Multi-element filter container

Abstract

A multi-element filter container comprising a plurality of filter elements is disclosed. The element has a cylindrical housing and an annular header. At least a portion of the annular header surrounds the cylindrical housing at one end. The cross-sectional width and thickness of the annular header are periodically varied such that the annular header has alternating thinner and thicker regions. The elements are loaded with the thinner sections of the headers aligned. This allows tight loading of the elements without excessively compromising the mechanical strength of the header.

Description

TECHNICAL FIELD

[0001] The present invention relates to multi-element filtration vessels. BACKGROUND

[0002] Multi-element filtration vessels are widely used in water treatment facilities to purify water. Semi-permeable membranes are assembled within elements that are mounted vertically in an array within a vessel. Feed fluid is supplied through the elements where it is separated by the membranes into concentrate and permeate. A mixed gas is typically introduced into the vessel at a point below the elements. The mixed gas can perform several useful functions. When the membranes are of the hollow fiber type, the gas bubbles provide buoyancy, thereby assisting in the transport of feed fluid between or through capillaries in the fibers. The mixed gas can also perform a cleaning function and improve fluid mixing.

[0003] The filtration elements typically include a cylindrical housing that contains the membranes. A header is fitted on one end of the cylindrical housing. The header is a ring that is fitted on the cylindrical housing. The header has an outer cross-sectional dimension that is larger than the outer dimension of the cylindrical housing. The larger the diameter, means that fewer filtration elements can be packed within a given size pressure vessel. Increasing the number of filtration elements within a vessel increases its capacity. For large facilities, this can mean that fewer filtration vessels are required and / or the cost of the vessels can be spread over a larger number of filtration elements. This will greatly reduce equipment costs and operating costs. An improved ratio of filtration element area to vessel cost can also be obtained through more efficient packing. For example, the presence of edge effects within the vessel of the filtration elements can make irregular packing geometries more efficient than close packing geometries.

[0004] The shape of the filtration elements is typically cylindrical and the headers used with the filtration elements are also typically cylindrical. Making the header wall thinner can reduce the overall cross-section of the filtration element, but doing so can weaken the header to the point where it cannot withstand the physical stresses that occur during handling, shipping and use. As a result, the header wall tends to be fairly thick, which in turn increases the cross-sectional dimension of the filtration element and limits how close the elements can be packed within a pressure vessel. SUMMARY

[0005] The invention solves this problem by providing a filtration element that includes:

[0006] a) a cylindrical housing having an outer diameter, the cylindrical housing enclosing an interior portion of the filtration element, the cylindrical housing having opposite first and second ends;

[0007] b) at least one filtration membrane disposed within the interior portion of the filtration element;

[0008] c) one or more openings for allowing feed fluid to pass through at least one of the first and second ends of the cylindrical housing and into the inner portion of the filter element;

[0009] d) an annular header comprising a surrounding section around one of the opposite ends of the cylindrical housing, the surrounding section having a periodically varying cross-sectional width and thickness, such that the surrounding section of the annular header has alternating thinner and thicker regions, and

[0010] e) separate openings for removing permeate and concentrate from the filter element.

[0011] Due to the periodically varying cross-sectional width, the surrounding section of the annular header has alternating thinner and thicker regions, the thinner regions being periodically (and preferably regularly) spaced around the circumference of the surrounding section of the annular header. The thinner regions can be small, such as 1-3 mm or less in thickness, as the mechanical strength is provided by the alternating thicker regions. When packed into a multi-element filtration device, the thinner regions of the annular header are aligned, thereby allowing adjacent filter elements to be packed closer together.

[0012] The invention is also a multi-element filtration device, the device comprising:

[0013] a) a pressure vessel having a cylindrical shell;

[0014] b) a plurality of filter elements of the invention arranged vertically inside the cylindrical shell of the pressure vessel, wherein adjacent filter elements are aligned along the thinner regions of the section of the annular header surrounding the respective permeate end of the cylindrical housing of the adjacent filter elements;

[0015] a feed inlet port for introducing feed fluid into the pressure vessel; a permeate discharge port for discharging permeate produced by the plurality of filter elements from the pressure vessel; and a concentrate discharge port for discharging concentrate produced by the plurality of filter elements from the pressure vessel. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 is a front sectional view of a first embodiment of the multi-element filtration device of the invention.

