GB2063089A - Filter cartridge components and manufacture - Google Patents

Abstract

A filter unit is described a number of which may be bonded together in stack formation to form a disposable cartridge, optionally including an integral disposable housing. The unit comprises a thin plastics support (88) having flat filter membranes (68), permanently bonded to its upper and lower surfaces. The support has a central opening, a sealing rim (60), and on each surface, inner circumferential lands (66, 76) outer circumferential lands on the rim and intervening ribs or similar projections, the membranes being bonded and sealed to the lands and ribs. Each support surface adjacent the central opening has a circumferential coupling and spacing means (74, 86) for bonding stacked units together in spaced relation. Radial passages (90, 92) communicate with the spaces between the ribs. A method for bonding the membranes (68) to the support (88), using a liquid that is a solvent for the material of the support but is not a solvent for the membrane material, is also disclosed. <IMAGE>

Description

SPECIFICATION Filter cartridges and methods and components for making them.

This invention relates to filter units to filter cartridges employing such units and to methods for making filter units.

Membrane filters of various polymeric materials are known and are generally thin porous structuures having porosities between about 50-80% by volume.

They are relatively fragile and are commonly used with various types of mechanical support or reinforcement. Flow rates of liquids through such membranes per unit of area are a function of pore size, flow rates decreasing with decreasing pore size. To obtain high flow rates through filters with fine pores, for example below about one micron, relatively large filter areas are needed. Such areas have heretofore been provided by using large individual filters or by using a number of smaller individual filters in parallel. For use in critical applications such as sterilization, such membranes and their supporting apparatus must be free of leaks or defects capable of passing small particles or organisms.

Numbers of small filters have heretofore been hand assembled for parallel flow with supporting plates and associated apparatus, then tested, and if necessary, sterilized, often at the user's site at considerable cost and inconvenience. The operations must be repeated if the hand assembly fails the necessary tests. The mechanical parts are generally cleaned and re-used, only the filters being replaced.

One assembly heretofore provided in disposable plastics has also been mechanically secured with relatively moveable parts.

Individual membrane filters of large area have been supported flat or cylindrically, or have been pleated for disposition in compact housings. Holders for flat membranes are large, for a given filter area, are usually not disposable, and also require disassembly, cleaning reassembly and testing with each change of filter. Pleating of fragile membranes creates stress concentrations at the folds, permits flexing of the fragile membranes in use, normally requires interleaving flow screens on one or both of the upstream and downstream sides, requires potting and adhesives to seal the ends and overlapping seams, and requires considerable hand operation.

Because of concern for possible failures at the folds, seams, or ends, a separate flat final filter is sometimes used in series with pleated cartridges for added assurance in critical applications, for example, in sterilizing pharmaceuticals and intravenous fluids. In addition, the use of a number of different materials in pleated cartridge construction increases the sources for extractibles into the filtrate.

In one aspect, the present invention provides a filter unit for bonding with similar units into integrated disposable stacks, said unit having a central opening for axial liquid flow therethrough and comprising a support having porous membrane filters secured to its upper and lower surfaces, each of said surfaces having inner and outer circumferential lands and intervening ribs or other formations defining substantially a common plane, the membranes being permanently sealed to the surfaces of said lands and ribs or other formations in said plane, each support surface between the central opening and inner land having coupling means for bonding to complimentary coupling means of a superposed filter unit, the coupling means on at least one of said support surfaces extending beyond the surface of the adjacent membrance, said support having internal passageways for liquid between said membranes and said central openings, said openings and passageways providing the exclusive liquid paths through the unit.

The support structure may be made in any suitable manner in one or more pieces. If the support comprises a single piece, the inner land and coupling means of one support surface may be offset from the inner land and coupling means of the other surface from the axis of the central opening, the inner passageways of the support terminating within the offset between them. Where the support comprises two or more pieces, they may be welded together to form a unitary structure of a single material and one of the pieces may contain both outer lands joined together liquid-tight to avoid leakage.

