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Polydopamine coating – Surface modification of polyester filter and fouling reduction
Separation and Purification Technology 118 (2013) 226–233
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Polydopamine coating – Surface modification of polyester filter and fouling reduction q Lifen Liu ⇑, Bing Shao, Fenglin Yang Key Laboratory of Industrial Ecology and Environmental Engineering, Ministry of Education, School of Environmental Science and Technology, Dalian University of Technology, Dalian 116024, China
a r t i c l e
i n f o
Article history: Received 16 March 2013 Received in revised form 30 June 2013 Accepted 2 July 2013 Available online 11 July 2013 Keywords: Polydopamine Surface modification Hydrophilicity Membrane bioreactor Dynamic membrane
a b s t r a c t To develop a high flux and antifouling membrane to be used in membrane bioreactor for water pretreatment, a low cost polyester filter cloth was modified by coating with polydopamine (PDA), based on the easy self-polymerization and strong adhesion characteristics of dopamine (DA) under mild conditions. The apertures and surface morphological changes were characterized using SEM. The influence of the modifying conditions such as coating time and DA concentration on the membrane properties was investigated. It was found that the most reasonable modification conditions to obtain the best antifouling performance are 1.5 g/L DA concentration and 24 h coating time. There was 142.67% increment of the stable flux during filtrating yeast suspension for the PDA modified membrane under the best conditions compared with that of the original membrane. The result showed that the PDA modified filter membrane has higher flux, excellent antifouling properties and good stability. The stable flux increase in the shortterm test in MBR was 36% after PDA coating. The results of the relatively longer term experiments in MBR indicate that the membrane modified by PDA reduced the membrane fouling in active sludge, especially the irreversible fouling. This work provides a simple method of modifying low cost filter membrane for better antifouling properties and potential application in MBR. Ó 2013 The Authors. Published by Elsevier B.V. All rights reserved.
1. Introduction Membrane bioreactor (MBR) has been widely applied in wastewater treatment and water purification due to the advantages of smaller footprint, better effluent qualities and higher sludge concentration [1]. Unfortunately, the high cost of the membrane materials and membrane fouling are critical challenges and barriers to the improvement of MBR systems [2,3]. To reduce membrane costs, low cost non-woven and mesh filters, had been tested in submerged MBR [4–6]. These textile materials are composed of a network of overlapping fibers, which create multiple connected pores. Though the large apertures materials themselves cannot adequately fulfill the solid–liquid separation functions, MF or UF equivalent separation result can be achieved due to the self-forming dynamic membrane (SFDM) [7,8]. Polyester and nylon mesh filters have been used as filtration materials in MBR to reduce manufacturing and operating cost, which could also provide high removal ratio of COD (chemical oxygen demand), TSS (total soluble
q This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial-No Derivative Works License, which permits non-commercial use, distribution, and reproduction in any medium, provided the original author and source are credited. ⇑ Corresponding author. E-mail addresses: [email protected], [email protected] (L. Liu).
solid) and TKN (total kjeldahl nitrogen) [9]. Fuchs et al. [10] had studied the effect of different pore sizes and different mesh module configurations on the effluent quality, which resembled the quality of activated sludge process treatment system. Excellent effluent quality were achieved and also compared to those of conventional activated sludge systems. However, similar to the MF membrane fouling, the fouling of mesh filter membrane caused by pore blocking is the main problem limiting its application, especially when the membrane was relatively hydrophobic [11,12]. Membrane fouling occurred is more serious on hydrophobic membranes than hydrophilic ones. As a result, much attention has been paid to reduce membrane fouling by modifying hydrophobic membranes to increase their hydrophilicity. There are some research papers about modifying the non-woven fabric for higher flux and better antifouling performance [13,14]. For the polyester filter, we have used it in studying a new helical membrane module for better antifouling properties [15] and have coated it with polypyrrole to make it hydrophilic as well as electrically conductive [16]. We have also modified the polyester filter cloth using TiO2/PVA to inhibit the irreversible fouling, and the results showed a remarkable improvement of hydrophilicity and low resistance compared with the commercial PVDF membrane [17]. Compared to other modification methods, the coating technique is simple and easy to use in large-scale industrial applications. A major issue with coating based method is to achieve permanent modification and increase
1383-5866/$ - see front matter Ó 2013 The Authors. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2013.07.003
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the stability of modifying substance on membrane surface. Hence, the development of novel methods overcoming this issue is desirable for surface modification of the membrane. Polydopamine (PDA) is a novel polymer sharing similar properties to the adhesive proteins in mussels. The adhesive mechanisms and outstanding adhesive behaviors of PDA have been reported [18]. DA (3,4-dihydroxy-phenylala-nine) is able to self-polymerize in aqueous solution and form a PDA layer, which can firmly attach to a wide range of substrates such as rocks, metals, polymers and wood [19]. The polar groups in PDA layer, such as hydroxyl and amine groups, endow the substrates with improved hydrophilicity and anti-fouling ability [20]. Additionally, PDA has been successfully applied in modifying PVDF, PE and PS polymer membranes for higher membrane fouling resistance [21–23]. However, so far, only limited information on the application of PDA modification is available. In this research, PDA is used for filter membrane modification because of its highly hydrophilic nature and the stable polymerization of simple ingredients under mild reaction condition. We choose the low cost polyester cloth as a cheap membrane and expect the firm PDA modification to reduce the irreversible fouling. The application performance of PDA coated membranes in MBR is also reported. We tried to demonstrate that the PDA modification can reduce fouling of the large aperture filter materials and prolong the membrane life in MBR. The effects of different coating conditions, such as coating time, solution concentration on membrane properties are also investigated in detail. 2. Methods 2.1. Materials Polyester filter cloth (Shanghai Suita Filter Material Co. Ltd.) and its specifications: the fiber weight per unit area is 275 g/m2, the thickness is 0.5 mm, and in 10 cm length. The number of fiber was 295 and the stretch rate was 30.87% in latitude directions. And in longitude direction, the number of the fiber was 200 and the stretch rate was 14.6%. Because of the irregularity of the pore shape and size, the real pore size is hard to define. Its pore size is not as equally accurate or important as the pore size of the UF or NF membrane. The average pore size value was 100 lm, measured using a bubble point method. 3,4-Dihydroxyphenethylamine (dopamine or DA) was purchased from J&K China Chemical Ltd. and used as received. The water used in the following experiments was deionized water. Ethanol, hydrochloric acid (Melonepharma Chemical Reagent Co. Ltd.) and hydroxymethyl aminomethane (Tris, Sinopharm Chemical Reagent Co. Ltd.) are all analytical reagents and used without further purification.
