- Email: [email protected]
A technique for the study of the fouling of microfiltration membranes using two membranes in series
journal of MEMBRANE SCIENCE ELSEVIER
Journal of Membrane Science 105 (1995) 23-30
A technique for the study of the fouling of microfiltration membranes using two membranes in series Yazhen Xu-Jiang a, John Dodds ~'*, Dominique Leclerc a, Michel Lenoel b a Laboratoire des Sciences du GEnie Chimique, CNRS-ENSC-Nancy 1, B.P. 451, 1 rue Grandville, 54001 Nancy, France b Centre de Recherche, Tepral-Kronenbourg, Rue Jacob, 67000 Strasbourg, France
Received 5 July 1994; accepted 21 December 1994
Abstract This paper describes a technique for investigating the fouling of microfiltration membranes by measuring the pressure drops across two membranes fed in series by a constant rate pump, which enables a distinction to be made between surface fouling and internal fouling of the membrane. In the case of the microfiltration of BSA solutions, the technique shows how the type of pump and the operating temperature influence membrane fouling and how protein denaturation and adsorption give rise to different types of fouling. The technique is also used to investigate the microfiltration of beer and shows how this is affected by membrane properties and the aging of beer. Keywords: Microfiltration;Fouling; Two membranes in series
1. Introduction Microfiltration is a technique widely used in biotechnology for clarifying liquids containing macromolmecules and proteins. Examples are in cell reconvery from fermentation broths, sterilizating filtration of protein solutions, clarifying beverages, etc. In all these processes the macromolecules and proteins involved are much smaller in size than the pores of the microfiltration membrane and should not normally be retained by the membranes. Even so, microfiltration membranes can have their permeability drastically reduced by fouling and the success or failure of a process can be dependant on membrane-protein interactions. Fouling can take several forms in particular; deposition of denatured or agglomerated proteins at the surface of the * Corresponding author. 0376-7388/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSD10376-7388(94)00323-8
membrane, or adsorption of proteins inside the pore structure of the membranes. Many authors have studied the fundamental mechanisms involved in membrane fouling by protein suspensions. These may be grouped as follows. - The formation of a gel layer due to concentration polarisation [ 1 ]. - Adsorption of species on the membrane surface and inside the pore structure [2]. - Deposition and pore blocking after the formation of protein aggregates due to denaturation [ 3 ]. Two techniques are currently used to characterise membrane fouling. Measurements of the flux decline at constant pressure, and the measurement of the pressure increase in constant flow rate permeation. Most studies of fouling have been concerned with ultrafiltration membranes [4] but from a technological point of view the fouling of microfiltration membranes by pro-
24
Y. Xu-Jiang et al. / Journal of Membrane Science 105 (1995) 23-30
tein adsorption can be more important [5 ], though very few investigations have been devoted to protein interactions with microfiltration membranes. Hlavacek and Bouchet [6] studied dead-end filtration o f a BSA solution on microfiltration membranes and quantified fouling by means of the constant rate filtration equation. However, the different roles of adsorption, gel formation and the deposition of aggregates was not clear. In addition protein denaturation by shear in the piston pump used in their experiments cannot be excluded. The aim here is to extend this latter work by examining some of the ways in which proteins cause fouling of microfiltration membrane. A new technique has been developed in which constant rate dead-end filtration experiments are performed using an apparatus having two filter cells in series. If fouling is caused by protein adsorption inside the pore structure of the membrane then, as both membranes 'see' the same solution, the pressure drops across the two filter cells should increase together. On the other hand, if fouling is due to surface deposits of protein aggregates then the pressure drop over the first cell in the series should increase whilst that across the second cell, which 'sees' only filtered solution, should remain constant. In the first part of this investigation the technique has been used with solutions of bovine serum albumin (BSA), taken as a model protein. The effect of temperature and the type of pump have been examined. In the second part, the technique is used to study fouling in beer filtration and in particular, to examine the influence of the type of beer and the type of membrane.
