Abstract:
A portable or personal water filtration apparatus comprising upstream and downstream removable reservoirs and a back-washable filter interposed therebetween. The back-washable filter comprises an upstream housing and a downstream housing, inside of which are fitted a membrane cartridge and a slidable plunger. The downstream housing is rotatable relative to the upstream housing between two or more positions, including, for example, an open flow position, a closed flow position, and a back-wash position. In the back-wash position, the downstream housing and plunger are axially extendable relative to the upstream housing and membrane cartridge in order to obtain a charge of clean water in the plunger that may then be forced backward through the filter by returning the plunger to its starting position. This dislodges accumulated debris from the membrane and reverses the slowing of flow observed through repeated use of the filter.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application Nos. 61/700,864, filed on Sep. 13, 2012, 61/828,514, filed on May 29, 2013, and 61/833,310, filed on Jun. 10, 2013; each entitled “Fluid Treatment Apparatus And Method Of Using Same,” by Bradley Pierik, Kevin Reilly and Ronald Pierik, which are hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     This invention relates to the field of fluid treatment apparatuses and methods of using same. Embodiments of the present invention relate to the field of portable or personal back-washable water filtration apparatuses, water filters therefor, and methods of using same, particularly back-washing methods. 
     BACKGROUND 
     Access to clean water is a problem faced by many regions of the world. Water is often treated to remove contaminants before it is consumed. Water filters are commonly used for this purpose. 
     In developing countries, potable water is often unavailable or difficult to obtain. 
     People who camp may want a water filter that is portable and easy to use because water is heavy and bulky to carry. Victims of disasters may require filtration devices if sources of treated water are not readily available; the devices need to be stored, transported, and distributed. These scenarios highlight the need for a cheap, portable, and easy-to-use filtration device to provide clean water for these types of applications. 
     A common problem with existing water filters is that it is difficult to generate sufficient pressure to drive or draw water through the filter. Typically, water filters rely on gravity, incorporate pumps, or require a user&#39;s lung power to drive water through the filter. However, each of these means of pressurization has drawbacks. In many parts of the world, pumps are prohibitively expensive or present maintenance challenges. Using gravity to generate pressure requires significant vertical distances to build up head pressure, or else the flow rate is inconveniently slow. It is challenging or impossible for many users, such as children and the elderly, to create sufficient pressure to use a filter with just their lung power. For this reason, a water treatment system with an easier means of generating pressure is needed. 
     An additional problem with water filtration systems is that the filters are easily blocked with debris and must be cleaned. After a period of use, many filters lose efficiency. Particulate matter filtered out of untreated water might have accumulated and clogged the filter. Efficiency may be restored by periodic back-washing, a process of driving water through the filter in the direction countercurrent to the normal filtering mode to dislodge particles accumulated in the filter and flush them away. Typical cleaning processes involve back-washing the filter by generating pressure in this countercurrent direction with a separate pump or bulb system, or disassembling the system and manually cleaning the filter. Cleaning a filter is often a complex operation and these complexities can be difficult for many users. 
     The foregoing examples of the related art and limitations related thereto are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specifications and a study of the drawings. 
     SUMMARY 
     The following embodiments and aspects thereof are described and illustrated in conjunction with systems, tools, and methods which are meant to be exemplary and illustrative, not limiting in scope. In various embodiments, one or more of the above-described problems have been reduced or eliminated, while other embodiments are directed to other improvements. In a first aspect of the invention, a fluid treatment system comprises: an upstream portion adapted to convey fluid and to treat impurities within the fluid; and a downstream portion in fluid communication with the upstream portion, wherein the downstream portion comprises a flow valve, and wherein the downstream portion is configured to selectably engage the upstream portion and alternate between, a first engagement position, wherein the flow valve is in an open position, thereby enabling fluid to flow through the downstream portion, and a second engagement position, wherein the flow valve is in a closed position, thereby prohibiting fluid from flowing through the downstream portion, wherein the upstream portion is configured to selectably extend from and retract to the downstream portion, whereby extending the upstream portion from the downstream portion causes fluid to flow through the upstream portion in a downstream direction, whereby an amount of fluid is retained within the downstream portion, and retracting the upstream portion causes said amount of fluid to flow through the upstream portion in an upstream direction. 
     In a second aspect of the invention, a back-washable fluid treatment apparatus comprises: an upstream portion adapted to convey fluid and to treat impurities within the fluid; and a downstream portion in fluid communication with the upstream portion, wherein the downstream portion comprises a flow valve, and wherein the wherein the downstream portion is configured to selectably engage the upstream portion and alternate between, a first engagement position, wherein the flow valve is in an open position, thereby enabling fluid to flow through the downstream portion, and (1) a second engagement position, wherein the flow valve is in a closed position, thereby prohibiting fluid from flowing through the downstream portion, wherein the upstream portion is configured to selectably extend from and retract to the downstream portion, whereby extending the upstream portion from the downstream portion causes fluid to flow through the upstream portion in a downstream direction, whereby an amount of fluid is retained within the downstream portion, and retracting the upstream portion causes fluid to flow through the upstream portion in an upstream direction. 
