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Microfiltration through an inorganic tubular membrane with high frequency retrofiltration
Journal of Membrane Science 188 (2001) 181–188
Microfiltration through an inorganic tubular membrane with high frequency retrofiltration M. Héran∗ , S. Elmaleh UMR 5669, UM II/CNRS/IRD, Hydrosciences, Equipe Génie des Procédés, CC 24, Université Montpellier II, 34095 Montpellier Cedex 5, France Received 28 November 2000; received in revised form 15 January 2001; accepted 17 January 2001
Abstract High frequency backpulsing is a promising technique of flux enhancement that could contribute to the development of cross-flow micro-/ultrafiltration in water and wastewater treatment. A systematic study of the influence of the operational parameters was carried out with three suspensions, bentonite in tap water, biologically treated wastewater and activated sludge. The alumina membranes were tubular (0.02, 0.05 or 0.2 m), with internal or external skin, the latter being not suitable. The technique was particularly efficient for bentonite; a minimal cross-flow velocity was required to reach a net flux independent of the cross-flow. The results are less good for the biological suspensions since the same fluxes could be reached by an increase of cross-flow velocity. However, the energy required by high frequency backpulsing is lower. The average reverse fluxes, measured by a tracer method, are surprisingly high and could hamper the development of the technique. At low Reynolds number (Re = 3500), the net flux increased with the reverse flux, then reached a plateau corresponding to the total penetration of the laminar layer against the membrane wall by the backwash water. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Fouling; Inorganic membrane; Microfiltration; Ultrafiltration; Transient response
1. Introduction Cross-flow microfiltration is a constantly developing technique in many fields, particularly in water and wastewater treatment. However, it is often hampered by mass transport limitation. Backflush is one of the most frequently used techniques of flux enhancement: periodic reverses of transmembrane pressure induce cleaning by the backflow. The deposit and even internal foulants are then lifted off the membrane and swept away by the cross-flow. However, even with very short backflush duration (1–5 s), performed 1–10 ∗ Corresponding author. Tel.: +33-4-67-14-37-23; fax: +33-4-67-54-48-10. E-mail address: [email protected] (M. H´eran).
times per minute, at 1–10 bar differential pressure, backflushing accounts for about 10–20% of the total operation time [1]. Moreover, the reverse flux must be taken into account and net results are then poor, i.e. the net flux can only be increased by 10–30%. However, good results were obtained with such short backwash times as 0.05 s with a period less than 5 s [2,3]. It is thought that the deposit is removed from the membrane wall before it has fully formed and compacted. This technique is called high frequency retrofiltration, high frequency backpulsing or backshock [4]. A wide variety of suspensions and membranes has been tested. Rodgers and Sparks [5] studied backpulsing in 10 g/l BSA ultrafiltration. Fluxes, for 0.05 s backflow at 0.5 s period, increased up to three times
0376-7388/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 6 - 7 3 8 8 ( 0 1 ) 0 0 3 5 1 - 9
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M. H´eran, S. Elmaleh / Journal of Membrane Science 188 (2001) 181–188
under laminar cross-flow (Re = 8) but the technique was inefficient in the turbulent regime. When the feed concentration was between 10 and 30 g/l, backpulsing was always efficient but at 100 g/l, the flux was no longer enhanced [6]. Wilharm and Rodgers [7] concluded that the flux increase was due to concentration polarisation disruption by motion of the operated flat sheet membrane and not by reverse flow through the membrane. Wenten et al. [3] showed that a reverse asymmetric membrane gave better results. An asymmetric hollow fibre membrane with external skin was particularly convenient in beer or fruit juice filtering. In the case of ceramic tubular membranes, good results were obtained in beer filtration [3] but bad ones in yeast filtration [8]. Moreover, a systematic study of the modelling of high frequency retrofiltration has recently been carried out [9]. The existing models, based on a particle cake deposit [10] removed with a variable efficiency [11], can predict the net flux and the optimal frequency range. However, no model can predict the transient flux responses or the average reverse fluxes, particularly when the particles can interact with the membrane. It was also shown that one experimental response to a backpulse enables predicting the performance of the process and the optimal conditions [9]. In spite of those many works, no systematic study of the influence of all the operational parameters has been carried out. These parameters can be classified in two categories, the variables of cross-flow filtration, such as cross-flow velocity and transmembrane pressure, and the variables of pulsing, such as forward filtration duration or backpressure. This paper intends to carry out a systematic study with three suspensions. Bentonite in tap water provides an example of inorganic and non-compressible particle suspension which does not interact with the membrane and which has been previously tested in high frequency retrofiltration [12]. Considering the increasing importance of tertiary microfiltration [13], it is urgent to evaluate the applicability of the technique to secondary water, a low concentrated biological suspension interacting with the membrane [14]. Finally, considering the development of membrane bioreactors [15], activated sludge will also be tested. The final aim of this work is to provide a method to evaluate the suitability of the technique and to predict the optimal conditions.
