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Enhancement of membrane permeability by gas-sparging in submerged hollow fibre ultrafiltration of macromolecular solutions: Role of module design
Journal of Membrane Science 274 (2006) 73–82
Enhancement of membrane permeability by gas-sparging in submerged hollow fibre ultrafiltration of macromolecular solutions: Role of module design Raja Ghosh ∗ Department of Chemical Engineering, McMaster University, 1280 Main Street West, Hamilton, Ont., Canada L8S 4L7 Received 23 May 2005; received in revised form 2 August 2005; accepted 3 August 2005 Available online 14 November 2005
Abstract Permeability in membrane filtration processes suffers from two major limiting factors: concentration polarization and membrane fouling. Gas-sparging, which involves bubbling of a gas in close proximity of a membrane, is known to minimise both of these. Gas-sparged membrane filtration is carried out either by pressurising a gas-sparged feed side or by using suction to draw the permeate through a membrane from the un-pressurised, gas-sparged feed side. The first approach is mainly used in ultrafiltration processes. The second approach which is easier to implement and is widely used in microfiltration processes. This paper discusses the enhancement of permeability by gas-sparging in suctiondriven, submerged hollow fibre ultrafiltration using two different membrane module types. These modules were prepared using hollow fibre membranes having nominal MWCO of 150 kDa and were used to ultrafilter polysaccharide solutions. Depending on the operating conditions and on the module design, gas-sparging enhanced effective hydraulic permeability by as much as 115%. The extent of membrane fouling was also significantly lower in the gas-sparged mode. The effectiveness of gas-sparging was found to be greater with one membrane module type, clearly highlighting the effect of module design on process efficiency. © 2005 Elsevier B.V. All rights reserved. Keywords: Ultrafiltration; Gas-sparging; Hollow fibre; Submerged membrane; Flux enhancement; Fouling; Membrane module
1. Introduction Ultrafiltration (UF) is widely used for processing natural polymers such as proteins, polysaccharides and nucleic acids. UF is mainly used for the concentration (i.e. removal of solvent) and desalting (i.e. removal of low molecular weight contaminants) of polymer solutions. In recent years, UF is also being tried out as a method for polymer fractionation. Membrane-based separation processes generally suffer from two major limiting factors: concentration polarization and fouling. Concentration polarization which is basically the build-up of the retained species near the membrane surface can limit the permeate flux, i.e. transport of liquid through the membrane and thus render a separation process unvi∗
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able or uncompetitive. Several concentration polarization controlling or minimizing strategies have been proposed. Gas-sparging, which involves bubbling of a gas, typically air, in close proximity of the membrane is one of the more recent approaches towards minimising concentration polarization [1–23]. Fouling which refers to the adsorption and deposition of material on the membrane, particularly in and around the pores is widely regarded as the Achilles’ heel of membrane filtration. This problem is linked to concentration polarization: generally, the greater the concentration polarization, the greater is the fouling. Gas-sparging has therefore been found to be useful in reducing membrane fouling. The application of gas-sparging in membrane filtration has been reviewed in a recent paper by Cui et al. [23]. The use of gas-sparging for improving membrane process efficiency has been reported for both ultrafiltration and microfiltration. In each case, several mechanisms are
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simultaneously responsible for the beneficial actions of gassparging and their relative contributions are system specific, i.e. depend on type of membrane filtration, type of membrane module, type of solute/particle and so on. Identifying which ones of these predominate in a particular system will help in rationalizing the use of gas, thus increasing the competitiveness of gas-sparged ultrafiltration. Gas-sparging is mainly used for high-bulk, low-value product processing where gassparging cost could be a significant factor. Some of the more recent reports have attempted to analyse the mechanisms responsible for the beneficial effects of gas-sparging and quantify them [6–10]. Gas-sparged membrane filtration processes can broadly be classified into two categories, one in which the feed is on the pressurised side (i.e. where permeate is driven by positive transmembrane pressure), and the other in which the permeate is driven by suction (as with submerged hollow fibre membranes). The earlier developments in the area of gassparged membrane filtration took place in the former category [1–5]. The main advantage of using positive pressure to drive filtration is that higher transmembrane pressures can be used. The disadvantages include the necessity to use a pressurized system and the requirement for compressed gas of appropriate pressure. Pressurization of the feed side using a valve on the retentate line, which is common in both microfiltration and ultrafiltration, does not work very well in gas-sparged
processes. This is due to the fact that the retentate stream is a two-phase system and it is difficult to control the transmembrane pressure. One way to overcome this problem is to use a pressurized feed tank [11,12]. One of the earlier reports on the use of a gas-sparged membrane system operated in the suction mode was by Shimizu et al. [13]. They discussed cross-flow microfiltration using a submerged hollow fibre membrane. Gas-sparging was shown to facilitate continuous solid–liquid separation using a system based on simple equipment, such as a low-rate suction pump, an air blower and an open feed vessel. There have since been several reports on the use of gas-sparged submerged hollow fibre systems, some in the context of aerated membrane bioreactors [13–22]. Most of these have dealt with gas-sparged microfiltration processes. Gas-sparged ultrafiltration of dextran and indeed other macromolecules has been discussed in several papers [1–5,24,25]. However, all of these are based on sparging on the pressurised feed fide of the membrane. To the best of the author’s knowledge there are no reports on suction-driven, gas-sparged submerged hollow fibre ultrafiltration of macromolecules. The fact that the transmembrane pressure is limited in suction-based processes could be one of the factors responsible for this since ultrafiltration processes are typically carried out at higher transmembrane pressures than microfiltration. However, with the increasing trend of
Fig. 1. Type 1 submerged hollow fibre membrane module.
Fig. 2. Type 2 submerged hollow fibre membrane module.
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carrying out ultrafiltration processes at “sub-critical flux values” to minimize fouling, the low transmembrane pressure in a suction-driven operation would probably not be considered a major limiting factor in the future [26,27]. This paper discusses the enhancement of permeability and reduction of membrane fouling by gas-sparging in suctiondriven, submerged hollow fibre ultrafiltration using two different module types (Types 1 and 2). These modules were prepared using hollow fibres having nominal MWCO of 150 kDa and were used to ultrafilter polysaccharide solutions (dextran MW = 464 kDa). The effects of operating conditions as well as the module type on membrane permeability and fouling are discussed.
