w a t e r r e s e a r c h 5 2 ( 2 0 1 4 ) 1 1 2 e1 2 1
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Sustainable water recovery from oily wastewater via forward osmosis-membrane distillation (FO-MD) Sui Zhang, Peng Wang, Xiuzhu Fu, Tai-Shung Chung* Department of Chemical and Biomolecular Engineering, National University of Singapore, 117576 Singapore, Singapore
article info
abstract
Article history:
This study proposed and investigated a hybrid forward osmosis e membrane distillation
Received 18 October 2013
(FO-MD) system for sustainable water recovery from oily wastewater by employing lab-
Received in revised form
fabricated FO and MD hollow fiber membranes. Stable oil-in-water emulsions of different
26 December 2013
concentrations with small droplet sizes (99.9 >99.9 e 99.99
82.1 2.0 74.0 2.5 e 47.1 3.1
26.7 40.2 26.3 5.8
3.5 2.1 2.9 0.5
a
The water flux in FO was obtained with DI water as the feed and 2 M NaCl as the draw solution, and in MD it was obtained when 2 M NaCl is the feed. b FO-re indicates the membrane performance at room temperature after immersing it in 60 C water for 24 h.
4.1 0.8 gMH at room temperature, verifying low salt permeation of the FO membrane to NaCl. The oil content is not detected by both UV and TOC measurements, which proves FO to be an effective process to remove oil from oily water. Since the detection limit of TOC is 1ppm, it can be calculated from Eqs. (4) and (5) that the minimal rejection to oil is 99.9%. However, acetic acid is not well rejected by the FO membrane possibly due to its small size and hydrophilic nature. A rejection rate of only 82.1 2.0% is achieved, or in other words, around 179 ppm acetic acid is still found in the permeate when the feed contains 1000 ppm acetic acid. Nevertheless, since acetic acid works as the chemical additive in the oil production process, it is preferred to remain in the permeate stream as much as possible so that minimal replenishment of acetic acid is needed for reuse. Since the diluted draw solution after FO operations is continuously concentrated by the MD operation which is normally operated at a higher temperature, the water flux and solute rejection of the FO operation at 60 C are also characterized. Table 2 shows that the water flux is largely enhanced from 26.7 3.5 LMH to 40.2 2.1 LMH when the operation temperature is elevated from 23 to 60 C using DI water as the feed. Several factors contribute to this increment: (1) the osmotic pressure is higher based on the van’t Hoff equation, (2) similar to that of reverse osmosis membranes (McCutcheon and Elemelech, 2006), the reduced water viscosity facilitates its diffusion rate across the membrane, and (3) the increased diffusion coefficient of NaCl lowers the solute resistivity in the support layer and hence reduces the impact of internal concentration polarization. Even though the salt reverse flux is increased as well, the ratio of reverse salt to water fluxes is still kept at around 0.18 g L1, making the membrane quite suitable for salt e water separation at 60 C. Interestingly, the oil rejection at 60 C is again higher than 99.9% in the FO process, while the rejection for acetic acid is lowered down to 74.0 2.5%. Clearly, operating the FO process at elevated temperatures not only draws water from oily wastewater at a higher permeation rate, but also enhances the amount of acetic acid in the permeate stream while maintaining a very low oil concentration in the permeate. In addition, to verify the membrane stability at different temperatures, the membranes were immersed in DI water at 60 C for 24 h and then re-tested in FO at room temperature (23 C). Similar results are obtained before and after hot water immersion as shown in Table 2. Similar characterizations were conducted on PVDF hollow fiber membranes for the MD operation. As shown in Fig. 2 and Table 1, the PVDF membranes have dual macrovoid layers in
the cross-section which offer a high porosity of 86 6%, a microporous selective layer on the outer surface and a fully open-channeled inner surface. The membrane is relatively hydrophobic with a contact angle of 88 2 and an effective mean pore size of 0.161 0.004 mm. Table 2 shows that clean water can be collected at the rate of 5.8 0.5 LMH in the MD process where 2 M NaCl flows through the shell side of the fibers as the feed at 60 C. The NaCl rejection reaches as high as 99.99%. In addition, acetic acid passes through the membrane at a ratio of 52.9%, or only 47.1% of acetic acid stays in the feed side. The permeation rates of both water and acetic acid are highly dependent on their saturated vapor pressures (Gryta, 2002; Li and Sirkar, 2004; Edwie and Chung, 2012). Since the water mole fraction in the feed mixture is close to 1, the vapor pressure of both liquids can be estimated individually by the Antoine Equation (Antoine, 1888). They are about 0.120 atm and 0.196 atm for acetic acid and water, respectively. In other words, the amount of acetic acid from the feed solution to the permeate is approximately 60% of that of water. However, this theoretical ratio is slightly higher than the experimental result of 52.9% probably due to other factors such as molecule size, viscosity and barrier effects of the membranes, etc.
