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Measurements of dissolved organic nitrogen (DON) in water samples with nanofiltration pretreatment
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Measurements of dissolved organic nitrogen (DON) in water samples with nanofiltration pretreatment Bin Xu a, Da-Peng Li a, Wei Li a, Sheng-Ji Xia a, Yi-Li Lin b,*, Chen-Yan Hu c, Cao-Jie Zhang a, Nai-Yun Gao a a
State Key Laboratory of Pollution Control and Resources Reuse, Key Laboratory of Yangtze Aquatic Environment, Ministry of Education, College of Environmental Science and Engineering, Tongji University, Shanghai 200092, PR China b Department of Safety, Health and Environmental Engineering, National Kaohsiung First University of Science and Technology, Kaohsiung 811, Taiwan c College of Energy and Environment Engineering, Shanghai University of Electric Power, Shanghai 200090, PR China
article info
abstract
Article history:
Dissolved organic nitrogen (DON) measurements for water samples with a high dissolved
Received 27 January 2010
inorganic nitrogen (DIN, including nitrite, nitrate and ammonia) to total dissolved nitrogen
Received in revised form
(TDN) ratio using traditional methods are inaccurate due to the cumulative analytical
11 June 2010
errors of independently measured nitrogen species (TDN and DIN). In this study, we
Accepted 14 June 2010
present a nanofiltration (NF) pretreatment to increase the accuracy and precision of DON
Available online 22 June 2010
measurements by selectively concentrating DON while passing through DIN species in water samples to reduce the DIN/TDN ratio. Three commercial NF membranes (NF90,
Keywords:
NF270 and HL) were tested. The rejection efficiency of finished water from the Yangshupu
Dissolved organic nitrogen (DON)
drinking water treatment plant (YDWTP) is 12%, 31%, 8% of nitrate, 26%, 28%, 23% of
Nanofiltration
ammonia, 77%, 78%, 82% of DOC (dissolved organic carbon), and 83%, 87% 88% of UV254 for
DON retention
HL, NF90 and NF270, respectively. NF270 showed the best performance due to its high DIN
Polluted surface water
permeability and DON retention (w80%). NF270 can lower the DIN/TDN ratio from around 1
Drinking water treatment
to less than 0.6 mg N/mg N, and satisfactory DOC recoveries as well as DON measurements in synthetic water samples were obtained using optimized operating parameters. Compared to the available dialysis pretreatment method, the NF pretreatment method shows a similar improved performance for DON measurement for aqueous samples and can save at least 20 h of operating time and a large volume of deionized water, which is beneficial for laboratories involved in DON analysis. DON concentration in the effluent of different treatment processes at the YDWTP and the SDWTP (Shijiuyang DWTP) in China were investigated with and without NF pretreatment; the results showed that DON with NF pretreatment and DOC both gradually decreased after each water treatment process at both treatment plants. The advanced water treatment line, including biological pretreatment, clarification, sand filtration, ozone-BAC processes at the SDWTP showed greater efficiency of DON removal from 0.37 to 0.11 mg N L1 than that at the YDWTP, including pre-ozonation, clarification and sand filtration processes from 0.18 to 0.11 mg N L1. ª 2010 Elsevier Ltd. All rights reserved.
* Corresponding author. Tel.: þ886 7 6011000x2328; fax: þ886 7 6011061. E-mail address: [email protected] (Y.-L. Lin). 0043-1354/$ e see front matter ª 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2010.06.034
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1.
Introduction
Natural waters contain significant levels of dissolved inorganic nitrogen (DIN, including nitrate, ammonium, and nitrite) as well as dissolved organic nitrogen (DON). Typical DON concentrations in surface waters vary, ranging from less than 0.1 to larger than 10 mg N L1, with a median value of approximately 0.3 mg N L1 (Westerhoff and Mash, 2002). Because DON comprises a relatively small portion of the dissolved organic matter (DOM), researchers tend to ignore DON and assume that dissolved organic carbon (DOC), nitrate and ammonium play a more important role in water treatment processes (Westerhoff and Mash, 2002). However, there are increasing concerns regarding DON in water treatment processes recently due to its preferential reaction with oxidants/disinfectants (e.g., chlorine, chloramines, ozone (O3) and potassium permanganate) to form nitrogenous disinfection by-products (N-DBPs) (e.g., nitrosamines including Nnitrosodimethylamine [NDMA], halonitromethanes, and haloacetonitriles) (Westerhoff and Mash, 2002; Richardson, 2003; Mitch et al., 2003). Toxicity results indicate that N-DBPs show much stronger carcinogenicity or mutagenicity than that of the regulated DBPs, such as trihalomethanes (THMs) and haloacetic acids (HAAs) (Richardson et al., 2007; Plewa et al., 2004; Lee et al., 2007). The presence of DON also contributes to membrane fouling in water treatment processes (Her et al., 2000). Despite considerable information detailing DOC characteristics (such as structures and specific functional groups) and fate during water treatment processes (Bruggen and Vandecasteele, 2003; Her et al., 2000; Lin et al., 2007; Richardson et al., 2007; Solinger et al., 2001), little is known about DON. It is important to use available analytical methods for DON determination to investigate its fate, properties and reactivity in waters and water treatment processes. The DON concentration in water samples cannot be quantified directly but must be determined by subtracting the sum of DIN species from the total dissolved nitrogen (TDN). Because DON only contributes a relatively small portion of the total mass of TDN in humanimpacted surface waters (Perakis and Hedin, 2002), and DON is the residual concentration after subtracting a much larger concentration (DIN species) from the measured TDN (see Eq. (1)), the accuracy of DON measurements is subject to significant cumulative analytical errors of each independently measured nitrogen species (TDN, nitrite, nitrate and ammonia). þ DON ¼ TDN DIN ¼ TDN (NO 3 þ NO2 þ NH3/NH4 )
(1)
Negative DON concentrations were reported in practical measurements in the case of high DIN/TDN ratios (Vandenbruwane et al., 2007; Solinger et al., 2001; Lee and Westerhoff, 2005). Lowering the DIN/TDN ratio in water samples is necessary to improve the accuracy of DON measurements. Lee and Westerhoff (2005) reported a dialysisbased pretreatment to reduce DIN species in bulk water. They found that 70% of DIN was removed, and more than 95% of DON recoveries were obtained for surface and finished water after 24 h dialysis using a cellulose ester dialysis tube. Although dialysis pretreatment leads to a more accurate DON
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determination for surface and drinking waters, the method may not be applicable for wide use because it is not time and cost effective. Catalytic reduction of nitrate is another way to lower the DIN/TDN ratio (Ambonguilat et al., 2006). However, DON and DOC can be adsorbed onto the catalyst and will likely compete with nitrate for active catalytic sites. Extensive testing has indicated that catalytic reduction of nitrate is not suitable for DON determination (Ambonguilat et al., 2006). Another possible method for reducing the DIN/TDN ratio is to increase the DON concentration in combination with complete or partial DIN removal by means of membrane separation. Reverse osmosis (RO) is a popular technology for DOM concentration and isolation in surface waters, with overall DOC retention close to 100% (Croue´, 2004). However, a lower DIN/TDN ratio cannot be achieved because DIN concentrates along with the organics and remains in the concentrated solution. Therefore, it is more practical to find a suitable separation process that allows DIN pass through freely and still retains most organics, including DON. A nanofiltration (NF) membrane has properties that lie somewhere between ultrafiltration (UF) and RO. NF has been shown to be effective for removing DOC and DBPs precursors and controlling salt passage with a different membrane (Teixeira and Rosa, 2006; Bruggen and Vandecasteele, 2003; Lin et al., 2006). The permeate flux of an NF membrane is based on the applied transmembrane pressure, membrane resistance, feed water temperature, and the composition of feed water. The selectivity of a membrane is due to the combined factors of membrane characteristics, solution properties, and operating conditions (Ahn et al., 1999; Nghiem et al., 2005; Garba et al., 2000; Lin et al., 2007). The major rejection mechanisms for NF membranes include molecular sieve effects and electrostatic effects. Although the molecular weight cutoff (MWCO) of commercial NF membranes is around 200 Da (Ahn et al., 1999; Nghiem et al., 2005; Garba et al., 2000; Lin et al., 2007), molecules below the MWCO of a membrane may also be rejected by the electrical attraction or repulsion of charged species onto a charged membrane surface. The assumption of this study is that NF pretreatment will retain most of the DON in the concentrate while allowing DIN species (nitrate, nitrite and ammonia) to pass through into the permeate. As a result of increasing DON concentration and decreasing the DIN/TDN ratio, a more accurate DON measurement in water samples with high DIN/TDN ratios can be obtained (Lee and Westerhoff, 2005). Because NF separation performance is significantly affected by membrane characteristics (such as MWCO, pore size distribution, charge and hydrophobicity), solution properties (such as solution composition, ionic strength and pH), and operating parameters (such as applied pressure and permeate flux), factors such as membrane type, solution pH, and the initial ratio of sample volume to NF surface area (R) were studied to determine the critical parameters needed to optimize NF pretreatment performance. The objectives of this study were: (1) to investigate the feasibility of NF pretreatment for DON measurement using different commercial NF membranes; (2) to optimize the operating parameters for the NF pretreatment system; and (3) to measure the DON concentration in raw water and the effluent of different treatment processes in two
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drinking water treatment plants (Yangshupu drinking water treatment plant [YDWTP] and Shijiuyang drinking water treatment plant [SDWTP]) to validate the proposed NF pretreatment method as well as to evaluate the performance of each plant.
