extraction and EPA Method 8260 (Volatile Organic Compound Analysis by GC/MS) for quantification. Detection limits for each compound are shown in the SI.

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RESULTS AND DISCUSSION Overall Results of Organic Rejections through 6 Membrane Products. A total of 73 organic rejections were obtained through 6 membrane products and 54 organic rejections were selected for model development based on the selection criteria using the RSD values. Table S2 in the SI summarizes all calculated MTC values through 6 membrane products. Figure 2 shows the selected average rejections of 6

log(k T,N) = m[log(kR,N) − log(kR,B)] + log(k T,B)

(5)

where kT,N is the MTC for a target species through a new membrane, kR,N is the MTC for a representative species through a new membrane, kR,B is the MTC for a representative species through a base membrane, kT,B is the MTC for a target species through a base membrane, and m is an empirical parameter determined by the slope of a linear correlation relating the rejection of species through various membrane products. Since the rejection of organics in Figure 2 follows the same trend, there may be a single value of m that would be suitable for all neutral species and polyamide thin-film composite membrane products. The first step in developing a correlation shown in eq 5 was the selection of a base membrane and a representative species. Each membrane used in this study was evaluated for use as the base membrane. The Hydranautics ESPA2-LD membrane was found to give the best model results based on the lowest coefficient of determination (R2). The ESPA2-LD’s selection as the best base membrane may have resulted from rejection performance being near the median of the membranes tested in this study. This membrane product is also useful as a base product because of its wide use in the potable reuse industry. Because the ESPA2-LD membrane was used as the base

Figure 2. Rejection of selected solutes through 6 membrane products (BCM = bromochloromethane, PCE = tetrachloroethylene, IPE = isopropyl ether). Error bars are standard deviation.

compounds including bromochloromethane (BCM), acetone, boron, tetrachloroethylene (PCE), benzene, and isopropyl ether (IPE) through 6 membrane products. Several observations are readily evident from these representative results in Figure 2. First, the rejection varies substantially between compounds: the rejection of BCM ranged from 7% to 34%, whereas the rejection of IPE ranged from 95% to 99%. Second, rejection varied substantially between the membrane products: the rejection of acetone ranged from 32% in the AG LF membrane to 80% in the SW30XHR membrane. The wide range of rejection of the same compound between membrane products is particularly noteworthy since all of these membrane products exceed 99.2% NaCl rejection and would be considered equally suitable for use in a potable reuse system based on California potable reuse regulations. Third, the trend for rejection across the 13874

DOI: 10.1021/acs.est.8b03390 Environ. Sci. Technol. 2018, 52, 13871−13878

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was reasonable for several membranes, but not for the AG LF membrane. When the kS for NaCl is used, the rejection of organics is underpredicted for most compounds through most membranes. Two factors may contribute to the poor correlation shown in Figure 3. First, the mechanisms for mass transfer of water and NaCl (a charged species) are different from the mechanism of mass transfer of a neutral organic compound. In the negatively charged polyamide RO membrane, the Cl− ion rejection is affected by electroreplusive forces, and the Na+ ion rejection is affected by electroneutrality. These mechanisms do not occur for neutral compounds. Second, the relationship between the MTC values and rejection is not linear, as shown in Figure 4. When the

membrane for the model, any compounds with an RSD value of 0.5 or above for the ESPA2-LD membrane were not used in model development because of the uncertainty of the MTC values. The second step was to select a representative species that can be used as a surrogate indicator for the membrane property. Ideally, it should be possible to determine the MTC value of the surrogate from readily available information. Because the MTCs for water and NaCl can be determined from performance data available on manufacturer specification sheets (shown in Table 1) using eqs 1−4, water and NaCl were the first species considered for the representative species. In addition, NaCl rejection is currently used as a parameter for specifying allowable membrane products for potable reuse in California. In the comparisons that follow, the rejection was calculated for the medium test pressure and compared to the experimental rejection for the medium pressure tests. Water or NaCl As a Surrogate Indicator. The relationship between the predicted and measured values of rejection at the medium test pressure for each organic (including 19 compounds that were not used in model development) using water or NaCl as the representative species are shown in Figure 3. The dashed lines in Figure 3 represent a predicted rejection that is within 10% of the measured rejection. In both cases, the correlation is not strong. When the kW value is used for model development, the relationship between measured and predicted rejection values

Figure 4. Mathematical relationship between rejection and solute mass transfer coefficient.

