Article pubs.acs.org/est
Mitigation of Salinity Buildup and Recovery of Wasted Salts in a Hybrid Osmotic Membrane Bioreactor−Electrodialysis System Yaobin Lu and Zhen He* Department of Civil and Environmental Engineering, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, United States S Supporting Information *
ABSTRACT: The osmotic membrane bioreactor (OMBR) is an emerging technology that uses water osmosis to accomplish separation of biomass from the treated effluent; however, accumulation of salts in the wastewater due to water flux and loss of draw solute because of reverse salt flux seriously hinder OMBR development. In this study, a hybrid OMBR−electrodialysis (ED) system was proposed and investigated to alleviate the salinity buildup. The use of an ED (3 V applied) could maintain a relatively low conductivity of 8 mS cm−1 in the feed solution, which allowed the OMBR to operate for 24 days, about 6 times longer than a conventional OMBR without a functional ED. It was found that the higher the voltage applied to the ED, the smaller area of ion-exchange membrane was needed for salt separation. The salts recovered by the ED were successfully reused as a draw solute in the OMBR. At an energy consumption of 1.88−4.01 kWh m−3, the hybrid OMBR-ED system could achieve a stable water flux of about 6.23 L m−2 h−1 and an efficient waste salt recovery of 1.26 kg m−3. The hybrid OMBR-ED system could be potentially more advantageous in terms of less waste saline water discharge and salt recovery compared with a combined OMBR and reverse osmosis system. It also offers potential advantages over the conventional OMBR+post ED treatment in higher water flux and less wastewater discharge.
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INTRODUCTION Reclaimed water is one possible alternative source for freshwater supply, in response to the water crisis due to increasing water demand and contamination.1,2 Among the newly developed technologies for sustainable wastewater treatment, the osmotic membrane bioreactor (OMBR), coupling forward osmosis (FO) with biological treatment, is of great interest because of its highquality effluent for direct discharge or reuse.3 FO membranes have small pore radius (mean radius of 0.25−0.30 nm) and wide rejection of contaminants,4−6 and thus an OMBR, as a highretention membrane bioreactor (HRMBR) system,3 can produce high-quality water, potentially superior to that obtained with conventional MBRs. OMBRs can be engineered for clean water extraction,7,8 nutrient recovery,9 trace organic chemical removal,10 and toxic organic compounds biodegradation11 from wastewater. Moreover, compared to the combination of MBRs with nanofiltration (NF) or reverse osmosis (RO),12,13 an OMBR based on the naturally osmotic process has a low requirement for hydraulic pressure, which results in potentially less membrane fouling.14−16 Water flux in an OMBR can gradually decrease due to membrane fouling, membrane scaling,17−19 and the reduction of osmotic driving force resulting from the salt accumulation in the feed solution. Both membrane fouling and scaling can be minimized by using physical cleaning, chemical cleaning, and back washing. However, salt accumulation in the feed solution will seriously damage the operation of an OMBR system, and © XXXX American Chemical Society
efforts to alleviate this problem are limited at this moment. Two mechanisms can contribute to salt accumulation in the feed solution: first, during wastewater treatment, most inorganic contaminants, especially inorganic salts, are rejected by the FO membrane and then accumulate in the feed solution, thereby increasing the salt concentration; and second, because of the inherent problems of FO membranes, reverse solute leakage (or reverse salt flux) from the draw solution to the feed solution is inevitable,20,21 which can also increase the salt concentration in the feed solution. The loss of draw solute due to reverse solute leakage will not only increase the operational cost due to replenishment of draw agents but also create environmental problems with discharge of the treated effluent (feed solution) that has a high concentration of salts. In addition to reduced water flux, the salt accumulation can also inhibit microbial growth and decrease the biodegradation performance of an OMBR.14,22,23 For example, it was reported that, when the feed solution conductivity increased from 2.0 to 13.6 mS cm−1, the chemical oxygen demand (COD) removal efficiency decreased from 90 to 75%.24 Several operational strategies have been developed for alleviating salt accumulation in OMBRs. The most common Received: March 11, 2015 Revised: July 11, 2015 Accepted: August 4, 2015
A
DOI: 10.1021/acs.est.5b01243 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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Figure 1. Schematic of the hybrid OMBR-ED system.