     In certain aspects, extending the upstream portion drives the fluid in the downstream direction and retracting the upstream portion drives the fluid in the upstream direction, thereby (i) backwashing the upstream portion, (ii) clearing bubbles to mitigate risk of an air-lock condition, or (iii) starting a siphon when used in conjunction with a hose in fluid communication with the upstream portion. 
     In certain aspects, the downstream portion is configured to selectably engage the upstream portion and alternate between a third engagement position, wherein the flow valve is in the closed position, thereby prohibiting fluid from flowing through the downstream portion, wherein the upstream portion is fixedly secured from extending and retracting relative to the downstream portion. 
     In certain aspects, the downstream portion and the upstream portion alternate between the first engagement position and the second engagement position when the upstream portion is rotated relative to the downstream portion. 
     In certain aspects, the downstream portion and the upstream portion alternate between the first engagement position and the second engagement position when an intermediate portion positioned between the upstream portion and the downstream portion is rotated relative to the upstream and the downstream portion. 
     In certain aspects, the upstream portion and the downstream portion are cylindrical in shape and the upstream portion or downstream portion comprise one or more surface protrusions to deter rolling. 
     In certain aspects, the upstream portion comprises an upstream threaded connector adapted to sealingly connect to an upstream threaded reservoir, and the downstream portion comprises a downstream threaded connector adapted to un-sealingly connect to a downstream threaded reservoir. 
     In certain aspects, the upstream portion is configured to treat impurities using a microfiltration porous membrane having hydrophilic and hydrophobic fibers. 
     In certain aspects, the upstream portion is configured to treat impurities using an ultrafiltration porous membrane having hydrophilic and hydrophobic fibers. 
     In certain aspects, said flow valve is a spring-and-ball valve assembly. 
     In a third aspect of the invention, a kit for filtering fluid comprises: an upstream reservoir; a downstream reservoir; a membrane cartridge having an upstream end and a downstream end, wherein the membrane cartridge comprises an actuating protrusion at the downstream end; an upstream housing having an upstream end and a downstream end, wherein the downstream end of the upstream housing is fixedly connected to the upstream end of the membrane cartridge and comprises one or more tabs positioned circumferentially along an inner surface of the upstream housing; a downstream housing having an upstream end and a downstream end, wherein the upstream end of the downstream housing is movably connected to the downstream end of the membrane cartridge and comprises one or more shaped slots positioned circumferentially along the downstream housing&#39;s outer surface and configured to engage the upstream housing&#39;s one or more tabs; wherein the downstream housing is configured to selectably engage the upstream housing and alternate between, (1) a first engagement position that permits fluid to flow through the membrane cartridge, (2) a second engagement position that prevents fluid from flowing and seals the downstream end of the back-washable fluid treatment apparatus, and (3) a third engagement position that allows the upstream housing to disengage from the shaped slots of the downstream housing thereby allowing a user to pump the back-washable fluid treatment apparatus; a valve cap positioned within said downstream housing, said valve cap having a flow valve configured to engage the actuating protrusion at the downstream end of the membrane cartridge. 
     In certain aspects, (i) the upstream housing may comprise an inlet for receiving fluid and an interior threaded portion around said inlet configured to provide a sealing engagement between said upstream housing and the upstream reservoir; and (ii) the downstream housing may comprise an outlet for discharging fluid and an interior threaded portion around said outlet configured to provide an un-sealing engagement between said downstream housing and the downstream reservoir. 
     In a fourth aspect of the invention, a back-washable fluid filtration system comprises: a top cap assembly having an upstream end and a downstream end, the top cap assembly comprising; one or more tabs positioned circumferentially along an inner surface of the top cap assembly; a membrane cartridge portion configured to house a water treatment material; and an actuating protrusion at the downstream end; a downstream assembly having an upstream end and a downstream end, the downstream assembly comprising: one or more shaped slots positioned circumferentially along the downstream assembly&#39;s outer surface and configured to engage the top cap assembly&#39;s one or more tabs; and a valve cap portion having a flow valve configured to engage the actuating protrusion of the membrane cartridge portion; wherein the downstream assembly is movably coupled with the top cap assembly and configured to selectably engage the top cap assembly via the shaped slots and alternate between, (1) a first engagement position that permits fluid to flow through the water treatment material, (2) a second engagement position that prevents fluid from flowing and seals the downstream end of the back-washable fluid filtration system, and (3) a third engagement position that allows the upstream housing to disengage from the shaped slots of the downstream housing thereby allowing a user to pump the back-washable fluid filtration system. 