2. Experimental 2.1. Experimental set-up The experimental unit, provided with a stirred 100 l feed tank, operated a 40 cm long and 7 mm internal diameter tubular alumina membrane of different rated pore diameters (0.02, 0.05 or 0.2 m) (SCT/US Filter) (Fig. 1). A 0.2 m external skin membrane was also used to establish its suitability. A volumetric pump ensured cross-flow of the retentate through the filtration element (flow-rate monitored with a Fischer Porter rotameter). The permeate was not recycled. Transmembrane pressure, measured with two Air Liquide manometers, was periodically reversed by connecting the permeate circuit with a pressure tank through a solenoid valve Burker, Model W4XUT, W4XUS, opened or closed in 0.03 s by computer control. The computer generated controlled counter pressure waves of short duration (35–999 ms), of 1–3 bar over the forward driving pressure and at a frequency between 0.06 and 10 Hz. The 10 l pressurised tank was filled of filtered water through a 0.2 m US Filter cartridge. The runs were carried out at 25◦ C temperature, maintained constant with a spiral heat exchanger. A high precision balance (0.0001 g), Sartorius BP 210, linked to a computer, measured the flux each 0.1 s enabling to observe the average reverse flux
Fig. 1. Experimental set-up.
M. H´eran, S. Elmaleh / Journal of Membrane Science 188 (2001) 181–188
183
Table 1 Main characteristics of Montpellier sewage plant effluent Parameter
Value
Suspended solid concentration (mg/l) Chemical oxygen demand (mg/l) NTK (mg/l) Orthophosphate concentration (mg/l) Conductivity (S/cm)
20 ± 8 70 ± 20 40 ± 10 7 1500
signals. The membrane was just cleaned prior to each run by a conventional acid/base routine [14]. All the reported data of flux took into account the reverse flow and will be given as net flux. The steady-state was usually obtained after a maximum of 1 h operation. 2.2. Suspensions The following three suspensions were tested: 1. 1 g/l Volclay bentonite in tap water (8 m mean size particle); 2. biologically treated wastewater (effluent of Montpellier sewage plant, Table 1); 3. activated sludge (0.8 g/l suspended solids) sampled from a continuous stirred tank reactor fed with a alcoholic synthetic substrate (C2 H5 OH, N2 H9 PO4 , NH4 NO3 with COD/N/P = 150/10/1) at 0.65 kg COD/kg SS per day loading rate. 2.3. Retrofiltration flux Measuring the mass of the tank containing backwash water has currently monitored average reverse flux [2]. However, this method is not precise since the tank could not be directly installed over the balance. The reverse flux was then measured, as the average over about 10 cycles, by two methods. The first technique consisted of direct collection of the reverse flow on a Sartorius BP 2100 balance (0.1 g precision) in a set-up operated without inlet flow and provided with a capillary, which quantified damping, and a stopper that prevented water contained between the module and the membrane from flowing down by gravity (Fig. 2). This set-up enabled limiting and evaluating the quantity of bypassed liquid that did not vary with the elapsed time.
Fig. 2. Set-up for reverse flux measurement.
The second technique was a tracer method based on conductivity. During these runs, the unit was operated with water of 1500 S/cm conductivity (secondary water conductivity or tap water with added sodium chloride) while the backwashing tank contained de-ionised water (5 S/cm). The permeate volume and its conductivity were sampled and measured every 15 s (beginning of secondary water filtration) or every 5 min (bentonite and end of secondary water filtration). The volume coming from the backwashing tank in the same time was given by (conductivityfeed − conductivitysample ) × volumesample conductivityfeed − conductivitybackpulse which allowed calculating the real reverse flux after subtracting the bypassed water.
2.4. Analyses All the physical and chemical analyses were carried out in accordance with the Standard Methods [16]. Conductivity was given by a WTW conductimeter. A Malvern Mastersizer/E laser granulometer measured the particle size distribution.
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3. Results and discussion 3.1. Net flux, cross-flow velocity and transmembrane pressure As in conventional cross-flow filtration, an initial flux decline was observed then followed by a stationary value (Fig. 3). Contrary to published results that recommend an external skin membrane [3], such a filter was significantly less adapted than an internal active layer membrane, giving fluxes similar to those of conventional cross-flow filtration. Moreover, the external skin membrane was more difficult to regenerate since the particles penetrated into the 3 mm porous structure before reaching the active layer. This situation is different from beer filtration through a hollow fibre membrane of 210 m depth that gave good results [4]. In the case of the bentonite suspension and with a conventional internal skin membrane, high frequency retrofiltration allowed a significant increase of the flux (Fig. 3). The stationary flux (1000 l/h m2 ) could be even larger than with tap water (200 l/h m2 ) but lesser than with low conductivity water (1800 l/h m2 ) [17]; tap water microfiltration raises indeed many problems [17]. Similar results were obtained by Jones et al. [18], with clay in purified water, and Ramirez and Davis [12], with bentonite in tap water. Bentonite forms, like clay, a non-adhesive cake layer on the membrane surface that is easily removed by the reverse flux [18]. The significant flux increase can be explained by diffusion
Fig. 3. Net flux against elapsed time for bentonite suspension (1 bar transmembrane pressure; 2 m/s cross-flow velocity; 2 bar backpressure; 5 s period; 50 ms backwash).
