- Email: [email protected]
A combined VSEP and membrane bioreactor system
Desalination 183 (2005) 353–362
A combined VSEP and membrane bioreactor system S.C. Lowa*, Han Hee Juanb, Low Kwok Siong a
School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, S 639798, Singapore Tel. þ65 67905516; Fax þ65 67910788; email: [email protected] b Singapore Cleanseas Pte Ltd, 1 Maritine Sq, 13-05, S099253. Received 15 March 2005; accepted 10 April 2005
Abstract In a MBR system, huge amount of solid suspension clogged the membrane and reduced the permeate flux. Mechanical motions were believed to be effective in preventing premature clogging of the membrane. Three different mechanical motions; namely cross oscillation, lengthwise oscillation and the VSEP were investigated. The VSEP combined mechanical motion and the high trans-membrane pressure achieved the best GFD for MBR membrane filtration. Keywords: MBR; Membrane bioreactor; SMBR; Submerged membrane bioreactor; Ultrafiltration; Mechanical motion; VSEP; Shear enhanced filtration; Mixed liquor suspended solids; Trans-membrane pressure
1. Introduction The membrane bioreactor (MBR) sewage treatment process combined the operations of aeration, secondary clarification and filtration into one single process. It simplified the system design and significantly reduced system size. Either microfiltration or the ultrafiltration membrane, replaced the solids separation function of conventional secondary clarifiers and sand filters. Its microscopic pore ensures that no particulate matter
greater than the membrane pore size was discharged in the effluent. The MBR process was typically operated at a very high mixed liquor suspended solids (MLSS). The large amount of solid suspension clogged the membrane and affected its performance. It was reported that in certain applications, the operating cycle could be suctioned for 8 min and stop for 2 min allowing the membrane to ‘relax’ [1]. Air bubbling was a common way to induce turbulences that scoured the external surface of the membrane and kept it clean [2]. Water jet was also used
*Corresponding author. Presented at the Conference on Desalination and the Environment, Santa Margherita, Italy, 22–26 May 2005. European Desalination Society. 0011-9164/05/$– See front matter Ó 2005 Elsevier B.V. All rights reserved doi:10.1016/j.desal.2005.04.028
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in conjunction with air bubbling to physically remove or prevent the solid suspension from clogging the MBR membrane. In most applications, the trans-membrane pressure (TMP) driving the permeate through the membrane was by means of hydrostatic pressure caused by the membrane submersion. Alternatively, the permeate could be sucked through the membrane by a vacuum pump. As a result, the TMP was always below 1 atmospheric pressure which limited the permeate flux through the membrane. In most MBR systems, the permeate flux was limited by the low trans-membrane pressure and the strength of turbulences to reduce clogging and fouling. It was believed that strong mechanical motion could maintain a cleaner membrane surface and enabled higher permeate flux through higher TMP. In this work, three different mechanical motions; namely cross oscillation, lengthwise oscillation and the VSEP [3] were investigated. The cross oscillation and lengthwise oscillation were tested for the submerged membrane bundle with negative suction pressure of 3.45 psi. For the VSEP, the feed was pressurized and the TMP was higher than 1 atmospheric pressure (3.5 bar or 52.5 psi) to drive a higher permeate flux if clogging and fouling was not a problem for a high mixed liquor suspended solid fluid. 2. Test set up 2.1. A submerged MBR membrane (SMBR) with cross oscillation The membrane was a PAN hollow fiber bundle using 610 fibers of length 1.4 m, 2.7 mm outer diameter and 1.6 mm inner diameter (Fig. 1). The total flow surface area was 78 ft2. The fibers were bundled together adopting the dead end concept. Permeate entered the membrane from the external
9
7
4 1 3 8 2
5
Legend : 1. Submerged membrane bundle 2. Feed tank (MBR sludge) 3. Feed recirculation pump 4. Flow control valve 5. Porous stone
6. 7. 8. 9.
Permeate tank Aluminum bracket i tank Aeration Motor 6
Fig. 1. A SMBR membrane with cross oscillation.
wall and sucked out from the open end on top of the bundle. The membrane was housed in the sludge tank with MLSS between 1800 and 2000 mg/l and was placed higher than the permeate tank to provide a hydrostatic pressure of 3.45 psi. The motor with a direction converter enabled a cross oscillation of 30 RPM and an angular amplitude of 180 . The average velocity at the outer membrane was 0.204 m/s. 2.2. A submerged MBR membrane with lengthwise oscillation The same PAN hollow fiber bundle membrane as in the cross oscillation SMBR described above was used in the test for the MBR with lengthwise oscillation (Fig. 2). The oscillation amplitude was 0.1 m and had a frequency of 30 Hz. This would provide an average membrane surface velocity of 0.2 m/ s. The membrane bundle was housed in the sludge tank with MLSS between 1800 and 2000 mg/l and was placed higher than the permeate tank to provide a hydrostatic pressure of 3.45 psi.
