Fluid filled and collapsing polymeric filter tube having a supporting sleeve

A filter consisting of a filter tube of a porous polymeric material having tendency to collapse towards a predetermined flattened configuration and a porous support sleeve disposed around the filter tube, so as to support the filter tube against internal fluid pressure, while providing no more resistance to flow of fluid than the filter tube.

This invention relates to a filter, to a method of making a filter, and to 
separation systems which include filters. 
U.S. Pat. No. 2,864,506 discloses a device for ultrafiltration, which 
comprises a tubular membrane and a porous sleeve which surrounds the 
membrane along its entire length. A fluid which is to be filtered can be 
passed through the membrane from the supply end to the collection end 
under pressure. Filtrate passes through the membrane and the sleeve. 
Material in the fluid which is unable to pass through the membrane remains 
in the fluid, so that the concentration of that material in fluid leaving 
the membrane at the collection end thereof is increased. This filtration 
technique is sometimes referred to as cross-flow filtration. 
Examples of materials said to suitable for use for the tubular membrane 
include cellulose, parchment, rubbers and plastic materials, selected 
according to the nature of the filtration to be accomplished. Examples of 
materials said to be suitable for the porous sleeving include cotton, 
nylon, muslin, copper, stainless steel and glass. 
A problem which is encountered in connection with cross-flow filtration 
systems is that of cleaning any residue which collects on the internal 
surfaces of the membrane filter. This problem is significant: if collected 
material cannot be displaced from the filter surface, the entire 
filtration device must be replaced. This is undesirable because of the 
costs involved. 
The problem is solved in the device disclosed in U.S. Pat. No. 4,765,906 by 
use of a double ply of a filter support material in which is disposed a 
layer of particulate filter material, such as floc, cellulose, kieselguhr, 
possibly with a second coating such as of a metal hydroxide. Residue is 
removed from the device by washing, which removes the residue together 
with the particulate filter material. The filter material must be replaced 
before the device can be used again. 
It has been found that the problem of cleaning a filter, such as might be 
used in a cross-flow filtration system, can be solved by use of a filter 
whose configuration tends to change, towards a flattened configuration, 
when fluid pressure within it is relaxed. 
Accordingly, in one aspect, the invention provides a filter which 
comprises: 
(a) a filter tube of a porous polymeric material, which is set so that it 
has a tendency to collapse towards a pre-determined flattened 
configuration, and 
(b) a porous support sleeve disposed around the filter tube to support it 
against internal fluid pressure while providing no more resistance to flow 
of fluid than the filter tube. 
In another aspect, the invention provides a method of making a filter, 
which comprises: 
(a) forming a porous filter tube of a polymeric material, which is set so 
that it has a tendency to collapse towards a pre-determined flattened 
configuration, and 
(b) providing a support sleeve around the filter tube to support the filter 
tube against internal fluid pressure while providing no more resistance to 
flow of fluid than the filter tube. 
The filter of the present invention has the advantage that material which 
collects on the internal surface of the filter tube during a filtration 
process, for example during cross-flow filtration, is dislodged when the 
tube collapses towards its pre-determined flattened configuration. The 
collapse takes place when pressure in the filter tube is relaxed. 
Preferably, the support sleeve is set so that it too has a tendency to 
collapse towards a pre-determined flattened configuration. This has the 
significant advantage of enhancing the collapse of the filter tube, and 
therefore the release of collected material from the internal surface of 
the filter tube, especially when the filter tube and the support sleeve 
are oriented relative to one another such that they align with one another 
when they collapse. 
The flattened configuration of the filter tube, and of the support sleeve 
when applicable, is a flattened configuration which is pre-determined, in 
the sense that the tube or sleeve reverts always to that configuration 
when internal fluid pressure is released. The flattened configuration will 
generally involve the tube or sleeve being folded along at least two fold 
lines, and the tube or sleeve will tend to fold along the same lines when 
it collapses. Especially when the tube or sleeve has two fold lines, they 
will be disposed diametrically opposite to one another, so that the tube 
or sleeve is essentially flat (subject to the ease with which the material 
of the tube or sleeve can be folded) when in the collapsed configuration. 
