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A comparison of hydrodynamic methods for mitigating particle fouling in submerged membrane filtration
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Journal of the Chinese Institute of Chemical Engineers 39 (2008) 257–264 www.elsevier.com/locate/jcice
A comparison of hydrodynamic methods for mitigating particle fouling in submerged membrane filtration Kuo-Jen Hwang *, Chih-Sheng Chan, Fung-Fu Chen Department of Chemical and Materials Engineering, Tamkang University, Tamsui, Taipei Hsien 25137, Taiwan Received 16 July 2007; accepted 5 December 2007
Abstract Hydrodynamic methods are used for mitigating particle fouling and for enhancing the filtrate flux in submerged membrane filtration. In the comparison membrane blocking-cake formation filtration system, the effects of filtration pressure, aeration intensity, backwash duration and stepwise increasing pressure on the filtration resistances and filtration flux are measured and discussed. Aeration is helpful for reducing particle deposition on the membrane surface, while stepwise increasing pressure can mainly mitigate internal fouling of the membrane. Periodic backwash can significantly reduce both the resistance caused by the membrane internal fouling and by cake formation; consequently, it can effectively recover the filtrate flux. In contrast, increasing the pressure in constant pressure filtration leads the flux to be decreased due to more severe membrane blockage. According to the comparison of the long-term flux and the received filtrate volume, among these hydrodynamic methods, the periodic backwash with longer duration is the optimal strategy for the filtration. # 2008 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Particle fouling; Microfiltration; Submerged membrane filtration; Filtration resistance; Backwash
1. Introduction A submerged membrane filtration system is frequently installed in bioreactors to obtain clean filtrate or to concentrate microbial cells. Since it has many advantages, such as compact installation, high efficiency and low investment and operation costs, this kind of operation has become increasingly important in biochemical and wastewater treatment processes in recent years. Particle fouling in membrane filtration is commonly attributed to particle deposition on the membrane surface or internal membrane clogging. Defrance et al. (2000) claimed that the suspended solids were the major foulants in a membrane bioreactor because they contributed 2/3 overall filtration resistance. Choo and Hakllee (1998) evaluated the flux decline in a membrane bioreactor in terms of hydraulic resistance, biosolid size and operating condition. They concluded that the permeate flux was significantly determined by the cake formation, especially under low sludge age conditions. Lee et al. (2003) claimed that clean
* Corresponding author. Tel.: +886 2 26215656x2726; fax: +886 2 26209887. E-mail address: [email protected] (K.-J. Hwang).
membrane resistance, cake resistance and membrane blocking resistance contributed 12%, 80% and 8%, respectively, to the overall filtration resistance in a submerged membrane bioreactor. However, Bouhabila et al. (1998, 2001) concluded that the colloids in wastewater caused the majority of membrane fouling and that the filtration resistance was mainly due to membrane clogging. These differing results may be caused by different hydrodynamic conditions or membrane characteristics. The occurrence of particle fouling on the membrane surface or in the membrane pores may result in a serious decline in the filtration flux. To mitigate particle fouling is no doubt the essential step to improve the filtration performance. Several hydraulic methods have been previously proposed in order to reduce particle fouling, such as increasing fluid velocity, backwash, sparging air bubbles, or introducing turbulent flow (Belfort et al., 1994). In general, increasing the shear stress on the membrane surface or increasing aeration intensity are commonly believed to be helpful for reducing particle fouling because they are fairly effective for scouring the particles off of the membrane (Hwang and Chen, 2007; Liu et al., 2000; Shim et al., 2002). A backwash operation may somewhat alleviate not only the membrane internal clogging but the surface fouling as well (Hong et al., 2005; Kuberkar et al., 1998). As a result, the
0368-1653/$ – see front matter # 2008 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jcice.2007.12.016
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2. Materials and methods
Nomenclature DP q qs Q Rc Rif Rm Rt t tb vt
filtration pressure (kPa) superficial velocity of filtrate or filtrate flux (m3/m2 s) filtrate flux at 12,000 s (m3/m2 s) air volumetric flow rate (m3/s) resistance of the filter cake (1/m) resistance of the membrane internal fouling (1/m) resistance of the clean membrane (1/m) overall filtration resistance (1/m) filtration time (s) backwash duration (min) received filtrate volume per unit area at 12,000 s (m3/m2)
Greek symbol m viscosity of liquid (kg/m s) flux can be partially recovered after a backwash. According to the results of Kuberkar et al. (1998), the net fluxes in cross-flow microfiltration with optimum backwashing conditions for washed bacteria were approximately 10 times higher than those obtained during normal cross-flow microfiltration. Although those hydrodynamic methods may mitigate particle fouling and enhance the filtrate flux, no comparison has been made in the past to evaluate their characteristics and effectiveness. Since the hydrodynamic conditions are of significance for retarding membrane fouling and maintaining stable permeating flux during a filtration, four hydrodynamic methods are used in this study for mitigating the particle fouling and enhancing the filtrate flux in a membrane blocking-cake formation comparable submerged membrane filtration system. These are increasing filtration pressure, increasing aeration intensity, periodic backwash and step increase in filtration pressure. The effects of these operations on the filtration resistances and the filtrate fluxes are discussed thoroughly.
