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An investigation of reciprocating filtration system
Journal of Membrane Science 202 (2002) 233–243
An investigation of reciprocating filtration system D.J. Chang∗ , S.H. Chen, S.S. Lin, K.C. Yu, C.Y. Chang Department of Environmental Engineering and Health, Chia-Nan University of Pharmacy and Science, Tainan 717, Taiwan, ROC Received 30 April 2001; accepted 19 December 2001
Abstract For combining certain advantages of dead-end filtration (low energy consumption), cross-flow filtration (sweeping of accumulated foulants) and backflushing (lifting of accumulated foulants), a reciprocating filtration system is presented in this study. In this system, a tubular ceramic membrane was used and driven by a piston and the forward–reverse filtration cycles were operated without cross-flow. At the beginning of each filtration cycle, the membrane was backflushed by filtrate in reverse direction by suction to reduce membrane fouling and then the filtration was in progress when backflushing was completed. After a few forward–reverse filtration cycles, the concentrated suspension of particles or solute near the membrane surface was discharged into a stock tank under the cross-flow condition. The polystyrene colloidal suspensions and the backwash wastewater were used as the feed streams and filtered. The ultrafiltration and microfiltration membranes with a range of pore sizes were also studied. In the experiment of maximizing the cumulative filtrate volume, it was found that the optimum time of a filtration cycle was 90 s, namely, forward and reverse was 87 and 3 s, respectively. The experiment was carried out using the particle size of 0.3 m and a concentration of 50 ppm in polystyrene suspensions. There were filtered by the membrane with a pore size of 0.2 m under the pressure drop of 50 lb/in.2 (psi). The optimum discharge frequency for retentate was one for every three cycles. It was also found that the cumulative filtrate volume increased with an increase in both particle size and membrane pore size, but it decreased with an increase in feed concentration. Furthermore, an optimal cumulative filtrate volume was found on effecting the pressure drop. The process used in the study could be used to efficiently treat filter backwash wastewater from a water treatment plant. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Reciprocating; Ceramic membrane; Backflush; Filtration cycle
1. Introduction There are two phenomena that inevitably limit the application of membrane processes to a wider range of separation problems; one is the cake layer that forms on the membrane and the other is the inner fouling of the membrane, which leads to decline in permeate flux [1–5]. Cross-flow filtration has been increasingly used in recent years in order to overcome the former ∗ Corresponding author. Tel.: +886-6-2660-250; fax: +886-6-266-7323. E-mail address: [email protected] (D.J. Chang).
phenomenon and to achieve a more efficient performance of membrane separation [6–9]. Nevertheless, cross-flow filtration requires more complex equipment and higher energy consumption than the conventional dead-end filtration [10]. To solve the problem of inner fouling, de-clogging techniques including pulsation by solution or filtrate, backflushed by filtrate or air and the use of ultrasonic or electronic field, have been used [10,11]. According to previous investigations, backflushing by filtrate was very economical and efficient for de-clogging fouled filters. In this process, the permeate flowed back through the membrane, lifting off cake and removing some inner foulants from
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the membrane surface and pores. Several theoretical investigations and practical applications have been developed to find the optimum forward and reverse filtration time by periodical backwash or rapid backpulsing in microfiltration process. They showed that permeate flux could be enhanced up to several times by backwash [12–14]. Therefore, for economical reason, the clarification of suspensions containing low concentration of fine particles, it may still be advantageous to use dead-end filtration with successive phases of cleaning by backflushing with filtrate. However, in the dead-end mode, the concentration of retentate was continuously increasing during the filtration process until significant permeate flux decline occurred. In this study, a reciprocating filtration system was designed using a tubular ceramic membrane and driven by piston force. The polystyrene colloidal suspensions and the backwash wastewater were filtered,
using an intermittent operation of several forward– reverse cycles filtration in dead-end mode, followed by cross-flow filtration to sweep away the accumulated particles. This operation mode had the merits of membrane recovery by backflushing (reverse filtration) and the discharge of retentate by cross-flow process, thus providing more cost-effective membrane filtration system. Finally, the effects of related parameters in the experiments are presented in this study.
2. Materials and methods The reciprocating filtration system presented in this study is shown in Fig. 1. The Micro-Carbosep 20 system manufactured by Tech-set Company was used as a tubular filtration module. The tubular ceramic membrane was 20 cm long, with 0.6 cm i.d. and membrane
Fig. 1. Schematic diagram of experimental set-up.
