Abstract:
An apparatus for filtering a liquid in a tank has a plurality of elements and a frame for holding the elements while they are immersed in the liquid. The elements have a plurality of hollow fibre membranes attached to and suspended between an upper header and a lower header. The membranes are in fluid communication with one or more permeate channels in one or more of the headers. Releasable attachments between the headers and the frame allow the frame to releasably hold the elements by their headers. The size and configuration of the frame determines the positions of the upper and lower headers of each element relative to each other. Connections between the permeate channels and one or more permeate collection tubes attached to the frame are releasable and resealable connections which are made or broken automatically by the movements involved in inserting or removing an element into or out of the frame.

Description:
[0001]     This is a continuation of U.S. application Ser. No. 10/146,934 filed May 17, 2002, which is a continuation-in-part of U.S. Ser. No. 09/889,352 filed Jul. 17, 2001, issued as U.S. Pat. No. 6,790,360 on Sep. 14, 2004, which is a National Stage entry of PCT/CA00/01359 filed Nov. 15, 2000. All of the applications listed above are incorporated herein, in their entirety, by this reference to them. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention relates to filtering membranes and particularly to modules of immersed, suction driven, ultrafiltration or microfiltration membranes used to filter water or wastewater.  
       BACKGROUND OF THE INVENTION  
       [0003]     Submerged membranes are used to treat liquids containing solids to produce a filtered liquid lean in solids and an unfiltered retentate rich in solids. For example, submerged membranes are used to withdraw substantially clean water from wastewater and to withdraw potable water from well water or surface water.  
         [0004]     Immersed membranes are generally arranged in modules which comprise the membranes and headers attached to the membranes. The modules are immersed in a tank of water containing solids. A transmembrane pressure (“TMP”) is applied across the membrane walls which causes filtered water to permeate through the membrane walls. Solids are rejected by the membranes and remain in the tank water to be biologically or chemically treated or drained from the tank.  
         [0005]     U.S. Pat. No. 5,639,373, issued to Zenon Environmental Inc. on Jun. 17, 1997, describes one such module using hollow fibre membranes. In this module, hollow fibre membranes are held in fluid communication with a pair of vertically spaced headers. TMP is provided by suction on the lumens of the fibres through the headers. Other modules are shown in U.S. Pat. No. 5,783,083 issued to Zenon Environmental Inc. on Jul. 21, 1998, PCT Publication No. WO 98/28066 filed on Dec. 18, 1997 by Memtec America Corporation and European Patent Application No. EP 0 931 582 filed Aug. 22, 1997 by Mitsubishi Rayon Co., Ltd. As discussed in these documents, various means are provided for fixing modules together generally permanently into larger units.  
       SUMMARY OF THE INVENTION  
       [0006]     It is an object of the present invention to improve on the prior art. It is another object of the present invention to provide a filtration apparatus comprising a plurality of elements, for example, elements of immersed, suction driven, hollow fibre membranes, mounted to a frame. Embodiments of the invention provide few components to interfere with the flow of tank water through the apparatus, efficient permeate pipe connections, elements that may be removed easily and without interfering with adjacent elements, elements that may be economically manufactured to a wide range of sizes, an apparatus that may be assembled with variable spacing between elements, and a distance between headers of the elements that can be altered to account for membrane shrinkage in use. The objects of the invention are met by the combinations of features, steps or both described in the claims. The following summary may not describe all necessary features of the invention which may reside in a sub-combination of the following features or in a combination of some or all of the following features and features described in other parts of this document.  
         [0007]     In various aspects of the invention, the invention is directed at an apparatus for filtering a liquid in a tank having a plurality of elements, and a frame for holding the elements while they are immersed in the liquid. The elements have a plurality of hollow fibre membranes attached to and suspended between an upper header and a lower header. The membranes are in fluid communication with one or more permeate channels in one or more of the headers. Releasable attachments between the headers and the frame allow the frame to releasably hold the elements by their headers. While the frame is holding the elements, the elements themselves do not have any means for holding the headers in position relative to each other. For example, if the frame were removed, the headers would be free to move out of position relative to each other. As a result, the size and configuration of the frame determines the positions of the upper and lower headers of each element relative to each other. When out of the frame, the elements may be inserted into a separate carrying frame, if desired, for transport or handling.  
