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Study of flux decline and solute diffusion on an osmotically driven membrane process potentially applied to municipal wastewater reclamation
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JIEC 2674 1–7 Journal of Industrial and Engineering Chemistry xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec 1 2 3 4 5 6
Study of flux decline and solute diffusion on an osmotically driven membrane process potentially applied to municipal wastewater reclamation Q1 Sungmin
Kim, Gyeong-Wan Go, Am Jang *
Graduate School of Water Resources, Sungkyunkwan University, 2066 Seobu-ro, Jangan-gu, Suwon 440-746, Gyeonggi-do, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 4 July 2015 Received in revised form 14 September 2015 Accepted 11 October 2015 Available online xxx
One major challenge in this study was to investigate the performance of an osmotically driven membrane process, such as forward osmosis (FO) in a case of using raw wastewater that was obtained from a municipal wastewater treatment plant, with a focus on the flux decline and solute diffusion. First, to determine the effect of suspended solids (SS) in wastewater, wastewater was used containing 20 SS mg/L and filtered by a 0.45 mm filter to remove the SS. The results showed that a noticeable flux decline was observed in the case of the existing SS, but flux was slightly decreased without the SS. Furthermore, a larger decline in reverse salt flux (Js) was also obtained with the SS, thus it can be implied that cake enhanced osmotic pressure (CEOP) phenomenon occurred. In other words, the SS could accelerate membrane fouling, resulting in a flux decline and hindered reverse salt diffusion. There was also a comparison of the FO performance using wastewater and a MBR permeate. As hypothesized, it has been found that wastewater resulted in a higher flux and reverse salt flux (Js) decline through the consecutive fouling experiments (four times), but a MBR permeate also brought about substantial flux decline, which was contrary to what was conjecture. These findings indicate that an effective method to control fouling, such as pretreatment and cleaning, may be required even if the treated water resulting from wastewater treatment is used as feed water. As the fouling was getting severe, UV254 rejection was gradually decreased. That phenomenon might possibly be attributed to the increased humic substances on the membrane surface, changes in the membrane characteristics, and Js decline. ß 2015 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
Keywords: Cake enhanced osmotic pressure (CEOP) Forward osmosis (FO) Fouling Municipal wastewater Suspended solids
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Introduction
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Water scarcity is one of the most serious global challenges of our time and needs to be urgently solved, as it is exacerbated by population growth, industrialization, and climate change [1]. Membrane-based seawater desalination and wastewater reclamation is widely regarded as promising solutions for the purpose of augmenting water supply and mitigating water shortage [2]. Currently, a reverse osmosis (RO) process is the leading technology in the field of desalination and wastewater reclamation. A RO process is cost-effective and has high energy efficiency, compared with conventional thermal desalination processes such as multi-stage flash (MSD) and multi-effect distillation (MED), its demand has been rapidly increasing in the desalination market. However, a
Q2 * Corresponding author. Tel.: +82 312907526. E-mail address: [email protected] (A. Jang).
high amount of energy is still required to produce fresh water in a RO process. Despite the advances in the technology, the energy costs can be as much as 75% of the operating costs of desalination plants, or between 30 and 50% of the total production cost of water [3]. Furthermore, the long-standing problem of fouling aggravates the water productivity [4], so the use of cleaning agents is required, thereby increasing the water production cost [2]. In recent years, a FO process has gained increasing attention in the field of water treatment[5], such as wastewater reclamation, desalination and even complex and difficult liquid streams [5–7]. Unlike the pressure-driven membrane process, a FO process is operated by the osmotic gradient between the feed and draw solutions [8,9], hence it requires low or no hydraulic pressure. Thus, it has several advantages, such as low fouling propensity, easy fouling removal, and high water recovery [10–12]. However, the lack of appropriate draw solutions, which can be easily regenerated with low energy costs, has been hampering its application [7,13–15]. Fortunately, there has been a proposal for an
http://dx.doi.org/10.1016/j.jiec.2015.10.005 1226-086X/ß 2015 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
Please cite this article in press as: S. Kim, et al., J. Ind. Eng. Chem. (2015), http://dx.doi.org/10.1016/j.jiec.2015.10.005
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39 osmotic dilution membrane process (ODMP) that does not require 40 Q3 a regeneration process of the draw solution [5,16–18]. The ODMP is 41 for seawater desalination and wastewater reclamation, using the 42 FO and RO processes. In these hybrid processes, the impaired water 43 and seawater or brine from a RO desalination plant are used, and 44 seawater or brine are diluted by the impaired water through a FO 45 process, thereby resulting in reducing the energy demand during a 46 RO desalination. Moreover the economic feasibility of this process 47 has been evaluated and it is confirmed to outperform existing RO 48 process in respect to energy consumption [5,7,16–18]. 49 In addition to energy concerns, membrane fouling is still 50 important for reducing O&M cost in operating desalination plant 51 using RO membrane, so much of investigation for the deep 52 understating of the membrane fouling is conducted in respect to 53 concentration polarization, pretreatment and membrane surface 54 properties [19–21]. Like RO membrane, for the successful 55 operation of a FO process, despite of high fouling propensity of 56 FO membrane, membrane fouling is still important and thus an 57 understanding of fouling behavior is needed. It has been found that 58 a FO process is not free from fouling, and the fouling factors and 59 mechanism are different from those of a RO process [2,8]. Lee et al. 60 reported that a large flux decline is brought about by an 61 accelerated cake enhanced osmotic pressure (CEOP) resulting 62 from reverse salt diffusion from the draw solution side to the feed 63 solution side in a FO process [2]. It was observed that in a FO 64 process, the thick and loose fouling layer is formed on the 65 membrane surface in the absence of hydraulic pressure while the 66 fouling layer formed in a RO process is thin and dense, such as in 67 the case of alginate fouling [4]. 68 The objectives of this study are to give a deeper understanding 69 of fouling behavior in the case of using real wastewater from a 70 wastewater treatment plant, by focusing on the flux decline and 71 solute diffusion. To verify the effect of the SS in wastewater, 72 wastewater was used under conditions, with and without the SS, 73 as feed solutions. In addition, we also examined the effect of 74 wastewater characteristics, before and after wastewater treat75 ment, on the performance, using wastewater and a MBR 76 permeate.
Materials and methods
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FO membrane
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The membrane used in the experiments was supplied by Hydration Technologies, Inc. (Albany, OR, USA). The membrane consists of a cellulose triacetate layer with an embedded polyester mesh for mechanical support and is approximately 50 mm of the total thickness [8]. The contact angle of the membrane is 658 for the dense active layer and 66.58 for the porous support layer. The mean roughness of the dense active and porous support layer is 66 nm and 105 nm, respectively [22].