[0017] Figure 2 is a partial front sectional view of a filter element of the invention.

[0018] Figure 3 is an isometric view of one end of a filter element of the invention with an attached end cap.

[0019] Figure 4is an isometric view of a ring manifold for use in the present invention.

[0020] Figure 5 is an isometric cross-sectional view of a ring manifold for use in the present invention.

[0021] Figure 5A is an isometric cross-sectional view of a top section of a filter element of the present invention.

[0022] Figure 6 is a top cross-sectional view of a ring manifold for use in the present invention.

[0023] Figure 7 is a side view of a ring manifold for use in the present invention.

[0024] Figure 7A is a Figure 7 rotational view of a ring manifold as shown.

[0025] Figure 8 is a top view of an arrangement of filter elements of the present invention.

[0026] Figure 9 is a top view of a module pack within a pressure vessel having a support plate.

[0027] Figure 10 is a front cross-sectional view of a second embodiment of a filter apparatus of the present invention. DETAILED DESCRIPTION

[0028] Turning to Figure 1 , the multi-element filter apparatus 15 includes a pressure vessel 1, which in the illustrated embodiment includes a housing 2 and a removable cover 9. Preferably, the housing 2 is cylindrical, and the pressure vessel 1 is adapted to operate with an internal pressure at least two bar higher than an external pressure. A filter chamber 16 is surrounded by the housing 2 and a pressure plate 18. Filter elements 3 are disposed vertically within the filter chamber 16. In this embodiment, the filter elements 3 are aligned and supported by at least one of the pressure plate 18, a support plate 13, and a bottom support plate 12. The support plate 13, as shown, includes an opening for receiving the filter elements 3, such that the manifolds 22 rest on the support plate 13.

[0029] In Figure 1 the illustrated embodiment, a supply fluid to be treated is introduced into the multi-element filter apparatus 15 under pressure through an inlet port 4, where the supply fluid passes through an opening in the bottom support plate 12 and enters the filter elements 3 through the holes 27 in the first ends 24 of the filter elements 3 (see Figure 2 ). The filter elements 3 contain one or more membranes 26 that allow a portion of the supply fluid to pass through ( Figure 2), thereby producing a permeate (or filtrate) stream and a concentrate (or retentate) stream, the latter containing a small portion of the feed fluid and material that did not pass through the pores of the membrane.

[0030] exist Figure 1 、 Figure 3 as well as Figure 4 In the illustrated embodiment, concentrate is withdrawn from the filter element 3 through opening 33 in the annular manifold 22 and from the pressure vessel 1 through concentrate port 6. In alternative embodiments, an opening for withdrawing concentrate from the filter element 3 may be present within the cylindrical housing 21, instead of or in addition to opening 33. In either case, permeate passes through central opening 35 of the annular manifold 22 and into the collection chamber 10 of the pressure vessel 1, from which it is removed via permeate port 5. In the particular embodiment shown, an optional end cap 23 is mounted on the annular manifold 22 and forms a conduit for permeate exiting central opening 35 and entering the collection chamber 10, where it flows through outlet 34 of the end cap 23. As shown, the end cap 23 extends through the pressure plate 18. Alternatively, the annular manifold 22 may be in direct fluid communication with the collection chamber 10 through an opening in the pressure plate 18. During operation, the collection chamber 10 generally maintains a lower pressure than the filter chamber 16. The filter element 3 and pressure vessel 1 may alternatively be designed so that the concentrate passes through outlet 34 and the permeate is removed via port 6. This may be accomplished, for example, by operating in an inside-out mode of operation with both the top and bottom ends of the hollow fiber membranes open.

[0031] The optional aerator 11 provides a mixed gas that is introduced into the first end 24 of the filter element 3. The introduction of both the gas and the feed fluid from the bottom end of the filter element 3 can generate an upward force on the feed fluid to drive the concentrate toward the openings 33. The openings 33 for the concentrate can be in the cylindrical housing 21 or the manifold 22, but these openings 33 are separated from the permeate outlet 34 by a seal (such as the potting layer 31), such as Figure 5 shown.