2. Experimental 2.1. Material The 150 kDa MWCO Tetronic (hydrophilic polyethersulfone) hollow fibre membranes used to construct the membrane modules were kindly donated by Hydranautics Inc. The hollow fibres had inner diameter of 1.2 mm and outer diameter of 2 mm. These fibres were stated to be of the “inside-out” type but were used in an “outside-in” mode in the experiments. This did not affect the sieving properties of the
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membrane with respect to the solute used in the experiments. The fibres were potted within tubular perspex shells using epoxy resin. The headers fitted on to the ends of the membrane modules were made of Delrin. Dextran (MW 464 kDa) was purchased from Sigma–Aldrich. Feed solutions for ultrafiltration experiment were prepared using 18.2 M cm water obtained from a SimplicityTM (Millipore) water purification unit. 2.2. Modules The ultrafiltration membrane modules used in this study were designed and fabricated in-house. Types 1 and 2 membrane modules are shown in Figs. 1 and 2, respectively. Type 1 design has conceptual similarities with that of a module discussed by Sunaoka et al. [28] for gas-scrubbed membrane filtration of inorganic suspensions such as fine iron oxide particle dispersed in water. The device discussed in the patent [28] worked by exfoliation of deposited material by the scrubbing action of gas bubbles. Most reported submerged hollow fibre membrane modules do not have shells, i.e. the hollow fibres are unconstrained and directly exposed to the bulk feed solution, e.g. [29]. A submerged hollow fibre membrane module where the fibres are enclosed within a screen cage has been proposed by Zha et al. [30]. The membrane modules discussed in this paper were both provided with
Fig. 3. Experimental set-up for ultrafiltration.
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shells. The tubular Perspex shells within which the hollow fibre membranes were potted had length of 200 mm, inner diameter of 25 mm and outer diameter of 30 mm. The hollow fibre membranes were potted as shown in Figs. 1 and 2. Each membrane module was made with 52 hollow fibres, each fibre having an effective filtration length of 180 mm. The fibre endings on one end of the modules were blocked off and sealed within the potting material. The fibre endings on the other end were connected to the permeate header through the potting material. Several aeration tubes (10 in number, each having 9 mm diameter) were integrated within the potting material on the lower ends and these were connected to the aeration header. These aeration tubes were dispersed in a manner, which gave uniform distribution of gas bubbles within the shell during filtration. The shell of Type 1 membrane module was provided with two rows of circular holes (six holes in each row, having 5 mm diameter each) 25 mm from each end. During gas-sparged ultrafiltration one row of holes served as the feed inlet while the other served as the retentate outlet. The shell of Type 2 module was provided with 36 uniformly distributed holes each having 10 mm diameter, arranged in nine rows along the length of the shell. 2.3. Experimental set-up and method Fig. 3 shows the set-up used for the ultrafiltration experiments. The membrane module was immersed within a 2 litre feed tank and mounted vertically with the aeration headers facing the bottom. The content of the feed tank was kept wellmixed using a magnetic stirrer (VWR). The permeate was drawn by suction using a peristaltic pump (Simon Varistaltic, Barnant) and was recycled back to the feed tank through a rotameter (Bel-Art Products), which was used to determine the permeate flux. The pressure drop driving ultrafiltration was measured using a differential pressure manometer (SPER Scientific). An air pump (Admiral, Cole Parmer) was used to
pump air into the aeration header. The airflow rate was measured using a rotameter (Bel-Art Products). Ultrafiltration experiments with Type 1 membrane module were carried out using three different dextran concentrations: 10, 20 and 40 kg/m3 . With Type 2 membrane module, experiments were carried out only using 40 kg/m3 feed concentration. Before each dextran ultrafiltration experiment the pure water permeability of the membrane was measured. The membrane module was then immersed in the appropriate dextran solution within the feed tank and ultrafiltration with and without gas-sparging were carried out. Permeates samples was analysed for presence of dextran using a refractometer (Atago). After each dextran experiment, the membrane was cleaned by ultrafiltering water with periodic back flushing after which the pure water permeability was determined. The membrane was subsequently cleaned by ultrafiltering 0.1N NaOH also with periodic back flushing and the pure water permeability of the alkali-cleaned membrane was measured.
3. Results and discussion Fig. 4 shows results obtained from ultrafiltration experiment carried out with Type 1 membrane module using 10 kg/m3 dextran solution. The experiment was carried out at two different permeate pump speed settings while only one gas-sparging rate (i.e. 3 × 10−5 m3 /s) was used. Ultrafiltration was started at the low pump speed setting with gas-sparging and this was carried out for 30 min. The air supply was then stopped and ultrafiltration was carried out for a further 30 min. The permeate flux dropped while the transmembrane pressure increased in un-sparged ultrafiltration. Resumption of aeration resulted in decrease in transmembrane pressure and increase in permeate flux. However, the permeate flux obtained after resumption of aeration was marginally lower while the transmembrane pressure was marginally higher than the corresponding values at the begin-
Fig. 4. Ultrafiltration results obtained with 10 kg/m3 feed solution using Type 1 membrane module.
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Fig. 5. Effective hydraulic permeability data for ultrafiltration with 10 kg/m3 feed solution using Type 1 membrane module.
ning (i.e. 0–30 min). This was due to membrane fouling. The pump speed was then increased while still continuing gassparging at the same rate. In response to the increased pump speed, the permeate flux increased but the transmembrane pressure also increased, almost proportionally. After 30 min of gas-sparging at the higher permeate pump speed, aeration was stopped and this resulted in decrease in permeate flux and increase in transmembrane pressure. After a further 30 min, the aeration was resumed. The permeate flux increased while the transmembrane pressure decreased in response to gas-sparging. Once again the effect of fouling could be noticed in terms of the marginally lower permeate flux and the marginally higher transmembrane pressure as compared to that between 100 and 120 min. Random permeate samples collected during the experiment were found to be free of dextran clearly indicating that this solute was totally retained by the membrane. The effective hydraulic permeability in membrane filtration can be defined as the volumetric permeate flux divided by the transmembrane pressure. This parameter represents
the productivity of the membrane per unit applied transmembrane pressure and is a convenient way of expressing the ultrafiltration rate since both permeate flux and transmembrane pressure changed in response to change in operating conditions, i.e. permeate pump speed setting and gas-sparging. Fig. 5 shows the effective hydraulic permeability in unsparged and gas-sparged ultrafiltration of 10 kg/m3 dextran solution. Quite clearly the effective hydraulic permeability was significantly higher in gas-sparged ultrafiltration. At the lower permeate pump speed setting the enhancement in permeability was about 34% while at the higher pump speed, the enhancement was about 28%. Fig. 6 shows results obtained from ultrafiltration experiment carried out with Type 1 membrane module using 20 kg/m3 dextran solution. This experiment was carried out at one permeate pump speed setting only (low) while three gas-sparging rates ((1.83, 3 and 4.33) × 10−5 m3 /s) were examined. The results clearly show that the permeate flux was significantly higher and the transmembrane pressure was significantly lower in gas-sparged ultrafiltration. The
Fig. 6. Ultrafiltration results obtained with 20 kg/m3 feed solution using Type 1 membrane module.