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355
2.3. A combined VSEP and MBR membrane 10 9
7
4 1 3 2 8
5
Legend : 1. Submerged membrane bundle 2. Feed tank (MBR sludge) 3. Feed recirculation pump 4. Flow control valve 5. Porous stone
Fig. 2. A oscillation.
MBR
6. Permeate tank 7. Aluminum bracket 8. Aeration tank 9. Motor 10. Oscillation mechanism
membrane
with
6
lengthwise
Fig. 3. A VSEP with a high MLSS sludge feed.
The VSEP [3] system used was the Series L version 2.1 equipped with a single annular membrane with an effective membrane area of 0.624 ft2. The experimental set-up is shown in Fig. 3. The sludge feed with MLSS of 1800 mg/l was fed from the feed tank. The TMP was set to 3.5 bar (52.5 psi) and the amplitude of the oscillation was set to 1.9 cm (3/4 inch) and the frequency of 70 Hz which offered an average oscillation velocity was 5.33 m/s. The fluid from the sludge tank was pumped to the inlet of the VSEP system. The flat membrane used here was the PAN ultra-filtration membrane using the same materials as in the hollow fibers membranes tested for cross and lengthwise oscillations.
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3. The membranes
Permeate flux Vs Time
Two types of membrane were considered in this study and they were all produced in-house. For tests with the cross oscillation and lengthwise oscillation, a bundled hollow fiber ultra-filtration membrane made of PAN was used. The bundle was ported to have an open end on the top and a dead end at the bottom. It had a total surface area of 78 ft2 and the operation was under trans-membrane pressure of 3.45 psi. The dimensions of the hollow fiber membrane bundle are listed in Table 1. For the VSEP, a single flat ultra-filtration membrane was used. It was made from the same PAN materials for spinning the PAN hollow fiber membrane used in the cross and lengthwise oscillation tests. The net surface area was 0.624 ft2 and the standard TMP was 3.5 bar (52.5 psi). 4. Results 4.1. Bundled hollow fiber ultrafiltration membrane without mechanical motion In this test, the bundled hollow fiber membrane was submerged in the MBR with MLSS of about 1800–2000 mg/l and no mechanical motion was introduced. The permeate flow rate and the total dissolved solids (TDS) change for a whole working day were investigated. Table 1 The dimensions of the hollow fiber membrane bundle Descriptions
Dimensions
Type of membrane Outer diameter of fiber Inner diameter of fiber Length of hollow fiber Number of hollow fibers Bundle diameter Porting Membrane surface area
PAN ultra-filtration 2.7 mm 1.6 mm 1.4 m 610 0.13 m Dead end 78 ft2
Permeate flux / GFD
8 6 4 2 0 0
50
100
150
200
250
300
350
Time / min Permeate flow rate of stationery membrane (Day 1) Permeate flow rate of stationery membrane (Day 2)
Fig. 4. Permeate flux of hollow fiber membrane without any mechanical motion.
As seen in Fig. 4, the permeate flux was high initially. Its rate of decrease was very rapid at the beginning and the reduction rate slowed down toward the end of the day. After a 5 h of continuous operation, the permeate flux was dropped to only a quarter of its initial value. Due to the constraint of no overnight test to be conducted in the test site, the test rig was shut down at the end of the day and continued in the following morning. It was found that the membrane ‘relax’ in overnight and gave a higher initial flux when filtration resumed in the following day as seen in the same figure. The super-imposed of permeate flux for the 2 different days showed that the trends of the permeate flux reduction were quite consistent. Initially, there was no significant different in the total dissolved solids (TDS) between the source and permeate. The TDS difference built up gradually toward the end of the day. It registered a 3.9% reduction of TDS after 5 h of continuous operation as illustrated in Fig. 5. In the second day, though the source TDS was substantially reduced, the TDS reduction trend by the membrane was the same as before and registered a 3.5% TDS reduction after 4.5 h of test. This test showed that there was change in the membrane ability in the TDS reduction after operating
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S.C. Low et al. / Desalination 183 (2005) 353–362 P e rm e a te T D S 1100 1000 TDS / ppm
900 800 700 600 500 0
60
120
180
270
300
350
T im e / m in
D a y 1 S o u rc e D a y 2 S o u rc e
D a y 1 P e r m e a te D a y 2 P e r m e a te
Fig. 5. Total dissolved solids (TDS) at the source and in permeate.