Fig. 4. Responses to high period pulses (2 min) for bentonite suspension (1 bar transmembrane pressure; 1 bar backpressure; 2 m/s cross-flow velocity; 50 ms backwash time; 2 min period).
of particles from the cake surface, erosion of the cake, decompression and subsequent washing away of the cake and a shock wave knocking off part of the cake layer. The interest of retrofiltering such a suspension was evidenced by the forward flux response to a backpressure pulse. After the stationary regime had been reached, pulses were imposed at a larger period. The important flux decrease and recovery are significant arguments in favour of high frequency retrofiltration [9] (Fig. 4). The efficiency of the technique depended on cross-flow velocity. In the case of bentonite, the stationary flux increased with cross-flow velocity then reached a plateau value at a velocity depending on the frequency (Fig. 5). An estimate of the minimum
Fig. 5. Stationary flux against cross-flow velocity at different frequencies (bentonite suspension; 1 bar transmembrane pressure; 2 m/s cross-flow velocity; 1 bar backpressure; 50 ms backwash).
M. H´eran, S. Elmaleh / Journal of Membrane Science 188 (2001) 181–188
cross-flow velocity required to sweep the foulants to the filter exit during the backpulse, with conditions of Fig. 5, is 8 m/s, which is much higher than the velocities leading to the plateau flux. In fact, the foulants are periodically re-deposited. However, the existence of a minimal kinetic energy required to reach the maximal flux is an important result that has not yet been reported. There is a domain where the enhancement was particularly high since the flux of conventional filtration was low; the flux was multiplied by 20 at 0.25 m/s cross-flow velocity and 1 Hz frequency, whereas the flux was multiplied by a factor 4–6 when the plateau was reached (Fig. 5). On the other hand, the technique was much less efficient with such active suspensions as secondary water or activated sludge where the flux monotonously increased with cross-flow velocity like in conventional filtration (Fig. 6). A slight flux enhancement was obtained between 1 and 3 m/s cross-flow velocity for secondary water but similar values could also be reached by simple increase of cross-flow velocity without backpulsing. Those deceptive results were predictable considering the flux response to a backpressure pulse where the flux variation was narrow (Fig. 7) [9]. The use of membranes of smaller pores did not give better results and all the following runs were carried out with a 0.2 m membrane (Fig. 8). The filtration of biological suspensions is characterised by a steep decrease from the initial flux and reverse pulsing could not reduce the subsequent foul-
Fig. 6. Stationary flux against cross-flow velocity at different frequencies for active suspensions (1 bar transmembrane pressure; 2 m/s cross-flow velocity; 2 bar backpressure; 0.2 Hz frequency; 50 ms backwash).
185
Fig. 7. Responses to high period pulses (200 s) for secondary water (1 bar transmembrane pressure; 1 bar backpressure; 2 m/s cross-flow velocity; 50 ms backwash time; 200 s period).
ing. Still worse results were obtained while filtering yeast and proteins with high frequency pulsing [8]. However, the technique should not be discarded. High frequency retrofiltering requires indeed less energy than high cross-flow velocity (>5 m/s) [18]. In the case of the bentonite suspension, high frequency retrofiltration allowed increasing the critical transmembrane pressure that corresponds to a plateau flux beyond 2 bar; without reverse pulsing, the critical pressure was 0.5 bar (Fig. 9). The overall shape of the flux against transmembrane pressure does not depend on frequency but the graph shows that an optimal frequency should exist at the vicinity of 0.2 Hz.
Fig. 8. Net flux through different membranes against elapsed time for secondary water (1 bar transmembrane pressure; 2 m/s cross-flow velocity; 3 bar backpressure; 5 s period; 50 ms backwash).
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Fig. 9. Net flux against transmembrane pressure in conventional filtration and with retrofiltration at different frequencies for bentonite suspension (2 m/s cross-flow velocity; 2 bar backpressure; 50 ms backwash).
transport difficult in both senses. However, the average reverse flux was always higher than the forward flux, three times for bentonite and twice for secondary water. Could this mean that retrofiltration, as carried out here, is not a simple reverse filtration through a porous structure supposed to have the same resistance in both directions? Moreover, such high reverse fluxes could compromise the economic achievement of the technique. A comparison of experimental and predicted reverse fluxes by models based on cake filtration and incomplete cleaning has been already done [9]. The predictions were always lower than the experimental results, which indicates that, contrarily to forward filtration, an incomplete cleaning does not influence the reverse flux. 3.3. Influence of backpressure and backwash time
Establishing the influence of the retrofiltration operational parameters requires an evaluation of the retroflux. 3.2. Reverse flux The average reverse flux curves, obtained by tracing, could compare with the classical flux against time curves in non-pulsed cross-flow microfiltration (Fig. 10). An initial decrease, steeper for secondary water, was followed by a plateau value; the fouling processes induced by secondary water made the mass
The influence of backpressure on the net flux was negligible when the cross-flow was in turbulent regime (Re = 14 000), which was also observed in protein filtration [7]. In same conditions of flow regime, the advantages of a longer cleaning were counterbalanced by a larger permeate consumption, above all at high frequency, e.g. the net flux decrease is sharper at 1 than at 0.2 Hz (Fig. 11). At hydraulic conditions near of laminar regime (Re = 3500), the influence of backpressure and reverse filtration duration were amplified; the net flux appeared then as an increasing function
Fig. 10. Reverse flux against elapsed time for bentonite suspension and secondary water (1 bar transmembrane pressure; 2 m/s cross-flow velocity; 5 s period; 50 ms backpulse).
M. H´eran, S. Elmaleh / Journal of Membrane Science 188 (2001) 181–188
187
Fig. 11. Influence of reverse filtration time on net flux in turbulent regime (bentonite suspension; 1 bar transmembrane pressure; 2 m/s cross-flow velocity; 1 bar backpressure).