         [0008]     An assembled filtration apparatus, which may be called a cassette, has a plurality of elements held such that the membranes are generally vertical when immersed in the liquid in the tank. The headers may be elongated in shape and held in a generally horizontal orientation when the membranes are immersed in the tank. The frame holds the elements so as to provide a spacing between adjacent elements and allows tank water to rise vertically through the frame and past the elements.  
         [0009]     To assemble a filtration apparatus, the upper headers are slid into the frame, for example, through track and slider mechanism that may support the element whenever about one quarter of the length of the upper header is inserted into the frame. The lower header may similarly slide into the frame, for example through another track and slider mechanism. Or, while the element is supported by the upper header, the lower header may be swung into position to attach to releasable supports which engage with the ends of the lower header.  
         [0010]     The frame holds or restrains the elements in place, but the restraint provided by the frame may be released for a selected element individually. The selected element may be removed by reversing the steps for assembly without disassembling the remainder of the module. Connections between the permeate channels and one or more permeate collection tubes attached to the frame are releasable and resealable connections which are made or broken automatically by the movements involved in inserting or removing an element into or out of the frame.  
         [0011]     The frame may have cross bars located on uprights, the cross bars holding the elements. The vertical location of the cross bars may be changed from time to time to maintain the membranes in a slightly slackened condition although their length may decrease in use.  
         [0012]     Aerators are mounted generally below the elements and supply scouring bubbles to the cassette and circulate tank water. The elements may be narrow, each element being a rectangular skein of hollow fibres having an effective thickness of between 4 and 8 rows of hollow fibres. The headers, which may be extruded, may be thin to not greatly increase the width of the element. The attachments between the frame and the elements are positioned to provide horizontal spaces between adjacent elements, preferably at least one third of the width of the headers measured in the direction of the horizontal spacing, to promote penetration of the bubbles and tank water into the elements.  
         [0013]     Elements may be placed back to back in pairs separated by permeate pipes. The connections between the permeate pipes and the elements release when an element is pulled out of the cassette and reseal when the element is replaced in the cassette. Thus a single element can be removed for maintenance without disconnecting other parts of the permeate pipe network. A large permeate collector may be connected to a small group of elements by a short local permeate pipe with a valve that permits the small group of elements to be isolated. Thus, while waiting for repair, permeation can continue with the remaining elements. The large permeate collector may be located above the water surface and connect to an even larger collector which may be located on the edges of a tank.  
         [0014]     The headers may be made of an extrusion which may be cut to any desired length and capped with caps. The horizontal distance between the cross bars of the frame can be altered by changing the dimensions of the frame or the location of the cross bars relative to the frame. Longer or shorter cross bars can be used which hold fewer or more elements. The vertical distance between cross bars can be altered by changing the dimensions of the frame or the location of the cross bars relative to the frame. Accordingly, a cassette may be produced in a variety of sizes by altering the length of cut of one or more of the header extrusion, the cross bars or the frame members. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     Preferred embodiments of the present invention will now be described with reference to the following figures.  
         [0016]      FIG. 1  is a somewhat schematic front elevation of a filtering element.  
         [0017]      FIG. 2  is a somewhat schematic side elevation of the filtering element of  FIG. 1 .  
         [0018]      FIG. 3  is an isometric view of a header of an element of a first embodiment.  
         [0019]      FIG. 4  is an elevation of the end of 4 adjacent headers of  FIG. 3 .  
         [0020]      FIG. 5  is an isometric view of a frame for a cassette with headers attached.  
         [0021]      FIG. 6  is a close up of the top of  FIG. 5 .  
         [0022]      FIG. 7  is a diagrammatic drawing of part of two elements placed back to back and connected to a permeate pipe.  
         [0023]      FIG. 8A  is a perspective view of a header and releasable attachment of a second embodiment.  
         [0024]      FIG. 8B  is a side view of the header of  FIG. 8A .  
         [0025]      FIGS. 9 and 10  are assembled and exploded views of a component of the releasable attachment of  FIG. 8 .  
         [0026]      FIG. 11  is a perspective view of parts of a frame and releasable attachments of the second embodiment.  
         [0027]      FIGS. 12, 13  and  14  are perspective, front and side views of a frame and parts of releasable attachments of the second embodiment.  
         [0028]      FIG. 15A  is a perspective view of an element of a third embodiment.  
         [0029]      FIG. 15B  is a close up view of part of  FIG. 15A .  
