Abstract:
A cartridge apparatus for water treatment includes a header having a permeate cavity, a plurality of support elements spaced apart from the header, and a plurality of subgroups of hollow fiber membranes, the membranes in each subgroup having first ends potted in the header, and second ends potted in a respective one of the plurality of support elements.

Description:
This application is a continuation of prior U.S. patent application Ser. No. 10/968,883, abandoned, which was filed on Oct. 21, 2004, which is an application claiming the benefit under 35 USC 119(e) of U.S. Provisional Application No. 60/512,700, which was filed on Oct. 21, 2003, and U.S. Provisional Patent Application No. 60/614,980, which was filed on Oct. 4, 2004, the entire contents of which are incorporated herein by reference. 
    
    
     FIELD 
     The teaching disclosed herein relates to membrane modules for water treatment. 
     BACKGROUND 
     Immersed membranes are used for extracting clean water (permeate) from a tank of contaminated water or mixed liquor. The membranes are often provided in the form of assembled modules, each module having many hollow fiber membranes extending between two headers for collecting permeate that passes through pores of the membrane walls into the lumina of the membranes. Streams of air bubbles may be provided in the tank to rub against the membranes for cleaning purposes. The air bubbles also help to create circulation patterns in the tank. 
     Although these features improve filtration along a central portion of the length of the fibers, a dead zone remains at either end of the fibers where sufficient cleaning of the fibers and circulation of the water does not occur, thereby causing the fibers at either end to become plugged or fouled by contaminants, and resulting in a corresponding decrease in the filtration capacity of the system. Presence of the dead zone is primarily caused by the headers themselves, which interfere with the flow of air bubbles and water circulation at either end of the fibers, and which inhibit movement of the fibers since the ends of the fibers are fixed to the headers. 
     Efforts have been made to overcome these drawbacks associated with two-header membrane modules by using single-header modules. However, single header modules involve various issues such as supporting the fibres, aerating the fibres and building groups of modules. There is a need for improvements relating to these and other issues. 
     SUMMARY 
     It is an object of the teaching disclosed herein to provide improvements on the prior art. It is another object to provide a membrane module and elements of a membrane system and a process for a liquid with a membrane module or system. These and other objects are provided by the features described in the claims. The following summary provides an introduction to one or more inventions which may reside in a combination or sub-combination of features provided in this summary or in other parts of this document. 
     According to one aspect, a filtration apparatus for filtering liquid has a filtration cell which may have generally vertical front and rear panels spaced apart to define a cell length, and generally vertical side panels extending between the front and rear panels and spaced apart to define a cell width. A plurality of header segments are provided in the cell, each header segment having a permeate channel. Spaces between the header segments allow water to flow upwards into the cell. A plurality of hollow fiber membranes are associated with each header segment and extend generally upwards from the header segments, each membrane having a lumen in flow communication with the permeate channel of the associated header segment. Each membrane has a lower end secured to the associated header segment and a loose upper end. 
     According to a second aspect, a filtration apparatus for filtering liquid has an enclosure with a divider structure in the enclosure and dividing the enclosure into a plurality of filtration cells. Each cell has generally vertical front and rear panels spaced apart to define a cell length, and generally vertical side panels extending between the front and rear panels and spaced apart to define a cell width. A plurality of header segments are provided in each cell, each header segment having a permeate channel. A plurality of hollow fiber membranes are associated with each header segment and extend generally upwards from the associated header segment. Each membrane has a lumen in flow communication with the permeate channel of the associated header segment, a lower end secured to the associated header, and a loose upper end. 
     According to a third aspect, a filtration apparatus for filtering liquid has a plurality of cartridges, each cartridge defining a filtration cell and having generally vertical front and rear panels spaced apart to define a cell length, and generally vertical side panels extending between the front and rear panels and spaced apart to define a cell width. A plurality of header segments are provided in each cell, each header segment having a permeate channel and a first and second releasable resealable connection fitting at either end of the permeate channel. A plurality of hollow fiber membranes are associated with each header segment and extend generally upwards from the associated header segment. Each membrane has a lumen in flow communication with the permeate channel of the associated header, a lower end secured to the associated header segment, and a loose upper end. A frame is provided for holding the cartridges in at least one row of aligned cartridges wherein the first and second connection fittings of adjacent aligned cartridges cooperate to connect the corresponding permeate channels of the header segments of the aligned cartridges in operative fluid communication. 
