Short- and Long-Term Performance of the Thin-Film Composite

Aug 21, 2014 - The water and salt reverse fluxes of the TFC-FO hollow fiber ... Overall, the newly developed FO membranes can recover ∼80% of the wa...
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Short- and Long-Term Performance of the Thin-Film Composite Forward Osmosis (TFC-FO) Hollow Fiber Membranes for Oily Wastewater Purification P. Li,†,‡ S. S. Lim,† J. G. Neo,† R. C. Ong,† M. Weber,‡ C. Staudt,§ N. Widjojo,∇ C. Maletzko,⊥ and T. S. Chung*,† †

Department of Chemical & Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117585 College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China § Advanced Materials & Systems Research, BASF SE, GMV/W-B001, 67056 Ludwigshafen, Germany ∇ A-GMM/F, BASF South East Asia Pte., Ltd., 61 Science Park Road, #03-01, The Galen, Singapore Science Park II, Singapore 117525 ⊥ Engineering Plastics, BASF SE, E-KTE/NE-F206, 67056 Ludwigshafen, Germany ‡

S Supporting Information *

ABSTRACT: Fouling behavior of thin-film composite (TFC) membranes synthesized on sulfonated polyphenylenesulfone (sPPSU) hollow fiber substrates was investigated for separating oil−water emulsions under forward osmosis (FO). The water and salt reverse fluxes of the TFC-FO hollow fiber membranes were found to be stable for a period of 28 days when deionized (DI) water was used as feed. A series of fouling experiments were carried out, and it was observed that water flux decreased faster with increasing oil concentration of the feed solution. In addition, the rate of flux decline was rapid in the initial few hours, because of the cake-enhanced concentration polarization (CECP). Furthermore, we observed that the ratio of salt reverse flux to water flux (Js/Jw) was much lower, compared to that of using DI water as feed. This may indicate that the oil fouling plugs some defects in the selective layer and the oil fouling layer hinders the salt transport, because of the CECP mechanism. Further investigations revealed that the FO fouling could be efficiently washed off using a solution containing 1 g/L NaOH and 0.3 g/L sodium dodecyl sulfate (SDS), and the water flux could be effectively recovered, to a large extent, within 5 days when separating a 500 ppm oil−water emulsion. Overall, the newly developed FO membranes can recover ∼80% of the water at a high average water flux of 10.4 LMH using 1 M NaCl as draw solution from a 500 ppm oil−water emulsion containing a low salt concentration of 0.5 g/L. This, in turn, demonstrates the potential of the FO membranes for oily wastewater reclamation. treatment systems, which are able to remove droplets >10 μm in size. As such, these small dispersed oils may move upward and spread on water surface and result in sheening as well as enhancing biological oxygen demand (BOD) near the wastewater discharged area.15,16 As listed in Table 1, the compositions of the contaminants in oily wastewater vary greatly from different regions and sources.17−19 In all three sources (oil field, gas platform, and shale gas), the concentration of salt in oily wastewater ranges from 100 mg per liter to 97 g per liter and the oil concentration is noted to be less than 655 ppm (weight/weight). Thus, if we target to recover 80% of produced water, the final oil concentration in the feed solution must be 99.3%, indicating that the newly developed FO membranes can efficiently separate high-purity water from the oil−water emulsions. Compared with the water flux and Js/Jw values when using DI water as the feed, the water fluxes of the oil−water separation are 10%−35% lower and the Js/Jw values are 3−4 times smaller. Clearly, the formation of the oil fouling layer hinders the water and salt transport. Figure 2 presents the declines in water flux over 12 h for separating water from oil− water emulsions. It can be seen that a higher NaCl concentration in the draw solution corresponds to a higher water flux; the decrease in water flux is more severe as the oil concentration increases; the flux reduction in the first several hours is more significant and, subsequently, the rate of flux reduction will decrease; and the decline rates of normalized