D.J. Chang et al. / Journal of Membrane Science 202 (2002) 233–243 Table 1 Characteristics of untreated backwash wastewater Parameter
Value
Turbidity (NTU) TOC (mg/l) Coliform (MPN/100 ml) pH Temperature (◦ C)
56.2 14.3 >1.6 × 109 8.0 27.5
pore sizes of M1 (MWCO 150000), M9 (MWCO 300000), 0.14, 0.2 and 0.45 m. For investigating the cumulative filtrate volume during the reciprocating mode of filtration with backflushing, experiments were conducted with polystyrene latex suspension and filter backwash wastewater taken from Tangding water treatment plant. The diameter of the polystyrene latex particles was 0.3, 0.6 and 1.0 m, and their density was 1.05 g/cm3 . The characteristics of the backwash wastewater are given in Table 1. The samples were delivered from a stock tank equipped with a temperature controller to membrane cell by a piston system, which was a 6 cm diameter and 30 cm length acrylic cylinder. At the beginning of each filtration cycle, the reverse and forward filtration intervals were set in advance and controlled by a timer, so that when the piston was started from the right side and pushed to the left side, valves 1 and 4 were closed, whereas, valves 2 and 3 were opened. The piston then pushed solution through valve 3 into the stock tank and the membrane was backflushed by filtrate due to the suction created by the piston, thus reducing membrane fouling. After the reverse filtration mode was over, valves 1 and 4 were opened, whereas, valves 2 and 3 were closed. The backflush action was stopped due to the suspension from stock tank flowing through valve 4 into the piston cylinder and the forward filtration was in progress. After a moment, the forward filtration interval was over while the pre-set time was reached and this filtration cycle was also completed simultaneously. When the piston was activated and pushed from the left side to the right, valves 2 and 3 were closed, whereas, valves 1 and 4 were opened, thus starting the next filtration cycle. After a few cycles, the permeate flux notably reduced due to the accumulation of a large amount of particles or solute on the retentate side of the membrane. The highly concentrated suspension of the retentate was then discharged into the stock tank
235
under the cross-flow condition when the forward filtration is proceeding. Each experiment was carried out and completed in 2 h. Thereafter, the tubular ceramic membrane was taken out the filtration module, cleaned by distilled water and followed by chemical solutions until the initial flux of the membrane was restored. The cumulative filtrate volume was measured by an electronic balance and recorded. In this study, the optimum time of forward and reverse filtration intervals were obtained by experimental results using polystyrene latex suspensions under various conditions including pressure drop, particle size, membrane pore size and particle concentration. Moreover, the performance of the membrane filtration system using backwash wastewater from Tangding water treatment plant under various pressure drops and with different pore sizes was also conducted in this study.
3. Results and discussion 3.1. Polystyrene latex suspension 3.1.1. Optimum time in each filtration cycle An increase in reciprocating frequency led to a decrease in membrane fouling due to more frequent backflush, resulting in higher permeate flux, but there were more filtration cycles during the 2 h filtration run. Therefore, it caused more frequent pressure drop when the piston reversed its direction and the permeate flux decreases. Consequently, the optimum filtration time in each filtration cycle was determined to maximize the cumulative filtrate volume in the 2 h filtration. In addition, the duration of reverse filtration interval (tb ) in each filtration cycle was kept longer to facilitated a more complete recovery of the membrane by filtrate backflushing, so that higher permeate flux could be obtained due to lower hydraulic resistance. However, this also caused higher filtrate consumption and shorter forward filtration interval (tf ). Therefore, the optimum time intervals for both the forward and reverse filtration were also determined in each filtration cycle. In the experiments, the particle size (dp ) and concentration (C) of polystyrene suspensions used were 0.3 m and 50 ppm, respectively. The suspension was filtered by the membrane with pore size (dm ) of 0.2 m under a pressure drop (P) of 50 lb/in.2
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Fig. 2. Optimum time of filtration and backwash interval in each cycle (P = 50 psi; C = 50 ppm; d p = 0.3 m; d m = 0.2 m).
(psi). Fig. 2 shows that the optimum filtration time for a complete filtration cycle to maximize the cumulative filtrate volume in the 2 h filtration run is 90 s and the optimum time interval for the forward and reverse filtration are 87 and 3 s, respectively, in each filtration cycle. These optimum forward and reverse filtration intervals were used in all the experiments of this study. 3.1.2. Optimum discharge frequency of retentate A higher discharge frequency of retentate led to a shorter filtration run and reduced cumulative filtrate volume in the 2 h filtration time, although it might lower retentate concentration and create higher permeate flux. Therefore, there was an optimum discharge frequency of retentate which could maximize the cumulative filtrate volume in the 2 h filtration time. In the experiments followed, the operating conditions were kept the same as the forward and reverse filtration intervals from those mentioned in Section 3.1.1. As shown in Fig. 3, the optimum discharge frequency was one for every three cycles. This frequency was used in all the experiments of the study. 3.1.3. Effect of membrane pore size Membranes with pore sizes of 0.14, 0.20 and 0.45 m were used to study the effect of membrane
pore size on the cumulative filtrate volume under the pressure drop of 50 psi. The diameter of suspension particles was 0.6 m, which was much larger than membrane pore size. Therefore, it could be assumed that no particles could penetrate into or through the membrane. Fig. 4 shows that the cumulative filtrate volume increases with an increase in the membrane pore size, due to the use of membranes with larger pores which have lower membrane resistance during the forward and reverse filtration intervals. According to Darcy’s equation, the permeate flux increases with decreasing membrane resistance, therefore, the higher permeate flux and better recovery of membrane led to higher cumulative filtrate volume. Moreover, the average permeate fluxes over the 2 h filtration using membranes with pore sizes of 0.14, 0.20 and 0.45 m were 0.68, 0.81 and 1.46 l/(m2 h), respectively. 3.1.4. Effect of pressure drop The effect of pressure drop across the membrane on the cumulative filtrate volume was studied using four different pressure drops; namely, 30, 50, 80 and 100 psi. In the experiments, the particle size and the concentration of polystyrene latex suspensions were 0.3 m and 50 ppm, respectively. The suspensions were filtered by the membrane with a pore size of 0.2 m. Fig. 5 indicates that the cumulative filtrate
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Fig. 3. Optimum discharge frequency of 2 h filtration run (P = 50 psi; C = 50 ppm; d p = 0.3 m; d m = 0.2 m; t f = 87 s; t b = 3 s).
volume first increases and then decreases with increasing pressure drop, except for the lowest one at 30 psi. It can be seen from Darcy’s equation that the permeate flux increases with increasing pressure drops. However, the voidage of the cake is lower at
higher pressure drop, leading to a higher cake resistance. In other words, the structure of the cake accumulated on the membrane surface at higher pressure drops was denser than that at lower pressure drops. In addition, the recovery of a membrane by filtrate
Fig. 4. Effect of membrane pore size in 2 h filtration run (P = 50 psi; d p = 0.6 m; C = 50 ppm; t f = 87 s; t b = 3 s).
