Surface modification of polypropylene microporous membrane to improve its antifouling characteristics in an SMBR: N2 plasma treatment

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Surface modification of polypropylene microporous membrane to improve its antifouling characteristics in an SMBR: N2 plasma treatment Hai-Yin Yua,, Xiao-Chun Hea,b, Lan-Qin Liua, Jia-Shan Gua, Xian-Wen Weia a College of Chemistry and Materials Science, Anhui Key Laboratory of Functional Molecular Solids, Anhui Normal University, Wuhu 241000, China b Nantong Vocational College, Nantong 226007, China

art i cle info

ab st rac t

Article history:

Fouling is the major obstacle in membrane processes applied in water and wastewater

Received 4 April 2007

treatment. The polypropylene hollow fiber microporous membranes (PPHFMMs) were

Received in revised form

surface modified by N2 low-temperature plasma treatment to improve the antifouling

15 June 2007

characteristics. Morphological changes on the membrane surface were characterized by

Accepted 19 June 2007

field emission scanning electron microscopy (FE-SEM). The change of surface wettability

Available online 21 June 2007

was monitored by contact angle measurements. The static water contact angle of the

Keywords: Submerged membrane bioreactor Wastewater treatment Antifouling characteristics Polypropylene hollow fiber microporous membrane N2 low-temperature plasma treatment

1.

modified membrane reduced obviously; the relative pure water flux of the modified membranes increased with the increase of plasma treatment time. To assess the relation between plasma treatment and membrane fouling in a submerged membrane bioreactor (SMBR), filtration of activated sludge was carried out by using synthetic wastewater. After continuous operation in the SMBR for about 90 h, flux recoveries for the N2 plasma-treated PPHFMM for 8 min were 62.9% and 67.8% higher than those of the virgin membrane after water and NaOH cleaning. The irreversible fouling resistance decreased after plasma treatment.

Introduction

In many areas of membrane separation process, the character of the surface, including the nature of chemical groups, spatial distribution, surface roughness or texture, especially their surface wettability, has a great effect on its performance. Polypropylene membrane exhibits high potentials for comprehensive applications due to high void volume, wellcontrolled porosity, high thermal and chemical stability, and low cost. However, the low energy surface or relatively high hydrophobicity probably lead to membrane fouling (Kilduff et al., 2005; Yang et al., 2005). It is widely accepted that the antifouling characteristics of the hydrophilic membrane are Corresponding author. Tel.: +86 553 5991165; fax: +86 553 3869303.

E-mail address: [email protected] (H.-Y. Yu). 0043-1354/$ - see front matter & 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2007.06.039

& 2007 Elsevier Ltd. All rights reserved.

better than those of the hydrophobic one. As a result, modification of polypropylene membrane from hydrophobicity to hydrophilicity is very important. Different methods such as UV irradiation, plasma treatment, gamma irradiation, and chemical reaction have been employed to modify the membrane surface (Barni et al., 2005; Cheng et al., 2006; Favia et al., 2006; Njatawidjaja et al., 2006). Among the various surface-modification techniques, lowtemperature plasma treatment is regarded as the most advantageous one. The active species generated in plasma can activate the upper molecular layers on the surface, thus making it possible to tailor surface properties to satisfy a particular request without affecting the bulk of the polymer;

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it can also provide a chemically free and environmental friendly alternative to enhance the separation performance of membranes (Gancarz et al., 2000; Upadhyay and Bhat, 2004). Intensive research on the surface modification of polypropylene microporous membranes by plasma treatment has been conducted (Kang et al., 2001; Kim and Kim, 2006); the fouling of bovine serum albumin was greatly reduced after plasma treatment. However, to our knowledge, few results have been reported to describe the effect of surface modification on the dynamic antifouling characteristics of a polymeric membrane in a membrane bioreactor (MBR) for wastewater treatment (Cho et al., 2004; Hilal et al., 2005). The MBR process has been deemed to be a promising technology for wastewater treatment and water reclamation (Kim et al., 2007; Miura et al., 2007; Pollice et al., 2007; Zhang et al., 2007). Compared with the conventional activated sludge process, an MBR system features advantages such as a small footprint, high-quality effluent, a low sludge production rate, and easy manipulation of the sludge retention time. With its effective biomass-effluent separation by membrane filtration, the MBR process is expected to lead the next generation of biological wastewater technologies. However, membrane fouling is still the major limitation to the large-scale application of the MBR process (Chae et al., 2006; Chen et al., 2007; Khongnakorn et al., 2007; Miura et al., 2007; Trussell et al., 2007; Zhang et al., 2007). Physical rinsing and chemical cleaning have to be applied frequently in the operation of an MBR (Ndinisa et al., 2006a, b; Ng et al., 2006; Wei et al., 2006), which increases the operation cost and shortens the life of the membrane. Thus, there is a need to obtain membranes with better performance for fouling control in MBR applications. The primary objective of this study is to investigate the effects of N2 low-temperature plasma treatment on membrane fouling during the filtration of activated sludge in a submerged aerobic MBR.