(5)

where Ct and Vt are the salt concentration and the feed volume at the end of FO tests, respectively. 2.5. Determination of Kinematic Viscosity of the Oil− Water Emulsions. Kinematic viscosities (mm2/s) of the oil− water emulsions were measured using an Ubbelohde viscometer (SCHOTT, Germany, Type No. 532 10/I) via eq 6: (6)

v = Kt

0.24 0.02 0.10 0.04 0.02

where V is the volume flow rate of oil−water emulsions (m /s), d the hollow fiber inner diameter (given in meters), and A the cross-section area of the hollow fiber inner surface (m2). In this study, the flow rate of the feed solutions (oil−water emulsions) was fixed at 0.1 L/min and that of the draw solution was kept at 0.2 L/min. The diameters of all hollow fiber lumen sides were 850 μm. As shown in Table 2, the estimated Re values of all oil−water emulsions and DI water were determined to be within a range of 518.7−536.5. Since Re < 2300, we could conclude that the flow at the lumen side was a laminar flow. 2.6. Fouling Evaluation. The water flux (given in terms of LMH), salt reverse flux (given in terms of gMH), oil rejection, and fouling behavior (i.e., decrease in water flux versus operation time) were our interests. For short-term measurements of fouling behavior, we carried out the FO performance for 12 h. To get a relatively stable condition for FO experiments, we used a large volume (3 L) of the feed and draw solutions to maintain the stability of the experimental condition. By using this method, changes in the concentrations of salt and oil in the draw and feed solutions were within 10%. For the study of the long-term fouling behavior, NaCl was added in the draw solution and fresh water was introduced to the feed for every 12−18 h.

where Cp is the oil concentration in the draw solution and Vdraw is the volume of the draw solution. Cf is estimated using eq 4:

Cf ≈

± ± ± ± ±

3

Cp(Vdraw + ΔV ) ΔV

100.25 100.48 100.73 101.68 103.71

ν (mm2/s)

number (Re) of all oil−water emulsions and DI water were also estimated using eq 7:

where R is the oil rejection, Cep the effective oil concentration in the draw solution, and Cf is the average oil concentration in the feed solution. Cep is estimated using eq 3: Cep =

flow time (s)

where ν is the kinematic viscosity (expresed in units of mm /s), K is the viscometer constant (given in mm2/s2), and t is the liquid flow time (given in seconds). To obtain the viscometer constant K, the flow time of DI water was measured at 23 °C. Since the kinematic viscosity of water was 0.933 mm2/s at the specific temperature,33,34 a value of K = 0.009307 mm2/s2 was deduced using eq 6 by measuring the flow time of DI water. Subsequently, the kinematic viscosities of all oil−water 2

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organic fouling occurs on salt-rejecting membranes, the fouling layer decreases the water flux via the following two mechanisms: (i) an increase in hydraulic resistance, because of low water convection flow in the fouling layer; and (ii) an enhancement in salt concentration at the interface between the membrane and organic fouling layer, as illustrated in Figure 3.35

Table 3. Average Water Flux, Salt Reverse Flux, and Oil Rejection of the 12-h FO Experiments feed and draw condition (under FO mode)

water flux (LMH)

salt reverse flux (gMH)

Js/Jw (g/L)

average oil rejection (%)

500 ppm, 0.5 M 500 ppm, 1.0 M 500 ppm, 2.0 M 1500 ppm, 0.5 M 1500 ppm, 1.0 M 1500 ppm, 2.0 M 3000 ppm, 0.5 M 3000 ppm, 1.0 M 3000 ppm, 2.0 M DI water, 0.5 M DI water, 1.0 M DI water, 2.0 M

12.3 16.6 23.3 10.9 14.9 20.9 8.5 14.2 16.6 16.1 19.2 25.4

1.30 1.57 3.84 1.21 2.33 2.33 0.71 1.65 2.98 5.41 5.64 7.28

0.11 0.09 0.16 0.11 0.16 0.11 0.08 0.12 0.18 0.34 0.29 0.44

99.96 99.35 99.62 99.51 99.87 99.90 99.68 99.83 99.88

water flux using 1.0 and 2.0 M NaCl draw solutions are less than that using 0.5 M NaCl as a draw solution. The first two observations are normal. A higher salt concentration in the draw solution leads to a higher osmotic pressure across the membrane, so that the water flux is higher. Since the increment in oil concentration in the feed solution causes a faster formation of the organic fouling layer that hinders the water transport, the water flux decreases more as the oil concentration increases. The third observation regarding flux reduction has been reported by many researchers who attribute this to the salt permeation to the feed solution that accelerate the cakeenhanced concentration polarization (CECP).35−39 When

Figure 3. A conceptual illustration of the effect of draw solute reverse diffusion on cake-enhanced concentration polarization (CECP) in FO. Reproduced from ref 35.