The support sleeve can advantageously be formed from a fabric, formed in 
turn from a fibre. For example, the fabric may be formed, for example, by 
knitting, braiding or weaving. A woven fabric can be formed with at least 
one of the fold lines by which the sleeve can be made to adopt a flattened 
configuration. By appropriate arrangement of the weaving equipment, the 
sleeve can be formed with a plurality of the fold lines, as required to 
cause the sleeve to adopt the flattened configuration. Similarly, a 
knitted fabric can be formed with required fold lines. 
The support sleeve can be formed separately from the filter tube, and the 
filter can be made by locating the filter tube inside the support sleeve. 
The support sleeve can however be produced directly around the filter 
tube. 
It can be preferred to provide discontinuities in the support sleeve which 
cause the internal cross-section of the support sleeve to vary along its 
length, and therefore also of the filter tube, at least when in contact 
with the internal surface of the support sleeve as a result of fluid 
pressure. This might be achieved when the support sleeve is formed by 
weaving by selection of an appropriate weave pattern. For example, 
combinations of twill, plain and hop sack with single or multiple weave 
patterns may be used. The characteristics of the sleeve can be varied from 
one region of the sleeve to another, for example by varying the weave 
density, the fibre diameter or stiffness, or by incorporating more fibres 
in the selvage of the sleeve, for example in order to introduce variations 
in the stiffness of the sleeve. Discontinuities might also be introduced 
by means of externally applied components, such as clips. Variations in 
the cross-section of the filter tube can give rise to turbulence in fluid 
flowing in the tube, which can lead to reductions in accumulation of 
material on the internal surface of the tube. 
The material of the sleeve can be varied from one region of the sleeve to 
another, for example to introduce variations in the stiffness of the 
sleeve. For example, portions of a stiff filament, or of an elastomeric 
material, can be incorporated in regions of the sleeve. 
A preferred support can provide support sleeves for a plurality of filter 
tubes. For example, a support formed from parallel spaced lengths of woven 
material, whose plies are seamed together along spaced longitudinal seams 
to provide separate sleeves between adjacent seams, can provide support 
for a plurality of filter tubes, one in each support sleeve. It will be 
preferred in this embodiment that the plane towards which the filter tube 
tends to flatten is substantially parallel to the plane of the sheet. 
A preferred material for the support sleeve is a non-woven fabric, for 
example formed by wet laying, air laying, melt spinning, spun bonding or 
needle punching processes, as appropriate to the material from which the 
fabric is manufactured. 
The sleeve can be formed from a fibre of a metallic material, or of a 
natural or synthetic polymeric material, or from fibres of different 
materials. Examples of suitable materials include stainless steels, 
polyamides, polyesters, polyolefins such as polypropylenes and high 
molecular weight polyethylenes, polyimides, naturally fibrous substances 
such as cotton and silk, inorganic fibrous substances such as might be 
made from carbon, glass and boron, and natural inorganic substances such 
as asbestos. Particularly preferred fibres include those formed from 
polypropylene. 
The support sleeve will be made to collapse towards its flattened 
configuration using a technique selected according to the material of the 
sleeve. For example, if the material is capable of being deformed 
plastically by the application of force, this technique can be used to 
crease the material of the sleeve. Another technique which might be 
employed involves the application to the sleeve of heat, while the sleeve 
is retained in its flattened configuration, so as to set the sleeve in its 
flattened configuration. This technique lends itself particularly to 
certain polymeric materials. A combination of techniques can of course be 
employed. 
Preferably, the external width of the filter tube when laid flat is less 
than about 95% of the internal width of the support sleeve when laid flat. 
This has the advantage that the filter tube is capable of expanding within 
the support tube when placed under pressure from fluid within it. More 
significantly, the tube tends to contract when fluid pressure is relaxed, 
causing material built up on the internal surface of the tube to be 
released. This facilitates cleaning of the tube of collected material. It 
is particularly preferred that the material of the filter tube is such 
that the filter tube stretches under fluid pressure within it, and that 
the tendency of the support sleeve to stretch is less than that of the 
filter tube. In this way, the filter tube can be stretched satisfactorily 
under the pressures exerted by fluid within it, to engage the internal 
surface of the support sleeve. 