retaining biomass. Two flat-sheet membrane modules made of cellulose triacetate (CTA) membranes (Hydration Technologies Inc., Albany, NY, USA) with a total osmosis area of 0.026 m2 were submerged in the OMBR. The active side of membranes faced the feed solution (activated sludge mixed liquor), while the support side faced the draw solution. The wastewater storage tank was connected to the OMBR with a float valve to maintain a constant water level in the reactor. An air pump was connected to gas diffusers for providing oxygen, mixing the biomass, minimizing the concentration polarization, and alleviating the fouling on the membrane surface. The ED contained one cell pair assembled with two cation-exchange membranes, one anionexchange membrane (PC-SK and PC-SA), and spacers;29 each membrane had a surface area of 0.0064 m2. The membrane spacings of the concentrated chamber and the desalination chamber were 0.5 and 3 mm, respectively. The 3-mm-thick spacer consisted of two 0.5 mm mesh spacers and a 2 mm rubber with a square space of 64 cm2. The anode electrode and cathode electrode were Titane metal coated with Pt/Ir. System Operation. The synthetic wastewater (influent COD of 300 mg L−1, mimicking primary effluent of municipal wastewater) was composed of 0.30 g L−1 glucose, 0.09 g L−1 NH4Cl, 0.30 g L−1 NaCl, 0.01 g L−1 MgSO4, 0.01 g L−1 CaCl2, 0.01 g L−1 KH2PO4, 0.03 g L−1 K2HPO4, 0.06 g L−1 NaHCO3, and 0.5 mL L−1 trace element.30 The OMBR was operated under either a continuous mode or a batch mode. Under the continuous mode, the conductivity of the draw solution (1.0 M NaCl solution) was 76.0−78.0 mS cm−1 and maintained by conductivity control connected to a 5 M NaCl concentrated salt tank. In the batch mode, the draw solution, with the original volume and conductivity of 500 mL and 76.0−78.0 mS cm−1, was applied and replaced every 24 h; during this operation, no concentrated salt solution was added. The flow rate of the draw solution of each membrane module was kept at 0.04 L min−1. The OMBR was inoculated with the aerobic sludge (3000 mg L−1 of mixed liquid suspended solids) from a local wastewater treatment plant (Peppers Ferry, Radford, VA, USA) and cultivated for one month with the SRT of 10 days. The operating condition of the OMBR is listed in Table S1 (Supporting Information, SI). In the ED, a 100 mM Na2SO4 solution was used as the electrolytes for both the anode and the cathode, and recirculated at a rate of 100 mL min−1 (flow velocity of 0.42 cm s−1). The mixed liquid of the OMBR (feed solution) was recirculated through the desalination chamber of the ED at a rate
method is periodic discharge of mixed liquid, which uses the operating solid retention time (SRT) to control the daily amount of salt and sludge wasted to achieve a stable salinity of the feed solution and thus a stable water flux.14,25 However, due to the high salinity of the feed solution, the water flux becomes much lower than the initial water flux,25,26 which may reduce the wastewater treatment capacity and increase the footprint of an OMBR system. Recently, ultrafiltration or microfiltration was combined with the OMBR process to control the salt accumulation, wherein the supernatant containing salt could be discharged from the OMBR through a microfiltration or ultrafiltration membrane while most of the biomass was retained in the reactor, thereby mitigating salt accumulation and maintaining stable water flux in the FO process.10,27,28 However, further disposal of the extracted supernatant containing a high concentration of salts will be challenging, and this method does not address the problem of the loss of draw solute. Therefore, it is necessary and important to develop alternative strategies to control the salt accumulation and possibly recover the useful draw agent in an OMBR. Electrodialysis (ED), based on transport of ions under the influence of an electrical field through ion-exchange membranes, has been widely used for producing fresh water from saline water. Because a concentrated brine solution is also generated as a result of desalination, ED may be used to reduce salt concentration in the feed solution and recover the salts as a draw solute. In this study, for the first time, a system comprising an OMBR and an ED was established and investigated for wastewater treatment and wasted salt recovery. In this system (Figure 1), wastewater treatment was accomplished by the OMBR (activated sludge process), and mitigation of salinity increase in the feed solution was achieved by the ED. The wasted salt in the feed was separated and recovered as the draw agent continuously. The objectives of this study are to (1) demonstrate the technical feasibility of this hybrid system, (2) investigate the effect of alleviating salt accumulation exerted by the applied voltage to the ED, and (3) examine the influence of the operational modes (continuous and batch) on the system’s performance.