In a second aspect of the present invention, a disposable filter cartridge is provided which comprises a plurality of filter units in accordance with the present invention permanently bonded together at said coupling means in stacked relationship to provide a central axial flow channel for liquids, the central flow channel being sealed at one end and having connecting means permanently bonded to the other end for connection to a liquid conduit, said bonded coupling means spacing adjacent membranes apart to define a flow channel for liquid there-between external to said units.

The filter units may have supports of the same plastics material and they may be bonded together at their coupling means by heat or solvent welding.

The cartridges can be used in conventional cartridge housings having an inlet and outlet for liquids communicating respectively with the periphery and central opening of the cartridge. The normal flow path for liquids through the cartridge is thus from the periphery, between the membranes of adjacent filter units, through the membranes and internal passageways, to the central opening. Reverse flow at somewhat reduced pressure is also possible since the membranes are bonded to the intervening support ribs, as well as the marginal lands, and are well supported against back pressure.

Alternatively, the filter cartridges may be mounted and sealed in their own disposable housings which may comprise end caps and a central spacer, the end caps providing liquid inlet and outlet to the cartridge.

The end caps may be identical and the number of filter units in the cartridge may be varied merely by changing the height of the central spacer. The end caps and spacer of the housing may be of the same plastics as the filter unit supports and they may also be joined and sealed by welding.

By this means only two materials are used, the plastics material of the support and housing, and the material of the membranes.

No adhesives, potting compounds, O-rings or the like are required, thereby minimizing extractibles and mechanical stresses from thermal expansion and contraction.

The present invention also comprehends a method for bonding a filter membrane to a plastics supportcomrpising absorbing a predetermined amount of solvent into the pores of the membrane, said solvent being a solventforthe support but not the membrane, contacting the dry support with the wetted membrane, maintaining contact until said solvent solvates said support and wicks a portion thereof into pores of the membrane, then removing the solvent.

Such a method is reliable and reproducible, readily automated, and minimizes damage to the filter membrane and occlusion of its filter volume.

The solvent may be removed by drying.

Preferably, said pores are substantially filled with solvent, without excess. Conveniently, the filter can be saturated with excess solvent and spun to remove the excess.

Using the method as defined, only a minimum contact pressure between support and filter is required which avoids mechanical damage to the membrane.

The structure of the resulting filter and support is believed to be different from and superior to structures obtained with prior techniques whereby heat, solvent or adhesive is applied to the support. By the present method, plastics material of the support is wicked or imbibed only into the membrane pores over the ribs or the like, without significant lateral spread which occludes filter area. Indeed, the imbibed plastics material need not, and normally does not extend through the membrane, thereby leaving portions over the ribs, remote therefrom, with oblique channels through the membrane. Experience to date, for example, indicate that for a given fluid, fluid pressure and support structure, the present procedure reduces flow rate by only about 15%. The ribs or other formations preferably have a curved surface to minimize contact.

The bonding procedure according to the present invention is applicable to the bonding of a filter membrane to any support structure having support features, as shown for example in U.S. patents 3,854,907; 3,730,353; and 3,471,019. Heretofore, fine ribbed or dimpled supports have not been bonded to the filters for reasons of oclusion, control and the like. Bonding by the method of the present invention is practical and can provide significant back-flow protection against pressure surges and, in some cases, can obviate the need for check valves. The method can also be conveniently applied to laminate membranes to fibrous plastics supports such as supports of woven cloth, non-woven felts, and spunbonded filaments for reinforcement.