2.

Materials and methods

2.1.

Materials

Polypropylene hollow fiber microporous membrane (PPHFMM) and polypropylene flat microporous membrane (PPFMM) with a porosity of 45–50% and an average pore diameter of 0.10 mm were prepared with a melt-extruded/ cold-stretched method in our laboratory (Yu et al., 2006). The inner and outer diameters of PPHFMM are 240 and 290 mm, respectively. Membrane samples were washed with acetone, dried in a vacuum oven at room temperature for 24 h, and stored in a desiccator. The area of each membrane module is about 100 cm2.

frequency generator. Membranes were mounted on a clean glass stand to allow modification on either side of the sample surface. The glass stand was then placed 60 mm from the edge of plasma. Before plasma treatment, the chamber was purged four times with high-purity nitrogen and evacuated to a pressure of 5.0 Pa. The working pressure was then adjusted to 10 Pa. On the basis of systematic experiments considering surface etching and modification induced by plasma, 30 W was chosen as the applied radio-frequency power. Treatment time was changed in the range of 0–20 min. Finally, the membrane was taken out of the chamber and used for characterization and/or filtration measurement.

2.3.

Characterization of the membrane surface

Surface morphology for nascent and modified PPHFMMs was observed by a field emission scanning electron microscope (FE-SEM) with a Sirion FEG-SEM (FEI, USA). To evaluate the hydrophilicity changes of the membrane surface, PPFMM with almost similar average pore size and porosity was treated by N2 plasma under the same condition. The water contact angle on the membrane surface was measured by the sessile drop method using a DATA Physics System (OCA20, Germany). Contact angle was measured at constant temperature (25 1C). A liquid drop of 1 ml was placed onto the membrane surface by a micro-syringe. The drop image was recorded by a video camera and digitalized. The average value was obtained from at least 10 measurements tested for each membrane. The standard deviation was about 1–31.

2.4.

Filtration and antifouling properties measurements

An SMBR was designed to characterize the filtration performance of unmodified and modified PPHFMMs (Fig. 1). U-shaped hollow fiber membrane modules were used in the SMBR. 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 15 days till chemical organic demand in the digest broth was nearly unchanged and lower than 50 mg/L. Then, the SMBR was operated in the constant conditions summarized in Table 1. Synthetic

1 2 3

6 7

10

4 8

2.2. Surface modification of PPHFMM by N2 lowtemperature plasma treatment A plasma generator from Peking KEEN Co. Ltd. (China) was used. A tubular-type Pyrex reactor (10  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-

5

9

11

Fig. 1 – Schematic diagram of the experimental setup: (1) feed tank, (2) valve, (3) air flow meter, (4) liquid flow meter, (5) air pump, (6) permeate collector, (7) hollow fiber membrane, (8) air diffuser, (9) membrane bioreactor, (10) manometer, (11) suction pump.

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Table 1 – Operating conditions of the SMBR Hydraulic retention time, HRT (h) Solid retention time, SRT (d) Dissolved oxygen, DO (mg O2/L) Mixed liquor suspended solids, MLSS (g/L) pH Feed concentration (mg COD/L) Temperature (1C) Air flow rate (L/min) Trans-membrane pressure (kPa) Working volume (L)

10 Infinite 6.070.2 6.2 7.070.2 350 Room temperature 10 45 10

wastewater used in this work was prepared from a sterile concentrated feed solution. The permeate was continuously removed by a suction pump at 45 kPa for about 90 h. The permeation flux when five recording values differed by less than 2% (1 h for each record) was designated as Jp. After being used in the SMBR, the membranes were taken out from the SMBR, washed with water and 5% NaOH solution, and then the de-ionized water flux J1 (after water cleaning) and J2 (after 5% NaOH solution cleaning) were measured. The volumetric flux was determined through the timed collection of the permeate, and adjusted to a reference temperature of 20 1C by accounting for the viscosity change of water (Yu et al., 2006). J20 ¼ JT ðmT =m20 Þ,

