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A surrogate-based approach for trace organic chemical removal by a high-rejection reverse osmosis membrane
Science of the Total Environment 696 (2019) 134002
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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv
Short Communication
A surrogate-based approach for trace organic chemical removal by a high-rejection reverse osmosis membrane Takahiro Fujioka a,⁎, Haruka Takeuchi b, Hiroaki Tanaka b, Hitoshi Kodamatani c a b c
Graduate School of Engineering, Nagasaki University, 1-14 Bunkyo-machi, Nagasaki 85 2-8521, Japan Research Center for Environmental Quality Management, Kyoto University, 1-2 Yumihama, Otsu 520-0811, Japan Division of Earth and Environmental Science, Graduate School of Science and Engineering, Kagoshima University, Korimoto 1-21-24, Kagoshima 890-0065, Japan
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
• A high-rejection RO membrane achieved high NDMA rejection (65–87%) • The NDMA rejection values were consistently lower than 23 other TOrCs • NDMA rejection was highly correlated with TOrC rejection across varied conditions • NDMA can be a conservative indicator to ensure a high level of TOrC removal by RO
a r t i c l e
i n f o
Article history: Received 8 May 2019 Received in revised form 12 August 2019 Accepted 18 August 2019 Available online 19 August 2019 Editor: Ching-Hua Huang Keywords: CECs Micropollutants Potable reuse RO membrane TOrCs NDMA
a b s t r a c t Public confidence in the safety of recycled water for potable water reuse can be improved by providing assurance regarding high removal of trace organic chemicals (TOrCs) by reverse osmosis (RO) treatment. This pilot-scale study assessed the effectiveness of a surrogate indicator—N-Nitrosodimethlyamine (NDMA)—for ensuring a high level of TOrC removal by a high-rejection RO membrane. The pilot-scale tests showed that the rejection of 23 TOrCs by the high-rejection RO membrane was consistently greater than NDMA rejection. In addition, NDMA rejection was highly correlated with TOrC rejection across varied operating conditions, indicating that NDMA can be used as a conservative surrogate indicator for TOrC removal. The RO treatment at a permeate flux of 20 L/m2 h and feed temperature of 13–27 °C resulted in as high as 75–87% NDMA rejection, which was considerably greater than a conventional low-pressure RO membrane (26–47%). However, the high-rejection RO membrane required a transmembrane pressure that was greater than that of the low-pressure RO membrane. Despite this disadvantage, this study suggests that the high-rejection RO membrane can effectively ensure a high level of TOrC removal (≥65%) when NDMA is used as a surrogate indicator, which cannot be ensured by assessing conventional conductivity rejection. © 2019 Elsevier B.V. All rights reserved.
1. Introduction ⁎ Corresponding author. E-mail address: [email protected] (T. Fujioka).
https://doi.org/10.1016/j.scitotenv.2019.134002 0048-9697/© 2019 Elsevier B.V. All rights reserved.
Potable water reuse is a method of recycling wastewater for potable water. Potable water reuse has been implemented to address the
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T. Fujioka et al. / Science of the Total Environment 696 (2019) 134002
shortages in water supplies that have occurred owing to prolonged droughts (Drewes and Khan, 2011; Tchobanoglous et al., 2015). The assurance of high recycled water quality is of paramount importance for potable water reuse because wastewater contains hazardous impurities such as pathogens and trace organic chemicals (TOrCs) (Luo et al., 2014; Taheran et al., 2016). These TOrCs include pharmaceuticals and personal care products, disinfection by-products (DBPs), pesticides, and endocrine-disrupting compounds (Snyder, 2014; Zeng et al., 2016). The TOrCs are known as potential toxins to humans (Asano and Cotruvo, 2004; Khan et al., 2019); thus, the concentrations of many TOrCs in drinking water or recycled water intended for potable water reuse have been regulated (NRMMC et al., 2008). Among advanced wastewater treatment processes, reverse osmosis (RO) treatment and ultraviolet (UV)-based advanced oxidation process (AOP) are capable of removing TOrCs as major barriers (Plumlee et al., 2008; Rizzo et al., 2019). It is noted that RO process also plays a role in rejecting a wide range of contaminants including ionic salts, heavy metals, and dissolved organics. The removal of TOrCs can be ensured through periodic water quality analyses (e.g., weekly or monthly) and continuous monitoring of surrogate or operating parameters (Drewes and Khan, 2015; Merel et al., 2015; Steinle-Darling et al., 2016). In California (USA), the AOP for potable water reuse is required to meet the following criteria for TOrC removal: (a) the removal of 1,4-dioxane greater or equal to 69% (0.5log) as a design criteria and (b) monitoring of the UV transmission as an operational parameter (SWRCB, 2015). In contrast, no design or operational criteria for TOrC removal has been established for RO treatment. Researchers (Fujioka et al., 2018a) have recently suggested the use of N-Nitrosodimethylamine (NDMA) as a surrogate indicator for TOrC removal by RO treatment. Suitable surrogate chemicals for TOrC removal must meet the following criteria: they are always present in the RO feed at high concentrations, they can be continuously monitored online, and the rejection is conservatively low compared with other targeted TOrCs. NDMA in the RO feed is typically identified at concentrations of N10 ng/L (Farré et al., 2011; Fujioka et al., 2012a), because NDMA is present in raw wastewater and NDMA concentration typically increases during advanced wastewater treatment due to its formation by prechloramination prior to RO process (Sgroi et al., 2015; Shah and Mitch, 2012). Additionally, NDMA can be monitored online using a newly developed NDMA analyzer, which includes high-performance liquid chromatography followed by a photochemical reaction and chemiluminescence (HPLC-PR-CL) (Fujioka et al., 2017; Kodamatani et al., 2018; Kodamatani et al., 2009). Many previous studies using NF and RO (Doederer et al., 2014) membranes identified that small and uncharged TOrCs show low rejections (Bellona et al., 2004; Verliefde et al., 2009). NDMA, which has a molecular weight of 74 Da, shows rejections lower than many other TOrCs upon a nanofiltration (NF) or RO process (Bellona et al., 2008; Yangali-Quintanilla et al., 2010). The major concerns associated with NDMA as a conservative surrogate indicator include: the consistency of NDMA rejection being lower than TOrC rejection and the assurance level for TOrC removal. Thus far, the validity of the surrogate-based approach has been demonstrated with a linear correlation between NDMA and only seven uncharged TOrCs including 1,4-dioxane across varied operating conditions in two previous studies (Fujioka et al., 2018a; Fujioka et al., 2018b), which do not fully support the validity of NDMA as a surrogate indicator. Moreover, the conventional low-pressure RO membranes used in these studies (EPSA2 RO membrane supplied by Hydranautics, CA, USA) at a permeate flux of 20 L/m2 h were capable of rejecting NDMA at only 15–45%, which is not sufficiently high to ensure the rejection of other TOrCs based on NDMA rejection. The low NDMA rejection by a conventional RO membrane and a small number of tested TOrCs remain a challenge for finding an advantage of the surrogate-based approach. This study aimed to evaluate the effectiveness of NDMA as a TOrC for ensuring a high level of TOrC removal by a high-rejection RO
membrane. The validity of NDMA as a surrogate indicator was examined by determining the correlation of rejection between NDMA and 23 TOrCs under different operating conditions. The feasibility of using the high-rejection RO membrane element was assessed by comparing the separation performance and permeability with a conventional low-pressure RO membrane element.
