Separation of protein from suspended particles using submerged membrane filtration

Journal of Membrane Science 362 (2010) 427–433

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Separation of protein from suspended particles using submerged membrane filtration Kuo-Jen Hwang a,∗ , Hung-Pin Lo a , Tung-Wen Cheng a , Kuo-Lun Tung b a b

Department of Chemical and Materials Engineering, Tamkang University, 151 Ying-Chuan Rd., Tamsui, Taipei Hsien 25137, Taiwan R&D Center for Membrane Technology and Department of Chemical Engineering, Chung Yuan Christian University, Taoyuan 320, Taiwan

a r t i c l e

i n f o

Article history: Received 25 April 2010 Received in revised form 25 June 2010 Accepted 29 June 2010 Available online 23 July 2010 Keywords: Microfiltration Membrane fouling Submerged membrane filtration Filtration resistance Protein separation

a b s t r a c t In this study, bovine serum albumin (BSA) molecules are separated from suspended particles using submerged membrane filtration. Several methods, including constant-pressure filtration with backwash, air bubble sparging and a stepwise increase in pressure, are used to reduce filtration resistance and to enhance filtration flux. The effects of filtration pressure, backwash flow rate, backwash duration, air flow rate and stepwise pressure increase on filtration flux, filtration resistances, BSA rejection and BSA production are discussed. The results show that internal fouling of the filter membrane is the most important contributor to the overall filtration resistance, while cake formation is the main determinant of BSA rejection. An increase in filtration pressure leads to lower filtration flux and lower BSA rejection due to more severe internal fouling of the membrane. Although additional periodic backwash or air bubble sparging increases the filtration flux relative to solely constant-pressure filtration, the filtration flux and BSA production when using a stepwise pressure increase is much higher than with the other operations, especially when the latter is combined with a periodic backwash. Moreover, the calculated filtration flux and BSA rejection results for constant-pressure filtration under various filtration pressures agree fairly well with experimental data. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Biochemical reactions and subsequent separations, such as microbial cell concentration, protein purification or clean filtrate acquisition, can be achieved in a compact unit by installing a submerged membrane filtration system into a bioreactor. Because the investment and operation costs are often considerably reduced compared to side-stream installations, this type of installation has been increasingly used in biochemical and wastewater treatment processes in recent years. However, reducing membrane fouling and improving separation efficiency are still the most important tasks for optimum process design and technology development. When membrane filtration is used for bio-separation, such as when proteins or enzymes are purified from microbial cells after a fermentation reaction, the separation performance is mainly determined by the purification efficiency and the operation rate. However, the occurrence of unexpected membrane fouling often causes a drastic decrease in filtration flux. Membrane fouling is frequently attributed to particle deposition on the membrane surface or to internal clogging of the membrane. In previous studies of membrane bioreactors, some researchers have claimed that

∗ Corresponding author. Tel.: +886 2 26215656x2726; fax: +886 2 26209887. E-mail address: [email protected] (K.-J. Hwang). 0376-7388/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2010.06.051

suspended solids were the major foulant [1] and that the main filtration resistance was attributed to the cake formed on the membrane surface [2,3]. In contrast, other researchers have indicated that internal fouling was the main cause of filtration resistance [4,5]. In fact, both of these results may occur under different hydrodynamic conditions, microbial characteristics or membrane types. In the membrane filtration of bio-products, the filter cake formed by microbial cells results in one of the primary sources of filtration resistance and also in a rejection effect on protein transmission [6]. Arora and Davis [7] and Guell et al. [8] studied the microfiltration of yeast/protein binary suspensions and found that a thinner yeast cake led to a higher filtration rate and higher protein transmission. Kuberkar and Davis [9] claimed that the formation of yeast cakes could markedly reduce membrane fouling and enhance the filtration flux by as much as a factor of two. Hwang and Cheng [6] and Hwang and Wu [10] studied the crossflow microfiltration of fine particle/macromolecule suspensions and indicated that the filter cake formed by these particles played an important role in macromolecule rejection. A theoretical equation for rejection estimation was derived using the concentration polarization model and the standard particle capture equation for depth filtration. In a membrane filtration system for recovering proteins from a cell culture, microbial cells are retained by the membrane, while proteins have opportunity to permeate through the membrane