[0032] Although Figure 1 The illustrated embodiment is designed to operate with feed fluid introduced from the bottom and concentrate and permeate removed from the top, but in alternative embodiments, multi-element filtration apparatus 15 can be configured to operate with feed fluid introduced from the top and concentrate withdrawn from the bottom. Figure 1 The illustrated embodiment may alternatively be designed with openings in the support plate 13 to allow the concentrate to drain into the filter chamber 16, from which it may be removed through a suitable port.

[0033] Steering Figure 2, the filter element 3 comprises a cylindrical housing 21 enclosing an interior space in which one or more filter membranes 26 are disposed. The filter element 3 has opposite first and second ends 24 and 25 at opposite ends of the cylindrical housing 21. During normal operation, feed fluid is introduced into the first end 24 through the inlet hole 27, and permeate and concentrate flow out of the second end 25 into the annular header 22. Figure 2 In the embodiment shown, the inlet hole 27 passes through the lower end of the potting layer 31'. The inlet hole 27 may alternatively pass through an optional collar 29 or through a location on the cylindrical shell 21 below the potting layer 31'. The optional collar 29 may cover the first end 24. An optional end cap 23 is mounted on the annular manifold 22; in the particular embodiment shown, the end cap 23 is in fluid communication with the annular manifold 22 and the collection chamber 10 of the pressure vessel 1 to allow permeate to flow out of the filter element 3 and into the collection chamber 10 of the pressure vessel 1. Figure 1 In the embodiment shown, the fluid flows into a collecting chamber 10 .

[0034] The annular header 22 is generally shaped like a ring having a central opening 35. The portion of the central opening 35 surrounding the cylindrical housing 21 is generally circular to correspond to the outer shape of the cylindrical housing 21.

[0035] The annular header 22 includes a surrounding section 30 surrounding an end (as shown, the second end 25) of the cylindrical housing 21. Figure 6 As shown in more detail in FIG, the cross-sectional width of the surrounding section 30 varies periodically. Starting from any arbitrary point A on the outer surface of the annular header 22 surrounding the section 30, the cross-sectional width of the surrounding section 30 varies in a periodic manner around the circumference of the surrounding section 30. Figure 6 The cross-sectional width of the surrounding section 30 at point A is W max .like Figure 6 As shown, moving clockwise around the segment 30, the cross-sectional width first decreases, reaching W at point B. min <W max , then increases and reaches W again at point C max , and thereafter in W min With W max The width of the substrate decreases and increases periodically between 52 and 53 until it returns to the starting point A. This creates alternating thinner regions 52 and thicker regions 53, reaching a relative minimum at a point such as point B, where the width decreases to W. min The thinner regions 52 are periodically (and preferably regularly) spaced about the circumference of the segments 30 of the annular header 22. The periodic variation in thickness around the segments 30 produces relative minima that preferably occur at repeating intervals of 30 to 120 degrees, and most preferably occur at repeating intervals of 60 degrees.

[0036] The thickness of the thinner sections 52 at their thinnest points can be in the range of, for example, 0.5 mm to 3 mm. The thickness of the thicker sections 53 at their thickest points can be, for example, 1.4 to 5 times, for example 1 mm to 15 mm, especially 2 mm to 8 mm, the minimum thickness of the thinner sections 52.

[0037] In Figures 3-7A The surrounding sections 30 of the annular header 22 in the particular embodiment shown have a preferably generally regular hexagonal outer cross-sectional shape. If desired, the vertices 50 can be rounded or bevelled, rather than forming sharp points as Figures 3-7A The sides 51 of the surrounding sections 30 are preferably flat, as Figure 5 Alternatively, the surrounding sections 30 can have any arbitrary number of sides, ranging from as few as three (in which case the cross-section is generally triangular), four or five, or as many as, for example, 12, 10 or 8 sides.