the membrane in the MBR system for more than 5 h. This perhaps, was the evidence of bio-membrane formed on the surface of the ultra-filtration membrane after long hours of operation. It was found that there was an improvement in the initial flux after the filtration was stopped for overnight. This relaxation of the membrane fibers had led to the improvement of the average flux.
motion. The effect of this type of motion on permeate flow rate and the total dissolved solids of permeate were studied. As seen in Fig. 6, the initial permeate flux was slightly higher (7.8 GFD) than that of the membrane not subjected to any forms of motion. The difference was that the drop in flux was more gradual and the flux after 5 h was still high (6.2 GFD). It was 78.5% of the initial flux as compared to only 25% for the membrane without motion. This implied that the cross oscillation had significant anti-clogging and antifouling effect for membrane operated in SMBR environment.
4.2. Hollow fiber membrane subjected to a continuous cross oscillation motion The bundle of hollow fiber membrane was submerged in the membrane bioreactor and subjected to a continuous cross oscillation
P e r m e a t e f lu x v s T im e
Perrmeate flux / GFD
10 8 6 4 2 0 0
50
100
150
200
250
300
T im e / m in C o n t in u o u s c ro s s o s c illa t io
Fig. 6. Permeate flux of hollow fiber bundle with continuous cross oscillation.
350
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S.C. Low et al. / Desalination 183 (2005) 353–362 T o t a l d is s o lv e d s o lid s V s T im e
TDS / ppm
590
580
570
560 0
50
100
150
200
250
T im e / m in T D S a t S o u rc e
T D S a t p e rm e a t e
Fig. 7. Total dissolved solids (TDS) at the source and permeate for membrane with continuous cross oscillation.
The TDS of source fluctuated during the test and the registered TDS’s in the permeate were marginally lesser than those in the source. The maximum TDS difference registered was only around 0.5%. The reduction in the TDS by the membrane at the end of the test was not as obvious as the membrane without motion which had a value of around 3.5%. 4.3. Hollow fiber membrane subjected to a continuous vertical oscillation The hollow fiber membrane bundle was submerged in the membrane bioreactor and subjected to a continuous vertical oscillation.
The effect of this type of motion on permeate flow rate and the total dissolved solids of permeate were investigated. The initial permeate flux of the hollow fiber membrane subjected to a continuous vertical oscillation was 7.8 GFD. The downwards trend in the permeate flux was quite negligible. At the end of the test after continuous operation of 5 h, the permeate flux dropped by 10.2% to 7 GFD which was 89.8% of the initial flux (Fig. 8). In general, the TDS of permeate was not much different from that in the source (Fig. 9). This finding was similar to the membrane subjected to a cross-oscillation motion.
Permeate flux with lengthwise oscillation 8
Permeate flux / GFD
7.5 7 6.5 6 5.5 5 4.5 4 0
20
40
60
80
100 120
140 160
180 200
220 240 260
280 300
Time / min 28-Jan-05
02-Feb-05
04-Feb-05
07-Feb-05
Fig. 8. Permeate flux of hollow fiber bundle with lengthwise oscillation.
07-Feb-05
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TDS / ppm
Total dissolved solids with lengthwise oscillation 590 570 550 530 0
50
100
150
200
250
300
350
Time / min TDS at source
TDS at permeate
Fig. 9. TDS before and after the membrane with vertical oscillation.
4.4. Comparison of the permeate flux of membranes subjected to different motions As we can see from Fig. 10, all the permeate fluxes had the decreasing trends whether or not having membrane motion. However, the downward trends with the introduction of mechanical motions to the bundled hollow fiber membrane were slower than that without the motion. From the diagram, the permeate flux after a long testing period were incredibly high. It was found that by subjecting the membrane to a lengthwise oscillation, we were able to obtain a higher permeate flux compared to the stationary
membrane and the membrane subjected to a cross oscillation motion. The permeate flux reduction was 76% for the bundled hollow fiber membrane without motion, 21% with cross oscillation motion and only dropped by 10% for the one with lengthwise oscillation. It was concluded that oscillation motion of the membrane created the alternating shear forces on the membrane surface. The alternating shear forces had effective anti-clogging and anti-fouling effects for the submerged membrane in MBR system. The lengthwise oscillation provided the alternating shear forces equally on all the fiber membranes.