The cake layer impedes salt from transporting to the bulk of the feed solution. This phenomenon results in a higher NaCl concentration at the membrane surface facing the feed, which not only exacerbates CECP but also reduces the net osmotic

Figure 2. Twelve-hour (12-h) fouling behaviors of the 1.5 mol % sPPSU TFC-FO hollow fiber membranes with different feed (500, 1500, and 3000 ppm oil/water emulsions) and draw solutions (0.5, 1.0, and 2.0 M NaCl) under the FO mode. 14059

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Figure 4. 48-h fouling behaviors of the TFC-FO hollow fiber membranes for separating a 500-ppm oil−water emulsion using draw solutions of 0.5, 1.0, and 2.0 M NaCl, respectively; the oil concentration in the feed and salt concentration in the draw solutions are topped up every 12 h to maintain a stable operation conditions.

pressure. Consequently, a sharp reduction in water flux is observed after the first few hours. Table 3 demonstrates that the salt reverse fluxes and the Js/Jw ratio of the oil−water separation systems are much lower than those FO systems using DI water as a feed solution. The defects in the selective layer of the FO membrane plugged by the oil fouling and/or the oil fouling layer may provide a greater resistance for NaCl transport, compared to that for water. To the best of our knowledge, it is the first time to observe the hindrance of salt reverse diffusion in a FO system. After the first few hours, since the salt reverse flux was greatly reduced, the effect of CECP became less significant and the rate of flux decline decreases radically. The last observation can be attributed to the selfcompensation mechanism.40 In a FO system, the supported layer of the membrane hinders the convection of NaCl and the draw solution. Because of the fresh water diffusing from the feed solution, the NaCl concentration on the surface of the active layer (facing the draw solution side) is lower than that of the bulk draw solution. Thus, the water flux across the membrane declines, because of this internal concentration polarization (ICP). However, the dilution effect by fresh water diffusing from the feed solution on the NaCl concentration of the active layer is reduced when fouling occurs. Thus, the net osmotic gradient across the membrane increases and alleviates the water flux decline. When the salt concentration in a draw solution is high, the ICP is more severe and the difference in the salt concentration between the active layer and the bulk phase is larger. Therefore, as the water flux from the feed solution decreases because of fouling, the salt concentration at the active layer increases more and results in a higher compensation to the decline of normalized water flux. If only the self-compensation mechanism is considered, using 2.0 M NaCl as a draw solution shall exhibit the best “anti-fouling” behavior, compared with 0.5 and 1.0 M NaCl draw solutions. However, the decline in normalized water fluxes is faster when using a 2.0 M NaCl draw solution than using a 1.0 M NaCl. This is due to the fact that the former has a higher rate of fouling formation than the latter. As the salt concentration increases, both the water flux and the salt reverse flux increase. A higher water flux leads to a higher hydro-

dynamic drag force for promoting foulant deposition.38 In addition, a higher salt reverse flux can further accelerate CECP. Therefore, the normalized water flux of the former decreases faster than the latter. 3.2. Long-Term Fouling Behaviors. Since the oil concentration in oily wastewater from gas platforms, oil and shale gas fields is below 655 ppm, we chose an oil−water emulsion containing 500 ppm oil for the study of long-term FO separation performance in order to mimic the real wastewater condition. Figure 4 gives the 48-h fouling behaviors for separating the 500-ppm oil−water emulsion. During the operation time, salt and DI water were added to the feed and draw solutions every 12 h to maintain stable concentrations in both the feed and draw solutions. The major declines in water flux occurred during the first 12 h, which was caused by the effects of fouling and CECP. When salt was added into the draw solution, the water flux increased due to the recovery of osmotic pressure. From 12 h to 48 h, the water fluxes of the FO experiments using 0.5, 1.0, and 2.0 M NaCl draw solutions decreased from 10.6, 15.8, and 20.3 LMH to 8.6, 14.8, and 19.2 LMH, respectively. As shown in Figure 4b, the normalized water fluxes of 1.0 and 2.0 M NaCl draw solutions only drop 5% compared with the fluxes obtained at the 12th hour; however, that of the 0.5 M NaCl draw solution decreases 20%. This result again proves that a higher salt concentration in the draw solution can compensate the CECP and mitigates fouling effects. 3.3. Long-Term FO Performance and Membrane Cleaning Procedures. Studies on fouling in FO processes have been reported by many groups.35−37,40−49 Without membrane cleaning, the water fluxes of the newly developed membranes drawn from a 500-ppm oil−water emulsion using 0.5, 1.0, and 2.0 M NaCl draw solutions decrease to 66%, 81%, and 82% of the original values, respectively, after 48-h operations. Therefore, the following section aims to find a suitable cleaning method to rejuvenate the membrane and recover its water flux. Initially, the cleaning process was done as follows. First, we stopped the FO experiment. Second, we started to prepare the washing solutions that took a few minutes. Third, the solutions flowed to the membrane modules to wash away the fouling. 14060