rejection is 99% and higher, significant differences in mass transfer can lead to very little change in rejection, whereas in the region of interest for neutral organics, differences in mass transfer lead to much more substantial differences in rejection. As a result, rejection of neutral organics is much more sensitive to mass transfer and may not adequately be described by the differences in NaCl rejection between membrane products. Thus, additional attempts at developing a correlation were conducted using neutral species. Benzene As a Surrogate Indicator. Since water and NaCl were demonstrated to be ineffective surrogates for rejection through multiple membrane products, a representative neutral organic was evaluated. Benzene was selected because it is the simplest aromatic and had a wide range of rejections in this study (57 to 93%). The relationship between the predicted and measured value of rejection at the medium test pressure for each of the organics in this study using benzene as the representative species are shown in Figure 5. The correlation is significantly better than the correlations shown in Figure 3. When benzene was used as the representative species, 86.0% of the predicted results were within 10% of the measured results. Thus, if the rejection of benzene were measured for a new membrane product, and the rejection of benzene and any target neutral compound were known for the ESPA2-LD membrane from existing literature, then it would be possible to estimate the rejection of the target compound in the new membrane within 10%. A disadvantage of using benzene as the representative compound is that it would be necessary to measure the benzene rejection through every new membrane product, and

Figure 3. Predicted rejection through membrane products using data available on manufacturer specification sheets (A) water mass transfer coefficient, calculated from permeate flow data, and (B) NaCl mass transfer coefficient, calculated from average NaCl rejection.25 13875

DOI: 10.1021/acs.est.8b03390 Environ. Sci. Technol. 2018, 52, 13871−13878

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linear regression model using the kS value for boron is presented in eq 6. log(k ORG,NEW ) = 0.82[log(kBORON,NEW ) − log(kBORON,ESPA2)] + log(k ORG,ESPA2)

(6)

Additional analysis was conducted to investigate how different groups of organic compounds are predicted by the linear regression model. The aromatics, alcohols, esters, and ethers had the highest rejection by all six membrane products. The haloalkane and haloalkene compounds were frequently rejected below 50% by the BW membranes. Although the rejections of haloalkane and haloalkene compounds were slightly higher through the two seawater membranes, the rejections were still relatively low compared to other groups of organic compounds. Generally, the lowest error between predicted and measured rejection occurred for compounds and membranes with high levels of rejection. The compounds with the lowest error in the predicted rejections were the aromatics, alcohols, esters, and ethers and, as noted above, these groups of compounds were generally rejected well by each membrane tested. The compounds with the highest error of the predicted rejections are compounds with low rejection. The effect of rejection on the accuracy of the prediction is demonstrated by Figure 4. When rejection is low, the rejection is more sensitive to variation in the mass transfer coefficient than when rejection is high. For instance, an organic compound with 98% rejection will experience only a slight decline in rejection, to 97.4%, when the solute mass transfer increases by 30%. In contrast, the same increase in mass transfer will cause an organic compound with 50% rejection to decrease to 40% rejection. The membranes with the least error in the predicted rejection are the two seawater membranes. Implications for Full-Scale RO Systems. This study highlights the usefulness of boron as a surrogate indictor for the rejection of a wide variety of organic compounds by various brackish and seawater RO membranes that are typically applied for potable reuse applications. As more organic compounds are found in wastewater effluent and RO is used as part of the multibarrier process for potable reuse applications, a surrogate indicator that is easily measured and represents the rejection of many organic compounds is needed for the selection of membrane products and design of RO systems. Ideally, RO membrane manufacturers should include boron rejection on the specification sheets of their BW RO products, as they do for their SW RO products. The addition of boron rejection data onto BW RO specification sheets does not represent a substantial burden for manufacturers since they already include this information in the specifications of other products. The experimental results presented in this study were derived from bench-scale flat-sheet testing. In full-scale operation, production of permeate increases the concentration of solutes on the feed side, which increases the solute flux because of increased concentration gradient and decreases the water flux because of increased osmotic pressure. These combined effects reduce the rejection compared to bench-scale flat-sheet studies that operate at very low recovery. A mathematical analysis of the first stage of a full-scale system containing 7 ESPA2-LD elements operating at 10.5 bar (150 psi) feed pressure and 48.5% water recovery was conducted. For sodium chloride, the specification sheet reports 99.6% NaCl rejection, and the modeled full-scale system achieved

Figure 5. Predicted rejection through membrane products using the mass transfer coefficient for benzene.25