A particulate sample made of polymethylmethacrylate (PMMA) was suspended in de-ionized water to prepare the 0.1 wt.% suspensions used in all experiments. The particles, with a mean diameter of 5.0 mm, were manufactured by Soken Co., Japan. The shape of the particles was spherical, and the density was 1190 kg/m3. The pH and temperature of the suspensions were kept at 7.0 and 20 8C, respectively, during filtration. In such a condition, the particle zeta potential was 20 mV. A ceramic membrane, manufactured by Orelis Co., France (Kerasep #06040), with a mean pore size of 5.6 mm on the outer surface was used in all experiments as the filter medium. That cylindrical membrane had an inner diameter of 6.0 103 m, an outer diameter of 1.0 102 m, and a length of 0.32 m. The filtration was operated in an outside-in scheme; therefore, the overall filtration area was 0.01 m2. The clean membrane resistance was measured as 1.3 1011 m1 under a pressure of 40 kPa. A schematic diagram of the submerged membrane filtration system is shown in Fig. 1. A mechanical mixer was installed in the suspension tank to maintain the homogeneity of particle concentration. The filtration pressure was adjusted to the designated value by a regulator that was installed before the vacuum pump. The flow rate of aerated air was adjusted and measured by a rotameter. The filtrate was received into a flask and weighed by the load cell during filtration. The data of filtrate weight vs. time was transferred to a personal computer for analysis. In this study, four hydrodynamic methods were selected for mitigating particle fouling and enhancing the filtrate flux. The filtration pressures ranged from 28 to 64 kPa in constant pressure filtration, while the air volumetric flow rates ranged from 0 to 3.3 104 m3/s in aeration filtration. Analysis of the bubble size distribution under various aeration intensities can be referred to our recent work (Hwang and Chen, 2007). In order to avoid severe membrane blocking occurring in the early period of filtration, pressure step increase (PSI) operations were carried out. The filtration pressures were set as 28 kPa at the
Fig. 1. A schematic diagram of the submerged membrane filtration system.
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Fig. 2 shows the time courses of the filtrate flux in a submerged membrane filtration under various conditions. As a comparison to constant pressure filtration, various durations of backwash and PSI operations were used in order to simultaneously mitigate particle fouling and enhance the filtrate flux. All conditions in this figure were set to be no aeration and be under a constant filtration pressure of 40 kPa except for the pressure in the PSI operation. For constant pressure filtration (CPF), with no additional procedures to mitigate particle fouling, the flux declines very quickly in the
early period of filtration. The flux decline becomes slower after 2000 s and then gradually approaches a pseudo-constant value after 10,000 s. This is because the extremely low flux at that time cannot supply enough drag for further stable particle deposition (Hwang and Chen, 2007). A typical result of the PSI operation is also shown in Fig. 2. The procedures for increasing pressure have been described in the previous section. If the membrane blockage is very serious in the filtration system, operating under a low pressure at the early stage may effectively prevent too many particles from migrating into the membrane pores. The later formed filter cake plays a similar role to a dynamic membrane to trap the coming particles (Guell et al., 1999; Hwang and Cheng, 2003; Hwang and Wu, 2007). Hence, the filtrate flux will be improved by reducing the membrane blocking. Comparing the results of CPF and PSI operations shown in Fig. 2, the flux in PSI is lower than that in CPF at the beginning of filtration due to the lower driving force. However, the flux in PSI operation becomes higher after 1000 s because of minor membrane blocking. When the filtration pressure is increased stepwise, the flux will suddenly increase due to the increase in driving force. Although the average filtration pressure during the whole courses of these two operations is almost the same, the PSI operation makes a longterm flux over two times higher than CPF. Another hydrodynamic method to enhance the filtrate flux is to backwash the fouled membrane following each 15 min of filtration. De-ionized water is pumped reversely in an inside-out scheme with a constant rate of 4.15 105 m3/m2 s. In the backwash experiments, three washing durations were set; they were 1, 2 and 4 min. The experimental data shown in Fig. 2 clearly indicates that the flux attenuation would be retarded by the backwash operations. The longer the backwash duration is, the higher the flux recovery will be. This fact can be attributed to the wash-away of fouled particles; a longer backwash duration leads to a cleaner filter membrane. A 4-min duration of the backwash makes the flux four to eight times higher than would be in CPF under the same filtration pressure. The effect of aeration on the flux enhancement is shown in Fig. 3. An increase in the air flow rate leads to an increase in the
Fig. 2. Time courses of the filtrate flux under various operating conditions.
Fig. 3. Effect of air flow rate on the filtrate flux during aeration filtration.
beginning of filtration, increased to 40 kPa at 6000 s and then increased 12 kPa per 3000 s until the final setting value. The pressure was kept constant in each time interval. In the backwash operations, the backwash durations were set as 1, 2 or 4 min, respectively. After each 15 min of filtration, it was necessary to clean the fouled membrane and recover the filtrate flux. The flow rate of de-ionized water for backwash was set to be a constant of 4.15 105 m3/m2 s. When an experiment was terminated, the cake formed on the filter membrane was washed out and its dry mass measured. After cleaning the membrane surface, the sum of the filtration resistances caused by the membrane blocking and the clean membrane could be measured by de-ionized water flux through the fouled membrane, while the cake resistance could be calculated by subtracting this sum from the overall filtration resistance. After the preceding experimental procedures and resistance analyses, a chemical–physical cleaning process was performed on the ceramic membrane. The membrane was submerged in a container filled with 1 L acetone. The container was then put in an ultrasonic vibrator and vibration with a frequency of 70 Hz was carried out for 5 h. After the chemical cleaning, the membrane was immersed in de-ionized water for 2 h and was washed using a back-pulse to remove the residual acetone. The membrane was cleaned and regenerated completely to its original resistance after these procedures. 3. Results and discussion
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Fig. 5. Comparison of the filtrate fluxes at 12,000 s under various conditions.