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MATERIALS AND METHODS
System Setup. A submerged OMBR combined with an electrodialysis cell unit (64002, PCCell GmbH, Heusweiler, Germany) was used in this study. The OMBR with a working volume of 3.6 L was equipped with several carbon brushes for B
DOI: 10.1021/acs.est.5b01243 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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Environmental Science & Technology of 100 mL min−1 (flow velocity of 0.69 cm s−1). A NaCl solution with initial conductivity of 1mS cm−1 (about 0.01 M) and an initial volume of 200 mL was used as the ED concentrate, and recirculated at 100 mL min−1 (flow velocity of 4.17 cm s−1). An external voltage was applied to the ED using a power supply (3465A, Circuit Specialists Inc., Mesa, AZ, USA) by connecting the cathode electrode to the negative pole of the power supply, while the anode electrode was connected to a 1-Ω resistor and then to the positive pole. The hybrid OMBR-ED system was operated under different applied voltages (2, 2.5, and 3 V) and different modes (continuous mode and batch mode). When changing the operational condition, physical cleaning and back wash of membrane modules were performed to remove membrane foulants/scaling and to recover the membrane permeability (SI). Meanwhile, the mixed liquid of feed solution in the hybrid OMBR-ED system was discharged and replaced with fresh synthetic wastewater (conductivity around 1.0 mS cm−1). The feasibility of the recovered salts as a draw agent for water extraction was investigated by comparing the performance of the conventional OMBR system between using fresh NaCl solution and the recovered salt solution under a batch mode. Before each batch experiment, physical cleaning and back wash of membrane modules was performed. The mixed liquid of the feed solution in the OMBR was discharged and replaced with fresh synthetic wastewater. During each batch experiment, the draw solution (recovered salt solution or fresh NaCl solution), with the original volume of 500 mL, was applied and operated for 24 h. The recovered salt solution was collected from the ED concentrate of the OMBR-ED system operated under the continuous mode and with applied 3 V. Analysis and Calculation. The concentration of COD was measured using a DR/890 colorimeter (HACH Co., Ltd., USA) according to manufacturer’s handbook. During the experiment, no COD was detected in either the draw solution or all the electrolytes in the ED. Therefore, the COD biodegradation can be calculated as the difference of the total COD in the influent and total COD in the feed solution: Rk =
JS =
ΔσFOVF − VEσI btAFO
(3)
JR =
σEFVCF − σEBVCB btAIC
(4)
where SA is the total salt accumulation in the feed solution, mol m−2 h−1; JS is the salt reverse flux from draw solution to feed solution, mol m−2 h−1; JR is the salt recovery flux of the ED, mol m−2 h−1; t is the operation time, h; ΔσFO is the change of conductivity in the feed solution during the operation, mS cm−1; σEB and σEF are the conductivity of ED concentrate at the beginning and the end of the operation, respectively, mS cm−1; σI is the conductivity of the influent; AFO and AIC are the working areas of FO membrane and ion-exchange membrane, respectively, m2; VCB and VCF are the volume of the ED concentrate at the beginning and the end of the operation, respectively, L;VE is the effluent volume of the OMBR, L; and b is the conversion ratio of conductivity and the concentration of total dissolved solids (TDS), mS L cm−1 mol −1. Considering that the NaCl is the main solute in this system (the percentage of its mole concentration was up to 63%), we used the conversion ratio of conductivity and NaCl concentration as the value of b, which is taken from the slope of the standard curve of the relationship between conductivity and NaCl concentration. In this study, b was calculated as 92.03 mS L cm−1 mol−1. The composition of the recovered salt solution (ED concentrate at the end of experiment) was analysis by the Thermo Electron X-Series ICPMS (Waltham, MA). The voltage (Uj, V) of the ED on the 1-Ω resistor (R, Ω) was recorded by a digital multimeter (2700, Keithley Instruments, Inc., Cleveland, OH, USA) with a time interval of 300 s (tj). Current efficiency is the amount of separated ions divided by the amount of electrons transferred at the electrodes in the ED. Energy consumption (EC, kWh) and current efficiency (ηi) of the ED under a constant voltage (Ue, V) within a set time period (t, h) were calculated as31 m