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Use the FO system for separating a 500-ppm oil−water emulsion by means of 0.5 M NaCl draw solution as an example. After washing, a decrease in water flux of more than 50% was observed no matter what types of washing solutions were tried in this report. The possible reason is that once the feed solution stops flowing, the type of filtration changes to the dead-end filtration. All newly formed organic fouling as well as the back diffusing salt cannot be flushed away but accumulate on the membrane surface and pores. This kind of fouling cannot be washed away efficiently by the membrane cleaning methods applied in this work. As a consequence, the water flux is greatly reduced. Therefore, we need to keep water continuously flowing through the membrane modules to avoid irreversible flux drops during the entire fouling experiments. In other words, the feed and draw solutions must be switched to the washing solution immediately after the fouling experiments. A similar operation protocol has been reported by Mi and Elimelech.42 By doing so, the fresh DI water in the original draw solution side may transport through the substrate, because of the osmotic gradient, and facilitate the detachment of foulants from the membrane surface. In addition, the fresh DI water in the original feed side may wash away the detached foulants. Therefore, continuous water flows in both the feed and draw sides of the membrane are essential to clean up FO membranes. The feed oil−water flow rate in the aforementioned studies was 60 cm/s (0.1 L/min). Mi and Elimelech observed that a better recovery of water flux and a shorter cleaning time could be obtained by increasing the flow rate of the wash solution.42 Therefore, we increase the flow rate of the wash solutions to 300 cm/s (0.5 L/min), the highest flow rate that the experiment setup can reach. Under this condition, we observe a pressure buildup of 0.6 bar at the feed side of the membrane; while at the draw solution side, the pressure buildup is minor. Three methods were utilized to clean the FO membranes. The first method was to wash both sides of the membrane with DI water for 30 min. The second method was to wash both sides with DI water for 5 min, and then washed thereafter with a 2 M NaCl solution in the feed side for the purpose of osmotic back wash for 15 min, followed by washing with DI water for 30 min. Lastly, the third method was to wash both sides with DI water for 5 min, and then wash the feed side using a solution containing 1 g/L NaOH and 0.3 g/L sodium dodecyl sulfate (SDS) for 15 min, followed by washing with DI water for 10 min. For the second method, the system must be washed for 30 min to remove the residue NaCl in the feed side. Figure 5 shows that the water flux decreases to 75% just after tops up the salt concentration. After three membrane cleaning processes, the water fluxes recover to 84%, 88%, and 90%, respectively. Consistent with our expectation, the high pH solutions (pH ∼11.5) consisting of SDS as a surfactant show the most effectiveness to remove the oil foulant. Note that this method is widely used to clean the organic foulants of natural origin on reverse-osmosis membranes.50,51 Figure 6 presents the 120-h (5-day) FO performance by using the third method for membrane cleaning every 12−18 h. The empty red dots represent the flux declines without membrane cleaning for comparison. The fluxes just after cleaning are significantly higher than the reference values for all three cases (i.e., NaCl concentrations at 0.5, 1.0, and 2.0 M, respectively). The high fluxes cause severe dilution in the draw solution and reduce their osmotic pressures. Therefore, the flux decline rates are higher than those of the reference curves and

Figure 5. A comparison of cleaning efficiency using different cleaning procedures. Membrane modules are applied to separate a 500-ppm oil−water emulsion using 0.5 M NaCl as a draw solution for 12 h. The initial water flux is 12.3 LMH.

