Chapter 5
Microfiltration
A. S. GRANDISON, Departrnent of Food Science and Technology, The University of
Reading, Reading RG6 6AP and T. J. A. FINNIGAN, Marlow Foods, Middlesbrough.
5.1 INTRODUCTION
Microfiltration (MF) is the oldest membrane technology, having been used several
decades before the first industrial use of reverse osmosis (Glimenius, 1985). However,
subsequent development of the technology has been slow. Until recently microfilters
were operated in the dead-end mode and were exclusively of the depth-filter type in
which particles become trapped within the filter structure, but recent developments have
led to membrane-type microfilters, with a narrow pore size distribution, which can be
operated in the cross-flow mode. This has led to an increase in possible applications,
including clarification of fluids in the food and beverage industries, recovery of cells and
cell debris in the biotechnology industries, and the treatment of wastes.
5.2 THEORY, MATERIALS AND EQUIPMENT
Like ultrafiltration (UF), MF is a pressure-driven process employing pressures consider-
ably lower than reverse osmosis. In fact the distinction between UF and MF is somewhat
arbitrary and there is no distinction on purely theoretical grounds. The distinction lies in
the size ranges of the materials which are separated. UF is considered to involve the
processing of dissolved macromolecules, while MF involves separation of dispersed
particles such as colloids, fat globules or cells. MF can be considered to fall between UF
and conventional filtration, although there is overlap at both ends of the spectrum. A
guide to the pore sizes used for MF could be 0.01-10 pm.
For many years, MF has been applied as a dead-end operation using highly
microporous symmetric membranes of the depth-filter type. Such membranes retain
particles and consequently result in the build-up of a filter cake. This reduces flow, and
when the pressure drop has reached a certain level the membrane must be replaced or
removed and regenerated. In addition, the presence of a filter cake radically alters the
filtering characteristics, effectively acting as a prefilter which removes particles which
142
could otherwise pass through the membrane itself. On a large scale, therefore, it is only
practicable to use this technique when small amounts of particles are present.
Cross-flow MF (CMF) is a development which combines the cross-flow technique, as
applied to UF and reverse osmosis, with MF. CMF can be used to minimise (although it
should be emphasised, not completely eradicate) the problems encountered in dead-end
MF, and thus permit the processing of fluids containing quite large amounts of suspended
solids on a large scale. The advantages result from the fact that the build-up of filter cake
is avoided due to the shearing effect of the feed stream flowing parallel to the membrane
(Fig, 5.1). CMF plants can be operated in the same batch or continuous modes as
described in Chapter 3.
A, S. Grandison and T. J. A. Finnigan
FEED
Filter cake
0 OOOc$ O O O
AP{ lo+:0 **;," L
' ' ' ' +A Membrane
PERMEATE
(a)
0
000 0
0 O O+
- 0 oo
-0 Ooo0 0
C+SCC++++
Concentrate
0-
Feed 0 00
Membrane
Ap-- - __---_ -
Permeate
(b)
FIG. 5. I. Principles of (a) 'dead-end' and (b) 'cross-tlow' filtration.
5.2.1 Membrane configurations and characteristics
The geometric designs of MF membranes are the same as for UF as described in Chapter
3. Hence the module housings and ancillary equipment are also similar. Also the mem-
brane types are the same as for UF, i.e. cellulose and synthetic polymers (described in
Chapter 3) or inorganic. It is notable that the development of inorganic membranes has
been towards applications in MF rather than UF and reverse osmosis. In fact some types
of inorganic membrane are only available with pore sizes in the MF range.
Various inorganic materials have been employed for membrane manufacture including
glass, metals and compounds of aluminium, zirconium and titanium, and the geometries
can vary radically from conventional membrane design.
The structures and methods of manufacture of inorganic membranes are described in
greater detail by Rios et al. (1989). Inorganic membranes consist of two parts-a macro-
porous support and the active membrane coated onto the surface. The supporting
Microfiltration 143
materials must drain away the permeate without any hydrodynamic resistance, and thus
have a pore diameter of about 1Opm or more. They are produced from sintered fine
powdered materials including alumina, carbon, stainless steel and nickel. The tubular or
multichannel geometries of modules (e.g. Fig. 5.2 (a-c)) are produced by extrusion of the
Filtrate flows in
conduits to downstream
end of module
144 A. S. Grandison and T. J. A. Finnigan
Surface view
Cross-section
I 1
I
Nickellbased Zirconia
meshsupport membranes
Fig. 5.2. Some designs of inorganic membranes: (a) CARBOSEP membrane composed of
zirconia on a carbon support (courtesy of SFEC); (b) different designs of Ceraver alumina
membranes (reproduced by permission of Membralox@); (c) alumina membrane module
(reproduced by permission of CeraMem); (d) ceramic/metal-mesh composite membrane
(reproduced by permission of NWW Acumem Ltd).