2. Materials and methods 2.1. Chemicals A total of 24 low molecular weight chemicals, including NDMA and three other N-Nitrosamines (N-Nitrosomethyelthylamine (NMEA), NNitrosopyrrolidine (NPYR), and N-Nitrosomorpholine (NMOR)) were evaluated as TOrCs in this study (Table 1 and S1). These chemicals have a broad range of molecular weight from 74 to 460 Da. They were primarily divided into two classes: neutral (≤50% ionized) or charged (N50% ionized). This was based on their dissociation levels at the tested pH of 7.5. The charged TOrCs were further categorized as positively or negatively charged compounds. A secondary effluent, which was collected after activated sludge process at a municipal treatment plant in Japan, was treated using an ultrafiltration (UF) membrane, and it was named UF-treated wastewater. The conductivity of the UF-treated wastewater was 1922 μS/cm.
2.2. Membrane and RO treatment system This study used a pilot-scale cross-flow RO treatment system holding a 4-inch HYDRApro 501–4040 RO membrane with an effective membrane area of 7.0 m2 (Hydranautics, Oceanside, CA, USA) (Fig. S1). The HYDRApro RO membrane, which was designed for industrial applications, was used as a high-rejection RO membrane. The system comprised of a 4-in. glass-fibre pressure vessel (ROPV, Nangang, China), 65-L stainless steel reservoir, a high-pressure pump (25NED15Z, Nikuni Co., Ltd., Kawasaki, Japan), digital flow meters (FDM, Keyence Co., Osaka, Japan), and digital pressure indicators (GPM, Keyence Co., Osaka, Japan). Feed solution temperature was controlled in the reservoir using a titanium heat exchanging pipe connected to a chiller unit (CA-1116A, Tokyo Rikakikai Co. Ltd., Tokyo, Japan).
2.3. Test protocols The membrane element was first rinsed with pure water to eliminate residual preservatives on the RO element. RO treatment was conducted at a constant cross-flow rate of 8.7 L/min by recirculating 50 L of the UF-treated wastewater. Rejection tests started by spiking TOrC stock solutions into the RO feed at approximately 100 ng/L (NDMA), 300 ng/L (NMEA, NPYR, and NMOR), and 100 μg/L (other TOrCs). The concentrations of TOrCs in the RO feed were determined to keep their RO permeate concentrations above the detection limits, so that their rejection were accurately calculated. The high TOrC concentrations in the RO feed may be a limitation of this study during the assessment of their rejection values. It is noted that NDMA concentration in the RO feed was determined to be lower than other N-Nitrosamine concentrations, because this study aimed to evaluate the validity of NDMA as a surrogate indicator using realistic concentrations of NDMA in treated wastewater. The system was operated for 18 h to stabilize the performance. Thereafter, operating conditions were adjusted stepwise at a permeate flux of 5–20 L/m2 h and a solution temperature of 13–34 °C. The varied permeate flux corresponded to a system recovery of 6–20%, which is equivalent to a 7–10% recovery from a single RO element at full scale. Samples for analysis were collected from the RO feed and permeate streams.
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Table 1 Physicochemical characteristics of the selected TOrCs. Compound
Structure
MW [Da]
MPAa,b [Å2]
LogD at pH 7.5a,c [−]
pKa (0 to 14)a [−]
Ionization at pH 7.5a [%]
Neutral NDMA NMEA NPYR NMOR Acetaminophen Ethenzamide Theophyline DEET
C2H6N2O C3H8N2O C4H8N2O C4H8N2O2 C8H9NO2 C9H11NO2 C7H8N4O2 C12H17NO
74.08 88.11 100.12 116.12 151.17 165.19 180.17 191.27
19.18 22.19 25.02 26.58 20.62 30.99 28.75 37.50
0.04 0.40 0.44 −0.18 0.90 1.56 −0.92 2.50
3.6 3.4 3.3 3.1 9.6 13.8 7.8 Not ionized
0 0 0 0 1 (−) 0 30 (−) 0
Positively charged Salbutamol Propranolol Atenolol Sulpiride Lincomycin
C13H21NO3 C16H2NO2 C14H22N2O3 C15H23N3O4S C18H34N2O6S
239.32 259.35 266.34 341.43 406.54
36.82 39.43 38.37 53.85 55.33
−1.23 0.45 −1.71 −0.61 −0.91
9.5, 10.2 9.8 9.8 8.5, 10.3, 13.7 8.1, 12.5, 13.1
99 (+) 99 (+) 99 (+) 91 (+) 80 (+)
Negatively charged Clofibric acid Nalidixic acid Sulfapyridine Ketoprofen Sulfathiazole Sulfamerazine Sulfadimidine Sulfamonomethoxine Sulfadimethoxine Tetracycline Oxytetracycline
C10H11ClO3 C12H12N2O3 C11H11N3O2S C16H14O3 C9H9N3O2S2 C11H12N4O2S C12H14N4O2S C11H12N4O3S C12H14N4O4S C22H24N2O8 C22H24N2O9
214.65 232.24 249.29 254.29 255.31 264.30 278.33 280.30 310.33 444.44 460.43
30.25 34.70 44.91 39.38 43.45 46.65 51.21 46.16 52.82 59.59 58.68
−0.53 −0.54 0.21 0.34 0.45 0.03 0.16 0.66 0.73 −3.87 −4.99
3.4 4.7, 5.8 0.5, 2.2, 6.3 3.9 7.0, 2.1 7.0, 2.0 7.0, 2.0 7.2, 2.6 6.9, 2.0 7.3, 7.9, 8.3 5.9, 7.3, 8.2, 9.2
100 (−) 98 (−) 94 (−) 100 (−) 77 (−) 74 (−) 74 (−) 67 (−) 78 (−) 58 (−), 26 (+/−) 53 (−), 24 (+/−)
a b c
Chemical properties: The information was obtained from ChemAxon (https://www.chemaxon.com/) on 26th March 2019. Minimum projection area is the area of the compound projection with the minimum plane of its circular disk, based on the van der Waals radius. LogD is the logarithm base 10 of the apparent water-octanol distribution coefficients at a specific pH, which represents a degree of its hydrophobicity.