Fig. 4. Appearance of the filter cakes formed on the membrane surfaces under (a) Q = 0 m3/s and (b) Q = 3.33 104 m3/s.
filtrate flux. This is because the particle fouling can be efficiently mitigated by the aeration. The upflow stream of the air bubbles produces extra shear stress acting on the membrane surface, which can provide a sweeping effect and restrain the particle deposition. Fig. 4 illustrates the filter cakes formed on the membrane surface under Q = 0 and 3.33 104 m3/s, respectively. When an experiment is terminated, the cake thickness can be measured by a vernier, and the cake appearance can be observed from a photograph taken by a digital camera. The cake is thicker and its surface is smoother under the condition of no aeration, as shown in Fig. 4(a). On the other hand, under the aeration condition, a thinner cake with a ripple surface can be observed in Fig. 4(b). This demonstrates that the upflow air bubbles can reduce the particle deposition on the membrane surface. Consequently, increasing the aeration intensity may be an efficient way to enhance the filtrate flux. Fig. 5 shows a comparison of the filtrate fluxes at 12,000 s under various operating conditions. For the filtration under constant pressure (CPF), it is clear that the effect of filtration pressure on the flux is negligible whether under aeration conditions or not. In fact, the flux is slightly decreased by increasing pressure. This is due to more severe internal fouling
of the membrane under higher pressure and has been discussed in the authors’ previous study (Hwang and Chen, 2007). For the aeration conditions, the air bubbles generated by the diffuser are in an oblate ellipsoidal shape; and their equivalent diameters range from 3.5 to 5.5 mm in this study. Comparing the results with those in CPF under the same pressure, a 20% increase in flux can be obtained when the gas volumetric flow rate increases from 0 to 3.33 104 m3/s. For the PSI operations, the filtration pressures are set at a low value, e.g., 28 kPa, at the beginning of the filtration to mitigate the membrane blocking, and then increased stepwise after the designated time increment is reached to increase the driving force. The pressures shown in Fig. 5 for PSI operations are their final values. The results clearly indicate that a remarkable increase in flux is obtained by using this method, and the flux can be enhanced two- to three-fold as compared to those under constant pressure filtration. In the PSI operations, a higher filtration pressure, however, leads to a lower filtrate flux due to more severe particle fouling. In addition, for PSI operations under the same pressures, higher fluxes can be obtained under the condition of aeration. Since the fouled particles may be washed away from the membrane by a periodic backwash, the filtrate flux can also be enhanced due to the decrease of the filtration resistance. Fig. 5 also shows the effect of the backwash duration on the filtrate flux under DP = 40 kPa. An increase in backwash duration causes the flux to increase linearly. When the duration exceeds 1 min, the backwash is more effective than the other methods from the viewpoint of flux enhancement. The maximum flux resulting from the backwash can attain over a six-fold increase, as compared to that under the constant pressure filtration. In most microfiltration, the major filtration resistance sources include the filtration resistances due to the internal fouling in the membrane pores (membrane blocking), Rif, due to the cake formation, Rc and the clean membrane resistance, Rm. Therefore, the basic cake filtration equation can be written
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in a resistance-in-series form as q¼
DP DP ¼ mðRt Þ mðRc þ Rif þ Rm Þ
(1)
Then, the overall filtration resistance, Rt, can be calculated from the filtrate flux and the filtration pressure, and the other resistance sources can be estimated by the method described in the previous section. For example, after removing the filter cake, the value of Rif can be obtained from the difference between the filtration resistances of the fouled and the clan membranes. In order to understand the filtration characteristics and the effectiveness of the hydrodynamic methods, the resistance sources are measured and analyzed as follows. Fig. 6 shows a pie chart of the filtration resistances in a constant pressure filtration at 12,000 s. The filtration was carried out under a pressure of 40 kPa with no aeration. It can be found that 64% of the overall filtration resistance is caused by the membrane internal fouling. The cake resistance accounts for around 1/3 of the overall resistance, while the clean membrane resistance is trivial compared to the others. These results imply that prevention or reduction of the membrane blocking becomes the most important strategy to enhance the filtrate flux in this submerged membrane filtration system. Fig. 7 depicts the filtration resistances under the condition of a 4 min backwash after each cycle of 15 min filtration. The filtration pressure and the aeration conditions are all the same as those in Fig. 6. The data was measured about 12,000 s before the initiation of the backwash in that cycle. Comparing this figure with Fig. 6, the percentage of Rif is increased while Rc is decreased in the backwash operation. Seventy-four percent of
Fig. 6. Pie chart of filtration resistance at 12,000 s in the filtration under the constant pressure of 40 kPa.
Fig. 7. Distribution of filtration resistance at 12,000 s in the filtration with a 4min duration of the periodic backwash.
the overall resistance is due to the membrane internal fouling. However, both Rif and Rc can be significantly reduced by the backwash operation. The values of Rif and Rc in the backwash operation are only 1/8 and 1/20, respectively, of their original values in CPF. Therefore, backwash is an efficient way of removing the fouled particles, especially for those deposited on the membrane surface. Fig. 8 shows a typical resistance distribution in a PSI operation at 12,000 s. Since the lower filtration pressure at the beginning can reduce the particle fouling, the value of Rif can be reduced by 40% while Rc can be reduced by 18% compared to those in Fig. 6. Therefore, the overall filtration resistance is significantly decreased by this PSI operation. Comparing the results shown in Figs. 8 and 6, the ratio of Rif to Rc decreases from 1.88 in the CPF to 1.33 in the PSI operation under the same conditions of no aeration and no backwash. This reveals that the PSI operation can reduce both Rif and Rc but it is more efficient in the mitigation of the membrane internal fouling. Fig. 9 shows the resistance distribution in the filtration under a constant pressure of 40 kPa and an air flow rate of 3.33 104 m3/s. Ninety percent of the overall filtration resistance is caused by the membrane internal fouling. However, the value of Rif is nearly the same as that in the CPF with no aeration shown in Fig. 6. This means that aeration has no effect on the mitigation of internal fouling of the membrane. However, the cake resistance can be significantly reduced by aeration. Only 10% of the overall resistance is contributed by the cake formation. Compared with the results of CPF without aeration shown in Fig. 6, only 1/4 of the cake mass in the former is formed in this aeration case. The filtration resistances due to membrane-internal fouling at 12,000 s under various hydrodynamic conditions are
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Fig. 10. Comparison of Rif under various operating conditions.