Permeate Flow rate Vs Time 9 8 Flow rate/GFD
7 6 5 4 3 2 1 0 0
50
100
Membrane in continuous vertical motion
150 200 Time/min
250
300
350
Membrane in continuous cross-rotational motion
Membrane w ith no mechanical motion
Fig. 10. Comparison of permeate flux of hollow fiber membrane subjected to no motion, lengthwise oscillation and cross oscillation.
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However, the cross oscillation provided unequalled shear forces on the membrane bundle, higher on the outer portion of the bundled and lower in the inner portion. This explained why the cross oscillation, though effective, was not as effective as compared with that of the lengthwise oscillation.
temperature due to the long hours of running the system. The membrane we used was the PAN ultra-filtration membrane, we did not expect the reduction the TDS for the tests. There was TDS reduction tendency similar to that of the hollow fiber tested with no mechanical motion. It exhibited the slight downward trends with respect to time as shown in Fig. 12. Roughly, there was a 5–10% reduction of the TDS at the end of the 7 h test. Fig. 13 compared the permeate flux between the VSEP MBR and the SMBR with lengthwise oscillation. The main differences between the two were the trans-membrane pressure on the membranes and the intensity of the mechanical motion that created the alternating shear forces on the membrane surfaces. The VSEP was placed in extreme operating conditions with the TMP 15 times higher than the SMBR that offered a 7.8 times higher in the initial flux. This membrane would be fouled up in a short period of time if not because of the VSEP membrane oscillation. The intensive membrane oscillation, 26 times higher than the SMBR with
4.5. VSEP for high MLSS sludge in MBR system The PAN membrane for the experiment was used continuously for a long period of 3 months. Every time it was run for 7 h and stop running at the end of the day. In the following morning, it was found that the membrane ‘relax’ after an overnight rest as exhibited by the hollow fiber membrane tested before. At the beginning, the membrane almost regained the originally high initial flux of about 65 GFD as shown in Fig. 11. It quickly attenuated to around 45 GFD, which was about 69% of the initial flux, within less than an hour. After that, the flux stabilized. It was also observed that there was a slight upward trend in the flux because of the increased in the sludge fluid
Permeate flux for VSEP 70
Permeate flux / GFD
65 60 55 50 45
420
400
380
360
340
320
300
280
260
240
220
200
180
160
140
120
80
100
60
40
0
20
40 Time / min 27-Jan-05
02-Feb-05
Fig. 11. The permeate flux for the VSEP MBR.
24-Feb-05
08-Mar-05
11-Mar-05
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S.C. Low et al. / Desalination 183 (2005) 353–362 Permeate TDS for VSEP 700
Permeate TDS/ ppm
650 600 550 500 450 400 350
420
400
380
360
340
320
300
280
260
240
220
200
180
160
140
120
80
100
60
40
0
20
300 Time / min 02-Feb-05
24-Feb-05
08-Mar-05
11-Mar-05
Fig. 12. The downward trends in the TDS for the VSEP MBR.
Comparison VSEP vs Other motions
Permeate flux / GFD
70 60 50 40 30 20 10 0 0
50
100
150
200
250
300
350
Time / min Permeate flux of UF membrane in VSEP Permeate flux of UF hollow fibre membrane in SMBR
Fig. 13. Comparison of permeate flux VSEP and lengthwise oscillation.
lengthwise oscillation, managed to maintain the stabilized flux at 70% of the initial value. This value was still 6.8 times higher than that of the SMBR with lengthwise oscillation. 5. Conclusion The mechanical motion helped to maintain the MBR membrane in a relatively ‘clean’
condition and kept the permeate flux close to that of the clean membrane. The high TMP increased the permeate flux significantly for MBR. It required proper matching between the mechanical motion and its TMP to ensure the high performance of the membranes. In our tests, 0.2 m/s of average oscillating velocity managed to keep the flux well above the 75% that of the clean membrane
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for SMBR. The VSEP with high TMP tested here offered 6.8 times higher flux than the SMBR and kept the stabilized flux up to 70% that of the clean membrane. References [1] T.C. Schwartz and B.R. Herring, The first year’s performance of a membrane bioreactor compared with conventional wastewater treatment of domestic waste, Proceedings of WEFTEC, 2001.
[2] K. Brindle, D. Passe, S. Wilkes and S.J. Judd, Performance and economic evaluation of a submerged membrane bioreactor on large diameter hollow fiber membranes for the treatment of raw sewage, Proceedings of the 8th World Filtration Congress, 3–7th April 2000, Brighton, 713–716. [3] S.C. Low, W.X. Jin and M. Tan, Characteristics of particle and membrane pore sizes on the performance of water recovery from a fine carbon loaded wastewater using a vibration membrane system, Desalination, 167 (2004) 217–226.