Specific embodiments of the present invention in all its aspects will now be described by way of example, and not by way of limitation, with refer ence # the accompanying drawings in which: Figure 1 is a plan view of a filter cartridge according to the present invention; Figure 2 is a side view of the cartridge shown in Figure 1; Figure 3 is vertical diametric section through the cartridge shown in Figures 1 and 2; Figure 4 is a partial plan view to enlarged scale of one part of a two part filter support according to the present invention; Figure 5 is a partial plain view similar to Figure 4 showing the second part of the two part support; Figure 6 is a section on the line 6-6 of Figure 4; Figure 7is a section on the line 7-7 of Figure 5; Figure 8 is a section on the line 8-8 of Figure 4;; Figure 9 is a section on the line 9-9 of Figure 5; Figure 10 is a section similar to Figures 6 and 7 showing the two parts assembled, together with superposed filter units shown in broken lines; Figure 11 is a section similar to Figures 8 and 9 showing the two support parts asembled; Figure 12 is a partial plan view, to reduced scale, showing the reverse side of the support part shown in Figure 4; Figure 13 is a partial plan, to reduced scale, of the reverse side of the part shown in Figure 5; Figure 14 is a diametric vertical section similar to Figure 3 showing a cartridge according to the present invention mounted in a conventional, reusable cartridge holder; Figure 15 is a partial plan view illustrating an alternative one-piece filter support; Figure 16 is a partial plan view of the reverse side of the support shown in Figure 15; and Figure 17 is a partial section on the line 17-17 of Figure 15.

Referring to the accompanying drawings, a selfcontained filter cartridge having, for example, 500 square centimeters of filter area, is shown in Figures 1 to 3. The cartridge comprises a housing 20 having an upper end cap 22, a lower end cap 24 and a spacer sleeve 26 therebetween. Preferably the end caps 22 and 24 and the spacer 26 are of the same plastics material and are welded togetherwith heat or solvent at 28 and 30, end caps 22 and 24 having a marginal tongue, and spacer 26 having complementary grooves. End cap 22 is provided with a fitting 32 adapted for connection to an outlet conduit of tubing or the like which is welded to the end cap at the weld line 34. Beneath the fitting 32, end cap 22 has a concentric ring 36 for bonding to a filter unit 38 as more fully hereinafter described. Ring 36 and the fitting 32 comprise connecting means for connecting a stack of filter units or discs 38 within the housing 20 to an outlet conduit (not shown).

A vent 40 of any suitable type is provided in the end cap 22 to permit the venting of air from the housing at start-up. This may comprise, for example, a manually opening valve which is opened to exhaust air and thereafter closed. Vent 40 is provided with a removable cap 41.

End cap 24 is similar to the end cap 22 and is provided with an inlet 42 bonded thereto at the weld line 44. If desired, end cap 24 may be provided with a vent 40 and is otherwise identical to the cap 22. A vent at each end of the housing permits use of the cartridge with either end uppermost. End cap 24 has a ring 46 similar to the ring 36. However, ring 36 is not bonded to the internal stack of filter discs 38.

Instead, the lowermost disc 38 is sealed with an end cap 48 provided with external ribs 49 or the like resting on the inner periphery of ring 46 to prevent sealing against inlet flow.

In use, a liquid to be filtered enters inlet 42, passes around the outer periphery of the stack of filter units 38, enters the spaces between the units, passes through the filters to a central channel within the stack, as hereinafter described, from which the filtrate passes out fitting 32 to the exit conduit.

One embodiment of the filter unit 38 is shown in Figures 4 to 13 comprising two interfitting parts 50 and 52. Part 50 comprises a flat disc portion 54 (Figure 13) which carries concentric upstanding ribs 56 and is provided with radial cutaway channels 58 beneath the ribs 56. Part 50 also has integral with the base 54 a solid rim 60 which has upper and lower flat lands 62 and 64. Part 50 is also provided with an inner circumferential land 66. Inner land 66 and outer land 64 together with the surfaces of the intervening ribs 56 define a common plane to which a membrane filter 68 is bonded as hereinafter described.

Both parts 50 and 52 have a central opening 70 and 72 respectively which are adapted for axial flow of liquid therethrough. Between the edge of opening 70 and the inner circumferential land 66, part 50 has upstanding circumferential coupling means 74 adapted for permanent bonding to the complementary coupling of superposed filter unit 38 to lock the two units together and space their adjacent filters 68 apart so that liquid may flow between them. Coupling 74 may have a surface of any desired configuration, but conveniently as shown is provided with an upstanding tongue on its surface.