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Fig. 5. Effect of pressure drop in 2 h filtration run (d m = 0.2 m; d p = 0.3 m; C = 50 ppm; t f = 87 s; t b = 3 s).
backflushing at the suction pressure of <1 atm caused lower efficiency with a denser cake. Therefore, these two counteracting effects decided the maximum cumulative filtrate volume obtained. In addition, the average permeate fluxes over the 2 h filtration using four different pressure drops, 30, 50, 80 and 100 psi were 0.22, 0.64, 0.55 and 0.45 l/(m2 h), respectively. 3.1.5. Effect of particle concentration in the suspension The effect of particle concentration in the suspension on the cumulative filtrate volume was studied using three different concentrations of 25, 50 and 100 ppm. In these experiments, the particle size of polystyrene latex suspensions was 0.3 m, filtered by the membrane with a pore size of 0.2 m under the pressure drop of 50 psi. As shown in Fig. 6, the cumulative filtrate volume decreases as particle concentration in the suspension is increased due to the increase in the cake thickness. As a consequence, the cake resistance increases and the cumulative filtrate volume decreases. In addition, the average permeate fluxes over the 2 h filtration using three different concentrations, 25, 50 and 100 ppm were 0.97, 0.64 and 0.49 l/(m2 h), respectively.
3.1.6. Effect of particle size The effect of suspension particle size on the cumulative filtrate volume was studied using three different particle sizes of 0.3, 0.6 and 1.1 m. In the experiments, the polystyrene latex suspensions with 50 ppm concentration were filtered by the membrane with a pore size of 0.2 m and under the pressure drop of 50 psi. As shown in Fig. 7, the cumulative filtrate volume increases with increasing particle size. Because the voidage of the cake increased as the particle size was increased, leading to a lower cake resistance. At the same time, the cake formed by larger particles was more easily lifted away by filtrate backflushing. As a result, the cumulative filtrate volume was greater for larger particle sizes. In addition, the average permeate fluxes over the 2 h filtration using three different particle sizes, 0.3, 0.6 and 1.1 m were 0.64, 0.81 and 0.90 l/(m2 h), respectively. 3.1.7. Effect of pH The effect of pH was studied at pH value of 4, 7 and 11. Solutions of HCl and NaOH were used to adjust the pH of the suspension and 1 M NaCl solution was used to keep the ionic strength of the suspension at the same value in all experiments. The diameter of the
D.J. Chang et al. / Journal of Membrane Science 202 (2002) 233–243
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Fig. 6. Effect of particle concentration in 2 h filtration run (P = 50 psi; d p = 0.3 m; d m = 0.2 m; t f = 87 s; t b = 3 s).
polystyrene latex suspensions was 0.3, 0.6 and 1.1 m and their concentration was 50 ppm. The suspensions were filtered by the membrane with a pore size of 0.2 m under the pressure drop of 50 psi. Fig. 8 shows that the maximum and minimum cumulative filtrate volumes in the 2 h filtration took place at pH values
7 and 4, caused by the difference in the zeta potential of particles. The highest zeta potential occurred at pH 7 because the double layer repulsion force between particles was the highest, leading to the lowest cake resistance on the membrane. At this pH value, the maximum cumulative filtrate volume also took place [7].
Fig. 7. Effect of particle size in 2 h filtration run (P = 50 psi; d m = 0.2 m; C = 50 ppm; t f = 87 s; t b = 3 s).
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Fig. 8. Effect of pH value after 2 h filtration run (P = 50 psi; d m = 0.2 m; t f = 87 s; t b = 3 s; C = 50 ppm).
3.2. Application to wastewater treatment The wastewater samples from Tangding water treatment plant were used for direct filtration or they were pre-treated by flocculation before filtration.
Figs. 9 and 10 show the experimental results with five different membrane pore sizes of M1, M9, 0.14, 0.20 and 0.45 m. Fig. 9 shows the maximum and the minimum cumulative filtrate volumes obtained, respectively, with M9 and 0.2 m membranes
Fig. 9. Direct filtration process for backwash wastewater in 2 h filtration run (P = 50 psi; t f = 87 s; t b = 3 s).