(1)

mT ¼ m20 expð0:0239ð20  TÞÞ,

(2)

where m and J refer to viscosity and flux, and subscripts T and 20 refer to the mean operating temperature in T and 20 1C. The average pure water fluxes ranged from 280 to 420 L/h/m2 at 20 1C under 45 kPa. To normalize the filtration differences that exist between different unmodified PPHFMM lots, five replicates of PPHFMMs originating from a single lot were used, and all flux data reported in this study are relative flux (J ¼ J20/J0). The antifouling characteristics, such as flux recovery after water (FR1) and 5% NaOH cleaning (FR2), were described by the following equations: FR1 ¼ J1 =J0 ,

(3)

FR2 ¼ J2 =J0 .

(4)

The resistance-in-series model was applied to evaluate the characteristics of membrane fouling (Choo and Lee, 1998). According to this model, the relationship between permeate flux and trans-membrane pressure (DPT) can be given by Eqs. (5)–(10): J¼

DPT , ZR

Rt ¼ Rm þ Rrf þ Rirf , Rm ¼

Rt ¼

DPT , Z  Jo

DPT , Z  Jp

(5) (6) (7)

(8)

Rirf ¼

DPT  Rm , Z  J1

Rrf ¼

DPT  ðRm þ Rirf Þ, Z  Jp

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(9)

(10)

where J is relative permeate flux (%), DPT (Pa) is transmembrane pressure, Z is viscosity (Pa s1) of the permeate, Rt is total membrane resistance (m1), Rm is intrinsic membrane resistance (m1), Rrf is reversible fouling resistance (m1) due to cake formation and polarization (which can be washed off by water from the membrane surface), and Rirf is irreversible fouling resistance (m1) due to irreversible adsorption and pore plugging (which cannot be washed off by water from the membrane surface). Flux and DPT data are used to calculate resistances by Eqs. (5)–(10). Filtration of pure water with a new membrane before operation in the SMBR gives Rm; Rt is calculated from the final flux (Jp) and DPT values at the end of the 90 h operation. (Rm+Rirf) is measured after removing the cake layer by washing the membrane with tap water after the operation followed by filtration of pure water. From these values each of Rt, Rm, Rrf, and Rirf can be obtained using Eq. (6).

3.

Results and discussion

3.1.

Characterization

Three main phenomena affecting membrane properties take place normally during plasma treatment (Gancarz et al., 1999). First, ablation and etching result in the increase of pore diameter and porosity. Second, chemical changes on the surface layer introduce various functional groups that can be further used for chemical reaction or hydrophilization, depending on the plasma conditions. Third, the deposition of polymer fragments formed by the gas used and/or by the volatile products produced from the etched surface can result in lower porosity. Therefore, the pore size and pore size distribution of a plasma-treated membrane can become larger or smaller, depending on which of the two competing processes prevails (ablation or deposition). To ascertain whether the plasma adversely altered the physical properties (i.e. pore size) of the membrane, FE-SEM images were obtained for treated membranes (Fig. 2). It was found that the surface morphologies remained nearly unchanged with the increase of N2 plasma treatment time up to 16 min. However, scission of the polymer chain due to ablation and etching mainly took place for the N2 plasma treatment time of 20 min, some cracks appeared on the membrane surface, and pore diameter and porosity increased to some extent. PPFMM with a similar porosity as the PPHFMM was used for water contact angle measurement in this work. The static contact angles of the unmodified and modified PPFMMs are depicted in Fig. 3. It can be seen that the unmodified PPFMM showed the highest contact angle (128.21). This membrane can be considered as the most hydrophobic one among the studied membranes. The contact angle of the modified membrane tended to decrease with an increase of plasma treatment time. The value of PPFMM treated by N2 plasma for

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Fig. 2 – FE-SEM images of PPHFMMs. (a) Nascent PPHFMM; (b)–(d) PPHFMMs treated with N2 plasma for 8, 16, and 20 min, respectively. Table 2 – Variation of contact angle with N2 treatment pressure and storage time (d)

Water contact angle, degree

130 120

Plasma treatment time (min)

110 100

0 4 8 12 16 20

90 80 70 0

5 10 15 N2 plasma treatment time, min

0

5

35

50

128.2 94.3 86.5 98.7 90.6 86.3

128.2 90.2 86.1 78.5 73.7 71.4

128.2 92.2 93.9 88.2 87.2 87.0

128.2 93.6 94.6 91.2 88.8 97.2

20

Fig. 3 – Effect of plasma treatment time on water contact angle of the studied PPFMMs.