The part 52 fits within the part 50 as shown in Figures 10 and 11 and is bonded permanently thereto to make a unitary support for filters 68 bonded to its upper and lower surface. Part 52 has a flat central portion 80 (Figure 12) similar to the portion 54 of the part 50 and is also provided with upstanding ribs 82 and radial channels 84 which are bridged by the ribs 82. The part 52 has an outer rim 76 which fits within the interior of the rim 60 of the part 50 inside the land 62. The part 52 has an inner circumferential land 78 which, when assembled with the part 50 as shown in Figures 10 and 11, defines a common plane with the land 62 and the surfaces of the ribs 82. Between the openings 72 and the land 78, the part 52 is provided with upstanding coupling means 86 which as shown has a groove complementary to the tongue of the coupling means 74 of part 50.

When the parts 50 and 52 are assembled and bonded together, preferably by welding with heat or solvent, a unitary support 88 for membrane filters 68 is provided. The membranes 68 one on each of the upper and lower surfaces of the support 88, are permanently bonded to the lands 66 and 64 and the tops of ribs 56, and the lands 62 and 78 and to the top of ribs 82.

As illustrated in Figure 10, the completed filter unit 88 has a hub about the opening 70,72 comprising the coupling means 74 on one side and the coupling means 86 on the other side, one coupling means being complementary to the other. As shown in Figure 10 in = broken lines, like filer units 38 are superimposed and bonded at their coupling means, preferably by solvent welding. The filter units 38, comprising a completed support 88 and the two filter membranes 68, may be bonded into stacks of any desired height and secured within the housing 20 as shown in Figure 3. The size of the cartridge can be varied by varying the number of units 38 included, and by varying the height of spacer 26.

The channels 58 and 84 of parts 50 and 52, respectively, terminate in openings 90 and 92 at the central openings 70 and 72, respectively, of the parts 50 and 52. In assembling the part 52 within the part 50, the channels 58 and 84 may be aligned, which will provide alignment for the openings 90 and 92, but such alignment is not required. The spaces between the ribs 56 and 82 provide arcuate channels by which liquid passing through the filters 68 may flow to the radial channels 58 and 84 and then through the openings 90 and 92 into the central opening 70,72 of the filter element 38. Such spaces between the ribs and radial channels provide internal passageways for liquid flowing through the membrane filters 68 to the central opening 70,72, from which filtrate flows axially to the fitting 32 and to an outlet conduit.

Preferably all parts of the cartridge shown in Figures 1 to 3, other than the membrane filters 68, are of a common plastics material, for example polycarbonate, and are welded together into a rigid unitary structure by means of heat or solvent. The resulting cartridge can be sterilized and used in an in-line filtering application, discarded, and readily replaced with a minimum of down-time, labour and inconvenience. While adhesive or other permanent bonding means can be employed to assemble the parts of the cartridge, welding as described is preferred and can be accomplished by automated operations. A cartridge thus assembled will consist of but two materials, the plastics comprising the housing and filter supports, and the material from which the membranes are made.This minimum number of materials not only decreases the cost and operations involved, but also minimizes extractable material.

The membranes 68 are microporous cast films or bonded or matted fibers, preferably having pores less than about 25 microns in size. Such membranes and methods for making them from a variety of polymeric materials are known. By being supported flat, without folds, and by being bonded to support ribs intermediate their peripheral margins, the membranes are well anchored against flow in either direction and are not subject to the stresses of folding or of flexing during use.

The membranes 68 may be bonded to the supports 88 in any desired manner. However, the following procedure is preferred. A predetermined amount of a solvent for the plastics material of support 88, which is a non-solvent for the membranes, is absorbed into the membrane pores. Each membrane is lightly pressed on the lands and rib surfaces of the support. The solvent solvates the plastics material, preferably a thermoplastics, mate rial of the support and wicks portions thereof into the membrane's pores lying above the surfaces of the lands and ribs, but not substantially into the flow passages of the pores between the ribs, thereby providing firm anchorage with a minimum of occlusion of filter volume. A predetermined amount of solvent can be premeasured and applied, and will spread throughout the membrane by capillarity.