Fig. 8. Pie chart of filtration resistance at 12,000 s in a filtration under the pressure increasing stepwise from 28 to 40 kPa.
compared in Fig. 10. The filtration pressures are all kept at 40 kPa except those in PSI operation and that in the CPF under 64 kPa. To compare the values in CPF under 40 and 64 kPa, the value of Rif is increased by raising the filtration pressure in the system of this study. This is because particles can more easily penetrate the membrane pores under a higher
Fig. 9. Distribution of filtration resistances at 12,000 s in the filtration under a constant pressure of 40 kPa and Q = 3.33 104 m3/s.
pressure. When constant–flux backwash is operated periodically, the value of Rif can be significantly reduced because some fouled particles may be washed out of the membrane pores. Since a longer backwash time results in a cleaner membrane, the value of Rif decreases with the increase in the backwash duration. The values of Rif under 1 and 4 min of backwash durations are only a half and 12.5%, respectively, of those in CPF under the same pressures. Furthermore, the value of Rif becomes smaller in PSI operation but a slightly larger in the condition with an aeration rate of 3.33 104 m3/s. This is because particles have more difficulty penetrating the membrane pores under a lower pressure in the early period of filtration, and the contrary is true if less cake is formed on the membrane surface. Fig. 11 shows the cake resistances under the same conditions as those shown in Fig. 10. Since more cake will be formed under a higher pressure, the value of Rc increases with increasing the filtration pressure in a CPF. When the filtration pressure increases from 40 to 64 kPa, the value of Rc increases over twofold. For those backwash operations, an increase in the wash duration leads to a higher flux recovery; therefore, the increase causes more cake to be formed during the following filtration period. The value of Rc then increases with increasing
Fig. 11. Comparison of Rc under various operating conditions.
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the backwash duration. As the filtration period in each cycle is only 900 s, the Rc values are much lower than those in CPF. Furthermore, a lower filtration pressure is applied in the initial stage of PSI operations; the values of Rc are slightly lower than those in CPF under the same filtration pressure. The other way to mitigate particle fouling is through aeration. Although the internal fouling of the membrane cannot be affected by aeration (as shown in Fig. 10), the resistance due to cake formation can be effectively reduced by this operation. Combining the results shown in Figs. 10 and 11, a PSI operation can significantly decrease Rif, while aeration can efficiently reduce the cake formation. The PSI operation is more effective than aeration from the viewpoint of reducing overall filtration resistance. In fact, the optimal way to mitigate particle fouling is to undergo a periodical backwash operation. Four hydrodynamic methods, such as increasing filtration pressure, increasing aeration intensity, increasing filtration pressure stepwise, and periodic backwashing of the fouled membrane have been tested for the purpose of mitigating particle fouling of a submerged membrane filtration system. In order to understand the effectiveness of these methods, the net filtrate volumes (i.e., the filtrate volume—the injected clean water volume for backwashing) received at 12,000 s under various conditions were measured and are shown in Fig. 12. The only method having a negative effect is to increase the filtration pressure as the filtrate volume decreases 30% when the working pressure increases from 40 to 64 kPa. The aeration intensity increasing from Q = 0 to 3.33 104 m3/s makes an increase in the filtrate volume as high as 50%. The PSI operation can obtain 70% more of the filtrate compared to that in CPF under the same pressure. The most efficient method is the periodical backwash if the duration is longer than 1 min.
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The backwash operation with 4 min duration can get a filtrate volume more than three-fold higher than those in CPF. 4. Conclusion Four hydrodynamic methods (increasing filtration pressure, increasing aeration intensity, increasing filtration pressure stepwise, and periodic backwashing of the fouled membrane) have been tested for mitigating particle fouling and enhancing the filtrate flux in a submerged membrane filtration system. Increasing the filtration pressure causes the flux to be decreased due to more severe membrane blocking. The filtrate received at 12,000 s decreases 30% as the pressure increases from 40 to 64 kPa. Aeration can reduce the particle deposition on the membrane surface but has no noticeable effect on the membrane internal fouling. Compared with the constant pressure filtration under the same pressure, the cake resistance can be reduced 75% and the flux can be enhanced 20% under an aeration rate of 3.33 104 m3/s. A stepwise increasing pressure can mitigate the membrane internal fouling as high as 40% and obtain two- to three-fold increase in flux as compared to constant pressure filtration. A periodic backwash can significantly reduce both the resistances due to membraneinternal fouling and cake formation. Comparing the long-term flux and the received filtrate volume among these hydrodynamic methods, the periodic backwash with a longer duration shows great effect on mitigating membrane fouling among four methods. Acknowledgement The authors wish to express their sincere gratitude to the National Science Council of the Republic of China for its financial support. References
Fig. 12. Comparison of received filtrate volumes at 12,000 s under various operating conditions.
Belfort, G., R. H. Davis, and A. L. Zydney, ‘‘The Behavior of Suspensions and Macromolecular Solutions in Crossflow Microfiltration,’’ J. Membr. Sci., 96, 1 (1994). Bouhabila, E. H., R. B. Aim, and H. Buisson, ‘‘Microfiltration of Activated Sludge Using Submerged Membrane with Air Bubbling: Application to Wastewater Treatment,’’ Desalination, 118, 315 (1998). Bouhabila, E. H., R. Ben Aim, and H. Buisson, ‘‘Fouling Characterisation in Membrane Bioreactors,’’ Sep. Purif. Technol., 22/23, 123 (2001). Choo, K. H. and C. Hakllee, ‘‘Hydrodynamic Behavior of Aerobic Biosolids during Crossflow Filtration in the Membrane Anaerobic Bioreactor,’’ Water Res., 32, 3387 (1998). Defrance, L., M. Y. Jaffrin, B. Gupta, P. Paullie, and V. Geaugey, ‘‘Contribution of Various Constituents of Activated Sludge to Membrane Bioreactor Fouling,’’ Bioresour. Technol., 73, 105 (2000). Guell, C., P. Czekaj, and R. H. Davis, ‘‘Microfiltration of Protein Mixtures and the Effects of Yeast on Membrane Fouling,’’ J. Membr. Sci., 155, 113 (1999). Hong, S., P. Krishna, C. Hobbs, D. Kim, and J. Cho, ‘‘Variations in Backwash Efficiency During Colloidal Filtration of Hollow–Fiber Microfiltration Membranes,’’ Desalination, 173, 257 (2005). Hwang, K. J. and Y. H. Cheng, ‘‘The Role of Dynamic Membrane in CrossFlow Microfiltration of Macromolecules,’’ Sep. Sci. Technol., 38, 779 (2003).