2.4. Analytical techniques The concentrations of the four N-Nitrosamines were determined via the HPLC-PR-CL method (Kodamatani et al., 2018). The method detection limits of NDMA, NMEA, NPYR, and NMOR for a 200-μL injection
volume were 0.40, 0.54, 0.60, and 1.4 ng/L, respectively. Because the HPLC-PR-CL method enables to analyze only N-Nitrosamines, the concentrations of the other TOrCs were determined using ultraperformance liquid chromatography coupled with an atmospheric pressure ionization and tandem mass spectrometer (Waters, MA, USA)
Fig. 1. (a) Rejection values and (b) log rejection of TOrCs by the HYDRApro RO membrane at a feed temperature of (i) 34, (ii) 20, and (iii) 13 °C as a function of their minimum projection area (MPA). Permeate flux was 20 L/m2 h. Rejection values reported for N-Nitrosamines and other TOrCs are the average of duplicate and triplicate samples, respectively.
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(Narumiya et al., 2013). The conductivity and temperature of the RO feed water and permeate were also measured using conductivity meters (Orion Star™ A325, Thermo Fisher Scientific, MA, USA). 3. Results and discussion 3.1. Role of molecular size on rejection The rejections of the TOrCs were analyzed based on their minimum projection area (MPA) rather than their molecular weights, because the MPA of uncharged TOrCs has been reported to be better correlated with their rejections (Fujioka et al., 2019). The MPA of a compound represents a projected area of the chemical, which was calculated based on the van der Waals radius (Fig. S2). Minimum projection area can play an important role in TOrC rejection, because TOrCs can pass through the free volume holes of the membrane structure more freely when the clearance between the two-dimensional area of the chemical (i.e., MPA) and the area of free volume holes is larger. NDMA, which has the lowest MPA (19 Å2) of the selected TOrCs, showed the lowest rejection (65–87%) at feed temperatures of 13–34 °C (Fig. 1a). The rejection of uncharged TOrCs clearly increased according to the increased MPA when the rejection data was presented in log rejection (Fig. 1b). This was consistent with other operating conditions (i.e., permeate
fluxes of 5 and 10 L/m2 h and a feed water temperature of 13–34 °C) (Figs. S3 and S4). It is important to note that hydrophobic interactions between the membrane and TOrCs on rejection was not observed in this study, because all of the selected TOrCs with the exception of DEET are hydrophilic (LogD = b2.0). The results indicate that size exclusion is the dominant mechanism governing the rejection of small and uncharged TOrCs by the high-rejection RO membrane. Unlike the uncharged TOrCs, positively and negatively charged TOrCs showed similar log rejection at approximately N3-log rejection (i.e., N99.9%) across a wide range of MPA (Fig. 1b). The charged TOrCs were mostly larger than the uncharged TOrCs in molecular size; thus, the influence of electrostatic interactions on rejection was only evaluated between the positively and negatively charged TOrCs in this study. The electrostatic interaction between the charged TOrCs and negatively charged RO membrane surface could be explained by the repulsive (or attractive) force of the negatively (or positively) charged chemicals to the negatively charged RO membrane, which can enhance (or deteriorate) TOrC rejection (Verliefde et al., 2008). However, the difference of charge (i.e., positive and negative) on rejection was not apparent. The influence of electrostatic interactions on rejection may not be apparent when charged TOrCs were highly rejected at N99.9% by the high-rejection RO membrane. Overall, the results here indicate that NDMA can be a conservative surrogate indicator for ensuring the
Fig. 2. The rejection of TOrCs by the HYDRApro RO membrane as a function of NDMA rejection over the varied operating conditions (permeate flux = 5, 10, or 20 L/m2 h and feed water temperature = 13, 20, 27, or 34 °C): (a) N-Nitrosamines (uncharged TOrCs), (b) other uncharged TOrCs, and (c) positively and (d) negatively charged TOrCs. The dot line is the equality line with a slope of 1.0.
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removal of TOrCs by the high-rejection RO membrane regardless of their charge properties. 3.2. Correlation between NDMA and TOrCs The validity of NDMA as a conservative indicator for TOrC removal was further evaluated based on the correlation of NDMA rejection with the rejection of other TOrCs over a permeate flux of 5–20 L/m2 h and feed water temperatures of 13–34 °C. In general, the rejection of TOrCs increased with increasing permeate flux (Fig. S5) or decreasing feed temperature (Fig. S6). When permeate (water) flux, which varies depending on the position of RO elements vessel at a full-scale plant, increases, solute rejection also increases because solute flux remains almost constant regardless of the change in permeate (water) flux (Wijmans and Baker, 1995). Solute rejection at a constant permeate flux can increase with decreasing RO feed temperature, which varies seasonally, because solute flux decreases according to the decreased diffusivity of solutes induced by lower temperature (Sharma et al., 2003; Tsuru et al., 2010). The results showed that NDMA rejection (34–87%) was highly correlated with the rejection of the uncharged TOrCs, as indicated by the Pearson correlation coefficients (r) of 0.95–0.96 (N-Nitrosamines) and 0.70–0.94 (other uncharged TOrCs) (Fig. 2a and b). A lower correlation was determined with the charged TOrCs according to the Pearson correlation coefficients (r) of 0.67–0.87 (positively charged TOrCs) and 0.84–0.92 (negatively charged TOrCs) (Fig. 2c and d). Because the rejection of these charged TOrCs was typically above 99.8%, the lower correlation was likely owing to the low TOrC concentrations in the RO permeate, which are unable to be accurately determined. Despite the limitation, the results here still support that NDMA can be used as the surrogate indicator for TOrC removal by the high-rejection RO membrane. Similar to NDMA, the conductivity rejection (98.6–99.8%) was also highly correlated with the TOrC rejection (Pearson correlation coefficient (r) = 0.59–0.94) (Fig. S7). The advantages of using NDMA over conductivity include: (a) its relevance to TOrCs, (b) its sensitivity to varying RO treatment conditions; and (c) its conservative rejection value for TOrC rejection. In fact, the conductivity rejection was higher than some of the uncharged TOrCs (NMEA, NPYR, NMOR, and acetaminophen). Overall, the results here indicate the potential of NDMA as a conservative indicator for the removal of TOrCs with a high correlation and high sensitivity. It is important to note that this pilot study did not demonstrate the influence of membrane fouling, chemical cleaning, or membrane aging on the correlation; thus, further long-term investigations on site are necessary in a future study.