[0038] At all points on the circumference of the filter element 3, the surrounding sections 30 preferably have the maximum cross-sectional width of any component of the filter element 3. In particular, at points on the circumference of the filter element 3 corresponding to regularly spaced relative minima in the surrounding sections 30, W min is preferably equal to or greater than the cross-sectional width of each of the cylindrical housing 21, any optional collar 29, and any end cap 23. With reference to Figure 7 and Figure 7A The minimum width W min of the surrounding sections 30 of the annular header 22 is equal to or greater than the cross-sectional width D of the cylindrical housing 21, and the maximum width W max of the surrounding sections 30 is greater than the cross-sectional width D. The end caps 23, as well as any collars 29 or other components that can be present, have a minimum width that is less than or equal to W min and preferably a maximum width that is no greater than W min Preferably, at any point on the circumference of the filter element 3, the cross-sectional width of any collars 29 and end caps 23 is equal to or less than the corresponding (at the same radial location) cross-sectional width of the surrounding sections 30 of the annular header 22 at that point. As described in relation to the surrounding sections 30 of the annular header 22, the cross-sectional width of the collars 29 (when present) can vary periodically.

[0039] Each membrane 26 can be, for example, a microfiltration membrane, an ultrafiltration membrane, a nanofiltration membrane, or a reverse osmosis membrane. Most preferred are ultrafiltration membranes. In preferred embodiments, the membranes take the form of hollow fibre membranes, but if desired they can be spiral wound or have other configurations.

[0040] A preferred hollow fiber membrane is potted within the filter element 3, typically at both opposite ends (24, 25) of the filter element 3. The capillary openings of the hollow fiber membranes are open at at least one end of the filter element 3. In certain embodiments adapted for outside-in mode of permeation operation, the capillary openings can be open at both ends, but they are preferably closed at one end. The capillary tubes can be plugged at the first end 24 (such as by embedding within the potting), while the capillary tubes at the opposite second end 25 can be open and extend through the potting to allow permeate to exit the element. Referring to Figure 2 The illustrated configuration, the potting layer 31' seals the capillary tubes of the hollow fiber membranes 26, but the potting layer 31' also includes an inlet aperture 27 through the potting layer 31' and allowing feed fluid to enter the filter element 3. In other configurations (e.g., Figure 10 ), the capillary openings can be closed at the second end 25, while the open capillary tubes at the first end 24 extend through the potting.

[0041] Permeate and concentrate are removed from the filter element 3 at separate openings. In preferred embodiments of outside-in permeation through the hollow fibers, the permeate is removed from the open capillary tubes of the hollow fibers at the top or bottom of the filter element 3. In filter elements adapted for inside-out permeation, the capillary openings are typically open through the potting on both ends of the filter element, allowing permeate to be removed from an opening (such as the opening 33 of the annular header 22 or other opening in the cylindrical housing 21).

[0042] In the illustrated embodiment, Figure 5 and Figure 5A , one end of the hollow fiber membranes 26 is potted within the header 22. In this particular embodiment, the header 22 includes an optional annular extension 32 that extends beyond the second end 25 of the cylindrical housing 21. As shown, the membranes 26 can be sealed within a potting layer 31 located within the annular extension 32, the membranes extending through the potting layer 31 and in fluid communication with the collection chamber 10 via the optional end cap 23. In the illustrated embodiment, Figure 5 , the opening 33 is provided in the annular header 22, between the potting layer 31 and the second end 25 of the cylindrical housing 21, for removal of concentrate (in the preferred outside-in mode of operation) or permeate (in the less preferred inside-out configuration). In the illustrated embodiment, Figure 5A , the opening 33 for removal of permeate or concentrate from the filter element is instead provided through the cylindrical housing 21.

[0043] In some embodiments, the hollow fiber membranes 26 are potted within the annular header 22 and / or within the cylindrical housing 21, with an opening positioned appropriately below the potting for removal of concentrate or permeate, as the case can be.

[0044] Even in the case where the membrane 26 does not extend into the annular header 22 or beyond the surrounding section 30 of the annular header 22, the annular header 22 can include an annular extension 32. For example, the annular extension 32 can serve as a mounting area for an optional end cap 23.

[0045] At a location radially aligned with the thin region (corresponding to a relative minimum in the thickness of the surrounding section 30), the cross-sectional width of the annular extension 32 is preferably no greater than the minimum width W of the surrounding section 30 of the annular header 22 min . Preferably, the cross-sectional width of the annular extension 32 at all points on the circumference of the filter element 3 is less than the cross-sectional width of the surrounding section 30 of the annular header 22. The diameter of the circular outer perimeter of the annular extension 32 is preferably less than or equal to the minimum cross-sectional width of the surrounding section 30 of the annular header 22. Similarly, the cross-sectional width of any end cap 23 at all points on the circumference of the filter element 3 is preferably equal to or less than the cross-sectional width of the surrounding section 30 of the annular header 22.