3.3.
Water recovery from the hybrid FO-MD process
To simulate the hybrid FO-MD process in the real applications, an 800 ml feed solution containing NaCl, petroleum, Tween 80 and acetic acid was continuously run through the CTA TFC FO hollow fiber module using concentrated NaCl as the draw solution until the recovery rate reaches more than 90%, and then the diluted draw solution was concentrated by the PVDF MD membrane module until a similar amount of the permeate water was collected from the other side. In both operations, the high salinity draw solution is kept at 60 C. As mentioned in Section 2.5, two sets of NaCl concentrations were selected to recover water from oily wastewater: series #1, where 0.04 M NaCl and 2 M NaCl were used as the feed and draw solutions, respectively; and series #2, where the corresponding NaCl concentrations were changed to 0.2 M and 5 M. Fig. 5 illustrates the water flux, calculated NaCl concentration in the feed and draw solutions and feed water recovery in the batch-mode FO-MD process as a function of time. The initial water flux in FO for series 1 and 2 begins from different values as shown in Fig. 5A. Series #1 starts from around 32 LMH, which is approximately 80% of the value using DI water as the feed, mainly due to the presence of oil droplets and NaCl in the current feed solution. A higher initial water flux is
119
w a t e r r e s e a r c h 5 2 ( 2 0 1 4 ) 1 1 2 e1 2 1
45
NaCl concentration, M
Water flux, LMH
40 35 30 25 20 15 10
0
10
20 Time, hr
30
Feed recovery, %
Water flux, LMH
3 2 1 0
5
4 2
10
15 20 25 Time, hr
30
60
35
(D)
#1: FO #2: FO #3: MD #4: MD
80
6
(B)
4
100
#1 #2
8
5
0
40
(C)
10
#1: draw solution #2: draw solution #1: feed solution #2: feed solution
6
(A)
#1 #2
40 20 0
0
0
10
20
30
40
50
Time, hr
0
10
20 30 Time, hr
40
50
Fig. 5 e (A) water flux in FO, (B) calculated NaCl concentrations in the draw and feed solutions in FO, (C) water flux in MD and (D) feed water recovery of the FO-MD hybrid system at 60 C. After the feed water recovery reached around 90%, the diluted draw solution was connected to a MD system (maintained at 60 C) and the permeate was collected. A dashed line is drawn in Fig. 5(A) to guide the readers of the flux reduction trend. #1 and #2 refer to series #1 and series #2 as specified in the text, respectively.