2.
Materials and methods
2.1.
NF pretreatment system
The NF membranes selected in this study were thin-film composite polyamide membranes, including NF90, NF270 (Dow/Filmtec, USA) and HL (GE Osmonics, USA). According to the manufacturers, each of the 3 membranes has a thin layer of polyamide on a microporous polysulfone support layer and is negatively charged at neutral pH. Although these polyamide layers have similar chemical compositions, the membranes show different characteristics due to unknown surface modifications (Nghiem, 2005; Boussu, 2007; Xu et al., 2005). The properties of the 3 selected membranes are summarized in Table 1. The membranes were received as flat sheets and were cut and stored in regularly replaced Mill-Q (Millipore) water at 4 C. Before proceeding with any experiments, the membranes were rinsed thoroughly with Mill-Q water until no UV absorbance at 254 nm (UV254) was found in the permeate, and DOC was less than 0.1 mg C L1. Two laboratory-scale dead-end filtration test units were used in this study. The experiments were conducted in duplicate in two 300 mL solvent-resistant stirred cells (Millipore, USA). The filter diameter was 75 mm with a filtration area of 40 cm2. A polytetrafluoroethene (PTFE) stirring bar was equipped at the center of the borosilicate glass cylinder. Stirrer speed was controlled by a magnetic stirrer to minimize the effects of concentration polarization. The NF system was pressurized by connecting it to a high purity nitrogen gas cylinder, and the maximum pressure was restricted to 0.62 MPa (70 psi) according to the manufacturer’s instructions.
Table 1 e Properties of NF membranes used in this study. Membrane type Manufacturer Water permeability (L m2 h1 bar1) MWCO (Daltons) Contact angle ( ) Zeta potential (mV) MgSO4 rejection (%) (manufacturer) NaCl retention (%) (manufacturer) a b c d e f
NF90
NF270
HL
DOW/Filmtec 6.4a
DOW/Filmtec 13.5a
GE Osmonics 11.4b
200c 42.5a 21.6c,e >97
170e300b,d 55.0a 19b,d,f >97
150e300b 47b 14b,f >96
85
40
e
Literature (Nghiem, 2005). Literature (Boussu, 2007). Literature (Xu et al., 2005). Literature (Lin et al., 2007). Zeta potential measured at pH 6 in 0.01 M NaCl solution. Zeta potential measured at pH 7 in 0.01 M KCl solution.
The applied pressure and initial ratio of sample volume to NF membrane surface area (R) was 0.5 MPa and 7.5 cm3/cm2, respectively, for pretreatment operation unless otherwise stated. Blank and control tests using Mill-Q water (pH 3e9) with and without the chosen NF membranes in the filtration system demonstrated that there were no organics and DIN leached from the clean NF membranes or the filtration unit and pipe fittings.
2.2.
Description of challenge waters
2.2.1.
Synthetic water
Synthetic water containing 5 mg C L1 humic acid (HA, measured by DOC), 0.1 mg N L1 nitrite, 1.0 mg N L1 nitrate, and 0.5 mg N L1 ammonia was used to test the performance of the chosen NF membranes to sift out the one with the most organic matter but the least DIN retention. A guaranteed grade of reagents KNO3, NaNO2, NH4Cl, NaOH and HCl and HA (C 60.3%; N 0.1%; H 5.4%, extracted from soil) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) and were not purified further. Commercial concentrated HA and DIN species were dissolved separately into ultra-pure water produced from a Milli-Q (Millipore, USA) water purification system to create the solution mentioned above, and the prepared solution was filtered with pre-washed 0.45 mm cellulose acetate membranes (Anpel Co. Ltd, Shanghai, China) before use.
2.2.2. Finished water from the Yangshupu drinking water treatment plant (YDWTP) The finished water from the YDWTP was also used to test the performance of the 3 chosen NF membranes to sift out the one with the most organic matter but the least DIN retention and to evaluate its effectiveness for reducing the DIN/TDN ratio under the influence of a water matrix.
2.3.
Performance of NF membranes
The best-performing NF membrane was then selected for further experiments using the finished water from the YDWTP to optimize the operating parameters of R (varying from 2.5, 5, 7.5, 12.5 to 25), feed water pH (varying from 3 to 10), and applied transmembrane pressure (varying from 0.2 to 0.6 MPa). The NF pretreatment system was kept running with applied pressure until the remaining solution (retentate) in the cell was around 50 mL. The process was completed in 1e4 h depending on the volume of feed solution, applied pressure, and composition of the feed solution. The permeate was collected using a beaker, and the volume of permeate was recorded precisely after each filtration experiment. Collected retentate and permeate were stored at 4 C for further analysis. Assuming that there is a negligible amount of DON in the permeate or adsorbed onto the selected NF membranes (further explanation shown in Section 3.1), the DON concentration of samples with NF pretreatment can be calculated according to the mass balance of DON: CF ¼
CR VR þ CP VP CR ðVF VP Þ z VF VF
(2)
where CF is the DON concentration in the feed water, VF is the volume of feed water; CR is the DON concentration in the
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retentate, VR is the volume of retentate (VR ¼ VF VP, around 50 mL), CP is the DON concentration in the permeate, and VP is the exact volume of collected permeate.
2.4.
Validation and application of NF pretreatment
Twenty seven grab water samples from the effluent of different treatment processes in the YDWTP and SDWTP, respectively, were collected to compare the DON measurements with and without NF pretreatment. The YDWTP has a capacity of 1,480,000 CMD (cubic meters per day) and is located close to Shanghai Bund, which supplies approximately 1/4 of the drinking water consumed by Shanghai, which has a population of 13.8 million. The source water is delivered upstream from the Huangpu River (30.97 N, 121.30 E) by a 40 km tunnel. The characteristics of the raw water have been presented elsewhere (Xu et al., 2007). There are seven conventional treatment lines in the YDWTP, including coagulation, sedimentation and sand filtration processes. An advanced treatment line (320,000 CMD) with pre-O3, high rate clarification, filtration, post-O3 and biological activated carbon (BAC) filtration is also used in this plant. The last two processes were under reconstruction during sampling and therefore were not sampled. The SDWTP has a capacity of 170,000 CMD with source water taken from the Xinchengtang River located in Jiaxi City, Zhejiang Province, China. There are several treatment processes in the SDWTP, including biological aerated filters, clarification, filtration, post-O3, BAC filtration, and post-chlorination to produce drinking water. Grab samples were collected in polypropylene containers, packed in ice and shipped to the laboratory where they were filtered through pre-washed 0.45 mm cellulose acetate membranes (Anpel Co. Ltd, Shanghai, China) to eliminate suspended solids and then kept at 4 C in the dark. The pH and conductivity of all samples were found to be in the range 6.9e7.3 and 221e480 mS/cm, respectively. DOC, UV254, DIN, and DON concentrations were measured for each sample with and without NF pretreatment. Note that DOC and UV254 were used as surrogate parameters to evaluate the loss of organics because the DON concentration of synthetic and field water samples varied greatly before NF pretreatment due to the high DIN/TDN ratios in the samples.
2.5.
Analytical methods
DOC and TDN were measured using a Shimadzu TOC-VCSH analyzer with a TNM-1 TN unit (Shimadzu, Japan). Nitrate and nitrite were measured by ion chromatography (Dionex ICS1000, AS14 column, USA). Nitrite concentrations of all the surface water and finished water tested in this study were below the method detection limit (MDL) (0.005 mg N L1). The MDL of nitrate was 0.005 mg N L1. Ammonia was measured using the colorimetric method with a Nessler reagent (APHA, 1992) on a UNICO 4802 UVeVis spectrophotometer (Shanghai UNICO, China) with a method detection limit (MDL) of 0.05 mg N L1. DON was then calculated by subtracting the value of DIN from TDN. The UV absorbance of the HA solution, as well as samples from the YDWTP and the SDWTP, were analyzed at a wavelength of 254 nm using a 1 cm quartz cell by a UNICO 4802
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UVeVis spectrophotometer (Shanghai UNICO, China). Solution pH was measured using a pH-meter (module PHS-3B, Shanghai LEICI Analysis Instrument Factory, China), which was calibrated on a regular basis. The measurements were carried out in duplicate for each sample, and more replications were executed in cases where the variation between each measurement exceeded 5%.
3.
Results and discussion
3.1.