CODIk VIk + CODFk − 1VF − CODFk VF × 100% CODIk VIk + CODFk − 1VF
EC =
j=0
(1)
where Rk is the COD biodegradation efficiency of the k day; CODIk is the COD of the influent in the k day, mg L−1; CODFk−1 and CODFk are the COD of the feed solution in the k−1 and k day, respectively; VIk is the volume of the influent in the k day, L; and VF is the volume of the feed solution, L. Under the continuous mode, the permeate flux was calculated by the difference between the increase of the effluent volume and the decrease of the concentrated salt volume, and then normalized for the membrane area and operation time. In the batch mode, the draw solution was replaced every 24 h, and the water flux was calculated by the increase volume of the draw solution during the 24 h operation and normalized for the area of FO membrane and time. The salt transportation in the hybrid OMBR-ED system, including salt accumulation in the feed solution, reverse salt transportation from draw solution to feed solution and salt recovery by the ED, was monitored by a conductivity meter (Mettler-Toledo, Columbus, OH, USA) and calculated by the following equations:
SA =
ΔσFOVF btAFO
∑
ni =
UeUj
tj
1000 × R 3600
m
−
∑ j=0
Uj 2
tj
1000 × R 3600
(5)
Fz(σEFVCF − σEBVCB) m
Uj
nb∑ j = 0 R t j
(6)
where m is the number of recorded data during the operation; F is the Faraday’s constant, C mol−1; z is the ionic charge; n is the number of cell pairs; and b is the conversion ratio of conductivity and NaCl concentration, mS L cm−1 mol −1.
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RESULTS AND DISCUSSION Feasibility of the Hybrid System. The feasibility of the hybrid system was examined through comparing it with a conventional OMBR (or 0 V applied to the ED). In the conventional OMBR, when the conductivity of the feed solution increased from 1.1 to 19.3 mS cm−1 within 10 days, the permeate flux decreased from 6.31 to 2.40 L per square meter per hour (LMH) (Figure 2A,B). The accumulation of salts also showed an inhibitive effect on the biodegradation of organic compounds. When the conductivity reached 19.3 mS cm−1, the COD biodegradation efficiency decreased to 13.9% (Figure 2C). As shown in Figure 2A, the use of an ED had a positive effect on mitigating water flux decline in the OMBR due to a slower
(2) C
DOI: 10.1021/acs.est.5b01243 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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continuous mode. In addition, with the low water flux mentioned above, the HRT of the batch operation was increased to 50.5 h, almost 1.6 times longer than that in the continuous mode. However, the low water flux also resulted in a low organic loading rate. Therefore, the total COD removal (biodegradation) under the batch operation was 0.14 ± 0.02 kg COD m−3 d−1, only 68% of that under the continuous mode. Coordination between OMBR and ED. The coupling of an ED with an OMBR could successfully alleviate the salt accumulation in the OMBR. For this hybrid system, proper coordination between the two units is critical to maximize the system performance with minimal expense on construction and operation. The coordination was represented by the area ratio between ion-exchange membrane (ED) and FO membrane (OMBR), determined by the salt accumulation in the OMBR and the salt recovery in the ED. The salt transportation was normalized for the membrane area (FO membrane or ionexchange membrane) and operating time, and represented as total salt accumulation, salt reverse flux and salt recovery flux. As shown in Figure 3, the total salt accumulation in the conventional
Figure 2. Performance of the OMBR under different applied voltages and/or operational modes: (A) water flux, (B) conductivity of the feed solution, and (C) COD biodegradation efficiency.