Fig. 13. Net flux against reverse flux (bentonite suspension; 1 bar transmembrane pressure; 0.5 m/s cross-flow velocity; 0.2 Hz frequency).
of both variables (Fig. 12). Backpressure and reverse filtration time determine the reverse flux whose link with net flux was evidenced. Net flux increased with reverse flux then reached a plateau that corresponds to backwash water complete penetration into the laminar layer (Fig. 13). Assuming a uniform reverse filtration, backwash water is distributed, at the end of the pulse, along the internal membrane wall constituting a layer of thickness ε called the backwash penetration layer: 1 4v b ε= d − d2 − (1) 2 πL
On the other hand, the thickness of the laminar layer e is given by [19]
where d is the internal diameter of the filtration element and L its length.
1 2f
Fig. 12. Influence of backpressure and backwash time on net flux in transition regime (bentonite suspension; 1 bar transmembrane pressure; 0.5 m/s cross-flow velocity; 1 bar backpressure; 0.2 Hz frequency).
e=K
ν uf
(2)
where ν is the kinematic viscosity, uf the friction velocity and K is a coefficient between 5 (Von Karman) and 10 (Carlier). The friction velocity is calculated by uf = u 21 f (3) where u is the mean cross-flow velocity and f/2 is the friction factor given by the Blasius relationship = 0.040Re−0.25
(4)
Fig. 14. Flux enhancement against ε/e (bentonite suspension; 1 bar transmembrane pressure; 0.5 m/s cross-flow velocity; 0.2 Hz frequency).
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The flux enhancement can be quantified by the ratio between the net flux obtained with backpulsing and the flux observed in conventional filtration. This ratio was an increasing function of the ratio ε/e between the thickness of the backwash penetration layer and the thickness of the laminar layer; a plateau value was reached when backwash water penetrated completely through the laminar layer (ε/e is then 1) (Fig. 14). 4. Conclusions 1. High frequency retrofiltration is effective for suspensions of non-deformable particles such as bentonite. 2. An external skin membrane is not suitable. 3. The technique is less efficient for secondary water or activated sludge; however, it allows flux increasing with a lower energy requirement than high cross-flow velocity. 4. A minimal cross-flow velocity is required to reach high net flux. 5. The retrofiltration flux can be surprisingly high. References [1] S.K. Su, J.C. Liu, R.C. Wiley, Cross-flow microfiltration with gas backwash of apple juice, J. Food Sci. 58 (1993) 638–641. [2] C.S. Parnham, R.H. Davis, Protein recovery from bacterial cell debris using cross-flow microfiltration with backpulsing, J. Membr. Sci. 118 (1996) 259–268. [3] I.G.Wenten, D.M. Koenhen, H.D.W. Roesink, A. Rasmussen, G. Jonsson, Method for removal of components causing turbidity, from a fluid, by means of microfiltration, US Patent 5,560,828 (1996). [4] I.G. Wenten, Mechanisms and control of fouling in crossflow microfiltration, Filtration Sep. 3 (1995) 252–254. [5] V.G.J. Rodgers, R.E. Sparks, Effect of transmembrane pressure pulsing on concentration polarization, J. Membr. Sci. 68 (1992) 149–168.
[6] V.G.J. Rodgers, R.E. Sparks, Effect of solution properties on polarization redevelopments and flux in pressure pulsed ultrafiltration, J. Membr. Sci. 78 (1993) 163–180. [7] C. Wilharm, V.G.J. Rodgers, Significance of duration and amplitude in transmembrane pressure pulsed ultrafiltration of binary protein mixtures, J. Membr. Sci. 121 (1996) 217–228. [8] J.A. Levesley, M. Hoare, The effect of high frequency backflushing on the microfiltration of yeast homogeneate suspensions for the recovery of solute proteins, J. Membr. Sci. 158 (1999) 29–39. [9] M. Héran, S. Elmaleh, Prediction of cross-flow microfiltration through an inorganic tubular membrane with high frequency retrofiltration, Chem. Eng. Sci., submitted for publication. [10] S. Redkar, R.H. Davis, Cross-flow microfiltration with high frequency reverse filtration, AIChE J. 41 (1995) 501– 508. [11] V. Kuberkar, P. Czekaj, R. Davis, Flux enhancement for membrane filtration of bacterial suspensions using high-frequency backpulsing, Biotech. Bioeng. 60 (1998) 77– 87. [12] J.A. Ramirez, R.H. Davis, Application of cross-flow microfiltration with rapid backpulsing to wastewater treatment, J. Hazard. Mater. 63 (1998) 179–197. [13] L. Vera, R. Villarroel, S. Delgado, S. Elmaleh, Can microfiltration of treated wastewater produce suitable water for irrigation? Water Sci. Tech. 38 (1998) 395–403. [14] 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) 3419–3428. [15] E.H. Bouhabila, R. Ben A¨ım, H. Buisson, Microfiltration of activated sludge using submerged membrane with air bubbling (application to wastewater treatment), Desalination 118 (1998) 315. [16] APHA, Standard Methods for the Examination of‘ Water and Wastewater, American Public Health Association, Washington, DC, 1985. [17] S. Elmaleh, W. Naceur, Transport of water through an inorganic composite membrane, J. Membr. Sci. 66 (1992) 227–234. [18] W.F. Jones, R.L. Valentine, V.G.J. Rodgers, Removal of suspended clay from water using transmembrane pressure pulsed microfiltration, J. Membr. Sci. 157 (1999) 199–210. [19] P. Le Goff, Energétique Industrielle, Lavoisier, Paris, 1974.