20 min was 86.31 and showed an approximately 421 lower value than that of the unmodified membrane. Variation of water contact angle with storage time was performed; the typical results are listed in Table 2. The water contact angle for the modified membranes decreased up to 5 d, and then increased steadily. The decrease in the water contact angle may be due to the following reason. In the case of nitrogen plasma treatment, polypropylene membrane surface interacted with N ion and electrons that are activated species in the plasma to make bond scission of C–H and C–C bonds. As a result, hydrogen atoms will be

removed from the polymer chain, and carbon radicals will be formed at the polymer chain. These carbon radicals will be successively oxidized into oxygen functional groups such as hydroxyl, carbonyl, carboxyl groups, etc., when the membrane is taken out from the plasma reactor (Wavhal and Fisher, 2002). However, with the increase of storage time, the water contact angle increased. It is commonly known that hydrophilicity gained by plasma modification is not stable; the effect can get lower and even disappear completely due to the following reasons: mobility of surface functionalities or sorption of hydrophobic moieties appearing in laboratory air. The increase of water contact angle, namely ‘‘hydrophobic recovery’’, may result from the surface rebuilding. After 35 d of storage, the surface wettability clearly changes. This is in

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good agreement with the assumption (Gancarz et al., 2000): some polar functionalities move toward bulk polymer; the rest of them modify the surface properties to make them different from what virgin samples have.

3.2.

Filtration and antifouling properties of PPHFMMs

The relative pure water flux through the membrane was measured as a function of plasma treatment time; typical results are shown in Fig. 4. The relative pure water fluxes increased with the increase of plasma treatment time. This could be explained by the increase of membrane surface hydrophilicity. Apart from that, enlargement of membrane pore size and/or the increase of porosity after plasma treatment play important roles for the enhancement of membrane permeability. To study the effect of N2 plasma treatment on the antifouling characteristics, filtration of activated sludge in an SMBR was carried out. The flux curves of unmodified and modified PPHFMMs are shown in Fig. 5. It can be seen that the flux curve for each membrane possessed the same trend, and

1.8

Relative pure water flux

1.7 1.6 1.5 1.4 1.3 1.2 1.1 1.0 0

5

10 15 N2 plasma treatment time, min

20

Fig. 4 – Dependence of relative pure water flux through PPHFMMs on N2 plasma treatment time.

0.8 Nascent PPHFMM N2 plasma treated PPHFMM for 4 min N2 plasma treated PPHFMM for 8 min N2 plasma treated PPHFMM for 12min N2 plasma treated PPHFMM for 20min

0.7

Relative flux

0.6 0.5 0.4 0.3 0.2 0.1 0.0 0

20

40

60

80

100

Operation time, h

Fig. 5 – Variation of the relative fluxes of PPHFMMs in the SMBR with the operation time.