Effect of membrane type
When choosing a suitable NF membrane with low DIN retention but high organic retention for the pretreatment method, it is essential to evaluate the distribution of DIN species and DOC/UV254 with different NF membranes. Experiments were performed using HL, NF90 and NF270 to treat both the synthetic water and the finished water from the YDWTP. A mass balance of each DIN compound and the DOC/ DON ratio were calculated using permeate and retentate concentrations to determine whether the compound was lost onto the membrane surface and/or into the membrane pores. With a transmembrane pressure of 0.5 MPa and R of 7.5 cm3/cm2, the mass percentages of nitrite, nitrate, ammonia and HA in the retentate and permeate after NF pretreatment are shown in Fig. 1A. Low molecular weight ions of nitrite and nitrate were found in a mass percentage of 30e60% in the retentate and 40e70% in the permeate, respectively (Fig. 1A). Negligible adsorption loss (<4%) was found for both nitrite and nitrate for all 3 tested NF membranes. The tighter NF90 had the highest retention rate for both nitrite and nitrate, while the looser NF270 had the lowest retention of 42.5% and 33.2% for nitrite and nitrate, respectively. The results were similar to those of Paugam et al. (2004) which were obtained with higher nitrate concentrations of feed solution. Approximately 50% of the ammonia was not recovered, which may have been due to the adsorption of ammonia onto/into the membrane surface or due to degassing and high back-pressure depending on the pH of the solution. Only 20% of the ammonia was left in the retention solution, which is beneficial for DON measurements because the decrease of ammonia in the retentate leads to a lower DIN/ TDN ratio. HA is a mixture of organic chemicals with a board molecular weight range and is always seen as a model for natural organic matter (Tang et al., 2007). Fig. 1A shows that no HA was detected (in mg C L1) in the permeate for all membranes. The mass percentage of HA in the retentate for HL, NF90, NF270 was 83%, 67% and 89%, respectively, which means that there was some mass loss for HA during the NF pretreatment. A possible explanation for the loss of HA is adsorption onto/into the membrane surface (Tang et al., 2007). NF270 has a semi-aromatic piperazine based polyamide layer (Lin et al., 2007) and shows the highest water permeability (Table 1) and lowest organics adsorption loss among the 3 membranes tested, as well as fairly high organic rejection and low retention of DIN species (Fig. 1A). Fig. 1B shows the results of experiments using the finished water from the YDWTP with the same operation conditions as mentioned above. Much less nitrate retention was observed
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100
Mass percent (%)
80
60 HL-R HL-P NF90-R NF90-P NF270-R NF270-P
40
20
0 Nitrite
B
Nitrate
Ammonia Humic acid
100
Mass percent (%)
80
60 HL-R HL-P NF90-R NF90-P NF270-R NF90-P
40
20
0
Nitrate
Ammonia
DOC
UV254
Fig. 1 e NF pretreatment for lab synthetic solutions (A) as well as a finished water samples from YDWTP (B) with different NF membranes. (A) Initial concentration of nitrite, nitrate, ammonia, and HA were 0.1, 1.0, 0.5 mg N LL1 and 5 mg C LL1, respectively. (B) Finished water from YDWTP with initial concentration of nitrate, ammonia, DOC, and UV254 as 2.51, 0.52 mg of N LL1, 3.99 mg C LL1 and 0.077 cmL1, respectively (R refers to retentate, and P refers to permeate in the figure legend).
Vandenbruwane et al. (2007), who reported an average of 16 14% DOM loss in surface water samples during dialysis pretreatment (Vandenbruwane et al., 2007). To validate the pretreatment method and evaluate the precision of DON measurement, DON concentrations were analyzed with and without NF pretreatment, as shown in Fig. 2. Because the DON concentration is fairly low in finished water, and the DIN/TDN ratio was as high as 0.95, the standard deviation of DON measurements was great, and negative concentration values were measured. The high standard deviation is related to the variances in DIN and TDN measurements (Lee and Westerhoff, 2005). Previous research has indicated that samples with a DIN/TDN ratio greater than 0.7 have high analytical variations, which increases as the DIN/TDN ratio increases (Lee and Westerhoff, 2005). On the other hand, no negative concentrations were measured after any one of the NF pretreatments. As shown in Fig. 2, the highest DON concentration was found for the pretreatment using NF270, which is related to satisfactory DOM retention and the lowest DOM lost to membrane adsorption as mentioned above. Because NF90 showed the highest DIN rejection and DOM adsorption in Fig. 1, most DIN would remain in the retention solution (DIN/TDN ratio around 0.90) and would affect the DON measurement accordingly. Consequently, the lowest DON concentration and largest standard deviation were observed when using NF90. Because DIN/TDN ratios were lowered using HL and NF270, the standard deviations were fairly low for both of these DON measurements. The results are in accordance with the reports from Vandenbruwane et al. (2007) and Lee and Westerhoff (2005), which indicated that the standard deviation of DON measurement depends greatly on the DIN/TDN ratio. Because the HL membrane showed lower DOC retention and more adsorption loss than that of NF270 (Fig. 1A), the DON concentrations appeared to be lower. The above findings suggest that NF pretreatment is a potential way to increase the precision of DON measurements in water samples with high DIN/TDN ratios. High DOM recovery and low DIN/TDN ratio in the retentate can be
1.2
DIN/TDN
1
100
-1
D ON concentration (mg N L )
120
DON (R=7.5) DON (R=12.5)
compared to that in the synthetic water filtration experiments, which can be explained by the Donnan effect (Garcia et al., 2006). Because the divalent anions in the finished water can be retained almost 100% by the NF membranes, the monovalent anions (particularly nitrate ions) are forced to pass through the membrane to assure the electroneutrality of the permeate (Garcia et al., 2006). The mass percentage of ammonia was similar to that of the synthetic water tests for the 3 chosen NF membranes. We were unable to account for approximately 50% of the ammonia. Recoveries of 76e81% and 83e88% for DOC and UV254, respectively, were observed in the retentate. NF270 shows the highest DOC and UV254 retention and the lowest organic adsorption loss. Because DOC has traditionally been used as a surrogate for DOM, DOC recoveries were used as an indicator for DOM losses with respect to DON. The DOC recoveries in this study were comparable to those of
0.8
80
0.6
60
0.4
40
0.2
20
0
-0.2
D IN/TD N (%)
A
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0
NP
HL
NF90
NF270
-20
Fig. 2 e Comparison of measured DON concentration and DIN/TDN ratio (A, :, -) for finished water in YDWTP with (n [ 3) and without (n [ 4) NF pretreatment using HL, NF90 and NF270, respectively, with R [ 7.5 and 12.5 (NP refers to without pretreatment).
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achieved after pretreatment by using suitable NF membranes, which is much more practical than the time-consuming and costly dialysis method. To examine the effects of operating parameters of this NF pretreatment on DON measurements, NF270, which had the best performance, was selected for further studies.
3.2. Effects of initial sample volume to membrane surface area ratio (R) Because R may influence DIN permeate, DOC retention and analysis time, it is essential to study the effect of this parameter on NF pretreatment. Experiments were conducted at different feed volumes ranging from 100 to 1000 mL according to the set R value (from 2.5 to 25 cm3/cm2). Fig. 3A shows the calculated DON and DIN/TDN ratios after NF pretreatment as a function of R, as well as the mass balance of DIN species, DOC, and UV254 in retention solutions. The DOC and UV254 rejections are quite stable in the retentions (>80%) when R is less than 12.5 cm3/cm2 (shown in Fig. 3A). Slight decreases of DOC and UV254 retention were observed when R was 25 cm3/cm2 due to a greater concentration of organics concentrated near the membrane surface that increased the concentration gradient across the membrane (the effect of concentration polarization) (Lin et al., 2006). It is interesting that the percentage of nitrate and ammonia retained tends to decrease as R increases, which results in the decrease of the DIN/TDN ratio for R between 2.5 and 12.5 cm3/cm2. The minimum DIN/TDN ratio (0.5) is reached when R is 12.5 cm3/ cm2, then the ratio begins to rise when R increases to 25 cm3/ cm2. The trend for the DON measurements is reversed, as the DIN/TDN ratio increases and R decreases. These findings are in accordance with the results of Lee and Westerhoff (2005), who found that the calculated DON concentration slightly decreased as DIN/TDN ratios increased in surface water spiked with different DIN concentrations. As mentioned above, the goal of NF pretreatment is to lower the DIN/TDN ratio and enrich DON by passing the DIN species through the NF membrane while retaining DOM. Therefore, the recommended value for R should be between 7.5 and 12.5 cm3/cm2 for NF pretreatment using NF270.
3.3.
Effects of solution pH
The solution pH can have a significant effect on membrane surface charge and is responsible for electrostatic repulsion causing the separation of charged solutes (Nghiem, 2005). Organics in water also can vary from neutral to charged species according to the solution pH and their pKa values (Lin et al., 2007). Thus, the effect of pH was evaluated in the NF pretreatment system using the finished water from the YDWTP. Fig. 3B shows the retention of DIN species, DOC and UV254 by NF270 membranes as well as the calculated DIN/TDN ratio and DON concentration as a function of pH. The results show a minimum of nitrate retention for pH 5, and the minimum and maximum retention of ammonia was observed at pH 4 and pH 10, respectively. Because the isoelectric point (ISP) of NF270 is around pH 4 (Boussu, 2007; Lin et al., 2007), the sieve effect for NF270 at pH 4 would help to retain solutes from
Fig. 3 e Effects of R (A), pH (B), and transmembrane pressure (C) on DON measurements with NF pretreatment using NF270 and the finished water from YDWTP. (A) Initial concentration of nitrate, ammonia, DOC, and UV254 were 2.71, 0.48 mg N LL1, 4.4806 mg C LL1 and 0.082 cmL1, respectively; (B) Initial concentration of nitrate, ammonia, DOC, and UV254 were 2.71, 0.53 mg N LL1, 4.51 mg C LL1 and 0.077 cmL1, respectively; (c) Initial concentration of nitrate, ammonia, DOC, and UV254 were 2.43, 0.50 mg N LL1, 5.49 mg C LL1 and 0.093 cmL1 respectively.