     In any one filtration apparatus according to the first, second, or third aspects, the header segments can be elongate and aligned generally parallel to the cell length. The header segments can be arranged in spaced apart parallel relationship across the cell width, the spacing between the header segments providing a passage for a liquid to be filtered to flow upwards into the cell. The cell length can be about 10 to 40 percent longer than the cell width. The cell length can be about 20 to 30 cm, and the cell width can be about 15 to 25 cm. The apparatus can have from 2 to 4 header segments, and in particular can have three header segments. The panels can include one or more structures of the group of structures consisting of generally solid plastic plates, apertures, and horizontal rungs. The apparatus can have about 700 to 1200 membranes associated with each header segment. The membranes can be potted to the header segment in about 8 to 12 rows, each row having about 70 to 130 membranes. 
     The filtration apparatus, according to the third aspect, can include releasable attachments between at least one of the header segments of each cartridge and the frame, the releasable attachments allowing the frame to releasably hold the cartridges by the at least one header segment. The releasable attachments can include keys extending from the headers and slots in the frame to receive the keys in sliding fit. 
     According to a fourth aspect, a method for filtering liquid includes providing a filtration apparatus having (i) a filtration cell with generally vertical front and rear panels spaced apart to define a cell length, and generally vertical side panels extending between the front and rear panels and spaced apart to define a cell width, (ii) a plurality of header segments the cell, each header segment having a permeate channel, and (iii) a plurality of hollow fiber membranes associated with each header segment and extending generally upwards from the associated header segment, each membrane having a lumen in flow communication with the permeate channel of the associated header segment and a lower end secured to the associated header segment and a loose upper end. The method further includes immersing the apparatus in a liquid at ambient pressure, applying suction to the lumens to withdraw permeate, and aerating the liquid such that fluid currents rise upwards along the outer surfaces of the hollow fiber membranes, urging the membranes to a generally vertical position when aerated. 
     According to a fifth aspect, a method for filtering liquid includes providing a filtration apparatus having (i) an enclosure, (ii) a divider structure in the enclosure and dividing the enclosure into a plurality of filtration cells, each cell having generally vertical front and rear panels spaced apart to define a cell length, and generally vertical side panels extending between the front and rear panels and spaced apart to define a cell width, (iii) a plurality of header segments in each cell, each header segment having a permeate channel, and (iv) a plurality of hollow fiber membranes associated with each header segment and extending generally upwards from the associated header segment, each membrane having a lumen in flow communication with the permeate channel of the associated header segment and a lower end secured to the associated header and a loose upper end. The method for filtering a liquid further includes immersing the apparatus in a liquid at ambient pressure, applying suction to the lumens to withdraw permeate, and aerating the liquid such that fluid currents rise upwards along the outer surfaces of the hollow fiber membranes, urging the membranes to a generally vertical position when aerated. 
     According to a sixth aspect, a method for filtering liquid includes providing a filtration apparatus having (i) a plurality of cartridges, each cartridge defining a filtration cell and having generally vertical front and rear panels spaced apart to define a cell length, and generally vertical side panels extending between the front and rear panels and spaced apart to define a cell width, (ii) a plurality of header segments in each cell, each header segment having a permeate channel and a first and second releasable resealable connection fitting at either end of the permeate channel, (iii) a plurality of hollow fiber membranes associated with each header segment and extending generally upwards from the associated header segment, each membrane having a lumen in flow communication with the permeate channel of the associated header and a lower end secured to the associated header segment and a loose upper end, and (iv) a frame for holding the cartridges in at least one row of aligned cartridges wherein the first and second connection fittings of adjacent aligned cartridges cooperate to connect the corresponding permeate channels of the header segments of the aligned cartridges in operative fluid communication. The method for filtering a liquid further includes immersing the apparatus in a liquid at ambient pressure, applying suction to the lumens to withdraw permeate, and aerating the liquid such that fluid currents rise upwards along the outer surfaces of the hollow fiber membranes, urging the membranes to a generally vertical position when aerated. 
     In any one of the filtration methods according to the fourth, fifth, and sixth aspects, the aeration can be intermittent, having a supply of air that is cycled on and off in cycles of between about 15 and 120 seconds in duration. Furthermore, the cycle time for which the air supply for aeration is off can have a duration limited to not more than 30 seconds. 