Figure 3. Salt flux in the OMBR and the salt recovery in the ED under different operational conditions, while the conductivity of feed solution was lower than 8 mS cm−1. Note: 0, 2, 2.5, and 3 V applied voltages; C, continuous mode; B, batch mode.
growth rate of salt concentration, and the flux decline rate was lowered with the increase of applied voltage. With the applied voltage of 3 V, the conductivity of the feed solution reached 8.0 mS cm−1 in 24 days, which was about 6 times longer than that in the conventional OMBR. Furthermore, after the 24 days of operation, the water flux and the COD biodegradation efficiency were above 3.70 LMH and 70%, respectively. In contrast, in the conventional OMBR, the water flux and COD biodegradation efficiency reduced to lower than 3.70 LMH and 70% in 4 days. These results demonstrate that coupling an ED with an OMBR could significantly enhance the wastewater treatment capacity of the OMBR. It was also shown that, under the batch mode with an applied voltage of 3 V, the average water flux was 2.74 ± 0.10 LMH, which was only 62% of that under the continuous mode. The relatively low water flux under the batch mode mainly resulted from the continuous decline of salt concentration in the draw solution. Due to the diluting effect of the water flux, the conductivity of the draw solution decreased from 78.7 ± 2.3 to 25.4 ± 1.6 mS cm−1 during 24 h of operation, which reduced the driving force for osmotic water transport. As shown in Figure 2B, under the batch mode, the average conductivity of the feed solution was 2.9 mS cm−1, which was only half of that under the
OMBR was 0.123 ± 0.017 mol m−2 h−1, of which 46% was attributed to concentrating the feed solutes (due to water loss to the draw), while 54% was ascribed to the salt reverse flux (inset, Figure 3). With the applied voltage of 2, 2.5, or 3 V, the salt recovery flux was 0.020 ± 0.003, 0.094 ± 0.016, or 0.205 ± 0.038 mol m−2 h−1. One can see that, under the area ratio of 1, the ED would need an applied voltage >2.5 V to recover all the salts that were accumulated in the OMBR. To achieve complete salt recovery, the area ratios between ion-exchange membrane and FO membrane were 6.18, 1.31, and 0.60, under the voltage of 2, 2.5, and 3 V, respectively. In addition to the applied voltage, the salt loading rate/wastewater loading rate of the hybrid OMBRED system determined by the operational mode, also affected the coordination between OMBR and ED. Under the batch mode, the area ratio of 0.26 (under 3 V applied) was required for complete salt recovery, which was half of that for the continuous mode and benefited from a lower salt loading rate. The results of comparative membrane ratio indicate that applying a high voltage appears to be economically more favorable in terms of capital cost, because of the relatively small area of ion-exchange membrane (or few cell pairs of ion-exchange membranes) required for complete salt separation; however, the operational cost will increase due to higher energy input to the ED. The D
DOI: 10.1021/acs.est.5b01243 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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affected by the applied voltage (a more comprehensive evaluation and comparison of the proposed system is presented in the next section). Energy consumption increased from 0.10 ± 0.01 to 2.08 ± 0.08 kWh m−3 with the applied voltage increasing from 2 to 3 V (the current density increased from 0.93 ± 0.29 to 11.49 ± 1.56 A m−2) (Figure 5). Likewise, when complete salt
trade-off between capital and operational costs needs to be further analyzed once the proposed system is scaled up. Salt Recovery and Reuse as Draw Solute. The ED separated the salts from the OMBR feed solution, and concentrated them in the ED concentrate. The salt concentration of the ED concentrate became higher with increasing the applied voltage (Figure 4). With an applied voltage of 3 V, the
Figure 4. Salt concentration and the volume of the concentrated solution in the ED unit under the different applied voltages of 2, 2.5, and 3 V.