2.3. The filtration experiments The filtration set-up used was shown in Fig. 1. A flat sheet membrane module (filtration area 0.0072 m2) was installed into a cylindrical container vertically, which had an effective volume of 3.925 L. Aeration was supplied by an air pump through air diffusers at the bottom of the cell, and the aeration intensity was 0.15 m3/h. In all experiments, the permeation was operated in a gravitational filtration mode, at constant trans-membrane pressure (TMP) 7.6 kPa. Before the filtration tests, the membrane was dipped into the deionized water for 30 min to be wetted. The pure water flux was examined first, after that the fresh bakers yeast suspension was used as foulant particles to test antifouling properties. The size of the particle suspension is 7 lm and the concentration of the yeast was 5 g/L in all experiments. Furthermore, to ensure the accuracy of the tests and verify the stability of the dopamine coated membrane, the filtration test was repeated for three cycles, in each cycle, the filtration lasted 1.5 h. At the end of each cycle, the membrane was only physically cleaned by washing with tap water. In the filtration test, the influence of the coating time and the DA concentration on the membrane flux and antifouling properties was studied. We chose 0.5 g/L DA concentration, when the influence of the coating time was studied. Then we changed the DA concentration and studied its influence, at the best coating time derived from our experiments results.
2.4. Antifouling properties in MBR To gain more information on filtration performance of the modified membranes under practical conditions, both relatively shortterm and long-term tests in the laboratory-scale MBR were designed to characterize their antifouling properties. The diagram of the test-rig MBR was shown in Fig. 2. The reactor had an effective volume of 10 L and the membrane area of each membrane module was 0.04 m2 (0.1 m 0.2 m 2). Constant aeration was maintained at 0.3 m3/h. A synthetic wastewater was used in the experiments, and the composition of the water was sucrose 5, NH4Cl 0.07, KH2PO4 0.003 and CaCl2 0.002 (g/L). The influent was controlled by a liquid level balance tank. There are only two types of membranes, which were original polyester cloth and PDA coated polyester cloth. Here we had chosen membrane modified at PDA concentration 1.5 g/L and coating time 24 h. The above operation conditions were adapted in both short-term and long-term test if there was no further explanation.
2.2. Preparation of PDA modified membrane Polyester filter cloth was dipped in deionized water and set in ultrasonic cleaner for 30 min at 30 kHz. After that, the cloth was dipped in ethanol for another 30 min, then washed by deionized water. Different amount of dopamine was dissolved in 200 ml deionized water to make a dilute aqueous solution, buffered to a pH typical of marine environments (10 mM Tris, pH = 8.5) to obtain a DA solution. The concentration of DA solution was set at 0.5, 1.0, 1.5 and 2.0 g/L respectively. After that, the polyester cloth was immersed into the DA solution immediately and shaken for a designed time at 30 °C and 150 rpm/min. After modification the membranes were taken out followed by a thorough washing with ethanol and deionized water. The filter membrane before and after PDA modification was compared by characterization with SEM (JEOL JSM-5600LV).
227
Fig. 1. The filtration test set-up.
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the cake layer removal, the pure water flux was measured again and used to calculate the resistance of Rm + Rp, thus the cake layer resistance Rc can be calculated Rc = Rt (Rm + Rp). 3. Results and discussion
Fig. 2. The schematic diagram of MBR set up.
For the short-time experiment, the test was operated in a gravitational filtration mode, at constant trans-membrane pressure (TMP) 7.6 kPa and mixed liquor suspended solids (MLSS) concentration of 12.45 g/L. The long-term test was used to investigate the irreversible fouling on the two types of membrane and demonstrate the application of the PDA coated membrane in MBR. The permeate was drawn using a pump at a constant suction rate, the duration of suction time and idle time was 5 min and 1 min, respectively. The trans-membrane pressure (TMP) and the flux were monitored for 25 days and the membrane resistances were calculated after the test. Once the trans-membrane pressure is over 0.03 MPa, the membrane was taken off and cleaned mechanically. Water flushing and back washing was carried out to remove the cake layer. 2.5. Analysis and calculation In the permeate experiments, the flux was determined by the permeate volume within identical filtration time.
F¼
V A Dt
ð1Þ
where F is the membrane flux (m3 m2 h1), Dt is the filtration time interval (h), A is the membrane area (m2), V is the volume of the collected permeate during the time interval (L). In this study, the standard filtration model is suitable for analyzing the fouling mechanisms, because the size of particle is much smaller than the membrane pore size [24]. To this type of membrane filtration under constant TMP (trans-membrane pressure), the plot of t/V t and the slopes can be used for comparing the different membrane modules, as it is a model suitable for standard filtration. The t/V t curves were drawn for comparing the slopes.
t 1 k ¼ þ t V Q0 2
ð2Þ
In this equation, t is filtration time (min); V is the cumulative permeate volume (L); 0 Q is the initial flux rate (L min1); k is the filtration constant (L1). The plot of t/V t and the slopes can be used for comparing the antifouling properties of different membranes. The smaller the slopes are, the lower the filtration resistances. The filtration resistances are calculated using Darcy equation [25]:
Rt ¼ Rm þ Rp þ Rc ¼
TMP
lJ
ð3Þ
where Rt is the total resistance (m1), calculated using the flux of the fouled membrane; Rm is the membrane resistance (m1), calculated using the pure water flux; Rc is the cake layer resistance (m1); Rp is the pore blocking resistance (m1). After flushing and
PDA is of interest in membrane modifications because a strong increase in hydrophilicity may be imparted to the membrane surface with a very thin PDA layer [20]. The possible structural evolution of dopamine was presented in Fig. 3. Dopamine is easily oxidized by dissolved oxygen under alkaline conditions, creating 5,6-dihydroxyindole and 5,6-indolequinone via intramolecular cyclization, oxidation, and isomerisation. After a multistep reaction proceeding, a mass of melanin-like PDA particles and aggregates was generated in the solution [26,27]. The cross-linking of the dopamine is introduced by the reverse dismutation reaction of dopamine and dopamine quinone. After that, dopamine self-polymerized and formed PDA, and then PDA strongly attached onto the substrate (polyester filter cloth). The mechanism/procedure of the polymerization of dopamine was shown in Fig. 3. Though the mechanism of the reaction between the PDA and the substrates is not very clear, it has been demonstrated that the adhesion is strong enough and the hydrophilicity can be improved due to the hydroxyl and amine groups [28,29]. The two main necessary conditions for the polymerization reaction are oxygen and water. The observed color change from white to black suggests the existence of PDA on the membrane surface. 3.1. PDA modified membrane properties The average pore size of the PDA modified membrane (dopamine concentration of 2 g/L and coating time of 24 h) was measured using a bubble point method. The pore size reduced only 1.13% compared to the original membrane, which suggests that the PDA formed as a very thin layer on the polyester fiber surface. SEM was used to observe the changes in surface morphology of original membrane and modified membrane, which is shown in Fig. 4. For the original polyester cloth, the surface of the fibers was clean and smooth. After modification, the polydopamine particles were deposited on the fiber surface or the gap between fibers. 3.2. Influence of PDA coating time on the antifouling performance of membranes First, the polyester filter cloth was modified by PDA at a low concentration of 0.5 g/L with different coating time. The permeate flux measurement was carried out to characterize the permeation and antifouling properties of the modified membrane using the set-up in Fig. 1. The pure water flux was 520 L m2 h1 at 7.6 kPa, there was no significant difference between the original membrane and the modified membrane. In contrast, the PDA coated membrane had slower flux reduction in the suspended yeast particles, comparing with the original membrane (Fig. 5). The better fluxes of the PDA coated membrane are thought to be the result of the hydrophilic layer on the membrane surface, which tends to form a thin water film between the membrane and bulk solutions and that avoided significant deterioration in permeation and decline in water flux. In Fig. 6, the average flux of three cycles was presented. Obviously, the average flux increased with the coating time when the coating time was shorter than 24 h. The flux of modified membranes of different coating time (6, 12, 24, 36 h) increased 0.6%, 3.08%, 13.94% and 10.7% respectively compared with the original membrane. Importantly, there was no further significant increase of the flux when the coating time reached 36 h, after 24 h. To our
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229
Fig. 3. The scheme for the dopamine cross-linking and polymerization on polyester filter cloth.