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Experimental setup
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The schematic diagram of the FO experimental setup is shown in Fig. 1. A FO membrane cell was a flat-and-frame design with a rectangular channel on each side of membrane, which allows the feed and draw solutions to flow, respectively. Each channel has the same dimensions of 77 mm length, 26 mm width, and 3 mm depth, thereby providing an effective area of 20.02 cm2. The variable speed gear pumps (Cole-Parmer, USA) were used to circulate the feed and draw solutions in each channel. The flow meters (Dwyer, USA) were installed for measuring the cross-flow. The feed solution was stirred by a magnetic stirrer in order to maintain a well-mixed homogeneous solution. The temperature of the feed and draw solutions was maintained using a chiller (CPT Inc., Korea). The changes in the weight of draw solution were monitored every 1 min with a digital scale (RADWAG, Poland) that was connected to a computer for the calculation of the permeate flux.
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Calculation of reverse draw solute flux
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The conductivity in the feed tank was measured and converted to NaCl concentration using predetermined calibration curve for conductivity versus NaCl concentration. This system composed of closed circuit and there was no draw regeneration system to let
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Fig. 1. Schematic diagram of the FO experimental setup.
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draw solution concentration be at certain level. As time goes by, permeate from feed to draw solution keep diluting draw solution concentration. So, when calculating reverse draw solute flux and NaCl mole balance in the feed tank, permeate volume should be considered using the following equation: ðC t V t C 0 V 0 Þ Js ¼ At
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In Eq. (1), C0 and Ct are the concentrations of the NaCl concentration in the feed tank at time 0 and t in each. Vo and Vt are the volume of feed tank at time 0 and t in each.
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Fouling experiments
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For the FO experiments, raw wastewater was collected from a wastewater treatment plant located on the S University in Korea. A MBR permeate was obtained from a lab-scale MBR pilot system that was installed at the municipal wastewater treatment plant. A 0.5 M or 4 M NaCl solution was used as a draw solution to produce the osmotic pressure. The cross-flow velocity for both feed and draw solutions was fixed at 8.5 cm/s. The temperature was maintained at 25 8C. The initial volume of feed and draw solutions was 3 L and 2 L, respectively. For all the experiments, a membrane active layer was placed against the feed solution. In the FO runs, the water flows from the feed solution to draw solution, so the feed solution is gradually concentrated while the draw solution is continuously diluted. A baseline test was performed using deionized water as the feed solution, in order to evaluate the flux decline resulting from the phenomena in the FO experiments. The protocol for all the fouling experiments is described as follows. First, a new membrane coupon was placed in the membrane cell and equilibrated with the deionized water for at least 2 h. Next, the feed and draw solutions were placed and circulated in their respective closed loops, without their passing through the membrane cell, until the temperature of both the solutions reached the desired temperature (i.e., 25 8C). After that, the fouling experiments started, thereby allowing the feed and draw solutions to flow through both sides of the membrane. During the experiments, the weight change of the draw solution was recorded in real time to calculate the permeate flux, whereas the data obtained for the initial 15 min was discarded, in consideration of the stabilization of the permeate flux. The conductivity of the feed solution was measured intermittently to determine the salt changes. The fouling experiment was performed until a 1 L permeate flowed from the feed solution to the draw solution, and after the fouling experiment, sampling was done. For the consecutive fouling experiment, the feed and draw solutions were replaced with new ones and the experiment resumed again.
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Analytical techniques
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A TOC analyzer (TOC-LCPH/CPN, Shimadzu, Japan) was used to determine the concentration of the dissolved organic carbon (DOC). A UV absorbance at 254 nm was measured using a UV visible spectrophotometer (DR6000, Hach, USA). The chemical oxygen demand (COD) of the feed solution was measured with a water analyzer apparatus (DR6000, Hach, USA). The analyses for the major components and complexity of organic matter in the feed solution were conducted by a fluorescence spectrophotometer (RF-5301PC, Shimadzu, Japan). The samples’ DOC concentration were adjusted at 1 mg/L for FEEM measurement. The pH and conductivity were measured using a pH meter (Professional Plus, YSI, USA) and a conductivity meter (Orion 4 Star, Thermo Scientific, USA) respectively.
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Results and discussion
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The effect of suspended solids in wastewater
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Wastewater characteristics The wastewater characteristics were determined for COD, SS, DOC, UV absorbance, conductivity, pH, cations and anions and the results are summarized in Table 1. The wastewater was filtered by a 5 mm filter in order to maintain a certain concentration of the SS (i.e., 20 SS mg/L). And then, to remove the SS completely, the wastewater was filtered by a 0.45 mm filter. Fig. 2 provides a fluorescence excitation–emission matrix (FEEM) of the wastewater. The region I, II mean an aromatic protein, while the regions III, IV, and V indicate a fulvic acid-like, soluble microbial by-product-like, and humic acid-like substances, respectively [1]. The wastewater had peaks in each region at an excitation (Ex) = 225 nm and emission (Em) = 305 nm, Ex = 230 nm and Em = 355 nm, Ex = 240 nm and Em = 440 nm, Ex = 280 nm and Em = 350 nm, and Ex = 340 nm and Em = 440 nm. These results mean that the wastewater contained aromatic protein, fulvic acidlike, soluble microbial by-product-like, and humic acid-like substances. Salt-rejecting membrane such as NF, RO and FO can separate not only soluble salts but also small colloidal matters (inorganic colloids and organic macromolecules). These colloidal matters cause membrane fouling and thus aggravate membrane performance [2–4]. In different with inorganic fouling, organic fouling
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Table 1 Wastewater characteristics used as a feed solution in this study. Characteristics
Raw wastewater
Filtered wastewater
COD (mg/L) SS (mg/L) DOC (mg/L) UV254 (cm1) Conductivity (mS/cm) pH
61 3.5 20 1.4 10.14 0.188 0 816 0.6 7.04 0.006
39 1.0 –
Fig. 2. FEEM spectra of wastewater used as the feed solution in this study.
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in membrane process can severe effect on membrane performance because organic-caused fouling is incorporated with biofouling [5]. Even though FO membrane has higher fouling resistance rather than NF and RO ones, it doesn’t mean FO membrane is free of fouling. In salt-rejection membrane, CEOP (cake-enhanced osmotic pressure) caused by inorganic and organic substances can occur in presence of any accumulated mass on the surface [6,7]. This extent of adverse effect broken out of CEOP phenomenon on membrane performance is more significant in FO process than other salt-rejecting membranebased process such as RO [8]. Moreover particle size of foulants significantly affects membrane fouling because shear-induce diffusion depends on the size of foulants. The big particle (size > 100 nm) is more sensitive of shear force while small particle (size < 100 nm) is not but dominated by Brownian diffusion [23].