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Fig. 7. Effective hydraulic permeability data for ultrafiltration with 20 kg/m3 feed solution using Type 1 membrane module.
air-sparging rate also seemed to have an effect on the enhancement in permeate flux, i.e. the flux was higher at (3 and 4.33) × 10−5 m3 /s air flow rates in comparison to that at 1.83 × 10−5 m3 /s. This dependence of ultrafiltration on gassparging is better illustrated in Fig. 7 where the effective hydraulic permeability data is presented. The effective permeability increased by 52%, 65% and 66%, respectively, at gas flow rates of (1.83, 3 and 4.33) × 10−5 m3 /s. These results indicate that once the gas flow was increased to a certain extent, further increase in permeability was not observed. Also the enhancement in effective hydraulic permeability was significantly higher in 20 kg/m3 dextran ultrafiltration than in 10 kg/m3 dextran ultrafiltration. Fig. 8 shows results obtained from ultrafiltration experiment carried out with Type 1 membrane module using 40 kg/m3 dextran solution. This experiment was carried out at two different permeate pump speed settings. At the lower pump speed the ultrafiltration was carried out at three different gas-sparging rates ((1.83, 3 and 4.33) × 10−5 m3 /s)
while at the higher pump speed ultrafiltration was carried out at gas flow rates of (3 and 4.33) × 10−5 m3 /s. As with the 20 kg/m3 dextran experiments the air-sparging rate seemed to have an effect on the increase in permeate flux, i.e. the flux was higher at (3 and 4.33) × 10−5 m3 /s air flow rates in comparison to that at 1.83 × 10−5 m3 /s. Once again this dependence is better illustrated by the effective hydraulic permeability data (see Fig. 9). At the lower permeate pump speed setting, the enhancement in permeability was 85%, 101% and 96%, respectively, at (1.83, 3 and 4.33) × 10−5 m3 /s gas flow rates. These results are in line with the earlier observation that once the gas flow was increased to a certain extent, further increase in permeability with gas flow rate was not observed. At the higher pump speed setting, the enhancement in permeability were 112% and 115%, respectively, at gas flow rates of (3 and 4.33) × 10−5 m3 /s. The enhancement in effective hydraulic permeability was significantly higher in 40 kg/m3 dextran ultrafiltration than in 10 and 20 kg/m3 dextran ultrafiltration.
Fig. 8. Ultrafiltration results obtained with 40 kg/m3 feed solution using Type 1 membrane module.
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Fig. 9. Effective hydraulic permeability data for ultrafiltration with 40 kg/m3 feed solution using Type 1 membrane module.
Fig. 10 shows results obtained from ultrafiltration experiment carried out with Type 2 membrane module using 40 kg/m3 dextran solution. This experiment was carried out at only one permeate pump speed setting (low) and two different gas-sparging rates ((3 and 4.33) × 10−5 m3 /s). Fig. 11 shows the corresponding enhancements in effective hydraulic permeability. These were 33% and 35%, respectively (before and after discontinuation of gas flow), at a gas flow rate of 3 × 10−5 and 36.5% at a gas flow rate of 4.33 × 10−5 m3 /s. Figs. 9 and 11 clearly demonstrate that membrane module design had a significant effect on the extent of permeability enhancement by gas-sparging. The positive effects of gas-sparging observed in the dextran ultrafiltration experiments could be explained by evoking several mechanisms which have been gleaned from Refs. [1–25]. The introduction of gas bubbles results in bubble induced secondary flow near the membrane surface and this reduces concentration polarization. The gas bubbles also reduce concentration polarization by direct scouring action
on the membrane. The random motion of the bubbles near the membrane surface results in random shear rate and pressure changes and this also destabilises concentration polarization. The motion of the gas bubbles within the shell side of the membrane module cause the hollow fibres to move (i.e. sway and oscillate) particularly when the fibres are slack and this reduces concentration polarization. In fact, the last mentioned mechanism is considered to be one of the major reasons for permeability enhancement in gas-sparged microfiltration. The module design clearly had an impact on the extent of permeability enhancement by gas-sparging with Type 1 membrane module being able to better utilize the positive effects of aeration. Fig. 12 illustrates the differences between the two. The manner in which the air was sparged was similar in both module types. What was different was the number and arrangement of the holes on the Perspex shells. The arrangement in Type 1 membrane module, i.e. two rows of holes on the shell produced a more pronounced air-lift induced liquid flow in an upward direction within the shell. This was
Fig. 10. Ultrafiltration results obtained with 40 kg/m3 feed solution using Type 2 membrane module.
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Fig. 11. Effective hydraulic permeability data for ultrafiltration with 40 kg/m3 feed solution using Type 2 membrane module.
Fig. 12. Feed and bubble flow in Types 1 and 2 membrane modules.
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verified using a tracer dye experiment. During gas-sparging, a red coloured dye introduced near the lower header of the membrane module was sucked in through the bottom row of holes and was rapidly transported to the row of holes near the top and finally expelled from the shell. A similar experiment carried out with Type 2 membrane module showed that the extent of air-lift induced liquid flow within the shell was considerably lower. This was due to the fact that both liquid and air bubbles could leave the shell at different points. With Type 1 module the liquid and air bubbles could only leave the shell through the top row of holes. With Type 2 module, the manner in which tracer dye entered the shell was also quite random in nature. The air-lift induced liquid flow contributed towards general bulk mixing of the feed within the shell in addition to reducing concentration polarization. The upward liquid flow also accelerated the gas bubbles, thus further increasing their beneficial effects. The loss of air bubbles at different points in Type 2 membrane module also resulted in energy dissipation. With Type 1 membrane module the gas bubbles were confined in close proximity of the hollow fibre membranes during their sojourn within the shell. It is anticipated that this would maximize the bubble-induced hydrodynamic effects. The useful role of the shell (which is not commonly used in submerged hollow fibre modules) is clearly evident from the experimental results. Type 2 module due to the large number of perforations present on the shell is somewhat between a module with a proper shell (embodied by Type 1 module) and one without. From the experimental results one may extrapolate that the permeability enhancement by gas-sparging for a module without the shell would be even lower than Type 2 membrane module. In order to determine the effect of gas-sparging on membrane fouling, ultrafiltration experiments with and without gas-sparging were carried out with Type 1 membrane module using 40 kg/m3 dextran solution for extended duration. Table 1 shows results obtained during these ultrafiltration experiments. Ultrafiltration was first started with gassparging and this was carried out for 180 min. The membrane was then cleaned with water and the pure water permeability was determined. The membrane was then cleaned with 0.1N NaOH and the pure water permeability of the alkali-cleaned membrane was determined. The same sequence of operations was performed for un-sparged dextran ultrafiltration. The results shown in Table 1 quite clearly indicate that the extent of membrane fouling was significantly lower in gas-sparged ultrafiltration. In gas-sparged dextran ultrafiltration, the overall hydraulic permeability after 180 min was about 2% lower than at 90 min. In un-sparged dextran ultrafiltration, the corresponding value was about 17.5% lower. The amount of irreversible fouling was significantly higher in un-sparged ultrafiltration. The lower fouling with gas-sparged ultrafiltration was likely due to the reduction in concentration polarization. The membrane modules discussed in this paper are primarily intended for large-scale ultrafiltration of natural polymers.