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Fig. 10. Flocculation as pre-treatment process for backwash wastewater in 2 h filtration run (P = 50 psi; t f = 87 s; t b = 3 s).
when wastewater was directly filtered without any pre-treatment. Fig. 10 indicates the maximum and the minimum values were also obtained with pore sizes of M9 and 0.45 m when the wastewater was pre-treated by flocculation. These results are quite different from those obtained when polystyrene suspensions were used where by the cumulative filtrate volume increased with an increase in membrane pore size. This could be explained by the total filtration resistance including membrane resistance, cake resistance and membrane inner clogging resistance. Membranes with larger pore sizes had lower membrane resistance but higher inner clogging resistance. In the experiments, the total resistance was dominated by membrane inner clogging resistance. Therefore, the total filtration resistance depended on the relationship between the average pore size of the membrane and the size distribution of particles in the wastewater suspension [15–17]. Accordingly, it could be concluded that the largest and the smallest ranges of particle size distribution were approximately 0.2 m and M9, respectively, for direct filtration process, while the largest and the smallest ranges of particle size distribution were approximately 0.45 m and M9, respectively, when flocculation was used as pre-treatment. Table 2 shows that the flocculation as pre-treatment
Table 2 The average permeate fluxes over the 2 h filtration of two treatment processes dm (m)
Direct filtration (l/(m2 h))
Flocculation process (l/(m2 h))
M9 M1 0.14 0.20 0.45
2.02 1.29 1.84 0.80 1.45
2.93 1.88 2.11 1.44 0.99
process yielded higher average permeate fluxes over the 2 h filtration than the direct filtration process for all membrane pore sizes, except 0.45 m. The performances of both types of filtration are shown in Tables 3 and 4. As shown, the treatment efficiency Table 3 Characteristics of treated water using direct filtration dm (m)
Turbidity (NTU)
TOC (mg/l)
Coliform (MPN/100 ml)
pH
M9 M1 0.14 0.20 0.45
0.17 0.28 0.35 0.47 0.57
1.89 2.51 6.23 9.12 9.51
0 0 0 0 9
8.15 8.05 7.94 8.15 7.76
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Table 4 Characteristics of treated water using flocculation as pre-treatment dm (m)
Turbidity (NTU)
TOC (mg/l)
Coliform (MPN/100 ml)
pH
M9 M1 0.14 0.20 0.45
0.04 0.05 0.12 0.17 0.44
1.27 2.09 4.23 4.32 6.18
0 0 0 0 0
7.82 7.96 7.75 7.90 7.88
in term of turbidity and coliform removals was better when flocculation was used. However, no significant different was observed in terms of TOC removals between both two types of filtration.
4. Conclusions The polystyrene latex suspensions and backwash wastewater were used as feed streams to investigate the performance of membrane filtration under several operating conditions in this study. In the former experiments with the particle size of polystyrene latex kept larger than membrane pore size, it was found that in order to get the maximum cumulative filtrate volume in the 2 h filtration run, the optimum of forward and reverse filtration intervals in each filtration cycle were 87 and 3 s, respectively. The particle size and the concentration of polystyrene suspensions were 0.3 m and 50 ppm, respectively, filtered by the membrane with a pore size of 0.2 m under the pressure drop of 50 psi. The optimum discharge frequency is one for every three cycles. It was also found that the cumulative filtrate volume increased with an increase in particle size and membrane pore size, but decreased with an increase in feed concentration. Furthermore, the optimal cumulative filtrate volume was found at the pressure drop of 50 psi and at pH 7 of the suspension. In the latter experiments, the maximum and the minimum cumulative filtrate volumes were obtained with the membrane pore size of M9 and 0.2 m, respectively, for direct filtration process. However, the maximum and the minimum values were obtained with pore sizes of M9 and 0.45 m, respectively, when flocculation was used as pre-treatment. The performance depended on the relationship between the
average membrane pore size and the size distribution of particles in wastewater samples. From the results of the latter experiments, it could be concluded that the largest and the smallest range of particle size distributions were approximately 0.2 m and M9 in untreated wastewater and approximately 0.45 m and M9 in the flocculated wastewater. It was also found that pre-treatment with flocculation was more efficient than direct filtration in term of turbidity, TOC and coliform removal. Moreover, this system could provide a cost-effective treatment process for membrane filtration systems.
Acknowledgements The authors wish to thank the National Science Council of ROC for financial support for this study and the Tangding water treatment plant for allowing the use of their filter backwash wastewater for the experiments. References [1] P. Czekaj, F. Lopez, C. Guell, Membrane fouling during microfiltration of fermented beverages, J. Membr. Sci. 166 (2000) 199–212. [2] P.S. Cartwright, Industrial wastewater treatment with membranes—a US perspective, Water Sci. Technol. 25 (1992) 373–390. [3] K.K. Sirkar, Membrane separation technologies: current developments, Chem. Eng. Commun. 157 (1997) 145–184. [4] E. Drioli, Membrane operations for the rationalization of industrial products, Water Sci. Technol. 25 (1992) 107–125. [5] A.B. Koltuniewicz, R.W. Field, T.C. Arnot, Cross-flow and dead-end microfiltration of oily-water emulsion. Part I. Experimental study and analysis of flux decline, J. Membr. Sci. 102 (1995) 193–207. [6] G. Belfort, R.H. Davis, A.L. Zydney, The behavior of suspensions and macromolecular solutions in cross-flow microfiltration, J. Membr. Sci. 96 (1994) 1–58. [7] D.J. Chang, S.J. Hwang, Unsteady state permeate flux of cross-flow microfiltration, Sep. Sci. Technol. 29 (1994) 1593–1608. [8] R. Jirraratananon, D. Uttapap, P. Sampranpiboon, Cross-flow microfiltration of a colloidal suspension with the presence of macromolecules, J. Membr. Sci. 140 (1998) 57–66. [9] R.H. Davis, Theory of cross-flow microfiltration, in: W.S.W. Ho, K.K. Sirkar (Eds.), Membrane Handbook, Van Nostrand Reinhold, New York, 1992, p. 457. [10] C. Serra, M.J. Clifton, P. Moulin, J.C. Rouch, P. Aptel, Dead-end ultrafiltration in hollow fiber modules: module
D.J. Chang et al. / Journal of Membrane Science 202 (2002) 233–243 design and process simulation, J. Membr. Sci. 145 (1998) 159–172. [11] S.S. Kulkarni, Module and process configuration, in: W.S.W. Ho, K.K. Sirkar (Eds.), Membrane Handbook, Van Nostrand Reinhold, New York, 1992, p. 393. [12] Y. Xu, J. Dodds, D. Leclerc, Optimization of discontinuous microfiltration backwash process, Chem. Eng. J. 57 (1995) 247–251. [13] W.D. Mores, C.N. Bowman, R.H. Davis, Theoretical and experimental flux maximization by optimization of backpulsing, J. Membr. Sci. 165 (2000) 225–236.