     In a fifth aspect of the invention, a back-washable fluid filtration apparatus may comprises: a membrane cartridge having an upstream end and a downstream end, wherein the membrane cartridge comprises an actuating protrusion at the downstream end; an upstream housing having an upstream end and a downstream end, wherein the downstream end of the upstream housing is fixedly connected to the upstream end of the membrane cartridge and comprises one or more tabs positioned circumferentially along an inner surface of the upstream housing; an upstream end cap fixedly connected to the upstream end of the upstream housing; a downstream housing having an upstream end and a downstream end, wherein the upstream end of the downstream housing is movably connected to the downstream end of the membrane cartridge and comprises one or more shaped slots positioned circumferentially along the downstream housing&#39;s outer surface and configured to engage the upstream housing&#39;s one or more tabs; wherein the downstream housing is configured to selectably engage the upstream housing and alternate between, (1) a first engagement position that permits fluid to flow through the water treatment material, (2) a second engagement position that prevents fluid from flowing and seals the downstream end of the back-washable fluid filtration apparatus, and (3) a third engagement position that allows the upstream housing to disengage from the shaped slots of the downstream housing thereby allowing a user to pump the back-washable fluid filtration apparatus; a downstream end cap fixedly connected to the downstream end of the downstream housing; and a valve cap positioned within said downstream housing, said valve cap having a flow valve configured to engage the actuating protrusion at the downstream end of the membrane cartridge. 
     In certain aspects, the upstream end of said top cap assembly may provide an inlet for receiving water. The upstream end may further comprise an interior threaded portion, around said inlet, configured to provide a sealing engagement between said top cap assembly and a first reservoir, that is, a connection providing fluid communication such that fluid cannot leak out between the connected surfaces. The top cap assembly may further comprise one or more small notches around an outside edge of the threaded portion for providing an air pathway, such that the threads may un-sealingly connect to a reservoir, that is, a connection providing fluid communication such that fluid can leak out between the connected surfaces. 
     In certain aspects, the downstream end of said downstream assembly provides at least an outlet for discharging water. The downstream assembly may further comprise an interior threaded portion, around said outlet, configured to provide a sealing engagement between said downstream assembly and a second reservoir. The downstream assembly may further comprise one or more small notches around an outside edge of the threaded portion for providing an air pathway. 
     In certain aspects, the water treatment material is a hollow-fiber membrane filter bundle. 
     In certain aspects, the downstream assembly comprises one or more surface protrusions on an outer surface of the downstream assembly to deter rolling. 
     In certain aspects, the flow valve is a spring-and-ball valve assembly. 
     In addition to the exemplary aspects and embodiments described above, further aspects and embodiments will become apparent by reference to the drawings and by study of the following detailed descriptions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments are illustrated in referenced figures of the drawings. It is intended that the embodiments and figures disclosed herein are to be considered illustrative rather than restrictive. 
         FIG. 1  is a perspective view of a water filtration device according to one embodiment of the invention. 
         FIG. 2  shows an exploded view of a first embodiment of a water filter of the water filtration device. 
         FIG. 3A  shows a side cross-sectional view of an upstream housing of the first water filter. 
         FIG. 3B  shows a perspective view of the upstream housing of the first water filter. 
         FIG. 4A  shows a side cross-sectional view of a membrane cartridge of the first water filter. 
         FIG. 4B  shows a side view of the membrane cartridge of the first water filter. 
         FIG. 5A  shows a side cross-sectional view of the plunger of the first water filter. 
         FIG. 5B  shows a side view of the plunger of the first water filter. 
         FIG. 6A  shows a side cross-sectional view of the downstream housing of the first water filter. 
         FIG. 6B  shows a perspective view of the downstream housing of the first water filter in an inverted orientation relative to  FIG. 6A . 
         FIG. 7  shows a side cross-sectional view of the assembled first water filter, along with hoses for indirectly connecting the filter to the first and second reservoirs. 
         FIGS. 8A-8C  illustrate a sequence of operations involved in back-washing the first water filter. 
         FIG. 9A  shows a side cross-sectional view of the first water filter illustrating a first flow configuration of the water filter. 
         FIG. 9B  illustrates a first engagement position of the tab in the shaped slot of the plunger when the filter is in the first flow configuration. 
         FIG. 10A  shows a side cross-sectional view of the first water filter illustrating a second flow configuration of the water filter. 
         FIG. 10B  illustrates a second engagement position of the tab in the shaped slot of the plunger when the filter is in the second flow configuration. 
         FIG. 11A  shows a side cross-sectional view of the first water filter illustrating a third flow configuration of the water filter. 
         FIG. 11B  illustrates a third engagement position of the tab in the shaped slot of the plunger when the first water filter is in the third flow configuration. 
         FIG. 12A  shows a side cross-sectional view of the first water filter illustrating a fourth flow configuration of the water filter. 
         FIG. 12B  illustrates a third engagement position of the tab in the shaped slot of the plunger when the first water filter is in the fourth flow configuration. 
         FIG. 13  illustrates a perspective view of a second embodiment of a water filter of the water filtration device in an open position. 
         FIG. 14A  illustrates a side view of an end cap of the second water filter. 
         FIG. 14B  illustrates a cross-sectional view of the end cap of the second water filter. 
         FIG. 14C  illustrates a top plan view of an end cap of the second water filter. 
         FIG. 14D  illustrates a bottom plan view of the end cap of the second water filter. 