1 ml of a 1 g/1 solution sodium azide to prevent bacteriological contamination. The resulting solution was finally completed to 1 I and the pH adjusted with NaoH or HC1 to the required value. Immediately before the experiment, the solution to be used was again pre-filtered at low pressure (0.1 bar) on an 0.2/zm Nuclepore membrane to eliminate any non-dissolved BSA. The membranes used in these experiments was a hydrophilic polycarbonate Nuclepore membrane, pore size 0.2 /zm having a porosity of about 10%. The BSA molecule has a dimension of about 10 nm, that is about 20 times smaller than the pores of the membrane. The second series of experiments were made with beer. To eliminate the problem of surface deposits of 'large' particles, an 0.8/zm pore size membrane was used and brewery filtered beer was provided by the Kronenbourg Brewery at Champigneulles, France. The first experiments were performed in the week in which the beer was provided. This is called 'fresh filtered beer'. The experiments were repeated after about 10 days storage at 0°C to test if there was a change in the beer with time. This is called 'old filtered beer'. All other experimental conditions were identical. Three types of PVDF [poly(difluorure vinylidine) ] membrane of pore size 0.8/xm were used. These were provided by Techsep, Miribel, France and were either untreated (PVDF, hydrophobic, negatively charged) or treated with poly(ethylenimine) (PEI, hydrophilic, positive charge) or poly(vinylic alcohol) (PVA, hydrophilic, neutral charge). 2.2. Experimental set-up
2. Materials and methods 2.1. Solutions and m e m b r a n e s
In the first series of experiments bovine serum albumin (BSA) (Sigma, fraction V, 98-99% albumin, No. A-7906) was used as a model protein. Solutions were prepared with water (freshly prepared for each experiment with a Milli-Q apparatus with a resistivity of 18 megohm c m - 1 and filtered on a 0.22/xm membrane) by adding 1 g of BSA to a litre flask containing ca. 500 ml of water followed by gentle stirring with a magnetic agitator to disolve the albumin. After dissolution was complete, a previously filtered strong solution of NaCI was added to obtain the required ionic strength, as was
The experimental set-up is shown in Fig. 1 and comprises three parts. - The feed section: the solution is filtered in situ in a closed circuit using a constant flow rate pump (PD), which was either a Gilson 302 piston pump, an Ismatec IPN or a Masterflex peristaltic pump. The filtered solution was fed into a glass reservoir (R) maintained at the required temperature in a thermostatic bath (T). - The filter section: two Sartorius stainless steel filter cells (SM 16278) are used in series (MF1 and MF2). In the first stage of the experiment the solution to be filtered passes first through cell 1 and then through cell 2 (R-PD-VT1-VT2-MF1-VT3-MF2VT4-R). In the second stage the solution passes in
Y. Xu-Jiang et al. / Journal of Membrane Science 105 (1995) 23-30
the reverse order (R-PD-VT1-VT3-MF2-VT2MFI-VT4-R). This change in order is made by means of four stainless steel 3-way valves (VTI, VT2, VT3 and VT4). Measurements and data acquisition: two pressure transducers (P1 and P2) (Synsym S C × 3 0 D N ) were located before the filter cell and signals fed to a data acquisition card in an IBM PC to give continuous recording of the pressure drop across the two filter cells.
0,20 -
L
A P2 (barl
0,15
A
0,10
A
A
0,05
0,00
VT2
25
|
I
|
t (min) VTI
Fig. 3. Change in pressure drop with time in filter cell 2 for two differenttemperatures. 3. Fouling of microfiltration membranes by BSA
In this series of experiments the solutions had an ionic strength of 0.15 M NaCI, a pH of 5.9 and the flow rate was 10 ml/min
VT4
3.1. Effect of the temperature ~2
Fig. 1. Experimental set-up. 1,0 "
A P1 (barl
0,8-
0,50,4,
0,2-
0,0 "g 50
i
i
100
150
t (min) Fig. 2. Change in pressure drop with time in filter cell 1 for two different temperatures.
It is known that protein solutions undergo irreversible denaturation above a certain threshold temperature. Meireles et al. [3] have already observed that, in the ultrafiltration of BSA, polymers and aggregates tend to form at temperatures above 8°C. This tendency was found to be further enhanced above 22°C thus confirming the need for temperature control in handling protein solutions. To examine the effect of temperature, filtration has been performed in the direction of filter cell 1 to filter cell 2 at both ambient temperature (20°C) and at 6°C. Fig. 2 shows that the pressure drop across the first filter cell increases more rapidly at ambient temperature (20°C) than at 6°C. At the same time, after a short initial period, the pressure drops across the second cell of the series (Fig. 3) remain constant (the difference in the absolute values of Ap at the two temperatures can be attributed to the increase in viscosity at low temperature). These results indicate that, as expected, the higher temperature favors the denaturation an aggregation of the BSA resulting in a deposit in the first filter cell and no effect in the second.