observation, the polyester cloth became darker and the solution became clearer during the polymerization. And there has no black particles after 24 h. This demonstrated a rapid and nearly complete consumption of dopamine monomers within 24 h, and there was no more dopamine depositing onto the membrane when the coating time was prolonged to 36 h. Moreover, the high flux recovery suggests the stability of the modification. 3.3. Influence of PDA modification concentration on the antifouling performance of membranes
We come to conclude that the hydrophilicity of membrane surface can be improved when the concentration of solution is increased to a proper level. This is because that the dopamine self-polymerization is not only occurring on the filter surface, but also occurring in solution. When the concentration increases, the monomer consumption in the solution will also increase. Importantly, an average flux in the three runs was observed for the polydopamine-coated membrane, even after physical cleaning of the membrane, suggesting good durability of PDA coating method. 3.4. Fouling mechanism and analysis
Second, the effects of PDA modification concentration on membrane antifouling properties were investigated and the results were presented in Figs. 7 and 8. Based on the previous study, the DA has polymerized on the polyester cloth within 24 h. Thus, with the coating time fixed at 24 h, the concentration of DA was changed from 0.5 g/L to 2 g/L. In comparison with the original polyester cloth membrane, the PDA modified membrane has slower flux reductions, evident in the first cycle. Furthermore, the higher the DA concentration was, the slower the flux went down, this demonstrated the positive effect of the PDA modification on the membrane antifouling properties. As observed, the average flux increased 13.94% for the 0.5 g/L membrane, 32.09% for the 1.0 g/L membrane, 67.55% for the 1.5 g/L membrane and 71.96% for the 2.0 g/L membrane. It can be seen that when the concentration of DA solution increased from 1.5 to 2.0 g/L, the average flux had only a slight enhancement.
For large pore size filter membrane materials, the filtration process depends on the formation of dynamic membrane on the membrane [8]. The mechanism has been divided into two categories in general filtration process, internal pore clogging and cake layer fouling [3]. As shown in Fig. 9, the t/V t curves of different membranes under different coating conditions were used to illustrate the fouling mechanism of the membrane. The parameters of the t/V t were summarized in Table 1. This fouling mechanism means the membrane was fouled by pore blocking followed by the deposit of a cake layer, since the yeast particles had a smaller size comparing to the polyester cloth pore size. In this fouling mode, the smaller the slopes of the curves are, the lower the filtration resistances. It was clearly shown that the slope decreased with the increase of coating time at a 0.5 g/L DA concentration, and reached minimum
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Fig. 4. SEM images of the original membrane (A and B) and PDA modified membrane (C and D).
400
-2
-1
Flux (Lm h )
500
300 200 100
220 204.483 198.657
200
Average flux (Lm -2 h -1)
original membrane 6h PDA coated 12h PDA coated 24h PDA coated 36h PDA coated
600
179.452
180.679
0
6
184.985
180
160
140
120
0 0
50
100
150
200
250
300
Time (min) Fig. 5. The influence of PDA coating time on permeate flux (PDA concentration = 0.5 g/L).
at 24 h. Moreover, the slope of the membrane modified by 24 h PDA coating was much smaller than the original membrane, demonstrating that the PDA coating could improved the membrane antifouling properties effectively. At a coating time of 24 h, the filtration resistance of the membranes was reduced significantly when the PDA concentration increased, due to the improvement of the membrane hydrophilicity. However, the slopes of the curves were similar when the concentration was 1.5 and 2.0 g/L, which agreed well with the flux trend in Fig. 7. This is because the increase of the
100 12
24
36
Time (h) Fig. 6. The stable flux of the original membrane and PDA modified membrane on different coating time.
PDA formed in the solution rather than on the membrane surface, when the concentration was above a proper value. Thus, we can choose the 1.5 g/L dopamine concentration to reduce the modification cost and a coating time at 24 h as the suitable modification conditions for the polyester filters. 3.5. Short-term experiment in MBR For the membrane modification, it is important to demonstrate the prevention of microbial adhesion and practical application
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original membrane 0.5g/l PDA coated 1 g/l PDA coated 1.5g/l PDA coated 2g/l PDA coated
600
400
-2
-1
Flux (Lm h )
500
300
200
100
0 0
50
100
150
200
250
300
Time (min) Fig. 7. The influence of PDA modification concentration on permeate flux (PDA coating time = 24 h).