         FIG. 15A  illustrates a side view of an upstream housing of the second water filter. 
         FIG. 15B  illustrates a cross-sectional view of the upstream housing of the second water filter. 
         FIG. 15C  illustrates a top plan view of the upstream housing of the second water filter. 
         FIG. 16A  illustrates a side view of a membrane cartridge of the second water filter. 
         FIG. 16B  illustrates a cross-sectional view of the membrane cartridge of the second water filter. 
         FIG. 16C  illustrates a top plan view of the membrane cartridge of the second water filter. 
         FIG. 17A  illustrates a top plan view of a valve cap of the second water filter. 
         FIG. 17B  illustrates a bottom plan view of the valve cap of the second water filter. 
         FIG. 17C  illustrates a side view of the valve cap of the second water filter. 
         FIG. 17D  illustrates a cross-sectional view of the valve cap of the second water filter. 
         FIG. 18A  illustrates a side view of a downstream housing of the second water filter having a first guide channel design. 
         FIG. 18B  illustrates a cross-sectional view of the downstream housing of the second water filter. 
         FIG. 18C  illustrates a side view of the shaped slot of the second water filter. 
         FIG. 18D  illustrates a top plan view of the downstream housing of the second water filter. 
         FIG. 19A  illustrates a perspective view of the second water filter in an open position. 
         FIG. 19B  illustrates a first engagement position of the tab in the shaped slot when the second water filter is in an open position. 
         FIG. 19C  illustrates a side view of the second water filter in an open position. 
         FIG. 19D  illustrates a cross-sectional view of the second water filter in an open position. 
         FIG. 20A  illustrates a perspective view of the second water filter in a closed position. 
         FIG. 20B  illustrates a second engagement position of the tab in the shaped slot when the second water filter is in a closed position. 
         FIG. 20C  illustrates a side view of the second water filter in a closed position. 
         FIG. 20D  illustrates a cross-sectional view of the second water filter in a closed position. 
         FIG. 21A  illustrates a perspective view of the second water filter in an unlocked position. 
         FIG. 21B  illustrates a third engagement position of the tab in the shaped slot when the second water filter is in an unlocked position. 
         FIG. 21C  illustrates a side view of the second water filter in an unlocked position. 
         FIG. 21D  illustrates a cross-sectional view of the second water filter in an unlocked position. 
         FIG. 22  illustrates a third embodiment of a water filter of the water filtration device. 
         FIG. 23  illustrates a fourth embodiment of a water filter of the water filtration device. 
         FIG. 24  illustrates a fifth embodiment of a water filter of the water filtration device. 
         FIG. 25  illustrates a sixth embodiment of a water filter of the water filtration device. 
         FIG. 26  illustrates a seventh embodiment of a water filter of the water filtration device. 
         FIG. 27  illustrates an eighth embodiment of a water filter of the water filtration device. 
         FIGS. 28 a -28 e    illustrate an embodiment of a water filter having surface protrusions. 
         FIGS. 29 a -29 b    illustrate an embodiment of a water filter having anti-seal notches. 
         FIG. 30 a -30 b    illustrate an embodiment of a water filter having a second guide channel design. 
     
    
    
     DESCRIPTION 
     Throughout the following description, specific details are set forth in order to provide a more thorough understanding to persons skilled in the art. However, well-known elements may not have been shown or described in detail to avoid unnecessarily obscuring the disclosure. Accordingly, the description and drawings are to be regarded in an illustrative, rather than a restrictive, sense. Like features of certain embodiments may be referenced in connection with other embodiments by like reference numerals. Accordingly, not all reference numerals shown in a particular drawing will necessarily be described in connection with that drawing. 
     As used herein, the terms “treat” and “filter” are used to refer to the process of removing or inactivating contaminants in fluid. In some embodiments, membranes, such as microfiltration membranes, or ultrafiltration membranes, are used. Microfiltration membranes may refer to membranes having pores in the range of 0.1 to 10 microns. Ultrafiltration membranes may refer to membranes having pores in the range of 0.001 to 0.1 microns. In some embodiments, activated carbon is used. In some embodiments, a chemical water treatment technology, such as chlorination, is used. In some embodiments, a radiative water treatment technology, such as ultraviolet light, is used. In some embodiments, a combination of multiple water treatment technologies is used. 
     As used herein, the terms “personal water filter” and “portable water filter” are used to refer to a device that is small enough to be easily carried and used by a single person, e.g., as distinguished from larger water filtration units that would generally be difficult to move and would provide filtered water for a large number of people. The amount of water that can be filtered before back-wash is required is a function of incoming water quality and user preference for an acceptable throughput as a function of time. In some embodiments, a personal water filter can produce sufficient quantities of filtered water to conveniently serve the filtered water needs of between one and eight people on a daily basis for a period of from 1-30 days, 1-12 months, 1-3 years, or any period therebetween. 
     As used herein, the term “downstream” means the direction in the typical direction of flow through a filter. The term “upstream” means the opposite of downstream, i.e., a direction opposite to the typical direction of flow through a filter. When functioning under the influence of gravity, the typical direction of flow through the filter is from an elevated upstream reservoir to a relatively lowered downstream reservoir. 