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[14] J.A. Ramirez, R.H. Davis, Application of cross-flow microfiltration with rapid backpulsing to wastewater treatment, J. Hazard. Mater. B63 (1998) 179–197. [15] D.J. Chang, S.H. Chen, C.Y. Chang, S.S. Lin, J.S. Chang, A new design of microfiltration system and application to wastewater treatment, Water Sci. Technol. 41 (2000) 181–188. [16] M.S. Le, J.A. Howell, Alternative model for ultrafiltration, Chem. Eng. Res. Des. 62 (1984) 373. [17] D.J. Chang, S.J. Hwang, Removal of metal ions from liquid solutions by cross-flow microfiltration, Sep. Sci. Technol. 31 (1996) 1831–1841.
An investigation of reciprocating filtration system D.J. Chang∗ , S.H. Chen, S.S. Lin, K.C. Yu, C.Y. Chang Department of Environmental Engineering and Health, Chia-Nan University of Pharmacy and Science, Tainan 717, Taiwan, ROC Received 30 April 2001; accepted 19 December 2001
Abstract For combining certain advantages of dead-end filtration (low energy consumption), cross-flow filtration (sweeping of accumulated foulants) and backflushing (lifting of accumulated foulants), a reciprocating filtration system is presented in this study. In this system, a tubular ceramic membrane was used and driven by a piston and the forward–reverse filtration cycles were operated without cross-flow. At the beginning of each filtration cycle, the membrane was backflushed by filtrate in reverse direction by suction to reduce membrane fouling and then the filtration was in progress when backflushing was completed. After a few forward–reverse filtration cycles, the concentrated suspension of particles or solute near the membrane surface was discharged into a stock tank under the cross-flow condition. The polystyrene colloidal suspensions and the backwash wastewater were used as the feed streams and filtered. The ultrafiltration and microfiltration membranes with a range of pore sizes were also studied. In the experiment of maximizing the cumulative filtrate volume, it was found that the optimum time of a filtration cycle was 90 s, namely, forward and reverse was 87 and 3 s, respectively. The experiment was carried out using the particle size of 0.3 m and a concentration of 50 ppm in polystyrene suspensions. There were filtered by the membrane with a pore size of 0.2 m under the pressure drop of 50 lb/in.2 (psi). The optimum discharge frequency for retentate was one for every three cycles. It was also found that the cumulative filtrate volume increased with an increase in both particle size and membrane pore size, but it decreased with an increase in feed concentration. Furthermore, an optimal cumulative filtrate volume was found on effecting the pressure drop. The process used in the study could be used to efficiently treat filter backwash wastewater from a water treatment plant. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Reciprocating; Ceramic membrane; Backflush; Filtration cycle
1. Introduction There are two phenomena that inevitably limit the application of membrane processes to a wider range of separation problems; one is the cake layer that forms on the membrane and the other is the inner fouling of the membrane, which leads to decline in permeate flux [1–5]. Cross-flow filtration has been increasingly used in recent years in order to overcome the former ∗ Corresponding author. Tel.: +886-6-2660-250; fax: +886-6-266-7323. E-mail address: [email protected] (D.J. Chang).
phenomenon and to achieve a more efficient performance of membrane separation [6–9]. Nevertheless, cross-flow filtration requires more complex equipment and higher energy consumption than the conventional dead-end filtration [10]. To solve the problem of inner fouling, de-clogging techniques including pulsation by solution or filtrate, backflushed by filtrate or air and the use of ultrasonic or electronic field, have been used [10,11]. According to previous investigations, backflushing by filtrate was very economical and efficient for de-clogging fouled filters. In this process, the permeate flowed back through the membrane, lifting off cake and removing some inner foulants from
0376-7388/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 6 - 7 3 8 8 ( 0 2 ) 0 0 0 1 2 - 1
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D.J. Chang et al. / Journal of Membrane Science 202 (2002) 233–243
the membrane surface and pores. Several theoretical investigations and practical applications have been developed to find the optimum forward and reverse filtration time by periodical backwash or rapid backpulsing in microfiltration process. They showed that permeate flux could be enhanced up to several times by backwash [12–14]. Therefore, for economical reason, the clarification of suspensions containing low concentration of fine particles, it may still be advantageous to use dead-end filtration with successive phases of cleaning by backflushing with filtrate. However, in the dead-end mode, the concentration of retentate was continuously increasing during the filtration process until significant permeate flux decline occurred. In this study, a reciprocating filtration system was designed using a tubular ceramic membrane and driven by piston force. The polystyrene colloidal suspensions and the backwash wastewater were filtered,
using an intermittent operation of several forward– reverse cycles filtration in dead-end mode, followed by cross-flow filtration to sweep away the accumulated particles. This operation mode had the merits of membrane recovery by backflushing (reverse filtration) and the discharge of retentate by cross-flow process, thus providing more cost-effective membrane filtration system. Finally, the effects of related parameters in the experiments are presented in this study.