In another aspect, the invention provides a filter which comprises: 
(a) a filter tube of a porous polymeric material, and 
(b) a porous support sleeve disposed around the filter tube to support it 
against internal fluid pressure while providing no more resistance to flow 
of fluid than the filter tube, 
in which the external width of the filter tube when laid flat is less than 
about 95% of the internal width of the support sleeve when laid flat, and 
in which the tendency of the support sleeve to stretch laterally under 
internal fluid pressure is less than that of the filter tube, so that the 
filter tube is capable of being stretched by fluid under pressure within 
it to engage the internal surface of the support sleeve. 
Preferably, the filter tube has a porosity of at least about 35%, 
especially at least about 50%, for example about 75%, determined by 
comparison of a calculated density of the membrane derived from the weight 
and dimensions of a sample, and the theoretical density of the components 
of that sample. It is preferred that the polymeric material of the filter 
tube be microporous, for example so that details of the structure of the 
pores of the tube are discernable only by microscopic examination which 
can resolve details of structure below 500 nm. It will generally preferred 
that there are many pathways between opposite surfaces of the membrane. 
Preferably, the mean pore size is less than about 10 .mu.m, more 
preferably less than about 4 .mu.m, especially less than about 1 .mu.m, 
for example about 0.3 .mu.m. The mean pore size could be significantly 
smaller than these values, for example less than 150 nm, making the filter 
of the invention suitable for use in ultrafiltration applications. 
Preferably, the thickness of the wall of the filter tube is less than about 
500 .mu.m, more preferably less than about 300 .mu.m, especially less than 
about 150 .mu.m, for example less than about 75 .mu.m. 
The filter tube can conveniently be formed from a polymeric material by one 
or more of techniques which include removing a pore forming substance from 
the material, and stretching the material. For example, the tube can be 
made from a polymeric material which includes a filler. A material can be 
selected for the filler which is inert towards substances with which the 
filter will come into contact when in use. This allows the filler to 
remain in the material of the filter tube. 
Preferably, the tube is made from a blend of a polymeric material and a 
filler, by forming the blend into a film and stretching the film to render 
it porous. The film may be formed into a tubular article by folding and 
sealing the folded film. The film may be formed directly as a tubular 
article, for example by extrusion. The blend may include additives which 
facilitate the deformation of the film to render it porous, such as, for 
example, appropriate plasticisers. Particularly appropriate additives 
include plasticisers which are substantially immiscible with the polymer 
of the blend, in the absence of filler. 
Preferably, the filler is present in the blend in an amount of about 50 to 
about 350 parts by weight, more preferably from about 100 to about 250 
parts, per hundred parts by weight of polymer. Plasticiser may be present 
in an amount from about 1 to 50 parts by weight, more preferably from 
about 20 to about 40 parts, per hundred parts by weight of polymer. 
The blend may include other additives, such as anti-oxidants, UV 
stabilisers, processing aids, dispersal aids and so on. A preferred 
dispersal aid comprises a fatty acid salt, especially a stearate. It may 
be added directly to the polymer composition, or be formed by reaction of 
the filler with stearic acid. Preferably, the ratio of the dispersal aid 
to filler is from about 1 to about 10 by weight. 
The degree to which the film formed from the blend is deformed may depend 
on a number of factors, including for example the nature of the polymer, 
the filler and any plasticiser present, whether the plasticiser or filler 
is to be extracted from the pores, the required pore size and so on. 
Generally, a high degree of deformation is preferred, to create a high 
degree of porosity. For example, the film may be stretched so that the 
dimension in the direction of stretching increases by at least about 50%, 
preferably, at least about 100%, more preferably at least about 250%, for 
example at least about 450%. By this process, the thickness of the film 
can be reduced by a factor of five or more. 
The polymeric material of the filter tube can be selected from polymers of 
compounds with one or more polymerisable double bonds, or condensation 
polymers of condensable compounds. 
Useful polymers of compounds with polymerisable double bonds may be 
selected from polymers of ethylenically unsaturated hydrocarbons, having 2 
to 12 carbon atoms, such as ethylene, propylene, n-dodecene, and of vinyl 
ethers such as methyl or ethyl vinyl ether. The compounds can be 
substituted, for example halogenated. Copolymers of these compounds can 
also be useful. 