       FIG. 1  shows a water treatment device  10  according to one embodiment of the invention. Water treatment device  10  includes a first reservoir  20 , a filter cartridge  30  and second reservoir  40 . First reservoir  20  is positioned upstream of filter cartridge  30  and is intended to contain unfiltered water  21 . Second reservoir  40  is positioned downstream of filter cartridge  30  and is intended to receive filtered water  23 . First reservoir  20  and second reservoir  40  are either directly or indirectly connected to filter cartridge  30 . When directly connected, there are no fluid conduits or hoses interposing first reservoir  20  and filter cartridge  30  or second reservoir  40  and filter cartridge  30 , as shown. When indirectly connected, such hoses may be provided, as further described hereinafter. 
     In some embodiments, first and second reservoirs  20 ,  40  are designed to be durable under repeated manual application of pressure, either through squeezing, rolling, or folding. In some embodiments, the material used to make first and second reservoirs  20 ,  40  is flexible, and is designed to withstand numerous squeezing cycles, while still functioning effectively to provide water containment and safe storage of filtered water. In some embodiments, the material used to make first and second reservoirs  20 ,  40  is sufficiently strong to withstand the interior pressure generated when first and second reservoirs  20 ,  40  are squeezed firmly by a user. In some embodiments, first and second reservoirs  20 ,  40  are made from a flexible non-porous material such as a rupture-resistant plastic. Examples of potentially suitable plastics that could be used to make first and second reservoirs  20 ,  40  include polyethylene, polypropylene, thermoplastic polyurethane, and laminates or co-extrusions in which several materials are layered together. 
     In some embodiments, the material used to make first and second reservoirs  20 ,  40  is transparent, so that a user can see both the unfiltered water  21  and the filtered water  23 . This may provide a user with visual confirmation that filter cartridge  30  is effectively treating the unfiltered water  21 . 
     Filter cartridge  30  may contain any suitable water treatment technology to treat contaminated or potentially contaminated water so that it is potable (i.e., safe to drink). In some embodiments, membranes, such as microporous membranes, or ultrafiltration membranes, are used in filter cartridge  30 . Microfiltration membranes may refer to membranes having pores in the range of 0.1 to 10 microns, Ultrafiltration membranes may refer to membranes having pores in the range of 0.001 to 0.1 microns. In some embodiments, activated carbon is used in filter cartridge  30 . In some embodiments, a chemical water treatment technology, such as chlorination, is used in filter cartridge  30 . In some embodiments, a radiative water treatment technology, such as ultraviolet light, is used in the filter cartridge  30 . In some embodiments, a combination of multiple water treatment technologies is used in filter cartridge  30 . 
     In the illustrated embodiment, first reservoir  20  includes a downstream opening  24  and a rigid member  22 . The downstream opening  24  is sized and configured to sealingly engage with an upstream receiving portion  32  at the upstream end of filter cartridge  30 . Opening  24  can sealingly engage with upstream receiving portion  32  in any suitable manner, for example, by friction fit, threaded engagement, or via a coupling that sealingly connects opening  24  with upstream receiving portion  32 . In the illustrated embodiment, upstream receiving portion  32  includes an upstream threaded portion  33  and opening  24  includes a complementary threaded portion  35 . Threaded portions  33  and  35  are dimensioned and configured to provide a sealing engagement between first reservoir  20  and filter cartridge  30 . 
     The embodiments in which opening  24  is detachably coupled to filter cartridge  30  allow first reservoir  20  to be easily detached for convenient storage, cleaning, or replacement of first reservoir  20 . In some embodiments, in which opening  24  is detachably coupled to filter cartridge  30 , a cap or other suitable closure may be provided so that water can be contained within first reservoir  20 . 
     In the illustrated embodiment, second reservoir  40  includes an upstream opening  44 , a rigid member  42 , and a downstream outlet  45 . In some embodiments, including the illustrated embodiment, downstream outlet  45  includes a flow controller  46 . 
     The upstream opening  44  is sized and configured to sealingly engage with a downstream receiving portion  34  at the downstream end of the filter cartridge  30 . Upstream opening  44  can sealingly engage with downstream receiving portion  34  in any suitable manner, for example by friction fit, threaded engagement, or via a coupling that sealingly connects upstream opening  44  with downstream receiving portion  34 . In the illustrated embodiment, downstream receiving portion  34  includes an interior downstream threaded portion  37  and opening  44  includes a complementary exterior threaded portion  39 . Threaded portions  37  and  39  are dimensioned and configured to provide a sealing engagement between second reservoir  40  and filter cartridge  30 . Similar structure may be provided in connection with upstream reservoir  20 . 
     Embodiments in which upstream opening  44  is detachably coupled to filter cartridge  30  allow second reservoir  40  to be easily detached for convenient storage, cleaning, or replacement of second reservoir  40 . In some embodiments, in which upstream opening  44  is detachably coupled to filter cartridge  30 , a cap or other suitable closure is provided so that water can be contained within first reservoir  20 . For example, a screw cap  48  may be provided that has a threaded interior surface  50 . Threaded interior surface  50  is dimensioned and configured to sealingly engage with upstream opening  44 . 