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Flux decline As said above, FO membrane is generally known to have lower fouling propensity than pressure-driven membrane processes [2,24] due to lower fouling compaction [25]. However it doesn’t mean that FO process is free of fouling. Cath et al. said that, in pilotscale FO process test, the SS contained in WWTP effluent exacerbated membrane fouling and water flux declined as the SS started depositing on the FO membrane surface. They also said that, with proper way to prevent the SS from depositing on the membrane surface, FO process showed reduced flux decline and stable water flux [26]. In this study, to verify the effect of the SS on the performance of a FO process, the flux decline was evaluated with and without SS. The raw wastewater contained 20 SS mg/L and the filtered wastewater had no SS. The flux decline curves obtained from the experiments are shown in Fig. 3. When using a 0.5 M draw solution, the overall flux decline was insignificant for the experiment. With raw wastewater, slightly more flux decline occurred, but compared with the baseline, the flux decline with time was insignificant with both raw wastewater and filtered wastewater. An initial flux decline may be due to the osmotic pressure of the feed solution and adsorption of organic materials to the membrane for the initial 15 min. In particular, the FO membrane is negatively charged at pH 7 [8], so organic materials with positive charges have stronger interactions with the membrane [27]. When using a 4 M draw solution, the effect of the SS on flux decline was obviously observed. At the middle phase of the experiment, the flux of the raw wastewater started to decline significantly and finally, a 20% decline occurred, compared with the final flux of the baseline. However, the flux of filtered wastewater was decreased by only 5%. This result indicates that the SS could accelerate membrane fouling and flux decline. Furthermore the lager decline in the flux of raw wastewater may be attributed to the cake enhance osmotic pressure (CEOP) phenomenon. In a FO process, the reverse salt captured by fouling layer could increase osmotic pressure on the membrane surface and thus lead to a higher flux decline [2]. The CEOP will be discussed in detail, in Section 3.1.3. Overall, a higher flux decline was observed with a 4 M draw solution. This result was due to the difference of the permeation drag force. The membrane fouling is generally affected by hydrodynamic conditions, such as the permeation drag force resulting from convective flow toward the membrane and the shear force caused by cross-flow velocity [28]. In this study, the shear force applied was identical since the cross-flow velocity was adjusted to be the same. Thus, the stronger permeation drag force, which was generated by a higher initial flux, caused the foulants to be more easily deposited or accumulated on the membrane surface.
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Solute transport In a FO process, the reverse salt diffusion from the draw solution side of the membrane to the feed side of the membrane is because of the large solute gradient. Fig. 4 shows the average reverse salt flux (Js) and water flux (Jw) for the experiments. The larger decline in Js occurred when there was fouling by the raw wastewater. 20
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Generally, a reverse salt diffusion may be affected by membrane fouling. In particular, a cake layer formed on the membrane surface hinders the back diffusion of salt, thereby leading to an elevated osmotic pressure near the membrane surface [2]. This phenomenon is called CEOP and is an important mechanism for flux decline. Furthermore, a high concentration of reverse salt, entrapped in the fouling layer, may decrease the driving force for the reverse salt diffusion from the draw solution side to the feed solution side [29]. In this experiment, we compared the fouling behavior with and without the SS. As a result, a higher decline in Js was observed with raw wastewater, and it was attributed to a difference on the membrane surface, that is, a fouling layer was formed. In other words, the existence of the SS could accelerate membrane fouling and the fouling layer formed with the SS could make it more difficult for the reverse salt to diffuse through the membrane. In the current study, we also evaluated the transport of organic matter at a UV absorbance of 254 nm. Comparing with using filtered wastewater as a feed solution (76.1% UV254 rejection), using raw wastewater (72.6% UV254 rejection) showed a slightly declined UV254 rejection. An increased rejection was hypothesized because a previous study observed that the accumulation of organic matter on the surface of the nanofiltration membranes improved the rejection of DOC [30]. However, although more fouling was observed with the raw wastewater, UV254 rejection decreased, and the fouling layer did not lead to an enhancement of the UV254 rejection. This result might possibly be attributed to the increased humic substances on the membrane surface by membrane fouling. Furthermore, an experimental result reported that the pore size of the fouled membranes was larger than that of the virgin membrane, especially for the CTA RO membranes [31].
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Fouling behavior when fouled by wastewater and MBR permeate
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Water characteristics The characteristics of wastewater and the MBR permeate are summarized in Table 2. The wastewater was pre-filtered by a 0.45 mm filter to remove the effect of the SS on membrane fouling. The membrane bioreactor (MBR) process was operated with an interval of 9 min filtration and 1 min backwashing, where mixed liquor suspended solids (MLSS), solids retention time (SRT) and hydraulic retention time (HRT) were approximately controlled at 6500 mg/L, 72 days and 8 h, respectively. Hollow fiber element consisted of 25 fibers whose total membrane area was 1147 cm2. The hollow fiber membrane made of polyvinylidene fluoride (PVDF) which has a pore size of 0.1 mm was submerged in 20 L reactor with an aeration of 3 L/min for fouling control. And also permeate flux was 13 LMH, so thus producing 36 L permeate per day. Fig. 5 shows the FEEM of wastewater and the MBR permeate. The wastewater had peaks at Ex = 235 nm and Em = 330 nm, Ex = 245 nm and Em = 440 nm, Ex = 285 nm and Em = 330 nm, Ex = 345 nm and Em 435 nm, thus indicating that the wastewater contained aromatic protein, fulvic acid-like, soluble microbial byproduct-like, and humic acid-like substances. Likewise the MBR permeate had peaks in a similar fashion with the wastewater. However intensity was different a little bit. It is thought that the
Table 2 Characteristics of wastewater and the MBR permeate used in this study.
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Characteristics
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MBR permeate
COD (mg/L) DOC (mg/L) UV254 (cm1) Conductivity (mS/cm) pH
31 0.6 6.02 0.168 0.0012 750 3.6 7.10 0.021
9 0.6refre 3.03 0.089 0.0021 896 1.5 7.20 0.025
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Fig. 5. FEEM spectra of (a) wastewater and (b) MBR permeate.
part of the fractions contained in wastewater were non-detectable substances by FEEM measurements and that fractions may be converted into detectable one by going through biological process (MBR). Even though it looked that all the two samples had consist of similar substances, in respect to quantitative profile, wastewater had a higher fouling potential compared with the MBR permeate as shown in Table 2. Moreover the particle size of MBR permeate was expected to be smaller than one of wastewater (<0.45 mm) due to size exclusion of hollow fiber membrane (pore size: 0.1 mm).
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Flux decline To verify the degree of fouling by wastewater and the MBR permeate, consecutive experiments were conducted using a 4 M draw solution. One stage was operated until 1 L of the permeate was produced. After finishing that stage, the feed and draw solutions were replaced with new feed and draw solutions, and the
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Fig. 6. Flux decline curves (a) with wastewater and (b) MBR permeate.