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However, the transmembrane pressure required for the 4-inch HYDRApro RO membrane was over three times greater than that of the ESPA2 RO membrane (Fig. 3b). This higher transmembrane pressure indicates a higher energy consumption; thus, the use of a high-rejection RO membrane can result in an increase in operating costs. One of the potential approaches to avoid considerable increases in energy consumption is reducing the permeate flux from 20 to 10 or 5 L/m2 h. Although the decrease in permeate flux correspondingly increases the membrane surface area (i.e., the number of membrane elements) to maintain water production, it provides the ability to considerably suppress the transmembrane pressure and possibly reduce membrane fouling owing to the reduction in the amount of foulants transported toward the membrane surface. More importantly, the decreased permeate flux still maintains a NDMA rejection that is greater than the ESPA2 RO membrane (Fig. 3a). The feasibility of this high-rejection RO membrane under a lower permeate flux will be evaluated in our future study. 3.4. Conclusions The results of the present study indicated that this 4-inch HYDRApro RO membrane can achieve a 65–87% NDMA rejection over a feed water temperature of 13–34 °C and permeate flux of 20 L/m2 h. NDMA rejection was consistently lower than the rejection of 23 other TOrCs over the varied operating conditions. In addition, a linear correlation of NDMA rejection with the rejection of the 23 TOrCs using the highrejection RO membrane was observed. Therefore, this suggests that the high-rejection RO membrane can effectively ensure a high level of TOrC removal (≥65%) when NDMA is used as a surrogate indicator.
3.3. Full-scale implications This study suggested that NDMA, which is usually identified in the RO feed at concentrations higher than its detection level (1–2 ng/L), can act as a conservative surrogate indicator for ensuring a high level of TOrC removal upon RO treatment. Adopting the high-rejection (HYDRApro) RO membrane resulted in as high as 75–87% NDMA rejection at a typical permeate flux of 20 L/m2 h and feed temperature of 13–27 °C. This was greater than NDMA rejection by a conventional low-pressure RO membrane (ESPA2, Hydranautics, CA, USA) (26–47%), which was evaluated at similar operating conditions (permeate flux = 20 L/m2 h and feed temperature = 15–26 °C) using UFtreated wastewater (Fujioka et al., 2018b) (Fig. 3a). It is noted that ionic compositions and their concentrations between these studies can differ, because the conductivity of RO feed during the evaluation of HYDRApro (1922 μS/cm) was approximately twice that of ESPA2 (895 μS/cm). According to a previous study (Fujioka et al., 2012b), such a difference in conductivity does not cause a major difference in NDMA rejection when evaluated at the same permeate flux; thus, the comparison between the two tests can be considered valid.
Fig. 3. (a) NDMA rejection and (b) Transmembrane pressure (TMP) as a function of feed temperature of RO feed (UF-treated wastewater). The data by 4-inch HYDRApro (HY) and ESPA2 (ES) RO membrane elements were obtained from this study and a previous study (Fujioka et al., 2018b), respectively. Error bars show the range of two test samples.
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Acknowledgement This work was supported by JSPS KAKENHI Grant Number JP18H01572. The authors acknowledge Hydranautics for providing an RO membrane element. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.scitotenv.2019.134002. References Asano, T., Cotruvo, J.A., 2004. Groundwater recharge with reclaimed municipal wastewater: health and regulatory considerations. Water Res. 38, 1941–1951. Bellona, C., Drewes, J.E., Xu, P., Amy, G., 2004. Factors affecting the rejection of organic solutes during NF/RO treatment - a literature review. Water Res. 38, 2795–2809. Bellona, C., Drewes, J.E., Oelker, G., Luna, J., Filteau, G., Amy, G., 2008. Comparing nanofiltration and reverse osmosis for drinking water augmentation. J. AWWA 100, 102–116. Doederer, K., Farré, M.J., Pidou, M., Weinberg, H.S., Gernjak, W., 2014. Rejection of disinfection by-products by RO and NF membranes: influence of solute properties and operational parameters. J. Membr. Sci. 467, 195–205. Drewes, J.E., Khan, S.J., 2015. Contemporary design, operation, and monitoring of potable reuse systems. Journal of Water Reuse and Desalination 5, 1–7. Drewes, J., Khan, S., 2011. Chapter 16: Water reuse for drinking water augmentation. In: American Water Works Association, Edzwald, J.K. (Eds.), Water Quality & Treatment: A Handbook on Drinking Water, 6th edition McGraw-Hill Professional. Farré, M.J., Döderer, K., Hearn, L., Poussade, Y., Keller, J., Gernjak, W., 2011. Understanding the operational parameters affecting NDMA formation at advanced water treatment plants. J. Hazard. Mater. 185, 1575–1581. Fujioka, T., Khan, S.J., Poussade, Y., Drewes, J.E., Nghiem, L.D., 2012a. N-nitrosamine removal by reverse osmosis for indirect potable water reuse – a critical review based on observations from laboratory-, pilot- and full-scale studies. Sep. Purif. Technol. 98, 503–515. Fujioka, T., Nghiem, L.D., Khan, S.J., McDonald, J.A., Poussade, Y., Drewes, J.E., 2012b. Effects of feed solution characteristics on the rejection of N-nitrosamines by reverse osmosis membranes. J. Membr. Sci. 409–410, 66–74. Fujioka, T., Tanisue, T., Roback, S.L., Plumlee, M.H., Ishida, K.P., Kodamatani, H., 2017. Near real-time N-nitrosodimethylamine monitoring in potable water reuse via online high-performance liquid chromatography-photochemical reactionchemiluminescence. Environmental Science: Water Research & Technology 3, 1032–1036. Fujioka, T., Kodamatani, H., Takeuchi, H., Tanaka, H., Nghiem, L.D., 2018a. Online monitoring of N-nitrosodimethylamine for the removal assurance of 1,4-dioxane and other trace organic compounds by reverse osmosis. Environmental Science: Water Research & Technology 4, 2021–2028. Fujioka, T., Takeuchi, H., Tanaka, H., Kodamatani, H., 2018b. Online monitoring of Nnitrosodimethylamine rejection as a performance indicator of trace organic chemical removal by reverse osmosis. Chemosphere 200, 80–85. Fujioka, T., Kodamatani, H., Nghiem, L.D., Shintani, T., 2019. Transport of N-nitrosamines through a reverse osmosis membrane: role of molecular size and nitrogen atoms. Environmental Science & Technology Letters 6, 44–48. Khan, S.J., Fisher, R., Roser, D.J., 2019. Potable reuse: which chemicals to be concerned about. Current Opinion in Environmental Science & Health 7, 76–82. Kodamatani, H., Yamazaki, S., Saito, K., Amponsaa-Karikari, A., Kishikawa, N., Kuroda, N., Tomiyasu, T., Komatsu, Y., 2009. Highly sensitive method for determination of Nnitrosamines using high-performance liquid chromatography with online UV irradiation and luminol chemiluminescence detection. J. Chromatogr. A 1216, 92–98. Kodamatani, H., Roback, S.L., Plumlee, M.H., Ishida, K.P., Masunaga, H., Maruyama, N., Fujioka, T., 2018. An inline ion-exchange system in a chemiluminescence-based