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Fig. 1. A schematic diagram of the submerged membrane filtration system.

into filtrate. Use of the membranes with pore sizes similar to protein size often leads to high protein rejection and relatively low filtration rate. The filtration flux and protein transmission may be increased, but the risk of membrane internal fouling due to cell penetration also increased by using a membrane with larger pore sizes. To increase protein recovery by considering both filtration flux and protein rejection is therefore an important issue in membrane bio-separation processes. A membrane blocking – cake formation comparable submerged membrane filtration system was then used to recover proteins from a model bio-product in this study. PMMA particles and BSA were selected as the sample cells and proteins, respectively. In the system, the membrane fouling including cake formation and membrane internal blocking were mainly attributed to PMMA particle deposition on the membrane surface and in the membrane pores, respectively. Several hydrodynamic operating conditions, including constant-pressure filtration, air bubble sparging, periodic backwash and a stepwise increase in filtration pressure, were used for reducing membrane fouling, enhancing the permeating flux and increasing the BSA production. The effects of these operations on the filtration resistances, filtration fluxes and BSA rejection were thoroughly discussed. 2. Materials and experiments Spherical particles made of poly(methyl methacrylate) (PMMA) were purchased from Soken Co. in Japan (Cat. no.: MX1500H) for use as the particulate sample in the experiments. The mean particle diameter and density were 15 ␮m and 1190 kg/m3 , respectively. Bovine serum albumin (BSA), manufactured by USB Co. in Canada, was used as a typical protein sample. The molecular weight of BSA was 67,000 Da. PMMA particles and BSA were suspended in deionized water to prepare a binary suspension with concentrations of 0.1 and 0.05 wt.%, respectively. The suspension pH and temperature were kept at 7.0 and 20 ◦ C, respectively. Under these conditions, the particle zeta potential was measured as −20 mV. The amounts of BSA adsorbed onto the PMMA particles were measured by a batch adsorption experiment. Only less than 1.5% of BSA would be adsorbed during the suspension preparation. A ceramic membrane tube manufactured by Orelis Co. in France (Kerasep #06040) was used as the filter medium. The membrane tube had an inner diameter of 6.0 × 10−3 m, an outer diameter of 1.0 × 10−2 m and a length of 0.32 m. The filtration was performed in an outsidein scheme, and the pore sizes on the outer surface ranged from 2 to 20 ␮m with a mean value of 5.6 ␮m. The virgin membrane resistance was measured as 1.3 × 1011 m−1 under a pressure of 40 kPa. A schematic diagram of the submerged membrane filtration system is shown in Fig. 1. The suspension was prepared in a suspension

tank and mixed well using a mechanical mixer. The filtration pressure was supplied by a vacuum pump and indicated on a pressure gauge. The filtrate was collected into a receiver and weighed using a load cell during filtration. The measured filtrate weight data were transferred to a personal computer for further analysis. The BSA concentration in the filtrate was measured with a UV/visible spectrometer at a wavelength of 280 nm. When each experiment was terminated, a chemical–physical cleaning process was used to clean the fouled membrane and to restore its original flux characteristics [11,12]. Besides constant-pressure filtration (CPF), several other operating conditions were used in this study to enhance the filtration flux and to reduce filtration resistance. When using aeration techniques, the air flow rate was adjusted and measured using a rotameter. Air bubbles were dispersed using a diffuser installed under the membrane tube. In a periodic backwash operation, a constant rate deionized water stream was flushed back through the membrane in an inside-out scheme, followed by 9000 s of filtration in each cycle. For the stepwise pressure increase (SPI) operation, the filtration pressure was increased by 10 kPa after every 3000 s of filtration. For instance, if the initial filtration pressure was set to 20 kPa, the filtration pressure would be increased to 70 kPa at 18,000 s, and the average filtration pressure during the entire process would be 45 kPa. 3. Results and discussion The predominant filtration resistances in a microfiltration process are caused by the filter cake Rc , the membrane internal fouling Rif and the clean membrane Rm . Therefore, considering these resistances in series, the basic filtration equation can be expressed as: q=