Fig. 7. Average Js and Jw (a) with wastewater and (b) MBR permeate.
experiment resumed. A direct comparison of flux decline was difficult because the two membranes used did not show similar performances. One had about 20 LMH of initial flux and the other one showed around 25 LMH. The flux decline curves obtained are shown in Fig. 6. A continuous flux decline was observed throughout the stages, and the flux decreased by 9%, 29%, 41%, and 46%, respectively, at each stage, compared with the final flux of baseline, using wastewater as feed solution. In particular, through the second stage, a large flux decline was obtained and this may be due to the cake layer formation and CEOP phenomenon. The slightly larger decline in Js supported the possibility of the occurrence of CEOP and this will be further discussed in Section 3.2.3. During the experiments using the MBR permeate, the overall flux decline was less severe, compared with that of wastewater. There were around 8%, 17%, 22%, and 30% declines in flux obtained through each stage, respectively. The wastewater had a higher fouling potential than the MBR permeate, based on the water characteristics shown in Table 2 and Fig. 5, so these results were reasonable and hypothesized. The membrane was slowly fouled by the MBR permeate, and if the fouling experiment with the MBR permeate were operated under the same condition of an initial flux (i.e., 20 LMH), the degree of fouling would be less than that. However, it should be noted that even though the permeate treated by the MBR wastewater treatment process was used as a feed solution, the FO process was not free from membrane fouling. To apply a FO process to a wastewater treatment or reclamation process, the appropriate pretreatment process or cleaning process is needed.
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Solute transport The average Js and Jw at each stage are shown in Fig. 7. For both of the cases, the Js was continuously decreased throughout the
stages. As discussed in the previous section, the fouling layer could affect the back diffusion of salt, thus resulting in an increased osmotic pressure on the membrane surface [2]. For consecutive experiments, the increased osmotic pressure may reduce the driving force for a reverse salt diffusion from the draw solution side to the feed solution side [29]. Using wastewater, which had higher fouling, the Js was declined by 16% from the first stage to fourth stage. However, 11% decline in the Js was obtained when there was fouling by a MBR permeate. However, the Js could be affected by not only the fouling layer characteristics (like thickness and compactness) but also by the membrane surface charge, hydrophilicity, degradation of a membrane, and the feed and draw
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solution characteristics [32,33]. Thus, to determine the complex mechanism of the Js decline, more studies are needed. Fig. 8 shows the UV254 rejection at each stage, and the rejection declined consistently throughout the stages. As stated in the previous section, the increased humic substances on the membrane surface and membrane deformation by fouling could be possible reasons. Furthermore, Xie et al. reported that a reverse salt flux could hinder the pore forward diffusion of the organic solute, thus resulting in a high rejection [34]. Hence, the decline in the Js might affect the decline in the UV254 rejection throughout the stages in this study.
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Conclusions
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In this study, we evaluated the flux decline and solute diffusion of the FO process potentially applied to municipal wastewater reclamation, with a focus on the effect of the SS and wastewater characteristics. It has been found that the SS significantly affected flux decline and the reverse salt diffusion near the membrane surface, with accelerating membrane fouling. When using a 4 M draw solution, a 20% decline in flux was obtained with the SS, whereas there was only a 5% decline in flux without the SS. The larger flux decline might be due to the CEOP phenomenon, which resulted from a more severe fouling layer being formed on the membrane surface. Additionally, the possible occurrence of CEOP was supported by more decline in the Js during the experiment with the SS. In addition, we compared the FO performance when there was fouling by wastewater and the MBR permeate. As hypothesized, the wastewater had a higher DOC and UV254 concentrations, thus it was regarded as having a higher fouling potential. Accordingly, a higher decline in the Jw and Js was observed in the case of wastewater. It should be noted that a substantial flux decline was also obtained in the experiment using the MBR permeate. Thus, even though a permeate having wastewater treatment is used as a feed solution in a FO process, fouling is still inevitable, and the appropriate pretreatment and cleaning process are needed. Furthermore, the UV254 rejection was decreased in both of the cases as the fouling was getting severe. Contrary to conjecture, the sieving effect of the fouling layer was insignificant in this study. The increased humic substances on the membrane surface, change in membrane characteristics, and decline in the Js, which could affect the forward diffusion of organic solute, might be possible reasons for the decline in the UV254 rejection.
Acknowledgment
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This research was supported by a grant (code 15IFIP-B088091- 422 02) from Industrial Facilities & Infrastructure Research Program 423 funded by Ministry of Land, Infrastructure and Transport of Korean Q4424 government. 425 References
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[1] M. Elimelech, W.A. Phillip, Science 333 (2011) 712. 427 [2] S. Lee, C. Boo, M. Elimelech, S. Hong, J. Membr. Sci. 365 (2010) 34. 428 [3] A.M. Farooque, A.T.M. Jamaluddin, A.R. Al-Reweli, P.A.M. Jalaluddin, S.M. Al429 Marwani, A.A. Al-Mobayed, A.H. Qasim, Desalination 219 (2008) 137. 430 [4] B.X. Mi, M. Elimelech, J. Membr. Sci. 348 (2010) 337. 431 [5] T.-S. Chung, X. Li, R.C. Ong, Q. Ge, H. Wang, G. Han, Curr. Opin. Chem. Eng. 1 (2012) 432 246. 433 [6] B.D. Coday, P. Xu, E.G. Beaudry, J. Herron, K. Lampi, N.T. Hancock, T.Y. Cath, 434 Desalination 333 (2014) 23. 435 [7] G. Blandin, A.R.D. Verliefde, C.Y. Tang, P. Le-Clech, Desalination 363 (2015) 26. 436 [8] C. Kim, S. Lee, S. Hong, Desalin. Water Treat. 43 (2012) 314. 437 [9] J.-J. Qin, W.C.L. Lay, K.A. Kekre, Desalin. Water Treat. 39 (2012) 123. 438 [10] T.Y. Cath, A.E. Childress, M. Elimelech, J. Membr. Sci. 281 (2006) 70. 439 [11] C.R. Martinetti, A.E. Childress, T.Y. Cath, J. Membr. Sci. 331 (2009) 31. 440 [12] S.F. Zhao, L. Zou, C.Y.Y. Tang, D. Mulcahy, J. Membr. Sci. 396 (2012) 1. 441 [14] R.K. McGovern, J.H. Lienhard V, J. Membr. Sci. 469 (2014) 245. 442 [15] R. Semiat, Environ. Sci. Technol. 42 (2008) 8193. 443 [16] T.Y. Cath, N.T. Hancock, C.D. Lundin, C. Hoppe-Jones, J.E. Drewes, J. Membr. Sci. 444 362 (2010) 417. 445 [17] V. Yangali-Quintanilla, Z. Li, R. Valladares, Q. Li, G. Amy, Desalination 280 (2011) 446 160. 447 [18] V. Yangali-Quintanilla, L. Olesen, J. Lorenzen, C. Rasmussen, H. Laursen, E. Ves448 tergaard, K. Keiding, Desalin. Water Treat. 55 (2014) 2437. 449 [19] J.-W. Nam, J.-Y. Park, J.-H. Kim, S. Kwon, K. Chon, E.-J. Lee, H.-S. Kim, A. Jang, J. Ind. 450 Eng. Chem. 20 (2014) 2354. 451 [20] S. Shanmuganathan, T.V. Nguyen, W.G. Shim, J. Kandasamy, A. Listowski, S. 452 Vigneswaran, J. Ind. Eng. Chem. 20 (2014) 4499. 453 [21] D.H. Kim, H.K. Shon, G. Sharma, J. Cho, J. Ind. Eng. Chem. 17 (2011) 109. 454 [22] V. Parida, H.Y. Ng, Desalination 312 (2013) 88. 455 [23] M. Park, J. Lee, C. Boo, S. Hong, S.A. Snyder, J.H. Kim, Desalination 314 (2013) 456 115. 457 [24] R.W. Holloway, A.E. Childress, K.E. Dennett, T.Y. Cath, Water Res. 41 (2007) 458 4005. 459 [25] B. Mi, M. Elimelech, J. Membr. Sci. 320 (2008) 292. 460 [26] J.E. Drewes, T.Y. Cath, C.D. Lundin, Water Res Found. (2009). Q5461 [27] C. Martin-Orue, S. Bouhallab, A. Garem, J. Membr. Sci. 142 (1998) 225. 462 [28] A. Seidel, M. Elimelech, J. Membr. Sci. 203 (2002) 245. 463 [29] N.T. Hancock, P. Xu, M.J. Roby, J.D. Gomez, T.Y. Cath, J. Membr. Sci. 445 (2013) 464 34. 465 [30] J.W. Cho, G. Amy, J. Pellegrino, Water Res. 33 (1999) 2517. 466 [31] P. Xu, J.E. Drewes, T.U. Kim, C. Bellona, G. Amy, J. Membr. Sci. 279 (2006) 467 165. 468 [32] D. Li, X.Y. Zhang, G.P. Simon, H.T. Wang, Water Res. 47 (2013) 209. 469 [33] M. Xie, W.E. Price, L.D. Nghiem, M. Elimelech, J. Membr. Sci. 438 (2013) 57. 470 [34] M. Xie, L.D. Nghiem, W.E. Price, M. Elimelech, Water Res. 46 (2012) 2683. 471