2. Materials and methods The reciprocating filtration system presented in this study is shown in Fig. 1. The Micro-Carbosep 20 system manufactured by Tech-set Company was used as a tubular filtration module. The tubular ceramic membrane was 20 cm long, with 0.6 cm i.d. and membrane
Fig. 1. Schematic diagram of experimental set-up.
D.J. Chang et al. / Journal of Membrane Science 202 (2002) 233–243 Table 1 Characteristics of untreated backwash wastewater Parameter
Value
Turbidity (NTU) TOC (mg/l) Coliform (MPN/100 ml) pH Temperature (◦ C)
56.2 14.3 >1.6 × 109 8.0 27.5
pore sizes of M1 (MWCO 150000), M9 (MWCO 300000), 0.14, 0.2 and 0.45 m. For investigating the cumulative filtrate volume during the reciprocating mode of filtration with backflushing, experiments were conducted with polystyrene latex suspension and filter backwash wastewater taken from Tangding water treatment plant. The diameter of the polystyrene latex particles was 0.3, 0.6 and 1.0 m, and their density was 1.05 g/cm3 . The characteristics of the backwash wastewater are given in Table 1. The samples were delivered from a stock tank equipped with a temperature controller to membrane cell by a piston system, which was a 6 cm diameter and 30 cm length acrylic cylinder. At the beginning of each filtration cycle, the reverse and forward filtration intervals were set in advance and controlled by a timer, so that when the piston was started from the right side and pushed to the left side, valves 1 and 4 were closed, whereas, valves 2 and 3 were opened. The piston then pushed solution through valve 3 into the stock tank and the membrane was backflushed by filtrate due to the suction created by the piston, thus reducing membrane fouling. After the reverse filtration mode was over, valves 1 and 4 were opened, whereas, valves 2 and 3 were closed. The backflush action was stopped due to the suspension from stock tank flowing through valve 4 into the piston cylinder and the forward filtration was in progress. After a moment, the forward filtration interval was over while the pre-set time was reached and this filtration cycle was also completed simultaneously. When the piston was activated and pushed from the left side to the right, valves 2 and 3 were closed, whereas, valves 1 and 4 were opened, thus starting the next filtration cycle. After a few cycles, the permeate flux notably reduced due to the accumulation of a large amount of particles or solute on the retentate side of the membrane. The highly concentrated suspension of the retentate was then discharged into the stock tank
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under the cross-flow condition when the forward filtration is proceeding. Each experiment was carried out and completed in 2 h. Thereafter, the tubular ceramic membrane was taken out the filtration module, cleaned by distilled water and followed by chemical solutions until the initial flux of the membrane was restored. The cumulative filtrate volume was measured by an electronic balance and recorded. In this study, the optimum time of forward and reverse filtration intervals were obtained by experimental results using polystyrene latex suspensions under various conditions including pressure drop, particle size, membrane pore size and particle concentration. Moreover, the performance of the membrane filtration system using backwash wastewater from Tangding water treatment plant under various pressure drops and with different pore sizes was also conducted in this study.
3. Results and discussion 3.1. Polystyrene latex suspension 3.1.1. Optimum time in each filtration cycle An increase in reciprocating frequency led to a decrease in membrane fouling due to more frequent backflush, resulting in higher permeate flux, but there were more filtration cycles during the 2 h filtration run. Therefore, it caused more frequent pressure drop when the piston reversed its direction and the permeate flux decreases. Consequently, the optimum filtration time in each filtration cycle was determined to maximize the cumulative filtrate volume in the 2 h filtration. In addition, the duration of reverse filtration interval (tb ) in each filtration cycle was kept longer to facilitated a more complete recovery of the membrane by filtrate backflushing, so that higher permeate flux could be obtained due to lower hydraulic resistance. However, this also caused higher filtrate consumption and shorter forward filtration interval (tf ). Therefore, the optimum time intervals for both the forward and reverse filtration were also determined in each filtration cycle. In the experiments, the particle size (dp ) and concentration (C) of polystyrene suspensions used were 0.3 m and 50 ppm, respectively. The suspension was filtered by the membrane with pore size (dm ) of 0.2 m under a pressure drop (P) of 50 lb/in.2
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Fig. 2. Optimum time of filtration and backwash interval in each cycle (P = 50 psi; C = 50 ppm; d p = 0.3 m; d m = 0.2 m).