achieved for series #2 despite of its high salinity in the feed, owing to the substantial increment in draw solution concentration. However, the water flux of both series undergoes similar trends against time. Guided by the black dashed line drawn in the figure, the flux pattern is characterized by three stages: a significant flux drop appears in the first 7e10 h, followed by mild flux decay until 20e25 h, and then a fast flux decline again in the third stage. The decline phenomena may be explained by Figs. 4 and 5B. Consistent with the long-term studies in Fig. 4, the water flux drops in the first several hours partially due to fouling. In addition, Fig. 5B shows that the feed salinity is continuously increased because water permeates from the feed to the draw solution. The salt accumulation in the feed becomes serious at the later stage when the recovery is high (Fig. 5D). Meanwhile, the draw solution is continuously diluted, leading to a decreased effective osmotic driving force across the membranes. Moreover, the increasing concentration of oil and acetic acid in the feed also contributes to the flux decline. As a result, the combined effects of increasing fouling and decreasing driving force cause a large flux drop in the 1st stage (i.e., the first 7e10 h). The flux decline becomes milder in the 2nd stage since fouling is gradually stabilized. Since the feed salinity is substantially increased when higher recovery is reached in the 3rd stage, a sharper decrease in flux is observed in this stage. The final feed salinity is 0.47 M and 2.11 M for series #1 and #2, respectively, which corresponds to the recovery of 91.5% and 90.5%, respectively. Note that the salt
reverse diffusion into the feed solution is not taken into account considering the low salt reverse flux of the FO membranes. Interestingly, series #2 starts the 3rd stage phenomenon (i.e., big flux drop) earlier than series #1. This is due to the fact that the former has a higher initial feed concentration than the latter. As a result, the effect of high feed salinity on flux at a high recovery is more significantly magnified in the former than the latter. However, it is worth noting that the water fluxes in both cases always stay above 14 LMH even at more than 90% recovery. Clearly, FO could effectively recover water from oily wastewater even in the presence of relatively high feed salinity. Table 3 summarizes the initial and final compositions of the feed, draw solution and permeate for the FO and MD operations. The permeate concentration is in fact calculated from the difference between the initial and final composition of the draw solution. Since oil was not detected by UV in the permeate, a word of “trace” is given in the table, again proving an ultrahigh oil rejection rate by FO. The concentrations of acetic acid in the permeate are 237 and 219 ppm for series #1 and #2, respectively, which correspond to more than 70% retention against their initial feed concentration of 1000 ppm. Consequently, the oil concentration in the draw solutions is negligible, and the acetic acid content is around 50 ppm. The subsequent regeneration of the diluted draw solution by MD is shown in Figs. 5C and D as well. Relatively constant water fluxes are found against operation time in Fig. 5C despite of the increment in NaCl concentration. Interestingly,
120
w a t e r r e s e a r c h 5 2 ( 2 0 1 4 ) 1 1 2 e1 2 1
Table 3 e The initial and final compositions of the feed solution, permeate and draw solution for FO and MD of the hybrid FO-MD system at 60 C. Composition
Initial feed solution (FO) Permeate (FO) Final feed solution (FO, calculated) Draw solution (FO) at the end of the FO operation (The initial composition for the MD) Permeate (MD) Final draw solution (MD, calculated)
2 M NaCl as the draw solution (series#1) Oil, ppm
Acetic acid, ppm
NaCl
Oil, ppm
Acetic acid, ppm
NaCl
4000 Trace 47,000
1000 237 9213
0.04 M e 0.47 M
4000 Trace 42,200
1000 219 8440
0.2 M e 2.11 M
Trace
54
1.55 M
Trace
49
3.87 M
e Trace
29 61
1 ppm 2.0 M
e Trace
28 55
3 ppm 5.0 M
the water fluxes of series #1 and #2 are almost the same. It shows that water permeation rate in MD is not significantly affected by the salt concentration. Therefore, a high draw solution concentration is preferred for FO-MD hybrid systems in pursuit of large water flux in FO. After 42e45 h, around 720 ml distilled water (or 90% recovery in regards to the initial feed volume of the FO-MD hybrid system) is collected (Fig. 5D). Table 3 shows that the content of NaCl and acetic acid in the final permeate are as low as 1 or 3 ppm, and the acetic acid concentration reaches 28 or 29 ppm. The draw solution salinity is recovered to their initial status. The experiments not only demonstrate efficient water recovery from oily wastewater by the proposed FO-MD process with at least 90% water recovery rate, but also partially recover acetic acid and almost completely reject oil droplets whose sizes are below 10 mm. In reality, the regenerated draw solution can be used for the next rounds of the FO-MD process. Since the acetic acid concentration in the draw solution will be increased in each round until an equilibrium state is reached between the feed solution and the draw solution, a higher acetic acid recovery ratio is expected in the repeated FO-MD processes.