conductivity of the ED concentrate could reach 60 mS cm−1, close to that of the fresh draw solution for the OMBR. As shown in Table 1, there was no significant difference in terms of COD and turbidity between fresh NaCl solution (60 mS cm−1) and the ED concentrate (3 V applied). When the ED concentrate was used as a draw solution in the OMBR, a water flux of 2.33 ± 0.07 LMH was achieved, and meanwhile the salt reverse flux was 0.017 ± 0.002 mol m−2 h−1; these results are comparable with those obtained with the fresh NaCl solution (Table 1). The composition of the recovered salt solution is shown in Table S2. The solute of the recovered salt solution was dominated by Na and Cl ions. Therefore, we concluded that the salts in the OMBR feed solution could be recovered by the ED and reused as draw solutes in the OMBR, thereby reducing the loss of draw solutes, with both environmental and economic benefits. As a byproduct of the ED process, water was extracted from the OMBR feed solution into the ED concentrate, due to the osmotic pressure difference and electro-osmosis. This water flux became higher with a higher salinity of the ED concentrate that was caused by a higher applied voltage. The ED with applied voltage of 2, 2.5, and 3 V had average water flux of 0.002, 0.167, and 0.247 LMH, respectively. In the presence of ion-exchange membrane, the extracted water had nondetectable COD and a low turbidity of 0.13 ± 0.02 NTU (Table 1). This additional water extraction in the ED could increase the water production of the hybrid OMBR-ED system. For example, under the applied voltage of 3 V, the water production was increased by about 5.6%, compared with that of the OMBR without a functional ED. Energy Consumption. Adding ED units to OMBRs will increase the energy consumption, as well as operational cost,
Figure 5. Energy consumption and current efficiency of the hybrid OMBR-ED system under different operational conditions. Note: 0, 2, 2.5, and 3 V applied voltages; C, continuous mode; B, batch mode; EI, energy consumption per cubic meter of treated wastewater with the same area of ion-exchange membrane (0.0064 m2); and EW, energy consumption per cubic meter of treated wastewater under the condition of complete salt recovery.
recovery was obtained in the hybrid OMBR-ED system, the energy consumption per unit volume of the treated wastewater was also increased from 1.72 to 3.68 kWh m−3 with the increased applied voltage, related to the decline of current efficiency in the ED. When 2 V was applied to the ED, the current efficiency was 76.2 ± 4.9%, which decreased to 41.6 ± 3.8% at 3 V applied. With the same operational time, the decrease in the current efficiency could be because of the stronger reverse salt diffusion due to a higher concentration gradient with a higher applied voltage. Compared with the continuous operation, the energy consumption under the batch mode with complete salt recovery was lower at 2.66 kWh m−3 (Figure 5), benefiting from the low area of ion-exchange membrane needed for salt separation at a lower salt loading rate.
Table 1. Performance of the OMBR Using the Recovered Salt Solution as Draw Solution Compared with the Fresh NaCl Solution draw solution
turbidity (NTU)
COD (mg L−1)
initial conductivity (mS cm−1)
water flux (LMH)
salt reverse flux (mol m−2 h−1)
NaCl solution Recovered solution
0.09 ± 0.01 0.13 ± 0.02
0 0
60 60
2.25 ± 0.03 2.33 ± 0.07
0.018 ± 0.005 0.017 ± 0.002
E
DOI: 10.1021/acs.est.5b01243 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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Figure 6. Comparison of a conventional OMBR (SRT = 10 d) and a hybrid OMBR-ED system for chemical/energy input and waste discharge. The energy input, waste discharge, salt recovery, and salt input are normalized for the volume of water production in the OMBR.