Preferably, however, it has been found convenient to spin the membrane, to apply excess solvent to the membrane, and to remove the excess by the centrifugal force. This procedure substantially saturated the membrane pores, without excess, and is readily automated. After light contact sufficient to wick solvated plastics material into the membrane's pores, the membrane is dried. This has not been found to be a critical operation and normal air drying after a dwell time of 60 seconds or less has been found suitable. For the preferred materials, a dwell time of three seconds is sufficient. Any suitable solvent for the plastics comprising support 88 may be used.

Careful testing has established that the above bonding procedure effectively bonds the membranes liquid-tight against flow of liquid except through the membranes. It has also shown that the area of each membrane occluded from the flow of liquid is substantially less than is the case when other bonding techniques are used, for example by applying the solvent directly to the support or by the use of heat and pressure. The resulting bond is believed to be at least partly mechanical, crosssections of such membranes having shown extensions of support material filling the pore portions adjacent to the support.

In the embodiment currently preferred, the support 88 and housing 20 are of polycarbonate, and the membranes 68 are of cast polyvinylidene fluoride.

Each membrane is solvent welded to its support, the filter units are welded together at their coupling means, and the housing parts are solvent welded together, with trichloroethylene as solvent. The fittings to end caps 22 and 24 are prewelded ultrasonically with heat. Other suitable materials may be used.

An alternative filter cartridge of stacked discs 38 is shown in Figure 14 disposed in a reusable cartridge holder 106 of conventional design. The cartridge comprises a plurality of filter units 38 bonded together at their coupling means to form a contral axial flow channel which is closed at its lower end by a bonded cap 100. The upper end of the central flow channel is provided with connecter means 102 comprising a hollow cylinder fitted with an O-ring seal 104. The holder 106 comprises a cap 112 having an inlet 108 admitting liquid to the periphery of filter units 38 and an outlet 110, and a base 114. As shown, base 114 and cap 112 have interfitting threads 116 which seal against an O-ring 118. Any other suitable sealing means can be employed. For the sterilization of fluids, a conventional stainless steel housing with a sealing gasket and external clamp is preferred.Cap 112 is recessed beneath the inlet 110 for receiving the connector means 102, the walls of the recess co-operating with the O-ring 104to provide a liquid seal.

An alternative one-piece support 150 for a filer unit is shown in Figures 15 to 17. In this embodiment a disc has a rim 120 at its outer circumference provided with lands 122 and 124 on its upper and lower surface. Innercircumferencial lands 126 and 128 are provided which define with the outer circumferential lands 122 and 124, respectively, common planes to which membrane filters are bonded as previously described. Intermediate the lands 122 and 126, ribs 130 are provided. Similarly, ribs 132 are provided intermediate the inner and outer lands 124 and 128 on the opposite surface of the support. Ribs 130 and 132 are regularly spaced across the support surface and intersect, preferably at an angle of 90 degrees, providing a gridded structure through which liquids can flow.

Inner circumferential land 128 is radially offset and lies within the land 126, the ribs 132 extending across the gap between them to provide structural support and liquid passageways into the central opening 134. Coupling means 136 and 138 are provided, respectively, between the central opening 134 and the land 126 an the land 128. As shown, coupling means 136 and 138 have substantially flat outer surfaces which extend beyond the surface of the membranes bonded adjacent thereto. The support discs may be stacked alternately right side up and upside down with the coupling means 136 of adjacent discs in contact for permanent bonding and with the coupling means 138 bonded to coupling means 138 of another superposed disc.Alternatively, two sets of support discs may be provided which are identical except that one disc has coupling means provided with a tongue and the other disc having coupling means provided with a complementary groove. The alternative supports shown in Figures 15 to 17 may be bonded in stacks to form a central flow channel and housed in its own disposable housing or in a conventional reusable housing as previously described.