analyzer for direct analysis of N-nitrosamines in treated wastewater. J. Chromatogr. A 1553, 51–56. Luo, Y., Guo, W., Ngo, H.H., Nghiem, L.D., Hai, F.I., Zhang, J., Liang, S., Wang, X.C., 2014. A review on the occurrence of micropollutants in the aquatic environment and their fate and removal during wastewater treatment. Sci. Total Environ. 473-474, 619–641. Merel, S., Anumol, T., Park, M., Snyder, S.A., 2015. Application of surrogates, indicators, and high-resolution mass spectrometry to evaluate the efficacy of UV processes for attenuation of emerging contaminants in water. J. Hazard. Mater. 282, 75–85. Narumiya, M., Nakada, N., Yamashita, N., Tanaka, H., 2013. Phase distribution and removal of pharmaceuticals and personal care products during anaerobic sludge digestion. J. Hazard. Mater. 260, 305–312. NRMMC, EPHC, AHMC, 2008. Australian Guidelines for Water Recycling: Managing Health and Environmental Risks (Phase 2): Augmentation of Drinking Water Supplies. Environment Protection and Heritage Council, National Health and Medical Research Council, Natural Resource Management Ministerial Council, Canberra. Plumlee, M.H., López-Mesas, M., Heidlberger, A., Ishida, K.P., Reinhard, M., 2008. Nnitrosodimethylamine (NDMA) removal by reverse osmosis and UV treatment and analysis via LC-MS/MS. Water Res. 42, 347–355. Rizzo, L., Malato, S., Antakyali, D., Beretsou, V.G., Đolić, M.B., Gernjak, W., Heath, E., Ivancev-Tumbas, I., Karaolia, P., et al., 2019. Consolidated vs new advanced treatment methods for the removal of contaminants of emerging concern from urban wastewater. Sci. Total Environ. 655, 986–1008. Sgroi, M., Roccaro, P., Oelker, G.L., Snyder, S.A., 2015. N-nitrosodimethylamine (NDMA) formation at an indirect potable reuse facility. Water Res. 70, 174–183. Shah, A.D., Mitch, W.A., 2012. Halonitroalkanes, halonitriles, haloamides, and Nnitrosamines: a critical review of nitrogenous disinfection byproduct formation pathways. Environmental Science & Technology 46, 119–131. Sharma, R.R., Agrawal, R., Chellam, S., 2003. Temperature effects on sieving characteristics of thin-film composite nanofiltration membranes: pore size distributions and transport parameters. J. Membr. Sci. 223, 69–87. Snyder, S.A., 2014. Emerging chemical contaminants: looking for greater harmony. Journal - American Water Works Association 106, 38–52. Steinle-Darling, E., Salveson, A., Sutherland, J., Dickenson, E., Hokanson, D., Trussell, S., Stanford, B., 2016. Direct Potable Reuse Monitoring: Testing Water Quality in a Municipal Wastewater Effluent Treated to Drinking Water Standards. Texas Water Development Board. SWRCB, 2015. California code of regulations, title 22: Social security, division 4: Environmental health. Chapter 3. Water Recycling Criteria. State Water Resources Control Board. Taheran, M., Brar, S.K., Verma, M., Surampalli, R.Y., Zhang, T.C., Valero, J.R., 2016. Membrane processes for removal of pharmaceutically active compounds (PhACs) from water and wastewaters. Sci. Total Environ. 547, 60–77. Tchobanoglous, G., Cotruvo, J., Crook, J., McDonald, E., Olivieri, A., Salveson, A., Trussell, R.S., 2015. Framework for Direct Potable Reuse. WateReuse Association, American Water Works Association, Water Environment Federation, National Water Research Institute, Alexandria, VA. Tsuru, T., Ogawa, K., Kanezashi, M., Yoshioka, T., 2010. Permeation characteristics of electrolytes and neutral solutes through titania nanofiltration membranes at high temperatures. Langmuir 26, 10897–10905. Verliefde, A.R.D., Cornelissen, E.R., Heijman, S.G.J., Verberk, J.Q.J.C., Amy, G.L., Van der Bruggen, B., van Dijk, J.C., 2008. The role of electrostatic interactions on the rejection of organic solutes in aqueous solutions with nanofiltration. J. Membr. Sci. 322, 52–66. Verliefde, A.R.D., Cornelissen, E.R., Heijman, S.G.J., Verberk, J.Q.J.C., Amy, G.L., Van der Bruggen, B., van Dijk, J.C., 2009. Construction and validation of a full-scale model for rejection of organic micropollutants by NF membranes. J. Membr. Sci. 339, 10–20. Wijmans, J.G., Baker, R.W., 1995. The solution-diffusion model: a review. J. Membr. Sci. 107, 1–21. Yangali-Quintanilla, V., Maeng, S.K., Fujioka, T., Kennedy, M., Amy, G., 2010. Proposing nanofiltration as acceptable barrier for organic contaminants in water reuse. J. Membr. Sci. 362, 334–345. Zeng, T., Plewa, M.J., Mitch, W.A., 2016. N-nitrosamines and halogenated disinfection byproducts in U.S. full advanced treatment trains for potable reuse. Water Res. 101, 176–186.
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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv
Short Communication
A surrogate-based approach for trace organic chemical removal by a high-rejection reverse osmosis membrane Takahiro Fujioka a,⁎, Haruka Takeuchi b, Hiroaki Tanaka b, Hitoshi Kodamatani c a b c
Graduate School of Engineering, Nagasaki University, 1-14 Bunkyo-machi, Nagasaki 85 2-8521, Japan Research Center for Environmental Quality Management, Kyoto University, 1-2 Yumihama, Otsu 520-0811, Japan Division of Earth and Environmental Science, Graduate School of Science and Engineering, Kagoshima University, Korimoto 1-21-24, Kagoshima 890-0065, Japan
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
• A high-rejection RO membrane achieved high NDMA rejection (65–87%) • The NDMA rejection values were consistently lower than 23 other TOrCs • NDMA rejection was highly correlated with TOrC rejection across varied conditions • NDMA can be a conservative indicator to ensure a high level of TOrC removal by RO
a r t i c l e
i n f o
Article history: Received 8 May 2019 Received in revised form 12 August 2019 Accepted 18 August 2019 Available online 19 August 2019 Editor: Ching-Hua Huang Keywords: CECs Micropollutants Potable reuse RO membrane TOrCs NDMA
a b s t r a c t Public confidence in the safety of recycled water for potable water reuse can be improved by providing assurance regarding high removal of trace organic chemicals (TOrCs) by reverse osmosis (RO) treatment. This pilot-scale study assessed the effectiveness of a surrogate indicator—N-Nitrosodimethlyamine (NDMA)—for ensuring a high level of TOrC removal by a high-rejection RO membrane. The pilot-scale tests showed that the rejection of 23 TOrCs by the high-rejection RO membrane was consistently greater than NDMA rejection. In addition, NDMA rejection was highly correlated with TOrC rejection across varied operating conditions, indicating that NDMA can be used as a conservative surrogate indicator for TOrC removal. The RO treatment at a permeate flux of 20 L/m2 h and feed temperature of 13–27 °C resulted in as high as 75–87% NDMA rejection, which was considerably greater than a conventional low-pressure RO membrane (26–47%). However, the high-rejection RO membrane required a transmembrane pressure that was greater than that of the low-pressure RO membrane. Despite this disadvantage, this study suggests that the high-rejection RO membrane can effectively ensure a high level of TOrC removal (≥65%) when NDMA is used as a surrogate indicator, which cannot be ensured by assessing conventional conductivity rejection. © 2019 Elsevier B.V. All rights reserved.