P P = (Rt ) (Rc + Rif + Rm )

(1)

where q is the filtration flux, P is the filtration pressure,  is the fluid viscosity and Rt is the overall filtration resistance. The Rt value can be calculated by substituting filtration flux data into Eq. (1), while the Rm value can be obtained from the pure water permeation data. When a filtration procedure is terminated, the cake formed on the membrane surface is washed away by spraying deionized water. Because the filtration resistance due to solute concentration polarization is negligibly small compared to the other sources of resistance in the methods used in study, the value of Rif can be determined from the difference between the pure water fluxes before and after cake removal. The Rc value is then calculated using Eq. (1) once the other resistances are known. Fig. 2 shows the values of Rif and Rc during the constant-pressure filtration of pure PMMA and mixed PMMA/BSA binary suspensions under various filtration pressures. Because some membrane pore sizes are larger than particle size, PMMA particles have opportunity to penetrate into the membrane pores. As a result, the Rif value is much higher than Rc , and this difference becomes larger under higher filtration pressures. The Rif value increases nearly linearly with increasing filtration pressure, as higher pressures result in higher fluxes in the initial periods and lead particles to penetrate more easily into the membrane pores. In contrast, the Rc value decreases only slightly as pressure increases. This behavior is observed because a decrease in filtration flux under higher pressure diminishes particle deposition on the membrane surface. The Rif value is markedly higher for the binary suspension than for the pure PMMA suspension, even under the same filtration pressure. Because some membrane pores are blocked by PMMA particles, BSA may be captured in the narrowed membrane pores and becomes a significant contributor to the internal fouling of the membrane. However, the presence of BSA molecules has only a trivial effect on the cake filtration resistance. This observation reveals that PMMA

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Fig. 2. Effects of filtration pressure on the filtration resistances due to internal fouling of the membrane and filter caking.

particles are the main constituent of the filter cake. BSA molecules will rarely be captured in the filter cake and thus will not result in noticeable filtration resistance. Because the trends of filtration flux attenuation under different pressures are quite similar, the pressure effect on the filtration flux is appropriately discussed through the comparison of pseudosteady filtration flux. Fig. 3 shows the effect of filtration pressure on the pseudo-steady filtration flux at 18,000 s. The pseudo-steady filtration flux decreases with increasing filtration pressure because more severe internal fouling of the membrane occurs under higher pressure, as shown in Fig. 2. A 50% decrease in filtration flux can occur as the filtration pressure increases from 20 to 60 kPa. Theoretical calculation results are shown as a solid curve in Fig. 3 for comparison. The pseudo-steady filtration flux can be theoretically calculated using a force balance model [11,13]. Because the filter cake is formed mostly from PMMA particles, the force analysis is only necessary for the depositing particles. A PMMA particle can deposit stably on the membrane surface only when the friction force exceeds the net tangential force. The external forces exerted on a depositing particle considered in this study include the drag

Fig. 3. Comparison of the filtrate flux in CPF at 18,000 s between calculated results and experimental data.

429

Fig. 4. Effects of backwash duration on various filtration resistances.