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Table 1 Membrane permeability data from fouling assessment studies using Type 1 module (a) Gas-sparged ultrafiltration Initial pure water permeability (m/s Pa) Effective hydraulic permeability after 90 min of dextran ultrafiltration (m/s Pa) Effective hydraulic permeability after 180 min of dextran ultrafiltration (m/s Pa) Pure water permeability after water cleaning (m/s Pa) Pure water permeability after alkali cleaning (m/s Pa) (b) Un-sparged ultrafiltration Initial pure water permeability (m/s Pa) Effective hydraulic permeability after 90 min of dextran ultrafiltration (m/s Pa) Effective hydraulic permeability after 180 min of dextran ultrafiltration (m/s Pa) Pure water permeability after water cleaning (m/s Pa) Pure water permeability after alkali cleaning (m/s Pa)
1.996 × 10−10 3.327 × 10−11 3.258 × 10−11 1.864 × 10−10 1.944 × 10−10 1.962 × 10−10 1.602 × 10−11 1.323 × 10−11 1.711 × 10−10 1.771 × 10−10
Feed: 40 kg/m3 solution of 464 kDa dextran; permeate pump speed: low; gas flow rate in gas-sparged ultrafiltration: 3 × 10−5 m3 /s; water/alkali cleaning procedure: ultrafiltration with periodic back flushing using the appropriate cleaning liquid.
However, these modules could also be used as components of aerated membrane bioreactors. The gas-sparging in such a situation would serve as a means for aeration in addition to enhancement of membrane permeability and reduction of membrane fouling.
4. Conclusions The experimental results demonstrated that gas-sparging could be used in suction-driven, submerged hollow fibre membrane modules to enhance permeability in the ultrafiltration of macromolecular solutions. The effectiveness of gas-sparging increased with increase in feed concentration. The enhancement in effective hydraulic permeability depended on the gas flow rate used. However, the results indicate that once the gas flow had been increased to a certain extent, further increase in gas flow rate did not increase the permeability any significantly further, i.e. some form of enhancement saturation limit was reached. The experimental results clearly show that the module design had a significant impact on the extent of permeability enhancement by gas-sparging. Type 1 membrane module was able to better utilize the positive effects of gas-sparging. Tracer studies showed that Type 1 membrane module gave a more defined air-lift induced liquid flow within the shell. Also in Type 1 membrane module the gas bubbles were confined in close proximity of the hollow fibre membranes during their sojourn within the shell while in Type 2 module, some bubbles escaped through the holes at different locations on the shell. Type 1 membrane module therefore maximized the bubbleinduced hydrodynamic effects. The extent of membrane fouling was found to be significantly lower in the gas-sparged ultrafiltration.
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Acknowledgements Hydranautics Inc. is thanked for donating the polyethersulfone hollow fibre ultrafiltration membranes. Paul Gatt of the Department of Chemical Engineering, McMaster University is thanked for fabricating the Perspex shell and Delrin headers used to make the membrane module.
References [1] Z.F. Cui, Experimental investigation on the enhancements of crossflow ultrafiltration with air sparging, in: R. Paterson (Ed.), Effective Membrane Processes—New Perspectives, Mechanical Engineering, London, 1993, p. 237. [2] Z.F. Cui, K.I.T. Wright, Gas–liquid two-phase cross-flow ultrafiltration of BSA and dextran solutions, J. Membr. Sci. 90 (1994) 183. [3] Z.F. Cui, K.I.T. Wright, Flux enhancement with gas sparging in downwards crossflow ultrafiltration: performance and mechanism, J. Membr. Sci. 117 (1996) 109. [4] S.R. Bellara, Z.F. Cui, D.S. Pepper, Gas sparging to enhance permeate flux in ultrafiltration using hollow fibre membranes, J. Membr. Sci. 121 (1996) 175. [5] Z.F. Cui, S.R. Bellara, P. Homewood, Airlift crossflow membrane filtration—a feasibility study with dextran ultrafiltration, J. Membr. Sci. 128 (1997) 83. [6] R. Ghosh, Z.F. Cui, Mass transfer in gas-sparged ultrafiltration: upward slug flow in tubular membranes, J. Membr. Sci. 162 (1999) 91. [7] T.W. Cheng, Influence of inclination on gas-sparged cross-flow ultrafiltration through an inorganic tubular membrane, J. Membr. Sci. 196 (2002) 103. [8] S. Laborie, C. Cabassud, L. Durand-Bourlier, J.M. Laine, Characterisation of gas–liquid two-phase flow inside capillaries, Chem. Eng. Sci. 54 (1999) 5723. [9] T.W. Cheng, J.G. Wu, Quantitative flux analysis of gas–liquid twophase ultrafiltration, Sep. Sci. Technol. 38 (2003) 817. [10] T. Taha, Z.F. Cui, CFD modelling of gas-sparged ultrafiltration in tubular membrane, J. Membr. Sci. 210 (2002) 13. [11] Q.Y. Li, Z.F. Cui, D.S. Pepper, Fractionation of HSA and IgG by gas sparged ultrafiltration, J. Membr. Sci. 136 (1997) 181. [12] R. Ghosh, Q.Y. Li, Z.F. Cui, Fractionation of BSA and lysozyme using ultrafiltration: effect of gas sparging, AIChE J. 44 (1998) 61. [13] Y. Shimizu, Y. Okuno, K. Uryu, S. Ohtsubo, A. Watanabe, Filtration characteristics of hollow fiber microfiltration membranes used in membrane bioreactor for domestic wastewater treatment, Water Res. 30 (1996) 2385.