A combined VSEP and membrane bioreactor system S.C. Lowa*, Han Hee Juanb, Low Kwok Siong a
School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, S 639798, Singapore Tel. þ65 67905516; Fax þ65 67910788; email: [email protected] b Singapore Cleanseas Pte Ltd, 1 Maritine Sq, 13-05, S099253. Received 15 March 2005; accepted 10 April 2005
Abstract In a MBR system, huge amount of solid suspension clogged the membrane and reduced the permeate flux. Mechanical motions were believed to be effective in preventing premature clogging of the membrane. Three different mechanical motions; namely cross oscillation, lengthwise oscillation and the VSEP were investigated. The VSEP combined mechanical motion and the high trans-membrane pressure achieved the best GFD for MBR membrane filtration. Keywords: MBR; Membrane bioreactor; SMBR; Submerged membrane bioreactor; Ultrafiltration; Mechanical motion; VSEP; Shear enhanced filtration; Mixed liquor suspended solids; Trans-membrane pressure
1. Introduction The membrane bioreactor (MBR) sewage treatment process combined the operations of aeration, secondary clarification and filtration into one single process. It simplified the system design and significantly reduced system size. Either microfiltration or the ultrafiltration membrane, replaced the solids separation function of conventional secondary clarifiers and sand filters. Its microscopic pore ensures that no particulate matter
greater than the membrane pore size was discharged in the effluent. The MBR process was typically operated at a very high mixed liquor suspended solids (MLSS). The large amount of solid suspension clogged the membrane and affected its performance. It was reported that in certain applications, the operating cycle could be suctioned for 8 min and stop for 2 min allowing the membrane to ‘relax’ [1]. Air bubbling was a common way to induce turbulences that scoured the external surface of the membrane and kept it clean [2]. Water jet was also used
*Corresponding author. Presented at the Conference on Desalination and the Environment, Santa Margherita, Italy, 22–26 May 2005. European Desalination Society. 0011-9164/05/$– See front matter Ó 2005 Elsevier B.V. All rights reserved doi:10.1016/j.desal.2005.04.028
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in conjunction with air bubbling to physically remove or prevent the solid suspension from clogging the MBR membrane. In most applications, the trans-membrane pressure (TMP) driving the permeate through the membrane was by means of hydrostatic pressure caused by the membrane submersion. Alternatively, the permeate could be sucked through the membrane by a vacuum pump. As a result, the TMP was always below 1 atmospheric pressure which limited the permeate flux through the membrane. In most MBR systems, the permeate flux was limited by the low trans-membrane pressure and the strength of turbulences to reduce clogging and fouling. It was believed that strong mechanical motion could maintain a cleaner membrane surface and enabled higher permeate flux through higher TMP. In this work, three different mechanical motions; namely cross oscillation, lengthwise oscillation and the VSEP [3] were investigated. The cross oscillation and lengthwise oscillation were tested for the submerged membrane bundle with negative suction pressure of 3.45 psi. For the VSEP, the feed was pressurized and the TMP was higher than 1 atmospheric pressure (3.5 bar or 52.5 psi) to drive a higher permeate flux if clogging and fouling was not a problem for a high mixed liquor suspended solid fluid. 2. Test set up 2.1. A submerged MBR membrane (SMBR) with cross oscillation The membrane was a PAN hollow fiber bundle using 610 fibers of length 1.4 m, 2.7 mm outer diameter and 1.6 mm inner diameter (Fig. 1). The total flow surface area was 78 ft2. The fibers were bundled together adopting the dead end concept. Permeate entered the membrane from the external
9
7
4 1 3 8 2
5
Legend : 1. Submerged membrane bundle 2. Feed tank (MBR sludge) 3. Feed recirculation pump 4. Flow control valve 5. Porous stone
6. 7. 8. 9.
Permeate tank Aluminum bracket i tank Aeration Motor 6
Fig. 1. A SMBR membrane with cross oscillation.
wall and sucked out from the open end on top of the bundle. The membrane was housed in the sludge tank with MLSS between 1800 and 2000 mg/l and was placed higher than the permeate tank to provide a hydrostatic pressure of 3.45 psi. The motor with a direction converter enabled a cross oscillation of 30 RPM and an angular amplitude of 180 . The average velocity at the outer membrane was 0.204 m/s. 2.2. A submerged MBR membrane with lengthwise oscillation The same PAN hollow fiber bundle membrane as in the cross oscillation SMBR described above was used in the test for the MBR with lengthwise oscillation (Fig. 2). The oscillation amplitude was 0.1 m and had a frequency of 30 Hz. This would provide an average membrane surface velocity of 0.2 m/ s. The membrane bundle was housed in the sludge tank with MLSS between 1800 and 2000 mg/l and was placed higher than the permeate tank to provide a hydrostatic pressure of 3.45 psi.