forces due to the suspension flow and the permeate flow, the inertial lift force, the net gravitational force, and the shear force due to bubble flow. Interparticle forces are reasonably neglected because the particles used in this study are far larger than 1 ␮m. The filtration flux can then be calculated by substituting those analyzed forces into a force balance model [11,13]. The calculated results agree fairly well with the experimental data, as shown in Fig. 3. A periodic backwash operation is believed to recover filtration flux partially by removing fouled particles. Fig. 4 shows the effects of backwash duration on the filtration resistances under a constant filtration pressure of 40 kPa. The backwash operations are carried out by pumping deionized water through the filter in the reverse direction in an inside-out scheme at a constant rate of 1.66 × 10−4 m3 /m2 s followed by 30 min of filtration in each operation cycle. Two washing durations, 2 and 4 min, were compared in these backwash operations. The filtration resistance data shown in Fig. 4 were obtained during the 10th operation cycle before the initiation of backwash. Comparing the backwash operation results with those from constant-pressure filtration without backwash demonstrates that the backwash operation can reduce Rif by nearly 30% and Rc by 60%. The filtration flux will then be significantly enhanced. However, a longer backwash duration has no further benefit for resistance reduction or flux recovery. A longer duration may be more effective in removing fouled particles in the first few operation cycles, but subsequent particles can then more easily penetrate into the cleaner membrane, resulting in a higher Rif value for longer backwash duration under the same filtration pressure and time. Because of this effect, although the Rc value is lower for longer backwash durations, the overall filtration flux should be lower when using a 4 min backwash duration because Rif is the dominant component of the overall filtration resistance. Fig. 5 shows the variations of Rif and Rc during a constantpressure filtration with periodic constant-flux backwash. The filtration time was 30 min, the backwash flux was 1.66 × 10−4 m3 /m2 s, and the backwash duration was 4 min in each cycle. Each data point shown in Fig. 5 was measured in one experiment. Each experiment was carried out until the set cycle, and the filtration resistances were then measured by the method described previously. These data indicate that backwash can remove only partially fouled particles, as both Rif and Rc values continuously increase during the entire filtration. The data shown in Fig. 5 can be regressed to the

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Fig. 5. Variations of Rif and Rc values with backwash cycle.

Fig. 6. Time series of filtration fluxes under various CPF and SPI operations.

following power-law empirical equations: Rif = Rif,N=1 · N 0.69

(2)

Rc = Rc,N=1 · N 0.14

(3)

where Rif,N=1 and Rc,N=1 are the Rif and Rc values in the first cycle, respectively. These power-law empirical equations are similar to those found in previous studies on backwash operation efficiency [12,14]. The data also reveal a more rapid increase in Rif after the backwash cleans the membrane. The Rif value is only 30% higher than Rc at the beginning of filtration; however, it becomes 4-fold higher than Rc after 10 cycles of operation. The filtration flux recovery results for various operating cycles under three different backwash flow rates are summarized in Table 1. A 4 min backwash operation was followed with 30 min filtration in each cycle, and the filtration pressure was kept under 40 kPa. The flux recovery can be found to decrease with each operation cycle. Because the internal fouling of the membrane continuously increases, only approximately 5% of the initial filtration flux can be recovered after 3 cycles. Furthermore, the membrane will be washed cleaner with a higher backwash flow rate. As a result, the filtration flux recovery increases accordingly, especially before the third cycle. Stepwise pressure increase (SPI) operations with different backwash flow rates were used in this study to mitigate particle fouling and to enhance filtrate flux. Fig. 6 shows the time series of filtration flux under various SPI conditions. All experiments shown in this figure were done without aeration. For constant-pressure filtration (CPF), the filtration pressure was fixed at 45 kPa during the entire process, while the pressure increased stepwise from 20 to 70 kPa in the SPI operations. The filtration flux in CPF decayed very quickly in the early filtration period and then gradually approached a pseudo-steady value after 4000 s. This behavior occurs because particles are difficult to deposit on the membrane surface under Table 1 Comparison of flux recovery after different backwash cycles. 0.1 wt.% PMMA + 0.05 wt.% BSA, P = 40 kPa, tf = 30 min, tb = 4 min Backwash flow rate (m3 /m2 s)

