Flux enhancement for polypropylene microporous membrane in a SMBR by the immobilization of poly(N-vinyl-2-pyrrolidone) on the membrane surface

Journal of Membrane Science 279 (2006) 148–155

Flux enhancement for polypropylene microporous membrane in a SMBR by the immobilization of poly(N-vinyl-2-pyrrolidone) on the membrane surface Hai-Yin Yu a,b , Zhi-Kang Xu a,∗ , Ya-Jie Xie c , Zhen-Mei Liu a , Shu-Yuan Wang a a

b

Institute of Polymer Science, Zhejiang University, Hangzhou 310027, China School of Chemistry and Materials Science, Anhui Normal University, Wuhu 241000, China c College of Biochemical Engineering, Jiaxing University, Jiaxing 314001, China

Received 28 July 2005; received in revised form 18 October 2005; accepted 25 November 2005 Available online 4 January 2006

Abstract To improve its limiting flux and antifouling characteristics in a submerged membrane-bioreactor (SMBR) for wastewater treatment, polypropylene hollow fiber microporous membrane (PPHFMM) was surface-modified by the immobilization of poly(N-vinyl-2-pyrrolidone) (PVP) through air plasma treatment. Attenuated total reflection-Fourier transform infrared spectroscopy (ATR/FT-IR), X-ray photoelectron spectroscopy (XPS) and scanning electron microscope (SEM) were used to characterize the structural and morphological changes on the membrane surface. Water contact angles were also measured by the sessile drop method. Results of ATR/FT-IR and XPS clearly indicated that PVP was successfully immobilized on the membrane surface. Water contact angle on the PVP-immobilized membrane showed a minimum value of 72.3◦ , approximately 57◦ lower than that on the unmodified one. The SMBR was operated at a fixed transmembrane pressure to determine the limiting fluxes of PPHFMMs with or without PVP immobilization. The maximum relative limiting flux appeared to be 159 L m−2 h−1 for the PVP-immobilized PPHFMM with an immobilization degree of 6.8 wt.%, 54.4% higher than that of the nascent PPHFMM. After continuous operation for about 50 h, flux recovery, reduction of flux, and relative flux ratio were 53% higher, 17.9% lower and 79% higher than those of the nascent PPHFMM respectively. It was found also that the Pearson correlation between total surface free energy and flux recovery was significant, which indicated that the PVP-immobilized membrane with higher total surface free energy would have excellent antifouling characteristics. © 2005 Elsevier B.V. All rights reserved. Keywords: Surface modification; Polypropylene hollow fiber membrane; Poly(N-vinyl-2-pyrrolidone); Limiting flux; Submerged membrane-bioreactor; Antifouling characteristics; Total surface free energy

1. Introduction The combination of membrane separation with the process of biological reactor is called a membrane-bioreactor (MBR). Studies on MBR have received considerable attention due to the deterioration of the water environment and the advantages it possesses over the conventional technologies [1–5]. There are mainly two types of MBR according to the allocation of membrane modules [6]. One is the submerged membrane-bioreactor (SMBR), in which the membrane modules are submerged in the bioreactor. The other is called side stream membrane-bioreactor (SSMBR), in which the membrane modules are located outside the bioreactor. Compared with SSMBR, SMBR is favored as its ∗

Corresponding author. Tel.: +86 571 87952605; fax: +86 571 87951773. E-mail address: [email protected] (Z.-K. Xu).

0376-7388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2005.11.046

energy consumption is lower than that of the former, and the aeration can also be used to generate a cleaning action over the membrane surface without circulation of the mixed liquid. However, membrane fouling in a SMBR has been one of the major drawbacks in extending its wide application [7,8]. Membrane fouling may result from many processes such as particle deposition, macromolecule adsorption and/or internal clogging and cake build-up. Moreover, the particle matrix within MBR liquor is composed of biological flocs formed by a large number of living microorganisms along with colloidal compounds. The biomass biological characteristics, as well as the physic-chemical properties of the suspension, such as the concentration of the mixed liquor suspended solids and extracellular polymer substances, change with the change of operating conditions. As a result, membrane fouling in an MBR is very complex, unpredictable and difficult to control.