[14] H. Nagaoka, S. Ueda, A. Miya, Influence of bacterial extracellular polymers on the membrane separation activated sludge process, Water Sci. Technol. 34 (1996) 165. [15] T. Ueda, K. Hata, Y. Kikuoka, O. Seino, Effects of aeration on suction pressure in a submerged membrane bioreactor, Water Res. 31 (1997) 489. [16] K. Suda, S. Shibuya, Y. Itoh, T. Kohno, Development of a tanksubmerged type membrane filtration system, Desalination 119 (1998) 151. [17] E.H. Bouhabila, R. Ben Aim, H. Buisson, Microfiltration of activated sludge using submerged membrane with air bubbling (application to wastewater treatment), Desalination 118 (1998) 315. [18] B. Gunder, K. Krauth, Replacement of secondary clarification by membrane separation—results with plate and hollow fibre modules, Water Sci. Technol. 38 (1998) 383. [19] C.M. Silva, D.W. Reeve, H. Husain, H.R. Rabie, K.A. Woodhouse, Model for flux prediction in high-shear microfiltration systems, J. Membr. Sci. 173 (2000) 87. [20] S. Chang, A.G. Fane, Characteristics of microfiltration of suspensions with inter-fiber two-phase flow, J. Chem. Technol. Biotechnol. 75 (2000) 533. [21] S. Chang, A.G. Fane, The effect of fibre diameter on filtration and flux distribution—relevance to submerged hollow fibre modules, J. Membr. Sci. 184 (2001) 221. [22] S. Chang, A.G. Fane, S. Vigneswaran, Modeling and optimizing submerged hollow fiber membrane modules, AIChE J. 48 (2002) 2213. [23] Z.F. Cui, S. Chang, A.G. Fane, The use of gas bubbles to enhance membrane processes, J. Membr. Sci. 221 (2003) 1. [24] T.W. Cheng, H.M. Yeh, C.T. Gau, Enhancement of permeate flux by gas slugs for crossflow ultrafiltration in tubular membrane module, Sep. Sci. Technol. 33 (1998) 2295. [25] L. Vera, S. Delgado, S. Elmaleh, Dimensionless numbers for the steady-state flux of cross-flow microfiltration and ultrafiltration with gas sparging, Chem. Eng. Sci. 55 (2000) 3419. [26] S.S. Madaeni, A.G. Fane, D.E. Wiley, Factors influencing critical flux in membrane filtration of activated sludge, J. Chem. Technol. Biotechnol. 74 (1999) 539. [27] S. Judd, A review of membrane bioreactors in sewage treatment, Water Sci. Technol. 49 (2004) 229. [28] Y. Sunaoka, K. Kitazato, S. Tsuda, Process for scrubbing porous hollow fiber membranes in hollow fiber membrane module, US Patent 5,209,852 (1993). [29] W.J. Henshaw, M. Mahendran, H. Behmann, Vertical cylindrical skein of hollow fiber membranes and method of maintaining clean fiber surface, US Patent 5,783,083 (1998). [30] F. Zha, C.V. Kopp, R.J. McMahon, W.T. Johnson, T.W. Beck, Gas-scrubbed hollow fiber membrane module, US Patent 6,156,200 (2000).
Enhancement of membrane permeability by gas-sparging in submerged hollow fibre ultrafiltration of macromolecular solutions: Role of module design Raja Ghosh ∗ Department of Chemical Engineering, McMaster University, 1280 Main Street West, Hamilton, Ont., Canada L8S 4L7 Received 23 May 2005; received in revised form 2 August 2005; accepted 3 August 2005 Available online 14 November 2005
Abstract Permeability in membrane filtration processes suffers from two major limiting factors: concentration polarization and membrane fouling. Gas-sparging, which involves bubbling of a gas in close proximity of a membrane, is known to minimise both of these. Gas-sparged membrane filtration is carried out either by pressurising a gas-sparged feed side or by using suction to draw the permeate through a membrane from the un-pressurised, gas-sparged feed side. The first approach is mainly used in ultrafiltration processes. The second approach which is easier to implement and is widely used in microfiltration processes. This paper discusses the enhancement of permeability by gas-sparging in suctiondriven, submerged hollow fibre ultrafiltration using two different membrane module types. These modules were prepared using hollow fibre membranes having nominal MWCO of 150 kDa and were used to ultrafilter polysaccharide solutions. Depending on the operating conditions and on the module design, gas-sparging enhanced effective hydraulic permeability by as much as 115%. The extent of membrane fouling was also significantly lower in the gas-sparged mode. The effectiveness of gas-sparging was found to be greater with one membrane module type, clearly highlighting the effect of module design on process efficiency. © 2005 Elsevier B.V. All rights reserved. Keywords: Ultrafiltration; Gas-sparging; Hollow fibre; Submerged membrane; Flux enhancement; Fouling; Membrane module
1. Introduction Ultrafiltration (UF) is widely used for processing natural polymers such as proteins, polysaccharides and nucleic acids. UF is mainly used for the concentration (i.e. removal of solvent) and desalting (i.e. removal of low molecular weight contaminants) of polymer solutions. In recent years, UF is also being tried out as a method for polymer fractionation. Membrane-based separation processes generally suffer from two major limiting factors: concentration polarization and fouling. Concentration polarization which is basically the build-up of the retained species near the membrane surface can limit the permeate flux, i.e. transport of liquid through the membrane and thus render a separation process unvi∗
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able or uncompetitive. Several concentration polarization controlling or minimizing strategies have been proposed. Gas-sparging, which involves bubbling of a gas, typically air, in close proximity of the membrane is one of the more recent approaches towards minimising concentration polarization [1–23]. Fouling which refers to the adsorption and deposition of material on the membrane, particularly in and around the pores is widely regarded as the Achilles’ heel of membrane filtration. This problem is linked to concentration polarization: generally, the greater the concentration polarization, the greater is the fouling. Gas-sparging has therefore been found to be useful in reducing membrane fouling. The application of gas-sparging in membrane filtration has been reviewed in a recent paper by Cui et al. [23]. The use of gas-sparging for improving membrane process efficiency has been reported for both ultrafiltration and microfiltration. In each case, several mechanisms are
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simultaneously responsible for the beneficial actions of gassparging and their relative contributions are system specific, i.e. depend on type of membrane filtration, type of membrane module, type of solute/particle and so on. Identifying which ones of these predominate in a particular system will help in rationalizing the use of gas, thus increasing the competitiveness of gas-sparged ultrafiltration. Gas-sparging is mainly used for high-bulk, low-value product processing where gassparging cost could be a significant factor. Some of the more recent reports have attempted to analyse the mechanisms responsible for the beneficial effects of gas-sparging and quantify them [6–10]. Gas-sparged membrane filtration processes can broadly be classified into two categories, one in which the feed is on the pressurised side (i.e. where permeate is driven by positive transmembrane pressure), and the other in which the permeate is driven by suction (as with submerged hollow fibre membranes). The earlier developments in the area of gassparged membrane filtration took place in the former category [1–5]. The main advantage of using positive pressure to drive filtration is that higher transmembrane pressures can be used. The disadvantages include the necessity to use a pressurized system and the requirement for compressed gas of appropriate pressure. Pressurization of the feed side using a valve on the retentate line, which is common in both microfiltration and ultrafiltration, does not work very well in gas-sparged
processes. This is due to the fact that the retentate stream is a two-phase system and it is difficult to control the transmembrane pressure. One way to overcome this problem is to use a pressurized feed tank [11,12]. One of the earlier reports on the use of a gas-sparged membrane system operated in the suction mode was by Shimizu et al. [13]. They discussed cross-flow microfiltration using a submerged hollow fibre membrane. Gas-sparging was shown to facilitate continuous solid–liquid separation using a system based on simple equipment, such as a low-rate suction pump, an air blower and an open feed vessel. There have since been several reports on the use of gas-sparged submerged hollow fibre systems, some in the context of aerated membrane bioreactors [13–22]. Most of these have dealt with gas-sparged microfiltration processes. Gas-sparged ultrafiltration of dextran and indeed other macromolecules has been discussed in several papers [1–5,24,25]. However, all of these are based on sparging on the pressurised feed fide of the membrane. To the best of the author’s knowledge there are no reports on suction-driven, gas-sparged submerged hollow fibre ultrafiltration of macromolecules. The fact that the transmembrane pressure is limited in suction-based processes could be one of the factors responsible for this since ultrafiltration processes are typically carried out at higher transmembrane pressures than microfiltration. However, with the increasing trend of
Fig. 1. Type 1 submerged hollow fibre membrane module.