4.
5 M NaCl as the draw solution (series#2)
Conclusion
This study has investigated the applicability of hybrid FO-MD systems for sustainable water recovery and acetic acid reuse from oily wastewater by employing lab-fabricated CTA TFC and PVDF hollow fiber membranes. Firstly, the effects of oil concentration and operation time on the FO process for oil/ water separation were studied, which revealed that 1) oil fouling on membrane surface is fast in a few hours and increases slightly with oil concentration; 2) the extent of oil fouling is related to water flux, 3) oil fouling is much less in the FO mode than the PRO mode and 4) no further fouling occurs in the extended operation time over 24 h. Secondly, the water flux and solute rejection in separate FO and MD operations were characterized at 60 C. A large water flux, a high oil removal ratio and a moderate acetic acid permeation rate were observed in the FO process, while an ultrahigh NaCl rejection was achieved in the MD process to regenerate the draw solution. Thirdly, in the batch-mode hybrid FO-MD system, a three-stage flux pattern was observed during the
FO operation while the flux maintained high throughout the tests, and almost constant water flux was obtained from MD regardless of the feed difference in salt concentration. Since the hybrid FO-MD system has (1) demonstrated a water recovery of 90% even in the presence of relatively high feed salinity, (2) almost completely rejected oil and NaCl, and (3) partially reused the acetic acid, it has great potential for the water reuse from oily wastewater.
Acknowledgment This research was funded by the Singapore National Research Foundation under its Competitive Research Program for the project entitled, “Advanced FO Membranes and Membrane Systems for Wastewater Treatment, Water Reuse and Seawater Desalination” (grant number: R-279-000-336-281 and R-279-000-339-281). The authors would like to thank Mr. Ruben Toh, Dr Li Pei and Dr Duong Hoang Hanh Phuoc for their help.
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.watres.2013.12.044.
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Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/watres
Sustainable water recovery from oily wastewater via forward osmosis-membrane distillation (FO-MD) Sui Zhang, Peng Wang, Xiuzhu Fu, Tai-Shung Chung* Department of Chemical and Biomolecular Engineering, National University of Singapore, 117576 Singapore, Singapore
article info
abstract
Article history:
This study proposed and investigated a hybrid forward osmosis e membrane distillation
Received 18 October 2013
(FO-MD) system for sustainable water recovery from oily wastewater by employing lab-
Received in revised form
fabricated FO and MD hollow fiber membranes. Stable oil-in-water emulsions of different
26 December 2013
concentrations with small droplet sizes (99.9 >99.9 e 99.99
82.1 2.0 74.0 2.5 e 47.1 3.1
26.7 40.2 26.3 5.8
3.5 2.1 2.9 0.5
a
The water flux in FO was obtained with DI water as the feed and 2 M NaCl as the draw solution, and in MD it was obtained when 2 M NaCl is the feed. b FO-re indicates the membrane performance at room temperature after immersing it in 60 C water for 24 h.