     According to a seventh aspect, a filtration apparatus for filtering liquid includes a filtration cell having a perimeter defined by generally vertical panels. A plurality of header segments are provided in the cell, the header segments oriented in spaced relationship providing a passage between the header segments for a liquid to be filtered to flow upwards into the cell, and each header segment having a permeate channel. A plurality of hollow fiber membranes are associated with each header segment and extend generally upwards from the associated header segment, each membrane having a lumen in flow communication with the permeate channel of the associated header segment and a lower end secured to the associated header segment and a loose upper end. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the teaching disclosed herein and to show more clearly how it may be carried into effect, reference will now be made by way of example, to the accompanying drawings in which: 
         FIG. 1  is a front view of a filtration apparatus; 
         FIG. 2  is a top view of the apparatus of  FIG. 1 ; 
         FIG. 3  is a partial perspective view of an apparatus for sealing membrane ends; 
         FIG. 4  is a side view of the apparatus if  FIG. 1 ; 
         FIG. 5  is a front view of another embodiment of a filtration apparatus, shown in combination with a tank containing liquid; 
         FIG. 6  is a top view of the apparatus of  FIG. 5 ; 
         FIG. 7  is a side view of the apparatus of  FIG. 5 ; 
         FIG. 8  is a perspective view of a divider structure of  FIG. 5 ; 
         FIG. 9  is a front sectional view of  FIG. 6  taken along the lines  9 - 9 ; 
         FIGS. 10   a  and  10   b  are perspective views showing optional embodiments of the structure of  FIG. 8 ; 
         FIG. 11  is a perspective view of an alternate embodiment of a filtration apparatus; 
         FIG. 12  is a perspective view of a single cartridge of the apparatus of  FIG. 11 ; 
         FIG. 13  is side view showing portions of two adjacent cartridges of  FIG. 12  in greater detail; 
         FIG. 14   a  is a side view of the apparatus of  FIG. 11 ; and 
         FIG. 14   b  is an exploded view showing a portion of the apparatus of  FIG. 11  in greater detail. 
         FIG. 15   a  side view of an optional embodiment of the apparatus of  FIG. 7 ; 
         FIG. 15   b  is a side view of another optional embodiment of the apparatus of  FIG. 7 ; 
         FIG. 16  is a perspective view of an alternate embodiment of loose upper ends of membranes; 
         FIG. 17  is a side view of the embodiment of  FIG. 16 ; 
         FIG. 18  is a front view of another alternate embodiment of loose upper ends of membranes; and 
         FIG. 19  is a side view of the embodiment of  FIG. 18 . 
     
    
    
     DETAILED DESCRIPTION 
     A filtration apparatus is shown generally at  100  in  FIG. 1 . In  FIG. 1 , the apparatus  100  is shown in combination with a tank  102  that contains liquid  103  from which filtered water (permeate) is to be withdrawn. The tank  102  may be part of a bioreactor, a drinking water filtration plant, or another type of solid/fluid separation system. In some applications, providing a tank  102  may not be required. For example, the apparatus  100  may be a self-contained unit holding liquid  103 . Alternately, the apparatus  100  may be co-extensive with a tank or used in an apparatus and/or process as described in PCT Patent Application No. PCT/CA00/01354, published as WO 01/36075 which is incorporated herein in its entirety by this reference to it. 
     The apparatus  100  has a housing  106  with vertical side panels that define a filtration cell. More specifically, the housing  106  has opposed vertical end panels  105 , and vertical side panels  107  extending between the end panels  105  ( FIG. 2 ). The top and bottom faces of the housing  106 , as well as the sides, can present openings to allow liquid  103  to circulate through the interior space of the housing  106 . The panels  105 ,  106  can comprise plastic plates, plates with apertures, or rungs of string, wire, plastic or other material. 
     The apparatus  100  is further provided with a plurality of header segments  108 , each having a plurality of hollow fiber membranes  110  extending therefrom. The header segments have permeate collection channels  109  for collecting permeate from the membranes  110 , as discussed further hereinafter. 
     Various constructions and dimensions of membranes  110  and headers  108  are possible for obtaining satisfactory results with the apparatus  100  of the present invention. It has been determined that particularly satisfactory filtration results may be obtained by using generally narrow, elongate header segments to form rectangular skeins of vertically oriented membranes  110 . Such a configuration permits close-packing of a plurality of skeins to efficiently use the available space in the tank for filtration, while allowing adequate penetration of aeration bubbles to interior fibers. Narrow spacing can be provided between adjacent skeins to permit efficient aeration and circulation around the membranes  110  of the apparatus  100 . 
     Accordingly, in the embodiment illustrated, three header segments  108   a ,  108   b  and  108   c , are provided in the cell  104 . The header segments  108  are elongate and are oriented generally horizontally and parallel to each other, extending between the opposed end panels  105  of the housing  106 . In particular, the header segments  108  in the illustrated embodiment have dimensions of about 6 cm high by 4 cm wide by 25 cm long. The header segments  108  are spaced apart by about 3 cm. 
     The header segments  108  need not be separate header elements, but may be distinct portions of a single header. In the embodiment illustrated in  FIGS. 1 and 2 , the three header segments  108   a ,  108   b , and  108   c  are part of a larger single header element  80  which fits snugly within the housing  106  of the cell  104 . Two elongate apertures  82  are provided in the header  80 , extending across the header  80  in parallel spaced-apart relationship. The spaces between and on either side of the apertures  82  define the header segments  108   a ,  108   b ,  108   c . Each header segment  108   a ,  108   b ,  108   c  may be provided with a distinct permeate channel  109   a ,  109   b ,  109   c , respectively, for example, as convenient for potting the membranes by a fugitive potting method as described in U.S. Pat. No. 5,639,373 which is incorporated herein in its entirety by this reference to it. 