Figure 6. Predicted rejection through membrane products using the mass transfer coefficient for boron.25

measuring benzene rejection is more challenging because the loss of benzene in either feed or permeate streams due to volatility can interfere with determining rejection. Boron As a Surrogate Indicator. An alternative to benzene as the representative compound is boron. The relationship between the predicted and measured value of rejection for each of the organics in this study using boron as the representative species are shown in Figure 6. The correlation is similar to that of benzene, with 86.3% of the predicted results within 10% of the measured results. When the predicted and measured rejection varied by more than 10%, the measured rejection was typically higher than the predicted rejection, which is a conservative result. Thus, boron rejection can serve as a reasonably reliable predictor of the effectiveness of an RO membrane product for rejecting organics. Some RO membrane manufacturers already list boron rejection for their SW membrane products because of concerns about boron regulations in the seawater RO market. Both of the SW products used in this study have boron rejection listed on their specification sheets. The addition of boron rejection on the specification sheets of BW membrane products would benefit the potable reuse industry with additional information about the effectiveness of membrane products for removing organics without imposing undue burden on membrane manufacturers since they already provide this data for the SW products. The 13876

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99.4% rejection, demonstrating that rejection is relatively insensitive to operating conditions when rejection is high. For acetone, a bench scale system would achieve 55% rejection at this pressure, but the modeled full-scale would achieve only 36% rejection, indicating that poorly rejection compounds have greater sensitivity to factors such as recovery during operation. These results also highlight the variability that can be present in the rejection of neutral organics in full-scale operation. This study focused on simple neutral organics but more complex organics with multiple functional groups, such as pharmaceuticals, personal care products, pesticides, and industrial solvents are also of concern in potable reuse. On the basis of the outcomes of the larger study with respect to the importance of functional chemistry on rejection25 it is expected that other neutral organics would follow similar rejection trends based on their properties and functional chemistry. However, additional research may be necessary to validate that this correlation is appropriate for pharmaceuticals, pesticides, and other complex organic compounds.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.8b03390.

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Article

The organic compounds used in this study with their molecular weight, low(kOW), target feed concentration, and detection limits; the compounds used for model development; and the ks values for each organic compound and membrane product (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Kerry J. Howe: 0000-0002-8187-9744 Daisuke Minakata: 0000-0003-3055-3880 Present Address §

Trussell Technologies.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the Water Research Foundation’s financial, technical, and administrative assistance in funding and managing the project through which this information was discovered, developed, and presented. Funding was provided by Project Reuse-14-19/4769. The comments and views detailed herein do not necessarily reflect the views of the Water Research Foundation, its officers, directors, employees, affiliates, or agents. Additional support for this project was provided by the National Science Foundation through Grant 1345169. Any opinions, findings, and conclusions or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. Membrane material was provided by Dow Water and Process Solutions, GE Water and Process Technologies, HydranauticsA Nitto Group Company, and Toray Membrane U.S.A., Inc. 13877

DOI: 10.1021/acs.est.8b03390 Environ. Sci. Technol. 2018, 52, 13871−13878

Article

Environmental Science & Technology by reverse osmosis membranes for water reuse applications: A pilotscale study. Desalination 2014, 350, 28−34. (21) Tu, K. L.; Fujioka, T.; Khan, S. J.; Poussade, Y.; Roux, A.; Drewes, J. E.; Chivas, A. R.; Nghiem, L. D. Boron as a Surrogate for N-Nitrosodimethylamine Rejection by Reverse Osmosis Membranes in Potable Water Reuse Applications. Environ. Sci. Technol. 2013, 47 (12), 6425−6430. (22) Farias, E. L.; Howe, K. J.; Thomson, B. M. Spatial and temporal evolution of organic foulant layers on reverse osmosis membranes in wastewater reuse applications. Water Res. 2014, 58, 102−10. (23) Marinas, B. J.; Urama, R. I. Modeling concentration polarization in reverse osmosis spiral-wound elements. J. Environ. Eng. 1996, 122 (4), 292−298. (24) U.S. EPA. Membrane filtration guidance manual; EPA 815-R-06009; United States Environmental Protection Agency: Washington, DC, November, 2005. (25) Howe, K. J.; Minakata, D.; Breitner, L. N.; Zhang, M. Predicting Reverse Osmosis Removal of Unique Organics; Water Research Foundation: Alexandria, VA, 2018.

13878

DOI: 10.1021/acs.est.8b03390 Environ. Sci. Technol. 2018, 52, 13871−13878