26 2,0
1,5
Y. Xu-Jiang et al. /Journal of Membrane Science 105 (1995) 23-30
A P1 (barl
S
,m peristaltic piston
1,0
MF2
to M F I
MF1 to MF2 /
>
0,5 '
0,0
|
100
•
v
200
•
i
300
•
i
400
t (min) Fig. 4. Change in pressure drop with time in the first filter cell: Comparisonof the type of pump. 3.2. Effect o f the type o f pump
The piston pump subjects the solution passing through it to a strong shear force which may be suspected of favoring protein denaturation. Comparison was therefore made with a peristaltic pump having a more gentle action. Experiments were performed at 6°C by filtering first one the direction (MF1 to MF2) and then by changing the direction of filtration (MF2 to MF1 ) by means of the valve system. The results, given in Figs. 4 and 5, show that the changes in pressure drop across the two filter cells are very different when the peristaltic pump is used from when the piston pump is used. When the piston pump is used in the circuit the pressure drop in the first cell in the series (Fig. 4, direction MF 1 to MF2) increases quasi-exponentially whilst the pressure drop in the second cell in the series (Fig. 5, direction MF1 to MF2) remains constant. When the order of the cells is reversed, the pressure drop in filter cell 2, now the first in the sequence (Fig. 5, direction MF2 to MF1), rises in a quasi-exponential fashion whilst the pressure drop in filter cell 1, the second in order (Fig. 4, direction MF2 to MF1 ) rises only linearly. With the peristaltic pump the changes in pressure drop are relatively insignificant. This suggests that aggregates are formed by denaturation by the piston pump and are filterd by the first membrane in the series. In the direction MF1 to MF2 the aggregates are stopped by filter cell 1 and AP1 increases very rapidly whilst AP2 remains roughly constant. When the order of the
filter cells is reversed to give the direction MF2 to MF1 the pressure drops of the two filter cells increase together. In this case it would be expected that the aggregates be stopped by MF2, the first filter in the series. The fact AP2 now increases rapidly whilst AP1 also continues to rise, though no longer exponentially, can be interpreted as being either due to filtration by the deposit at the surface of the membrane, or as a synergetic adsorption of BSA by the protein aggregates. That is the existance of protein aggregates at the membrane favourises protein adsorption or filtration. With the peristaltic pump the pressure drops across the two membranes are similar and only increase slightly. This small effect may be attributed essentially to a reduction in membrane permeability by protein adsorption. As all other condition are the same, the differences found in experiments performed with the piston pump and the peristaltic pump it may be attributed to the denaturation of the BSA by shear in the head and valves of the piston pump. 3.3. SEM observation o f the membranes after filtration
After the experiments described above, the membranes were examined with a scanning electron microscope to try to confirm the hypothesis of BSA denaturation by the piston pump. Fig. 6a shows a Nuclepore membrane after constant rate filtration using the piston pump. It can be seen that there is a thick, dense deposit on the membrane. Fig. 6b shows a Nucle-
0,5 -
A P2 (barl
o,4,
o
'
peristaltic I
1. piston
,)'
[ /
0,3-
0,1 ~ 0,0
~ •
~ ,
100
~ -
~ B
200
~ ~1 •
~
> •
,
300 400 t (min)
Fig. 5. Change in pressuredrop with time in the second filter cell: Comparisonof the type of pump.
Y. Xu-Jiang et aL /Journal of Membrane Science 105 (1995) 23-30
27
Fig. 6. Observation by SEM of the membranes atter filtration of BSA solutions: (a) MF1 with the piston pump (Nuclepore 0.2 gm); (b) MF1 with the peristaltic pump (Nuclepore 0.2/zm).
pore membrane after constant rate filtration using the peristaltic pump. It can be seen that the pores of the membrane are still visible and that there is only a slight deposit, perhaps due to cristallization of NaCI during storage prior to examination. Despite the inevitable questions raised by the need to dry membranes for
observation with a scanning electron microscope, these figures do tend to confirm the conclusions from the experiments. The BSA is more denatured by the piston pump than by the peristaltic pump. The relative absence of deposit, in the case where the peristaltic pump was used, indicates that the increase in pressure drop during
28
Y. Xu-Jiang et al. /Journal of Membrane Science 105 (1995) 23-30
filtration may be attributed to protein adsorption in the pores.
2,0
3.4. Conclusion from the first series of experiments using BSA
1,5
The experiments show that, at room temperature and with a pump giving high shear conditions, BSA is susceptible to denaturation causing rapid rise in pressure drop in constant rate filtration. This obviously indicates that filtration should be performed under conditions were denaturation is minimal. In such conditions membrane fouling will then be essentially due to protein adsorption, even so these experiments, over some 7 h, show that this may not be too important.
o,5.1'° ~
A P (bar)
,
,
o
l
d filtered , ,beer
o,o
i
0
30
60
90
120 t
150
(rain)
Fig. 8. Change in the pressure drop of the two filter cells with 'old
filteredbeer' (PVDFmembrane)
4. Membrane fouling in beer microfiltration In this series of experiments the temperature was maintained at 0°C, the whole circuit was maintained under a pressure of CO2 of 1.5 bar and the flow rate of 13 ml/min was provided by a Masterflex peristaltic pump
4.1. Effect of ageing the beer Figs. 7 and 8 show the changes in the pressure drop with time of the two filter cells in series, fitted with 0.8 /zm PVDF membranes, when 'fresh' and 'old' filtered
2,0- AP (bar) "fresh"filteredbeer 1,5. 1,0'
~
0,5~ 0,0
'
0
30
60
90
120
150
t (min) Fig.7.Changeinthepressuredropofthetwofiltercellswith'fresh filteredbeer' (PVDFmembrane)
beer is passed through them. The filtered beer should not contain particles greater than 0.8/zm, but nevertheless it can be seen that there is a rise in pressure drop during filtration and that the 'old' beer gives a different response from that of the 'fresh' beer. With the 'fresh' beer the pressure drops over the two filter cells are similar and the reduction in membrane premeability can be mainly attributed to protein adsorption. With the 'old' filtered beer the pressure drop in the first filter cell rises rapidly whilst that in the second filter cell remains low thus indicating pore blocking in the first membrane by particle aggregates formed during stockage. It should however be mentioned that the conditions in the laboratory are not ideal and a microbial growth in the the beer can not be excluded. No significant change in the distribution of the size of particles in the beer was detected with a HIAC/ Royco model 346 particle counter. However this is not unexpected as the statistical significance of a few large aggregates in an essentially 'clean' solution is difficult to establish. On the other hand, the two membrane technique used here, involving the passage of a large volume of solution through the two membranes, does allow confidence as a means of detection. The method may therefore be considered as a good way of characterising small changes in the degree of aggregation of a solution.