While the coupling means on the surfaces of the units 38 and the supports 150 may conveniently extend each from the surface of the disc above the adjacent membrane to provide spacing between adjacent discs, one of them can provide all or a majority of the required height and spacing.

While the filter supports are herein described for convenience and clarity as being round and having.

inner and outer circumferential lands, the supports may have any other convenient shape, oval or polygonal, and such shapes are intended to be included within the scope of the present invention.

Similarly, while the central flow opening and cartridge channel is preferably round and axially located, the opening may be off centre and of other shapes if desired. Also, the term "ribs" as herein used is intended to include any fine support structure, continuous or discontinuous, useful for providing mechanical support to a membrane filter while providing flow paths for liquids passing through the filter. In addition to discontinuous or continuous ribs as shown, for example, aligned or randomly arranged upstanding dimples of any desired crosssection may be used.

The present invention is useful with fibrous or cast microporous and ultrafiltration membranes having a pore size less than 25 microns, and particularly less than 1 micron. It may be used as a prefilter or as a final filter for blood, bacteria or the like. It is especially useful as a sterilizing unit for liquids.

When used as a final filter, prefilter materials can be included in the cartridges to protect the final membranes 68. Such coarse filters may be superposed over the membranes 68, or a cylinder of prefilter material can be bonded to the periphery of the stack of units 38 as shown in broken lines in Figure 3 at 160. Cast microporous membrane filters are preferred.

As previously described, the three part integral housing shown in Figures 2 to 3 facilitates changes in cartridge size. If desired, however, spacer 26 can be omitted and the end caps of proper size can be provided with complimentary margins and be bound together directly.

Claims (18)

1. A filter unit for bonding with similar units into integrated disposable stacks, said unit having a central opening for axial liquid flow therethrough and comprising a support having porous membrane filters secured to its upper and lower surfaces, each of said surfaces having inner and outer circumferential lands and intervening ribs or other formations defining substantially a common plane, the membranes being permanently sealed to the surfaces of said lands and ribs or other formations in said plane, each support surface between the central opening and inner land having coupling means for bonding to complimentary coupling means of a superposed filter unit, the coupling means on at leat one of said support surfaces extending beyond the surface of the adjacent membrane, said support having internal passageways for liquid between said membranes and said central opening, said opening and passageways providing the exclusive liquid paths through the unit.

2. A filter unit according to claim 1 wherein said support comprises two pieces bonded together, each piece carrying the ribs or other formations, inner land and coupling means of one surface, one of said pieces carrying an outer rim which includes the outer lands of both surfaces.

3. Afilter unit according to claim 1 wherein said support is a one-piece plastics moulding, the inner land and coupling means of one surface being within and spaced from the inner land and coupling means of the other surface, said internal passageways terminating in the space between respective inner lands.

4. A filter unit acording to claim 3 wherein ribs on one support surface extend across the space between the two inner lands, the gaps between the extended ribs forming a portion of the internal passageways communicating with the central opening.

5. A filter unit according to any of claims 1 to 4 wherein said support is of a plastics material different from said membranes, and wherein the membranes are bonded thereto by means of plastics material from the support extending into the pores of the membranes in contact with said lands and intervening ribs or other formation, the pores of the membranes between said ribs or other formations being substantially free of material from the support.

6. A disposable filter cartridge comprising a plurality of filter units according to any of claims 1 to 5, the units being permanently bonded together at said coupling means in stacked relationship to provide a central axial flow channel for liquids, the central flow channel being sealed at one end and having connecting means permanently bonded to the other end for connection to a liquid conduit, said bonded coupling means spacing adjacent membranes apart to define a flow channel for liquid therebetween external to said units.