Useful condensation polymers include polyamides of diamines and 
dicarboxylic acids. 
Examples of particularly preferred polymers for the filter tube include: 
Polyethylene 
Polypropylene 
Polybutylene 
Poly(4-tert-butylstyrene) 
Poly(vinyl methyl ether) 
Poly (vinylidene fluoride) 
Ethylene/tetrafluoroethylene copolymer 
Tetrafluoroethylene hexafluoropropylene copolymer 
Ethylene/chlorotrifluoroethylene copolymer 
Poly(6-aminocaproic acid) 
Poly(11-aminoundecanoic acid) 
Poly(ethyleneterephthalate) 
Poly(butyleneterephthalate) 
Poly(decamethylene sebacamide) 
Poly(heptamethylene pimelamide) 
Poly(octamethylene suberamide) 
Poly(nonamethylene azelamide) 
Poly(hexamethylene adipamide) 
Examples of plasticisers which might be used in the blend from which the 
filter tube is made include ethylene carbonate, propylene carbonate, 
ethylene glycol, dimethylether, tetrahydrofuran, tryglyme, tetraglyme and 
selected polyethylene oxides and polyethylene glycols. It can be 
appropriate to extract the plasticiser from the blend after the article 
formed from the blend has been stretched. This can best be done by means 
of an appropriate solvent. 
Examples of suitable fillers for the material of the filter tube include: 
salts such as metal oxides and hydroxides (for example of calcium, 
magnesium, barium, aluminium, titanium, iron and tin); carbonates (for 
example of calcium, magnesium, and lithium); and chlorides and sulphates 
(for example of sodium, potassium, calcium and lithium). 
minerals, such as mica, montmorillorite, kaolinite, cellopulgite, asbestos, 
talk, diatomaceous earth and vermiculate, synthetic and natural zeolites, 
and Portland cement. 
silica, precipitated metal silicates such as calcium silicate, aluminium 
polysilicate, aluminium silica gels, glass particles including solid and 
hollow microspheres, flakes and fibres. 
Preferably, the filler comprises particles which are approximately 
spherical. Suitable particles have a diameter less than about 10 .mu.m, 
especially less than about 5 .mu.m, for example less than about 3 .mu.m. 
It has been found that these materials can enable membranes to be produced 
with uniform pores having a narrow spread of diameters, which can be 
predicted from the mean diameter of the filler spheres. Furthermore, the 
physical properties of the membranes can be enhanced compared with the 
properties of materials made using filler particles which are not 
spherical. 
The filter of the invention can include means for deforming the filter 
mechanically, to cause material accumulated on the internal surface of the 
filter tube to be dislodged. For example, the filter assembly might 
include one or more rollers which can be passed over the filter, from one 
end thereof towards its other end. It is particularly preferred that there 
be two rollers, to act on opposite sides of the filter. The filter can 
also include means for driving the deformation means along the filter. 
In a further aspect, the invention provides a separation system which 
includes a filter of the type referred to above. The filter can be 
arranged spirally. Preferably, the filter is arranged so that the plane 
towards which the filter tube tends to flatten deviates significantly from 
perpendicular to the axis of the spiral, and preferably is substantially 
parallel to that axis. For example, the angle between the plane and the 
axis might be less than about 45.degree., preferably less than about 
30.degree., especially less than about 10.degree.. 
Preferably, the separation system including the filter includes means for 
supporting the filter in its arrangement, for example its spiral 
arrangement. This might comprise, for example, an array of pins which the 
filter is wound around. In this embodiment, it is preferred that the plane 
towards which the filter tube tends to flatten deviates significantly from 
perpendicular to the axis of the spiral. The turbulence imparted at each 
pin, where the tube is deformed, can reduce accumulation of material on 
the internal surface of the filter tube. 
The separation system of the invention might include a plurality of the 
filters of the invention, connected in parallel in the direction of flow 
of liquid to be filtered by means of appropriate headers. For example, 
three of the filters might be fitted in the direction of flow of liquid by 
means of two headers; a first header splits the flow from a supply conduit 
into the three filters, and a second header combines the flow from the 
three filters into a collection conduit. 