4.2. Effect of the type of membrane Figs. 9 and 10 show the changes in the pressure drop of the first and second filter cells during the filtration
Y. Xu-Jiang et al. / Journal of Membrane Science 105 (1995) 23-30
1,0
A P1 (barl •
0,8
[]
0,6 x
0,4 ' [] PVDF PEI
Op2 0,0
*
•
!
i
30
}
|
60
i
i
90
150
120 t (min)
Fig. 9. Changein pressure dropin the first filtercell: Comparisonof three types of membrane.
1,0
A I'2 (bar)
0,8
0,6 0,4 0,2'
,
e
,
/~,,~"
0,0 ,v 0
~
" x
, 30
, 60
, 120
' 150
t (min) Fig. 10. Changein pressure dropin the secondfiltercell:Comparison of three types of membrane. Table 1 Forces actingbetweenthe membranesand the beer Membrane
PVDF ( - ) PEI ( + ) PVA (0)
module in series, the membrane treated with PVA has the lowest increase in pressure drop. Independant measurements by a streaming current technique, reported elsewhere [7], show that the untreated PVDF membrane has a small negative charge, the membrane treated with PEI has a low positive charge and that the membrane treated with PVA is neutral. Measurement of the electrophoretic mobility of the particles in beer using a Rank Mark II apparatus show that they have an overall negative charge with a low zeta potential of the order of - 3 to - 5 mV. In addition, the untreated PVDF membrane is hydrophobic and the membranes treated with PEI and PVA are hydrophilic. Table 1 gives an overall view of these different factors. Assuming that beer-membrane interactions can be limited to only the electrical interactions between the particles in the beer and surface of the membrane, and to the effects of the hydrophobic nature of the membrane, these experiments indicate that. There is opposition between the two effects in the case of the PVDF and PEI treated membranes. That there is an overall repulsion in the case of the PVA treated membrane. The results observed in the experiments, where the PVA treated membrane has a lower pressure drop than the other two, is in agreement with this reasoning.
4.3. Conclusion from the second series of experiments using beer
PEI PVA
, 90
29
Forces Electrostatic
Hydrophobicity
Overall
repulsive attractive neutral
attractive repulsive repulsive
opposition opposition repulsion
of 'fresh' beer for three different types of membrane. It can be seen that for both the first and the second
These experiments show that the membrane permeability is reduced during microfiltration of pre-filtered beer. When the beer is fresh the increase in pressure drop may be principally attributed to adsorption in the pores of the membrane of species present in the beer. After a period of storage, even though the beer had been filtered, there occurs an agglomeration of the particles in the beer which is filtered out causing a deposit on the first membrane in the series and leaving the second membrane only subject to adsorption in the pores. Furthermore, the general behaviour of the beermembrane pair is in agreement with an analysis based on beer-membrane interactions due to electrical charge and the hydrophobicity of the membrane. Adsorption plays an important role in membrane fouling as the retention efficiency and fouling may be taken to depend not only on the diameter of the pores but also on the
30
Y. Xu-Jiang et al. /Journal of Membrane Science 105 (1995) 23-30
adsorption capacity o f the membrane. These are obviously important considerations in the choice of membrane for a given service.
6. Symbols used Ap~ pressure drop in first filter cell (Pa) AP2 pressure drop in second filter cell (Pa) t time of filtration (s)
5. General conclusions The technique of using two membranes in series has been shown to provide a way of distinguishing between external and internal fouling of microfiltration membranes. The method has been used to demonstrate that fouling of microfiltration membranes by protein solutions depends on the way the solution is handled and also on the properties of the solution and the membrane. This indicates that modifications o f the surface properties of the membrane may be useful in promoting repusive conditions and reducing fouling. When applied to beer filtration, the technique has been used to confirm the influence o f the surface properties o f membranes on fouling during filtration. In addition the technique demonstrates that particles present in beer can agglomerate during storage.
References [ 1] D.R. Trettin and M.R. Doshi, Chem. Eng. Commun., 1 (1979) 507-522. [2] P. Aimar, S. Baklouti and V. Sanchez, J. Membrane Sci., 29 (1986) 207-224. [ 3 ] M. Meireles, P. Aimar and V. Sanchez, Biotechnol. Bioeng., 38 (1991) 528-534. [4] W.M. Clark, A. Banal, M. Sontakkeand Y.H. Ma, J. Membrane Sci., 55 (1991) 21-38. [5] W.R. Bowen and Q. Gan, J. Colloid Interface Sci., 144 ( 1991 ) 254-262. [ 6 ] M. Hlavacekand F. Bouchet,J. MembraneSci., 82 (1993) 285295. [7] Y. Xu-Jiang, Ph.D. thesis, INPL Nancy, 1994.