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Hwang, K. J. and R. M. Wu, ‘‘Use of Models in the Design of Cross-flow Microfilters for the Purification of Protein from Bio-mixture,’’ J. Chin. Inst. Chem. Engrs., 38, 125 (2007). Hwang, K. J. and F. F. Chen, ‘‘Modeling of Particle Fouling and Membrane Blocking in Submerged Membrane Filtration,’’ Sep. Sci. Technol., 42, 2595 (2007). Kuberkar, V., P. Czekaj, and R. Davis, ‘‘Flux Enhancement for Membrane Filtration of Bacterial Suspensions Using High-Frequency Backpulsing,’’ Biotechnol. Bioeng., 60, 77 (1998).
Lee, W., S. Kang, and H. S. Shin, ‘‘Sludge Characteristics and their Contribution to Microfiltration in Submerged Membrane Bioreactors,’’ J. Membr. Sci., 216, 217 (2003). Liu, R., X. Huang, C. Wang, L. Chen, and Y. Qian, ‘‘Study on Hydraulic Characteristics in a Submerged Membrane Bioreactor Process,’’ Process. Biochem., 36, 249 (2000). Shim, J. K., I. Yoo, and Y. M. Lee, ‘‘Design and Operation Considerations for Wastewater Treatment Using a Flat Submerged Membrane Bioreactor,’’ Process. Biochem., 38, 279 (2002).
Journal of the Chinese Institute of Chemical Engineers 39 (2008) 257–264 www.elsevier.com/locate/jcice
A comparison of hydrodynamic methods for mitigating particle fouling in submerged membrane filtration Kuo-Jen Hwang *, Chih-Sheng Chan, Fung-Fu Chen Department of Chemical and Materials Engineering, Tamkang University, Tamsui, Taipei Hsien 25137, Taiwan Received 16 July 2007; accepted 5 December 2007
Abstract Hydrodynamic methods are used for mitigating particle fouling and for enhancing the filtrate flux in submerged membrane filtration. In the comparison membrane blocking-cake formation filtration system, the effects of filtration pressure, aeration intensity, backwash duration and stepwise increasing pressure on the filtration resistances and filtration flux are measured and discussed. Aeration is helpful for reducing particle deposition on the membrane surface, while stepwise increasing pressure can mainly mitigate internal fouling of the membrane. Periodic backwash can significantly reduce both the resistance caused by the membrane internal fouling and by cake formation; consequently, it can effectively recover the filtrate flux. In contrast, increasing the pressure in constant pressure filtration leads the flux to be decreased due to more severe membrane blockage. According to the comparison of the long-term flux and the received filtrate volume, among these hydrodynamic methods, the periodic backwash with longer duration is the optimal strategy for the filtration. # 2008 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Particle fouling; Microfiltration; Submerged membrane filtration; Filtration resistance; Backwash
1. Introduction A submerged membrane filtration system is frequently installed in bioreactors to obtain clean filtrate or to concentrate microbial cells. Since it has many advantages, such as compact installation, high efficiency and low investment and operation costs, this kind of operation has become increasingly important in biochemical and wastewater treatment processes in recent years. Particle fouling in membrane filtration is commonly attributed to particle deposition on the membrane surface or internal membrane clogging. Defrance et al. (2000) claimed that the suspended solids were the major foulants in a membrane bioreactor because they contributed 2/3 overall filtration resistance. Choo and Hakllee (1998) evaluated the flux decline in a membrane bioreactor in terms of hydraulic resistance, biosolid size and operating condition. They concluded that the permeate flux was significantly determined by the cake formation, especially under low sludge age conditions. Lee et al. (2003) claimed that clean
* Corresponding author. Tel.: +886 2 26215656x2726; fax: +886 2 26209887. E-mail address: [email protected] (K.-J. Hwang).
membrane resistance, cake resistance and membrane blocking resistance contributed 12%, 80% and 8%, respectively, to the overall filtration resistance in a submerged membrane bioreactor. However, Bouhabila et al. (1998, 2001) concluded that the colloids in wastewater caused the majority of membrane fouling and that the filtration resistance was mainly due to membrane clogging. These differing results may be caused by different hydrodynamic conditions or membrane characteristics. The occurrence of particle fouling on the membrane surface or in the membrane pores may result in a serious decline in the filtration flux. To mitigate particle fouling is no doubt the essential step to improve the filtration performance. Several hydraulic methods have been previously proposed in order to reduce particle fouling, such as increasing fluid velocity, backwash, sparging air bubbles, or introducing turbulent flow (Belfort et al., 1994). In general, increasing the shear stress on the membrane surface or increasing aeration intensity are commonly believed to be helpful for reducing particle fouling because they are fairly effective for scouring the particles off of the membrane (Hwang and Chen, 2007; Liu et al., 2000; Shim et al., 2002). A backwash operation may somewhat alleviate not only the membrane internal clogging but the surface fouling as well (Hong et al., 2005; Kuberkar et al., 1998). As a result, the
0368-1653/$ – see front matter # 2008 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jcice.2007.12.016
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K.-J. Hwang et al. / Journal of the Chinese Institute of Chemical Engineers 39 (2008) 257–264
2. Materials and methods
Nomenclature DP q qs Q Rc Rif Rm Rt t tb vt
filtration pressure (kPa) superficial velocity of filtrate or filtrate flux (m3/m2 s) filtrate flux at 12,000 s (m3/m2 s) air volumetric flow rate (m3/s) resistance of the filter cake (1/m) resistance of the membrane internal fouling (1/m) resistance of the clean membrane (1/m) overall filtration resistance (1/m) filtration time (s) backwash duration (min) received filtrate volume per unit area at 12,000 s (m3/m2)
Greek symbol m viscosity of liquid (kg/m s) flux can be partially recovered after a backwash. According to the results of Kuberkar et al. (1998), the net fluxes in cross-flow microfiltration with optimum backwashing conditions for washed bacteria were approximately 10 times higher than those obtained during normal cross-flow microfiltration. Although those hydrodynamic methods may mitigate particle fouling and enhance the filtrate flux, no comparison has been made in the past to evaluate their characteristics and effectiveness. Since the hydrodynamic conditions are of significance for retarding membrane fouling and maintaining stable permeating flux during a filtration, four hydrodynamic methods are used in this study for mitigating the particle fouling and enhancing the filtrate flux in a membrane blocking-cake formation comparable submerged membrane filtration system. These are increasing filtration pressure, increasing aeration intensity, periodic backwash and step increase in filtration pressure. The effects of these operations on the filtration resistances and the filtrate fluxes are discussed thoroughly.