H.-Y. Yu et al. / Journal of Membrane Science 279 (2006) 148–155

Permeate flux is one of the main parameters determining the economic viability of an MBR process. Many researches have been done for its enhancement, and a series of methods, such as using unsteady flow [9], limiting the flux and flushing by big bubbles [10], have been described in the literatures. For example, using a shell-side feed axial hollow fiber in a SSMBR, Yu et al. [11] sparged air to the liquid feed stream at various aeration intensities from 0 to 254.6 m3 m−2 h−1 . As expected, the critical flux was increased from null to 41.7–50.16 L m−2 h−1 at the aeration intensity of 0 and 254.6 m3 m−2 h−1 , respectively. Chang and Judd [12] adopted two coarse bubble aeration modes in a submerged tubular MBR, and they found that the flux increased by 43% when aeration was introduced into the module. As is well known that the membrane process play an important role in an MBR, however, little work has been done to improve the antifouling characteristics of the membrane itself applied in the MBR. On the other hand, poly(N-vinyl-2-pyrrolidone) (PVP) is widely used in different aspects, such as additives, cosmetics, coatings and biomedicines due to its excellent biocompatibility and hydrophilicity [13–16]. PVP has also been used to modify the membrane surface. Higuchi et al. [17] covalently conjugated PVP onto the surface of polysulfone membranes, and they found that the modified membrane was the most hydrophilic, gave lower protein adsorption from a plasma solution and showed a much suppressed number of adherent platelets on the surface than polysulfone and other surface-modified membranes. Rovira-Bru et al. [15] used PVP as a zirconia surface modifier. It was reported the maximum adsorption capacity of protein decreased by up to about 76%. Chen and Belfort [18] had successfully modified poly(ether sulfone) ultrafiltration membrane by low-temperature helium plasma treatment followed by grafting N-vinyl-2-pyrrolidone onto the membrane surface. They also found that the modified membrane was less susceptible to BSA fouling and easier to be cleaned. On the other hand, using a chemical cross-linking method by Kang et al. [19], PVP was immobilized within chlorinated poly(vinyl chloride) membrane to improve its hydraulic permeation behaviors. It is well recognized that the antifouling characteristics of hydrophilic membranes is usually better than that of the hydrophobic ones [8,20,21], thus the modification of commonly used membranes such as polypropylene microporous membrane is very important. Many efforts have been made to reduce membrane fouling by modifying hydrophobic surface relatively to hydrophilic [21–28]. In our previous work [28], it was found that introducing hydrophilic polymers, such as PVP, on the membrane surface can enhance the resistant characteristics of protein adsorption for polypropylene microporous membrane. These results indicate that the static antifouling characteristics of polypropylene microporous membrane can be improved by such surface modification. However, to our knowledge, few results [29] have been reported to describe the effect of surface modification on the dynamic antifouling characteristics of a polymeric membrane in an MBR for wastewater treatment. In the present work, therefore, PVP was tethered onto the surface of PPHFMM, and the effects of PVP immobilization on the limiting flux and the antifouling characteristics during the membrane filtration of