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permeate. When pH departs from the membrane ISP, the degree of membrane ionization is pH-dependent, and membrane separation mechanisms include both sieve and charge repulsion effects. Therefore, we can state that the minimum retention of nitrate occurs at a pH near the ISP of NF270, and increases slightly when pH departs from the ISP, which is in accordance with Paugam et al. (2004), who treated pure NaNO3 solutions using NF membranes. The ammonia retention increases that occur at pH 3 can also be explained as positive charge repulsion between the solute and membrane. The increased ammonia rejections from pH 4 to 10 might be due to the adsorption (or partition) onto/into membrane surfaces or the increased concentration of NH3 relative to NHþ 4 , where NH3 is volatile but also neutral at higher pH and therefore is expected to be unaffected by electrostatic repulsion. DOM in water tends to become negatively charged when the pH increases from acid to basic (Lin et al., 2006). This could explain the increase of DOC and UV254 retention as the solution pH increases above 3 because the electrostatic repulsion between DOM and membrane is enhanced. The DIN/TDN ratio decreases at pH less than 5 mainly due to the decrease of nitrate retention, and a marginal increase at pH 10 occurs due to the drastic rising retention of ammonia. As mentioned above, DON measurement increased with decreasing DIN/TDN. Therefore, the measured DON concentration is fairly stable for pH between 6 and 9, with the highest DON measured at 0.27 mg N L1 in neutral conditions, which fits the stable DIN/TDN ratio as well as DOC and UV254 in that pH range.
3.4.
Effects of transmembrane pressure
Experiments were carried out for finished water from the YDWTP with NF270 pretreatment for different transmembrane pressures ranging from 0.2 to 0.6 MPa. The time to completion decreased from 4.3 to 1.7 h, and the permeate flux increased from 1.48 to 3.71 mL cm2 h1. Fig. 3C shows the retention of nitrate, ammonia, DOC and UV254 retention as a function of transmembrane pressure for the YDWTP finished water using NF270. Because the DIN and DOC retentions remained constant, and the calculated DIN/ TDN ratio and DON concentration also appeared to be constant, no significant effect of transmembrane pressure was found. These results suggest that we can apply high transmembrane pressure to accelerate the filtration process.
3.5. Studies of measured DON concentration and the effectiveness of the treatment processes in the YDWTP and the SDWTP 3.5.1. Comparison of DON concentration for drinking water treatment processes with and without NF pretreatment The varying trends of DOC and DON in raw and treated water throughout the drinking water treatment processes in the YDWTP and the SDWTP are shown in Fig. 4. DON concentration of water samples were compared with and without NF pretreatment. The DIN/TDN of all samples in Fig. 4 were higher than 0.9 without NF pretreatment (data not shown). While 10 of the 27 samples (7 from the YDWTP and 3 from the SDWTP) had negative DON concentration without NF pretreatment, no
Fig. 4 e Comparison of DON and DOC concentrations in different treatment processes in YDWTP and SDWTP with (WP) and without (NP) the NF pretreatment. (A) YDWTP with pre-ozone (Pre-O3), clarification (Cla), and sand filtration (Fil) processes (n [ 4); (B) SDWTP with biological pretreatment (BP), Cla, Fil, and biological activated carbon (BAC) processes (Fin refers to finished water.)
negative DON concentration was found for those with NF270 pretreatment. The retention of DOC for all samples during NF pretreatment was higher than 75%, which implies that satisfactory DON recovery can be obtained for raw and treated waters.
3.5.2. The effectiveness of treatment processes in the YDWTP and the SDWTP As shown in Fig. 4A, the averaged DON concentration of samples collected after pre-O3, clarification, and finished water were negative without NF pretreatment. Moreover, standard deviations of direct DON measurements were relatively high for all samples without NF pretreatment. Because all the samples show low DON and high DIN concentrations, it is impossible to identify the fate of DON during a treatment process without sample pretreatment. DON measurements with NF pretreatment were in the range 0.05e0.35 mg N L1 for all the samples. The YDWTP removed both DOC and DON with similar trends across various treatment processes. Because
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ozonation cannot only oxidize organic chemicals but also release DOM from particulate organic materials, nitrogen incorporated into the particulate can breakdown, releasing DON and raising the DON concentration after pre-ozonation (Westerhoff and Mash, 2002). The application of polyaluminum chloride (w60 mg/L) and polyacrylamide (w0.12 mg/L) in the high rate clarifier can remove around 55% of the DON in this process. These findings are consistent with the results of Lee and Westerhoff (2006), who used aluminum sulfate and a cationic polymer to improve surface water DOC and DON removal in a jar test. The DON in raw water decreased from 0.18 to 0.11 mg N L1, and 36.2% of DON was removed after the YDWTP treatment processes. As shown in Fig. 4B, DON concentrations of the 3 samples from filtration, post-O3 and finished in the SDWTP also showed negative values by direct measurement without NF pretreatment; however, no negative DON concentration values were found with NF pretreatment for all the samples. DON and DOC also showed a similar decreasing trend during the treatment processes. Because some DON species such as amino acids are biodegradable (Westerhoff and Mash, 2002), around 20% of DON was removed by biofiltration in the SDWTP. The advanced treatment process at the SDWTP (post-O3) could increase the biodegradability of DOM coming from the breakdown of larger compounds into smaller molecules. As a result, more non-biodegradable organic matter can be converted to biodegradable organic carbon and then removed in the subsequent BAC process. Therefore, about 70% of the DON was removed after the BAC process when combined with post-O3.
4.
Conclusions
Nanofiltration is a feasible pretreatment for DON measurement with selective NF membranes. NF270 showed the highest DIN permeability and DON retention (w80%) among the selected membranes, and was able to lower the DIN/TDN ratio from around 1 to less than 0.6 mg of N/mg N. Satisfactory DOC recoveries and DON measurements in synthetic water samples were obtained with optimized operating parameters (the sample volume to membrane surface area ratio was 7.5 or 12.5 cm3/cm2 under neutral conditions), and the effect of applied transmembrane pressure on DON measurements was negligible. The proposed NF pretreatment method performed similarly for DON measurement as the dialysis method for aqueous samples. At least 20 h of operating time and a large volume of deionized water can be saved, which is beneficial for laboratories undertaking DON analysis. On the basis of DON measurements in the effluent from different treatment processes at both the YDWTP and the SDWTP, NF pretreatment can solve the problem of reporting negative DON concentrations. DON measurements with NF pretreatment and DOC similarly gradually decreased after each water treatment process in both drinking water treatment plants. The advanced water treatment line at the SDWTP showed higher efficiency for DON removal from 0.37 to 0.11 mg N L1 than what was found at the YDWTP from 0.18 to 0.11 mg N L1.
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Acknowledgments This study was supported by the Natural Science Foundation of China (No. 50708066), High Technology Research and Development (863) Program (No. 2008AA06Z302), the Foundation of the State Key Laboratory of Pollution Control and Resource Reuse (PCRRY08011), the National Major Project of Science & Technology Ministry of China (2008ZX07421-002), and the National Science Council of Taiwan (NSC 98-2218-E-327-002). Dow and GE are thanked for providing the NF membranes.
references
Ahn, K.H., Song, K.G., Cha, H.Y., Yeom, I.T., 1999. Removal of ions in nickel electroplating rinse water using low-pressure nanofiltration. Desalination 122, 77e84. Ambonguilat, S., Gallard, H., Garron, A., Epron, F., Croue´, J.P., 2006. Evaluation of the catalytic reduction of nitrate for the determination of dissolved organic nitrogen in natural waters. Water Res. 40, 675e682. APHA, 1992. Standard Methods for the Examination of Water and Wastewater, eighteenth ed. American Public Health Association, Washington, DC. Boussu, K., 2007. Influence of Membrane Characteristics on Flux Decline and Retention in Nanofiltration. Ph.D. thesis, Department of Chemical Engineering, Katholieke University Leuven, Belgium. Bruggen, B.V., Vandecasteele, C., 2003. Removal of pollutants from surface water and groundwater by nanofiltration: overview of possible applications in the drinking water industry. Environ. Pollut. 122, 435e445. Croue´, J.P., 2004. Isolation of humic and non-humic NOM fractions: structural characterization. Environ. Monit. Assess. 92, 193e207. Garba, Y., Taha, S., Gondrexon, N., Dorange, G., 2000. Mechanisms involved in cadmium salts transport through a nanofiltration membrane: characterization and distribution. J. Memb. Sci. 168, 135e141. Garcia, F., Ciceron, D., Saboni, A., Alexandrova, S., 2006. Nitrate ions elimination from drinking water by nanofiltration: membrane choice. Sep. Purif. Technol. 52, 196e200. Her, N., Amy, G., Jarusutthirak, C., 2000. Seasonal variations of nanofiltration (NF) foulants: identification and control. Desalination 132, 143e160. Lee, W., Westerhoff, P., 2005. Dissolved organic nitrogen measurement using dialysis pretreatment. Environ. Sci. Technol. 39, 879e884. Lee, W., Westerhoff, P., 2006. Dissolved organic nitrogen removal during water treatment by aluminum sulfate and cationic polymer coagulation. Water Res. 40, 3767e3774. Lee, W., Westerhoff, P., Croues, J.P., 2007. Dissolved organic nitrogen as a precursor for chloroform, dichloroacetonitrile, N-nitrosodimethylamine, and trichloronitromethane. Environ. Sci. Technol. 41, 5485e5490. Lin, Y.L., Chiang, P.C., Chang, E.E., 2006. Reduction of disinfection by-products precursors by nanofiltration process. J. Hazard. Mater. 137, 324e331. Lin, Y.L., Chiang, P.C., Chang, E.E., 2007. Removal of small THMPs by nanofiltration membranes. J. Hazard. Mater. 146, 20e29. Mitch, W.A., Gerecke, A.C., Sedlak, D.L., 2003. A N-nitrosodimethylamine (NDMA) precursor analysis for chlorination of water and wastewater. Water Res. 37, 3733e3741. Nghiem, L.D., 2005. Removal of Emerging Trace Organic Contaminants by Nanofiltration and Reverse Osmosis. Ph.D.