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355
2.3. A combined VSEP and MBR membrane 10 9
7
4 1 3 2 8
5
Legend : 1. Submerged membrane bundle 2. Feed tank (MBR sludge) 3. Feed recirculation pump 4. Flow control valve 5. Porous stone
Fig. 2. A oscillation.
MBR
6. Permeate tank 7. Aluminum bracket 8. Aeration tank 9. Motor 10. Oscillation mechanism
membrane
with
6
lengthwise
Fig. 3. A VSEP with a high MLSS sludge feed.
The VSEP [3] system used was the Series L version 2.1 equipped with a single annular membrane with an effective membrane area of 0.624 ft2. The experimental set-up is shown in Fig. 3. The sludge feed with MLSS of 1800 mg/l was fed from the feed tank. The TMP was set to 3.5 bar (52.5 psi) and the amplitude of the oscillation was set to 1.9 cm (3/4 inch) and the frequency of 70 Hz which offered an average oscillation velocity was 5.33 m/s. The fluid from the sludge tank was pumped to the inlet of the VSEP system. The flat membrane used here was the PAN ultra-filtration membrane using the same materials as in the hollow fibers membranes tested for cross and lengthwise oscillations.
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3. The membranes
Permeate flux Vs Time
Two types of membrane were considered in this study and they were all produced in-house. For tests with the cross oscillation and lengthwise oscillation, a bundled hollow fiber ultra-filtration membrane made of PAN was used. The bundle was ported to have an open end on the top and a dead end at the bottom. It had a total surface area of 78 ft2 and the operation was under trans-membrane pressure of 3.45 psi. The dimensions of the hollow fiber membrane bundle are listed in Table 1. For the VSEP, a single flat ultra-filtration membrane was used. It was made from the same PAN materials for spinning the PAN hollow fiber membrane used in the cross and lengthwise oscillation tests. The net surface area was 0.624 ft2 and the standard TMP was 3.5 bar (52.5 psi). 4. Results 4.1. Bundled hollow fiber ultrafiltration membrane without mechanical motion In this test, the bundled hollow fiber membrane was submerged in the MBR with MLSS of about 1800–2000 mg/l and no mechanical motion was introduced. The permeate flow rate and the total dissolved solids (TDS) change for a whole working day were investigated. Table 1 The dimensions of the hollow fiber membrane bundle Descriptions
Dimensions
Type of membrane Outer diameter of fiber Inner diameter of fiber Length of hollow fiber Number of hollow fibers Bundle diameter Porting Membrane surface area
PAN ultra-filtration 2.7 mm 1.6 mm 1.4 m 610 0.13 m Dead end 78 ft2
Permeate flux / GFD
8 6 4 2 0 0
50
100
150
200
250
300
350
Time / min Permeate flow rate of stationery membrane (Day 1) Permeate flow rate of stationery membrane (Day 2)
Fig. 4. Permeate flux of hollow fiber membrane without any mechanical motion.
As seen in Fig. 4, the permeate flux was high initially. Its rate of decrease was very rapid at the beginning and the reduction rate slowed down toward the end of the day. After a 5 h of continuous operation, the permeate flux was dropped to only a quarter of its initial value. Due to the constraint of no overnight test to be conducted in the test site, the test rig was shut down at the end of the day and continued in the following morning. It was found that the membrane ‘relax’ in overnight and gave a higher initial flux when filtration resumed in the following day as seen in the same figure. The super-imposed of permeate flux for the 2 different days showed that the trends of the permeate flux reduction were quite consistent. Initially, there was no significant different in the total dissolved solids (TDS) between the source and permeate. The TDS difference built up gradually toward the end of the day. It registered a 3.9% reduction of TDS after 5 h of continuous operation as illustrated in Fig. 5. In the second day, though the source TDS was substantially reduced, the TDS reduction trend by the membrane was the same as before and registered a 3.5% TDS reduction after 4.5 h of test. This test showed that there was change in the membrane ability in the TDS reduction after operating
357
S.C. Low et al. / Desalination 183 (2005) 353–362 P e rm e a te T D S 1100 1000 TDS / ppm
900 800 700 600 500 0
60
120
180
270
300
350
T im e / m in
D a y 1 S o u rc e D a y 2 S o u rc e
D a y 1 P e r m e a te D a y 2 P e r m e a te
Fig. 5. Total dissolved solids (TDS) at the source and in permeate.