5.53 × 10−5 8.30 × 10−5 1.66 × 10−4

Backwash cycle no. 0

1

3

5

7

100% 100% 100%

16% 23% 31%

9% 16% 18%

6% 7% 8%

5% 5% 6%

such low filtration fluxes after long time periods. Because internal fouling is very severe in the filtration system, operating at a low pressure early on in the filtration process is an effective method for diminishing the drastic increase in filtration resistance caused by fouling. When a filter cake forms on the membrane surface, it acts similarly to a secondary membrane, preventing further internal fouling of the membrane [6,8,10]. Thus, the increase in filtration pressure late in the filtration operation enhances the filtration flux. As can be seen in the comparison of CPF and SPI results shown in Fig. 6, the flux will suddenly increase when filtration pressure is increased. The fluxes in the SPI operations are indeed much higher than those using CPF, especially in the earlier periods. Although the average filtration pressure over the course of each of these processes is nearly the same, the SPI operations result in a cumulative flux that is more than 3-fold higher than that achieved by CPF. The triangle and square symbols shown in the figure represent backwash operations performed before pressure increases in the SPI operations. Backwash operations were maintained for 4 min under different water flow rates. Because PMMA particles more easily migrate into the membrane pores after a backwash, the backwash flow rate exhibits no obvious effect on the filtration flux. A comparison of the results from SPI operations reveals that backwashing significantly enhances the filtration flux only for a short period of time. Fig. 7 shows the filtration resistances at 18,000 s under the same operating conditions as those in Fig. 6. The data indicate that the SPI technique reduces the Rif value to be roughly half of that observed for the CPF process under the same average pressure. In contrast, backwashing, especially under a higher backwash flow rate, increases the Rif value, as removal of the filter cake allows for greater internal fouling of the membrane. However, SPI operations have a smaller effect on the Rc value. A higher filtration flux in SPI mode causes more cake formation, but backwashing can reduce this filter cake, and a higher backwash flow rate leads to less cake formation. In conclusion, SPI operation can significantly reduce overall filtration resistance, but applying additional backwash under SPI conditions has no obvious effects on flux enhancement or resistance reduction. Another method for reducing filtration resistance is to sparge air bubbles using a diffuser located under the membrane tube. The effects of the air volumetric flow rate on the filtration resistances are shown in Fig. 8. The aeration bubble sizes ranged from 3 to 6 mm depending on the air flow rate. Compared with the CPF data

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431

Fig. 7. Effects of SPI operations on various filtration resistances. Fig. 9. Comparison of BSA rejection under various operating conditions.

in Fig. 2, which was collected for the same pressure conditions but without aeration, the Rif and Rc values were reduced by 60% and 40%, respectively, under an air flow rate of 5.9 × 10−4 m3 /s, which implies that sparging air bubbles can effectively enhance the filtration flux. Furthermore, an increase in the air flow rate leads to lower Rc but higher Rif values. The formation of a cake benefits to prevent the migration of PMMA particles into the membrane to result in internal fouling. Because the up-flowing air bubbles produce a resultant shear stress acting on the membrane surface that increases with an increasing air flow rate, particle deposition is more difficult under a higher air flow rate. Consequently, the internal fouling of the membrane becomes more severe when aeration is used because less caking occurs. These effects offset each other, such that increasing the air flow rate has no effect on the filtration resistance or flux. The received filtrate volumes per unit area at 18,000 s under various operating conditions are summarized in Table 2. For those constant-pressure filtration, the filtrate volume decreases with increasing filtration pressure because of more severe internal fouling of the membrane, which has been discussed previously. The most amount of filtrate is received under SPI operations due to the least membrane fouling. However, an additional backwash during SPI operation has only a little effect on the filtrate volume. A peri-

Fig. 8. Effects of the air flow rate on various filtration resistances.

odical backwash operation causes reduction in filtration resistance. Therefore, more filtrate volumes are received in backwash operations compared to that in CPF under the same filtration pressure. An increase in backwash flux leads to more received filtrate. However, increasing backwash duration results in a contrary effect. Because an aeration operation may reduce more filtration resistance than those done by backwash operations, more filtrate is received in an aeration operation. However, the filtrate volume decreases with increasing air flow rate due to the increase in membrane internal fouling. Because most membrane pores are larger than the size of a BSA molecule, some BSA is able to penetrate the filter cake and membrane into the filtrate. The BSA rejection coefficient is defined as: Rrej = 1 −