     A plurality of hollow fiber membranes  110  extends upwardly from each of the header segments  108 . The membranes  110  are flexible, tube-like elements, each having a hollow interior channel or lumen  111  and a water permeable wall structure. Each membrane  110  is secured to its associated header segment  108  such that the lumen  111  is in flow communication with the permeate collecting channel  109  of the header segment  108 . 
     In the illustrated embodiment, the membranes  110  are of a braid reinforced construction, having a tubular braided inner layer that is coated with a coagulated film-forming polymer. Further details of such membranes can be found in U.S. Pat. No. 5,472,607 (Zenon Environmental Inc.), which is incorporated herein in its entirety by this reference to it. The membranes  110  can have an outer diameter of about 2.3 mm, and a length of about 160 cm. This configuration provides membranes  110  that are flexible and nearly buoyant so that they stand generally upright in the liquid  103  when aerated, and can sway back and forth under the influence of currents in the liquid  103 , despite having some stiffness to inhibit their collapse when not aerated. 
     The membranes  110  have lower ends  112  that are secured to the header segments  108 , and upper ends  114  that are loose. As mentioned previously, the header segments  108  each have a hollow channel  109  for collecting permeate. The lower ends  112  of the membranes  110  are secured to the header segments  108  such that the lumens  111  of the membranes  110  are in flow communication with the permeate collecting channels of the header segments  108 . In the illustrated embodiment, the lower ends  112  of the membranes  110  are potted in the header segments  108  using known potting techniques. The membranes  110  are potted in rows of about 9 by 100 membranes  110  along the upper surface of the header segments  108 . 
     The upper ends  114  of the membranes  110  are loose, meaning that they are not secured in a fixed position relative to the housing  106  of the cell  104  or to a fixed upper header, but are able to sway back and forth. The loose upper ends  114  may be free from connection to any other member of the apparatus  100 , or the ends  114  may be tethered to each other and to the housing  106 . In this embodiment, the upper ends  114  of the membranes are free, having no connection to other membranes or to any fixed structural number. Other configurations of the loose upper ends  114  of the membranes  110  will be described hereinafter. 
     The upper ends  114  of each individual membrane  110  may be plugged, sealed, or otherwise obstructed to prevent the flow of raw liquid  103  into the lumens of the membranes  110 . In the illustrated embodiment, the upper ends  114  of the membranes  110  are sealed by subjecting the ends  114  to ultrasonic waves and squeezing or pinching the ends  114  closed. In particular, as best seen in  FIG. 3 , an ultrasonic machine  90  with a 1.3 cm by 25 cm horn  92  can be used to seal about 20 to 30 fiber membranes  110  simultaneously. The fiber membranes  110  can be pre-wetted and evenly positioned on a receiving plate  94  having spacers  96 , prior to sealing. Alternately, the fibre membranes  110  can be distributed randomly in the receiving plate  94  and some connection between adjacent membranes  110  tolerated. 
     The filtration apparatus  100  can advantageously be provided with aerators  118  for providing streams of air bubbles directed upwards along the outer surfaces of the hollow fiber membranes  110 . In the embodiment illustrated, two aerators  118  are provided, positioned below and between the pairs of headers  108   a ,  108   b , and  108   b ,  108   c . Other locations and numbers of aerators may also be used. Further, the header  108  may be adapted to form part of an inverted air box aerator used in conjunction with continuous cyclic air as described in U.S. application Ser. No. 10/171,997 which is incorporated herein in its entirety by this reference to it. The air bubbles can be provided continuously or cyclically as described in U.S. Pat. No. 6,550,747 which is incorporated herein in its entirety by this reference to it. 
     During normal operation of the filtration apparatus  100 , a pressure gradient (or trans-membrane pressure) is applied across the walls of the hollow fiber membranes, such that the liquid  103  side of the membranes is at a higher pressure than the permeate side of the membranes. For example, the trans-membrane pressure may be provided by suction on the lumens. As a result, permeate is drawn from the liquid  103  through the porous layers of the hollow fiber membranes  110  and into the lumens  111 . The permeate is then collected by the header segments  108 . The streams of air bubbles from the aerators  118  help to keep the membranes from becoming plugged or fouled by scrubbing the outer surfaces of the membranes, and by inducing flow currents in the liquid  103  that circulate liquid  103  with more concentrated contaminant levels away from the fibers  110 , being replaced by liquid  103  with less concentrated contaminant levels. Aeration may be provided continuously or intermittently. 