Journal of Membrane Science 105 (1995) 23-30
A technique for the study of the fouling of microfiltration membranes using two membranes in series Yazhen Xu-Jiang a, John Dodds ~'*, Dominique Leclerc a, Michel Lenoel b a Laboratoire des Sciences du GEnie Chimique, CNRS-ENSC-Nancy 1, B.P. 451, 1 rue Grandville, 54001 Nancy, France b Centre de Recherche, Tepral-Kronenbourg, Rue Jacob, 67000 Strasbourg, France
Received 5 July 1994; accepted 21 December 1994
Abstract This paper describes a technique for investigating the fouling of microfiltration membranes by measuring the pressure drops across two membranes fed in series by a constant rate pump, which enables a distinction to be made between surface fouling and internal fouling of the membrane. In the case of the microfiltration of BSA solutions, the technique shows how the type of pump and the operating temperature influence membrane fouling and how protein denaturation and adsorption give rise to different types of fouling. The technique is also used to investigate the microfiltration of beer and shows how this is affected by membrane properties and the aging of beer. Keywords: Microfiltration;Fouling; Two membranes in series
1. Introduction Microfiltration is a technique widely used in biotechnology for clarifying liquids containing macromolmecules and proteins. Examples are in cell reconvery from fermentation broths, sterilizating filtration of protein solutions, clarifying beverages, etc. In all these processes the macromolecules and proteins involved are much smaller in size than the pores of the microfiltration membrane and should not normally be retained by the membranes. Even so, microfiltration membranes can have their permeability drastically reduced by fouling and the success or failure of a process can be dependant on membrane-protein interactions. Fouling can take several forms in particular; deposition of denatured or agglomerated proteins at the surface of the * Corresponding author. 0376-7388/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSD10376-7388(94)00323-8
membrane, or adsorption of proteins inside the pore structure of the membranes. Many authors have studied the fundamental mechanisms involved in membrane fouling by protein suspensions. These may be grouped as follows. - The formation of a gel layer due to concentration polarisation [ 1 ]. - Adsorption of species on the membrane surface and inside the pore structure [2]. - Deposition and pore blocking after the formation of protein aggregates due to denaturation [ 3 ]. Two techniques are currently used to characterise membrane fouling. Measurements of the flux decline at constant pressure, and the measurement of the pressure increase in constant flow rate permeation. Most studies of fouling have been concerned with ultrafiltration membranes [4] but from a technological point of view the fouling of microfiltration membranes by pro-
24
Y. Xu-Jiang et al. / Journal of Membrane Science 105 (1995) 23-30
tein adsorption can be more important [5 ], though very few investigations have been devoted to protein interactions with microfiltration membranes. Hlavacek and Bouchet [6] studied dead-end filtration o f a BSA solution on microfiltration membranes and quantified fouling by means of the constant rate filtration equation. However, the different roles of adsorption, gel formation and the deposition of aggregates was not clear. In addition protein denaturation by shear in the piston pump used in their experiments cannot be excluded. The aim here is to extend this latter work by examining some of the ways in which proteins cause fouling of microfiltration membrane. A new technique has been developed in which constant rate dead-end filtration experiments are performed using an apparatus having two filter cells in series. If fouling is caused by protein adsorption inside the pore structure of the membrane then, as both membranes 'see' the same solution, the pressure drops across the two filter cells should increase together. On the other hand, if fouling is due to surface deposits of protein aggregates then the pressure drop over the first cell in the series should increase whilst that across the second cell, which 'sees' only filtered solution, should remain constant. In the first part of this investigation the technique has been used with solutions of bovine serum albumin (BSA), taken as a model protein. The effect of temperature and the type of pump have been examined. In the second part, the technique is used to study fouling in beer filtration and in particular, to examine the influence of the type of beer and the type of membrane.