powder and binder in aqueous media. The membrane layer may be coated directly onto
the macroporous support where pore size is quite large, but for UF and the smaller pore-
size MF membranes an intermediate sintered, ceramic layer is necessary due to the
surface rugosity of the support. The membrane layer (usually composed of alumina,
titania or zirconia) is formed by coating the support with a colloidal suspension and firing
at a lower temperature than the firing temperature of the support. To prevent rapid flux
decline, the membrane thickness must not exceed a few microns - titanium and zirco-
nium membranes of thickness 3-5 pn have been achieved. Accurate control of the col-
loidal particle size allows the possibility of producing membranes with an extremely
narrow pore size distribution compared to conventional membranes. The final pore size is
also related to the sintering temperature. An example of the structure of a sintered ce-
ramic membrane is shown in Fig. 5.3.
Microfiltration 145
Microporous -
membrane
Macroporous -
support
Fig. 5.3. Electron micrograph showing structure of Ceraver sintered alumina membrane with
0.2 pm pore size (reproduced by permission of Membralox@).
Early work on inorganic membranes used glass, but the first reliable cross-flow system
was the CARBOSEP membrane made of a microporous layer of zirconia coated onto a
macroporous carbon support (Fig. 5.2(a)). New products have subsequently appeared
including several designs of alumina membrane (e.g. Ceraver and CeraMem designs -
Figs. 5.2(b) and 5.2(c)). A novel design is the composite membrane produced by Acumem
composed of a zirconia ceramic membrane with nickel-based superalloy mesh support
(Fig. 5.2(d)).
The potential advantages of inorganic membranes result from their greater structural
strength and resistance to abrasive degradation, as well as improved chemical and
temperature properties. Their rigidity and strength allow the processing of feed materials,
such as cheese curd (Mahaut et al., 1982) or particulate materials which would not be
possible using conventional designs. Another possibility is that they could be used in
conjunction with fluidised turbulence promoters to increase permeate flux, which would
be out of the question with the more fragile surfaces of organic membranes. The wide pH
ranges of inorganic membranes (e.g. alumina membranes are resistant to pHs ranging
from 0.5 to 13.5, although phosphoric and hydrofluoric acids should be avoided) are a
great advantage during cleaning and sterilisation. In-place cleaning regimes using high
concentrations of caustic soda (3%), nitric acid (2%) and sodium hypochlorite are
possible. The modules can withstand temperatures of several hundred "C, which is far
beyond the temperatures used for food processing. However, this is an advantage during
cleaning cycles and the modules can be sterilised by steam. In practice, operating
temperatures are limited by other components such as gaskets, but it could be feasible to
+j
E
&
a
i
*- With backflush
x\
zi
i.
\.
\.
\.
<.
+.- .-.-.-.-.-._._. Without backflush
Microfiltration 147
membranes of the appropriate pore size in relation to the characteristics of the particles
suspended in the feed. Unfortunately it is very difficult to predict the optimal pore size
from first principles, and experimentation is required. However, a recent study of Tarleton
and Wakeman (1993) has attempted to understand the relationship between pore size and
particle size in flux decline. Clearly the particle size distribution is an important factor,
especially when even a small proportion of fines are present in the feed. Many anomalies
remain, and further systematic studies are required.
A large amount of effort has been put into devising means of avoiding or counter-
acting the effects of fouling in MF, and several promising techniques have evolved. The
first requirement is to maintain high shear in the retentate and thus reduce the thickness
of the boundary layer at the membrane surface and remove deposits from the membrane.
It is desirable that turbulent flow be maintained on the retentate side of the membrane.
Murkes (1989) has described a new high shear cross-flow method for MF, and claimed
an order of magnitude increase in flux rates over conventional CMF.