[0046] The optional end cap 23 is hollow and preferably includes at least one outlet 34 in fluid communication with both the annular header 22 and the collection chamber 10, thereby forming a fluid passageway between the annular header and the collection chamber.

[0047] The end cap 23 is preferably removably mounted on the annular header 22, as this allows the end cap 23 to be removed from the annular header 22 to, for example, perform cleaning and / or maintenance (e.g., sealing broken fibers). In the illustrated embodiment, the annular extension 32 of the annular header 22 includes threads 36 on its outer surface. In this case, complementary threads are provided on the mating inner surface of the end cap 23. In another embodiment, the annular extension 32 includes threads 36 on its inner surface, with complementary threads provided on the mating outer surface of the end cap 23. As an alternative or in addition to threads, other mounting provisions such as grooves and ridge-shaped mounts, various types of snap mounts, etc. can be provided.

[0048] A plurality of filter elements 3 of the present invention are arranged vertically inside the shell 2 of the pressure vessel 1 to form a module pack. The module pack can contain, for example, 2 to 150 or more filter elements 3, with a number of elements of 55 to 131 being particularly useful for many applications. In other preferred embodiments, the number of filter elements 3 within the module pack can be selected from 55, 61, 73, 85, 91, 97, 119, or 131.

[0049] Figure 8 and Figure 9 A suitable packing pattern using preferred filter elements 3 having cylindrical housings 21 and headers 22 is shown, in which the shape of the surrounding section 30 is a regular hexagon. As Figure 8As shown, adjacent filter elements 3 are aligned along one side of their respective circumferential segments 30 of the annular header 22. The aligned side of the respective circumferential segments 30 of the annular header 22 is the thinner region 52 of the respective header. The thicker region 53 is disposed within the gap 60 formed by the two or more filter elements 3 being brought together. This allows for close packing of the filter elements 3, as the packing density is determined by the width W of the circumferential segments 30 of the annular header 22 min The thicker segments 53 provide mechanical strength to the annular header 22. Note that a conventional circular cross-section header having constant width and thickness would typically need to be thicker than the thinner segments 52 to provide the necessary mechanical properties. This added thickness increases the spacing between elements and reduces the number of elements that can be packed within a given volume.

[0050] A module pack including the filter elements of the present invention typically has an irregular outer perimeter, as shown in Figure 9 In some embodiments, the support plate 13 occupies some or all of the space between the irregular outer perimeter of the module pack and the housing 2, or otherwise provides flow resistance between the irregular outer perimeter of the module pack and the housing 2. The support plate 13 can form a complete barrier to fluid flow from above to below the support plate 13, or can be a partial barrier having openings that allow fluid to flow from above to below the support plate only at specified locations and / or at specified rates. The support plate 13, or any portion thereof, can be sloped or otherwise adapted to promote fluid flow in one or more particular directions, such as toward a concentrate port (such as concentrate port 6 in Figure 1 In another embodiment, the support plate 13 occupies some or all of the space between the irregular outer perimeter of the module pack and the housing 2.

[0051] Figure 8 Another optional but preferred feature is shown, namely a locking device for locking adjacent filter elements together. In the particular embodiment shown, the locking device includes a clip 41 and receiving notches 40 adapted to receive and hold the clip 41. Other mechanical locking devices include various latches and / or clips. Magnetic locking devices are also useful. The locking device is preferably adapted to be opened and reclosed to allow easy removal or disassembly of filter elements from the module pack.

[0052] The multi-element filtration apparatus 15 can further include various ancillary apparatus, such as pumps, valves, seals, instrumentation, tubing, plumbing, and ancillary apparatus as can be desired or useful, etc.