Microfiltration through an inorganic tubular membrane with high frequency retrofiltration M. Héran∗ , S. Elmaleh UMR 5669, UM II/CNRS/IRD, Hydrosciences, Equipe Génie des Procédés, CC 24, Université Montpellier II, 34095 Montpellier Cedex 5, France Received 28 November 2000; received in revised form 15 January 2001; accepted 17 January 2001
Abstract High frequency backpulsing is a promising technique of flux enhancement that could contribute to the development of cross-flow micro-/ultrafiltration in water and wastewater treatment. A systematic study of the influence of the operational parameters was carried out with three suspensions, bentonite in tap water, biologically treated wastewater and activated sludge. The alumina membranes were tubular (0.02, 0.05 or 0.2 m), with internal or external skin, the latter being not suitable. The technique was particularly efficient for bentonite; a minimal cross-flow velocity was required to reach a net flux independent of the cross-flow. The results are less good for the biological suspensions since the same fluxes could be reached by an increase of cross-flow velocity. However, the energy required by high frequency backpulsing is lower. The average reverse fluxes, measured by a tracer method, are surprisingly high and could hamper the development of the technique. At low Reynolds number (Re = 3500), the net flux increased with the reverse flux, then reached a plateau corresponding to the total penetration of the laminar layer against the membrane wall by the backwash water. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Fouling; Inorganic membrane; Microfiltration; Ultrafiltration; Transient response
1. Introduction Cross-flow microfiltration is a constantly developing technique in many fields, particularly in water and wastewater treatment. However, it is often hampered by mass transport limitation. Backflush is one of the most frequently used techniques of flux enhancement: periodic reverses of transmembrane pressure induce cleaning by the backflow. The deposit and even internal foulants are then lifted off the membrane and swept away by the cross-flow. However, even with very short backflush duration (1–5 s), performed 1–10 ∗ Corresponding author. Tel.: +33-4-67-14-37-23; fax: +33-4-67-54-48-10. E-mail address: [email protected] (M. H´eran).
times per minute, at 1–10 bar differential pressure, backflushing accounts for about 10–20% of the total operation time [1]. Moreover, the reverse flux must be taken into account and net results are then poor, i.e. the net flux can only be increased by 10–30%. However, good results were obtained with such short backwash times as 0.05 s with a period less than 5 s [2,3]. It is thought that the deposit is removed from the membrane wall before it has fully formed and compacted. This technique is called high frequency retrofiltration, high frequency backpulsing or backshock [4]. A wide variety of suspensions and membranes has been tested. Rodgers and Sparks [5] studied backpulsing in 10 g/l BSA ultrafiltration. Fluxes, for 0.05 s backflow at 0.5 s period, increased up to three times
0376-7388/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 6 - 7 3 8 8 ( 0 1 ) 0 0 3 5 1 - 9
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under laminar cross-flow (Re = 8) but the technique was inefficient in the turbulent regime. When the feed concentration was between 10 and 30 g/l, backpulsing was always efficient but at 100 g/l, the flux was no longer enhanced [6]. Wilharm and Rodgers [7] concluded that the flux increase was due to concentration polarisation disruption by motion of the operated flat sheet membrane and not by reverse flow through the membrane. Wenten et al. [3] showed that a reverse asymmetric membrane gave better results. An asymmetric hollow fibre membrane with external skin was particularly convenient in beer or fruit juice filtering. In the case of ceramic tubular membranes, good results were obtained in beer filtration [3] but bad ones in yeast filtration [8]. Moreover, a systematic study of the modelling of high frequency retrofiltration has recently been carried out [9]. The existing models, based on a particle cake deposit [10] removed with a variable efficiency [11], can predict the net flux and the optimal frequency range. However, no model can predict the transient flux responses or the average reverse fluxes, particularly when the particles can interact with the membrane. It was also shown that one experimental response to a backpulse enables predicting the performance of the process and the optimal conditions [9]. In spite of those many works, no systematic study of the influence of all the operational parameters has been carried out. These parameters can be classified in two categories, the variables of cross-flow filtration, such as cross-flow velocity and transmembrane pressure, and the variables of pulsing, such as forward filtration duration or backpressure. This paper intends to carry out a systematic study with three suspensions. Bentonite in tap water provides an example of inorganic and non-compressible particle suspension which does not interact with the membrane and which has been previously tested in high frequency retrofiltration [12]. Considering the increasing importance of tertiary microfiltration [13], it is urgent to evaluate the applicability of the technique to secondary water, a low concentrated biological suspension interacting with the membrane [14]. Finally, considering the development of membrane bioreactors [15], activated sludge will also be tested. The final aim of this work is to provide a method to evaluate the suitability of the technique and to predict the optimal conditions.