Forcing a quantity of permeate back through the membrane (i.e. backflushing) can
drive deposited particles away from the membrane surface, as well as breaking up con-
centration polarisation and/or gel layers in the bulk flow. This can lead to regeneration of
permeate flux, but it is necessary to ensure that a higher volume of permeate is not used
for the backflushing than is gained from the resulting increased permeate flux (Milisic
and Bersillon, 1986). However, assuming the correct sequencing and volume of
pulsations are used, the technique can be most beneficial (Fig. 5.4). An alternative method
of backwashing is to use cleaning sequences consisting of blowing air through the mem-
branes - i.e. gas backflushing (Dietrich et al., 1988; Peters, 1989). This may prove
beneficial with problem feed streams. Another approach to maintaining flux is to use the
‘uniform transmembrane pressure’ mode of operation. This requires simultaneous
operation of a retentate pumping loop and a permeate pumping loop, adjusted so that the
pressure drop across the membrane is small, and is uniform along the length of the
membrane. This system has allowed very high fluxes (500-700 1 m-* h-’) to be main-
tained during processing of skimmed milk using ceramic membranes in the ‘Bactocatch’
system (see Section 5.3.1).
The application of an electric field has been shown to reduce fouling due to colloids
and particles during CMF (Visvanathan and Ben Aim, 1989) and thus prevent flux
decline (Tarleton and Wakeman, 1988), although the mechanism of action is not fully
understood. Bowen et al. (1989) reported that electrical pulses were effective in both
improving flux rates and restoring flux rates in fouled systems, and could be used as an
alternative to backwashing and conventional cleaning techniques.
Other approaches to reducing fouling during MF have included the use of abrasives to
break down the fouling layers, and the application of pulsations to the feed stream
(Milisic and Bersillon, 1986). Chemical pretreatments of the feed streams can also be
used to reduce the problems of fouling (Bedwell et al., 1988), although this is frequently
not a suitable option. Pretreatment of the membrane may reduce the flux decline during
fouling. Taddei and Howell (1989) reported a 70% improvement of flux during
harvesting of yeast cells by conditioning the MF membranes with respect to pH.
The initial challenge to the membrane has been found to affect the subsequent flux
during MF. Care must be taken during start-up procedures to prevent too-rapid flux
148
decline, It is frequently an advantage to start the run in the dynamic mode @e. with water
circulating prior to introduction of the feed), or with the permeate side filled with solvent.
While the above methods of combating fouling can be very effective, it is essential to
combine them with efficient chemical cleaning procedures.
A characteristic of MF is that the separation may alter during processing due to
fouling, such that the MF operation, in reality, becomes UF through a ‘secondary
membrane’ formed at the surface of the MF membrane, consisting of macromolecules or
colloids. This generally constitutes a problem. However, this property has been used to
advantage in the French dairy industry for the concentration of milk. Bennassar et al.
(1982) described the use of the selective UF properties of the secondary membrane
formed while using 0.2 pm membranes. Almost total retention of milk protein was
possible with this membrane.
A. S. Grandison and T. J. A. Finnigan
5.3 APPLICATIONS IN THE FOOD AND BIOTECHNOLOGY INDUSTRIES
MF is generally used to separate particles suspended in liquid media, and may frequently
be considered as an alternative to conventional filtration or centrifugation. For industrial
use the aim is usually to obtain either a clear permeate or the concentrate. Therefore most
applications are either clarification, or the recovery of suspended particles such as cells or
colloids, or the concentration of slurries. MF is a useful process for the treatment of
fermentation broths, frequently as an alternative to centrifugation. During recovery of
intracellular components, the cells can be enriched and washed by diafiltration prior to
disintegration of the cells, and cell debris can be removed from the products of lysis.
MF membranes can be incorporated in water polishing systems as they are effective in
removing suspended solids and bacteria. They may also be applied to a variety of liquid
food streams, and may give the accompanying benefit of chemical and/or micro-
biological stabilisation. Clarification of biochemical or microbiological reaction products
or effluents is also possible. MF is a well-established laboratory technique for the
production of sterile fluids without the application of heat.
5.3.1 Food industry
Perhaps the majority of applications in the food industry have been in the treatment of
juices and beverages. As MF is a purely physical process, it can have advantages over
traditional methods involving chemical additives, in terms of the quality of the product as
well as the costs of processing.
Finnigan and Skudder (1989) discussed the application of ceramic microfilters to the
processing of beer and cider. Very good quality clear permeate was found for both
products with high flux rates. Fluxes of 200-400 1 m-2 h-’ were obtained with beer at
low temperatures with no rejection of essential components. Recovery of beer from tank
bottoms IS also possible using ceramic MF, but this is less important in the UK now that
duty is not paid at the wort stage of production.