         [0030]      FIGS. 16A, 16B  and  16 C are schematic views of elements and permeate connections of a third embodiment.  
         [0031]      FIG. 17  is a perspective view of a header and parts of a releasable attachment of the third embodiment.  
         [0032]      FIGS. 18A and 18B  are side views of the releasable attachment between a lower header and frame of the third embodiment. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
     A First Embodiment  
       [0033]     The following paragraphs describe a first embodiment that is shown in FIGS.  1  to  7 . Although the description below may at times refer to specific figures, some components discussed may be shown only in others of FIGS.  1  to  7 .  
         [0034]      FIGS. 1 and 2  show simplified front and side elevations respectively of a filtering element  10 . The element  10  has a plurality of hollow fibre membranes  12  in the form of a rectangular skein  14  suspended between an upper header  16  and a lower header  18 . The rectangular skeins  14  may be between four and eight layers of membranes  12  deep (five layers being shown in  FIG. 2 ), optionally up to 12 layers deep, and are in the range of several tens of membranes  12  wide. The element  10  itself does not include any permanently attached means for holding the headers  16 ,  18  in position relative to each other but the element  10  may be connected to a carrying frame if required for transport or handling. The lack of means for holding the headers  16 ,  18  in position relative to each other improves the flow of tank water about the element  10  and avoids a possible source of damage to the membranes  12 .  
         [0035]     The membranes  12  typically have an outside diameter between 0.4 mm and 4.0 mm. The length of the membranes  12  is chosen to maximize flux for a given cost according to relationships known in the art and is typically between 400 mm and 1,800 mm. The membranes  12  have an average pore size in the microfiltration or ultrafiltration range, preferably between 0.003 microns and 10 microns and more preferably between 0.02 microns and 1 micron.  
         [0036]     The upper header  16  has a permeate channel  20  in fluid communication with the lumens of the membranes  12 . The membranes  12  in  FIGS. 1 and 2  are sealed in the lower header  18 , but the lower header  18  may also have a permeate channel in fluid communication with the lumens of the membranes  12  to permit permeation from both ends of the membranes  12 . The membranes  12  are potted into the upper header  16  (and any other permeating header) such that the membranes  12  are all closely spaced apart from each other. Potting resin completely surrounds the outsides of the end of each membrane  12  to provide a watertight seal so that water can only enter the permeate channel after first flowing though the membranes  12 . Suitable potting resins include polyurethane, epoxy, rubberized epoxy and silicone resin. One or more resins may also be used in combination to meet objectives of strength and providing a soft interface with the membranes  12  and avoiding cutting edges.  
         [0037]     A potting method like that described in U.S. Pat. No. 5,639,373, which is incorporated herein by this reference, may be used to pot layers of membranes  12 . Other potting methods known in the art, include methods that produce non-layered or random arrangements of the membranes, may also be used. In particular, the methods described in Canadian Patent Application No. 2,308,234, filed May 5, 2000 by Zenon Environmental Inc., and in U.S. application Ser. No. 09/847,338, filed on May 3, 2001 by Rabie et al., both of which are incorporated herein by this reference, may be used. The thickness of the assembled mass of membranes  12  may be between 18 and 40 mm. Headers  16 ,  18  to accommodate such masses of membranes may be 40 to 50 mm wide, typically 40 mm. The potting densities may be between 10% and 40%. For example, an element  10  may use membranes  12  as used in commercially available ZW 500 (TM) modules made by Zenon Environmental Inc. which have an outside diameter of about 2 mm, an un-potted length (meaning the unsupported length of membrane  12  between the upper header  16  and lower header  18 ) of 1,600 to 1,900 mm, and a pore size of approximately 0.1 microns.  