Evaluation of the Hybrid System. Despite higher energy consumption with added ED units, the hybrid system has advantages in salt recovery and reuse and should be properly evaluated through a comparison with the OMBR with or without other (potential) post waste treatment (RO or ED). Figure 6 shows several scenarios with the estimated input/output of water and energy. The evaluation methods are described in the SI. The proposed system has a potential to minimize waste discharge, compared with a conventional OMBR without post-treatment. The hybrid OMBR-ED system could achieve waste salt recovery of 0.64 kg m−3 with 1.88−4.01 kWh m−3 energy input under the applied voltage of 2−3 V, while in a conventional OMBR, 1.22 kg salt per m3 of water production will be needed for supplementing the salt loss due to reverse salt flux, and 0.15 m3 m−3 of waste saline water will discharged (which may require additional treatment). If the waste saline water from the conventional OMBR is further treated by RO or ED, additional energy input will be required (Figure 6). When the conventional OMBR is combined with RO, due to the limitation by the high osmosis pressure under a high solute concentration, 100% salt separation from waste saline water cannot be achieved in the RO system; thus, a portion of the waste saline water entering into RO will be discharged as brine water. In the case of 50% water recovery, 0.10 kWh m−3 of energy is required for the initial waste saline water of 19.3 mS cm−1,32 while 0.075 m3 m−3 of high-salinity wastewater (e.g., 40 mS cm−1) and 0.075 m3 m−3 of fresh water will be produced; the brine solution from the RO contains both salts and organic contaminants (e.g., biomass and organic compounds) and will not be suitable for being reused as a draw in the OMBR. Thus, the hybrid OMBR-ED system is more advantageous in terms of less waste saline water discharge, less draw solute consumption, and salt recovery/reuse, though at a cost of higher energy consumption, compared with the OMBR+RO system. When the conventional OMBR combined with ED for the post treatment of waste saline water, 1.77 kg m−3 of salt discharged from the OMBR can be recovered as draw solute. With the same salt loading rate due to feed solutes concentration, the salt loading rate of ED is determined by the accumulation
induced by reverse salt transportation, and represented by the reverse salt flux divided by the water flux of OMBR. Unlike the proposed OMBR-ED system, in which the water flux can maintain at 6.23 LMH (ignoring the membrane fouling/scaling and the accumulation of nonionic solutes), the water flux in the OMBR+post ED will decline with time. The corresponding average water flux in the OMBR+post ED will be 3.83 LMH, only 61% of that in the hybrid OMBR-ED system. Therefore, compared with the hybrid OMBR-ED system, under the similar reverse salt flux (0.066 mol m−2 h−1), the OMBR+post ED treatment had a higher salt loading rate. The hybrid OMBR-ED system offers advantages over the OMBR+post ED in terms of water flux and wastewater discharge. With the relatively low voltage of 2 V, the energy input of the hybrid OMBR-ED system was 1.88 kWh m−3, which was comparable to that obtained with OMBR+post ED treatment. This study has demonstrated an alternative approach for mitigating the salt buildup and recovering waste salts as draw agents in OMBRs toward sustainable operation. Further efforts need to reduce energy consumption through optimizing system operation and/or using renewable energy (e.g., solar and wind), and to examine the hybrid system with actual wastewater for long-term stability and treatment performance. In addition, longterm operation will expect fouling of both FO and ion-exchange membranes, and appropriate antifouling/cleaning strategies should be investigated. The system should also be evaluated through comparison with conventional MBRs from the aspects of energy consumption, treatment performance, and membrane fouling.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.5b01243. Details of membrane cleaning, OMBR operating conditions, evaluation methods, and composition of the recovered salt solution in the ED (PDF) F
DOI: 10.1021/acs.est.5b01243 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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Environmental Science & Technology
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AUTHOR INFORMATION
Corresponding Author
*Phone: 1-540-231-1346; fax: 1-540-231-7916; e-mail: zhenhe@ vt.edu. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
This research was supported by a faculty startup fund of Virginia Polytechnic Institute and State University and a grant from National Science Foundation (award # 1358145).
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DOI: 10.1021/acs.est.5b01243 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