(psi). Fig. 2 shows that the optimum filtration time for a complete filtration cycle to maximize the cumulative filtrate volume in the 2 h filtration run is 90 s and the optimum time interval for the forward and reverse filtration are 87 and 3 s, respectively, in each filtration cycle. These optimum forward and reverse filtration intervals were used in all the experiments of this study. 3.1.2. Optimum discharge frequency of retentate A higher discharge frequency of retentate led to a shorter filtration run and reduced cumulative filtrate volume in the 2 h filtration time, although it might lower retentate concentration and create higher permeate flux. Therefore, there was an optimum discharge frequency of retentate which could maximize the cumulative filtrate volume in the 2 h filtration time. In the experiments followed, the operating conditions were kept the same as the forward and reverse filtration intervals from those mentioned in Section 3.1.1. As shown in Fig. 3, the optimum discharge frequency was one for every three cycles. This frequency was used in all the experiments of the study. 3.1.3. Effect of membrane pore size Membranes with pore sizes of 0.14, 0.20 and 0.45 m were used to study the effect of membrane
pore size on the cumulative filtrate volume under the pressure drop of 50 psi. The diameter of suspension particles was 0.6 m, which was much larger than membrane pore size. Therefore, it could be assumed that no particles could penetrate into or through the membrane. Fig. 4 shows that the cumulative filtrate volume increases with an increase in the membrane pore size, due to the use of membranes with larger pores which have lower membrane resistance during the forward and reverse filtration intervals. According to Darcy’s equation, the permeate flux increases with decreasing membrane resistance, therefore, the higher permeate flux and better recovery of membrane led to higher cumulative filtrate volume. Moreover, the average permeate fluxes over the 2 h filtration using membranes with pore sizes of 0.14, 0.20 and 0.45 m were 0.68, 0.81 and 1.46 l/(m2 h), respectively. 3.1.4. Effect of pressure drop The effect of pressure drop across the membrane on the cumulative filtrate volume was studied using four different pressure drops; namely, 30, 50, 80 and 100 psi. In the experiments, the particle size and the concentration of polystyrene latex suspensions were 0.3 m and 50 ppm, respectively. The suspensions were filtered by the membrane with a pore size of 0.2 m. Fig. 5 indicates that the cumulative filtrate
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Fig. 3. Optimum discharge frequency of 2 h filtration run (P = 50 psi; C = 50 ppm; d p = 0.3 m; d m = 0.2 m; t f = 87 s; t b = 3 s).
volume first increases and then decreases with increasing pressure drop, except for the lowest one at 30 psi. It can be seen from Darcy’s equation that the permeate flux increases with increasing pressure drops. However, the voidage of the cake is lower at
higher pressure drop, leading to a higher cake resistance. In other words, the structure of the cake accumulated on the membrane surface at higher pressure drops was denser than that at lower pressure drops. In addition, the recovery of a membrane by filtrate
Fig. 4. Effect of membrane pore size in 2 h filtration run (P = 50 psi; d p = 0.6 m; C = 50 ppm; t f = 87 s; t b = 3 s).
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Fig. 5. Effect of pressure drop in 2 h filtration run (d m = 0.2 m; d p = 0.3 m; C = 50 ppm; t f = 87 s; t b = 3 s).
backflushing at the suction pressure of <1 atm caused lower efficiency with a denser cake. Therefore, these two counteracting effects decided the maximum cumulative filtrate volume obtained. In addition, the average permeate fluxes over the 2 h filtration using four different pressure drops, 30, 50, 80 and 100 psi were 0.22, 0.64, 0.55 and 0.45 l/(m2 h), respectively. 3.1.5. Effect of particle concentration in the suspension The effect of particle concentration in the suspension on the cumulative filtrate volume was studied using three different concentrations of 25, 50 and 100 ppm. In these experiments, the particle size of polystyrene latex suspensions was 0.3 m, filtered by the membrane with a pore size of 0.2 m under the pressure drop of 50 psi. As shown in Fig. 6, the cumulative filtrate volume decreases as particle concentration in the suspension is increased due to the increase in the cake thickness. As a consequence, the cake resistance increases and the cumulative filtrate volume decreases. In addition, the average permeate fluxes over the 2 h filtration using three different concentrations, 25, 50 and 100 ppm were 0.97, 0.64 and 0.49 l/(m2 h), respectively.
3.1.6. Effect of particle size The effect of suspension particle size on the cumulative filtrate volume was studied using three different particle sizes of 0.3, 0.6 and 1.1 m. In the experiments, the polystyrene latex suspensions with 50 ppm concentration were filtered by the membrane with a pore size of 0.2 m and under the pressure drop of 50 psi. As shown in Fig. 7, the cumulative filtrate volume increases with increasing particle size. Because the voidage of the cake increased as the particle size was increased, leading to a lower cake resistance. At the same time, the cake formed by larger particles was more easily lifted away by filtrate backflushing. As a result, the cumulative filtrate volume was greater for larger particle sizes. In addition, the average permeate fluxes over the 2 h filtration using three different particle sizes, 0.3, 0.6 and 1.1 m were 0.64, 0.81 and 0.90 l/(m2 h), respectively. 3.1.7. Effect of pH The effect of pH was studied at pH value of 4, 7 and 11. Solutions of HCl and NaOH were used to adjust the pH of the suspension and 1 M NaCl solution was used to keep the ionic strength of the suspension at the same value in all experiments. The diameter of the
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Fig. 6. Effect of particle concentration in 2 h filtration run (P = 50 psi; d p = 0.3 m; d m = 0.2 m; t f = 87 s; t b = 3 s).
polystyrene latex suspensions was 0.3, 0.6 and 1.1 m and their concentration was 50 ppm. The suspensions were filtered by the membrane with a pore size of 0.2 m under the pressure drop of 50 psi. Fig. 8 shows that the maximum and minimum cumulative filtrate volumes in the 2 h filtration took place at pH values
7 and 4, caused by the difference in the zeta potential of particles. The highest zeta potential occurred at pH 7 because the double layer repulsion force between particles was the highest, leading to the lowest cake resistance on the membrane. At this pH value, the maximum cumulative filtrate volume also took place [7].