         [0038]     Referring to  FIG. 3 , the upper header  16  is shown. The lower header  18  is the same would be mounted in an inverted position. The upper header  16  includes a body  22  preferably extruded from a suitable plastic such as PVC or ABS. The extrusion can be cut to a wide range of sizes as desired. A back cap  24  is attached to the body  22  by gluing or welding. The body  22  includes a key  26  running the length of the top of the upper header  16 . The back cap  24  is shaped to extend the key  26 . The key  26  fits into slots in cross bars  30  of which only short sections are shown. The back cap  24  has an upper wing  32  and a lower wing  34 . The back cap  24  and the body  22  each have an upper channel  36  and a lower channel  38 . A front cap is attached to the front of the body  22  but has been omitted from  FIG. 3  to show the cross-section of the body  22 . The front cap need not have any wings  32 ,  34  but it does have channels  36 ,  38 ,  
         [0039]     Referring to  FIG. 4 , four upper headers  16  are attached to a section of cross bar  30  spaced to leave about 20 to 25 mm between adjacent upper header  16 . The lower headers  18  are similarly attached to another crossbar  30  but in an inverted position. The cross bar  30  can be cut to any desired length. To avoid the need to cut slots  28  into a long cross bar, the one piece cross bar  30  shown can be replaced with a standard extruded section, such as an inverted “C” channel, which supports any suitable hanger containing a slot  28 . In that case, the standard extrusion is cut to a desired length and an appropriate number of hangers are attached or slid into it which allows the number of elements  10  to be easily varied.  
         [0040]     The upper headers  16  and their associated upper wings  32 , lower wings  34 , upper channels  36  and lower channels  38  are all designated a, b, c, d to indicate which of those parts is associated with which upper header  16 . As shown, the upper wing  32  of a first upper header  16  engages the upper channel  36  of an adjacent upper header  16  and the lower wing  34  of the first upper header  16  engages the lower channel  38  of an adjacent upper header  16  on the other side. But, the upper wings  32  and lower wings  34  do not interfere with each other in the direction of the length of the upper headers  16 . Accordingly, each upper header  16  can be moved into or out of its position in a direction parallel to the upper header  16 . Further, although the cross bar  30  provides support at only one point, a moving upper header  16  is supported and vertically positioned by its adjacent upper headers  16  aong its travel. This makes it much easier to insert or withdraw an element  10  despite the lack of (a) means within the element  10  itself for maintaining separation between the headers  16 ,  18  or (b) continuous frame channels paralleling the length of each header  16 ,  18  which would add many parts, add to the overall cost and manufacturing time, as well as interfere with bubbles and tank water moving past the headers  16 ,  18 . A releasable catch can be incorporated into the slot  28  and key  26  structure, typically at the front only, to provide a releasable restraint in the direction of the headers  16 ,  18 .  
         [0041]     Referring to  FIGS. 5 and 6 , a cassette  50  includes a frame  40  for holding several elements  10 . The frame  40  includes top and bottom, front and back cross bars  30 , uprights  42  and struts  44  as shown. Three elements  10  (with membranes  12  removed for clarity) are shown being withdrawn from the frame  40 . Extra blank (ie. unpotted) headers  48  are optionally included between the uprights  42  to provide support for the wings  32 ,  34  of the first element  10  on each side. An element  10  may be completely withdrawn and then supported by hand or a single element carrying frame (not shown) may be placed against the frame  40 . The element  10  is then slid into the carrying frame which may allow the element  10  to be more easily worked with.  
         [0042]     The length of the uprights  42  is chosen as appropriate for any desired length of membranes  12 . The vertical distance between cross bars  30  is chosen so that the membranes  12  will be slightly slacked, their free length being, for example, 0.1% to 2% more than the distance between proximal faces of the headers  16 ,  18 . Particularly in wastewater applications where the tank water will be warm, ie. 30-50 C, the membranes  12  may shrink within the first few weeks or months of operation. To account for this shrinkage, the uprights  42  may be provided with a series of mounting holes  46  which allow at least one set of the upper or lower cross bars  30  to be moved to maintain the membranes  12  in a slightly slackened position. Although not shown, a suitable aerator (designs are known in the art) may be mounted to the frame  40  or placed on a tank floor below the frame  40  to provide bubbles from below the cassette  50 . The aerator is designed and positioned to encourage bubbles and tank water to flow upwards through the frame  40  and past the elements  10 , through the spaces between adjacent elements  10  and between the membranes  12  within the elements  10 .  
         [0043]     To connect the headers  16 ,  18  to permeate pipes, the back of any permeating headers  16 ,  18  are fitted with header permeate connections  52  that can be released and resealed to a permeate pipe located behind the headers  16 ,  18  and permit movement of the element  10  parallel to the headers  16 ,  18 . For example,  FIGS. 5 and 6  show commercially available clip on adapters sold under the trade mark UNI-SPRAY. These connectors  52 , however, require a clip to be released at the back of the element  10  which is difficult to do if the elements  10  are placed back to back to share common permeate pipes.  