Fig. 2. Type 2 submerged hollow fibre membrane module.
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carrying out ultrafiltration processes at “sub-critical flux values” to minimize fouling, the low transmembrane pressure in a suction-driven operation would probably not be considered a major limiting factor in the future [26,27]. This paper discusses the enhancement of permeability and reduction of membrane fouling by gas-sparging in suctiondriven, submerged hollow fibre ultrafiltration using two different module types (Types 1 and 2). These modules were prepared using hollow fibres having nominal MWCO of 150 kDa and were used to ultrafilter polysaccharide solutions (dextran MW = 464 kDa). The effects of operating conditions as well as the module type on membrane permeability and fouling are discussed.
2. Experimental 2.1. Material The 150 kDa MWCO Tetronic (hydrophilic polyethersulfone) hollow fibre membranes used to construct the membrane modules were kindly donated by Hydranautics Inc. The hollow fibres had inner diameter of 1.2 mm and outer diameter of 2 mm. These fibres were stated to be of the “inside-out” type but were used in an “outside-in” mode in the experiments. This did not affect the sieving properties of the
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membrane with respect to the solute used in the experiments. The fibres were potted within tubular perspex shells using epoxy resin. The headers fitted on to the ends of the membrane modules were made of Delrin. Dextran (MW 464 kDa) was purchased from Sigma–Aldrich. Feed solutions for ultrafiltration experiment were prepared using 18.2 M cm water obtained from a SimplicityTM (Millipore) water purification unit. 2.2. Modules The ultrafiltration membrane modules used in this study were designed and fabricated in-house. Types 1 and 2 membrane modules are shown in Figs. 1 and 2, respectively. Type 1 design has conceptual similarities with that of a module discussed by Sunaoka et al. [28] for gas-scrubbed membrane filtration of inorganic suspensions such as fine iron oxide particle dispersed in water. The device discussed in the patent [28] worked by exfoliation of deposited material by the scrubbing action of gas bubbles. Most reported submerged hollow fibre membrane modules do not have shells, i.e. the hollow fibres are unconstrained and directly exposed to the bulk feed solution, e.g. [29]. A submerged hollow fibre membrane module where the fibres are enclosed within a screen cage has been proposed by Zha et al. [30]. The membrane modules discussed in this paper were both provided with
Fig. 3. Experimental set-up for ultrafiltration.
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shells. The tubular Perspex shells within which the hollow fibre membranes were potted had length of 200 mm, inner diameter of 25 mm and outer diameter of 30 mm. The hollow fibre membranes were potted as shown in Figs. 1 and 2. Each membrane module was made with 52 hollow fibres, each fibre having an effective filtration length of 180 mm. The fibre endings on one end of the modules were blocked off and sealed within the potting material. The fibre endings on the other end were connected to the permeate header through the potting material. Several aeration tubes (10 in number, each having 9 mm diameter) were integrated within the potting material on the lower ends and these were connected to the aeration header. These aeration tubes were dispersed in a manner, which gave uniform distribution of gas bubbles within the shell during filtration. The shell of Type 1 membrane module was provided with two rows of circular holes (six holes in each row, having 5 mm diameter each) 25 mm from each end. During gas-sparged ultrafiltration one row of holes served as the feed inlet while the other served as the retentate outlet. The shell of Type 2 module was provided with 36 uniformly distributed holes each having 10 mm diameter, arranged in nine rows along the length of the shell. 2.3. Experimental set-up and method Fig. 3 shows the set-up used for the ultrafiltration experiments. The membrane module was immersed within a 2 litre feed tank and mounted vertically with the aeration headers facing the bottom. The content of the feed tank was kept wellmixed using a magnetic stirrer (VWR). The permeate was drawn by suction using a peristaltic pump (Simon Varistaltic, Barnant) and was recycled back to the feed tank through a rotameter (Bel-Art Products), which was used to determine the permeate flux. The pressure drop driving ultrafiltration was measured using a differential pressure manometer (SPER Scientific). An air pump (Admiral, Cole Parmer) was used to
pump air into the aeration header. The airflow rate was measured using a rotameter (Bel-Art Products). Ultrafiltration experiments with Type 1 membrane module were carried out using three different dextran concentrations: 10, 20 and 40 kg/m3 . With Type 2 membrane module, experiments were carried out only using 40 kg/m3 feed concentration. Before each dextran ultrafiltration experiment the pure water permeability of the membrane was measured. The membrane module was then immersed in the appropriate dextran solution within the feed tank and ultrafiltration with and without gas-sparging were carried out. Permeates samples was analysed for presence of dextran using a refractometer (Atago). After each dextran experiment, the membrane was cleaned by ultrafiltering water with periodic back flushing after which the pure water permeability was determined. The membrane was subsequently cleaned by ultrafiltering 0.1N NaOH also with periodic back flushing and the pure water permeability of the alkali-cleaned membrane was measured.