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activated sludge in an aerobic SMBR for synthetic wastewater treatment were investigated. 2. Materials and methods 2.1. Materials Polypropylene hollow fiber microporous membrane (PPHFMM) and polypropylene flat microporous membrane (PPFMM) with a porosity of 45.9 ± 3.1% and an average pore diameter of 0.10 ± 0.04 ␮m were prepared with a meltextruded/cold-stretched method according to that reported by Kim et al. [30] with only minor modification. Typically, fiber grade isotactic polypropylene (FY-4012) was pretreated at 250 ◦ C in the spinning apparatus. The spinning temperature was set at 200 ◦ C, and the melt-draw ratio was 1000%. The hollow fibers were subsequently annealed at 145 ◦ C for 30 min to increase crystallinity. Then, to form microspores, the hollow fibers were cold-stretched at 60 and 110 ◦ C with a cold-stretching ratio of 120 and 300%, respectively, and reannealed at 145 ◦ C for 1 h. The inner and outer diameters of PPHFMM are 240 and 290 ␮m respectively. PPFMM with a thickness of 100 ± 3.5 ␮m was prepared by a similar process from corresponding dense film. PVP (K-30, number average molecular weight = 45,000–55,000 g/mol) was used as received. In this study, U-shape PPHFMM modules were carefully fabricated by hand. There were 100 bundles of hollow fibers within each module, and the total area of the membrane module was about 90 cm2 . All other chemicals were AR grade and used without further purification. 2.2. Surface modification of PPHFMM by air-plasma induced immobilization of PVP Before plasma treatment, PPHFMM was washed with acetone to remove any chemicals and wetting agents absorbed on the membrane surface, dried in a vacuum oven at room temperature for 24 h, and soaked in 10% PVP-ethanol solution at 30 ◦ C for 48 h. A plasma generator from Peking KEEN Co. Ltd. (China) was used. Tubular type Pyrex reactor (10 cm × 150 cm) was rounded with a pair of copper electrodes. These two electrodes were powered through a matching network by a 13.56 MHz radio-frequency generator. On the basis of systematic experiments considering surface etching and modification induced by plasma, 30 W was chosen as the applied rf power for all the experiments described here. U-shaped membrane module and PPFMM sample (soaked in 10% PVP-ethanol solution for 48 h also) were fully stretched out in a rectangular frame using rubber bands, then the frame was put in the center of the plasma reactor chamber. To control the immobilization degree, the membranes were exposed to the air plasma for different durations. After plasma treatment, the membrane was taken out from the chamber, washed intensively with methanol, aqueous sodium hypochlorite (NaOCl) solution, and de-ionized water respectively. Finally, the membrane was washed ultrasonically in deionized water for 30 s, then dried under reduced pressure. Prior

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to the measurements, the membranes were conditioned in a desiccator for at least 1 month in the presence of P2 O5 to eliminate the effect of polar groups (such as C O and C OH) generated by plasma treatment [28]. The PVP immobilization degree was obtained by the following equation: immobilization degree (wt.%) =

wa − wb × 100 wb

(1)

where wb and wa are the weight of a membrane before and after PVP immobilization, respectively. 2.3. Characterization of the membrane surface ATR/FT-IR spectra were recorded on an infrared spectrometer (Brucker Vector 22 FT-IR, Switzerland). The ATR accessory contained a ZnSe crystal at a nominal incident angle of 45◦ , yielding about 12 internal reflections at the sample surface. All spectra (16 scans at 4.0 cm−1 resolution and ratio to the appropriate background spectra) were recorded at 25 ◦ C. The chemical composition of PPHFMM surface was analyzed by XPS with a PHI 5000c XPS spectrometer (PerkinElmer Instruments, USA). As a photon source, Al K␣ radiation (1486.6 eV) was used, and the energy scale of the spectrometer was calibrated using the lowest binding energy component present in the superficial layer. A lower energy (−5 eV) electron gun was used for charge neutralization on the non-conducting samples. The composition of the membrane surface was determined from 0 to 1000 eV survey scans acquired with analyzer pass energy of 150 eV. The surface morphologies of the nascent and modified PPHFMMs were observed by SEM with a Stereoscan 260 system (Cambridge, UK). To evaluate the hydrophilicity and the total surface free energy changes of the membrane surface, PPFMM with almost similar average pore size and porosity to PPHFMM was treated under the same conditions. Then, the samples were kept in a desiccator at room temperature for at least 1 month in the presence of P2 O5 before the contact angle measurements were conducted. The contact angles of water and diiodomethane on these membrane surfaces were measured by the sessile drop method using an OCA20 system (DATA Physics, Germany), and the advancing contact angles were immediately (within 30 s) measured to avoid the influence of water and diiodomethane penetrating into the membrane pores. At least 20 contact angle measurements were obtained for each membrane, and the values besides the maximum and minimum were used for the calculation of an average data. Standard deviation between 1◦ and 4◦ was also obtained. Then the total surface free energy, the dispersion and the polar components of the total surface free energy were calculated according to literature [31]. 2.4. Filtration and antifouling properties measurements Each membrane was first pre-compacted for 30 min at 66 kPa. Then, the pressure was lowered and the de-ionized water flux (J0 ) was obtained at fixed transmembrane pressure by measuring