Connections to the filter of the invention can be made conveniently using 
conventional fittings used for forming connections to fluid carrying 
tubes. This feature has the advantage of allowing connections to be made 
simply and conveniently, and at low cost. The advantage arises, in part, 
from the robust nature of the filter of the invention, in particular of 
the filter tube component. 
The transverse dimensions of the filter will be selected according to the 
characteristics of the filtration operation; smaller filters can give rise 
to a greater efficiency, because of the smaller volumes of liquid which 
pass through the filter. It has been found that a convenient size for the 
support sleeve is an external diameter which is less than about 50 mm, for 
example about 12 mm, but possibly less than about 8 mm for some 
applications. The diameter is preferably greater than about 3 mm, for 
example greater than about 5 mm. 
While the filter of the invention finds particular application in 
cross-flow filtration systems, it is also envisaged that it can be used in 
filtration systems in which a fluid to be filtered flows into the filter 
tube at one end, the tube being sealed at its other end, so that fluid 
leaves the filter only through the filtration surfaces of the filter tube, 
and subsequently the support sleeve. With this device, the residue 
retained by the filter tube can easily be recovered by periodically 
opening the closed end of the filter and flushing it out into a separate 
vessel.

The present invention will be now be described with reference to examples. 
1. FORMATION OF POROUS POLYMERIC MATERIAL TUBE 
(a) The following materials were mixed and pelletised by melt blending on a 
twin screw extruder proportions indicated by weight: 
______________________________________ 
Linear low density polyethylene (Sclair 8405) 
100 
Lithium carbonate (particle size &lt;5 .mu.m) 
110 
Lithium stearate 1.1 
______________________________________ 
The resulting blend was formed into a flat sheet by extrusion. The sheet 
was stretched in the machine direction 450%. A strip of the sheet was 
folded, and a seam was formed about 20 mm from the fold to form a tube. 
The tube was set in a flattened configuration by the application of heat 
and pressure. The tube had a mean pore size of about 0.25 .mu.m measured 
using a Coulter porometer, and a wall thickness of about 30 .mu.m. 
(b) The following materials were mixed and pelletised by melt blending in a 
twin screw extruder, proportions indicated by weight: 
______________________________________ 
Linear low density polyethylene (Sclair 8405) 
100 
Spherical glass (Potters-Ballotini Grade 5000) 
140 
Lithium stearate 1.4 
______________________________________ 
The blend formed into a tube by melt extrusion through a cross-head die 
using a 25 mm pin and die set. The extruded tube was sized by inflation 
with air, and drawn down into a flattened configuration. The flattened 
tube was reheated to 90.degree. C., and stretched longitudinally by 530% 
between nip rollers of differing speeds. 
The tube had a diameter of about 12.5 mm, a mean pore size of about 1.7 
.mu.m measured using a Coulter porometer, and a wall thickness of about 60 
.mu.m. 
(c) The following materials were mixed and pelletised by melt blending in a 
twin screw extruder, proportions indicated by weight: 
______________________________________ 
Polypropylene (Appryl Grade 3030FNI) 
100 
Spherical glass (Potters-Ballotini Grade 0-3CP00) 
150 
Lithium stearate 1.5 
______________________________________ 
The blend was formed into a tube by melt extrusion through a cross-head die 
using a 25 mm pin and die set. The extruded tube was sized by inflation 
with air, and drawn down into a flattened configuration. The flattened 
tube was reheated to 105.degree. C., and stretched longitudinally by 530% 
between nip rollers of differing speeds. The tube had a mean pore size of 
about 0.24 .mu.m measured using a Coulter porometer. 
(d) The following materials were mixed and pelletised by melt blending in a 
twin screw extruder, proportions indicated by weight: 
______________________________________ 
Linear low density polyethylene (Sclair 8405) 
100 
Lithium carbonate (maximum particle size 6 .mu.m) 
200 
Lithium stearate 2 
______________________________________ 
The blend was formed into a tube by melt extrusion through a cross-head die 
using a 25 mm pin and die set. The extruded tube was sized by inflation 
with air, and drawn down into a flattened configuration. The flattened 
tube was reheated to 90.degree. C., and stretched longitudinally by 530% 
between nip rollers of differing speeds. 