1. Introduction ⁎ Corresponding author. E-mail address: [email protected] (T. Fujioka).
https://doi.org/10.1016/j.scitotenv.2019.134002 0048-9697/© 2019 Elsevier B.V. All rights reserved.
Potable water reuse is a method of recycling wastewater for potable water. Potable water reuse has been implemented to address the
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shortages in water supplies that have occurred owing to prolonged droughts (Drewes and Khan, 2011; Tchobanoglous et al., 2015). The assurance of high recycled water quality is of paramount importance for potable water reuse because wastewater contains hazardous impurities such as pathogens and trace organic chemicals (TOrCs) (Luo et al., 2014; Taheran et al., 2016). These TOrCs include pharmaceuticals and personal care products, disinfection by-products (DBPs), pesticides, and endocrine-disrupting compounds (Snyder, 2014; Zeng et al., 2016). The TOrCs are known as potential toxins to humans (Asano and Cotruvo, 2004; Khan et al., 2019); thus, the concentrations of many TOrCs in drinking water or recycled water intended for potable water reuse have been regulated (NRMMC et al., 2008). Among advanced wastewater treatment processes, reverse osmosis (RO) treatment and ultraviolet (UV)-based advanced oxidation process (AOP) are capable of removing TOrCs as major barriers (Plumlee et al., 2008; Rizzo et al., 2019). It is noted that RO process also plays a role in rejecting a wide range of contaminants including ionic salts, heavy metals, and dissolved organics. The removal of TOrCs can be ensured through periodic water quality analyses (e.g., weekly or monthly) and continuous monitoring of surrogate or operating parameters (Drewes and Khan, 2015; Merel et al., 2015; Steinle-Darling et al., 2016). In California (USA), the AOP for potable water reuse is required to meet the following criteria for TOrC removal: (a) the removal of 1,4-dioxane greater or equal to 69% (0.5log) as a design criteria and (b) monitoring of the UV transmission as an operational parameter (SWRCB, 2015). In contrast, no design or operational criteria for TOrC removal has been established for RO treatment. Researchers (Fujioka et al., 2018a) have recently suggested the use of N-Nitrosodimethylamine (NDMA) as a surrogate indicator for TOrC removal by RO treatment. Suitable surrogate chemicals for TOrC removal must meet the following criteria: they are always present in the RO feed at high concentrations, they can be continuously monitored online, and the rejection is conservatively low compared with other targeted TOrCs. NDMA in the RO feed is typically identified at concentrations of N10 ng/L (Farré et al., 2011; Fujioka et al., 2012a), because NDMA is present in raw wastewater and NDMA concentration typically increases during advanced wastewater treatment due to its formation by prechloramination prior to RO process (Sgroi et al., 2015; Shah and Mitch, 2012). Additionally, NDMA can be monitored online using a newly developed NDMA analyzer, which includes high-performance liquid chromatography followed by a photochemical reaction and chemiluminescence (HPLC-PR-CL) (Fujioka et al., 2017; Kodamatani et al., 2018; Kodamatani et al., 2009). Many previous studies using NF and RO (Doederer et al., 2014) membranes identified that small and uncharged TOrCs show low rejections (Bellona et al., 2004; Verliefde et al., 2009). NDMA, which has a molecular weight of 74 Da, shows rejections lower than many other TOrCs upon a nanofiltration (NF) or RO process (Bellona et al., 2008; Yangali-Quintanilla et al., 2010). The major concerns associated with NDMA as a conservative surrogate indicator include: the consistency of NDMA rejection being lower than TOrC rejection and the assurance level for TOrC removal. Thus far, the validity of the surrogate-based approach has been demonstrated with a linear correlation between NDMA and only seven uncharged TOrCs including 1,4-dioxane across varied operating conditions in two previous studies (Fujioka et al., 2018a; Fujioka et al., 2018b), which do not fully support the validity of NDMA as a surrogate indicator. Moreover, the conventional low-pressure RO membranes used in these studies (EPSA2 RO membrane supplied by Hydranautics, CA, USA) at a permeate flux of 20 L/m2 h were capable of rejecting NDMA at only 15–45%, which is not sufficiently high to ensure the rejection of other TOrCs based on NDMA rejection. The low NDMA rejection by a conventional RO membrane and a small number of tested TOrCs remain a challenge for finding an advantage of the surrogate-based approach. This study aimed to evaluate the effectiveness of NDMA as a TOrC for ensuring a high level of TOrC removal by a high-rejection RO
membrane. The validity of NDMA as a surrogate indicator was examined by determining the correlation of rejection between NDMA and 23 TOrCs under different operating conditions. The feasibility of using the high-rejection RO membrane element was assessed by comparing the separation performance and permeability with a conventional low-pressure RO membrane element.
2. Materials and methods 2.1. Chemicals A total of 24 low molecular weight chemicals, including NDMA and three other N-Nitrosamines (N-Nitrosomethyelthylamine (NMEA), NNitrosopyrrolidine (NPYR), and N-Nitrosomorpholine (NMOR)) were evaluated as TOrCs in this study (Table 1 and S1). These chemicals have a broad range of molecular weight from 74 to 460 Da. They were primarily divided into two classes: neutral (≤50% ionized) or charged (N50% ionized). This was based on their dissociation levels at the tested pH of 7.5. The charged TOrCs were further categorized as positively or negatively charged compounds. A secondary effluent, which was collected after activated sludge process at a municipal treatment plant in Japan, was treated using an ultrafiltration (UF) membrane, and it was named UF-treated wastewater. The conductivity of the UF-treated wastewater was 1922 μS/cm.
2.2. Membrane and RO treatment system This study used a pilot-scale cross-flow RO treatment system holding a 4-inch HYDRApro 501–4040 RO membrane with an effective membrane area of 7.0 m2 (Hydranautics, Oceanside, CA, USA) (Fig. S1). The HYDRApro RO membrane, which was designed for industrial applications, was used as a high-rejection RO membrane. The system comprised of a 4-in. glass-fibre pressure vessel (ROPV, Nangang, China), 65-L stainless steel reservoir, a high-pressure pump (25NED15Z, Nikuni Co., Ltd., Kawasaki, Japan), digital flow meters (FDM, Keyence Co., Osaka, Japan), and digital pressure indicators (GPM, Keyence Co., Osaka, Japan). Feed solution temperature was controlled in the reservoir using a titanium heat exchanging pipe connected to a chiller unit (CA-1116A, Tokyo Rikakikai Co. Ltd., Tokyo, Japan).