A particulate sample made of polymethylmethacrylate (PMMA) was suspended in de-ionized water to prepare the 0.1 wt.% suspensions used in all experiments. The particles, with a mean diameter of 5.0 mm, were manufactured by Soken Co., Japan. The shape of the particles was spherical, and the density was 1190 kg/m3. The pH and temperature of the suspensions were kept at 7.0 and 20 8C, respectively, during filtration. In such a condition, the particle zeta potential was 20 mV. A ceramic membrane, manufactured by Orelis Co., France (Kerasep #06040), with a mean pore size of 5.6 mm on the outer surface was used in all experiments as the filter medium. That cylindrical membrane had an inner diameter of 6.0 103 m, an outer diameter of 1.0 102 m, and a length of 0.32 m. The filtration was operated in an outside-in scheme; therefore, the overall filtration area was 0.01 m2. The clean membrane resistance was measured as 1.3 1011 m1 under a pressure of 40 kPa. A schematic diagram of the submerged membrane filtration system is shown in Fig. 1. A mechanical mixer was installed in the suspension tank to maintain the homogeneity of particle concentration. The filtration pressure was adjusted to the designated value by a regulator that was installed before the vacuum pump. The flow rate of aerated air was adjusted and measured by a rotameter. The filtrate was received into a flask and weighed by the load cell during filtration. The data of filtrate weight vs. time was transferred to a personal computer for analysis. In this study, four hydrodynamic methods were selected for mitigating particle fouling and enhancing the filtrate flux. The filtration pressures ranged from 28 to 64 kPa in constant pressure filtration, while the air volumetric flow rates ranged from 0 to 3.3 104 m3/s in aeration filtration. Analysis of the bubble size distribution under various aeration intensities can be referred to our recent work (Hwang and Chen, 2007). In order to avoid severe membrane blocking occurring in the early period of filtration, pressure step increase (PSI) operations were carried out. The filtration pressures were set as 28 kPa at the
Fig. 1. A schematic diagram of the submerged membrane filtration system.
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Fig. 2 shows the time courses of the filtrate flux in a submerged membrane filtration under various conditions. As a comparison to constant pressure filtration, various durations of backwash and PSI operations were used in order to simultaneously mitigate particle fouling and enhance the filtrate flux. All conditions in this figure were set to be no aeration and be under a constant filtration pressure of 40 kPa except for the pressure in the PSI operation. For constant pressure filtration (CPF), with no additional procedures to mitigate particle fouling, the flux declines very quickly in the
early period of filtration. The flux decline becomes slower after 2000 s and then gradually approaches a pseudo-constant value after 10,000 s. This is because the extremely low flux at that time cannot supply enough drag for further stable particle deposition (Hwang and Chen, 2007). A typical result of the PSI operation is also shown in Fig. 2. The procedures for increasing pressure have been described in the previous section. If the membrane blockage is very serious in the filtration system, operating under a low pressure at the early stage may effectively prevent too many particles from migrating into the membrane pores. The later formed filter cake plays a similar role to a dynamic membrane to trap the coming particles (Guell et al., 1999; Hwang and Cheng, 2003; Hwang and Wu, 2007). Hence, the filtrate flux will be improved by reducing the membrane blocking. Comparing the results of CPF and PSI operations shown in Fig. 2, the flux in PSI is lower than that in CPF at the beginning of filtration due to the lower driving force. However, the flux in PSI operation becomes higher after 1000 s because of minor membrane blocking. When the filtration pressure is increased stepwise, the flux will suddenly increase due to the increase in driving force. Although the average filtration pressure during the whole courses of these two operations is almost the same, the PSI operation makes a longterm flux over two times higher than CPF. Another hydrodynamic method to enhance the filtrate flux is to backwash the fouled membrane following each 15 min of filtration. De-ionized water is pumped reversely in an inside-out scheme with a constant rate of 4.15 105 m3/m2 s. In the backwash experiments, three washing durations were set; they were 1, 2 and 4 min. The experimental data shown in Fig. 2 clearly indicates that the flux attenuation would be retarded by the backwash operations. The longer the backwash duration is, the higher the flux recovery will be. This fact can be attributed to the wash-away of fouled particles; a longer backwash duration leads to a cleaner filter membrane. A 4-min duration of the backwash makes the flux four to eight times higher than would be in CPF under the same filtration pressure. The effect of aeration on the flux enhancement is shown in Fig. 3. An increase in the air flow rate leads to an increase in the
Fig. 2. Time courses of the filtrate flux under various operating conditions.