     The panels  105 ,  107  of the housing  106  of each cell  104  corral or fence-in a group  124  of membranes  110 . The group  124  of membranes  110  is supported in a generally upright configuration by the housing  106 . The vertical panels  105 ,  107  of the housing  106  of the cell  104  provide support surfaces that limit the lateral distance that the membranes  110  can curl under the force of gravity when no liquid  103  surrounds the membranes  110  or when they are not aerated. 
     An example of the support provided to the groups  124  of hollow fiber membranes  110  by the housing  106  is shown in  FIG. 4 . In particular, it has been observed that when the tank  102  contains no liquid  103  or the membranes are not aerated, the hollow fiber membranes  110  curl into a generally “s”-shaped or similarly curved configuration. By suitably spacing the panels  105 ,  106  apart from each other in relation to the headers  108 , the curved portions at either horizontal boundary of the “s”-shaped configuration of each group  124  of membranes  110  contact and lean against the housing  106 . Each group  124  of membranes  110  can wind its way back and forth between, and in contact with, the housing  106  of the cell  104 . In this way the group  124  of membranes  110  is supported by the housing  106  so that the membranes  110  remain satisfactorily unentangled and in a generally upright orientation. 
     It has been determined that the spacing apart of the panels  105 ,  107  of the housing  106  requires careful consideration to achieve optimal results with the apparatus  100  of the present invention. If the panels  105 ,  107  are spaced too far apart, entanglement of the membranes  110  may not be sufficiently alleviated. Also, too great a space between the partitions  121  may reduce the effectiveness of aeration supplied in the apparatus  100 , since streams of air bubbles emitted from the aerators may more easily disperse horizontally, particularly since the upper ends  114  of the membranes  110  are loose. If the panels  105 ,  107  are spaced too closely together, the swaying motion of the membranes  110  may be reduced, and it may be more difficult to circulate highly contaminated water away from the membranes  110 . This may increase the susceptibility of the membranes  110  to becoming fouled with contamination. The total number of cells and panels is also increased unnecessarily. 
     It has been determined that satisfactory results can be obtained for the apparatus  100  having headers  108  and membranes  110  as described above, using rectangular shaped cells  104  having a width W and a length L. The width W can be sized to accommodate from 2 to 4 header segments  108 , spaced apart as described above. The length L can be sized to be slightly greater than the selected width. The width W, being perpendicular to the axis of the header segments  108 , is advantageously smaller than the length L because the membranes tend to collapse into the space between the header segments  108 . 
     In the embodiment illustrated, each cell  104  of the apparatus  100  has a length L of about 30 cm and a width W of about 25 cm. Each cell  104  holds three header segments  108  and a group  124  of membranes  110  having about 2700 hollow fiber membranes  110 . 
     It will be appreciated that the shape of the apparatus  100  facilitates the efficient packing of multiple filtration elements  100  in a side-by-side arrangement to obtain a desired filtration capacity. Furthermore, the filtration capacity provided by the apparatus  100 , either singly or in multiple elements, provides an efficient use of space, i.e. high filtration capacity for a given sized tank  102 . 
     Referring now to  FIGS. 5 and 6 , an alternative embodiment of a filtration apparatus according to the present invention is shown generally at  200 . The filtration apparatus  200  has an enclosure  206  which can contain or be immersed in liquid  103 . The enclosure  206  has opposed vertical end plates  205  and side plates  207  extending between the end plates ( FIG. 5 ). The top and bottom faces of the enclosure  206  can be open to facilitate the circulation of liquid  103  from a tank  102  when the apparatus is used in a tank immersion configuration. 
     Three headers  208   a ,  208   b  and  208   c  are provided in the illustrated embodiment of the apparatus  200 . The headers  208  are arranged in parallel in the enclosure  206 , each extending between the opposed end plates  205 . A plurality of hollow fiber membranes  110  extend from each header  208 . 
     The apparatus  200  is fitted with a divider structure  220  having generally vertical partitions  221  that divide the interior space of the housing  206  into a plurality of fenced cells  104 . The partitions  221  of the divider structure  220  can by themselves or in combination with the plates  205 ,  207  provide the panels  105 ,  107  of the housing  106  for each cell  104 . In other words, the partitions  221  of the divider structure  220  can comprise unique longitudinal panels  226  and transverse panels  228  to define the housing  106  of the cells  104 , or alternatively, the partitions  221  can comprise some unique panels  226 ,  228  and some panels  205 ,  207  of the enclosure  206  to define the cells  104 . Some unique panels  226 ,  228  of the divider structure  220  can be provided in flush contact with panels  205 ,  207  of the enclosure  206  to provide additional support and reinforcement for the enclosure  206  and cells  104 . 