2.3. Test protocols The membrane element was first rinsed with pure water to eliminate residual preservatives on the RO element. RO treatment was conducted at a constant cross-flow rate of 8.7 L/min by recirculating 50 L of the UF-treated wastewater. Rejection tests started by spiking TOrC stock solutions into the RO feed at approximately 100 ng/L (NDMA), 300 ng/L (NMEA, NPYR, and NMOR), and 100 μg/L (other TOrCs). The concentrations of TOrCs in the RO feed were determined to keep their RO permeate concentrations above the detection limits, so that their rejection were accurately calculated. The high TOrC concentrations in the RO feed may be a limitation of this study during the assessment of their rejection values. It is noted that NDMA concentration in the RO feed was determined to be lower than other N-Nitrosamine concentrations, because this study aimed to evaluate the validity of NDMA as a surrogate indicator using realistic concentrations of NDMA in treated wastewater. The system was operated for 18 h to stabilize the performance. Thereafter, operating conditions were adjusted stepwise at a permeate flux of 5–20 L/m2 h and a solution temperature of 13–34 °C. The varied permeate flux corresponded to a system recovery of 6–20%, which is equivalent to a 7–10% recovery from a single RO element at full scale. Samples for analysis were collected from the RO feed and permeate streams.
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Table 1 Physicochemical characteristics of the selected TOrCs. Compound
Structure
MW [Da]
MPAa,b [Å2]
LogD at pH 7.5a,c [−]
pKa (0 to 14)a [−]
Ionization at pH 7.5a [%]
Neutral NDMA NMEA NPYR NMOR Acetaminophen Ethenzamide Theophyline DEET
C2H6N2O C3H8N2O C4H8N2O C4H8N2O2 C8H9NO2 C9H11NO2 C7H8N4O2 C12H17NO
74.08 88.11 100.12 116.12 151.17 165.19 180.17 191.27
19.18 22.19 25.02 26.58 20.62 30.99 28.75 37.50
0.04 0.40 0.44 −0.18 0.90 1.56 −0.92 2.50
3.6 3.4 3.3 3.1 9.6 13.8 7.8 Not ionized
0 0 0 0 1 (−) 0 30 (−) 0
Positively charged Salbutamol Propranolol Atenolol Sulpiride Lincomycin
C13H21NO3 C16H2NO2 C14H22N2O3 C15H23N3O4S C18H34N2O6S
239.32 259.35 266.34 341.43 406.54
36.82 39.43 38.37 53.85 55.33
−1.23 0.45 −1.71 −0.61 −0.91
9.5, 10.2 9.8 9.8 8.5, 10.3, 13.7 8.1, 12.5, 13.1
99 (+) 99 (+) 99 (+) 91 (+) 80 (+)
Negatively charged Clofibric acid Nalidixic acid Sulfapyridine Ketoprofen Sulfathiazole Sulfamerazine Sulfadimidine Sulfamonomethoxine Sulfadimethoxine Tetracycline Oxytetracycline
C10H11ClO3 C12H12N2O3 C11H11N3O2S C16H14O3 C9H9N3O2S2 C11H12N4O2S C12H14N4O2S C11H12N4O3S C12H14N4O4S C22H24N2O8 C22H24N2O9
214.65 232.24 249.29 254.29 255.31 264.30 278.33 280.30 310.33 444.44 460.43
30.25 34.70 44.91 39.38 43.45 46.65 51.21 46.16 52.82 59.59 58.68
−0.53 −0.54 0.21 0.34 0.45 0.03 0.16 0.66 0.73 −3.87 −4.99
3.4 4.7, 5.8 0.5, 2.2, 6.3 3.9 7.0, 2.1 7.0, 2.0 7.0, 2.0 7.2, 2.6 6.9, 2.0 7.3, 7.9, 8.3 5.9, 7.3, 8.2, 9.2
100 (−) 98 (−) 94 (−) 100 (−) 77 (−) 74 (−) 74 (−) 67 (−) 78 (−) 58 (−), 26 (+/−) 53 (−), 24 (+/−)
a b c
Chemical properties: The information was obtained from ChemAxon (https://www.chemaxon.com/) on 26th March 2019. Minimum projection area is the area of the compound projection with the minimum plane of its circular disk, based on the van der Waals radius. LogD is the logarithm base 10 of the apparent water-octanol distribution coefficients at a specific pH, which represents a degree of its hydrophobicity.
2.4. Analytical techniques The concentrations of the four N-Nitrosamines were determined via the HPLC-PR-CL method (Kodamatani et al., 2018). The method detection limits of NDMA, NMEA, NPYR, and NMOR for a 200-μL injection
volume were 0.40, 0.54, 0.60, and 1.4 ng/L, respectively. Because the HPLC-PR-CL method enables to analyze only N-Nitrosamines, the concentrations of the other TOrCs were determined using ultraperformance liquid chromatography coupled with an atmospheric pressure ionization and tandem mass spectrometer (Waters, MA, USA)
Fig. 1. (a) Rejection values and (b) log rejection of TOrCs by the HYDRApro RO membrane at a feed temperature of (i) 34, (ii) 20, and (iii) 13 °C as a function of their minimum projection area (MPA). Permeate flux was 20 L/m2 h. Rejection values reported for N-Nitrosamines and other TOrCs are the average of duplicate and triplicate samples, respectively.
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(Narumiya et al., 2013). The conductivity and temperature of the RO feed water and permeate were also measured using conductivity meters (Orion Star™ A325, Thermo Fisher Scientific, MA, USA). 3. Results and discussion 3.1. Role of molecular size on rejection The rejections of the TOrCs were analyzed based on their minimum projection area (MPA) rather than their molecular weights, because the MPA of uncharged TOrCs has been reported to be better correlated with their rejections (Fujioka et al., 2019). The MPA of a compound represents a projected area of the chemical, which was calculated based on the van der Waals radius (Fig. S2). Minimum projection area can play an important role in TOrC rejection, because TOrCs can pass through the free volume holes of the membrane structure more freely when the clearance between the two-dimensional area of the chemical (i.e., MPA) and the area of free volume holes is larger. NDMA, which has the lowest MPA (19 Å2) of the selected TOrCs, showed the lowest rejection (65–87%) at feed temperatures of 13–34 °C (Fig. 1a). The rejection of uncharged TOrCs clearly increased according to the increased MPA when the rejection data was presented in log rejection (Fig. 1b). This was consistent with other operating conditions (i.e., permeate
fluxes of 5 and 10 L/m2 h and a feed water temperature of 13–34 °C) (Figs. S3 and S4). It is important to note that hydrophobic interactions between the membrane and TOrCs on rejection was not observed in this study, because all of the selected TOrCs with the exception of DEET are hydrophilic (LogD = b2.0). The results indicate that size exclusion is the dominant mechanism governing the rejection of small and uncharged TOrCs by the high-rejection RO membrane. Unlike the uncharged TOrCs, positively and negatively charged TOrCs showed similar log rejection at approximately N3-log rejection (i.e., N99.9%) across a wide range of MPA (Fig. 1b). The charged TOrCs were mostly larger than the uncharged TOrCs in molecular size; thus, the influence of electrostatic interactions on rejection was only evaluated between the positively and negatively charged TOrCs in this study. The electrostatic interaction between the charged TOrCs and negatively charged RO membrane surface could be explained by the repulsive (or attractive) force of the negatively (or positively) charged chemicals to the negatively charged RO membrane, which can enhance (or deteriorate) TOrC rejection (Verliefde et al., 2008). However, the difference of charge (i.e., positive and negative) on rejection was not apparent. The influence of electrostatic interactions on rejection may not be apparent when charged TOrCs were highly rejected at N99.9% by the high-rejection RO membrane. Overall, the results here indicate that NDMA can be a conservative surrogate indicator for ensuring the
Fig. 2. The rejection of TOrCs by the HYDRApro RO membrane as a function of NDMA rejection over the varied operating conditions (permeate flux = 5, 10, or 20 L/m2 h and feed water temperature = 13, 20, 27, or 34 °C): (a) N-Nitrosamines (uncharged TOrCs), (b) other uncharged TOrCs, and (c) positively and (d) negatively charged TOrCs. The dot line is the equality line with a slope of 1.0.