7. A disposable filter cartridge comprising a plurality of filter units according to any of claims 1 to 5, the units being permanently bonded together at said coupling means in stacked relationship to provide a central axial flow channel for liquids, the central flow channel being sealed at one end and having connecting means permanently bonded to the other end for connection to a liquid conduit, said bonded coupling means spacing adjacent membranes apart to define a flow channel for liquid therebetween external to said units, said support comprising a plastics material different from the material of said membranes, said membranes being bonded to the support by means of support material extending into the pores of the membrane in contact with the lands and ribs or other formations of the support, the pores of the membranes between ribs being substantially free of support material.

8. A disposable filter cartridge comprising a plurality of filter units according to any of claims 1 to 5, the units being permanently bonded together at said coupling means in stacked relationship to provide a central axial flow channel for liquids, the central flow channel being sealed at one end and having connecting means permanently bonded to the other end for connection to a liquid conduit, said bonded coupling means spacing adjacent membranes apart to define a flow channel for liquid therebetween external to said units, said support comprising plastics material different from the material of said membranes, said membranes being bonded to the support by means of support material extending into the pores of the membranes in contact with the lands and ribs or other formations of the support, the pores of the membranes between ribs or other formations being substantially free of support material, all parts of the cartridge other than the membranes being of a common plastics material, all of said parts being integrally welded together.

9. A disposable filter cartridge comprising a plurality of filter units according to any of claims 1 to 5, the units being permanently bonded together at said coupling means in stacked relationship to provide a central axial flow channel for liquids, the central flow channel being sealed at one end having connecting means permanently bonded to the other end for connection to a liquid conduit, said bonded coupling means spacing adjacent membranes apart to define a flow channel for liquid therebetween external to said units, said support comprising plastics material different from the material of said membranes, said membranes being bonded to the support by means of support material extending into the pores of the membranes in contact with the lands and ribs or other formations of the support, the pores of the membranes between ribs or other formations being substantially free of support material, said cartridge having membranes with pore sizes below one micron and being adapted to sterilize liquids.

10. A disposable filter cartridge comprising a plurality of filter units according to any of claims 1 to 5, the units being permanently bonded together at said coupling means in stacked relationship to provide a central axial flow channel for liquids, the central flow channel being sealed at one end and having connecting means permanently bonded to the other end for connection to a liquid conduit, said bonded coupling means spacing adjacent membranes apart to define a flow channel for liquid therebetween external to said units, said support comprising plastics material different from the material of said membranes, said membranes being bonded to the support by means of support material extending into the pores of the membrane in contact with the lands and ribs or other formations of the support, the pores of the membranes between ribs or other formations being substantially free of support material, said cartridge comprising a housing having a cap for each end and a spacer therebetween, said spacer having mating means for each end cap and being inegralyy welded thereto, one end cap at the sealed end of said central flow channel having an inlet for liquid, and the other end cap including said connecting means bonded to the other end of said central channel.

11. A method for bonding a filter membrane to a plastics support comprising absorbing a predetermined amount of solvent into the pores of the membrane, said solvent being a solvent for the support but not the membrane, contacting the dry support with the wetted membrane, maintaining contact until said solvent solvates said support and wicks a portion thereof into pores of the membrane, then removing the solvent.

12. A method according to claim 11 wherein said pores of the membrane are substantially filled with solvent with excess.

13. A method according to claim 11 wherein excess solvent is applied to the membrane to fill the pores, and the excess is removed by spinning the membrane.

14. A filter unit substantially herein before described with reference to, and as shown in, Figures 4 to 13 of the accompanying drawings.

15. A filter unit substantially as herein before described with reference to, and as shown in, Figures 15, 16 and 17 of the accompanying draw ings.

16. A disposable filter cartridge substantially as herein before described with reference to, and as shown in, Figures 1 to 3 and employing filter units as claimed in claim 14 or claim 15.

17. A disposable filter cartridge substantially as herein before described with reference to, and as shown in, Figure 14 and employing filter units as claimed in claim 14 or claim 15.

18. A method of bonding a filter membrane to a plastics support substantially as herein before described with reference to the accompanying drawings.