     As best seen in  FIG. 6 , about one third the length of each of the headers  208   a ,  208   b ,  208   c  are contained within each cell  104  formed by the partitions  221  of the divider structure  220 . Accordingly, each third of the headers  208   a ,  208   b ,  208   c  provides a separate and distinct header segment  108   a ,  108   b , and  108   c  for each cell  104  while requiring only 3 potting operations. It is to be understood that the headers  208  may extend below the lower edges  236  ( FIG. 7 ) of the transverse panels  228  of the divider structure  220 , but that the segments of the headers  208  defining the header segments  108  associated with the cells  104  are nevertheless considered to be part of the corresponding cells  104 . 
     As best seen in  FIG. 7 , the partitions  221  of the divider structure  220  form the housing  106  for the cells  104 . Each cell  104  contains a group  224  of membranes  110 , and the housing  106  of each cell  104  supports the group  224  of membranes in a generally upright orientation. As described earlier for the cell  104  in  FIG. 4 , the hollow fiber membranes  110  curl into a generally “s”-shaped or similarly curved configuration when the no liquid  103  surrounds the membranes  110 , or when the membranes  110  are not aerated. The partitions  221  of the divider structure  220  (comprising panels  105 ,  106  of the housing  106  of each cell  104 ) are spaced so that the curved portions at either horizontal boundary of the “s”-shaped configuration of each group  224  of membranes  110  contact and lean against the panels  221 . Each group  224  of membranes  110  can wind its way back and forth between, and in contact with, the housing  106  of the cell  104 . In this way the group  224  of membranes  110  is supported by the housing  106  so that the membranes  110  remain satisfactorily unentangled and in a generally upright orientation. 
     Referring now to  FIG. 8 , further details of the divider structure  220  will be provided. In the embodiment illustrated, the divider structure  220  has generally vertical partitions  221  comprising longitudinal and transverse panels  226  and  228 , respectively. The partitions  226 ,  228  may be individual panels fastened together to form an assembled divider structure  220 , or the panels  226 ,  228  may be integrally formed by, for example, an injection moulding process, to produce a unitary divider structure  220 . 
     The longitudinal panels  226  can be in the form of plastic panels, having upper and lower edges  230  and  232 , respectively. The transverse panels  228  can also be plastic panels, having upper and lower edges  234  and  236  respectively. The upper edges  230  and  234  of the panels  226 ,  228  need not lie in the same horizontal plane, but can be offset. In particular, the upper edges  234  of the transverse panels  228  can be lower than the upper edge  230  of the longitudinal panels  226  so that circulation of liquid  103  between adjacent cells  104  is facilitated. Similarly, the lower edges  232  and  236  can be offset from each other to enhance flow characteristics between adjacent cells  104 . 
     Referring now to  FIG. 9 , the lower edges  232  of the longitudinal panels  226  of the illustrated embodiment of the apparatus  100  extend below the vertical position of the headers  208  and aerators  218  in the tank  102 . The lower edges  236  of the transverse panels  228  are positioned above the headers  208  and aerators  218 . 
     By having the lower edges  232  of the longitudinal divider panels  226  extend below the aerators  218 , the stream of air bubbles emitted from the aerators  218  remains confined in a narrower vertical channel which can improve the cleaning action of the membranes  110  by the air bubbles as they travel upwards in the tank  102 . 
     The apparatus  200  may have the aerators  218  situated below the headers  208  (as shown in  FIG. 5 ), or in an alternate position above the headers  108  (as shown in  FIG. 8 ). By positioning the aerators  218  slightly above the top surface of the headers  208 , as shown in  FIG. 8 , the dead zone above the headers  208  may be further reduced in some embodiments. 
     The apparatus  200  can be advantageously supported above the bottom of the tank  102  to further facilitate circulation of liquid  103  through the housing  206  of the apparatus  200 , or to provide a sludge collection and removal zone in the tank  102  below the apparatus  200 . 
     Referring now to  FIG. 10   a , an alternate divider structure  220   a  has transverse divider panels  228   a  having apertures  227  therethrough. In  FIG. 10   b , another alternate divider structure  220   b  has transverse panels  228   b  comprised of spaced apart horizontal rungs  229 . The rungs  229  can be constructed of wire or yarn, and may be of the same material as the hollow fiber membranes  210 . The alternate divider structures  220   a  and  220   b  can advantageously facilitate fluid communication between cells  104  on either side of the partitions  221 . 
     Another alternate embodiment of a filtration apparatus is shown generally at  300  in  FIG. 11 . The filtration apparatus  300  has a plurality of filtration cartridges  302  supported by a frame  304 . Each cartridge  302  generally corresponds to a single cell  104  of the apparatus  100 , and multiple cartridges  302  can be packed together side-by-side to obtain a desired filtration capacity. 