     In some embodiments, the downstream opening  24  and the upstream opening  44  are sized and/or configured so that the upstream reservoir  20  (e.g., a first reservoir) and the downstream reservoir  40  (e.g., a second reservoir) cannot be interchanged. One embodiment may have upstream receiving portion  32  include an exterior threaded portion and opening  24  include a complementary interior threaded portion, while downstream receiving portion  34  includes interior threaded portion  37  and opening  44  includes complementary exterior threaded portion  39 . This may be advantageous because it reduces the risk of accidentally switching the upstream reservoir and the downstream reservoir, and thus reduces the risk of cross-contamination. 
     In some embodiments, the threaded portions of filter cartridge  30  are dimensioned and configured to engage with conventional liquid storage containers, such as pop bottles or water bottles. In some embodiments, the downstream outlet  45  is dimensioned and configured to engage with conventional liquid storage containers, such as pop bottles or water bottles. Such embodiments allow water treatment device  10  to conveniently provide filtered water for storage in containers that are available to users. 
     Rigid members  22  and  42  are optional features. In those embodiments that include rigid member  22  and/or  42 , rigid members  22  and  42  can be used to facilitate use of water treatment device  10  by allowing a user to easily roll portions of first reservoir  20  or second reservoir  40  over rigid member  22  or  42 , to allow the user to provide greater squeezing force against liquid contained therein. In a water-filtering mode (i.e., while water is being passed through filter cartridge  30  in the conventional direction to produce filtered water), the user can more easily apply force to squeeze unfiltered water  21  through filter cartridge  30  and into second reservoir  40 , thereby producing filtered water more rapidly. In a filter cleaning mode (i.e., while filtered water is being back-washed through filter cartridge  30  in the upstream direction), the user can more easily apply force to squeeze clean water through filter cartridge  30  and into first reservoir  20 , potentially increasing the efficiency with which filter cartridge  30  is cleaned. 
     The upper portion of first reservoir  20  optionally includes one or more apertures  26  or a hook, which allows water treatment device  10  to be easily suspended, for example, by a piece of rope tied to a tree branch or other tall structure, to facilitate use of gravity to force unfiltered water  21  through filter cartridge  30 . 
     Rigid member  22  optionally includes one or more apertures which allow rigid member  22  to function as a handle so that water treatment device  10  may be carried by the user. Such an embodiment allows water treatment device  10  to be easily suspended, for example, by a tree branch or other tall structure, to facilitate use of gravity to force unfiltered water  21  through filter cartridge  30 . 
     In some embodiments, the configuration of reservoirs  20  and/or  40  is such as to optimize gravity and/or manual pressure-assisted filtration. In some embodiments, first reservoir  20  is shaped to be taller in height than the width of first reservoir  20 . In some embodiments, first reservoir  20  is shaped such that its height is at least double the width of first reservoir  20 . This configuration enables a larger pressure head to be produced by the unfiltered water  21  in first reservoir  20 , thereby improving the efficiency of gravity-assisted filtration, because the pressure head is proportional to the vertical height of the water. This configuration may also increase the pressure that a user can generate within first reservoir  20  by squeezing, rolling, or folding reservoir  20 . 
     In some embodiments, the configuration of filter cartridge  30  is such as to optimize gravity, assisted filtration. For example, using a longer filter cartridge (i.e., a filter cartridge having a greater height) will similarly increase the height through which the water must flow before it encounters ambient air pressure. The increase in height results in a higher pressure head within water treatment device  10  (i.e., a greater amount of water pressure is generated to drive unfiltered water  21  through filter cartridge  30 ). 
     In the use of the illustrated embodiment, unfiltered water may be loaded into the first reservoir  20  through the opening  24 . Threaded portions  33  and  35  are unscrewed to separate first reservoir  20  from filter cartridge  30 . Unfiltered water  21  is loaded into first reservoir  20  through opening  24 . Optionally, a cap or other suitable closure can be secured over opening  24  to store unfiltered water  21  in first reservoir  20  until a user is ready to begin filtering. Optionally, a user can wash or wipe down the exterior surfaces of first reservoir  20 , to minimize the risk of pathogens or other contaminants being transferred from first reservoir  20  to filtered water  23  that is removed from second reservoir  40  for use. 
     The opening  24  of the first reservoir  20  is then coupled to the upstream receiving portion  32  of the filter cartridge  30 . In the illustrated embodiment, first reservoir  20  is coupled to filter cartridge  30  by engaging threaded portion  35  of opening  24  with threaded portion  33  of upstream receiving portion  32 . Second reservoir  40  is also coupled to filter cartridge  30  by engaging threaded portions  37  and  39 , either prior to or after engagement of first reservoir  20  with filter cartridge  30 . 