permeate until the consecutive five recorded values differed by less than 2%. A SMBR [29] was designed to characterize the filtration performance of the unmodified and modified PPHFMMs. U-Shaped hollow fiber membrane modules were used in the SMBR. To eliminate the influence of different physiological states of the activated sludge suspension on the filtration performance [32], all membrane modules were installed into the MBR at the same time. The bioreactor was filled with activated sludge sampled from a biological reactor in the Wastewater Treatment Plant of Westlake Beer Co. Ltd. (Hangzhou, China). The activated sludge was cultivated for 60 days till CODcr in the digest broth was nearly unchanged and lower than 50 mg/L. Synthetic wastewater used in this work was prepared from a sterile concentrated feed solution [29]. This solution was stocked in the refrigerator and was diluted 100 times with pure water (CODcr was diluted to 700 mg/L) prior to feeding into the SMBR. Permeate was continuously removed by a suction pump at fixed pressure to determine the limiting flux. After that, the applied pressure was set at 42 kPa for 50 h, and permeate was registered again. The permeation flux when five recording values differed by less than 2% (1 h for each record) was designated as Jp . After used in the SMBR, the membranes were taken out from the SMBR and cleaned with de-ionized water, and then the de-ionized water flux (J1 ) was measured at transmembrane pressure of 42 kPa. The volume flux was determined through the timed collection of permeate, and adjusted to a reference temperature of 20 ◦ C by accounting for the viscosity change of water [29]
 µT J20 = JT (2) µ20 where µ and J refer to the viscosity and the flux, and the subscripts T and 20 refer to the mean operating temperature in T and 20 ◦ C. To normalize the filtration differences that exist between different unmodified PPHFMM lots, five replicates of PPHFMMs originated from a single lot were used, and all flux data reported in this study are adjusted to 600 L m−2 h−1 at 20 ◦ C under 42 kPa. Therefore, these fluxes are relative ones. The antifouling characteristics, such as flux recovery, reduction of flux in filtration, and relative flux ratio were calculated by the following equations [33]: flux recovery =

J1 J0

(3) Jp J0

(4)

(Jp )m (Jp )u

(5)

reduction of flux = 1 − relative flux ratio =

where the subscripts m and u refer to the modified and unmodified membrane respectively. Analysis of the mixed liquid suspension solid, the mixed liquid volatile suspension solid, chemical oxygen demand, ammonia–nitrogen and dissolved oxygen for the activated sludge suspension were respectively conducted using the standard procedures described elsewhere [34].

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The statistics package of social science (SPSS, Version 11.5) was used to find out the Pearson correlations of immobilization degree, contact angle, total surface free energy, the dispersion and the polar components of total surface free energy with flux recovery, reduction of flux, and relative flux ratio respectively. 3. Results and discussion 3.1. Immobilization of PVP induced by air-plasma treatment PPHFMMs were immobilized with PVP through air-plasma treatment at 70 Pa for a given time. This could be confirmed by the ATR/FT-IR (Fig. 1) and XPS analysis (Fig. 2). It was found that the IR spectrum of PVP-immobilized membrane represented a C O absorption peak arising from the amide group of PVP at 1660 cm−1 , whilst two peaks appeared at 532.7 and 401 eV corresponding to O1s and N1s in the XPS spectrum, respectively. All these results suggested the successful immobilization of PVP induced by air-plasma treatment.

Fig. 1. ATR/FT-IR spectra of the nascent PPHFMM (a), the PVP-immobilized PPHFMMs with immobilization degree of 4.8 wt.% (b) and 6.8 wt.% (c), respectively.