4.1 0.8 gMH at room temperature, verifying low salt permeation of the FO membrane to NaCl. The oil content is not detected by both UV and TOC measurements, which proves FO to be an effective process to remove oil from oily water. Since the detection limit of TOC is 1ppm, it can be calculated from Eqs. (4) and (5) that the minimal rejection to oil is 99.9%. However, acetic acid is not well rejected by the FO membrane possibly due to its small size and hydrophilic nature. A rejection rate of only 82.1 2.0% is achieved, or in other words, around 179 ppm acetic acid is still found in the permeate when the feed contains 1000 ppm acetic acid. Nevertheless, since acetic acid works as the chemical additive in the oil production process, it is preferred to remain in the permeate stream as much as possible so that minimal replenishment of acetic acid is needed for reuse. Since the diluted draw solution after FO operations is continuously concentrated by the MD operation which is normally operated at a higher temperature, the water flux and solute rejection of the FO operation at 60 C are also characterized. Table 2 shows that the water flux is largely enhanced from 26.7 3.5 LMH to 40.2 2.1 LMH when the operation temperature is elevated from 23 to 60 C using DI water as the feed. Several factors contribute to this increment: (1) the osmotic pressure is higher based on the van’t Hoff equation, (2) similar to that of reverse osmosis membranes (McCutcheon and Elemelech, 2006), the reduced water viscosity facilitates its diffusion rate across the membrane, and (3) the increased diffusion coefficient of NaCl lowers the solute resistivity in the support layer and hence reduces the impact of internal concentration polarization. Even though the salt reverse flux is increased as well, the ratio of reverse salt to water fluxes is still kept at around 0.18 g L1, making the membrane quite suitable for salt e water separation at 60 C. Interestingly, the oil rejection at 60 C is again higher than 99.9% in the FO process, while the rejection for acetic acid is lowered down to 74.0 2.5%. Clearly, operating the FO process at elevated temperatures not only draws water from oily wastewater at a higher permeation rate, but also enhances the amount of acetic acid in the permeate stream while maintaining a very low oil concentration in the permeate. In addition, to verify the membrane stability at different temperatures, the membranes were immersed in DI water at 60 C for 24 h and then re-tested in FO at room temperature (23 C). Similar results are obtained before and after hot water immersion as shown in Table 2. Similar characterizations were conducted on PVDF hollow fiber membranes for the MD operation. As shown in Fig. 2 and Table 1, the PVDF membranes have dual macrovoid layers in
the cross-section which offer a high porosity of 86 6%, a microporous selective layer on the outer surface and a fully open-channeled inner surface. The membrane is relatively hydrophobic with a contact angle of 88 2 and an effective mean pore size of 0.161 0.004 mm. Table 2 shows that clean water can be collected at the rate of 5.8 0.5 LMH in the MD process where 2 M NaCl flows through the shell side of the fibers as the feed at 60 C. The NaCl rejection reaches as high as 99.99%. In addition, acetic acid passes through the membrane at a ratio of 52.9%, or only 47.1% of acetic acid stays in the feed side. The permeation rates of both water and acetic acid are highly dependent on their saturated vapor pressures (Gryta, 2002; Li and Sirkar, 2004; Edwie and Chung, 2012). Since the water mole fraction in the feed mixture is close to 1, the vapor pressure of both liquids can be estimated individually by the Antoine Equation (Antoine, 1888). They are about 0.120 atm and 0.196 atm for acetic acid and water, respectively. In other words, the amount of acetic acid from the feed solution to the permeate is approximately 60% of that of water. However, this theoretical ratio is slightly higher than the experimental result of 52.9% probably due to other factors such as molecule size, viscosity and barrier effects of the membranes, etc.
3.3.