[0053] The multi-element filtration apparatus of the present invention is generally operable in the same manner as conventional multi-element filtration apparatus. Feed fluid can be driven to flow within the filter elements 3 based on the pressure differential induced. In the embodiment shown in Figure 1During operation of the pressure vessel shown, for example, a feed fluid can be introduced into the pressure vessel 1 of the multi-element filter apparatus 15 via the feed inlet port 4, and if sufficient resistance to the flow of the feed fluid around the filter elements 3 is created, this alone can cause the feed fluid to flow upward within the filter elements 3. This resistance can be provided, for example, by close positioning of adjacent modules with an annular header. Alternatively, the upward movement of the feed fluid within the filter elements 3 can be caused by air bubbles. Again referring to Figure 1 , the mixed gas is supplied to the filter element 3 through the aerator 11, and the bubbles used to keep the membrane 26 clean can also cause the feed fluid to flow upward within the filter element 3. During operation, it is desirable to cause the feed fluid to flow upward within the filter element 3, but it is also possible to cause the feed fluid to flow downward by reversing the positions of the inlet port 4 and the outlet port 6.

[0054] Upon entering the interior portion of the filter element 3, the mixed gas and feed fluid travel upward through the filter element 3, contacting the filter membrane 26, which separates the feed fluid into a permeate that passes through the membrane and a concentrate or retentate that includes one or more concentrated substances that are retained by the membrane and are therefore prevented from passing through the membrane. The permeate and concentrate are removed separately from the second end 25 of the filter element 3 or thereabouts. Figure 1 In a preferred embodiment of the multi-element filter apparatus 15, for example, permeate is removed from the pressure vessel 1 via the permeate port 5. Concentrate is removed from the pressure vessel 1 via the concentrate port 6. The mixed gas may be exhausted through the concentrate port 6 or a separate exhaust port (not shown).

[0055] When operating the hollow fiber membranes in an outside-in permeation mode, the mixed gas and feed fluid are supplied to and contact the outer surfaces of the hollow fiber membranes 26. A portion of the fluid passes through the hollow fiber membranes 26 and enters the corresponding capillaries of the membranes to produce permeate, in which case the concentrate is the portion of the feed fluid and retentate that does not pass through and enter the hollow fibers. The apparatus of the present invention can also be operated in an inside-out mode, in which the feed fluid is supplied to the capillaries.

[0056] In other embodiments, multi-element filter apparatus 15 is designed and operated for top-to-bottom feed fluid flow, wherein the feed fluid is introduced above filter elements 3 and concentrate and permeate are removed from the bottom of filter elements 3. The direction of feed fluid flow can be selected independently of whether permeate is removed from the top or bottom of filter elements 3.

[0057] exist Figure 10In the illustrated embodiment, the multi-element filtration apparatus 115 includes a pressure vessel 101 that includes a housing 102 and a removable lid 109. Preferably, the housing 102 is cylindrical, and the pressure vessel 101 is adapted to operate with an internal pressure that exceeds an external pressure by at least two bar. A filtration chamber 116 is enclosed by the housing 102 and a pressure plate 118. Filtration elements 103 are disposed vertically within the filtration chamber 116. In this embodiment, the filtration elements 103 are aligned by at least one of a locator 108 and a support plate 113. The locator 108 can provide alignment, mechanical support, and / or function as a flow restrictor to maintain a pressure drop required for operation. The support plate 113, as illustrated, includes openings for receiving the filtration elements 103 such that the headers 122 rest on the support plate 113. In this embodiment, the filtration elements are inverted compared to the embodiment of FIG. 1, and the headers 122 are mounted at the bottom of the elements. Figure 1

[0058] In operation, a feed fluid enters the pressure vessel 101 via the feed inlet 104 and enters the filtration elements 122 via the openings 133. As previously described, the optional sparger 111 supplies bubbles that enter the openings 133 with the feed fluid to provide buoyancy. In this particular embodiment, the capillary tubes of the hollow fiber membranes are open at the bottom of the filtration elements 103 and closed at the top. In an outside-in mode of operation, a portion of the feed fluid that enters the openings 133 passes through the pores in the hollow fibers to create a permeate within the capillary tubes. Depending on the geometry of the Figure 10

[0059] The multi-element filtration apparatus of the present application is used to filter a wide variety of fluids, particularly aqueous fluids such as ground water, surface water, seawater, process streams from chemical operations and / or power plants, and many others. In particular embodiments, the multi-element filtration apparatus is a seawater ultrafiltration and / or microfiltration apparatus, and can be used, for example, as a prefilter for preparing seawater for reverse osmosis to produce potable water.