     Once water treatment device  10  has been filled with unfiltered water  21  and assembled, in a water-filtering mode, unfiltered water  21  is driven through filter cartridge  30  to produce filtered water  23 . The force required to cause unfiltered water  21  to pass through filter cartridge  30  in a water-filtering mode can be provided by manual pressure or by gravity. In some cases, manual pressure is used to assist gravity filtration, i.e., manual pressure-assisted filtration. 
     In a gravity water-filtering mode, unfiltered water  21  is forced through filter cartridge  30  by gravity. In a gravity water-filtering mode, water treatment device  10  can optionally be suspended from a suitable point of support in any suitable manner, e.g., by using a rope to tie water treatment device  10  to a taller structure via aperture  26 , by connecting a hook provided on an upper portion of first reservoir  20  to a suitable support, or the like. Water treatment device  10  is permitted to hang from such a point of support. Alternatively, a user can simply hold water treatment device  10  in a suspended fashion. The force of gravity acting against unfiltered water  21  forces unfiltered water  21  through filter cartridge  30 , and filtered water  23  flows into second reservoir  40 . 
     In a manual pressure-filtering mode, the user applies pressure to the first reservoir  20  in any suitable manner, for example, by squeezing, clenching, or rolling first reservoir  20  to apply pressure against unfiltered water  21  to force the unfiltered water through filter cartridge  30 . In embodiments in which rigid member  22  is present, a user can roll first reservoir  20  around rigid member  22  to apply force against unfiltered water  21 . 
     Treated water may be stored by removing second reservoir  40  from the filter cartridge  30  and optionally closing the opening  44 , for example, with a screw cap  48  that includes a threaded interior surface  50  that is dimensioned and configured to sealingly engage with exterior threaded portion  39 . The filtered water may alternatively or also be dispensed from second reservoir  40  through the flow controller  46 . The flow controller  46  may be any suitable device for regulating the release of fluid, such as a flow valve, a spigot, a spout, a hose and clamp, or the like. 
     In some embodiments, the flow controller  46  may be located on the lower side of the second reservoir  40 . In another embodiment, the flow controller  46  may be located on the bottom edge or end of the second reservoir  40 . In cases where the rigid member  42  is present, the flow controller  46  may be incorporated into the rigid member  42 . 
     Embodiments in which the flow controller  46  is located along the side edge of reservoir  40  (as opposed to embodiments in which the flow controller  46  is located on the bottom edge or end of reservoir  40 ) allow the rigid member  42  to be used for rolling to increase pressure in reservoir  40  without interference from flow controller  46 . 
     In some embodiments, filter cartridge  30  is configured so that its length is equal to the width of reservoirs  20  and  40 , so that reservoirs  20  and  40  may be rolled up around filter cartridge  30  for convenient storage. 
     Referring to  FIG. 2 , a first embodiment of a filter cartridge  30  is shown. The filter cartridge  30  comprises an upstream housing  51 , a membrane cartridge  60 , a plunger  70  and a downstream housing  90 . The upstream housing  51  comprises the upstream receiving portion  32 , an upstream housing recess  52 , an upstream tab  53 , a plunger slot  59  and a plunger stop  58 . The membrane cartridge  60  comprises a cartridge lip  61 , an outer O-ring recess  62  and an inner O-ring recess  64 . The plunger  70  comprises a plunger tab  79 , a sealable port  71  and a shaped slot  80 . The downstream housing  90  comprises a downstream housing recess  97 , a bayonet slide  99   a , a bayonet slot  98   a , the downstream receiving portion  34  and the downstream threaded portion  37 . 
     The upstream housing  51 , membrane cartridge  60 , plunger  70  and downstream housing  90  may be constructed from a substantially rigid non-porous plastic material. Examples of potentially suitable plastics that could be used include polypropylene (PP), various copolymers, polycarbonate (PC), acrylonitrile butadiene styrene (ABS), a polycarbonate/acrylonitrile butadiene styrene blend (PC/ABS), high-density polyethylene (HDPE), low-density polyethylene (LDPE), polyester, and copolyester. 
     Referring to  FIG. 3A  and  FIG. 3B , the upstream receiving portion  32  of the upstream housing  51  is for receiving an incoming flow of water to be treated and comprises the upstream threaded portion  33 , as previously described. A suitable fitting, such as an upstream hose fitting  56 , may optionally be provided for connection of the upstream reservoir  20  via a flexible fluid conduit (e.g., an upstream hose  27 , as illustrated in  FIG. 7 ) as an alternative to the use of the upstream threaded portion  33 . In some embodiments, it may be beneficial for the hose  27  to be transparent, to facilitate a visual comparison of the water entering the device and the water exiting the device, thus providing the user with added confidence regarding the treatment process. 
     Regardless of whether or not the upstream hose fitting  56  is present, an upstream fluid opening  57  is provided within the upstream receiving portion  32  to permit ingress of water to be treated. 