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JIEC 2674 1–7 Journal of Industrial and Engineering Chemistry xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec 1 2 3 4 5 6
Study of flux decline and solute diffusion on an osmotically driven membrane process potentially applied to municipal wastewater reclamation Q1 Sungmin
Kim, Gyeong-Wan Go, Am Jang *
Graduate School of Water Resources, Sungkyunkwan University, 2066 Seobu-ro, Jangan-gu, Suwon 440-746, Gyeonggi-do, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 4 July 2015 Received in revised form 14 September 2015 Accepted 11 October 2015 Available online xxx
One major challenge in this study was to investigate the performance of an osmotically driven membrane process, such as forward osmosis (FO) in a case of using raw wastewater that was obtained from a municipal wastewater treatment plant, with a focus on the flux decline and solute diffusion. First, to determine the effect of suspended solids (SS) in wastewater, wastewater was used containing 20 SS mg/L and filtered by a 0.45 mm filter to remove the SS. The results showed that a noticeable flux decline was observed in the case of the existing SS, but flux was slightly decreased without the SS. Furthermore, a larger decline in reverse salt flux (Js) was also obtained with the SS, thus it can be implied that cake enhanced osmotic pressure (CEOP) phenomenon occurred. In other words, the SS could accelerate membrane fouling, resulting in a flux decline and hindered reverse salt diffusion. There was also a comparison of the FO performance using wastewater and a MBR permeate. As hypothesized, it has been found that wastewater resulted in a higher flux and reverse salt flux (Js) decline through the consecutive fouling experiments (four times), but a MBR permeate also brought about substantial flux decline, which was contrary to what was conjecture. These findings indicate that an effective method to control fouling, such as pretreatment and cleaning, may be required even if the treated water resulting from wastewater treatment is used as feed water. As the fouling was getting severe, UV254 rejection was gradually decreased. That phenomenon might possibly be attributed to the increased humic substances on the membrane surface, changes in the membrane characteristics, and Js decline. ß 2015 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
Keywords: Cake enhanced osmotic pressure (CEOP) Forward osmosis (FO) Fouling Municipal wastewater Suspended solids
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Introduction
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Water scarcity is one of the most serious global challenges of our time and needs to be urgently solved, as it is exacerbated by population growth, industrialization, and climate change [1]. Membrane-based seawater desalination and wastewater reclamation is widely regarded as promising solutions for the purpose of augmenting water supply and mitigating water shortage [2]. Currently, a reverse osmosis (RO) process is the leading technology in the field of desalination and wastewater reclamation. A RO process is cost-effective and has high energy efficiency, compared with conventional thermal desalination processes such as multi-stage flash (MSD) and multi-effect distillation (MED), its demand has been rapidly increasing in the desalination market. However, a
Q2 * Corresponding author. Tel.: +82 312907526. E-mail address: [email protected] (A. Jang).
high amount of energy is still required to produce fresh water in a RO process. Despite the advances in the technology, the energy costs can be as much as 75% of the operating costs of desalination plants, or between 30 and 50% of the total production cost of water [3]. Furthermore, the long-standing problem of fouling aggravates the water productivity [4], so the use of cleaning agents is required, thereby increasing the water production cost [2]. In recent years, a FO process has gained increasing attention in the field of water treatment[5], such as wastewater reclamation, desalination and even complex and difficult liquid streams [5–7]. Unlike the pressure-driven membrane process, a FO process is operated by the osmotic gradient between the feed and draw solutions [8,9], hence it requires low or no hydraulic pressure. Thus, it has several advantages, such as low fouling propensity, easy fouling removal, and high water recovery [10–12]. However, the lack of appropriate draw solutions, which can be easily regenerated with low energy costs, has been hampering its application [7,13–15]. Fortunately, there has been a proposal for an
http://dx.doi.org/10.1016/j.jiec.2015.10.005 1226-086X/ß 2015 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
Please cite this article in press as: S. Kim, et al., J. Ind. Eng. Chem. (2015), http://dx.doi.org/10.1016/j.jiec.2015.10.005
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39 osmotic dilution membrane process (ODMP) that does not require 40 Q3 a regeneration process of the draw solution [5,16–18]. The ODMP is 41 for seawater desalination and wastewater reclamation, using the 42 FO and RO processes. In these hybrid processes, the impaired water 43 and seawater or brine from a RO desalination plant are used, and 44 seawater or brine are diluted by the impaired water through a FO 45 process, thereby resulting in reducing the energy demand during a 46 RO desalination. Moreover the economic feasibility of this process 47 has been evaluated and it is confirmed to outperform existing RO 48 process in respect to energy consumption [5,7,16–18]. 49 In addition to energy concerns, membrane fouling is still 50 important for reducing O&M cost in operating desalination plant 51 using RO membrane, so much of investigation for the deep 52 understating of the membrane fouling is conducted in respect to 53 concentration polarization, pretreatment and membrane surface 54 properties [19–21]. Like RO membrane, for the successful 55 operation of a FO process, despite of high fouling propensity of 56 FO membrane, membrane fouling is still important and thus an 57 understanding of fouling behavior is needed. It has been found that 58 a FO process is not free from fouling, and the fouling factors and 59 mechanism are different from those of a RO process [2,8]. Lee et al. 60 reported that a large flux decline is brought about by an 61 accelerated cake enhanced osmotic pressure (CEOP) resulting 62 from reverse salt diffusion from the draw solution side to the feed 63 solution side in a FO process [2]. It was observed that in a FO 64 process, the thick and loose fouling layer is formed on the 65 membrane surface in the absence of hydraulic pressure while the 66 fouling layer formed in a RO process is thin and dense, such as in 67 the case of alginate fouling [4]. 68 The objectives of this study are to give a deeper understanding 69 of fouling behavior in the case of using real wastewater from a 70 wastewater treatment plant, by focusing on the flux decline and 71 solute diffusion. To verify the effect of the SS in wastewater, 72 wastewater was used under conditions, with and without the SS, 73 as feed solutions. In addition, we also examined the effect of 74 wastewater characteristics, before and after wastewater treat75 ment, on the performance, using wastewater and a MBR 76 permeate.