300.687
308.588
-2 -1
Average flux (Lm h )
300
250
237.045 204.483
200
179.452
150
100 0.0
0.5
1.0
1.5
2.0
Concentration (g/l) Fig. 8. The stable flux of the original membrane and PDA modified membrane on different PDA concentration.
properties in MBR [30,31]. In this part of test, the PDA modified membrane (1.5 g/L and 24 h) and the original membrane were placed in a MBR to compare their antifouling properties for a short time filtration, by gravity flow at a constant but low water head drop of 0.08 m. As shown in Fig. 10, the flux of the PDA coated membrane was lower than the original membrane at the first 2 h. This because the membrane pore size had been reduced due to the PDA modification. For the yeast solution, the size of the particles was much smaller than the pore size so there was no effect on the flux. But the size of the sludge in MBR was much bigger than the yeast, so there has pore block at the beginning of the filtration. But its better antifouling properties was shown since 150 min and even was 36% higher than the original membrane (after >300 min). Though the large apertures of polyester filters lead to a high flux, the microbes will block the membrane apertures and form a biofilm on the membrane surface. The result suggests that the PDA membrane better inhibited microbial adhesion and biofouling in MBR. As a result, the application of the PDA modified membrane in MBR will be further tested. 3.6. Long-term experiment in MBR Polydopamine modification can improve the membrane hydrophilicity [21] and this attributes to the fouling resistance of the modified membrane in a short-time test. Miller et al. [32] demonstrated that short-term membrane surface adhesion tests cannot be suitable predictors of membrane biofouling. In this paper, the properties of the membrane were studied by filtration test rather than the adsorption test. A relatively longer time filtration test in MBR was carried out. The trans-membrane pressure (TMP) was shown in Fig. 11. Both the two types of membranes were installed in one reactor (initial MLSS = 13.55 g/L and the pump speed at 16 rpm, constant flux). The membrane was taken out for physical cleaning when the TMP was over 0.03 MPa. The results showed that the increase of TMP for the PDA coated membrane was slower than the original membrane. The modified membrane was washed for only one time at day 18, comparing to the three times washing of the original membrane at day 13, 20 and 25. After the long-time filtration test, the resistances of the membranes were determined through the flux measurement method (Table 2). It was clear that the total resistance of the modified membrane was lower than the original membrane. The cake layer resistance, which contributes 91% of the total resistance for both
45
45
original membrane 6h PDA coated 12h PDA coated 24h PDA coated 36h PDA coated
t/v (min/l)
35
35
30
25
30
25
20
20
15
15 0
20
40
t (min)
original membrane 0.5g/l PDA coated 1 g/l PDA coated 1.5g/l PDA coated 2g/l PDA coated
40
t/V (min/l)
40
60
80
100
0
20
40
60
80
100
t (min)
Fig. 9. The t/V t curves of different PDA coated conditions corresponding to the flux of the first cycle in Figs. 1 and 2, respectively.
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Table 1 Parameters of the t/V t curves. Variables
PDA coating time (h)
PDA concentration (g/L)
Q0 (ml min1)
Slope
R2
PDA coating time
0 6 12 24 36
0 0.5 0.5 0.5 0.5
60.2 61 59 57.5 57.0
0.296 0.292 0.260 0.211 0.215
0.999 0.999 0.999 0.998 0.999
PDA concentration
0 24 24 24 24
0 0.5 1.0 1.5 2.0
60.2 57.5 61.9 61.2 69.7
0.296 0.211 0.168 0.119 0.118
0.999 0.998 0.997 0.998 0.999
320
Table 2 The resistance analysis of the two membranes in long-term test.
280
Resistance
Flux (Lm-2 h-1)
240 200 Rm Rc Rp Rt
160
original membrane PDA coated membrane
120
Original membrane
PDA modified membrane
Value (1013 m1)
Percentage (%)
Value (1013 m1)
Percentage (%)
0.115 1.442 0.016 1.574
7.34 91.62 1.04 100
0.106 1.150 0.004 1.261
8.45 91.20 0.35 100
80 40 0 0
50
100
150
200
250
300
350
T (min) Fig. 10. The flux of the original membrane and the PDA coated membrane in MBR in short-term test.
0.04
original membrane PDA modified membrane
TMP (Mpa)
0.03
0.02
0.01
0.00 0
5
10
15
20
25
Time (day) Fig. 11. TMP of the original membrane and PDA modified membrane in long-term test.
membranes, is the main fouling form due to the high sludge concentration. Because of PDA modification, though Rm is a little lower, the Rm ratio is a little higher than the original membrane, but the Rp ratio is much lower (being 1/4 that of the original membrane). This means that the accumulation of the foulants particles in the pores was inhibited by PDA layer on the polyester. The hydrophilicity improvement of different polymer membranes by PDA coating has been demonstrated in many studies [20–23]. Though Miller et al. reports PDA modification did not reduce biofouling of the ultrafiltration and nanofiltration membranes
treating surface river water with added acetate [32], effective reduction of the irreversible fouling for the large apertures polyester cloth was shown here. The PDA layer on the surface of the fibers made the adhesion of microbes difficult, but the removal of microbes easy. The no effect report [32] highlights the need for authors to characterize fully both the fouling process and mechanisms, to allow clear understandings of the role of membrane modification, and allow comparison and/or precise prediction of membrane performance in real membrane separation applications. The fouling resistance performance of membranes is affected by membrane property (hydrophilicity and surface charge, pore size and morphology) and separation process (sludge property and hydrodynamic conditions), also by fouling process and mechanisms. For membrane, the fouling resistance depends on its composition and surface charge, pore size and hydrophilicity, also on the property of the interface between liquid and membrane surface, which may undergo changes during the formation of biofilm or adsorption of different foulants. After formation of fully covered biofilm or surface, the further fouling property and propensity will be shaped by the formed biofilm or cake and even the TMP, instead of by the modified membrane itself. For fouling mechanisms characterization, depending on the filtration time, membrane property and foulants concentration, biofouling may be caused by protein adsorption, bacteria adhesion, and present itself in the form of clogged pores or formed gel layer/biofilm or cake layers, thus may affect flux and TMP differently in the short term test and long term test. One paper [33] showed that the water quality significantly affected bio-fouling of PDA modified NF/RO with bacteria under 15 bar pressure. For our short term and long term filtration tests with modified membrane, similar fouling reduction was observed. The effluent of the reactor was not studied in our work considering the membrane pore size had no significant difference after PDA modification, and the effluent qualities was only related to the characterization of the sludge and the operation conditions.
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4. Conclusion A low cost polyester filter cloth was modified by PDA coating under mild conditions. The hydrophilicity and antifouling properties were improved effectively, and it was most evident at 1.5 g/L DA concentration and 24 h coating time. The pore size change was very small compared to the original membrane, due to the formation of a thin layer of PDA on fiber surface. During the long-time filtration test in MBR, the PDA modified membrane has slower TMP rise, longer filtration cycle and lower irreversible fouling resistance. The dopamine consumption was very little and will not affect the low cost of the membrane. It is believed that this hydrophilic, large pore filter membrane has great application potential in wastewater treatment.