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thesis, School of Civil Mining and Environmental Engineering, University of Wollongong, Wollongong, Canada. Nghiem, L.D., Schafer, A.I., Elimelech, M., 2005. Pharmaceutical retention mechanisms by nanofiltration membranes. Environ. Sci. Technol. 39, 7698e7705. Paugam, L., Taha, S., Dorange, G., Jaouen, P., Que´me´neur, F., 2004. Mechanism of nitrate ions transfer in nanofiltration depending pressure, pH, concentration and medium composition. J. Memb. Sci. 231, 37e46. Perakis, S.S., Hedin, L.O., 2002. Nitrogen loss from unpolluted South American forests mainly via dissolved organic compounds. Nature 415, 416e419. Plewa, M.J., Wagner, E.D., Jazwierska, P., Richardson, S.D., Chen, P.H., McKague, A.B., 2004. Halonitromethane drinking water disinfection byproducts: chemical characterization and mammalian cell cytotoxicity and genotoxicity. Environ. Sci. Technol. 38, 62e68. Richardson, S.D., 2003. Disinfection by-products and other emerging contaminants in drinking water. Trends Anal. Chem. 22, 666e684. Richardson, S.D., Plewa, M.J., Wagner, E.D., Schoeny, R., DeMarini, D.M., 2007. Occurrence, genotoxicity, and carcinogenicity of regulated and emerging disinfection byproducts in drinking water: a review and roadmap for research. Mutat. Res. Rev. Mutat. Res. 636, 178e242.
Solinger, S., Kalbitz, K., Matzner, E., 2001. Controls on the dynamics of dissolved organic carbon and nitrogen in a Central European deciduous forest. Biogeochemistry 55, 327e349. Tang, C.Y., Kwon, Y.N., Leckie, J.O., 2007. Fouling of reverse osmosis and nanofiltration membranes by humic acid e effects of solution composition and hydrodynamic conditions. J. Memb. Sci. 290, 86e94. Teixeira, M.R., Rosa, M.J., 2006. The impact of the water background inorganic matrix on the natural organic matter removal by nanofiltration. J. Memb. Sci. 279, 513e520. Vandenbruwane, J., Neve, S.D., Qualls, R.G., Salomez, J., Hofman, G., 2007. Optimization of dissolved organic nitrogen (DON) measurements in aqueous samples with high inorganic nitrogen concentrations. Sci. Total. Environ. 386, 103e113. Westerhoff, P., Mash, H., 2002. Dissolved organic nitrogen in drinking water supplies: a review. J. Water Supply Res. Technol. Aqua 51, 415e448. Xu, B., Gao, N.Y., Sun, X.F., Xia, S.J., Simonnot, M.O., Causserand, C., Rui, M., Wu, H.H., 2007. Characteristics of organic material in Huangpu River and treatability with the O3-BAC process. Sep. Purif. Technol. 57, 348e355. Xu, P., Drewes, J.E., Bellona, C., Amy, G., Kim, T.U., Adam, M., Heberer, T., 2005. Rejection of emerging organic micropollutants in nanofiltrationereverse osmosis membrane applications. Water Environ. Res. 77, 40e48.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/watres
Measurements of dissolved organic nitrogen (DON) in water samples with nanofiltration pretreatment Bin Xu a, Da-Peng Li a, Wei Li a, Sheng-Ji Xia a, Yi-Li Lin b,*, Chen-Yan Hu c, Cao-Jie Zhang a, Nai-Yun Gao a a
State Key Laboratory of Pollution Control and Resources Reuse, Key Laboratory of Yangtze Aquatic Environment, Ministry of Education, College of Environmental Science and Engineering, Tongji University, Shanghai 200092, PR China b Department of Safety, Health and Environmental Engineering, National Kaohsiung First University of Science and Technology, Kaohsiung 811, Taiwan c College of Energy and Environment Engineering, Shanghai University of Electric Power, Shanghai 200090, PR China
article info
abstract
Article history:
Dissolved organic nitrogen (DON) measurements for water samples with a high dissolved
Received 27 January 2010
inorganic nitrogen (DIN, including nitrite, nitrate and ammonia) to total dissolved nitrogen
Received in revised form
(TDN) ratio using traditional methods are inaccurate due to the cumulative analytical
11 June 2010
errors of independently measured nitrogen species (TDN and DIN). In this study, we
Accepted 14 June 2010
present a nanofiltration (NF) pretreatment to increase the accuracy and precision of DON
Available online 22 June 2010
measurements by selectively concentrating DON while passing through DIN species in water samples to reduce the DIN/TDN ratio. Three commercial NF membranes (NF90,
Keywords:
NF270 and HL) were tested. The rejection efficiency of finished water from the Yangshupu
Dissolved organic nitrogen (DON)
drinking water treatment plant (YDWTP) is 12%, 31%, 8% of nitrate, 26%, 28%, 23% of
Nanofiltration
ammonia, 77%, 78%, 82% of DOC (dissolved organic carbon), and 83%, 87% 88% of UV254 for
DON retention
HL, NF90 and NF270, respectively. NF270 showed the best performance due to its high DIN
Polluted surface water
permeability and DON retention (w80%). NF270 can lower the DIN/TDN ratio from around 1
Drinking water treatment
to less than 0.6 mg N/mg N, and satisfactory DOC recoveries as well as DON measurements in synthetic water samples were obtained using optimized operating parameters. Compared to the available dialysis pretreatment method, the NF pretreatment method shows a similar improved performance for DON measurement for aqueous samples and can save at least 20 h of operating time and a large volume of deionized water, which is beneficial for laboratories involved in DON analysis. DON concentration in the effluent of different treatment processes at the YDWTP and the SDWTP (Shijiuyang DWTP) in China were investigated with and without NF pretreatment; the results showed that DON with NF pretreatment and DOC both gradually decreased after each water treatment process at both treatment plants. The advanced water treatment line, including biological pretreatment, clarification, sand filtration, ozone-BAC processes at the SDWTP showed greater efficiency of DON removal from 0.37 to 0.11 mg N L1 than that at the YDWTP, including pre-ozonation, clarification and sand filtration processes from 0.18 to 0.11 mg N L1. ª 2010 Elsevier Ltd. All rights reserved.
* Corresponding author. Tel.: þ886 7 6011000x2328; fax: þ886 7 6011061. E-mail address: [email protected] (Y.-L. Lin). 0043-1354/$ e see front matter ª 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2010.06.034
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1.
Introduction
Natural waters contain significant levels of dissolved inorganic nitrogen (DIN, including nitrate, ammonium, and nitrite) as well as dissolved organic nitrogen (DON). Typical DON concentrations in surface waters vary, ranging from less than 0.1 to larger than 10 mg N L1, with a median value of approximately 0.3 mg N L1 (Westerhoff and Mash, 2002). Because DON comprises a relatively small portion of the dissolved organic matter (DOM), researchers tend to ignore DON and assume that dissolved organic carbon (DOC), nitrate and ammonium play a more important role in water treatment processes (Westerhoff and Mash, 2002). However, there are increasing concerns regarding DON in water treatment processes recently due to its preferential reaction with oxidants/disinfectants (e.g., chlorine, chloramines, ozone (O3) and potassium permanganate) to form nitrogenous disinfection by-products (N-DBPs) (e.g., nitrosamines including Nnitrosodimethylamine [NDMA], halonitromethanes, and haloacetonitriles) (Westerhoff and Mash, 2002; Richardson, 2003; Mitch et al., 2003). Toxicity results indicate that N-DBPs show much stronger carcinogenicity or mutagenicity than that of the regulated DBPs, such as trihalomethanes (THMs) and haloacetic acids (HAAs) (Richardson et al., 2007; Plewa et al., 2004; Lee et al., 2007). The presence of DON also contributes to membrane fouling in water treatment processes (Her et al., 2000). Despite considerable information detailing DOC characteristics (such as structures and specific functional groups) and fate during water treatment processes (Bruggen and Vandecasteele, 2003; Her et al., 2000; Lin et al., 2007; Richardson et al., 2007; Solinger et al., 2001), little is known about DON. It is important to use available analytical methods for DON determination to investigate its fate, properties and reactivity in waters and water treatment processes. The DON concentration in water samples cannot be quantified directly but must be determined by subtracting the sum of DIN species from the total dissolved nitrogen (TDN). Because DON only contributes a relatively small portion of the total mass of TDN in humanimpacted surface waters (Perakis and Hedin, 2002), and DON is the residual concentration after subtracting a much larger concentration (DIN species) from the measured TDN (see Eq. (1)), the accuracy of DON measurements is subject to significant cumulative analytical errors of each independently measured nitrogen species (TDN, nitrite, nitrate and ammonia). þ DON ¼ TDN DIN ¼ TDN (NO 3 þ NO2 þ NH3/NH4 )
(1)
Negative DON concentrations were reported in practical measurements in the case of high DIN/TDN ratios (Vandenbruwane et al., 2007; Solinger et al., 2001; Lee and Westerhoff, 2005). Lowering the DIN/TDN ratio in water samples is necessary to improve the accuracy of DON measurements. Lee and Westerhoff (2005) reported a dialysisbased pretreatment to reduce DIN species in bulk water. They found that 70% of DIN was removed, and more than 95% of DON recoveries were obtained for surface and finished water after 24 h dialysis using a cellulose ester dialysis tube. Although dialysis pretreatment leads to a more accurate DON