Fig. 3. Effect of air flow rate on the filtrate flux during aeration filtration.
beginning of filtration, increased to 40 kPa at 6000 s and then increased 12 kPa per 3000 s until the final setting value. The pressure was kept constant in each time interval. In the backwash operations, the backwash durations were set as 1, 2 or 4 min, respectively. After each 15 min of filtration, it was necessary to clean the fouled membrane and recover the filtrate flux. The flow rate of de-ionized water for backwash was set to be a constant of 4.15 105 m3/m2 s. When an experiment was terminated, the cake formed on the filter membrane was washed out and its dry mass measured. After cleaning the membrane surface, the sum of the filtration resistances caused by the membrane blocking and the clean membrane could be measured by de-ionized water flux through the fouled membrane, while the cake resistance could be calculated by subtracting this sum from the overall filtration resistance. After the preceding experimental procedures and resistance analyses, a chemical–physical cleaning process was performed on the ceramic membrane. The membrane was submerged in a container filled with 1 L acetone. The container was then put in an ultrasonic vibrator and vibration with a frequency of 70 Hz was carried out for 5 h. After the chemical cleaning, the membrane was immersed in de-ionized water for 2 h and was washed using a back-pulse to remove the residual acetone. The membrane was cleaned and regenerated completely to its original resistance after these procedures. 3. Results and discussion
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Fig. 5. Comparison of the filtrate fluxes at 12,000 s under various conditions.
Fig. 4. Appearance of the filter cakes formed on the membrane surfaces under (a) Q = 0 m3/s and (b) Q = 3.33 104 m3/s.
filtrate flux. This is because the particle fouling can be efficiently mitigated by the aeration. The upflow stream of the air bubbles produces extra shear stress acting on the membrane surface, which can provide a sweeping effect and restrain the particle deposition. Fig. 4 illustrates the filter cakes formed on the membrane surface under Q = 0 and 3.33 104 m3/s, respectively. When an experiment is terminated, the cake thickness can be measured by a vernier, and the cake appearance can be observed from a photograph taken by a digital camera. The cake is thicker and its surface is smoother under the condition of no aeration, as shown in Fig. 4(a). On the other hand, under the aeration condition, a thinner cake with a ripple surface can be observed in Fig. 4(b). This demonstrates that the upflow air bubbles can reduce the particle deposition on the membrane surface. Consequently, increasing the aeration intensity may be an efficient way to enhance the filtrate flux. Fig. 5 shows a comparison of the filtrate fluxes at 12,000 s under various operating conditions. For the filtration under constant pressure (CPF), it is clear that the effect of filtration pressure on the flux is negligible whether under aeration conditions or not. In fact, the flux is slightly decreased by increasing pressure. This is due to more severe internal fouling
of the membrane under higher pressure and has been discussed in the authors’ previous study (Hwang and Chen, 2007). For the aeration conditions, the air bubbles generated by the diffuser are in an oblate ellipsoidal shape; and their equivalent diameters range from 3.5 to 5.5 mm in this study. Comparing the results with those in CPF under the same pressure, a 20% increase in flux can be obtained when the gas volumetric flow rate increases from 0 to 3.33 104 m3/s. For the PSI operations, the filtration pressures are set at a low value, e.g., 28 kPa, at the beginning of the filtration to mitigate the membrane blocking, and then increased stepwise after the designated time increment is reached to increase the driving force. The pressures shown in Fig. 5 for PSI operations are their final values. The results clearly indicate that a remarkable increase in flux is obtained by using this method, and the flux can be enhanced two- to three-fold as compared to those under constant pressure filtration. In the PSI operations, a higher filtration pressure, however, leads to a lower filtrate flux due to more severe particle fouling. In addition, for PSI operations under the same pressures, higher fluxes can be obtained under the condition of aeration. Since the fouled particles may be washed away from the membrane by a periodic backwash, the filtrate flux can also be enhanced due to the decrease of the filtration resistance. Fig. 5 also shows the effect of the backwash duration on the filtrate flux under DP = 40 kPa. An increase in backwash duration causes the flux to increase linearly. When the duration exceeds 1 min, the backwash is more effective than the other methods from the viewpoint of flux enhancement. The maximum flux resulting from the backwash can attain over a six-fold increase, as compared to that under the constant pressure filtration. In most microfiltration, the major filtration resistance sources include the filtration resistances due to the internal fouling in the membrane pores (membrane blocking), Rif, due to the cake formation, Rc and the clean membrane resistance, Rm. Therefore, the basic cake filtration equation can be written
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in a resistance-in-series form as q¼
DP DP ¼ mðRt Þ mðRc þ Rif þ Rm Þ
(1)
Then, the overall filtration resistance, Rt, can be calculated from the filtrate flux and the filtration pressure, and the other resistance sources can be estimated by the method described in the previous section. For example, after removing the filter cake, the value of Rif can be obtained from the difference between the filtration resistances of the fouled and the clan membranes. In order to understand the filtration characteristics and the effectiveness of the hydrodynamic methods, the resistance sources are measured and analyzed as follows. Fig. 6 shows a pie chart of the filtration resistances in a constant pressure filtration at 12,000 s. The filtration was carried out under a pressure of 40 kPa with no aeration. It can be found that 64% of the overall filtration resistance is caused by the membrane internal fouling. The cake resistance accounts for around 1/3 of the overall resistance, while the clean membrane resistance is trivial compared to the others. These results imply that prevention or reduction of the membrane blocking becomes the most important strategy to enhance the filtrate flux in this submerged membrane filtration system. Fig. 7 depicts the filtration resistances under the condition of a 4 min backwash after each cycle of 15 min filtration. The filtration pressure and the aeration conditions are all the same as those in Fig. 6. The data was measured about 12,000 s before the initiation of the backwash in that cycle. Comparing this figure with Fig. 6, the percentage of Rif is increased while Rc is decreased in the backwash operation. Seventy-four percent of
Fig. 6. Pie chart of filtration resistance at 12,000 s in the filtration under the constant pressure of 40 kPa.