         [0044]     Referring to  FIG. 7 , pairs of cassettes  50  (partially shown, frames  40  omitted, for example) are placed back to back with a local permeate pipe  60  in between them. The frames  40  (not shown) of the two cassettes  50  are tied together to maintain a fixed distance between them. The upper headers  16  (and lower headers  18  if they are permeating) include male fittings  54  which releasably form a seal with a female fitting  56  attached to the local permeate pipe  60 . The seal is made by means of O-rings  58  fitted into O-ring grooves  66  in the male fittings  54 . The male fittings  54  are thus connected to a local permeate pipe  60  which may service a small number of elements  10 , ie. 2-6 elements  10 . The local permeate pipe  60  has an isolation valve  62 , for example a ball valve located above the water line, which permits the small group of elements  10  to be isolated from the rest of the cassette  50 . The local permeate pipes  60  connect into a larger permeate collector  64  which may be located at the level of even larger collector which may be located at the edge of a tank. Thus, the necessary connections may be made simply and without expensive flexible pipes. If the bottom headers  18  are also permeating, appropriate male fittings  54  are attached to the bottom headers  18  at the level of female fittings  56  on or in communication with a local permeate pipe extension  68  which may be an extension of the local permeate pipe  60 . If the bottom headers  18  are permeating headers, then the top headers may not be.  
       A Second Embodiment  
       [0045]     The following paragraphs describe a second embodiment, parts of which are shown in FIGS.  8  to  14 . Although the description below may at times refer to specific figures, some components discussed may be shown only in others of FIGS.  8  to  14  or in figures discussed with other embodiments. The second embodiment is similar to the first embodiment in many respects. Aspects of the second embodiment that do not differ substantially from the first embodiment may not be described in the following paragraphs which will concentrate on the features of the second embodiment which differ from the first.  
         [0046]     A second lower header  118  is shown in  FIGS. 8A and 8B . A second upper header  116  (not shown in this figure) is similar, but mounted in an inverted position. The second lower header  118  has a second key  126  on its lower surface that may be continuous like that of the second header  18 . Optionally, the second key  126  may be segmented, for example as shown in  FIG. 8B , which helps prevent the second key  126  from sticking in the second slot  128 , which will be described below.  
         [0047]     The second lower header  118  does not have an upper channel  36  or a lower channel  38 . A second back cap  124  of the second lower header  188  also does not have an upper wing  32  or a lower wing  34 , but rather is of a similar section as the second body  122  of the second lower header. A second front cap  125  is fitted to the front of the second body and has a pull tool fitting  180  adapted to allow a tool to pull on the second lower header  118  for removal.  
         [0048]      FIG. 8A  also shows a track piece  182  located below the second lower header  118 . A similar track piece  182  would be located above the second upper header  116 . The track piece  182  provides part of a continuous second slot  128  that the second keys  126  may slide into and be supported by. The track piece  182  is supported at both ends by the cross bars  30 . For example, in the embodiment shown, one end of the track piece  182  fits over and is supported by an abutment  184  attached to the side of a permeate pipe stub  186  resting on a cross bar  30 . The permeate pipe stub  186  is sealed at its lower end and ready to be connected to a local permeate pipe extension  68  (not shown in  FIG. 8A , refer to  FIG. 7 ) at its upper end. The permeate pipe stub  186  also has female fittings  56  in fluid communication with the inside of the permeate pipe stub  186 . The female fittings  56  are located and oriented so that when the second lower header  118  is fully inserted in the second slot  128 , a male fitting  54  (not visible) is sealingly connected to the female fitting  56 . The other end of the track piece  182  in the embodiment shown is supported by a locking clip  188  which both supports the track piece  182  relative to the cross bar  30 , but also completes the second slot  128  and releasably locks the second lower header  118  in position when the second lower header  118  is fully inserted in the second slot  128 . The locking clip  188  is held in place by fitting into a cross bar channel  192  and is located along the length of the cross bar  30  by interaction with a positioning hole  190 . The description above also applies, but with inverted orientation, for the second upper headers  116 .  