Clarification and biological stabilisation of wine musts and unprocessed wine have
been described. This avoids the requirement for fining and, possibly, pasteurisation. Dau
et al. (1988) used membrane filters with pore size 0.1-5 pm to clarify wine preheated to
Microfiltration 149
30-40”C to increase permeate flux. However, Rios et al. (1989) selected ceramic mem-
branes with pore diameter 0.2 pm at temperatures <20”C to avoid sensory changes.
Conventional methods of clarification of fruit juices incorporate the use of
diatomaceous earth and enzymes to remove a variety of materials including suspended
solids, colloidal particles, proteins and condensed polyphenols. Also they are quite time
consuming in that the juice must be stored in settling tanks. Juice clarification using MF
is becoming an established commercial practice as it is a continuous process which can
result in significant savings in time, materials and labour as well as improved yields.
(Note: The use of diatomaceous earth results in loss of yield.) Short (1988) described the
commercial application of CMF to the processing of a range of fruit juices. A plant
producing more than 5 x lo5 litres per day of apple juice exists in the USA. This has the
major benefit of increasing juice yield from 95 to 97% as well as eliminating the require-
ment for diatomaceous earth as a pretreatment, and the use of enzymes and fining agents.
A saving of $350 000 per annum was estimated. With highly coloured juices, a further
benefit of MF is that the colour is retained very well in the permeate. Commercial
systems are also known to exist for grape, pear, kiwifruit, pineapple, cranberry and citrus
juices. Clarifications of vinegar, vodka, pickling brines and liqueurs by MF have also
been described (Le and Billigheimer, 1985; Short, 1988).
It seems likely that CMF could replace carbonatation in the treatment of raw sugar
beet juice, although this is not carried out commercially to the author’s knowledge.
Another section of the industry with several applications is dairy processing. Piot et al.
(1984) and Merin (1986) have clarified sweet cheese whey using CMF. This has the dual
benefit of removing fat and reducing the bacterial population and could eliminate the
need for fat separation and heat treatment in the production of whey protein powders
prior to UF. The former authors reported that a decimal reduction of microorganisms of 5
could be obtained in the microfiltrate compared to the whey, although some loss of whey
protein was observed. Hanemaaijer (1985) described a scheme for whey treatment incor-
porating MF and UF to produce ‘tailor-made’ whey products with specific properties for
specific applications. The products include whey protein concentrates which are rich in
whey lipids, as well as highly purified protein.
Bacterial removal from whole milk by MF is a problem because the size range of the
bacteria overlaps with the fat globules, and to a lesser extent with the casein micelles.
However, some success has been achieved with skim milk. The ‘Bactocatch’ system can
remove 99.6% of the bacteria from skim milk using ceramic membranes on commercial
scale (Malmbert and Holm, 1988). The retentate (approximately 10% of the feed) can
then be sterilised by a UHT process, mixed with the permeate and the mixture
pasteurised, to give a product with 50% longer shelf-life but no deterioration in
organoleptic properties compared to milk that has only been pasteurised. The
combination of MF and heat reduces the bacterial numbers by 99.99% (4D).
Alternatively the permeate could be used for cheesemaking, or the production of low-heat
milk powder (Hansen, 1988). Piot et al. (1987) described the use of membranes of pore
diameter 1.8 pm to produce skim milk of low bacterial content. Recovery of fat from
buttermilk has also been described (Rios et al., 1989).
Membranes have been used to concentrate milk prior to the manufacture of many
cheese types. This results in improved yields and other associated benefits such as
150
reduced requirement for rennet and starter, and the ability to produce much more cheese
per vat (Grandison and Glover, 1994). In most cases UF membranes have been used as it
is necessary to retain all the protein in the concentrate. An alternative approach is to
concentrate the curd after coagulation of the milk in which case a solution of lactose and
minerals is removed from the semi-solid protein gel. This can be done using centrifuga-
tion in the manufacture of some soft cheese types.
However, the use of MF is an attractive alternative. Rios et al. (1989) have carried out
extensive trials on this application and concluded that the use of 0.2 pm pore diameter
membranes gave a product with better texture and yield than with centrifugation. The
choice of ceramic membranes allowed the curd to be contacted directly with the
membrane.
Other food applications have been reported with meat and vegetable products includ-
ing the following.
Devereux and Hoare (1986) described the use of MF to recover precipitated soya
protein. This could have advantages over recovery of the dissolved protein using UF.
Gelatin is a proteinaceous material derived by hydrolysis of collagen. This is purified
by filtration incorporating diatomaceous earth. The latter process can be replaced by
CMF which effectively removes dirt, coagulated proteins, fats and other particulate
materials from the feed. Again the CMF method gives higher yields of high quality
product on a continuous basis. Short (1988) calculated that incorporating CMF plants for
gelatin would have a payback time of 3 years for a capacity of 30 tonnes/h.