2. Experimental 2.1. Experimental set-up The experimental unit, provided with a stirred 100 l feed tank, operated a 40 cm long and 7 mm internal diameter tubular alumina membrane of different rated pore diameters (0.02, 0.05 or 0.2 m) (SCT/US Filter) (Fig. 1). A 0.2 m external skin membrane was also used to establish its suitability. A volumetric pump ensured cross-flow of the retentate through the filtration element (flow-rate monitored with a Fischer Porter rotameter). The permeate was not recycled. Transmembrane pressure, measured with two Air Liquide manometers, was periodically reversed by connecting the permeate circuit with a pressure tank through a solenoid valve Burker, Model W4XUT, W4XUS, opened or closed in 0.03 s by computer control. The computer generated controlled counter pressure waves of short duration (35–999 ms), of 1–3 bar over the forward driving pressure and at a frequency between 0.06 and 10 Hz. The 10 l pressurised tank was filled of filtered water through a 0.2 m US Filter cartridge. The runs were carried out at 25◦ C temperature, maintained constant with a spiral heat exchanger. A high precision balance (0.0001 g), Sartorius BP 210, linked to a computer, measured the flux each 0.1 s enabling to observe the average reverse flux
Fig. 1. Experimental set-up.
M. H´eran, S. Elmaleh / Journal of Membrane Science 188 (2001) 181–188
183
Table 1 Main characteristics of Montpellier sewage plant effluent Parameter
Value
Suspended solid concentration (mg/l) Chemical oxygen demand (mg/l) NTK (mg/l) Orthophosphate concentration (mg/l) Conductivity (S/cm)
20 ± 8 70 ± 20 40 ± 10 7 1500
signals. The membrane was just cleaned prior to each run by a conventional acid/base routine [14]. All the reported data of flux took into account the reverse flow and will be given as net flux. The steady-state was usually obtained after a maximum of 1 h operation. 2.2. Suspensions The following three suspensions were tested: 1. 1 g/l Volclay bentonite in tap water (8 m mean size particle); 2. biologically treated wastewater (effluent of Montpellier sewage plant, Table 1); 3. activated sludge (0.8 g/l suspended solids) sampled from a continuous stirred tank reactor fed with a alcoholic synthetic substrate (C2 H5 OH, N2 H9 PO4 , NH4 NO3 with COD/N/P = 150/10/1) at 0.65 kg COD/kg SS per day loading rate. 2.3. Retrofiltration flux Measuring the mass of the tank containing backwash water has currently monitored average reverse flux [2]. However, this method is not precise since the tank could not be directly installed over the balance. The reverse flux was then measured, as the average over about 10 cycles, by two methods. The first technique consisted of direct collection of the reverse flow on a Sartorius BP 2100 balance (0.1 g precision) in a set-up operated without inlet flow and provided with a capillary, which quantified damping, and a stopper that prevented water contained between the module and the membrane from flowing down by gravity (Fig. 2). This set-up enabled limiting and evaluating the quantity of bypassed liquid that did not vary with the elapsed time.
Fig. 2. Set-up for reverse flux measurement.
The second technique was a tracer method based on conductivity. During these runs, the unit was operated with water of 1500 S/cm conductivity (secondary water conductivity or tap water with added sodium chloride) while the backwashing tank contained de-ionised water (5 S/cm). The permeate volume and its conductivity were sampled and measured every 15 s (beginning of secondary water filtration) or every 5 min (bentonite and end of secondary water filtration). The volume coming from the backwashing tank in the same time was given by (conductivityfeed − conductivitysample ) × volumesample conductivityfeed − conductivitybackpulse which allowed calculating the real reverse flux after subtracting the bypassed water.
2.4. Analyses All the physical and chemical analyses were carried out in accordance with the Standard Methods [16]. Conductivity was given by a WTW conductimeter. A Malvern Mastersizer/E laser granulometer measured the particle size distribution.
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3. Results and discussion 3.1. Net flux, cross-flow velocity and transmembrane pressure As in conventional cross-flow filtration, an initial flux decline was observed then followed by a stationary value (Fig. 3). Contrary to published results that recommend an external skin membrane [3], such a filter was significantly less adapted than an internal active layer membrane, giving fluxes similar to those of conventional cross-flow filtration. Moreover, the external skin membrane was more difficult to regenerate since the particles penetrated into the 3 mm porous structure before reaching the active layer. This situation is different from beer filtration through a hollow fibre membrane of 210 m depth that gave good results [4]. In the case of the bentonite suspension and with a conventional internal skin membrane, high frequency retrofiltration allowed a significant increase of the flux (Fig. 3). The stationary flux (1000 l/h m2 ) could be even larger than with tap water (200 l/h m2 ) but lesser than with low conductivity water (1800 l/h m2 ) [17]; tap water microfiltration raises indeed many problems [17]. Similar results were obtained by Jones et al. [18], with clay in purified water, and Ramirez and Davis [12], with bentonite in tap water. Bentonite forms, like clay, a non-adhesive cake layer on the membrane surface that is easily removed by the reverse flux [18]. The significant flux increase can be explained by diffusion
Fig. 3. Net flux against elapsed time for bentonite suspension (1 bar transmembrane pressure; 2 m/s cross-flow velocity; 2 bar backpressure; 5 s period; 50 ms backwash).