Materials and methods
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FO membrane
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The membrane used in the experiments was supplied by Hydration Technologies, Inc. (Albany, OR, USA). The membrane consists of a cellulose triacetate layer with an embedded polyester mesh for mechanical support and is approximately 50 mm of the total thickness [8]. The contact angle of the membrane is 658 for the dense active layer and 66.58 for the porous support layer. The mean roughness of the dense active and porous support layer is 66 nm and 105 nm, respectively [22].
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Experimental setup
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The schematic diagram of the FO experimental setup is shown in Fig. 1. A FO membrane cell was a flat-and-frame design with a rectangular channel on each side of membrane, which allows the feed and draw solutions to flow, respectively. Each channel has the same dimensions of 77 mm length, 26 mm width, and 3 mm depth, thereby providing an effective area of 20.02 cm2. The variable speed gear pumps (Cole-Parmer, USA) were used to circulate the feed and draw solutions in each channel. The flow meters (Dwyer, USA) were installed for measuring the cross-flow. The feed solution was stirred by a magnetic stirrer in order to maintain a well-mixed homogeneous solution. The temperature of the feed and draw solutions was maintained using a chiller (CPT Inc., Korea). The changes in the weight of draw solution were monitored every 1 min with a digital scale (RADWAG, Poland) that was connected to a computer for the calculation of the permeate flux.
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Calculation of reverse draw solute flux
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The conductivity in the feed tank was measured and converted to NaCl concentration using predetermined calibration curve for conductivity versus NaCl concentration. This system composed of closed circuit and there was no draw regeneration system to let
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Fig. 1. Schematic diagram of the FO experimental setup.
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draw solution concentration be at certain level. As time goes by, permeate from feed to draw solution keep diluting draw solution concentration. So, when calculating reverse draw solute flux and NaCl mole balance in the feed tank, permeate volume should be considered using the following equation: ðC t V t C 0 V 0 Þ Js ¼ At
(1)
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In Eq. (1), C0 and Ct are the concentrations of the NaCl concentration in the feed tank at time 0 and t in each. Vo and Vt are the volume of feed tank at time 0 and t in each.
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Fouling experiments
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For the FO experiments, raw wastewater was collected from a wastewater treatment plant located on the S University in Korea. A MBR permeate was obtained from a lab-scale MBR pilot system that was installed at the municipal wastewater treatment plant. A 0.5 M or 4 M NaCl solution was used as a draw solution to produce the osmotic pressure. The cross-flow velocity for both feed and draw solutions was fixed at 8.5 cm/s. The temperature was maintained at 25 8C. The initial volume of feed and draw solutions was 3 L and 2 L, respectively. For all the experiments, a membrane active layer was placed against the feed solution. In the FO runs, the water flows from the feed solution to draw solution, so the feed solution is gradually concentrated while the draw solution is continuously diluted. A baseline test was performed using deionized water as the feed solution, in order to evaluate the flux decline resulting from the phenomena in the FO experiments. The protocol for all the fouling experiments is described as follows. First, a new membrane coupon was placed in the membrane cell and equilibrated with the deionized water for at least 2 h. Next, the feed and draw solutions were placed and circulated in their respective closed loops, without their passing through the membrane cell, until the temperature of both the solutions reached the desired temperature (i.e., 25 8C). After that, the fouling experiments started, thereby allowing the feed and draw solutions to flow through both sides of the membrane. During the experiments, the weight change of the draw solution was recorded in real time to calculate the permeate flux, whereas the data obtained for the initial 15 min was discarded, in consideration of the stabilization of the permeate flux. The conductivity of the feed solution was measured intermittently to determine the salt changes. The fouling experiment was performed until a 1 L permeate flowed from the feed solution to the draw solution, and after the fouling experiment, sampling was done. For the consecutive fouling experiment, the feed and draw solutions were replaced with new ones and the experiment resumed again.
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Analytical techniques
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A TOC analyzer (TOC-LCPH/CPN, Shimadzu, Japan) was used to determine the concentration of the dissolved organic carbon (DOC). A UV absorbance at 254 nm was measured using a UV visible spectrophotometer (DR6000, Hach, USA). The chemical oxygen demand (COD) of the feed solution was measured with a water analyzer apparatus (DR6000, Hach, USA). The analyses for the major components and complexity of organic matter in the feed solution were conducted by a fluorescence spectrophotometer (RF-5301PC, Shimadzu, Japan). The samples’ DOC concentration were adjusted at 1 mg/L for FEEM measurement. The pH and conductivity were measured using a pH meter (Professional Plus, YSI, USA) and a conductivity meter (Orion 4 Star, Thermo Scientific, USA) respectively.