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determination for surface and drinking waters, the method may not be applicable for wide use because it is not time and cost effective. Catalytic reduction of nitrate is another way to lower the DIN/TDN ratio (Ambonguilat et al., 2006). However, DON and DOC can be adsorbed onto the catalyst and will likely compete with nitrate for active catalytic sites. Extensive testing has indicated that catalytic reduction of nitrate is not suitable for DON determination (Ambonguilat et al., 2006). Another possible method for reducing the DIN/TDN ratio is to increase the DON concentration in combination with complete or partial DIN removal by means of membrane separation. Reverse osmosis (RO) is a popular technology for DOM concentration and isolation in surface waters, with overall DOC retention close to 100% (Croue´, 2004). However, a lower DIN/TDN ratio cannot be achieved because DIN concentrates along with the organics and remains in the concentrated solution. Therefore, it is more practical to find a suitable separation process that allows DIN pass through freely and still retains most organics, including DON. A nanofiltration (NF) membrane has properties that lie somewhere between ultrafiltration (UF) and RO. NF has been shown to be effective for removing DOC and DBPs precursors and controlling salt passage with a different membrane (Teixeira and Rosa, 2006; Bruggen and Vandecasteele, 2003; Lin et al., 2006). The permeate flux of an NF membrane is based on the applied transmembrane pressure, membrane resistance, feed water temperature, and the composition of feed water. The selectivity of a membrane is due to the combined factors of membrane characteristics, solution properties, and operating conditions (Ahn et al., 1999; Nghiem et al., 2005; Garba et al., 2000; Lin et al., 2007). The major rejection mechanisms for NF membranes include molecular sieve effects and electrostatic effects. Although the molecular weight cutoff (MWCO) of commercial NF membranes is around 200 Da (Ahn et al., 1999; Nghiem et al., 2005; Garba et al., 2000; Lin et al., 2007), molecules below the MWCO of a membrane may also be rejected by the electrical attraction or repulsion of charged species onto a charged membrane surface. The assumption of this study is that NF pretreatment will retain most of the DON in the concentrate while allowing DIN species (nitrate, nitrite and ammonia) to pass through into the permeate. As a result of increasing DON concentration and decreasing the DIN/TDN ratio, a more accurate DON measurement in water samples with high DIN/TDN ratios can be obtained (Lee and Westerhoff, 2005). Because NF separation performance is significantly affected by membrane characteristics (such as MWCO, pore size distribution, charge and hydrophobicity), solution properties (such as solution composition, ionic strength and pH), and operating parameters (such as applied pressure and permeate flux), factors such as membrane type, solution pH, and the initial ratio of sample volume to NF surface area (R) were studied to determine the critical parameters needed to optimize NF pretreatment performance. The objectives of this study were: (1) to investigate the feasibility of NF pretreatment for DON measurement using different commercial NF membranes; (2) to optimize the operating parameters for the NF pretreatment system; and (3) to measure the DON concentration in raw water and the effluent of different treatment processes in two
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drinking water treatment plants (Yangshupu drinking water treatment plant [YDWTP] and Shijiuyang drinking water treatment plant [SDWTP]) to validate the proposed NF pretreatment method as well as to evaluate the performance of each plant.
2.
Materials and methods
2.1.
NF pretreatment system
The NF membranes selected in this study were thin-film composite polyamide membranes, including NF90, NF270 (Dow/Filmtec, USA) and HL (GE Osmonics, USA). According to the manufacturers, each of the 3 membranes has a thin layer of polyamide on a microporous polysulfone support layer and is negatively charged at neutral pH. Although these polyamide layers have similar chemical compositions, the membranes show different characteristics due to unknown surface modifications (Nghiem, 2005; Boussu, 2007; Xu et al., 2005). The properties of the 3 selected membranes are summarized in Table 1. The membranes were received as flat sheets and were cut and stored in regularly replaced Mill-Q (Millipore) water at 4 C. Before proceeding with any experiments, the membranes were rinsed thoroughly with Mill-Q water until no UV absorbance at 254 nm (UV254) was found in the permeate, and DOC was less than 0.1 mg C L1. Two laboratory-scale dead-end filtration test units were used in this study. The experiments were conducted in duplicate in two 300 mL solvent-resistant stirred cells (Millipore, USA). The filter diameter was 75 mm with a filtration area of 40 cm2. A polytetrafluoroethene (PTFE) stirring bar was equipped at the center of the borosilicate glass cylinder. Stirrer speed was controlled by a magnetic stirrer to minimize the effects of concentration polarization. The NF system was pressurized by connecting it to a high purity nitrogen gas cylinder, and the maximum pressure was restricted to 0.62 MPa (70 psi) according to the manufacturer’s instructions.
Table 1 e Properties of NF membranes used in this study. Membrane type Manufacturer Water permeability (L m2 h1 bar1) MWCO (Daltons) Contact angle ( ) Zeta potential (mV) MgSO4 rejection (%) (manufacturer) NaCl retention (%) (manufacturer) a b c d e f
NF90
NF270
HL
DOW/Filmtec 6.4a
DOW/Filmtec 13.5a
GE Osmonics 11.4b
200c 42.5a 21.6c,e >97
170e300b,d 55.0a 19b,d,f >97
150e300b 47b 14b,f >96
85
40
e
Literature (Nghiem, 2005). Literature (Boussu, 2007). Literature (Xu et al., 2005). Literature (Lin et al., 2007). Zeta potential measured at pH 6 in 0.01 M NaCl solution. Zeta potential measured at pH 7 in 0.01 M KCl solution.
The applied pressure and initial ratio of sample volume to NF membrane surface area (R) was 0.5 MPa and 7.5 cm3/cm2, respectively, for pretreatment operation unless otherwise stated. Blank and control tests using Mill-Q water (pH 3e9) with and without the chosen NF membranes in the filtration system demonstrated that there were no organics and DIN leached from the clean NF membranes or the filtration unit and pipe fittings.
2.2.
Description of challenge waters
2.2.1.
Synthetic water
Synthetic water containing 5 mg C L1 humic acid (HA, measured by DOC), 0.1 mg N L1 nitrite, 1.0 mg N L1 nitrate, and 0.5 mg N L1 ammonia was used to test the performance of the chosen NF membranes to sift out the one with the most organic matter but the least DIN retention. A guaranteed grade of reagents KNO3, NaNO2, NH4Cl, NaOH and HCl and HA (C 60.3%; N 0.1%; H 5.4%, extracted from soil) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) and were not purified further. Commercial concentrated HA and DIN species were dissolved separately into ultra-pure water produced from a Milli-Q (Millipore, USA) water purification system to create the solution mentioned above, and the prepared solution was filtered with pre-washed 0.45 mm cellulose acetate membranes (Anpel Co. Ltd, Shanghai, China) before use.
2.2.2. Finished water from the Yangshupu drinking water treatment plant (YDWTP) The finished water from the YDWTP was also used to test the performance of the 3 chosen NF membranes to sift out the one with the most organic matter but the least DIN retention and to evaluate its effectiveness for reducing the DIN/TDN ratio under the influence of a water matrix.
2.3.
Performance of NF membranes
The best-performing NF membrane was then selected for further experiments using the finished water from the YDWTP to optimize the operating parameters of R (varying from 2.5, 5, 7.5, 12.5 to 25), feed water pH (varying from 3 to 10), and applied transmembrane pressure (varying from 0.2 to 0.6 MPa). The NF pretreatment system was kept running with applied pressure until the remaining solution (retentate) in the cell was around 50 mL. The process was completed in 1e4 h depending on the volume of feed solution, applied pressure, and composition of the feed solution. The permeate was collected using a beaker, and the volume of permeate was recorded precisely after each filtration experiment. Collected retentate and permeate were stored at 4 C for further analysis. Assuming that there is a negligible amount of DON in the permeate or adsorbed onto the selected NF membranes (further explanation shown in Section 3.1), the DON concentration of samples with NF pretreatment can be calculated according to the mass balance of DON: CF ¼
CR VR þ CP VP CR ðVF VP Þ z VF VF
(2)
where CF is the DON concentration in the feed water, VF is the volume of feed water; CR is the DON concentration in the
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retentate, VR is the volume of retentate (VR ¼ VF VP, around 50 mL), CP is the DON concentration in the permeate, and VP is the exact volume of collected permeate.
2.4.
Validation and application of NF pretreatment
Twenty seven grab water samples from the effluent of different treatment processes in the YDWTP and SDWTP, respectively, were collected to compare the DON measurements with and without NF pretreatment. The YDWTP has a capacity of 1,480,000 CMD (cubic meters per day) and is located close to Shanghai Bund, which supplies approximately 1/4 of the drinking water consumed by Shanghai, which has a population of 13.8 million. The source water is delivered upstream from the Huangpu River (30.97 N, 121.30 E) by a 40 km tunnel. The characteristics of the raw water have been presented elsewhere (Xu et al., 2007). There are seven conventional treatment lines in the YDWTP, including coagulation, sedimentation and sand filtration processes. An advanced treatment line (320,000 CMD) with pre-O3, high rate clarification, filtration, post-O3 and biological activated carbon (BAC) filtration is also used in this plant. The last two processes were under reconstruction during sampling and therefore were not sampled. The SDWTP has a capacity of 170,000 CMD with source water taken from the Xinchengtang River located in Jiaxi City, Zhejiang Province, China. There are several treatment processes in the SDWTP, including biological aerated filters, clarification, filtration, post-O3, BAC filtration, and post-chlorination to produce drinking water. Grab samples were collected in polypropylene containers, packed in ice and shipped to the laboratory where they were filtered through pre-washed 0.45 mm cellulose acetate membranes (Anpel Co. Ltd, Shanghai, China) to eliminate suspended solids and then kept at 4 C in the dark. The pH and conductivity of all samples were found to be in the range 6.9e7.3 and 221e480 mS/cm, respectively. DOC, UV254, DIN, and DON concentrations were measured for each sample with and without NF pretreatment. Note that DOC and UV254 were used as surrogate parameters to evaluate the loss of organics because the DON concentration of synthetic and field water samples varied greatly before NF pretreatment due to the high DIN/TDN ratios in the samples.