     Details of the cartridge  302  are best seen in  FIG. 12 . Each cartridge  302  has a housing  306  that comprises generally vertical panels  321 . The housing  306  of each cartridge  302  corresponds to the housing  106  of each cell  104 . In the embodiment illustrated, the panels  321  of the housing  306  include longitudinal side panels  326 , and front and rear transverse panels  328 ,  329  respectively. The housing  306  has a length L of 25 cm and a width W of 20 cm. 
     Each cartridge  302  further has a plurality of header segments  308  in the housing interior. In the embodiment illustrated, each header segment  308  is elongate and has a permeate channel  309  extending through the header segment  308  ( FIG. 13 ). First and second releasable resealable connection fittings  340  and  342 , respectively, are provided at either end of the permeate channels  309  of each header segment  308 . Further details of the connection fittings  340 ,  342  are provided hereinafter. 
     A plurality of hollow fiber membranes  110  are associated with each header segment  308  in each cartridge  302 . The membranes  110  extend generally vertically when aerated between the panels  321  of the housing  306 . The membranes  110  of each plurality of membranes are in flow communication with the permeate channel  309  of the corresponding associated header segment  308 . The membranes  110  have lower ends  112  secured to the header segments  308 , and loose upper ends  114 , as described previously. In the embodiment illustrated, the upper ends  114  of the membranes  110  are free. 
     Referring again to  FIG. 11 , the frame  304  for holding the cartridges  302  of the apparatus  300  has a base  360  with longitudinal side members  362  and transverse front and back members  364 ,  366 , respectively. The frame  304  can have additional intermediate transverse members  367  positioned parallel to, and spaced between, the front and back members  364 ,  366 . In addition, the frame  304  can be provided with uprights  368  and upper braces  369 , if desired, for reinforcing the apparatus  300  and further supporting the upper portions of the housings  306  of the cartridges  302 . 
     The base  360  of the frame  304  is sized to hold the cartridges  302  in at least one row  370  of aligned cartridges  302 . In the illustrated embodiment, three rows  370  of three aligned cartridges  302  are held in the frame  304 . In each row  370  of aligned cartridges  302 , the header segments  308  of each cartridge  302  extend parallel to the row  370 , and the first and second connection fittings  340 ,  342  of adjacent cartridges  302  cooperate to connect the corresponding permeate channels  309  of the header segments  308  of the aligned cartridges  202  in operative fluid communication. 
     More specifically, as best seen in  FIG. 13 , the first connection fitting  340  can have an axially extending male fitting  344  with an o-ring  346  seated in an o-ring groove  348 . The second fitting  342  can comprise a female fitting  350  having an axial bore  352 , the inner surface of which is shaped to engage the o-ring  346  of the first connection fitting  340  to provide a sealed connection between the permeate channels  309  of the adjacent headers  308 . 
     To pack or assemble multiple cartridges  302  into filtration units of larger capacity, rows  370  of cartridges  302  can easily be formed by aligning the connection fittings  340 ,  342  of two adjacent cartridges  302  and moving the cartridges together in the direction of arrow  372  in  FIG. 13 . This procedure can be repeated with additional cartridges  302  to form a row  370  of a desired length. For example, an assembled row  370  of three cartridges  302  can be seen in  FIG. 14   a.    
     Referring again to  FIG. 14   a , each row  370  of cartridges  302  will have a front cartridge  302   f  adjacent the front member  364  of the frame  304 , and a rear cartridge  302   r  adjacent the rear member  366  of the frame  304 . Accordingly, first connection fittings  340  are exposed towards the rear of the frame  304 , and second connection fittings  342  are exposed towards the front to the frame  304 . A permeate pipe  376  having transverse connection fittings  342  along its length can conveniently be provided adjacent the rear member  366  of the frame  204  to connect with the exposed connection fittings  242  of the rear cartridges  302   r . Similarly, a permeate pipe  374  having transverse connection fittings  344  along its length can be provided adjacent the front member  364  of the frame  304  to connect with the exposed connection fittings  342  of the front cartridges  302   f . The permeate pipes  374 ,  376  can further facilitate the assembly of a filtration unit having a desired capacity. 
     Referring now to  FIG. 14   b , the apparatus  300  can further be provided with releasable attachments  380  between the header segments  308  of the cartridges  302  and the frame  304 . The releasable attachments  380  can permit the frame  304  to releasably hold the cartridges  304  by the header segments  208 . This can facilitate removal and installation of the cartridges  302  in the frame  304  at times of, for example, maintenance of the apparatus  300 . 