4707

the relative flux decreased rapidly within 1 h. Flux decline was a result of the increase of membrane resistance to the permeate flow, which was caused by membrane fouling or particle deposition onto the membrane surface or into the membrane pores. The antifouling characteristics of the studied membranes, such as Jp, flux recovery after water (FR1) and 5% NaOH cleaning (FR2) for the virgin and N2 plasma-treated PPHFMMs, are shown in Table 3. It can be seen that the surface-modified membranes showed better regeneration performance, i.e. higher FR1 and FR2 than those of the nascent PPHFMM. After continuous operation in the SMBR for about 90 h, FR1 and FR2 for the N2 plasma-treated PPHFMM for 8 min were 62.9% and 67.8% higher than those of the virgin membrane. FR2 for N2 plasma-treated PPHFMM was even higher than 100% (initial water flux before surface modification). It was substantiated that surface modification by N2 plasma treatment could improve filtration performance and make fouling less troublesome and membrane regeneration more efficient. However, the water fluxes after being used in the SMBR (Jp) for the modified PPHFMMs were lower than that of the nascent one. This can be attributed to the fact that the hydrophilic surface decreases overall resistance, causing an increase in flow rate, resulting in significant cake formation and compaction. Cake formation and compaction on the membrane surface increase (porosity decreases), causing the resistance to increase, which results in a decrease of the flow rate. This result confirms that if the membrane surface is more hydrophilic, the cake thickness increases and thus Rc (cake resistance) increases, causing the decrease in the flow rate (le Roux et al., 2005). The intrinsic membrane resistance (Trussell et al.), reversible (Rrf) and irreversible (Rirf) membrane fouling resistance, total membrane resistance after being used in the SMBR (Rt), followed by water cleaning (Rt0 ) for each PPHFMM are listed in Table 4. It was found that the Rm of PPHFMMs decreased with the increase of plasma treatment time, which was due to the hydrophilization of the membrane surface by plasma treatment. Rt of the plasma-treated PPHFMMs increased because modified membranes have a higher flow rate, resulting in significant cake formation and compaction, leading to the increase of Rt. This is in good agreement with the results mentioned above. The total membrane resistance of PPHFMMs used in the SMBR followed by water cleaning decreased after plasma treatment (Rt0 ). It was also found that Rirf decreased while Rrf increased after plasma treatment. These results strongly suggested that the more hydrophilic the membrane surface was, the more easily the cake formed on the membrane surface would be washed off the membrane surface. The membrane resistance ratios, such as Rrf/Rt and Rrf/(Rrf+Rirf), are also listed in Table 4. Rrf/Rt showed that 49.4% and 95.6% of the total resistance were attributed to the cake layer for the virgin and 8 min plasma-treated PPHFMM. The Rrf/(Rrf+Rirf) ratios indicated that 57.4% and 98.4% of resistance for the virgin and 8 min plasma-treated PPHFMM causing the flux decline resulted from the cake layer. It indicated that the surface modification of PPHFMM by N2 plasma treatment can reduce the irreversible resistance.

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Table 3 – Antifouling characteristics, Jp, flux recovery after water (FR1) and 5% NaOH cleaning (FR2) for virgin and N2 plasma-treated PPHFMMs Plasma treatment time (min) Jp (%) FR1 (%) FR2 (%)

0

4

8

12

16

20

14.3 28.2 59.8

4.1 52.9 88.9

4.0 91.1 127.6

3.7 53.1 90.7

8.5 26.8 79.8

5.6 53.3 103.6

Table 4 – Intrinsic membrane resistance (Trussell et al.), reversible membrane fouling resistance (Rrf), total membrane fouling resistance (Rt), total membrane fouling resistance after water washing (Rt0 ), irreversible membrane fouling resistance (Rirf), and reversible membrane fouling resistance (Rrf) for each PPHFMM, resistance ratios of Rrf/Rt and Rrf/ (Rrf+Rirf) Resistance (  109 m1)

Plasma treatment time (min)

0 4 8 12 16 20

Resistance ratio (%)

Rm

Rt

Rt0

Rirf

Rrf

Rrf/Rt

Rrf/(Rrf+Rirf)

4.7 3.4 3.4 3.2 3.8 2.8

33.6 118.0 119.0 129.0 56.6 86.1

17 9.1 5.3 9.1 17.9 9.0

12.3 5.7 1.8 5.9 14.1 6.3

16.6 108.9 113.7 119.9 38.7 77.1

49.4 92.3 95.6 93.0 68.4 89.5

57.4 95.1 98.4 95.3 73.3 92.5

Rt0 : Rt after water washing.

4.

Conclusions

Commercial polypropylene hollow fiber microporous membranes were surface modified by N2 low-temperature plasma treatment. The wetting properties measured by a goniometer showed that the hydrophilicity of the N2 plasma-treated membranes increased obviously. The antifouling characteristics of the modified membranes in the submerged membrane bioreactor were investigated. The modified membranes showed better filtration behaviors in SMBR than the unmodified membrane, and flux recovery after water cleaning was higher. The irreversible fouling resistance decreased after plasma treatment.

Acknowledgments Financial supports from the Science and Technological Fund of Anhui Province for Outstanding Youth (No. 04046065), the National Natural Science Foundation (No. 20671002), the Education Department (No. 2006 KJ 006 TD) of Anhui province and ‘‘15’’ important item of science and technologies of Jiangsu province (Grant no. BE200231) are gratefully acknowledged. R E F E R E N C E S

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