The tube had a diameter of about 12.5 mm, a mean pore size of about 0.7 
.mu.m measured using a Coulter porometer, and a wall thickness of about 70 
.mu.m. 
2. FORMATION OF SUPPORT SLEEVE 
Two sheet of a non-woven polypropylene fabric, supplied by Freudenberg type 
FS 2123, were placed in face to face contact, and joined to each other by 
two parallel welds, about 20 mm apart, using an industrial bag sealer. 
Excess fabric was removed, to produce a flat tube. 
3. DEAD-END FILTRATION 
A filter formed from the filter tube of Example 1a and the support sleeve 
of Example 2 was connected to a domestic water supply at one end using 
nylon tube connection fittings conventionally used to form connection to 
domestic and industrial hoses. The filter was closed at its other end by 
folding the end of the filter and retaining the fold in place by means of 
a clip. 
Water was supplied to the filter and was seen to permeate through the walls 
of the filter tube and the support sleeve. Subsequent examination of the 
internal surface of the filter tube revealed a layer of retained material 
with a yellow/brown appearance. 
4. CROSS-FLOW FILTRATION OF YEAST SOLUTION 
4.1 A filter formed from the filter tube of Example 1a and a woven 
polyester tube supplied by InHome Limited under the designation "Standard 
Green Tube", was challenged with mains water using the test rig shown 
schematically in FIG. 1 of the accompanying drawings. The test rig 
includes the sample 1 of the filter, inlet and outlet pressure gauges 2, 
3, a pump 4 for liquid to be circulated through the filter, a reservoir 5 
for the liquid, a heat exchanger 6, a pressure control valve 7, a flow 
control valve 8, a flow gauge 9, and a sampling point 10 from which 
filtrate can be collected. The water was supplied to the filter at a 
pressure of 2 bar and a cross-flow rate of 2 l.min.sup.-1 for 2 hours. The 
membrane flux was calculated to be 88.7 l.sup.-1.m.sup.2.h.sup.-1 after 2 
hours. 
A similar filter was challenged with a 0.5% yeast solution made by 
dispersing a commercially available dried baker's yeast in tap water at a 
pressure of 2 bar and a cross-flow rate of 2 l.min.sup.-1 for 2 hours. The 
membrane filtrate was calculated to be 18 l.sup.-1.m.sup.2.h.sup.-1 after 
2 hours. The turbidity of the filtrate was determined by its UV absorbance 
at 595 nm. It was found to be less than that of the feed solution by a 
factor greater than 1000. 
4.2 A sample of the filter tube of Example 1b, length 160 mm, was inserted 
into the woven polyester tube referred to in Example 3.2 above. The 
resulting filter was installed on the test rig shown schematically in FIG. 
1 of the accompanying drawings. 
The filter was challenged with mains water at an input pressure of 2 bar 
and a crossflow rate of 4 l.min.sup.-1. After 30 minutes, the 
transmembrane flux was calculated to be 142 l.sup.-1.m.sup.2.h.sup.-1. 
The filter was then challenged with a 0.5% yeast solution, made as 
described above. After 300 minutes, the transmembrane flux was calculated 
to be 14.4 l.sup.-1.m.sup.2.h.sup.-1. The turbidity of the filtrate was 
determined by its UV absorbance at 595 nm. It was found to be less than 
that of the feed solution by a factor greater than 500. 
4.3 A sample of the filter tube of Example 1c, length 160 mm, was inserted 
into the woven polyester tube referred to above in Example 3.2. The 
resulting filter was installed on the test rig shown schematically in FIG. 
1 of the accompanying drawings. 
The filter was challenged with mains water at an input pressure of 2 bar 
and a cross-flow rate of 2 l.min.sup.-1. After 30 minutes, the 
transmembrane flux was calculated to be 99 l.sup.-1.m.sup.2.h.sup.-1. 
The filter was then challenged with a 0.5% yeast solution, made as 
described above. After 30 minutes, the transmembrane flux was calculated 
to be 18 l.sup.-1.m.sup.2.h.sup.-1. The turbidity of the filtrate was 
determined by its UV absorbance at 595 nm. It was found to be 1000 times 
less than that of the feed solution. 