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removal of TOrCs by the high-rejection RO membrane regardless of their charge properties. 3.2. Correlation between NDMA and TOrCs The validity of NDMA as a conservative indicator for TOrC removal was further evaluated based on the correlation of NDMA rejection with the rejection of other TOrCs over a permeate flux of 5–20 L/m2 h and feed water temperatures of 13–34 °C. In general, the rejection of TOrCs increased with increasing permeate flux (Fig. S5) or decreasing feed temperature (Fig. S6). When permeate (water) flux, which varies depending on the position of RO elements vessel at a full-scale plant, increases, solute rejection also increases because solute flux remains almost constant regardless of the change in permeate (water) flux (Wijmans and Baker, 1995). Solute rejection at a constant permeate flux can increase with decreasing RO feed temperature, which varies seasonally, because solute flux decreases according to the decreased diffusivity of solutes induced by lower temperature (Sharma et al., 2003; Tsuru et al., 2010). The results showed that NDMA rejection (34–87%) was highly correlated with the rejection of the uncharged TOrCs, as indicated by the Pearson correlation coefficients (r) of 0.95–0.96 (N-Nitrosamines) and 0.70–0.94 (other uncharged TOrCs) (Fig. 2a and b). A lower correlation was determined with the charged TOrCs according to the Pearson correlation coefficients (r) of 0.67–0.87 (positively charged TOrCs) and 0.84–0.92 (negatively charged TOrCs) (Fig. 2c and d). Because the rejection of these charged TOrCs was typically above 99.8%, the lower correlation was likely owing to the low TOrC concentrations in the RO permeate, which are unable to be accurately determined. Despite the limitation, the results here still support that NDMA can be used as the surrogate indicator for TOrC removal by the high-rejection RO membrane. Similar to NDMA, the conductivity rejection (98.6–99.8%) was also highly correlated with the TOrC rejection (Pearson correlation coefficient (r) = 0.59–0.94) (Fig. S7). The advantages of using NDMA over conductivity include: (a) its relevance to TOrCs, (b) its sensitivity to varying RO treatment conditions; and (c) its conservative rejection value for TOrC rejection. In fact, the conductivity rejection was higher than some of the uncharged TOrCs (NMEA, NPYR, NMOR, and acetaminophen). Overall, the results here indicate the potential of NDMA as a conservative indicator for the removal of TOrCs with a high correlation and high sensitivity. It is important to note that this pilot study did not demonstrate the influence of membrane fouling, chemical cleaning, or membrane aging on the correlation; thus, further long-term investigations on site are necessary in a future study.
5
However, the transmembrane pressure required for the 4-inch HYDRApro RO membrane was over three times greater than that of the ESPA2 RO membrane (Fig. 3b). This higher transmembrane pressure indicates a higher energy consumption; thus, the use of a high-rejection RO membrane can result in an increase in operating costs. One of the potential approaches to avoid considerable increases in energy consumption is reducing the permeate flux from 20 to 10 or 5 L/m2 h. Although the decrease in permeate flux correspondingly increases the membrane surface area (i.e., the number of membrane elements) to maintain water production, it provides the ability to considerably suppress the transmembrane pressure and possibly reduce membrane fouling owing to the reduction in the amount of foulants transported toward the membrane surface. More importantly, the decreased permeate flux still maintains a NDMA rejection that is greater than the ESPA2 RO membrane (Fig. 3a). The feasibility of this high-rejection RO membrane under a lower permeate flux will be evaluated in our future study. 3.4. Conclusions The results of the present study indicated that this 4-inch HYDRApro RO membrane can achieve a 65–87% NDMA rejection over a feed water temperature of 13–34 °C and permeate flux of 20 L/m2 h. NDMA rejection was consistently lower than the rejection of 23 other TOrCs over the varied operating conditions. In addition, a linear correlation of NDMA rejection with the rejection of the 23 TOrCs using the highrejection RO membrane was observed. Therefore, this suggests that the high-rejection RO membrane can effectively ensure a high level of TOrC removal (≥65%) when NDMA is used as a surrogate indicator.
3.3. Full-scale implications This study suggested that NDMA, which is usually identified in the RO feed at concentrations higher than its detection level (1–2 ng/L), can act as a conservative surrogate indicator for ensuring a high level of TOrC removal upon RO treatment. Adopting the high-rejection (HYDRApro) RO membrane resulted in as high as 75–87% NDMA rejection at a typical permeate flux of 20 L/m2 h and feed temperature of 13–27 °C. This was greater than NDMA rejection by a conventional low-pressure RO membrane (ESPA2, Hydranautics, CA, USA) (26–47%), which was evaluated at similar operating conditions (permeate flux = 20 L/m2 h and feed temperature = 15–26 °C) using UFtreated wastewater (Fujioka et al., 2018b) (Fig. 3a). It is noted that ionic compositions and their concentrations between these studies can differ, because the conductivity of RO feed during the evaluation of HYDRApro (1922 μS/cm) was approximately twice that of ESPA2 (895 μS/cm). According to a previous study (Fujioka et al., 2012b), such a difference in conductivity does not cause a major difference in NDMA rejection when evaluated at the same permeate flux; thus, the comparison between the two tests can be considered valid.
Fig. 3. (a) NDMA rejection and (b) Transmembrane pressure (TMP) as a function of feed temperature of RO feed (UF-treated wastewater). The data by 4-inch HYDRApro (HY) and ESPA2 (ES) RO membrane elements were obtained from this study and a previous study (Fujioka et al., 2018b), respectively. Error bars show the range of two test samples.
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