Figure 6. Comparison of FO performance between with and without membrane cleaning using 0.5, 1.0, and 2.0 M NaCl as draw solutions and a 500-ppm oil−water emulsion as the feed.

sometimes the fluxes are lower than the reference fluxes, especially at the moment just before membrane cleaning. During a period of 5 days, the average water fluxes using 0.5, 1.0, and 2.0 M NaCl as draw solutions are 9.5, 14.4, and 18.2 LMH, respectively. On the other hand, the fluxes just after the last membrane cleaning are 9.7, 15.4, and 20.0 LMH, respectively. In other words, the series of membrane cleaning restore the fluxes to 81%, 83%, and 86% of the initial values. In some literatures,35−40,42,43 the flux was recorded after 5−24 h of FO experiments for system stabilization. To properly compare the recovery of water flux, we choose the fluxes of the first hour in the second FO circle to be the initial fluxes, then the normalized final fluxes decrease to 92%, 93%, and 95%, respectively. Hence, the membrane cleaning method is effective and the long-term FO performance is very stable for oil−water separation. 3.4. The Impact of Salt Concentration in Feed Solutions on FO Performance. In the real world, the amount of salts in the oily wastewater varies greatly by different sources and regions.17−19 Two 500-ppm oil−water emulsions containing (1) a low NaCl concentration of 0.5 g/L and (2) a high NaCl concentration of 29.2 g/L (i.e., 0.5 M) were 14061

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Figure 7. Comparison of FO performance using different feed conditions and 1 M NaCl as a draw solution: (a) declines in water fluxes of oil−water emulsions with the NaCl concentrations of 0.5 g/L or 29.2 g/L (empty squares and diamonds represent the flux declines of the feed solutions without any salt and acting as the reference lines); and (b) a comparison of fouling behavior between a fixed feed condition and an 80% weight loss in the feed solution.

therefore prepared to test these membranes by measuring their 12-h flux declines using 1 M NaCl as a draw solution. Compared with emulsions containing no salt (i.e., the reference lines), Figure 7a shows sharp declines in water flux when emulsions contain salt. A low concentration of NaCl (0.5 g/L) in the feed causes a flux reduction of 24% (11.8 LMH), while a high amount of NaCl (29.2 g/L) leads to a reduction of 71% (2.9 LMH). These significant flux drops may be due to the combined effects of external concentration polarization (ECP) and CECP. The oily fouling layer not only hinders the convection of NaCl but also accumulates NaCl at the interface between the membrane and the feed. Thus, the flux decreases rapidly. Given that the industry aims to achieve an 80% recovery from oily wastewater,14 we set up a FO experiment for separating 80% water from a 500-ppm oil−water emulsion containing 0.5 g/L NaCl as feed and 1 M NaCl as the draw solution. The salt and oil concentrations in the residual feed solutions after the experiment are 2500 ppm and 2.8 g/L. The latter is slightly higher than the theoretical value of 2.5 g/L, because of reverse salt flux. Figure 7b shows the fouling curve, compared with that with the constant oil and salt concentrations (solid square data points shown in Figure 7a). The flux of the former decreases faster since the salt and oil concentrations in the feed increase with time. The average flux is 10.4 LMH. This result implies that our TFC-FO hollow fiber membranes can efficiently separate oily wastewater with a low salt concentration.

polarization (CECP) and/or pore plugging by the oil fouling, the observed water flux decreases faster in the first few hours. However, a higher NaCl concentration in the draw solution can compensate the CECP effect and slow the decrease in normalized water flux. (2) Fouling for oil−water separation can be efficiently controlled by membrane cleaning with the aid of high pH solutions (pH ∼11.5) consisting of sodium dodecyl sulfate (SDS) as a surfactant, and the resultant water fluxes are stable for a relatively long period of at least 5 days. (3) When the oil emulsions contains salt, the water flux decrease more rapidly due to both external concentration polarization (ECP) and CECP effects. (4) The newly developed FO membrane can effectively recover 80% of water from an oil−water emulsion with a similar industrial wastewater condition.

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This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: (65)-67791936. E-mail: [email protected]. Notes

4. CONCLUSIONS We have systematically studied the fouling behavior of TFC membranes synthesized on sulfonated polyphenylene sulfone (sPPSU) hollow fiber substrates for separating oil−water emulsions under forward osmosis (FO). The following conclusions can be drawn: (1) A higher NaCl concentration in the draw solution increases the water flux but results in more-severe fouling. Because of the cake-enhanced concentration

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank BASF SE, Germany for funding this work (Grant No. R-279-000-363-597). The authors would also like to thank Miss Xiu Zhu Fu for the guidance in the preparation of oil−water emulsions. The authors would also like to thank Mr. Gang Han for his valuable suggestion on interfacial polymerization. 14062

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