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Results and discussion
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The effect of suspended solids in wastewater
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Wastewater characteristics The wastewater characteristics were determined for COD, SS, DOC, UV absorbance, conductivity, pH, cations and anions and the results are summarized in Table 1. The wastewater was filtered by a 5 mm filter in order to maintain a certain concentration of the SS (i.e., 20 SS mg/L). And then, to remove the SS completely, the wastewater was filtered by a 0.45 mm filter. Fig. 2 provides a fluorescence excitation–emission matrix (FEEM) of the wastewater. The region I, II mean an aromatic protein, while the regions III, IV, and V indicate a fulvic acid-like, soluble microbial by-product-like, and humic acid-like substances, respectively [1]. The wastewater had peaks in each region at an excitation (Ex) = 225 nm and emission (Em) = 305 nm, Ex = 230 nm and Em = 355 nm, Ex = 240 nm and Em = 440 nm, Ex = 280 nm and Em = 350 nm, and Ex = 340 nm and Em = 440 nm. These results mean that the wastewater contained aromatic protein, fulvic acidlike, soluble microbial by-product-like, and humic acid-like substances. Salt-rejecting membrane such as NF, RO and FO can separate not only soluble salts but also small colloidal matters (inorganic colloids and organic macromolecules). These colloidal matters cause membrane fouling and thus aggravate membrane performance [2–4]. In different with inorganic fouling, organic fouling
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Table 1 Wastewater characteristics used as a feed solution in this study. Characteristics
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Filtered wastewater
COD (mg/L) SS (mg/L) DOC (mg/L) UV254 (cm1) Conductivity (mS/cm) pH
61 3.5 20 1.4 10.14 0.188 0 816 0.6 7.04 0.006
39 1.0 –
Fig. 2. FEEM spectra of wastewater used as the feed solution in this study.
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in membrane process can severe effect on membrane performance because organic-caused fouling is incorporated with biofouling [5]. Even though FO membrane has higher fouling resistance rather than NF and RO ones, it doesn’t mean FO membrane is free of fouling. In salt-rejection membrane, CEOP (cake-enhanced osmotic pressure) caused by inorganic and organic substances can occur in presence of any accumulated mass on the surface [6,7]. This extent of adverse effect broken out of CEOP phenomenon on membrane performance is more significant in FO process than other salt-rejecting membranebased process such as RO [8]. Moreover particle size of foulants significantly affects membrane fouling because shear-induce diffusion depends on the size of foulants. The big particle (size > 100 nm) is more sensitive of shear force while small particle (size < 100 nm) is not but dominated by Brownian diffusion [23].
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Flux decline As said above, FO membrane is generally known to have lower fouling propensity than pressure-driven membrane processes [2,24] due to lower fouling compaction [25]. However it doesn’t mean that FO process is free of fouling. Cath et al. said that, in pilotscale FO process test, the SS contained in WWTP effluent exacerbated membrane fouling and water flux declined as the SS started depositing on the FO membrane surface. They also said that, with proper way to prevent the SS from depositing on the membrane surface, FO process showed reduced flux decline and stable water flux [26]. In this study, to verify the effect of the SS on the performance of a FO process, the flux decline was evaluated with and without SS. The raw wastewater contained 20 SS mg/L and the filtered wastewater had no SS. The flux decline curves obtained from the experiments are shown in Fig. 3. When using a 0.5 M draw solution, the overall flux decline was insignificant for the experiment. With raw wastewater, slightly more flux decline occurred, but compared with the baseline, the flux decline with time was insignificant with both raw wastewater and filtered wastewater. An initial flux decline may be due to the osmotic pressure of the feed solution and adsorption of organic materials to the membrane for the initial 15 min. In particular, the FO membrane is negatively charged at pH 7 [8], so organic materials with positive charges have stronger interactions with the membrane [27]. When using a 4 M draw solution, the effect of the SS on flux decline was obviously observed. At the middle phase of the experiment, the flux of the raw wastewater started to decline significantly and finally, a 20% decline occurred, compared with the final flux of the baseline. However, the flux of filtered wastewater was decreased by only 5%. This result indicates that the SS could accelerate membrane fouling and flux decline. Furthermore the lager decline in the flux of raw wastewater may be attributed to the cake enhance osmotic pressure (CEOP) phenomenon. In a FO process, the reverse salt captured by fouling layer could increase osmotic pressure on the membrane surface and thus lead to a higher flux decline [2]. The CEOP will be discussed in detail, in Section 3.1.3. Overall, a higher flux decline was observed with a 4 M draw solution. This result was due to the difference of the permeation drag force. The membrane fouling is generally affected by hydrodynamic conditions, such as the permeation drag force resulting from convective flow toward the membrane and the shear force caused by cross-flow velocity [28]. In this study, the shear force applied was identical since the cross-flow velocity was adjusted to be the same. Thus, the stronger permeation drag force, which was generated by a higher initial flux, caused the foulants to be more easily deposited or accumulated on the membrane surface.
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Solute transport In a FO process, the reverse salt diffusion from the draw solution side of the membrane to the feed side of the membrane is because of the large solute gradient. Fig. 4 shows the average reverse salt flux (Js) and water flux (Jw) for the experiments. The larger decline in Js occurred when there was fouling by the raw wastewater. 20
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Generally, a reverse salt diffusion may be affected by membrane fouling. In particular, a cake layer formed on the membrane surface hinders the back diffusion of salt, thereby leading to an elevated osmotic pressure near the membrane surface [2]. This phenomenon is called CEOP and is an important mechanism for flux decline. Furthermore, a high concentration of reverse salt, entrapped in the fouling layer, may decrease the driving force for the reverse salt diffusion from the draw solution side to the feed solution side [29]. In this experiment, we compared the fouling behavior with and without the SS. As a result, a higher decline in Js was observed with raw wastewater, and it was attributed to a difference on the membrane surface, that is, a fouling layer was formed. In other words, the existence of the SS could accelerate membrane fouling and the fouling layer formed with the SS could make it more difficult for the reverse salt to diffuse through the membrane. In the current study, we also evaluated the transport of organic matter at a UV absorbance of 254 nm. Comparing with using filtered wastewater as a feed solution (76.1% UV254 rejection), using raw wastewater (72.6% UV254 rejection) showed a slightly declined UV254 rejection. An increased rejection was hypothesized because a previous study observed that the accumulation of organic matter on the surface of the nanofiltration membranes improved the rejection of DOC [30]. However, although more fouling was observed with the raw wastewater, UV254 rejection decreased, and the fouling layer did not lead to an enhancement of the UV254 rejection. This result might possibly be attributed to the increased humic substances on the membrane surface by membrane fouling. Furthermore, an experimental result reported that the pore size of the fouled membranes was larger than that of the virgin membrane, especially for the CTA RO membranes [31].
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Fouling behavior when fouled by wastewater and MBR permeate
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Water characteristics The characteristics of wastewater and the MBR permeate are summarized in Table 2. The wastewater was pre-filtered by a 0.45 mm filter to remove the effect of the SS on membrane fouling. The membrane bioreactor (MBR) process was operated with an interval of 9 min filtration and 1 min backwashing, where mixed liquor suspended solids (MLSS), solids retention time (SRT) and hydraulic retention time (HRT) were approximately controlled at 6500 mg/L, 72 days and 8 h, respectively. Hollow fiber element consisted of 25 fibers whose total membrane area was 1147 cm2. The hollow fiber membrane made of polyvinylidene fluoride (PVDF) which has a pore size of 0.1 mm was submerged in 20 L reactor with an aeration of 3 L/min for fouling control. And also permeate flux was 13 LMH, so thus producing 36 L permeate per day. Fig. 5 shows the FEEM of wastewater and the MBR permeate. The wastewater had peaks at Ex = 235 nm and Em = 330 nm, Ex = 245 nm and Em = 440 nm, Ex = 285 nm and Em = 330 nm, Ex = 345 nm and Em 435 nm, thus indicating that the wastewater contained aromatic protein, fulvic acid-like, soluble microbial byproduct-like, and humic acid-like substances. Likewise the MBR permeate had peaks in a similar fashion with the wastewater. However intensity was different a little bit. It is thought that the
Table 2 Characteristics of wastewater and the MBR permeate used in this study.