[0060] While the application has been described in terms of specific example embodiments, it is to be understood that numerous modifications and variations could be made by those skilled in the art without departing from the spirit and scope of the application, as set forth in the following claims.​​

Claims

1. A filter element comprising: a) a cylindrical housing having an outer diameter, the cylindrical housing enclosing an interior portion of the filter element, the cylindrical housing having opposite first and second ends; b) at least one filter membrane disposed within the interior portion of the filter element; c) one or more openings for allowing feed fluid to pass through at least one of the opposite first and second ends of the cylindrical housing and into the interior portion of the filter element; d) an annular header comprising a surrounding segment around one of the opposite ends of the cylindrical housing, the surrounding segment having a periodically varying cross-sectional width and thickness, such that the surrounding segment of the annular header has alternating thinner and thicker regions, and e) separate openings for removing permeate and concentrate from the filter element.

2. The filter element of claim 1, wherein, The thickness of the surrounding segment of the annular header varies periodically, creating relative minima at regular intervals of 30 to 120 degrees.

3. The filter element of claim 2, wherein, The thickness of the surrounding segment of the annular header varies periodically, creating relative minima at repeating intervals of 60 degrees.

4. The filter element of any one of claims 1 to 3, wherein, The segment of the annular header surrounding the outlet of the cylindrical housing has six flat exterior faces regularly arranged on the circumference of the segment, with a vertex between each pair of adjacent flat exterior faces, the vertex being optionally curved or beveled.

5. The filter element of claim 4, wherein, The segment of the annular header surrounding the outlet of the cylindrical housing has a regular hexagonal cross-section.

6. The filter element of any of the preceding claims, wherein, The at least one filter membrane extends into and is potted within the annular header.

7. The filter element of claim 6, wherein, The at least one filter membrane extends through the potting and is in fluid communication with a fluid collection area external to both the cylindrical housing and the annular header for receiving permeate.

8. The filter element of claim 7, wherein, The annular header further comprises an annular extension extending beyond the surrounding end of the cylindrical housing, the at least one filter membrane is potted within the annular extension, and the annular header has an opening between the potting and the end of the cylindrical housing for removing concentrate from the annular header.

9. The filter element of claim 8, wherein, The annular extension has a circular outer periphery having a diameter less than or equal to the minimum cross-sectional width of the surrounding segment of the annular header.

10. The filter element of claim 8 or 9, further comprising: An end cap attached to and in fluid communication with both the annular header and the fluid collection area.

11. The filter element of claim 10, wherein, The end cap is removably attached to the annular extension of the annular header.

12. A multi-element filter apparatus, the apparatus comprising: a) a pressure vessel having an outer shell; b) a plurality of filter elements as claimed in any one of claims 1 to 11, the plurality of filter elements being arranged vertically inside the outer shell of the pressure vessel, wherein adjacent filter elements are aligned along the thinner regions of the surrounding segments of the annular headers of the adjacent filter elements; a feed inlet port for introducing a feed fluid into the pressure vessel; a permeate discharge port for discharging permeate produced by the plurality of filter elements from the pressure vessel; and a concentrate discharge port for discharging concentrate produced by the plurality of filter elements from the pressure vessel.

13. The multi-element filtration apparatus of claim 12, wherein, The plurality of filter elements are removably mounted on the support.

14. The multi-element filtration apparatus of claim 13, wherein, The support includes a support plate within the pressure vessel, the support plate having openings for receiving filter elements, and the annular headers rest on and are supported by the support plate.

15. The multi-element filtration apparatus of any one of claims 12 to 14, the filtration apparatus further comprising: one or more aerators for supplying mixed gas to the plurality of filter elements.

16. The multi-element filtration apparatus of any one of claims 12 to 15, wherein, The concentrate discharge port is located above the annular header of the filter element.

17. The multi-element filtration apparatus of any one of claims 12 to 16, wherein, The plurality of filter elements form a module pack having an irregular outer periphery.

18. The multi-element device of claim 17, wherein, The annular header rests on and is supported by a support plate, and the support plate provides flow resistance between the irregular outer periphery of the module pack and the outer shell.

19. The multi-element filtration apparatus of any one of claims 12 to 18, further comprising: locking means for locking adjacent filter elements together.

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