     In the embodiment illustrated, the releasable attachments  380  comprise T-shaped slots  382  provided in the transverse members  364 ,  366 ,  367  of the base  360  of the frame  304 . Matching keys  384  are provided on the underside of the header segments  308  so that the cartridges  302  can be assembled to the frame  304  by sliding the keys  384  into the slots. As well, releasable clamps (not shown) can be provided to clamp the cartridges  302  to the frame  304 . 
     As best seen in  FIG. 15   a , the cartridges  302  can be adapted to permit vertical assembly into the frame  304  of the apparatus  300 . In particular, the header segments  308  can be provided with down-turned ends  390  that engage U-shaped couplers  392  supported by the base  360  of the frame  304  at a location below and between adjacent cartridges  302 . The exposed ends of the foremost and rearmost cartridges  302   f ,  302   r  can be coupled to permeate pipes  374 ,  376  using elbow couplers  394 . Releasable, resealable connection fittings  304 ,  342  can be provided between the header segments  308  and couplers  392 ,  394 . 
     As best seen in  FIG. 15   b , the cartridges  302  can also be adapted to permit formation of vertical rows  370 . In this embodiment, the cartridges  302  are fitted with vertical permeate tubes  396  at either end of the header segments  308 . The vertical tubes  396  are in flow communication with the header segments  308 , and extend the full height of the cartridge  302  so that the tubes  396  of vertically adjacent cartridges  302  in the row  370  can engage each other in flow communication. Upper and lower permeate collection pipes  398  can be provided to engage the exposed upper and lower ends of the tubes  396  of the cartridges  302  in the row  370 . Releasable resealable connection fittings  340 ,  342  can be provided between adjacent tubes  396  and between the tubes  396  and the collection pipes  398  to facilitate installation and removal of the cartridges  302 . 
     Other configurations of a filtration apparatus are also possible. For example, a filtration apparatus can have fiber membranes that are 80 cm long, contained in a cell that has L and W dimensions of 25 cm and 8 cm, respectively. Such a configuration was found to provide satisfactory (no tangling, no sludge fouling) operation in mixed liquor having solids concentration levels of 5-35 g/L, using 5-10 scfm cyclic aeration. Furthermore, the aerators can be optimized for use with membranes having free upper ends. In particular, fewer holes can be provided along the sides of aerators adjacent the panels defining each cell, and more holes can be provided for aeration between header segments in a cell. 
     As well, the loose upper ends  114  of the membranes  110  need not be free from all attachment to fall within the scope of the present invention. For example, with reference now to  FIG. 16 , the upper ends  114  of a portion of adjacent membranes  110  associated with a single header segment  108  may be attached to each other to form a sub-group  400  of membranes. This attachment may take the form of a support strip  402  extending along a row of membranes  110  in a direction parallel to the header segment  108 , so that the sub-group  400  of membranes comprises a sheet  404  of membranes. The sheet  404  of membranes  110  may have only one row of membranes  110 , or may be two or more rows of membranes  110  thick. In the embodiment illustrated, each sheet  404  comprises two rows of membranes  110 , and a total of five sheets  404  are associated with each header segment  108 . Gaps  406  are presented between adjacent support strips  402 , which, combined with the mobility of the support strips  402  in the liquid  103 , may inhibit formation of a dead zone below the support strips  402 . 
     Securing the upper ends  114  of the membranes  110  to the support strips  402  can advantageously eliminate the need for sealing the upper ends  114 , as described earlier. For example, the support strip can take the form of plastic or resin into which the upper ends  114  are potted, thereby simultaneously closing off the upper ends  114  of the membranes  110 . Furthermore, the support strips  402  can be tethered to the housing  106  of the cell  104  by tethering strings  410  ( FIG. 17 ), so that mobility of the support strips is maintained, while at the same time entanglement and collapse of the membranes is prevented. 
     Another embodiment providing loose upper ends  114  of the membranes  110  in the apparatus  100  is shown in  FIGS. 18 and 19 . The upper support strip  402  tethered to the housing  106  by strings  410  is used as a support surface over which the membranes  110  are hung in a U-shaped configuration. Accordingly, each single membrane  110  has two lower ends  112  secured to a header segment  108 , and two upper ends  114  adjacent the support strip  402 . A plurality of membranes  110  provided along the length of a header segment  108  and a single support strip  402  define a membrane sheet  404 . A spacing member  412  ( FIG. 19 ) can be provided across the sheet  404  adjacent the support member  402  to facilitate holding the membranes  110  in position. The spacing member  412  can comprise, for example, but not limited to, a length of string or yarn weaved across the membranes  110 . 
     While preferred embodiments of the invention have been described herein in detail, it is to be understood that this description is by way of example only, and is not intended to be limiting. The full scope of the invention is to be determined by reference to the appended claims.