2.5.
Analytical methods
DOC and TDN were measured using a Shimadzu TOC-VCSH analyzer with a TNM-1 TN unit (Shimadzu, Japan). Nitrate and nitrite were measured by ion chromatography (Dionex ICS1000, AS14 column, USA). Nitrite concentrations of all the surface water and finished water tested in this study were below the method detection limit (MDL) (0.005 mg N L1). The MDL of nitrate was 0.005 mg N L1. Ammonia was measured using the colorimetric method with a Nessler reagent (APHA, 1992) on a UNICO 4802 UVeVis spectrophotometer (Shanghai UNICO, China) with a method detection limit (MDL) of 0.05 mg N L1. DON was then calculated by subtracting the value of DIN from TDN. The UV absorbance of the HA solution, as well as samples from the YDWTP and the SDWTP, were analyzed at a wavelength of 254 nm using a 1 cm quartz cell by a UNICO 4802
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UVeVis spectrophotometer (Shanghai UNICO, China). Solution pH was measured using a pH-meter (module PHS-3B, Shanghai LEICI Analysis Instrument Factory, China), which was calibrated on a regular basis. The measurements were carried out in duplicate for each sample, and more replications were executed in cases where the variation between each measurement exceeded 5%.
3.
Results and discussion
3.1.
Effect of membrane type
When choosing a suitable NF membrane with low DIN retention but high organic retention for the pretreatment method, it is essential to evaluate the distribution of DIN species and DOC/UV254 with different NF membranes. Experiments were performed using HL, NF90 and NF270 to treat both the synthetic water and the finished water from the YDWTP. A mass balance of each DIN compound and the DOC/ DON ratio were calculated using permeate and retentate concentrations to determine whether the compound was lost onto the membrane surface and/or into the membrane pores. With a transmembrane pressure of 0.5 MPa and R of 7.5 cm3/cm2, the mass percentages of nitrite, nitrate, ammonia and HA in the retentate and permeate after NF pretreatment are shown in Fig. 1A. Low molecular weight ions of nitrite and nitrate were found in a mass percentage of 30e60% in the retentate and 40e70% in the permeate, respectively (Fig. 1A). Negligible adsorption loss (<4%) was found for both nitrite and nitrate for all 3 tested NF membranes. The tighter NF90 had the highest retention rate for both nitrite and nitrate, while the looser NF270 had the lowest retention of 42.5% and 33.2% for nitrite and nitrate, respectively. The results were similar to those of Paugam et al. (2004) which were obtained with higher nitrate concentrations of feed solution. Approximately 50% of the ammonia was not recovered, which may have been due to the adsorption of ammonia onto/into the membrane surface or due to degassing and high back-pressure depending on the pH of the solution. Only 20% of the ammonia was left in the retention solution, which is beneficial for DON measurements because the decrease of ammonia in the retentate leads to a lower DIN/ TDN ratio. HA is a mixture of organic chemicals with a board molecular weight range and is always seen as a model for natural organic matter (Tang et al., 2007). Fig. 1A shows that no HA was detected (in mg C L1) in the permeate for all membranes. The mass percentage of HA in the retentate for HL, NF90, NF270 was 83%, 67% and 89%, respectively, which means that there was some mass loss for HA during the NF pretreatment. A possible explanation for the loss of HA is adsorption onto/into the membrane surface (Tang et al., 2007). NF270 has a semi-aromatic piperazine based polyamide layer (Lin et al., 2007) and shows the highest water permeability (Table 1) and lowest organics adsorption loss among the 3 membranes tested, as well as fairly high organic rejection and low retention of DIN species (Fig. 1A). Fig. 1B shows the results of experiments using the finished water from the YDWTP with the same operation conditions as mentioned above. Much less nitrate retention was observed
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Fig. 1 e NF pretreatment for lab synthetic solutions (A) as well as a finished water samples from YDWTP (B) with different NF membranes. (A) Initial concentration of nitrite, nitrate, ammonia, and HA were 0.1, 1.0, 0.5 mg N LL1 and 5 mg C LL1, respectively. (B) Finished water from YDWTP with initial concentration of nitrate, ammonia, DOC, and UV254 as 2.51, 0.52 mg of N LL1, 3.99 mg C LL1 and 0.077 cmL1, respectively (R refers to retentate, and P refers to permeate in the figure legend).
Vandenbruwane et al. (2007), who reported an average of 16 14% DOM loss in surface water samples during dialysis pretreatment (Vandenbruwane et al., 2007). To validate the pretreatment method and evaluate the precision of DON measurement, DON concentrations were analyzed with and without NF pretreatment, as shown in Fig. 2. Because the DON concentration is fairly low in finished water, and the DIN/TDN ratio was as high as 0.95, the standard deviation of DON measurements was great, and negative concentration values were measured. The high standard deviation is related to the variances in DIN and TDN measurements (Lee and Westerhoff, 2005). Previous research has indicated that samples with a DIN/TDN ratio greater than 0.7 have high analytical variations, which increases as the DIN/TDN ratio increases (Lee and Westerhoff, 2005). On the other hand, no negative concentrations were measured after any one of the NF pretreatments. As shown in Fig. 2, the highest DON concentration was found for the pretreatment using NF270, which is related to satisfactory DOM retention and the lowest DOM lost to membrane adsorption as mentioned above. Because NF90 showed the highest DIN rejection and DOM adsorption in Fig. 1, most DIN would remain in the retention solution (DIN/TDN ratio around 0.90) and would affect the DON measurement accordingly. Consequently, the lowest DON concentration and largest standard deviation were observed when using NF90. Because DIN/TDN ratios were lowered using HL and NF270, the standard deviations were fairly low for both of these DON measurements. The results are in accordance with the reports from Vandenbruwane et al. (2007) and Lee and Westerhoff (2005), which indicated that the standard deviation of DON measurement depends greatly on the DIN/TDN ratio. Because the HL membrane showed lower DOC retention and more adsorption loss than that of NF270 (Fig. 1A), the DON concentrations appeared to be lower. The above findings suggest that NF pretreatment is a potential way to increase the precision of DON measurements in water samples with high DIN/TDN ratios. High DOM recovery and low DIN/TDN ratio in the retentate can be
1.2
DIN/TDN
1
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-1
D ON concentration (mg N L )
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DON (R=7.5) DON (R=12.5)
compared to that in the synthetic water filtration experiments, which can be explained by the Donnan effect (Garcia et al., 2006). Because the divalent anions in the finished water can be retained almost 100% by the NF membranes, the monovalent anions (particularly nitrate ions) are forced to pass through the membrane to assure the electroneutrality of the permeate (Garcia et al., 2006). The mass percentage of ammonia was similar to that of the synthetic water tests for the 3 chosen NF membranes. We were unable to account for approximately 50% of the ammonia. Recoveries of 76e81% and 83e88% for DOC and UV254, respectively, were observed in the retentate. NF270 shows the highest DOC and UV254 retention and the lowest organic adsorption loss. Because DOC has traditionally been used as a surrogate for DOM, DOC recoveries were used as an indicator for DOM losses with respect to DON. The DOC recoveries in this study were comparable to those of
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w a t e r r e s e a r c h 4 4 ( 2 0 1 0 ) 5 3 7 6 e5 3 8 4
0
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Fig. 2 e Comparison of measured DON concentration and DIN/TDN ratio (A, :, -) for finished water in YDWTP with (n [ 3) and without (n [ 4) NF pretreatment using HL, NF90 and NF270, respectively, with R [ 7.5 and 12.5 (NP refers to without pretreatment).
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achieved after pretreatment by using suitable NF membranes, which is much more practical than the time-consuming and costly dialysis method. To examine the effects of operating parameters of this NF pretreatment on DON measurements, NF270, which had the best performance, was selected for further studies.
3.2. Effects of initial sample volume to membrane surface area ratio (R) Because R may influence DIN permeate, DOC retention and analysis time, it is essential to study the effect of this parameter on NF pretreatment. Experiments were conducted at different feed volumes ranging from 100 to 1000 mL according to the set R value (from 2.5 to 25 cm3/cm2). Fig. 3A shows the calculated DON and DIN/TDN ratios after NF pretreatment as a function of R, as well as the mass balance of DIN species, DOC, and UV254 in retention solutions. The DOC and UV254 rejections are quite stable in the retentions (>80%) when R is less than 12.5 cm3/cm2 (shown in Fig. 3A). Slight decreases of DOC and UV254 retention were observed when R was 25 cm3/cm2 due to a greater concentration of organics concentrated near the membrane surface that increased the concentration gradient across the membrane (the effect of concentration polarization) (Lin et al., 2006). It is interesting that the percentage of nitrate and ammonia retained tends to decrease as R increases, which results in the decrease of the DIN/TDN ratio for R between 2.5 and 12.5 cm3/cm2. The minimum DIN/TDN ratio (0.5) is reached when R is 12.5 cm3/ cm2, then the ratio begins to rise when R increases to 25 cm3/ cm2. The trend for the DON measurements is reversed, as the DIN/TDN ratio increases and R decreases. These findings are in accordance with the results of Lee and Westerhoff (2005), who found that the calculated DON concentration slightly decreased as DIN/TDN ratios increased in surface water spiked with different DIN concentrations. As mentioned above, the goal of NF pretreatment is to lower the DIN/TDN ratio and enrich DON by passing the DIN species through the NF membrane while retaining DOM. Therefore, the recommended value for R should be between 7.5 and 12.5 cm3/cm2 for NF pretreatment using NF270.