Fig. 7. Distribution of filtration resistance at 12,000 s in the filtration with a 4min duration of the periodic backwash.
the overall resistance is due to the membrane internal fouling. However, both Rif and Rc can be significantly reduced by the backwash operation. The values of Rif and Rc in the backwash operation are only 1/8 and 1/20, respectively, of their original values in CPF. Therefore, backwash is an efficient way of removing the fouled particles, especially for those deposited on the membrane surface. Fig. 8 shows a typical resistance distribution in a PSI operation at 12,000 s. Since the lower filtration pressure at the beginning can reduce the particle fouling, the value of Rif can be reduced by 40% while Rc can be reduced by 18% compared to those in Fig. 6. Therefore, the overall filtration resistance is significantly decreased by this PSI operation. Comparing the results shown in Figs. 8 and 6, the ratio of Rif to Rc decreases from 1.88 in the CPF to 1.33 in the PSI operation under the same conditions of no aeration and no backwash. This reveals that the PSI operation can reduce both Rif and Rc but it is more efficient in the mitigation of the membrane internal fouling. Fig. 9 shows the resistance distribution in the filtration under a constant pressure of 40 kPa and an air flow rate of 3.33 104 m3/s. Ninety percent of the overall filtration resistance is caused by the membrane internal fouling. However, the value of Rif is nearly the same as that in the CPF with no aeration shown in Fig. 6. This means that aeration has no effect on the mitigation of internal fouling of the membrane. However, the cake resistance can be significantly reduced by aeration. Only 10% of the overall resistance is contributed by the cake formation. Compared with the results of CPF without aeration shown in Fig. 6, only 1/4 of the cake mass in the former is formed in this aeration case. The filtration resistances due to membrane-internal fouling at 12,000 s under various hydrodynamic conditions are
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Fig. 10. Comparison of Rif under various operating conditions.
Fig. 8. Pie chart of filtration resistance at 12,000 s in a filtration under the pressure increasing stepwise from 28 to 40 kPa.
compared in Fig. 10. The filtration pressures are all kept at 40 kPa except those in PSI operation and that in the CPF under 64 kPa. To compare the values in CPF under 40 and 64 kPa, the value of Rif is increased by raising the filtration pressure in the system of this study. This is because particles can more easily penetrate the membrane pores under a higher
Fig. 9. Distribution of filtration resistances at 12,000 s in the filtration under a constant pressure of 40 kPa and Q = 3.33 104 m3/s.
pressure. When constant–flux backwash is operated periodically, the value of Rif can be significantly reduced because some fouled particles may be washed out of the membrane pores. Since a longer backwash time results in a cleaner membrane, the value of Rif decreases with the increase in the backwash duration. The values of Rif under 1 and 4 min of backwash durations are only a half and 12.5%, respectively, of those in CPF under the same pressures. Furthermore, the value of Rif becomes smaller in PSI operation but a slightly larger in the condition with an aeration rate of 3.33 104 m3/s. This is because particles have more difficulty penetrating the membrane pores under a lower pressure in the early period of filtration, and the contrary is true if less cake is formed on the membrane surface. Fig. 11 shows the cake resistances under the same conditions as those shown in Fig. 10. Since more cake will be formed under a higher pressure, the value of Rc increases with increasing the filtration pressure in a CPF. When the filtration pressure increases from 40 to 64 kPa, the value of Rc increases over twofold. For those backwash operations, an increase in the wash duration leads to a higher flux recovery; therefore, the increase causes more cake to be formed during the following filtration period. The value of Rc then increases with increasing
Fig. 11. Comparison of Rc under various operating conditions.
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the backwash duration. As the filtration period in each cycle is only 900 s, the Rc values are much lower than those in CPF. Furthermore, a lower filtration pressure is applied in the initial stage of PSI operations; the values of Rc are slightly lower than those in CPF under the same filtration pressure. The other way to mitigate particle fouling is through aeration. Although the internal fouling of the membrane cannot be affected by aeration (as shown in Fig. 10), the resistance due to cake formation can be effectively reduced by this operation. Combining the results shown in Figs. 10 and 11, a PSI operation can significantly decrease Rif, while aeration can efficiently reduce the cake formation. The PSI operation is more effective than aeration from the viewpoint of reducing overall filtration resistance. In fact, the optimal way to mitigate particle fouling is to undergo a periodical backwash operation. Four hydrodynamic methods, such as increasing filtration pressure, increasing aeration intensity, increasing filtration pressure stepwise, and periodic backwashing of the fouled membrane have been tested for the purpose of mitigating particle fouling of a submerged membrane filtration system. In order to understand the effectiveness of these methods, the net filtrate volumes (i.e., the filtrate volume—the injected clean water volume for backwashing) received at 12,000 s under various conditions were measured and are shown in Fig. 12. The only method having a negative effect is to increase the filtration pressure as the filtrate volume decreases 30% when the working pressure increases from 40 to 64 kPa. The aeration intensity increasing from Q = 0 to 3.33 104 m3/s makes an increase in the filtrate volume as high as 50%. The PSI operation can obtain 70% more of the filtrate compared to that in CPF under the same pressure. The most efficient method is the periodical backwash if the duration is longer than 1 min.
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The backwash operation with 4 min duration can get a filtrate volume more than three-fold higher than those in CPF. 4. Conclusion Four hydrodynamic methods (increasing filtration pressure, increasing aeration intensity, increasing filtration pressure stepwise, and periodic backwashing of the fouled membrane) have been tested for mitigating particle fouling and enhancing the filtrate flux in a submerged membrane filtration system. Increasing the filtration pressure causes the flux to be decreased due to more severe membrane blocking. The filtrate received at 12,000 s decreases 30% as the pressure increases from 40 to 64 kPa. Aeration can reduce the particle deposition on the membrane surface but has no noticeable effect on the membrane internal fouling. Compared with the constant pressure filtration under the same pressure, the cake resistance can be reduced 75% and the flux can be enhanced 20% under an aeration rate of 3.33 104 m3/s. A stepwise increasing pressure can mitigate the membrane internal fouling as high as 40% and obtain two- to three-fold increase in flux as compared to constant pressure filtration. A periodic backwash can significantly reduce both the resistances due to membraneinternal fouling and cake formation. Comparing the long-term flux and the received filtrate volume among these hydrodynamic methods, the periodic backwash with a longer duration shows great effect on mitigating membrane fouling among four methods. Acknowledgement The authors wish to express their sincere gratitude to the National Science Council of the Republic of China for its financial support. References
Fig. 12. Comparison of received filtrate volumes at 12,000 s under various operating conditions.
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