1 ml of a 1 g/1 solution sodium azide to prevent bacteriological contamination. The resulting solution was finally completed to 1 I and the pH adjusted with NaoH or HC1 to the required value. Immediately before the experiment, the solution to be used was again pre-filtered at low pressure (0.1 bar) on an 0.2/zm Nuclepore membrane to eliminate any non-dissolved BSA. The membranes used in these experiments was a hydrophilic polycarbonate Nuclepore membrane, pore size 0.2 /zm having a porosity of about 10%. The BSA molecule has a dimension of about 10 nm, that is about 20 times smaller than the pores of the membrane. The second series of experiments were made with beer. To eliminate the problem of surface deposits of 'large' particles, an 0.8/zm pore size membrane was used and brewery filtered beer was provided by the Kronenbourg Brewery at Champigneulles, France. The first experiments were performed in the week in which the beer was provided. This is called 'fresh filtered beer'. The experiments were repeated after about 10 days storage at 0°C to test if there was a change in the beer with time. This is called 'old filtered beer'. All other experimental conditions were identical. Three types of PVDF [poly(difluorure vinylidine) ] membrane of pore size 0.8/xm were used. These were provided by Techsep, Miribel, France and were either untreated (PVDF, hydrophobic, negatively charged) or treated with poly(ethylenimine) (PEI, hydrophilic, positive charge) or poly(vinylic alcohol) (PVA, hydrophilic, neutral charge). 2.2. Experimental set-up
2. Materials and methods 2.1. Solutions and m e m b r a n e s
In the first series of experiments bovine serum albumin (BSA) (Sigma, fraction V, 98-99% albumin, No. A-7906) was used as a model protein. Solutions were prepared with water (freshly prepared for each experiment with a Milli-Q apparatus with a resistivity of 18 megohm c m - 1 and filtered on a 0.22/xm membrane) by adding 1 g of BSA to a litre flask containing ca. 500 ml of water followed by gentle stirring with a magnetic agitator to disolve the albumin. After dissolution was complete, a previously filtered strong solution of NaCI was added to obtain the required ionic strength, as was
The experimental set-up is shown in Fig. 1 and comprises three parts. - The feed section: the solution is filtered in situ in a closed circuit using a constant flow rate pump (PD), which was either a Gilson 302 piston pump, an Ismatec IPN or a Masterflex peristaltic pump. The filtered solution was fed into a glass reservoir (R) maintained at the required temperature in a thermostatic bath (T). - The filter section: two Sartorius stainless steel filter cells (SM 16278) are used in series (MF1 and MF2). In the first stage of the experiment the solution to be filtered passes first through cell 1 and then through cell 2 (R-PD-VT1-VT2-MF1-VT3-MF2VT4-R). In the second stage the solution passes in
Y. Xu-Jiang et al. / Journal of Membrane Science 105 (1995) 23-30
the reverse order (R-PD-VT1-VT3-MF2-VT2MFI-VT4-R). This change in order is made by means of four stainless steel 3-way valves (VTI, VT2, VT3 and VT4). Measurements and data acquisition: two pressure transducers (P1 and P2) (Synsym S C × 3 0 D N ) were located before the filter cell and signals fed to a data acquisition card in an IBM PC to give continuous recording of the pressure drop across the two filter cells.
0,20 -
L
A P2 (barl
0,15
A
0,10
A
A
0,05
0,00
VT2
25
|
I
|
t (min) VTI
Fig. 3. Change in pressure drop with time in filter cell 2 for two differenttemperatures. 3. Fouling of microfiltration membranes by BSA
In this series of experiments the solutions had an ionic strength of 0.15 M NaCI, a pH of 5.9 and the flow rate was 10 ml/min
VT4
3.1. Effect of the temperature ~2
Fig. 1. Experimental set-up. 1,0 "
A P1 (barl
0,8-
0,50,4,
0,2-
0,0 "g 50
i
i
100
150
t (min) Fig. 2. Change in pressure drop with time in filter cell 1 for two different temperatures.
It is known that protein solutions undergo irreversible denaturation above a certain threshold temperature. Meireles et al. [3] have already observed that, in the ultrafiltration of BSA, polymers and aggregates tend to form at temperatures above 8°C. This tendency was found to be further enhanced above 22°C thus confirming the need for temperature control in handling protein solutions. To examine the effect of temperature, filtration has been performed in the direction of filter cell 1 to filter cell 2 at both ambient temperature (20°C) and at 6°C. Fig. 2 shows that the pressure drop across the first filter cell increases more rapidly at ambient temperature (20°C) than at 6°C. At the same time, after a short initial period, the pressure drops across the second cell of the series (Fig. 3) remain constant (the difference in the absolute values of Ap at the two temperatures can be attributed to the increase in viscosity at low temperature). These results indicate that, as expected, the higher temperature favors the denaturation an aggregation of the BSA resulting in a deposit in the first filter cell and no effect in the second.