Fig. 2. Survey XPS spectra of the nascent PPHFMM (a), and the PVPimmobilized PPHFMM with an immobilization degree of 6.8 wt.% (b).

Fig. 3. SEM images of the nascent PPHFMM (a), the PVP-immobilized PPHFMMs with immobilization degree of 4.8 wt.% (b) and 6.8 wt.% (c), respectively.

The surface morphologies of the nascent and PVPimmobilized PPHFMMs were observed by SEM, and the typical images are shown in Fig. 3. Compared with Fig. 3(a), it can be clearly seen from Fig. 3(b) and (c) that after air-plasma treatment, a large quantity of PVP had been immobilized on the membrane surface. The immobilization degree of PVP and the static water contact angles of the unmodified and the modified membranes are depicted in Fig. 4. It was found that, the immobilization degree increased with plasma treatment time up to 12 min, and then decreased with further increasing the time. This might be simply postulated as follows. Two main actions would take place simultaneously during plasma treatment [33]. Ablation and etching might result in the scission of the polymer chains producing free radicals, and the

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Fig. 4. Effects of air plasma treatment time on water contact angle of the membrane surface and the immobilization degree of PVP.

grafting or immobilizing of polymer fragments produced from the etched surface would introduce various functional groups on the membrane surface. Therefore, the immobilization degree of PVP might increase or decrease with plasma treatment time, depending on which of the two competing processes prevails. The results of immobilization degree of PVP on the membrane surface indicated that the latter process mainly took place before 12 min, while the former after 12 min. On the other hand, water contact angle on the modified membrane surface tended to decrease with an increase of plasma treatment time up to 12 min and then it increased. This trend was in good agreement with the relation between the immobilization degree of PVP and the plasma treatment time, namely, the higher the immobilization degree was, the lower water contact angle would be, and vise versa. For example, water contact angle on the PVP-immobilized membrane surface with an immobilization degree of 6.8 wt.% was 72.3◦ , showing approximately 57◦ lower than that of the unmodified membrane, while for the membrane with an immobilization degree of 4.0 wt.%, water contact angle was 98.2◦ . This change of water contact angle meant that the hydrophobic polypropylene membrane could be hydrophilized to certain extent by the immobilization of PVP. 3.2. Filtration and antifouling characteristics of the PPHFMMs The relative pure water fluxes before and after PVP immobilization were measured as a function of transmembrane pressure and the typical results are shown in Fig. 5. It was found that the relative pure water fluxes of PPHFMM after PVP immobilization were higher than that of the unmodified one. This was due to that after PVP immobilization on the membrane surface, its hydrophilicity increased and the hydraulic resistance decreased. Correspondingly, the relative pure water flux for the PVP-immobilized membranes increased also. The variation of relative permeate flux with step increment of transmembrane pressure was studied using a SMBR. Transmembrane pressure was first set at 6 kPa for 1 h to obtain a stabilized flux (in the present work, permeate flux stabilized within 1 h). This pressure was then raised increasingly every

Fig. 5. Dependence of relative pure water flux on pressure for the PPHFMMs before and after PVP immobilization with different immobilization degrees.

1 h to permit flux stabilizing. The simultaneous variation of transmembrane pressure and relative permeate flux with time is illustrated in Fig. 6. It was interesting to note that when transmembrane pressure was increased, the relative flux rose instantaneously to a peak value and then decayed to a plateau in about 30 min because of cake layer build-up or compression and the level increased slightly with pressure. When the transmembrane pressure exceeded a certain value, the relative flux stabilized itself nearly at the same value, and was independent of the transmembrane pressure due to the build-up of the concentration polarization adjusting itself to the new pressure. Similar observation was reported by Defrance and Jaffrin [35]. To study the effect of PVP immobilization on the limiting flux of the membranes, the relative pure water flux and stabilized flux while used in the SMBR for the membranes with different immobilization degrees of PVP were measured with the variation of transmembrane pressure, and typical results are shown in Fig. 7. As can be seen from these figures, the relative pure water flux increased linearly with the increase of transmembrane pressure. Although the relative flux for the membrane used in SMBR increased with the increase of transmembrane pressure. However, when the transmembrane pressure exceeded 20 and 27 kPa, the relative flux for the nascent and the PVP-immobilized PPHFMMs stabilized themselves, and was independent with the transmembrane pressure. The flux which is independent with transmembrane pressure is called limiting flux. It was found that