Water recovery from the hybrid FO-MD process
To simulate the hybrid FO-MD process in the real applications, an 800 ml feed solution containing NaCl, petroleum, Tween 80 and acetic acid was continuously run through the CTA TFC FO hollow fiber module using concentrated NaCl as the draw solution until the recovery rate reaches more than 90%, and then the diluted draw solution was concentrated by the PVDF MD membrane module until a similar amount of the permeate water was collected from the other side. In both operations, the high salinity draw solution is kept at 60 C. As mentioned in Section 2.5, two sets of NaCl concentrations were selected to recover water from oily wastewater: series #1, where 0.04 M NaCl and 2 M NaCl were used as the feed and draw solutions, respectively; and series #2, where the corresponding NaCl concentrations were changed to 0.2 M and 5 M. Fig. 5 illustrates the water flux, calculated NaCl concentration in the feed and draw solutions and feed water recovery in the batch-mode FO-MD process as a function of time. The initial water flux in FO for series 1 and 2 begins from different values as shown in Fig. 5A. Series #1 starts from around 32 LMH, which is approximately 80% of the value using DI water as the feed, mainly due to the presence of oil droplets and NaCl in the current feed solution. A higher initial water flux is
119
w a t e r r e s e a r c h 5 2 ( 2 0 1 4 ) 1 1 2 e1 2 1
45
NaCl concentration, M
Water flux, LMH
40 35 30 25 20 15 10
0
10
20 Time, hr
30
Feed recovery, %
Water flux, LMH
3 2 1 0
5
4 2
10
15 20 25 Time, hr
30
60
35
(D)
#1: FO #2: FO #3: MD #4: MD
80
6
(B)
4
100
#1 #2
8
5
0
40
(C)
10
#1: draw solution #2: draw solution #1: feed solution #2: feed solution
6
(A)
#1 #2
40 20 0
0
0
10
20
30
40
50
Time, hr
0
10
20 30 Time, hr
40
50
Fig. 5 e (A) water flux in FO, (B) calculated NaCl concentrations in the draw and feed solutions in FO, (C) water flux in MD and (D) feed water recovery of the FO-MD hybrid system at 60 C. After the feed water recovery reached around 90%, the diluted draw solution was connected to a MD system (maintained at 60 C) and the permeate was collected. A dashed line is drawn in Fig. 5(A) to guide the readers of the flux reduction trend. #1 and #2 refer to series #1 and series #2 as specified in the text, respectively.
achieved for series #2 despite of its high salinity in the feed, owing to the substantial increment in draw solution concentration. However, the water flux of both series undergoes similar trends against time. Guided by the black dashed line drawn in the figure, the flux pattern is characterized by three stages: a significant flux drop appears in the first 7e10 h, followed by mild flux decay until 20e25 h, and then a fast flux decline again in the third stage. The decline phenomena may be explained by Figs. 4 and 5B. Consistent with the long-term studies in Fig. 4, the water flux drops in the first several hours partially due to fouling. In addition, Fig. 5B shows that the feed salinity is continuously increased because water permeates from the feed to the draw solution. The salt accumulation in the feed becomes serious at the later stage when the recovery is high (Fig. 5D). Meanwhile, the draw solution is continuously diluted, leading to a decreased effective osmotic driving force across the membranes. Moreover, the increasing concentration of oil and acetic acid in the feed also contributes to the flux decline. As a result, the combined effects of increasing fouling and decreasing driving force cause a large flux drop in the 1st stage (i.e., the first 7e10 h). The flux decline becomes milder in the 2nd stage since fouling is gradually stabilized. Since the feed salinity is substantially increased when higher recovery is reached in the 3rd stage, a sharper decrease in flux is observed in this stage. The final feed salinity is 0.47 M and 2.11 M for series #1 and #2, respectively, which corresponds to the recovery of 91.5% and 90.5%, respectively. Note that the salt
reverse diffusion into the feed solution is not taken into account considering the low salt reverse flux of the FO membranes. Interestingly, series #2 starts the 3rd stage phenomenon (i.e., big flux drop) earlier than series #1. This is due to the fact that the former has a higher initial feed concentration than the latter. As a result, the effect of high feed salinity on flux at a high recovery is more significantly magnified in the former than the latter. However, it is worth noting that the water fluxes in both cases always stay above 14 LMH even at more than 90% recovery. Clearly, FO could effectively recover water from oily wastewater even in the presence of relatively high feed salinity. Table 3 summarizes the initial and final compositions of the feed, draw solution and permeate for the FO and MD operations. The permeate concentration is in fact calculated from the difference between the