the membrane in the MBR system for more than 5 h. This perhaps, was the evidence of bio-membrane formed on the surface of the ultra-filtration membrane after long hours of operation. It was found that there was an improvement in the initial flux after the filtration was stopped for overnight. This relaxation of the membrane fibers had led to the improvement of the average flux.
motion. The effect of this type of motion on permeate flow rate and the total dissolved solids of permeate were studied. As seen in Fig. 6, the initial permeate flux was slightly higher (7.8 GFD) than that of the membrane not subjected to any forms of motion. The difference was that the drop in flux was more gradual and the flux after 5 h was still high (6.2 GFD). It was 78.5% of the initial flux as compared to only 25% for the membrane without motion. This implied that the cross oscillation had significant anti-clogging and antifouling effect for membrane operated in SMBR environment.
4.2. Hollow fiber membrane subjected to a continuous cross oscillation motion The bundle of hollow fiber membrane was submerged in the membrane bioreactor and subjected to a continuous cross oscillation
P e r m e a t e f lu x v s T im e
Perrmeate flux / GFD
10 8 6 4 2 0 0
50
100
150
200
250
300
T im e / m in C o n t in u o u s c ro s s o s c illa t io
Fig. 6. Permeate flux of hollow fiber bundle with continuous cross oscillation.
350
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S.C. Low et al. / Desalination 183 (2005) 353–362 T o t a l d is s o lv e d s o lid s V s T im e
TDS / ppm
590
580
570
560 0
50
100
150
200
250
T im e / m in T D S a t S o u rc e
T D S a t p e rm e a t e
Fig. 7. Total dissolved solids (TDS) at the source and permeate for membrane with continuous cross oscillation.
The TDS of source fluctuated during the test and the registered TDS’s in the permeate were marginally lesser than those in the source. The maximum TDS difference registered was only around 0.5%. The reduction in the TDS by the membrane at the end of the test was not as obvious as the membrane without motion which had a value of around 3.5%. 4.3. Hollow fiber membrane subjected to a continuous vertical oscillation The hollow fiber membrane bundle was submerged in the membrane bioreactor and subjected to a continuous vertical oscillation.
The effect of this type of motion on permeate flow rate and the total dissolved solids of permeate were investigated. The initial permeate flux of the hollow fiber membrane subjected to a continuous vertical oscillation was 7.8 GFD. The downwards trend in the permeate flux was quite negligible. At the end of the test after continuous operation of 5 h, the permeate flux dropped by 10.2% to 7 GFD which was 89.8% of the initial flux (Fig. 8). In general, the TDS of permeate was not much different from that in the source (Fig. 9). This finding was similar to the membrane subjected to a cross-oscillation motion.
Permeate flux with lengthwise oscillation 8
Permeate flux / GFD
7.5 7 6.5 6 5.5 5 4.5 4 0
20
40
60
80
100 120
140 160
180 200
220 240 260
280 300
Time / min 28-Jan-05
02-Feb-05
04-Feb-05
07-Feb-05
Fig. 8. Permeate flux of hollow fiber bundle with lengthwise oscillation.
07-Feb-05
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TDS / ppm
Total dissolved solids with lengthwise oscillation 590 570 550 530 0
50
100
150
200
250
300
350
Time / min TDS at source
TDS at permeate
Fig. 9. TDS before and after the membrane with vertical oscillation.
4.4. Comparison of the permeate flux of membranes subjected to different motions As we can see from Fig. 10, all the permeate fluxes had the decreasing trends whether or not having membrane motion. However, the downward trends with the introduction of mechanical motions to the bundled hollow fiber membrane were slower than that without the motion. From the diagram, the permeate flux after a long testing period were incredibly high. It was found that by subjecting the membrane to a lengthwise oscillation, we were able to obtain a higher permeate flux compared to the stationary
membrane and the membrane subjected to a cross oscillation motion. The permeate flux reduction was 76% for the bundled hollow fiber membrane without motion, 21% with cross oscillation motion and only dropped by 10% for the one with lengthwise oscillation. It was concluded that oscillation motion of the membrane created the alternating shear forces on the membrane surface. The alternating shear forces had effective anti-clogging and anti-fouling effects for the submerged membrane in MBR system. The lengthwise oscillation provided the alternating shear forces equally on all the fiber membranes.