H.-Y. Yu et al. / Journal of Membrane Science 279 (2006) 148–155

Fig. 6. Variations of relative permeate flux with time under step increments of transmembrane pressure: the nascent PPHFMM (a), the PVP-immobilized PPHFMMs with immobilization degree of 4.8 wt.% (b) and 6.8 wt.% (c), respectively.

the relative limiting fluxes for the nascent and PVP-immobilized membranes with immobilization degree of 4.8 and 6.8 wt.% were 103, 142 and 159 L m−2 h−1 , respectively. The membrane antifouling characteristics, such as flux recovery, reduction of flux, and relative flux ratio are shown in Table 1. It can be seen that the PVP-immobilized membranes showed better performance, i.e. lower reduction of flux, higher flux recovery and higher the relative flux ratio than those of the unmodified PPHFMM. The PVP-immobilized membrane with an immobilization degree of 6.8 wt.% showed the lowest reduction of flux (59.2%), the highest flux recovery (89.8%) and the highest the relative flux ratio (1.79). This was due to that the foulant was

153

Fig. 7. Relative pure water flux and stabilized flux while used in the SMBR for membranes with different immobilization degrees of PVP as a function of pressure: the nascent PPHFMM (a), the PVP-immobilized PPHFMMs with immobilization degree of 4.8 wt.% (b) and 6.8 wt.% (c), respectively.

Table 1 Flux recovery, reduction of flux and the relative flux ratio for the studied PPHFMMs Immobilization degree (wt.%)

Flux recovery (%) Reduction of flux (%) Relative flux ratio

0

4.0

4.8

5.5

6.8

36.8 77.1 1.00

42.5 60.4 1.39

47.1 62.7 1.63

62.3 63.7 1.58

89.8 59.2 1.79

Note: flux recovery = J1 /J0 , reduction of flux = 1 − Jp /J0 , the relative flux ratio = (Jp )m /(Jp )u .

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Table 2 Pearson correlations between immobilization degree, contact angle, total surface free energy, dispersion of the total surface free energy between flux recovery, reduction of flux, relative flux ratios before and after surface modification

Immobilization degree Water contact angle (◦ ) Total surface free energy (mN/m) Dispersion component (mN/m) The square root of the dispersion component (mN/m) a

Flux recovery

Reduction of flux

Relative flux ratio

0.622 −0.650 0.832a −0.173 −0.180

−0.536 0.544 −0.665 0.125 0.129

0.533 −0.551 0.667 −0.124 −0.128

Correlation is significant at the 0.05 level (two-tailed).

less adherent to the more hydrophilic membrane surface and could be more easily washed off from the surface. It indicated that the surface modification of PPHFMM with PVP immobilization could improve the antifouling characteristics for this hydrophobic membrane. The interactions between the membrane surfaces and solutes in solution play an important role in determining the extent of membrane fouling. Choo and Lee [36] predicted membrane fouling trend based on the dispersion component of surface tension and they found that it was in better agreement with the experimental results rather than that based on surface hydrophobicity. To verify this and to find out the relations of immobilization degree, contact angle, total surface free energy, the dispersion component of total surface free energy, the square root of dispersion component with flux recovery, reduction of flux, and relative flux ratio, Pearson correlations (rxy ) were calculated according to the following equation by SPSS software (Table 2): i=n (xi − x¯ )(yi − y¯ ) (6) rxy = i=1 i=n 2 2 (x − x ¯ ) (y − y ¯ ) i i i=1 where x is the observed values of immobilization degree, contact angle, total surface free energy, the dispersion component of total surface free energy, and the square root of the dispersion component; xi is the number i value of x. y is the observed values of flux recovery, reduction of flux, and relative flux ratio; yi is the number i value of y. Results demonstrated that the Pearson correlation between total surface free energy and flux recovery was significant at the 0.05 level (two-tailed). It indicated that the PVP-immobilized membrane which had a higher total surface free energy would have more excellent antifouling properties, i.e., higher flux recovery. The Pearson correlation between immobilization degree and flux recovery was positive significant while the Pearson correlation between water contact angle and flux recovery was negative significant. These were in good agreement with the relation between water contact angle and immobilization degree as mentioned above. The Pearson correlation between water contact angle and flux recovery was also high. It indicated that hydrophilic membranes would have better antifouling characteristics than the hydrophobic ones. However, the Pearson correlation between the square root of the dispersion component