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Contents lists available at SciVerse ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Polydopamine coating – Surface modification of polyester filter and fouling reduction q Lifen Liu ⇑, Bing Shao, Fenglin Yang Key Laboratory of Industrial Ecology and Environmental Engineering, Ministry of Education, School of Environmental Science and Technology, Dalian University of Technology, Dalian 116024, China
a r t i c l e
i n f o
Article history: Received 16 March 2013 Received in revised form 30 June 2013 Accepted 2 July 2013 Available online 11 July 2013 Keywords: Polydopamine Surface modification Hydrophilicity Membrane bioreactor Dynamic membrane
a b s t r a c t To develop a high flux and antifouling membrane to be used in membrane bioreactor for water pretreatment, a low cost polyester filter cloth was modified by coating with polydopamine (PDA), based on the easy self-polymerization and strong adhesion characteristics of dopamine (DA) under mild conditions. The apertures and surface morphological changes were characterized using SEM. The influence of the modifying conditions such as coating time and DA concentration on the membrane properties was investigated. It was found that the most reasonable modification conditions to obtain the best antifouling performance are 1.5 g/L DA concentration and 24 h coating time. There was 142.67% increment of the stable flux during filtrating yeast suspension for the PDA modified membrane under the best conditions compared with that of the original membrane. The result showed that the PDA modified filter membrane has higher flux, excellent antifouling properties and good stability. The stable flux increase in the shortterm test in MBR was 36% after PDA coating. The results of the relatively longer term experiments in MBR indicate that the membrane modified by PDA reduced the membrane fouling in active sludge, especially the irreversible fouling. This work provides a simple method of modifying low cost filter membrane for better antifouling properties and potential application in MBR. Ó 2013 The Authors. Published by Elsevier B.V. All rights reserved.
1. Introduction Membrane bioreactor (MBR) has been widely applied in wastewater treatment and water purification due to the advantages of smaller footprint, better effluent qualities and higher sludge concentration [1]. Unfortunately, the high cost of the membrane materials and membrane fouling are critical challenges and barriers to the improvement of MBR systems [2,3]. To reduce membrane costs, low cost non-woven and mesh filters, had been tested in submerged MBR [4–6]. These textile materials are composed of a network of overlapping fibers, which create multiple connected pores. Though the large apertures materials themselves cannot adequately fulfill the solid–liquid separation functions, MF or UF equivalent separation result can be achieved due to the self-forming dynamic membrane (SFDM) [7,8]. Polyester and nylon mesh filters have been used as filtration materials in MBR to reduce manufacturing and operating cost, which could also provide high removal ratio of COD (chemical oxygen demand), TSS (total soluble
q This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial-No Derivative Works License, which permits non-commercial use, distribution, and reproduction in any medium, provided the original author and source are credited. ⇑ Corresponding author. E-mail addresses: [email protected], [email protected] (L. Liu).
solid) and TKN (total kjeldahl nitrogen) [9]. Fuchs et al. [10] had studied the effect of different pore sizes and different mesh module configurations on the effluent quality, which resembled the quality of activated sludge process treatment system. Excellent effluent quality were achieved and also compared to those of conventional activated sludge systems. However, similar to the MF membrane fouling, the fouling of mesh filter membrane caused by pore blocking is the main problem limiting its application, especially when the membrane was relatively hydrophobic [11,12]. Membrane fouling occurred is more serious on hydrophobic membranes than hydrophilic ones. As a result, much attention has been paid to reduce membrane fouling by modifying hydrophobic membranes to increase their hydrophilicity. There are some research papers about modifying the non-woven fabric for higher flux and better antifouling performance [13,14]. For the polyester filter, we have used it in studying a new helical membrane module for better antifouling properties [15] and have coated it with polypyrrole to make it hydrophilic as well as electrically conductive [16]. We have also modified the polyester filter cloth using TiO2/PVA to inhibit the irreversible fouling, and the results showed a remarkable improvement of hydrophilicity and low resistance compared with the commercial PVDF membrane [17]. Compared to other modification methods, the coating technique is simple and easy to use in large-scale industrial applications. A major issue with coating based method is to achieve permanent modification and increase
1383-5866/$ - see front matter Ó 2013 The Authors. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2013.07.003
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the stability of modifying substance on membrane surface. Hence, the development of novel methods overcoming this issue is desirable for surface modification of the membrane. Polydopamine (PDA) is a novel polymer sharing similar properties to the adhesive proteins in mussels. The adhesive mechanisms and outstanding adhesive behaviors of PDA have been reported [18]. DA (3,4-dihydroxy-phenylala-nine) is able to self-polymerize in aqueous solution and form a PDA layer, which can firmly attach to a wide range of substrates such as rocks, metals, polymers and wood [19]. The polar groups in PDA layer, such as hydroxyl and amine groups, endow the substrates with improved hydrophilicity and anti-fouling ability [20]. Additionally, PDA has been successfully applied in modifying PVDF, PE and PS polymer membranes for higher membrane fouling resistance [21–23]. However, so far, only limited information on the application of PDA modification is available. In this research, PDA is used for filter membrane modification because of its highly hydrophilic nature and the stable polymerization of simple ingredients under mild reaction condition. We choose the low cost polyester cloth as a cheap membrane and expect the firm PDA modification to reduce the irreversible fouling. The application performance of PDA coated membranes in MBR is also reported. We tried to demonstrate that the PDA modification can reduce fouling of the large aperture filter materials and prolong the membrane life in MBR. The effects of different coating conditions, such as coating time, solution concentration on membrane properties are also investigated in detail. 2. Methods 2.1. Materials Polyester filter cloth (Shanghai Suita Filter Material Co. Ltd.) and its specifications: the fiber weight per unit area is 275 g/m2, the thickness is 0.5 mm, and in 10 cm length. The number of fiber was 295 and the stretch rate was 30.87% in latitude directions. And in longitude direction, the number of the fiber was 200 and the stretch rate was 14.6%. Because of the irregularity of the pore shape and size, the real pore size is hard to define. Its pore size is not as equally accurate or important as the pore size of the UF or NF membrane. The average pore size value was 100 lm, measured using a bubble point method. 3,4-Dihydroxyphenethylamine (dopamine or DA) was purchased from J&K China Chemical Ltd. and used as received. The water used in the following experiments was deionized water. Ethanol, hydrochloric acid (Melonepharma Chemical Reagent Co. Ltd.) and hydroxymethyl aminomethane (Tris, Sinopharm Chemical Reagent Co. Ltd.) are all analytical reagents and used without further purification.