Cp Co

(4)

where Co and Cp are the BSA concentrations in the original suspension and filtrate, respectively. Therefore, a small rejection parameter implies that more BSA will be collected in the filtrate. Fig. 9 shows the variation of the BSA rejection parameter during filtration under various conditions. BSA rejection continuously increases under CPF because the thicker cake rejects more BSA molecules [6]. Because more caking occurs under lower pressures in CPF, as shown in Fig. 2, BSA rejection is higher at a pressure of 20 kPa than at 40 kPa. These results also imply that the effect of cake formation on BSA rejection is more dominant than that of internal fouling of the membrane. Therefore, BSA rejection will become lower at the same pressure in CPF if a backwash operation is used. This effect is more significant when the cake grows to a considerable thickness, i.e., at late times in filtration periods shown in Fig. 9. Furthermore, BSA rejection is higher in a SPI operation under nearly the same average pressure as with CPF at 40 kPa. This result is expected because the cake is thicker in a SPI operation (the Rc value is higher, as shown in Fig. 7). Although the BSA rejection is less than 0.2 at most operating conditions, the value can be as high as 0.43 under aeration conditions and remains nearly constant during filtration (not shown in the figure). This characteristic is attributed to the decrease in BSA transmission opportunities caused by the sweeping effect of the lifting bubbles [10]. Although the filtration resistance due to BSA concentration polarization is negligibly small compared to the other sources of resistance, the BSA concentration polarization should be taken into account to estimate the concentration distribution near the

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Table 2 The received filtrate volumes per unit area at 18,000 s under various operating conditions. CPF

SPI (average filtration pressure = 45 kPa)

Backwash (P = 40 kPa)

Aeration (P = 40 kPa)

P (kPa)

vt (m /m )

qb (m /m s) tb (min)

vt (m /m )

qb (m /m s)

tb (min)

vt (m /m )

Q × 103 (m3 /s)

vt (m3 /m2 )

20 32 40 45 60

0.5523 0.4242 0.3718 0.3535 0.3272

No backwash qb = 1.66 × 10−4 tb = 2 qb = 8.30 × 10−5 tb = 2

1.0817 1.1687

1.66 × 10−4

2 4 2 4

0.5374 0.4684 0.4758 0.4552

0.59 1.18 1.77 2.36

0.6675 0.6330 0.6291 0.5334

3

2

3

2

3

2

3

8.30 × 10−5 1.1565

Fig. 10. Comparison of BSA rejection in CPF at 18,000 s between calculated results and experimental data.

cake surface as well as BSA rejection. Combining a concentration polarization model and the standard capture equation for depth filtration, Hwang and co-workers [6,10] derived a theoretical equation to correlate rejection, filtration flux, cake thickness and a mass transfer coefficient in cross-flow microfiltration of particle/macromolecule binary suspensions:



ln

exp

 q   R rej s k

·

1 − Rrej



+ 1 = (Lc + Lm )

2

(5)

where k is the mass transfer coefficient,  is a screening parameter representing the fraction of macromolecules rejected by the cake per unit thickness and Lc and Lm are the cake thickness and the equivalent membrane thickness, respectively. The BSA rejection under various conditions can then be estimated using Eq. (5) once the cake thickness and filtration flux are known. Fig. 10 shows a comparison of BSA rejection between calculated results and experimental data for CPF under various filtration pressures. As described previously, BSA rejection decreases with increasing filtration pressure due to the decrease in cake thickness. Although the filtration flux is higher under lower filtration pressures, as shown in Fig. 3, the BSA concentration in the filtrate is lower under such conditions. Furthermore, a good prediction of BSA rejection can be found. This finding demonstrates the appropriate use of the theoretical derivation shown above. In this case, cake formation is the main determinant of BSA rejection. To compare the effectiveness of different operating conditions, the BSA production in the filtrate after 18,000 s under roughly the same filtration pressure but varying other conditions is shown in Fig. 11. The most important factors affecting BSA production include the filtration flux and BSA rejection. However, the effects of the operating conditions on BSA rejection are negligibly small, except when aeration is used, as shown in Fig. 9. The filtration flux