5. COLLAPSE OF FILTER 
A filter formed from the filter tube of Example 1d and a woven polyester 
tube supplied by InHome Limited under the designation "Standard Green 
Tube". The resulting filter was installed on the test rig shown 
schematically in FIG. 1 of the accompanying drawings. 
The filter was challenged with contaminated industrial water at an input 
pressure of 2 bar and a cross-flow rate of 2.2 l.min.sup.-1 for a period 
of 48 hours. The supply of water was then cut off and the filter allowed 
to empty for 5 minutes. The filter tube substantially regained its 
original flattened shape. It was then reconnected to the supply of water 
for a period of 55 minutes. This 60 minute cycle was repeated a further 7 
times and the transmembrane flux was measured periodically. It was noted 
that, at the start of each cycle, the water discharged from the collection 
end of the filter was discoloured, indicating that the regained shape of 
the filter was accompanied by release of accumulated material from the 
internal surface of the tube. 
FIG. 2 shows how transmembrane flux varies with time, providing a 
comparison of performance under constant pressure conditions, and 
conditions in which the pressure is varied cyclically, respectively. 
6. ADDITIONAL SEATION SYSTEMS 
6.1 The filter referred to above in Example 4.3 was adapted by fitting to 
it of four spring clips at intervals of 2.7 cm. As a result, the 
transverse dimension of the filter was reduced from about 15 mm to about 7 
mm. 
It was found that the transmembrane flux tested in crossflow mode with the 
0.5% yeast solution was about 50% higher after 300 minutes than was found 
without the clips, tested at the same crossflow rate and pressure. There 
was no significant difference in the turbidity of the filtrate from the 
two experiments. 
6.2 A separation system was made from three filters referred to above in 
Example 4.3, each of length 1.0 m. The filters were connected to receive 
liquid in parallel by means of appropriate manifolds. 
The system was challenged with contaminated industrial water at an input 
pressure of 3 bar and a combined cross-flow rate of 6 l.min.sup.-1. During 
the test, the transverse dimension of the membrane was reduced from about 
15 mm to about 9 mm at each of three points spaced about 25 cm apart, by 
means of rollers connected to a motorised drive unit. The drive unit 
causes the rollers to move backwards and forwards along the tubes. The 
transmembrane flux was measured periodically. 
Changes in the transmembrane flux are shown in the graph of flux versus run 
time presented as FIG. 3, with comparative values for filters with 
stationary rollers, and with no rollers at all. 
6.3 A separation system was made from a filter referred to above in Example 
4.3, of length 13 m. It was wound into a spiral on a support provided by 
an array of pins, diameter 6 mm, height 25 mm, located at approximately 15 
mm centres on lines at 45.degree., mounted on an acrylic sheet. The 
internal diameter of the wound filter was about 20 cm. A further acrylic 
sheet was mounted on the wound filter to retain it in place. The 
transverse dimension of the filter when pressurised was reduced from about 
15 mm to about 9 mm at each of the pins, as a result of the distortion of 
the filter by the pins. The resulting system is shown in FIGS. 4a and 4b. 
The system was supplied with a 1% solution of baker's yeast at an input 
pressure of 3 bar and a crossflow rate of 2 l.min.sup.-1. The 
transmembrane flux was measured periodically. It was seen to drop from an 
initial value of at least 152 l.sup.-1.m.sup.2.h.sup.-1 to a value of 39 
l.sup.-1.m.sup.2.h.sup.-1 after 400 minutes. After 400 minutes, the 
turbidity of the filtrate was less than that of the yeast solution by a 
factor of at least 1000. 
After 400 minutes, the crossflow rate and pressure were reduced to zero for 
1 minute, allowing the filter tube to regain its original flattened shape. 
The retentate flow was then resumed for 60 seconds, at an increased rate 
of 8 l.min.sup.-1, and reduced pressure of less than 0.5 bar. It was noted 
that the turbidity of the retentate increased dramatically during the 60 
second flushing as the yeast accumulated on the internal surface of the 
filter tube was dislodged. 
The filtration was resumed after the flushing operation, under the original 
test conditions. The initial transmembrane flux on resumption of the 
filtration was calculated to be 61 l.sup.-1.m.sup.2.h.sup.-1, while the 
turbidity of the filtrate remained less than that of the yeast solution be 
a factor of at least 1000.