26 2,0
1,5
Y. Xu-Jiang et al. /Journal of Membrane Science 105 (1995) 23-30
A P1 (barl
S
,m peristaltic piston
1,0
MF2
to M F I
MF1 to MF2 /
>
0,5 '
0,0
|
100
•
v
200
•
i
300
•
i
400
t (min) Fig. 4. Change in pressure drop with time in the first filter cell: Comparisonof the type of pump. 3.2. Effect o f the type o f pump
The piston pump subjects the solution passing through it to a strong shear force which may be suspected of favoring protein denaturation. Comparison was therefore made with a peristaltic pump having a more gentle action. Experiments were performed at 6°C by filtering first one the direction (MF1 to MF2) and then by changing the direction of filtration (MF2 to MF1 ) by means of the valve system. The results, given in Figs. 4 and 5, show that the changes in pressure drop across the two filter cells are very different when the peristaltic pump is used from when the piston pump is used. When the piston pump is used in the circuit the pressure drop in the first cell in the series (Fig. 4, direction MF 1 to MF2) increases quasi-exponentially whilst the pressure drop in the second cell in the series (Fig. 5, direction MF1 to MF2) remains constant. When the order of the cells is reversed, the pressure drop in filter cell 2, now the first in the sequence (Fig. 5, direction MF2 to MF1), rises in a quasi-exponential fashion whilst the pressure drop in filter cell 1, the second in order (Fig. 4, direction MF2 to MF1 ) rises only linearly. With the peristaltic pump the changes in pressure drop are relatively insignificant. This suggests that aggregates are formed by denaturation by the piston pump and are filterd by the first membrane in the series. In the direction MF1 to MF2 the aggregates are stopped by filter cell 1 and AP1 increases very rapidly whilst AP2 remains roughly constant. When the order of the
filter cells is reversed to give the direction MF2 to MF1 the pressure drops of the two filter cells increase together. In this case it would be expected that the aggregates be stopped by MF2, the first filter in the series. The fact AP2 now increases rapidly whilst AP1 also continues to rise, though no longer exponentially, can be interpreted as being either due to filtration by the deposit at the surface of the membrane, or as a synergetic adsorption of BSA by the protein aggregates. That is the existance of protein aggregates at the membrane favourises protein adsorption or filtration. With the peristaltic pump the pressure drops across the two membranes are similar and only increase slightly. This small effect may be attributed essentially to a reduction in membrane permeability by protein adsorption. As all other condition are the same, the differences found in experiments performed with the piston pump and the peristaltic pump it may be attributed to the denaturation of the BSA by shear in the head and valves of the piston pump. 3.3. SEM observation o f the membranes after filtration
After the experiments described above, the membranes were examined with a scanning electron microscope to try to confirm the hypothesis of BSA denaturation by the piston pump. Fig. 6a shows a Nuclepore membrane after constant rate filtration using the piston pump. It can be seen that there is a thick, dense deposit on the membrane. Fig. 6b shows a Nucle-
0,5 -
A P2 (barl
o,4,
o
'
peristaltic I
1. piston
,)'
[ /
0,3-
0,1 ~ 0,0
~ •
~ ,
100
~ -
~ B
200
~ ~1 •
~
> •
,
300 400 t (min)
Fig. 5. Change in pressuredrop with time in the second filter cell: Comparisonof the type of pump.
Y. Xu-Jiang et aL /Journal of Membrane Science 105 (1995) 23-30
27
Fig. 6. Observation by SEM of the membranes atter filtration of BSA solutions: (a) MF1 with the piston pump (Nuclepore 0.2 gm); (b) MF1 with the peristaltic pump (Nuclepore 0.2/zm).
pore membrane after constant rate filtration using the peristaltic pump. It can be seen that the pores of the membrane are still visible and that there is only a slight deposit, perhaps due to cristallization of NaCI during storage prior to examination. Despite the inevitable questions raised by the need to dry membranes for
observation with a scanning electron microscope, these figures do tend to confirm the conclusions from the experiments. The BSA is more denatured by the piston pump than by the peristaltic pump. The relative absence of deposit, in the case where the peristaltic pump was used, indicates that the increase in pressure drop during
28
Y. Xu-Jiang et al. /Journal of Membrane Science 105 (1995) 23-30
filtration may be attributed to protein adsorption in the pores.
2,0
3.4. Conclusion from the first series of experiments using BSA
1,5
The experiments show that, at room temperature and with a pump giving high shear conditions, BSA is susceptible to denaturation causing rapid rise in pressure drop in constant rate filtration. This obviously indicates that filtration should be performed under conditions were denaturation is minimal. In such conditions membrane fouling will then be essentially due to protein adsorption, even so these experiments, over some 7 h, show that this may not be too important.
o,5.1'° ~
A P (bar)
,
,
o
l
d filtered , ,beer
o,o
i
0
30
60
90
120 t
150
(rain)
Fig. 8. Change in the pressure drop of the two filter cells with 'old
filteredbeer' (PVDFmembrane)
4. Membrane fouling in beer microfiltration In this series of experiments the temperature was maintained at 0°C, the whole circuit was maintained under a pressure of CO2 of 1.5 bar and the flow rate of 13 ml/min was provided by a Masterflex peristaltic pump
4.1. Effect of ageing the beer Figs. 7 and 8 show the changes in the pressure drop with time of the two filter cells in series, fitted with 0.8 /zm PVDF membranes, when 'fresh' and 'old' filtered
2,0- AP (bar) "fresh"filteredbeer 1,5. 1,0'
~
0,5~ 0,0
'
0
30
60
90
120
150
t (min) Fig.7.Changeinthepressuredropofthetwofiltercellswith'fresh filteredbeer' (PVDFmembrane)
beer is passed through them. The filtered beer should not contain particles greater than 0.8/zm, but nevertheless it can be seen that there is a rise in pressure drop during filtration and that the 'old' beer gives a different response from that of the 'fresh' beer. With the 'fresh' beer the pressure drops over the two filter cells are similar and the reduction in membrane premeability can be mainly attributed to protein adsorption. With the 'old' filtered beer the pressure drop in the first filter cell rises rapidly whilst that in the second filter cell remains low thus indicating pore blocking in the first membrane by particle aggregates formed during stockage. It should however be mentioned that the conditions in the laboratory are not ideal and a microbial growth in the the beer can not be excluded. No significant change in the distribution of the size of particles in the beer was detected with a HIAC/ Royco model 346 particle counter. However this is not unexpected as the statistical significance of a few large aggregates in an essentially 'clean' solution is difficult to establish. On the other hand, the two membrane technique used here, involving the passage of a large volume of solution through the two membranes, does allow confidence as a means of detection. The method may therefore be considered as a good way of characterising small changes in the degree of aggregation of a solution.