and reduction of flux was very low, which indicated that the present results were not in agreement with the results by Choo and Lee [36]. 4. Conclusion Poly(N-vinyl-2-pyrrolidone) (PVP) was successfully immobilized onto PPHFMM surface through air plasma treatment. ATR/FT-IR and XPS studies confirmed this immobilization. The immobilization degree of PVP increased while water contact angle decreased with plasma treatment time up to 12–16 min. The antifouling performances were investigated using a submerged membrane-bioreactor. Filtration results obviously demonstrated that the PVP-immobilized PPHFMMs showed better filtration behaviors in the submerged membrane bioreactor than its nascent analogue. The limiting flux for the PVP-immobilized membrane was 54.4% higher than that of the nascent membrane. After continuous operation for about 50 h, flux recovery, reduction of flux, and relative flux ratio were 53% higher, 17.9% lower and 79% higher than those of the nascent membrane respectively. Analysis of Pearson correlations indicated that the surface modified membrane with higher total surface free energy would possess excellent antifouling characteristics. Acknowledgements Financial supports from the High-Tech Research and Development Program of China (Grant No. 2002AA601230) and the National Basic Research Program of China (Grant No. 2003CB15705) are gratefully acknowledged. References [1] C. Visvanathan, R. Ben Aim, K. Parameshwaran, Membrane separation bioreactors for wastewater treatment, Crit. Rev. Environ. Sci. Technol. 30 (2000) 1–48. [2] T. Reemtsma, B. Zywicki, M. Stueber, A. Kloepfer, M. Jekel, Removal of sulfur-organic polar micropollutants in a membrane bioreactor treating industrial wastewater, Environ. Sci. Technol. 36 (2002) 1102–1106. [3] P. Le-Clech, B. Jefferson, S.J. Judd, Impact of aeration, solids concentration and membrane characteristics on the hydraulic performance of a membrane bioreactor, J. Membr. Sci. 218 (2003) 117–129. [4] S. Ognier, C. Wisniewski, A. Grasmick, Influence of macromolecule adsorption during filtration of a membrane bioreactor mixed liquor suspension, J. Membr. Sci. 209 (2002) 27–37. [5] E. Tardieu, A. Grasmick, V. Geaugey, J. Manem, Influence of hydrodynamics on fouling velocity in a recirculated MBR for wastewater treatment, J. Membr. Sci. 156 (1999) 131–140. [6] R. Liu, X. Huang, C.W. Wang, L.J. Chen, Y. Qian, Study on hydraulic characteristics in a submerged membrane bioreactor process, Process Biochem. 36 (2000) 249–254. [7] A.L. Lim, R. Bai, Membrane fouling and cleaning in microfiltration of activated sludge wastewater, J. Membr. Sci. 216 (2003) 279–290. [8] D.S. Kim, J.S. Kang, Y.M. Lee, The influence of membrane surface properties on fouling in a membrane bioreactor for wastewater treatment, Sep. Sci. Technol. 39 (2004) 833–854. [9] M. Mercier-Bonin, I. Daubert, D. Leonard, C. Maranges, C. Fonade, C. Lafforgue, How unsteady filtration conditions can improve the process efficiency during cell cultures in membrane bioreactors, Sep. Pur. Technol. 22–23 (2001) 601–615.

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