Fig. 4. Responses to high period pulses (2 min) for bentonite suspension (1 bar transmembrane pressure; 1 bar backpressure; 2 m/s cross-flow velocity; 50 ms backwash time; 2 min period).
of particles from the cake surface, erosion of the cake, decompression and subsequent washing away of the cake and a shock wave knocking off part of the cake layer. The interest of retrofiltering such a suspension was evidenced by the forward flux response to a backpressure pulse. After the stationary regime had been reached, pulses were imposed at a larger period. The important flux decrease and recovery are significant arguments in favour of high frequency retrofiltration [9] (Fig. 4). The efficiency of the technique depended on cross-flow velocity. In the case of bentonite, the stationary flux increased with cross-flow velocity then reached a plateau value at a velocity depending on the frequency (Fig. 5). An estimate of the minimum
Fig. 5. Stationary flux against cross-flow velocity at different frequencies (bentonite suspension; 1 bar transmembrane pressure; 2 m/s cross-flow velocity; 1 bar backpressure; 50 ms backwash).
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cross-flow velocity required to sweep the foulants to the filter exit during the backpulse, with conditions of Fig. 5, is 8 m/s, which is much higher than the velocities leading to the plateau flux. In fact, the foulants are periodically re-deposited. However, the existence of a minimal kinetic energy required to reach the maximal flux is an important result that has not yet been reported. There is a domain where the enhancement was particularly high since the flux of conventional filtration was low; the flux was multiplied by 20 at 0.25 m/s cross-flow velocity and 1 Hz frequency, whereas the flux was multiplied by a factor 4–6 when the plateau was reached (Fig. 5). On the other hand, the technique was much less efficient with such active suspensions as secondary water or activated sludge where the flux monotonously increased with cross-flow velocity like in conventional filtration (Fig. 6). A slight flux enhancement was obtained between 1 and 3 m/s cross-flow velocity for secondary water but similar values could also be reached by simple increase of cross-flow velocity without backpulsing. Those deceptive results were predictable considering the flux response to a backpressure pulse where the flux variation was narrow (Fig. 7) [9]. The use of membranes of smaller pores did not give better results and all the following runs were carried out with a 0.2 m membrane (Fig. 8). The filtration of biological suspensions is characterised by a steep decrease from the initial flux and reverse pulsing could not reduce the subsequent foul-
Fig. 6. Stationary flux against cross-flow velocity at different frequencies for active suspensions (1 bar transmembrane pressure; 2 m/s cross-flow velocity; 2 bar backpressure; 0.2 Hz frequency; 50 ms backwash).
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Fig. 7. Responses to high period pulses (200 s) for secondary water (1 bar transmembrane pressure; 1 bar backpressure; 2 m/s cross-flow velocity; 50 ms backwash time; 200 s period).
ing. Still worse results were obtained while filtering yeast and proteins with high frequency pulsing [8]. However, the technique should not be discarded. High frequency retrofiltering requires indeed less energy than high cross-flow velocity (>5 m/s) [18]. In the case of the bentonite suspension, high frequency retrofiltration allowed increasing the critical transmembrane pressure that corresponds to a plateau flux beyond 2 bar; without reverse pulsing, the critical pressure was 0.5 bar (Fig. 9). The overall shape of the flux against transmembrane pressure does not depend on frequency but the graph shows that an optimal frequency should exist at the vicinity of 0.2 Hz.
Fig. 8. Net flux through different membranes against elapsed time for secondary water (1 bar transmembrane pressure; 2 m/s cross-flow velocity; 3 bar backpressure; 5 s period; 50 ms backwash).
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Fig. 9. Net flux against transmembrane pressure in conventional filtration and with retrofiltration at different frequencies for bentonite suspension (2 m/s cross-flow velocity; 2 bar backpressure; 50 ms backwash).
transport difficult in both senses. However, the average reverse flux was always higher than the forward flux, three times for bentonite and twice for secondary water. Could this mean that retrofiltration, as carried out here, is not a simple reverse filtration through a porous structure supposed to have the same resistance in both directions? Moreover, such high reverse fluxes could compromise the economic achievement of the technique. A comparison of experimental and predicted reverse fluxes by models based on cake filtration and incomplete cleaning has been already done [9]. The predictions were always lower than the experimental results, which indicates that, contrarily to forward filtration, an incomplete cleaning does not influence the reverse flux. 3.3. Influence of backpressure and backwash time
Establishing the influence of the retrofiltration operational parameters requires an evaluation of the retroflux. 3.2. Reverse flux The average reverse flux curves, obtained by tracing, could compare with the classical flux against time curves in non-pulsed cross-flow microfiltration (Fig. 10). An initial decrease, steeper for secondary water, was followed by a plateau value; the fouling processes induced by secondary water made the mass
The influence of backpressure on the net flux was negligible when the cross-flow was in turbulent regime (Re = 14 000), which was also observed in protein filtration [7]. In same conditions of flow regime, the advantages of a longer cleaning were counterbalanced by a larger permeate consumption, above all at high frequency, e.g. the net flux decrease is sharper at 1 than at 0.2 Hz (Fig. 11). At hydraulic conditions near of laminar regime (Re = 3500), the influence of backpressure and reverse filtration duration were amplified; the net flux appeared then as an increasing function
Fig. 10. Reverse flux against elapsed time for bentonite suspension and secondary water (1 bar transmembrane pressure; 2 m/s cross-flow velocity; 5 s period; 50 ms backpulse).
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Fig. 11. Influence of reverse filtration time on net flux in turbulent regime (bentonite suspension; 1 bar transmembrane pressure; 2 m/s cross-flow velocity; 1 bar backpressure).