Permeate Flow rate Vs Time 9 8 Flow rate/GFD
7 6 5 4 3 2 1 0 0
50
100
Membrane in continuous vertical motion
150 200 Time/min
250
300
350
Membrane in continuous cross-rotational motion
Membrane w ith no mechanical motion
Fig. 10. Comparison of permeate flux of hollow fiber membrane subjected to no motion, lengthwise oscillation and cross oscillation.
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However, the cross oscillation provided unequalled shear forces on the membrane bundle, higher on the outer portion of the bundled and lower in the inner portion. This explained why the cross oscillation, though effective, was not as effective as compared with that of the lengthwise oscillation.
temperature due to the long hours of running the system. The membrane we used was the PAN ultra-filtration membrane, we did not expect the reduction the TDS for the tests. There was TDS reduction tendency similar to that of the hollow fiber tested with no mechanical motion. It exhibited the slight downward trends with respect to time as shown in Fig. 12. Roughly, there was a 5–10% reduction of the TDS at the end of the 7 h test. Fig. 13 compared the permeate flux between the VSEP MBR and the SMBR with lengthwise oscillation. The main differences between the two were the trans-membrane pressure on the membranes and the intensity of the mechanical motion that created the alternating shear forces on the membrane surfaces. The VSEP was placed in extreme operating conditions with the TMP 15 times higher than the SMBR that offered a 7.8 times higher in the initial flux. This membrane would be fouled up in a short period of time if not because of the VSEP membrane oscillation. The intensive membrane oscillation, 26 times higher than the SMBR with
4.5. VSEP for high MLSS sludge in MBR system The PAN membrane for the experiment was used continuously for a long period of 3 months. Every time it was run for 7 h and stop running at the end of the day. In the following morning, it was found that the membrane ‘relax’ after an overnight rest as exhibited by the hollow fiber membrane tested before. At the beginning, the membrane almost regained the originally high initial flux of about 65 GFD as shown in Fig. 11. It quickly attenuated to around 45 GFD, which was about 69% of the initial flux, within less than an hour. After that, the flux stabilized. It was also observed that there was a slight upward trend in the flux because of the increased in the sludge fluid
Permeate flux for VSEP 70
Permeate flux / GFD
65 60 55 50 45
420
400
380
360
340
320
300
280
260
240
220
200
180
160
140
120
80
100
60
40
0
20
40 Time / min 27-Jan-05
02-Feb-05
Fig. 11. The permeate flux for the VSEP MBR.
24-Feb-05
08-Mar-05
11-Mar-05
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S.C. Low et al. / Desalination 183 (2005) 353–362 Permeate TDS for VSEP 700
Permeate TDS/ ppm
650 600 550 500 450 400 350
420
400
380
360
340
320
300
280
260
240
220
200
180
160
140
120
80
100
60
40
0
20
300 Time / min 02-Feb-05
24-Feb-05
08-Mar-05
11-Mar-05
Fig. 12. The downward trends in the TDS for the VSEP MBR.
Comparison VSEP vs Other motions
Permeate flux / GFD
70 60 50 40 30 20 10 0 0
50
100
150
200
250
300
350
Time / min Permeate flux of UF membrane in VSEP Permeate flux of UF hollow fibre membrane in SMBR
Fig. 13. Comparison of permeate flux VSEP and lengthwise oscillation.
lengthwise oscillation, managed to maintain the stabilized flux at 70% of the initial value. This value was still 6.8 times higher than that of the SMBR with lengthwise oscillation. 5. Conclusion The mechanical motion helped to maintain the MBR membrane in a relatively ‘clean’
condition and kept the permeate flux close to that of the clean membrane. The high TMP increased the permeate flux significantly for MBR. It required proper matching between the mechanical motion and its TMP to ensure the high performance of the membranes. In our tests, 0.2 m/s of average oscillating velocity managed to keep the flux well above the 75% that of the clean membrane
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for SMBR. The VSEP with high TMP tested here offered 6.8 times higher flux than the SMBR and kept the stabilized flux up to 70% that of the clean membrane. References [1] T.C. Schwartz and B.R. Herring, The first year’s performance of a membrane bioreactor compared with conventional wastewater treatment of domestic waste, Proceedings of WEFTEC, 2001.
[2] K. Brindle, D. Passe, S. Wilkes and S.J. Judd, Performance and economic evaluation of a submerged membrane bioreactor on large diameter hollow fiber membranes for the treatment of raw sewage, Proceedings of the 8th World Filtration Congress, 3–7th April 2000, Brighton, 713–716. [3] S.C. Low, W.X. Jin and M. Tan, Characteristics of particle and membrane pore sizes on the performance of water recovery from a fine carbon loaded wastewater using a vibration membrane system, Desalination, 167 (2004) 217–226.