         [0049]      FIGS. 9 and 10  show the locking clip  188  in greater detail. A locking clip abutment  194  is sized and shaped to fit into and support the track piece  182 . A peg  196  fits into a peg slot  198  to provide a means for locating the locking clip  188  over a positioning hole  190 . A catch  200  fits over the body of the locking clip  188  and, in an unbent position, fills a part of the second slot  128 . However, the catch  200  has tapered faces so that the catch  200  can move out of the second slot when a second key  126  is slid into the second slot  128 . After the end of a second key  126 , or series of discontinuous second keys  126  passes the catch  200 , they are prevented from moving back out of the second slot  126 . However, a release hole  202  also provides access to the tapered faces of the catch  200 . By inserting a rod into the release hole  202 , the catch  200  can be held open to allow a second key  126  to be pulled back out of the second slot  128 . The locking clip  188  also has a foot  204  sized to engage with the cross bar channel  192 .  
         [0050]      FIG. 11  shows how the bottom part of a frame  40  ready to receive second elements  110 . As shown, a pair of struts  44  are attached to a central cross bar  30   a  and two end cross bars  30   b , only one visible. Four brackets  206  are provided to attach to uprights  42 . The central cross bar  30   a  supports a number of permeate pipe stubs  186  which in turn have abutments  184  holding track pieces  182 . The end cross bars  30   b  support locking clips  188  which support the other ends of the track pieces  182 . If the second lower headers  118  were not permeating headers, then the central cross bar  30   a  would also be used to support locking clips  188 .  
         [0051]     FIGS.  12  to  14  show a more fully assembled frame  40  forming part of a second cassette  150 . A second assembly like that shown in  FIG. 11  is inverted and placed over the assembly of  FIG. 11 . Uprights  42  hold the two assemblies together. The connection between the uprights  42  and one or both of the assemblies may be made though slots  208  which allow the distance between the two assemblies to be adjusted to fit the second elements  110 . The distance between the two assemblies may also be adjusted after the membranes  12  have been used, for example, to account for shrinking. The upper track pieces  182  are held at the central cross bar  30   a  by flow through permeate stubs  210  which connect the local permeate pipe extensions  68  to local permeate pipes  60  which are in turn connected to a permeate collector  64  mounted to upper mounting tabs  212  at the top of the frame. Lower mounting tabs  214  at the bottom of the frame  40  may be used to mount an aerator grid below the second cassette  150 . Only one of various components, such as second elements  110 , local permeate pipe extensions  68 , local permeate pipes  60  and isolation valves  62 , are shown for clarity, but these components would be repeated across the second cassette  150 . Also, although the second cassette  150  is shown as configured to collect permeate from second upper headers  116  and second lower headers  118 , it may be adapted for use with permeating second upper headers  116  only by replacing the permeate pipe stubs  186  shown on the lower central cross bar  30   a  with locking clips  188  and replacing the flow through permeate stubs  210  shown at the upper central cross bar  30   b  with permeate pipe stubs  186 . For use with permeating second lower headers  118  only, the female fittings  56  of the flow through permeate stubs  210  are plugged up or altered flow through permeate stubs not having female fittings  56  are provided.  
       A Third Embodiment  
       [0052]     The following paragraphs describe a third embodiment, parts of which are shown in FIGS.  15  to  18 . Although the description below may at times refer to specific figures, some components discussed may be shown only in others of FIGS.  15  to  19  or in figures discussed with other embodiments. The third embodiment is similar to the first and second embodiments in many respects. Aspects of the third embodiment that do not differ substantially from the first or second embodiment may not be described in the following paragraphs which will concentrate on the features of the third embodiment which differ from the first or second.  
         [0053]      FIGS. 15A and 15B  show a third element  310 . The third element  310  has a third lower header  318  and a third upper header  316  which are similar to the second lower header  118  and second upper header  116 . However, the third headers  316 ,  318  differ, for example, in having third keys  326 , third back caps  324  and third front caps  325  unlike related components of the second headers  116 ,  118 . The third element  310  shown has two permeating third headers  316 ,  318 , but like previous elements may be made with either the third lower header  318  not a permeating header or the third upper header  316  not a permeating header.  