5.3.2 Applications for biotechnology
Applications of MF in the biotechnology industry are very promising although the
process has not yet made the breakthroughs that may have been expected. The major
problem with MF of suspensions of cells or cell debris is the exponential flux decay
resulting from adhesion of cells or cell fragments to the membrane - i.e. biofouling. This
has limited the application of CMF into biotechnological downstream processing.
Solutions to this problem, other than general methods of avoiding fouling, are discussed
by Defrise and Gekas (1988) and include choice of biocompatible membrane materials
and the use of surfactants and polyelectrolytes. Most biotechnological applications of MF
are as a competitor to centrifugation, and MF is becoming recognised as a viable alterna-
tive in many cases. CMF may have advantages over centrifugation when containment is
required (e.g. when handling pathogenic organisms or in recombinant DNA technology)
as aerosols are not produced during the operation (Kroner et d., 1984). The capital and
maintenance costs of MF are lower than for centrifugation, although membrane replace-
ment is expensive. Tutunjian ( 1985) compared the costs of process-scale harvesting and
washing of Escherichia coli cells using MF with a hollow fibre system to centrifugation.
The MF system had about 70% lower capital, and 25% lower running costs than
Centrifugation. Also virtually 100% recovery of solids is possible with MF, whereas it is
often less than 90% with centrifugation.
CMF is a versatile technique for cell harvesting and cell debris removal during
recovery of intracellular products. The viability and enzyme content of the cells is
unaffected by MF, and it is possible to concentrate cell suspensions to 20-25% dry
weight, limited by pumping considerations (Le and Atkinson, 1985). A 0.1 pm
A. S. Grandison and T. J. A. Finnigan
Microfiltration 151
microporous hollow fibre membrane has been used for the purification of bovine growth
hormone from Escherichia coli lysate (Tutunjian, 1985). Although the growth hormone
was partially rejected from the membrane, this could be recovered using diafiltration such
that over 99% recovery from the original lysate was possible. Kroner et al. (1984) used
MF to recover a range of soluble enzymes from cell debris, although they concluded that
the separation was not satisfactory due to high retention of the enzymes in the feed.
Transmission of enzymes through microporous membranes is rather anomalous. Le and
Billigheimer (1985) found that transmission of arylamidase increased rapidly over the
first 5-10 minutes of processing and then declined steadily. This was ascribed to
adsorption of protein to the membrane surface during the initial phase, to form a
monolayer lining. Transmission of the enzyme occurred when the surface was saturated,
and subsequently declined due to the formation of a secondary membrane of cell debris
and proteinaceous matter. In general, careful manipulation of pressures and flow rates can
be used to optimise protein transmission through MF membranes.
Alternatively, valuable extracellular biotechnological products can be separated from
cell suspensions by MF. Raehse et al. (1986) employed polysulphone tubular membranes
with a pore size of 0.3-0.5 pm to separate alkaline protease from a fermentation broth.
The ratio of the mean pore diameter of the membrane to the size of the microorganisms
was between 0.15 and 0.85, which gave a rapid separation suitable for large-scale
applications.
A combination of CMF with affinity chromatography provides an innovative develop-
ment for purifying biochemicals from fermentation broths or other aqueous media, such
as trypsin from pancreatic extract. The principle is to bind the biochemical to a
macroligand and remove contaminants by CMF. Following dissociation, the purified
material can then be recovered by CMF, and the macroligand recycled (Luong et al.,
1987).
It is possible to incorporate MF membranes into membrane reactors. In this way a
continuous process can be developed in which the membranes are retentive to the cells
(or other biocatalyst), but permeable to the reaction products. Most reported develop-
ments in this field have been on laboratory scale, and use UF rather than MF membranes.
One promising report with MF is the incorporation of zirconia membranes into a
continuous fermentation system with cell recycle for production of alcohol which
permitted very high yeast concentrations to be used (Lafforgue et ul., 1987). This has led
to the possibility of extremely high alcohol production capacity (possibly
150 kg m-2 h-I).
5.4 CONCLUSIONS
MF has made significant advances in new applications in the food and biotechnology
industries. However, the technique has not yet realised its full potential, largely due to the
severe problems of flux decline due to fouling. It is believed that further developments in
membrane design and a greater knowledge of fouling mechanisms will result in greater
application in the future, especially in the field of downstream processing.
152
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