Fig. 7. Effect of particle size in 2 h filtration run (P = 50 psi; d m = 0.2 m; C = 50 ppm; t f = 87 s; t b = 3 s).
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Fig. 8. Effect of pH value after 2 h filtration run (P = 50 psi; d m = 0.2 m; t f = 87 s; t b = 3 s; C = 50 ppm).
3.2. Application to wastewater treatment The wastewater samples from Tangding water treatment plant were used for direct filtration or they were pre-treated by flocculation before filtration.
Figs. 9 and 10 show the experimental results with five different membrane pore sizes of M1, M9, 0.14, 0.20 and 0.45 m. Fig. 9 shows the maximum and the minimum cumulative filtrate volumes obtained, respectively, with M9 and 0.2 m membranes
Fig. 9. Direct filtration process for backwash wastewater in 2 h filtration run (P = 50 psi; t f = 87 s; t b = 3 s).
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Fig. 10. Flocculation as pre-treatment process for backwash wastewater in 2 h filtration run (P = 50 psi; t f = 87 s; t b = 3 s).
when wastewater was directly filtered without any pre-treatment. Fig. 10 indicates the maximum and the minimum values were also obtained with pore sizes of M9 and 0.45 m when the wastewater was pre-treated by flocculation. These results are quite different from those obtained when polystyrene suspensions were used where by the cumulative filtrate volume increased with an increase in membrane pore size. This could be explained by the total filtration resistance including membrane resistance, cake resistance and membrane inner clogging resistance. Membranes with larger pore sizes had lower membrane resistance but higher inner clogging resistance. In the experiments, the total resistance was dominated by membrane inner clogging resistance. Therefore, the total filtration resistance depended on the relationship between the average pore size of the membrane and the size distribution of particles in the wastewater suspension [15–17]. Accordingly, it could be concluded that the largest and the smallest ranges of particle size distribution were approximately 0.2 m and M9, respectively, for direct filtration process, while the largest and the smallest ranges of particle size distribution were approximately 0.45 m and M9, respectively, when flocculation was used as pre-treatment. Table 2 shows that the flocculation as pre-treatment
Table 2 The average permeate fluxes over the 2 h filtration of two treatment processes dm (m)
Direct filtration (l/(m2 h))
Flocculation process (l/(m2 h))
M9 M1 0.14 0.20 0.45
2.02 1.29 1.84 0.80 1.45
2.93 1.88 2.11 1.44 0.99
process yielded higher average permeate fluxes over the 2 h filtration than the direct filtration process for all membrane pore sizes, except 0.45 m. The performances of both types of filtration are shown in Tables 3 and 4. As shown, the treatment efficiency Table 3 Characteristics of treated water using direct filtration dm (m)
Turbidity (NTU)
TOC (mg/l)
Coliform (MPN/100 ml)
pH
M9 M1 0.14 0.20 0.45
0.17 0.28 0.35 0.47 0.57
1.89 2.51 6.23 9.12 9.51
0 0 0 0 9
8.15 8.05 7.94 8.15 7.76
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Table 4 Characteristics of treated water using flocculation as pre-treatment dm (m)
Turbidity (NTU)
TOC (mg/l)
Coliform (MPN/100 ml)
pH
M9 M1 0.14 0.20 0.45
0.04 0.05 0.12 0.17 0.44
1.27 2.09 4.23 4.32 6.18
0 0 0 0 0
7.82 7.96 7.75 7.90 7.88
in term of turbidity and coliform removals was better when flocculation was used. However, no significant different was observed in terms of TOC removals between both two types of filtration.
4. Conclusions The polystyrene latex suspensions and backwash wastewater were used as feed streams to investigate the performance of membrane filtration under several operating conditions in this study. In the former experiments with the particle size of polystyrene latex kept larger than membrane pore size, it was found that in order to get the maximum cumulative filtrate volume in the 2 h filtration run, the optimum of forward and reverse filtration intervals in each filtration cycle were 87 and 3 s, respectively. The particle size and the concentration of polystyrene suspensions were 0.3 m and 50 ppm, respectively, filtered by the membrane with a pore size of 0.2 m under the pressure drop of 50 psi. The optimum discharge frequency is one for every three cycles. It was also found that the cumulative filtrate volume increased with an increase in particle size and membrane pore size, but decreased with an increase in feed concentration. Furthermore, the optimal cumulative filtrate volume was found at the pressure drop of 50 psi and at pH 7 of the suspension. In the latter experiments, the maximum and the minimum cumulative filtrate volumes were obtained with the membrane pore size of M9 and 0.2 m, respectively, for direct filtration process. However, the maximum and the minimum values were obtained with pore sizes of M9 and 0.45 m, respectively, when flocculation was used as pre-treatment. The performance depended on the relationship between the
average membrane pore size and the size distribution of particles in wastewater samples. From the results of the latter experiments, it could be concluded that the largest and the smallest range of particle size distributions were approximately 0.2 m and M9 in untreated wastewater and approximately 0.45 m and M9 in the flocculated wastewater. It was also found that pre-treatment with flocculation was more efficient than direct filtration in term of turbidity, TOC and coliform removal. Moreover, this system could provide a cost-effective treatment process for membrane filtration systems.
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