3. Results and discussion Fig. 4 shows results obtained from ultrafiltration experiment carried out with Type 1 membrane module using 10 kg/m3 dextran solution. The experiment was carried out at two different permeate pump speed settings while only one gas-sparging rate (i.e. 3 × 10−5 m3 /s) was used. Ultrafiltration was started at the low pump speed setting with gas-sparging and this was carried out for 30 min. The air supply was then stopped and ultrafiltration was carried out for a further 30 min. The permeate flux dropped while the transmembrane pressure increased in un-sparged ultrafiltration. Resumption of aeration resulted in decrease in transmembrane pressure and increase in permeate flux. However, the permeate flux obtained after resumption of aeration was marginally lower while the transmembrane pressure was marginally higher than the corresponding values at the begin-
Fig. 4. Ultrafiltration results obtained with 10 kg/m3 feed solution using Type 1 membrane module.
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Fig. 5. Effective hydraulic permeability data for ultrafiltration with 10 kg/m3 feed solution using Type 1 membrane module.
ning (i.e. 0–30 min). This was due to membrane fouling. The pump speed was then increased while still continuing gassparging at the same rate. In response to the increased pump speed, the permeate flux increased but the transmembrane pressure also increased, almost proportionally. After 30 min of gas-sparging at the higher permeate pump speed, aeration was stopped and this resulted in decrease in permeate flux and increase in transmembrane pressure. After a further 30 min, the aeration was resumed. The permeate flux increased while the transmembrane pressure decreased in response to gas-sparging. Once again the effect of fouling could be noticed in terms of the marginally lower permeate flux and the marginally higher transmembrane pressure as compared to that between 100 and 120 min. Random permeate samples collected during the experiment were found to be free of dextran clearly indicating that this solute was totally retained by the membrane. The effective hydraulic permeability in membrane filtration can be defined as the volumetric permeate flux divided by the transmembrane pressure. This parameter represents
the productivity of the membrane per unit applied transmembrane pressure and is a convenient way of expressing the ultrafiltration rate since both permeate flux and transmembrane pressure changed in response to change in operating conditions, i.e. permeate pump speed setting and gas-sparging. Fig. 5 shows the effective hydraulic permeability in unsparged and gas-sparged ultrafiltration of 10 kg/m3 dextran solution. Quite clearly the effective hydraulic permeability was significantly higher in gas-sparged ultrafiltration. At the lower permeate pump speed setting the enhancement in permeability was about 34% while at the higher pump speed, the enhancement was about 28%. Fig. 6 shows results obtained from ultrafiltration experiment carried out with Type 1 membrane module using 20 kg/m3 dextran solution. This experiment was carried out at one permeate pump speed setting only (low) while three gas-sparging rates ((1.83, 3 and 4.33) × 10−5 m3 /s) were examined. The results clearly show that the permeate flux was significantly higher and the transmembrane pressure was significantly lower in gas-sparged ultrafiltration. The
Fig. 6. Ultrafiltration results obtained with 20 kg/m3 feed solution using Type 1 membrane module.
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Fig. 7. Effective hydraulic permeability data for ultrafiltration with 20 kg/m3 feed solution using Type 1 membrane module.
air-sparging rate also seemed to have an effect on the enhancement in permeate flux, i.e. the flux was higher at (3 and 4.33) × 10−5 m3 /s air flow rates in comparison to that at 1.83 × 10−5 m3 /s. This dependence of ultrafiltration on gassparging is better illustrated in Fig. 7 where the effective hydraulic permeability data is presented. The effective permeability increased by 52%, 65% and 66%, respectively, at gas flow rates of (1.83, 3 and 4.33) × 10−5 m3 /s. These results indicate that once the gas flow was increased to a certain extent, further increase in permeability was not observed. Also the enhancement in effective hydraulic permeability was significantly higher in 20 kg/m3 dextran ultrafiltration than in 10 kg/m3 dextran ultrafiltration. Fig. 8 shows results obtained from ultrafiltration experiment carried out with Type 1 membrane module using 40 kg/m3 dextran solution. This experiment was carried out at two different permeate pump speed settings. At the lower pump speed the ultrafiltration was carried out at three different gas-sparging rates ((1.83, 3 and 4.33) × 10−5 m3 /s)
while at the higher pump speed ultrafiltration was carried out at gas flow rates of (3 and 4.33) × 10−5 m3 /s. As with the 20 kg/m3 dextran experiments the air-sparging rate seemed to have an effect on the increase in permeate flux, i.e. the flux was higher at (3 and 4.33) × 10−5 m3 /s air flow rates in comparison to that at 1.83 × 10−5 m3 /s. Once again this dependence is better illustrated by the effective hydraulic permeability data (see Fig. 9). At the lower permeate pump speed setting, the enhancement in permeability was 85%, 101% and 96%, respectively, at (1.83, 3 and 4.33) × 10−5 m3 /s gas flow rates. These results are in line with the earlier observation that once the gas flow was increased to a certain extent, further increase in permeability with gas flow rate was not observed. At the higher pump speed setting, the enhancement in permeability were 112% and 115%, respectively, at gas flow rates of (3 and 4.33) × 10−5 m3 /s. The enhancement in effective hydraulic permeability was significantly higher in 40 kg/m3 dextran ultrafiltration than in 10 and 20 kg/m3 dextran ultrafiltration.
Fig. 8. Ultrafiltration results obtained with 40 kg/m3 feed solution using Type 1 membrane module.
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Fig. 9. Effective hydraulic permeability data for ultrafiltration with 40 kg/m3 feed solution using Type 1 membrane module.
Fig. 10 shows results obtained from ultrafiltration experiment carried out with Type 2 membrane module using 40 kg/m3 dextran solution. This experiment was carried out at only one permeate pump speed setting (low) and two different gas-sparging rates ((3 and 4.33) × 10−5 m3 /s). Fig. 11 shows the corresponding enhancements in effective hydraulic permeability. These were 33% and 35%, respectively (before and after discontinuation of gas flow), at a gas flow rate of 3 × 10−5 and 36.5% at a gas flow rate of 4.33 × 10−5 m3 /s. Figs. 9 and 11 clearly demonstrate that membrane module design had a significant effect on the extent of permeability enhancement by gas-sparging. The positive effects of gas-sparging observed in the dextran ultrafiltration experiments could be explained by evoking several mechanisms which have been gleaned from Refs. [1–25]. The introduction of gas bubbles results in bubble induced secondary flow near the membrane surface and this reduces concentration polarization. The gas bubbles also reduce concentration polarization by direct scouring action
on the membrane. The random motion of the bubbles near the membrane surface results in random shear rate and pressure changes and this also destabilises concentration polarization. The motion of the gas bubbles within the shell side of the membrane module cause the hollow fibres to move (i.e. sway and oscillate) particularly when the fibres are slack and this reduces concentration polarization. In fact, the last mentioned mechanism is considered to be one of the major reasons for permeability enhancement in gas-sparged microfiltration. The module design clearly had an impact on the extent of permeability enhancement by gas-sparging with Type 1 membrane module being able to better utilize the positive effects of aeration. Fig. 12 illustrates the differences between the two. The manner in which the air was sparged was similar in both module types. What was different was the number and arrangement of the holes on the Perspex shells. The arrangement in Type 1 membrane module, i.e. two rows of holes on the shell produced a more pronounced air-lift induced liquid flow in an upward direction within the shell. This was
Fig. 10. Ultrafiltration results obtained with 40 kg/m3 feed solution using Type 2 membrane module.