4.2. Effect of the type of membrane Figs. 9 and 10 show the changes in the pressure drop of the first and second filter cells during the filtration
Y. Xu-Jiang et al. / Journal of Membrane Science 105 (1995) 23-30
1,0
A P1 (barl •
0,8
[]
0,6 x
0,4 ' [] PVDF PEI
Op2 0,0
*
•
!
i
30
}
|
60
i
i
90
150
120 t (min)
Fig. 9. Changein pressure dropin the first filtercell: Comparisonof three types of membrane.
1,0
A I'2 (bar)
0,8
0,6 0,4 0,2'
,
e
,
/~,,~"
0,0 ,v 0
~
" x
, 30
, 60
, 120
' 150
t (min) Fig. 10. Changein pressure dropin the secondfiltercell:Comparison of three types of membrane. Table 1 Forces actingbetweenthe membranesand the beer Membrane
PVDF ( - ) PEI ( + ) PVA (0)
module in series, the membrane treated with PVA has the lowest increase in pressure drop. Independant measurements by a streaming current technique, reported elsewhere [7], show that the untreated PVDF membrane has a small negative charge, the membrane treated with PEI has a low positive charge and that the membrane treated with PVA is neutral. Measurement of the electrophoretic mobility of the particles in beer using a Rank Mark II apparatus show that they have an overall negative charge with a low zeta potential of the order of - 3 to - 5 mV. In addition, the untreated PVDF membrane is hydrophobic and the membranes treated with PEI and PVA are hydrophilic. Table 1 gives an overall view of these different factors. Assuming that beer-membrane interactions can be limited to only the electrical interactions between the particles in the beer and surface of the membrane, and to the effects of the hydrophobic nature of the membrane, these experiments indicate that. There is opposition between the two effects in the case of the PVDF and PEI treated membranes. That there is an overall repulsion in the case of the PVA treated membrane. The results observed in the experiments, where the PVA treated membrane has a lower pressure drop than the other two, is in agreement with this reasoning.
4.3. Conclusion from the second series of experiments using beer
PEI PVA
, 90
29
Forces Electrostatic
Hydrophobicity
Overall
repulsive attractive neutral
attractive repulsive repulsive
opposition opposition repulsion
of 'fresh' beer for three different types of membrane. It can be seen that for both the first and the second
These experiments show that the membrane permeability is reduced during microfiltration of pre-filtered beer. When the beer is fresh the increase in pressure drop may be principally attributed to adsorption in the pores of the membrane of species present in the beer. After a period of storage, even though the beer had been filtered, there occurs an agglomeration of the particles in the beer which is filtered out causing a deposit on the first membrane in the series and leaving the second membrane only subject to adsorption in the pores. Furthermore, the general behaviour of the beermembrane pair is in agreement with an analysis based on beer-membrane interactions due to electrical charge and the hydrophobicity of the membrane. Adsorption plays an important role in membrane fouling as the retention efficiency and fouling may be taken to depend not only on the diameter of the pores but also on the
30
Y. Xu-Jiang et al. /Journal of Membrane Science 105 (1995) 23-30
adsorption capacity o f the membrane. These are obviously important considerations in the choice of membrane for a given service.
6. Symbols used Ap~ pressure drop in first filter cell (Pa) AP2 pressure drop in second filter cell (Pa) t time of filtration (s)
5. General conclusions The technique of using two membranes in series has been shown to provide a way of distinguishing between external and internal fouling of microfiltration membranes. The method has been used to demonstrate that fouling of microfiltration membranes by protein solutions depends on the way the solution is handled and also on the properties of the solution and the membrane. This indicates that modifications o f the surface properties of the membrane may be useful in promoting repusive conditions and reducing fouling. When applied to beer filtration, the technique has been used to confirm the influence o f the surface properties o f membranes on fouling during filtration. In addition the technique demonstrates that particles present in beer can agglomerate during storage.
References [ 1] D.R. Trettin and M.R. Doshi, Chem. Eng. Commun., 1 (1979) 507-522. [2] P. Aimar, S. Baklouti and V. Sanchez, J. Membrane Sci., 29 (1986) 207-224. [ 3 ] M. Meireles, P. Aimar and V. Sanchez, Biotechnol. Bioeng., 38 (1991) 528-534. [4] W.M. Clark, A. Banal, M. Sontakkeand Y.H. Ma, J. Membrane Sci., 55 (1991) 21-38. [5] W.R. Bowen and Q. Gan, J. Colloid Interface Sci., 144 ( 1991 ) 254-262. [ 6 ] M. Hlavacekand F. Bouchet,J. MembraneSci., 82 (1993) 285295. [7] Y. Xu-Jiang, Ph.D. thesis, INPL Nancy, 1994.