2.3. The filtration experiments The filtration set-up used was shown in Fig. 1. A flat sheet membrane module (filtration area 0.0072 m2) was installed into a cylindrical container vertically, which had an effective volume of 3.925 L. Aeration was supplied by an air pump through air diffusers at the bottom of the cell, and the aeration intensity was 0.15 m3/h. In all experiments, the permeation was operated in a gravitational filtration mode, at constant trans-membrane pressure (TMP) 7.6 kPa. Before the filtration tests, the membrane was dipped into the deionized water for 30 min to be wetted. The pure water flux was examined first, after that the fresh bakers yeast suspension was used as foulant particles to test antifouling properties. The size of the particle suspension is 7 lm and the concentration of the yeast was 5 g/L in all experiments. Furthermore, to ensure the accuracy of the tests and verify the stability of the dopamine coated membrane, the filtration test was repeated for three cycles, in each cycle, the filtration lasted 1.5 h. At the end of each cycle, the membrane was only physically cleaned by washing with tap water. In the filtration test, the influence of the coating time and the DA concentration on the membrane flux and antifouling properties was studied. We chose 0.5 g/L DA concentration, when the influence of the coating time was studied. Then we changed the DA concentration and studied its influence, at the best coating time derived from our experiments results.
2.4. Antifouling properties in MBR To gain more information on filtration performance of the modified membranes under practical conditions, both relatively shortterm and long-term tests in the laboratory-scale MBR were designed to characterize their antifouling properties. The diagram of the test-rig MBR was shown in Fig. 2. The reactor had an effective volume of 10 L and the membrane area of each membrane module was 0.04 m2 (0.1 m 0.2 m 2). Constant aeration was maintained at 0.3 m3/h. A synthetic wastewater was used in the experiments, and the composition of the water was sucrose 5, NH4Cl 0.07, KH2PO4 0.003 and CaCl2 0.002 (g/L). The influent was controlled by a liquid level balance tank. There are only two types of membranes, which were original polyester cloth and PDA coated polyester cloth. Here we had chosen membrane modified at PDA concentration 1.5 g/L and coating time 24 h. The above operation conditions were adapted in both short-term and long-term test if there was no further explanation.
2.2. Preparation of PDA modified membrane Polyester filter cloth was dipped in deionized water and set in ultrasonic cleaner for 30 min at 30 kHz. After that, the cloth was dipped in ethanol for another 30 min, then washed by deionized water. Different amount of dopamine was dissolved in 200 ml deionized water to make a dilute aqueous solution, buffered to a pH typical of marine environments (10 mM Tris, pH = 8.5) to obtain a DA solution. The concentration of DA solution was set at 0.5, 1.0, 1.5 and 2.0 g/L respectively. After that, the polyester cloth was immersed into the DA solution immediately and shaken for a designed time at 30 °C and 150 rpm/min. After modification the membranes were taken out followed by a thorough washing with ethanol and deionized water. The filter membrane before and after PDA modification was compared by characterization with SEM (JEOL JSM-5600LV).
227
Fig. 1. The filtration test set-up.
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the cake layer removal, the pure water flux was measured again and used to calculate the resistance of Rm + Rp, thus the cake layer resistance Rc can be calculated Rc = Rt (Rm + Rp). 3. Results and discussion
Fig. 2. The schematic diagram of MBR set up.
For the short-time experiment, the test was operated in a gravitational filtration mode, at constant trans-membrane pressure (TMP) 7.6 kPa and mixed liquor suspended solids (MLSS) concentration of 12.45 g/L. The long-term test was used to investigate the irreversible fouling on the two types of membrane and demonstrate the application of the PDA coated membrane in MBR. The permeate was drawn using a pump at a constant suction rate, the duration of suction time and idle time was 5 min and 1 min, respectively. The trans-membrane pressure (TMP) and the flux were monitored for 25 days and the membrane resistances were calculated after the test. Once the trans-membrane pressure is over 0.03 MPa, the membrane was taken off and cleaned mechanically. Water flushing and back washing was carried out to remove the cake layer. 2.5. Analysis and calculation In the permeate experiments, the flux was determined by the permeate volume within identical filtration time.
F¼
V A Dt
ð1Þ
where F is the membrane flux (m3 m2 h1), Dt is the filtration time interval (h), A is the membrane area (m2), V is the volume of the collected permeate during the time interval (L). In this study, the standard filtration model is suitable for analyzing the fouling mechanisms, because the size of particle is much smaller than the membrane pore size [24]. To this type of membrane filtration under constant TMP (trans-membrane pressure), the plot of t/V t and the slopes can be used for comparing the different membrane modules, as it is a model suitable for standard filtration. The t/V t curves were drawn for comparing the slopes.
t 1 k ¼ þ t V Q0 2
ð2Þ
In this equation, t is filtration time (min); V is the cumulative permeate volume (L); 0 Q is the initial flux rate (L min1); k is the filtration constant (L1). The plot of t/V t and the slopes can be used for comparing the antifouling properties of different membranes. The smaller the slopes are, the lower the filtration resistances. The filtration resistances are calculated using Darcy equation [25]:
Rt ¼ Rm þ Rp þ Rc ¼
TMP
lJ
ð3Þ
where Rt is the total resistance (m1), calculated using the flux of the fouled membrane; Rm is the membrane resistance (m1), calculated using the pure water flux; Rc is the cake layer resistance (m1); Rp is the pore blocking resistance (m1). After flushing and
PDA is of interest in membrane modifications because a strong increase in hydrophilicity may be imparted to the membrane surface with a very thin PDA layer [20]. The possible structural evolution of dopamine was presented in Fig. 3. Dopamine is easily oxidized by dissolved oxygen under alkaline conditions, creating 5,6-dihydroxyindole and 5,6-indolequinone via intramolecular cyclization, oxidation, and isomerisation. After a multistep reaction proceeding, a mass of melanin-like PDA particles and aggregates was generated in the solution [26,27]. The cross-linking of the dopamine is introduced by the reverse dismutation reaction of dopamine and dopamine quinone. After that, dopamine self-polymerized and formed PDA, and then PDA strongly attached onto the substrate (polyester filter cloth). The mechanism/procedure of the polymerization of dopamine was shown in Fig. 3. Though the mechanism of the reaction between the PDA and the substrates is not very clear, it has been demonstrated that the adhesion is strong enough and the hydrophilicity can be improved due to the hydroxyl and amine groups [28,29]. The two main necessary conditions for the polymerization reaction are oxygen and water. The observed color change from white to black suggests the existence of PDA on the membrane surface. 3.1. PDA modified membrane properties The average pore size of the PDA modified membrane (dopamine concentration of 2 g/L and coating time of 24 h) was measured using a bubble point method. The pore size reduced only 1.13% compared to the original membrane, which suggests that the PDA formed as a very thin layer on the polyester fiber surface. SEM was used to observe the changes in surface morphology of original membrane and modified membrane, which is shown in Fig. 4. For the original polyester cloth, the surface of the fibers was clean and smooth. After modification, the polydopamine particles were deposited on the fiber surface or the gap between fibers. 3.2. Influence of PDA coating time on the antifouling performance of membranes First, the polyester filter cloth was modified by PDA at a low concentration of 0.5 g/L with different coating time. The permeate flux measurement was carried out to characterize the permeation and antifouling properties of the modified membrane using the set-up in Fig. 1. The pure water flux was 520 L m2 h1 at 7.6 kPa, there was no significant difference between the original membrane and the modified membrane. In contrast, the PDA coated membrane had slower flux reduction in the suspended yeast particles, comparing with the original membrane (Fig. 5). The better fluxes of the PDA coated membrane are thought to be the result of the hydrophilic layer on the membrane surface, which tends to form a thin water film between the membrane and bulk solutions and that avoided significant deterioration in permeation and decline in water flux. In Fig. 6, the average flux of three cycles was presented. Obviously, the average flux increased with the coating time when the coating time was shorter than 24 h. The flux of modified membranes of different coating time (6, 12, 24, 36 h) increased 0.6%, 3.08%, 13.94% and 10.7% respectively compared with the original membrane. Importantly, there was no further significant increase of the flux when the coating time reached 36 h, after 24 h. To our
L. Liu et al. / Separation and Purification Technology 118 (2013) 226–233
229
Fig. 3. The scheme for the dopamine cross-linking and polymerization on polyester filter cloth.