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Fig. 11. Effective hydraulic permeability data for ultrafiltration with 40 kg/m3 feed solution using Type 2 membrane module.
Fig. 12. Feed and bubble flow in Types 1 and 2 membrane modules.
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verified using a tracer dye experiment. During gas-sparging, a red coloured dye introduced near the lower header of the membrane module was sucked in through the bottom row of holes and was rapidly transported to the row of holes near the top and finally expelled from the shell. A similar experiment carried out with Type 2 membrane module showed that the extent of air-lift induced liquid flow within the shell was considerably lower. This was due to the fact that both liquid and air bubbles could leave the shell at different points. With Type 1 module the liquid and air bubbles could only leave the shell through the top row of holes. With Type 2 module, the manner in which tracer dye entered the shell was also quite random in nature. The air-lift induced liquid flow contributed towards general bulk mixing of the feed within the shell in addition to reducing concentration polarization. The upward liquid flow also accelerated the gas bubbles, thus further increasing their beneficial effects. The loss of air bubbles at different points in Type 2 membrane module also resulted in energy dissipation. With Type 1 membrane module the gas bubbles were confined in close proximity of the hollow fibre membranes during their sojourn within the shell. It is anticipated that this would maximize the bubble-induced hydrodynamic effects. The useful role of the shell (which is not commonly used in submerged hollow fibre modules) is clearly evident from the experimental results. Type 2 module due to the large number of perforations present on the shell is somewhat between a module with a proper shell (embodied by Type 1 module) and one without. From the experimental results one may extrapolate that the permeability enhancement by gas-sparging for a module without the shell would be even lower than Type 2 membrane module. In order to determine the effect of gas-sparging on membrane fouling, ultrafiltration experiments with and without gas-sparging were carried out with Type 1 membrane module using 40 kg/m3 dextran solution for extended duration. Table 1 shows results obtained during these ultrafiltration experiments. Ultrafiltration was first started with gassparging and this was carried out for 180 min. The membrane was then cleaned with water and the pure water permeability was determined. The membrane was then cleaned with 0.1N NaOH and the pure water permeability of the alkali-cleaned membrane was determined. The same sequence of operations was performed for un-sparged dextran ultrafiltration. The results shown in Table 1 quite clearly indicate that the extent of membrane fouling was significantly lower in gas-sparged ultrafiltration. In gas-sparged dextran ultrafiltration, the overall hydraulic permeability after 180 min was about 2% lower than at 90 min. In un-sparged dextran ultrafiltration, the corresponding value was about 17.5% lower. The amount of irreversible fouling was significantly higher in un-sparged ultrafiltration. The lower fouling with gas-sparged ultrafiltration was likely due to the reduction in concentration polarization. The membrane modules discussed in this paper are primarily intended for large-scale ultrafiltration of natural polymers.
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Table 1 Membrane permeability data from fouling assessment studies using Type 1 module (a) Gas-sparged ultrafiltration Initial pure water permeability (m/s Pa) Effective hydraulic permeability after 90 min of dextran ultrafiltration (m/s Pa) Effective hydraulic permeability after 180 min of dextran ultrafiltration (m/s Pa) Pure water permeability after water cleaning (m/s Pa) Pure water permeability after alkali cleaning (m/s Pa) (b) Un-sparged ultrafiltration Initial pure water permeability (m/s Pa) Effective hydraulic permeability after 90 min of dextran ultrafiltration (m/s Pa) Effective hydraulic permeability after 180 min of dextran ultrafiltration (m/s Pa) Pure water permeability after water cleaning (m/s Pa) Pure water permeability after alkali cleaning (m/s Pa)
1.996 × 10−10 3.327 × 10−11 3.258 × 10−11 1.864 × 10−10 1.944 × 10−10 1.962 × 10−10 1.602 × 10−11 1.323 × 10−11 1.711 × 10−10 1.771 × 10−10
Feed: 40 kg/m3 solution of 464 kDa dextran; permeate pump speed: low; gas flow rate in gas-sparged ultrafiltration: 3 × 10−5 m3 /s; water/alkali cleaning procedure: ultrafiltration with periodic back flushing using the appropriate cleaning liquid.
However, these modules could also be used as components of aerated membrane bioreactors. The gas-sparging in such a situation would serve as a means for aeration in addition to enhancement of membrane permeability and reduction of membrane fouling.
4. Conclusions The experimental results demonstrated that gas-sparging could be used in suction-driven, submerged hollow fibre membrane modules to enhance permeability in the ultrafiltration of macromolecular solutions. The effectiveness of gas-sparging increased with increase in feed concentration. The enhancement in effective hydraulic permeability depended on the gas flow rate used. However, the results indicate that once the gas flow had been increased to a certain extent, further increase in gas flow rate did not increase the permeability any significantly further, i.e. some form of enhancement saturation limit was reached. The experimental results clearly show that the module design had a significant impact on the extent of permeability enhancement by gas-sparging. Type 1 membrane module was able to better utilize the positive effects of gas-sparging. Tracer studies showed that Type 1 membrane module gave a more defined air-lift induced liquid flow within the shell. Also in Type 1 membrane module the gas bubbles were confined in close proximity of the hollow fibre membranes during their sojourn within the shell while in Type 2 module, some bubbles escaped through the holes at different locations on the shell. Type 1 membrane module therefore maximized the bubbleinduced hydrodynamic effects. The extent of membrane fouling was found to be significantly lower in the gas-sparged ultrafiltration.
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Acknowledgements Hydranautics Inc. is thanked for donating the polyethersulfone hollow fibre ultrafiltration membranes. Paul Gatt of the Department of Chemical Engineering, McMaster University is thanked for fabricating the Perspex shell and Delrin headers used to make the membrane module.
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