         [0054]     Referring to FIGS.  15 B and  16 A,B,C, the third headers  316 ,  318  have third back end caps  324  with male fittings  54  that are offset from the center of the third back end caps  324 . Third back end caps  324 A (shown schematically in FIGS.  16 A,B,C as having truncated tops) have a male fittings  54  offset to one side of the center while third back end caps  324 B (shown schematically in  FIG. 16A ,B,C as having rounded tops) have a male fitting  54  offset to the other side of the center. By placing one of third back end cap  324 A and one third back end cap  324 B on the third headers  316 ,  318 , third elements  310 I and  310 II can be made have male fittings  54  offset to opposite sides. If both of the third headers  316 ,  318  are permeating, then separate third elements  310 I and  310 II need not be made, as one will be an inverted version of the other. In conjunction with third permeate pipe stubs  386  having female fittings  56  on either side, a variable horizontal spacing between third elements  310 I and  310 II can be achieved with a single design of third permeate pipe stub  386  and without needing a third permeate pipe stub  386  for each third element  310 . In particular, as shown in  FIGS. 16A and 16C , swapping third element  310 I for third element  310 II (or turning each third element  310 I or  310 II over if both third headers  316 ,  318  are permeating) significantly alters the space between third elements  310 I and  310 II. By using third permeate pipe stubs  386  that can be mounted at various positions along a cross bar  30 , two different spacings of all of the third elements  310  of a third cassette  350  can be achieved without requiring a separate local permeate pipe  60  and local permeate pipe extension  68  for each third element  310  and with only one small component, the third back caps  324 , manufactured in two versions. The ability to have variable spacing is useful, for example, because a wider spacing can be chosen for wastewater applications and a narrower spacing chosen for drinking water filtration. As shown in  FIG. 16B , an intermediate spacing may also be achieved by using a pair of third elements  310 II. The same intermediate spacing may also be achieved by using a pair of third elements  310 I.  
         [0055]     The comments made in the paragraph above regarding the third permeate pipe stubs  386  similarly apply to third flow through permeate stubs  310 . The third permeate stubs  310  also have a pair of mounting pins  220  on each side to support the end of the third track piece  382  (to be described below) at either spacing. Similar pairs of mounting pins  220  may also be provided on the third permeate pipe stubs  386  if they will also be used to support the ends of third track pieces  383 , although this is optional as will be described further below.  
         [0056]      FIGS. 15B and 17  show a third key  326  that which has a key ridge  224 . Although  FIGS. 15B and 17  show only a third upper header  316 , the third lower header  318  is the same, but is mounted in an inverted orientation. Similarly, other components of  FIG. 17  may all be used in inverted orientation at the bottom of a third cassette  350 . The key ridge  324  provides a line of contact between the distal surface of the third key  326  and the third track piece  382 . Similarly, the edges of the third track piece  382  curl inwards to provide a line of contact with the proximal surfaces of the third key  326 . These lines of contact are less prone to fouling than planes of contact.  
         [0057]      FIG. 17  also shows the connection between one end of the third track pieces  382  and a cross bar  30 . The connection between the other end of the third track piece  382  and the third permeate pipe stubs  386 , or third flow through permeate stubs  310 , was discussed above. The end shown in  FIGS. 15B and 17  is supported on a pin (not visible) one a track mounting plate  226  mounted on a cross bar  30 . The track mounting plate  226  also supports a third lock  388  that assists in keeping the third track piece  382  in position. The third lock  388  also mates with a twist knob  228  to allow the third upper header  316  to be releasably secured when it has been fully inserted into the third slot  328 .  
         [0058]     As an alternative to using an inverted version of the components shown in  FIG. 17  to releasably attach the third lower header  318 ,  FIGS. 18A and 18B  show how the third lower header  318  may be Releasably attached to the frame  40  without using a third track piece  382 . Referring to  FIG. 18A , the third top header  316  (not shown) is partially inserted, for example between about one half to three quarters of the way, into the third track piece  382  (not shown). At this point, the third lower header  318  hangs from the membranes  12 . The third lower header  318  is then pushed into its final position which is shown in  FIG. 18B . Because the third upper header  316  was only partially inserted, the third lower header  318  arcs upwards slightly. Through trial and error or measurement and calculation, a position of the third upper header  316  can be determined at which the upward movement of the third lower header  318 , despite the excess length of the membranes  12  required to produce slackened membranes  12  when the third element  310  is fully installed, allows the male fitting  54  to meet the female fitting  56  and allows the twist knob  228  to meet and be releasably connected to the third lock  388 . The third upper header  318  is then fully inserted which restores the slack in the membranes  12 . The twist knob  228  of the third upper header  316  is then engaged with the third lock  388  of the upper part of the frame  40 .  
         [0059]     The embodiments described above are examples of the invention only. Modifications and other embodiments within the scope of the invention will be apparent to those skilled in the art. The scope of the invention is defined by the following claims.