observation, the polyester cloth became darker and the solution became clearer during the polymerization. And there has no black particles after 24 h. This demonstrated a rapid and nearly complete consumption of dopamine monomers within 24 h, and there was no more dopamine depositing onto the membrane when the coating time was prolonged to 36 h. Moreover, the high flux recovery suggests the stability of the modification. 3.3. Influence of PDA modification concentration on the antifouling performance of membranes
We come to conclude that the hydrophilicity of membrane surface can be improved when the concentration of solution is increased to a proper level. This is because that the dopamine self-polymerization is not only occurring on the filter surface, but also occurring in solution. When the concentration increases, the monomer consumption in the solution will also increase. Importantly, an average flux in the three runs was observed for the polydopamine-coated membrane, even after physical cleaning of the membrane, suggesting good durability of PDA coating method. 3.4. Fouling mechanism and analysis
Second, the effects of PDA modification concentration on membrane antifouling properties were investigated and the results were presented in Figs. 7 and 8. Based on the previous study, the DA has polymerized on the polyester cloth within 24 h. Thus, with the coating time fixed at 24 h, the concentration of DA was changed from 0.5 g/L to 2 g/L. In comparison with the original polyester cloth membrane, the PDA modified membrane has slower flux reductions, evident in the first cycle. Furthermore, the higher the DA concentration was, the slower the flux went down, this demonstrated the positive effect of the PDA modification on the membrane antifouling properties. As observed, the average flux increased 13.94% for the 0.5 g/L membrane, 32.09% for the 1.0 g/L membrane, 67.55% for the 1.5 g/L membrane and 71.96% for the 2.0 g/L membrane. It can be seen that when the concentration of DA solution increased from 1.5 to 2.0 g/L, the average flux had only a slight enhancement.
For large pore size filter membrane materials, the filtration process depends on the formation of dynamic membrane on the membrane [8]. The mechanism has been divided into two categories in general filtration process, internal pore clogging and cake layer fouling [3]. As shown in Fig. 9, the t/V t curves of different membranes under different coating conditions were used to illustrate the fouling mechanism of the membrane. The parameters of the t/V t were summarized in Table 1. This fouling mechanism means the membrane was fouled by pore blocking followed by the deposit of a cake layer, since the yeast particles had a smaller size comparing to the polyester cloth pore size. In this fouling mode, the smaller the slopes of the curves are, the lower the filtration resistances. It was clearly shown that the slope decreased with the increase of coating time at a 0.5 g/L DA concentration, and reached minimum
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Fig. 4. SEM images of the original membrane (A and B) and PDA modified membrane (C and D).
400
-2
-1
Flux (Lm h )
500
300 200 100
220 204.483 198.657
200
Average flux (Lm -2 h -1)
original membrane 6h PDA coated 12h PDA coated 24h PDA coated 36h PDA coated
600
179.452
180.679
0
6
184.985
180
160
140
120
0 0
50
100
150
200
250
300
Time (min) Fig. 5. The influence of PDA coating time on permeate flux (PDA concentration = 0.5 g/L).
at 24 h. Moreover, the slope of the membrane modified by 24 h PDA coating was much smaller than the original membrane, demonstrating that the PDA coating could improved the membrane antifouling properties effectively. At a coating time of 24 h, the filtration resistance of the membranes was reduced significantly when the PDA concentration increased, due to the improvement of the membrane hydrophilicity. However, the slopes of the curves were similar when the concentration was 1.5 and 2.0 g/L, which agreed well with the flux trend in Fig. 7. This is because the increase of the
100 12
24
36
Time (h) Fig. 6. The stable flux of the original membrane and PDA modified membrane on different coating time.
PDA formed in the solution rather than on the membrane surface, when the concentration was above a proper value. Thus, we can choose the 1.5 g/L dopamine concentration to reduce the modification cost and a coating time at 24 h as the suitable modification conditions for the polyester filters. 3.5. Short-term experiment in MBR For the membrane modification, it is important to demonstrate the prevention of microbial adhesion and practical application
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original membrane 0.5g/l PDA coated 1 g/l PDA coated 1.5g/l PDA coated 2g/l PDA coated
600
400
-2
-1
Flux (Lm h )
500
300
200
100
0 0
50
100
150
200
250
300
Time (min) Fig. 7. The influence of PDA modification concentration on permeate flux (PDA coating time = 24 h).