3.3.
Effects of solution pH
The solution pH can have a significant effect on membrane surface charge and is responsible for electrostatic repulsion causing the separation of charged solutes (Nghiem, 2005). Organics in water also can vary from neutral to charged species according to the solution pH and their pKa values (Lin et al., 2007). Thus, the effect of pH was evaluated in the NF pretreatment system using the finished water from the YDWTP. Fig. 3B shows the retention of DIN species, DOC and UV254 by NF270 membranes as well as the calculated DIN/TDN ratio and DON concentration as a function of pH. The results show a minimum of nitrate retention for pH 5, and the minimum and maximum retention of ammonia was observed at pH 4 and pH 10, respectively. Because the isoelectric point (ISP) of NF270 is around pH 4 (Boussu, 2007; Lin et al., 2007), the sieve effect for NF270 at pH 4 would help to retain solutes from
Fig. 3 e Effects of R (A), pH (B), and transmembrane pressure (C) on DON measurements with NF pretreatment using NF270 and the finished water from YDWTP. (A) Initial concentration of nitrate, ammonia, DOC, and UV254 were 2.71, 0.48 mg N LL1, 4.4806 mg C LL1 and 0.082 cmL1, respectively; (B) Initial concentration of nitrate, ammonia, DOC, and UV254 were 2.71, 0.53 mg N LL1, 4.51 mg C LL1 and 0.077 cmL1, respectively; (c) Initial concentration of nitrate, ammonia, DOC, and UV254 were 2.43, 0.50 mg N LL1, 5.49 mg C LL1 and 0.093 cmL1 respectively.
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permeate. When pH departs from the membrane ISP, the degree of membrane ionization is pH-dependent, and membrane separation mechanisms include both sieve and charge repulsion effects. Therefore, we can state that the minimum retention of nitrate occurs at a pH near the ISP of NF270, and increases slightly when pH departs from the ISP, which is in accordance with Paugam et al. (2004), who treated pure NaNO3 solutions using NF membranes. The ammonia retention increases that occur at pH 3 can also be explained as positive charge repulsion between the solute and membrane. The increased ammonia rejections from pH 4 to 10 might be due to the adsorption (or partition) onto/into membrane surfaces or the increased concentration of NH3 relative to NHþ 4 , where NH3 is volatile but also neutral at higher pH and therefore is expected to be unaffected by electrostatic repulsion. DOM in water tends to become negatively charged when the pH increases from acid to basic (Lin et al., 2006). This could explain the increase of DOC and UV254 retention as the solution pH increases above 3 because the electrostatic repulsion between DOM and membrane is enhanced. The DIN/TDN ratio decreases at pH less than 5 mainly due to the decrease of nitrate retention, and a marginal increase at pH 10 occurs due to the drastic rising retention of ammonia. As mentioned above, DON measurement increased with decreasing DIN/TDN. Therefore, the measured DON concentration is fairly stable for pH between 6 and 9, with the highest DON measured at 0.27 mg N L1 in neutral conditions, which fits the stable DIN/TDN ratio as well as DOC and UV254 in that pH range.
3.4.
Effects of transmembrane pressure
Experiments were carried out for finished water from the YDWTP with NF270 pretreatment for different transmembrane pressures ranging from 0.2 to 0.6 MPa. The time to completion decreased from 4.3 to 1.7 h, and the permeate flux increased from 1.48 to 3.71 mL cm2 h1. Fig. 3C shows the retention of nitrate, ammonia, DOC and UV254 retention as a function of transmembrane pressure for the YDWTP finished water using NF270. Because the DIN and DOC retentions remained constant, and the calculated DIN/ TDN ratio and DON concentration also appeared to be constant, no significant effect of transmembrane pressure was found. These results suggest that we can apply high transmembrane pressure to accelerate the filtration process.
3.5. Studies of measured DON concentration and the effectiveness of the treatment processes in the YDWTP and the SDWTP 3.5.1. Comparison of DON concentration for drinking water treatment processes with and without NF pretreatment The varying trends of DOC and DON in raw and treated water throughout the drinking water treatment processes in the YDWTP and the SDWTP are shown in Fig. 4. DON concentration of water samples were compared with and without NF pretreatment. The DIN/TDN of all samples in Fig. 4 were higher than 0.9 without NF pretreatment (data not shown). While 10 of the 27 samples (7 from the YDWTP and 3 from the SDWTP) had negative DON concentration without NF pretreatment, no
Fig. 4 e Comparison of DON and DOC concentrations in different treatment processes in YDWTP and SDWTP with (WP) and without (NP) the NF pretreatment. (A) YDWTP with pre-ozone (Pre-O3), clarification (Cla), and sand filtration (Fil) processes (n [ 4); (B) SDWTP with biological pretreatment (BP), Cla, Fil, and biological activated carbon (BAC) processes (Fin refers to finished water.)
negative DON concentration was found for those with NF270 pretreatment. The retention of DOC for all samples during NF pretreatment was higher than 75%, which implies that satisfactory DON recovery can be obtained for raw and treated waters.
3.5.2. The effectiveness of treatment processes in the YDWTP and the SDWTP As shown in Fig. 4A, the averaged DON concentration of samples collected after pre-O3, clarification, and finished water were negative without NF pretreatment. Moreover, standard deviations of direct DON measurements were relatively high for all samples without NF pretreatment. Because all the samples show low DON and high DIN concentrations, it is impossible to identify the fate of DON during a treatment process without sample pretreatment. DON measurements with NF pretreatment were in the range 0.05e0.35 mg N L1 for all the samples. The YDWTP removed both DOC and DON with similar trends across various treatment processes. Because
w a t e r r e s e a r c h 4 4 ( 2 0 1 0 ) 5 3 7 6 e5 3 8 4
ozonation cannot only oxidize organic chemicals but also release DOM from particulate organic materials, nitrogen incorporated into the particulate can breakdown, releasing DON and raising the DON concentration after pre-ozonation (Westerhoff and Mash, 2002). The application of polyaluminum chloride (w60 mg/L) and polyacrylamide (w0.12 mg/L) in the high rate clarifier can remove around 55% of the DON in this process. These findings are consistent with the results of Lee and Westerhoff (2006), who used aluminum sulfate and a cationic polymer to improve surface water DOC and DON removal in a jar test. The DON in raw water decreased from 0.18 to 0.11 mg N L1, and 36.2% of DON was removed after the YDWTP treatment processes. As shown in Fig. 4B, DON concentrations of the 3 samples from filtration, post-O3 and finished in the SDWTP also showed negative values by direct measurement without NF pretreatment; however, no negative DON concentration values were found with NF pretreatment for all the samples. DON and DOC also showed a similar decreasing trend during the treatment processes. Because some DON species such as amino acids are biodegradable (Westerhoff and Mash, 2002), around 20% of DON was removed by biofiltration in the SDWTP. The advanced treatment process at the SDWTP (post-O3) could increase the biodegradability of DOM coming from the breakdown of larger compounds into smaller molecules. As a result, more non-biodegradable organic matter can be converted to biodegradable organic carbon and then removed in the subsequent BAC process. Therefore, about 70% of the DON was removed after the BAC process when combined with post-O3.
4.
Conclusions
Nanofiltration is a feasible pretreatment for DON measurement with selective NF membranes. NF270 showed the highest DIN permeability and DON retention (w80%) among the selected membranes, and was able to lower the DIN/TDN ratio from around 1 to less than 0.6 mg of N/mg N. Satisfactory DOC recoveries and DON measurements in synthetic water samples were obtained with optimized operating parameters (the sample volume to membrane surface area ratio was 7.5 or 12.5 cm3/cm2 under neutral conditions), and the effect of applied transmembrane pressure on DON measurements was negligible. The proposed NF pretreatment method performed similarly for DON measurement as the dialysis method for aqueous samples. At least 20 h of operating time and a large volume of deionized water can be saved, which is beneficial for laboratories undertaking DON analysis. On the basis of DON measurements in the effluent from different treatment processes at both the YDWTP and the SDWTP, NF pretreatment can solve the problem of reporting negative DON concentrations. DON measurements with NF pretreatment and DOC similarly gradually decreased after each water treatment process in both drinking water treatment plants. The advanced water treatment line at the SDWTP showed higher efficiency for DON removal from 0.37 to 0.11 mg N L1 than what was found at the YDWTP from 0.18 to 0.11 mg N L1.
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Acknowledgments This study was supported by the Natural Science Foundation of China (No. 50708066), High Technology Research and Development (863) Program (No. 2008AA06Z302), the Foundation of the State Key Laboratory of Pollution Control and Resource Reuse (PCRRY08011), the National Major Project of Science & Technology Ministry of China (2008ZX07421-002), and the National Science Council of Taiwan (NSC 98-2218-E-327-002). Dow and GE are thanked for providing the NF membranes.
references
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