initial and final composition of the draw solution. Since oil was not detected by UV in the permeate, a word of “trace” is given in the table, again proving an ultrahigh oil rejection rate by FO. The concentrations of acetic acid in the permeate are 237 and 219 ppm for series #1 and #2, respectively, which correspond to more than 70% retention against their initial feed concentration of 1000 ppm. Consequently, the oil concentration in the draw solutions is negligible, and the acetic acid content is around 50 ppm. The subsequent regeneration of the diluted draw solution by MD is shown in Figs. 5C and D as well. Relatively constant water fluxes are found against operation time in Fig. 5C despite of the increment in NaCl concentration. Interestingly,
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w a t e r r e s e a r c h 5 2 ( 2 0 1 4 ) 1 1 2 e1 2 1
Table 3 e The initial and final compositions of the feed solution, permeate and draw solution for FO and MD of the hybrid FO-MD system at 60 C. Composition
Initial feed solution (FO) Permeate (FO) Final feed solution (FO, calculated) Draw solution (FO) at the end of the FO operation (The initial composition for the MD) Permeate (MD) Final draw solution (MD, calculated)
2 M NaCl as the draw solution (series#1) Oil, ppm
Acetic acid, ppm
NaCl
Oil, ppm
Acetic acid, ppm
NaCl
4000 Trace 47,000
1000 237 9213
0.04 M e 0.47 M
4000 Trace 42,200
1000 219 8440
0.2 M e 2.11 M
Trace
54
1.55 M
Trace
49
3.87 M
e Trace
29 61
1 ppm 2.0 M
e Trace
28 55
3 ppm 5.0 M
the water fluxes of series #1 and #2 are almost the same. It shows that water permeation rate in MD is not significantly affected by the salt concentration. Therefore, a high draw solution concentration is preferred for FO-MD hybrid systems in pursuit of large water flux in FO. After 42e45 h, around 720 ml distilled water (or 90% recovery in regards to the initial feed volume of the FO-MD hybrid system) is collected (Fig. 5D). Table 3 shows that the content of NaCl and acetic acid in the final permeate are as low as 1 or 3 ppm, and the acetic acid concentration reaches 28 or 29 ppm. The draw solution salinity is recovered to their initial status. The experiments not only demonstrate efficient water recovery from oily wastewater by the proposed FO-MD process with at least 90% water recovery rate, but also partially recover acetic acid and almost completely reject oil droplets whose sizes are below 10 mm. In reality, the regenerated draw solution can be used for the next rounds of the FO-MD process. Since the acetic acid concentration in the draw solution will be increased in each round until an equilibrium state is reached between the feed solution and the draw solution, a higher acetic acid recovery ratio is expected in the repeated FO-MD processes.
4.
5 M NaCl as the draw solution (series#2)
Conclusion
This study has investigated the applicability of hybrid FO-MD systems for sustainable water recovery and acetic acid reuse from oily wastewater by employing lab-fabricated CTA TFC and PVDF hollow fiber membranes. Firstly, the effects of oil concentration and operation time on the FO process for oil/ water separation were studied, which revealed that 1) oil fouling on membrane surface is fast in a few hours and increases slightly with oil concentration; 2) the extent of oil fouling is related to water flux, 3) oil fouling is much less in the FO mode than the PRO mode and 4) no further fouling occurs in the extended operation time over 24 h. Secondly, the water flux and solute rejection in separate FO and MD operations were characterized at 60 C. A large water flux, a high oil removal ratio and a moderate acetic acid permeation rate were observed in the FO process, while an ultrahigh NaCl rejection was achieved in the MD process to regenerate the draw solution. Thirdly, in the batch-mode hybrid FO-MD system, a three-stage flux pattern was observed during the
FO operation while the flux maintained high throughout the tests, and almost constant water flux was obtained from MD regardless of the feed difference in salt concentration. Since the hybrid FO-MD system has (1) demonstrated a water recovery of 90% even in the presence of relatively high feed salinity, (2) almost completely rejected oil and NaCl, and (3) partially reused the acetic acid, it has great potential for the water reuse from oily wastewater.
Acknowledgment This research was funded by the Singapore National Research Foundation under its Competitive Research Program for the project entitled, “Advanced FO Membranes and Membrane Systems for Wastewater Treatment, Water Reuse and Seawater Desalination” (grant number: R-279-000-336-281 and R-279-000-339-281). The authors would like to thank Mr. Ruben Toh, Dr Li Pei and Dr Duong Hoang Hanh Phuoc for their help.
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.watres.2013.12.044.
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