Q6
Characteristics
Wastewater
MBR permeate
COD (mg/L) DOC (mg/L) UV254 (cm1) Conductivity (mS/cm) pH
31 0.6 6.02 0.168 0.0012 750 3.6 7.10 0.021
9 0.6refre 3.03 0.089 0.0021 896 1.5 7.20 0.025
5
Fig. 5. FEEM spectra of (a) wastewater and (b) MBR permeate.
part of the fractions contained in wastewater were non-detectable substances by FEEM measurements and that fractions may be converted into detectable one by going through biological process (MBR). Even though it looked that all the two samples had consist of similar substances, in respect to quantitative profile, wastewater had a higher fouling potential compared with the MBR permeate as shown in Table 2. Moreover the particle size of MBR permeate was expected to be smaller than one of wastewater (<0.45 mm) due to size exclusion of hollow fiber membrane (pore size: 0.1 mm).
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Flux decline To verify the degree of fouling by wastewater and the MBR permeate, consecutive experiments were conducted using a 4 M draw solution. One stage was operated until 1 L of the permeate was produced. After finishing that stage, the feed and draw solutions were replaced with new feed and draw solutions, and the
330 331 332 333 334 335
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Fig. 6. Flux decline curves (a) with wastewater and (b) MBR permeate.
Fig. 7. Average Js and Jw (a) with wastewater and (b) MBR permeate.
experiment resumed. A direct comparison of flux decline was difficult because the two membranes used did not show similar performances. One had about 20 LMH of initial flux and the other one showed around 25 LMH. The flux decline curves obtained are shown in Fig. 6. A continuous flux decline was observed throughout the stages, and the flux decreased by 9%, 29%, 41%, and 46%, respectively, at each stage, compared with the final flux of baseline, using wastewater as feed solution. In particular, through the second stage, a large flux decline was obtained and this may be due to the cake layer formation and CEOP phenomenon. The slightly larger decline in Js supported the possibility of the occurrence of CEOP and this will be further discussed in Section 3.2.3. During the experiments using the MBR permeate, the overall flux decline was less severe, compared with that of wastewater. There were around 8%, 17%, 22%, and 30% declines in flux obtained through each stage, respectively. The wastewater had a higher fouling potential than the MBR permeate, based on the water characteristics shown in Table 2 and Fig. 5, so these results were reasonable and hypothesized. The membrane was slowly fouled by the MBR permeate, and if the fouling experiment with the MBR permeate were operated under the same condition of an initial flux (i.e., 20 LMH), the degree of fouling would be less than that. However, it should be noted that even though the permeate treated by the MBR wastewater treatment process was used as a feed solution, the FO process was not free from membrane fouling. To apply a FO process to a wastewater treatment or reclamation process, the appropriate pretreatment process or cleaning process is needed.
364 365 366
Solute transport The average Js and Jw at each stage are shown in Fig. 7. For both of the cases, the Js was continuously decreased throughout the
stages. As discussed in the previous section, the fouling layer could affect the back diffusion of salt, thus resulting in an increased osmotic pressure on the membrane surface [2]. For consecutive experiments, the increased osmotic pressure may reduce the driving force for a reverse salt diffusion from the draw solution side to the feed solution side [29]. Using wastewater, which had higher fouling, the Js was declined by 16% from the first stage to fourth stage. However, 11% decline in the Js was obtained when there was fouling by a MBR permeate. However, the Js could be affected by not only the fouling layer characteristics (like thickness and compactness) but also by the membrane surface charge, hydrophilicity, degradation of a membrane, and the feed and draw
100 W astewater M BR permea te
UV254 rejection (%)
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80
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60
0 1st
2nd
3rd
4th
Each experiment Fig. 8. UV254 rejection with wastewater and MBR permeate.
Please cite this article in press as: S. Kim, et al., J. Ind. Eng. Chem. (2015), http://dx.doi.org/10.1016/j.jiec.2015.10.005
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G Model
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solution characteristics [32,33]. Thus, to determine the complex mechanism of the Js decline, more studies are needed. Fig. 8 shows the UV254 rejection at each stage, and the rejection declined consistently throughout the stages. As stated in the previous section, the increased humic substances on the membrane surface and membrane deformation by fouling could be possible reasons. Furthermore, Xie et al. reported that a reverse salt flux could hinder the pore forward diffusion of the organic solute, thus resulting in a high rejection [34]. Hence, the decline in the Js might affect the decline in the UV254 rejection throughout the stages in this study.
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Conclusions
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In this study, we evaluated the flux decline and solute diffusion of the FO process potentially applied to municipal wastewater reclamation, with a focus on the effect of the SS and wastewater characteristics. It has been found that the SS significantly affected flux decline and the reverse salt diffusion near the membrane surface, with accelerating membrane fouling. When using a 4 M draw solution, a 20% decline in flux was obtained with the SS, whereas there was only a 5% decline in flux without the SS. The larger flux decline might be due to the CEOP phenomenon, which resulted from a more severe fouling layer being formed on the membrane surface. Additionally, the possible occurrence of CEOP was supported by more decline in the Js during the experiment with the SS. In addition, we compared the FO performance when there was fouling by wastewater and the MBR permeate. As hypothesized, the wastewater had a higher DOC and UV254 concentrations, thus it was regarded as having a higher fouling potential. Accordingly, a higher decline in the Jw and Js was observed in the case of wastewater. It should be noted that a substantial flux decline was also obtained in the experiment using the MBR permeate. Thus, even though a permeate having wastewater treatment is used as a feed solution in a FO process, fouling is still inevitable, and the appropriate pretreatment and cleaning process are needed. Furthermore, the UV254 rejection was decreased in both of the cases as the fouling was getting severe. Contrary to conjecture, the sieving effect of the fouling layer was insignificant in this study. The increased humic substances on the membrane surface, change in membrane characteristics, and decline in the Js, which could affect the forward diffusion of organic solute, might be possible reasons for the decline in the UV254 rejection.
Acknowledgment
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This research was supported by a grant (code 15IFIP-B088091- 422 02) from Industrial Facilities & Infrastructure Research Program 423 funded by Ministry of Land, Infrastructure and Transport of Korean Q4424 government. 425 References
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Please cite this article in press as: S. Kim, et al., J. Ind. Eng. Chem. (2015), http://dx.doi.org/10.1016/j.jiec.2015.10.005