3

2

Fig. 11. Comparison of BSA production under various operating conditions.

is therefore the most important factor in determining BSA production. Referring to Figs. 3 and 10, both the filtration flux and BSA rejection decrease with increasing filtration pressure in CPF. Therefore, an increase in filtration pressure in CPF is unimportant with regards to increasing BSA production. Because periodic backwash benefits both flux enhancement and cake removal, BSA production in backwash operations is higher than that in CPF alone for the same pressure of 40 kPa, and BSA production increases with increasing backwash flow rate. Although air bubble sparging can also increase the filtration flux, BSA rejection rises more than 3fold under aeration conditions. Therefore, the BSA production is lower when aeration is used than with CPF alone. Furthermore, the filtration flux is significantly increased for SPI operations, though the BSA rejection is only slightly higher. As a result, BSA production for SPI operation is much higher than for the other operating modes, especially when SPI is combined with a periodic backwash. However, the effectiveness of backwash operations in SPI is not noticeable because of the increase in Rif value, which is shown in Figs. 6 and 7. 4. Conclusions Several hydrodynamic methods, such as increasing the filtration pressure, employing additional periodic backwashes, using air bubble sparging and applying stepwise increases in pressure, were used to reduce filtration resistance, enhance filtration flux and increase BSA production in the membrane blocking/cake formation comparable filtration system. The filtration resistance due to internal fouling of the membrane was much higher than that due to filter caking under most conditions. BSA molecules played an important role in determining the filtration resistance caused by internal fouling while displaying a trivial effect on the cake

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filtration resistance. The BSA rejection was mainly determined by the cake formation. An increase in filtration pressure led to lower filtration fluxes and lower BSA rejection due to more severe internal fouling of the membrane. A backwash operation caused reductions in Rif and Rc of approximately 30% and 60%, respectively, compared to constant-pressure filtration alone under the same pressure. However, longer backwash durations had no further benefit in terms of resistance reduction or flux recovery. In the aeration operation, the Rif and Rc values were reduced by 60% and 40%, respectively, under an air flow rate of 5.9 × 10−4 m3 /s. However, 3-fold higher BSA rejection caused BSA production in aeration conditions to be lower than that for constant-pressure filtration. The filtration flux and BSA production in the SPI operation was much higher than in the other techniques, especially when SPI was combined with a periodic backwash. Moreover, the calculated results for the filtration flux and BSA rejection under CPF for various filtration pressures agreed fairly well with experimental data. Acknowledgements The authors wish to express their sincere gratitude to the National Science Council (NSC), the Center-of-Excellence (COE) Program on Membrane Technology from the Ministry of Education (MOE), R.O.C., and the Technology Development Program for Academia (TDPA) from the Ministry of Economic Affairs (MOEA), R.O.C., for financial support.

Nomenclature Co Cp k Lc Lm N P PBSA Q q qb qs Rc

BSA concentration in original suspension, [kg/m3 ] BSA concentration in filtrate, [kg/m3 ] mass transfer coefficient of BSA, [m s] cake thickness, [m] equivalent membrane thickness, [m] operation cycle number filtration pressure, [kPa] production of BSA in filtrate, [kg/m2 ] air volumetric flow rate, [m3 s] superficial velocity of filtrate or filtrate flux, [m3 /m2 s] backwash flow rate, [m3 /m2 s] filtrate flux at 18,000 s, [m3 /m2 s] resistance of the filter cake, [m−1 ]

Rif Rm Rrej Rt t tb tf

vt

433

resistance of the internal fouling of the membrane, [m−1 ] resistance of the clean membrane, [m−1 ] BSA rejection, defined in Eq. (4) overall filtration resistance, [m−1 ] filtration time, [s] backwash duration, [min] filtration time in each cycle, [min] received filtrate volume per unit area at 18,000 s, [m3 /m2 ]

Greek Letters  screening parameter, [m−1 ]  viscosity of liquid, [kg/m s]

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