300.687
308.588
-2 -1
Average flux (Lm h )
300
250
237.045 204.483
200
179.452
150
100 0.0
0.5
1.0
1.5
2.0
Concentration (g/l) Fig. 8. The stable flux of the original membrane and PDA modified membrane on different PDA concentration.
properties in MBR [30,31]. In this part of test, the PDA modified membrane (1.5 g/L and 24 h) and the original membrane were placed in a MBR to compare their antifouling properties for a short time filtration, by gravity flow at a constant but low water head drop of 0.08 m. As shown in Fig. 10, the flux of the PDA coated membrane was lower than the original membrane at the first 2 h. This because the membrane pore size had been reduced due to the PDA modification. For the yeast solution, the size of the particles was much smaller than the pore size so there was no effect on the flux. But the size of the sludge in MBR was much bigger than the yeast, so there has pore block at the beginning of the filtration. But its better antifouling properties was shown since 150 min and even was 36% higher than the original membrane (after >300 min). Though the large apertures of polyester filters lead to a high flux, the microbes will block the membrane apertures and form a biofilm on the membrane surface. The result suggests that the PDA membrane better inhibited microbial adhesion and biofouling in MBR. As a result, the application of the PDA modified membrane in MBR will be further tested. 3.6. Long-term experiment in MBR Polydopamine modification can improve the membrane hydrophilicity [21] and this attributes to the fouling resistance of the modified membrane in a short-time test. Miller et al. [32] demonstrated that short-term membrane surface adhesion tests cannot be suitable predictors of membrane biofouling. In this paper, the properties of the membrane were studied by filtration test rather than the adsorption test. A relatively longer time filtration test in MBR was carried out. The trans-membrane pressure (TMP) was shown in Fig. 11. Both the two types of membranes were installed in one reactor (initial MLSS = 13.55 g/L and the pump speed at 16 rpm, constant flux). The membrane was taken out for physical cleaning when the TMP was over 0.03 MPa. The results showed that the increase of TMP for the PDA coated membrane was slower than the original membrane. The modified membrane was washed for only one time at day 18, comparing to the three times washing of the original membrane at day 13, 20 and 25. After the long-time filtration test, the resistances of the membranes were determined through the flux measurement method (Table 2). It was clear that the total resistance of the modified membrane was lower than the original membrane. The cake layer resistance, which contributes 91% of the total resistance for both
45
45
original membrane 6h PDA coated 12h PDA coated 24h PDA coated 36h PDA coated
t/v (min/l)
35
35
30
25
30
25
20
20
15
15 0
20
40
t (min)
original membrane 0.5g/l PDA coated 1 g/l PDA coated 1.5g/l PDA coated 2g/l PDA coated
40
t/V (min/l)
40
60
80
100
0
20
40
60
80
100
t (min)
Fig. 9. The t/V t curves of different PDA coated conditions corresponding to the flux of the first cycle in Figs. 1 and 2, respectively.
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Table 1 Parameters of the t/V t curves. Variables
PDA coating time (h)
PDA concentration (g/L)
Q0 (ml min1)
Slope
R2
PDA coating time
0 6 12 24 36
0 0.5 0.5 0.5 0.5
60.2 61 59 57.5 57.0
0.296 0.292 0.260 0.211 0.215
0.999 0.999 0.999 0.998 0.999
PDA concentration
0 24 24 24 24
0 0.5 1.0 1.5 2.0
60.2 57.5 61.9 61.2 69.7
0.296 0.211 0.168 0.119 0.118
0.999 0.998 0.997 0.998 0.999
320
Table 2 The resistance analysis of the two membranes in long-term test.
280
Resistance
Flux (Lm-2 h-1)
240 200 Rm Rc Rp Rt
160
original membrane PDA coated membrane
120
Original membrane
PDA modified membrane
Value (1013 m1)
Percentage (%)
Value (1013 m1)
Percentage (%)
0.115 1.442 0.016 1.574
7.34 91.62 1.04 100
0.106 1.150 0.004 1.261
8.45 91.20 0.35 100
80 40 0 0
50
100
150
200
250
300
350
T (min) Fig. 10. The flux of the original membrane and the PDA coated membrane in MBR in short-term test.
0.04
original membrane PDA modified membrane
TMP (Mpa)
0.03
0.02
0.01
0.00 0
5
10
15
20
25
Time (day) Fig. 11. TMP of the original membrane and PDA modified membrane in long-term test.
membranes, is the main fouling form due to the high sludge concentration. Because of PDA modification, though Rm is a little lower, the Rm ratio is a little higher than the original membrane, but the Rp ratio is much lower (being 1/4 that of the original membrane). This means that the accumulation of the foulants particles in the pores was inhibited by PDA layer on the polyester. The hydrophilicity improvement of different polymer membranes by PDA coating has been demonstrated in many studies [20–23]. Though Miller et al. reports PDA modification did not reduce biofouling of the ultrafiltration and nanofiltration membranes
treating surface river water with added acetate [32], effective reduction of the irreversible fouling for the large apertures polyester cloth was shown here. The PDA layer on the surface of the fibers made the adhesion of microbes difficult, but the removal of microbes easy. The no effect report [32] highlights the need for authors to characterize fully both the fouling process and mechanisms, to allow clear understandings of the role of membrane modification, and allow comparison and/or precise prediction of membrane performance in real membrane separation applications. The fouling resistance performance of membranes is affected by membrane property (hydrophilicity and surface charge, pore size and morphology) and separation process (sludge property and hydrodynamic conditions), also by fouling process and mechanisms. For membrane, the fouling resistance depends on its composition and surface charge, pore size and hydrophilicity, also on the property of the interface between liquid and membrane surface, which may undergo changes during the formation of biofilm or adsorption of different foulants. After formation of fully covered biofilm or surface, the further fouling property and propensity will be shaped by the formed biofilm or cake and even the TMP, instead of by the modified membrane itself. For fouling mechanisms characterization, depending on the filtration time, membrane property and foulants concentration, biofouling may be caused by protein adsorption, bacteria adhesion, and present itself in the form of clogged pores or formed gel layer/biofilm or cake layers, thus may affect flux and TMP differently in the short term test and long term test. One paper [33] showed that the water quality significantly affected bio-fouling of PDA modified NF/RO with bacteria under 15 bar pressure. For our short term and long term filtration tests with modified membrane, similar fouling reduction was observed. The effluent of the reactor was not studied in our work considering the membrane pore size had no significant difference after PDA modification, and the effluent qualities was only related to the characterization of the sludge and the operation conditions.
L. Liu et al. / Separation and Purification Technology 118 (2013) 226–233
4. Conclusion A low cost polyester filter cloth was modified by PDA coating under mild conditions. The hydrophilicity and antifouling properties were improved effectively, and it was most evident at 1.5 g/L DA concentration and 24 h coating time. The pore size change was very small compared to the original membrane, due to the formation of a thin layer of PDA on fiber surface. During the long-time filtration test in MBR, the PDA modified membrane has slower TMP rise, longer filtration cycle and lower irreversible fouling resistance. The dopamine consumption was very little and will not affect the low cost of the membrane. It is believed that this hydrophilic, large pore filter membrane has great application potential in wastewater treatment.
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