Fig. 13. Net flux against reverse flux (bentonite suspension; 1 bar transmembrane pressure; 0.5 m/s cross-flow velocity; 0.2 Hz frequency).
of both variables (Fig. 12). Backpressure and reverse filtration time determine the reverse flux whose link with net flux was evidenced. Net flux increased with reverse flux then reached a plateau that corresponds to backwash water complete penetration into the laminar layer (Fig. 13). Assuming a uniform reverse filtration, backwash water is distributed, at the end of the pulse, along the internal membrane wall constituting a layer of thickness ε called the backwash penetration layer: 1 4v b ε= d − d2 − (1) 2 πL
On the other hand, the thickness of the laminar layer e is given by [19]
where d is the internal diameter of the filtration element and L its length.
1 2f
Fig. 12. Influence of backpressure and backwash time on net flux in transition regime (bentonite suspension; 1 bar transmembrane pressure; 0.5 m/s cross-flow velocity; 1 bar backpressure; 0.2 Hz frequency).
e=K
ν uf
(2)
where ν is the kinematic viscosity, uf the friction velocity and K is a coefficient between 5 (Von Karman) and 10 (Carlier). The friction velocity is calculated by uf = u 21 f (3) where u is the mean cross-flow velocity and f/2 is the friction factor given by the Blasius relationship = 0.040Re−0.25
(4)
Fig. 14. Flux enhancement against ε/e (bentonite suspension; 1 bar transmembrane pressure; 0.5 m/s cross-flow velocity; 0.2 Hz frequency).
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The flux enhancement can be quantified by the ratio between the net flux obtained with backpulsing and the flux observed in conventional filtration. This ratio was an increasing function of the ratio ε/e between the thickness of the backwash penetration layer and the thickness of the laminar layer; a plateau value was reached when backwash water penetrated completely through the laminar layer (ε/e is then 1) (Fig. 14). 4. Conclusions 1. High frequency retrofiltration is effective for suspensions of non-deformable particles such as bentonite. 2. An external skin membrane is not suitable. 3. The technique is less efficient for secondary water or activated sludge; however, it allows flux increasing with a lower energy requirement than high cross-flow velocity. 4. A minimal cross-flow velocity is required to reach high net flux. 5. The retrofiltration flux can be surprisingly high. References [1] S.K. Su, J.C. Liu, R.C. Wiley, Cross-flow microfiltration with gas backwash of apple juice, J. Food Sci. 58 (1993) 638–641. [2] C.S. Parnham, R.H. Davis, Protein recovery from bacterial cell debris using cross-flow microfiltration with backpulsing, J. Membr. Sci. 118 (1996) 259–268. [3] I.G.Wenten, D.M. Koenhen, H.D.W. Roesink, A. Rasmussen, G. Jonsson, Method for removal of components causing turbidity, from a fluid, by means of microfiltration, US Patent 5,560,828 (1996). [4] I.G. Wenten, Mechanisms and control of fouling in crossflow microfiltration, Filtration Sep. 3 (1995) 252–254. [5] V.G.J. Rodgers, R.E. Sparks, Effect of transmembrane pressure pulsing on concentration polarization, J. Membr. Sci. 68 (1992) 149–168.
[6] V.G.J. Rodgers, R.E. Sparks, Effect of solution properties on polarization redevelopments and flux in pressure pulsed ultrafiltration, J. Membr. Sci. 78 (1993) 163–180. [7] C. Wilharm, V.G.J. Rodgers, Significance of duration and amplitude in transmembrane pressure pulsed ultrafiltration of binary protein mixtures, J. Membr. Sci. 121 (1996) 217–228. [8] J.A. Levesley, M. Hoare, The effect of high frequency backflushing on the microfiltration of yeast homogeneate suspensions for the recovery of solute proteins, J. Membr. Sci. 158 (1999) 29–39. [9] M. Héran, S. Elmaleh, Prediction of cross-flow microfiltration through an inorganic tubular membrane with high frequency retrofiltration, Chem. Eng. Sci., submitted for publication. [10] S. Redkar, R.H. Davis, Cross-flow microfiltration with high frequency reverse filtration, AIChE J. 41 (1995) 501– 508. [11] V. Kuberkar, P. Czekaj, R. Davis, Flux enhancement for membrane filtration of bacterial suspensions using high-frequency backpulsing, Biotech. Bioeng. 60 (1998) 77– 87. [12] J.A. Ramirez, R.H. Davis, Application of cross-flow microfiltration with rapid backpulsing to wastewater treatment, J. Hazard. Mater. 63 (1998) 179–197. [13] L. Vera, R. Villarroel, S. Delgado, S. Elmaleh, Can microfiltration of treated wastewater produce suitable water for irrigation? Water Sci. Tech. 38 (1998) 395–403. [14] 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) 3419–3428. [15] E.H. Bouhabila, R. Ben A¨ım, H. Buisson, Microfiltration of activated sludge using submerged membrane with air bubbling (application to wastewater treatment), Desalination 118 (1998) 315. [16] APHA, Standard Methods for the Examination of‘ Water and Wastewater, American Public Health Association, Washington, DC, 1985. [17] S. Elmaleh, W. Naceur, Transport of water through an inorganic composite membrane, J. Membr. Sci. 66 (1992) 227–234. [18] W.F. Jones, R.L. Valentine, V.G.J. Rodgers, Removal of suspended clay from water using transmembrane pressure pulsed microfiltration, J. Membr. Sci. 157 (1999) 199–210. [19] P. Le Goff, Energétique Industrielle, Lavoisier, Paris, 1974.