     Turning specifically to  FIG. 3A , the interior of the upstream housing  51  comprises a cartridge recess  54  that is bounded at its lower end by a cartridge retainer  55 . The cartridge recess  54  and cartridge retainer  55  are annular and complementary in shape to the cartridge lip  61  of membrane cartridge  60 . Referring additionally to  FIG. 7 , upon assembly, the membrane cartridge  60  is inserted within the upstream housing  51  until an upper edge of the cartridge lip  61  engages a lower edge of the cartridge retainer  55 . Further insertion of the membrane cartridge  60  into the upstream housing  51  causes deformation of the cartridge lip  61  until it passes the cartridge retainer  55  and returns to its original shape. This ensures a semi-permanent snap-fit between the upstream housing  51  and the membrane cartridge  60 . 
     The interior of the upstream housing  51  further comprises a plunger slot  59  with an upstream end terminating below the cartridge retainer  55  and a downstream end that is bounded by a plunger stop  58 . The plunger slot  59  is complementary in shape to plunger tab  79 . At least a pair of plunger slots  59  and a pair of plunger tabs  79  are provided. Returning again to  FIG. 7 , upon insertion of the plunger  70  within the upstream housing  51 , following the aforementioned snap-fit of the membrane cartridge  60  therewithin, the plunger tabs  79  are aligned with the plunger slots  59  and engage the plunger stop  58 . Further insertion of the plunger lower end  78  within the upstream housing  51  causes deformation of the plunger tabs  79  until they pass the plunger stop  58  and return to their original shape. This ensures a semi-permanent snap-fit between the upstream housing  51  and the plunger  70  that substantially prevents removal of the plunger. Thereafter, the plunger  70  is permitted to slide axially relative to the upstream housing  51  by movement of the plunger tabs  79  along the plunger slots  59 . However, the plunger  70  is prevented from rotating relative to the upstream housing  51 . 
     Once the membrane cartridge  60  has been snap-fit within the upstream housing  51  and the plunger  70  has been inserted over the membrane cartridge  60  to slidingly snap-fit within the upstream housing  51 , sealing engagement between the membrane cartridge  60  and the interior of the plunger  70  is provided by a pair of O-rings within the outer O-ring recesses  62 . This prevents contaminated water entering the upstream housing  51  via the upstream fluid opening  57  from bypassing the membrane cartridge  60  and instead forces it to flow through the membrane receiving portion  63  ( FIG. 4A ) of the membrane cartridge  60 , as will be more completely described hereinafter. Similarly, the inner O-ring recess  64  accommodates an O-ring for sealing against a membrane  112  ( FIG. 9A ) to prevent contaminated water from passing between the membrane  112  and the membrane receiving portion  63 . The downstream housing  90  may then be installed over the plunger  70 . 
     Referring to  FIG. 6A  and  FIG. 6B , the downstream housing  90  comprises a downstream housing recess  97  upon which is provided a bayonet slide  99   a . The bayonet slide  99   a  protrudes radially from an outer annular surface of the downstream housing recess  97  by an amount slightly less than the depth of the recess. Therefore, the bayonet slide  99   a  does not protrude past the outer diameter of the downstream housing  90 . A bayonet slot  98   a  is formed between the bayonet slide  99   a , the outer annular surface of the downstream housing recess  97  and a lower edge of the downstream housing  90 . Since the bayonet slide  99   a  is substantially L-shaped, the bayonet slot  98   a  has a bayonet open end  98   b  and a bayonet closed end  99   b.    
     Upon insertion of the downstream housing recess  97  within the upstream housing recess  52 , the downstream housing  90  is oriented such that the upstream tab  53  is aligned with the bayonet open end  98   b . Thus, upon rotation of the downstream housing  90  relative to the upstream housing  51  about a central axis of the filter cartridge  30 , the upstream tab  53  enters the bayonet open end  98   b  and engages within the bayonet slot  98   a . Thereafter, the downstream housing  90  is prevented from being axially withdrawn from the upstream housing  51  by interference between the upstream tab  53  and the bayonet slide  99   a , while being permitted to rotate further only to the extent permitted by engagement of the upstream tab  53  with the bayonet closed end  99   b.    
     Referring specifically to  FIG. 6B , a tab  96  protrudes radially inwardly from within an interior of the downstream housing  90 . The tab  96  has a pair of parallel spaced apart faces normal to the central axis of the filter cartridge  30 , but rotated about that axis relative to one another, connected to one another at each end by a slanted face. 
     Turning to  FIG. 5B , the tab  96  engages within a shaped slot  80  formed in an exterior of the plunger  70 . When the downstream housing  90  is initially positioned over the plunger  70 , it is oriented such that the upstream tabs  53  are just outside the bayonet open end  98   b . The shaped slot  80  is positioned axially and circumferentially on the plunger  70  such that the tab  96  engages a third engagement portion  85  of the shaped slot  80  when the plunger  70  is initially inserted. Since the plunger  70  is prevented from rotating relative to the upstream housing  51  by engagement of the plunger tabs  79  within the plunger slots  59 , further rotation of the downstream housing  90  causes the tab  96  to move along the shaped slot in a manner as will be described hereinafter. 
     Rotation of the downstream housing  90  causes the tab  96  to move from the third engagement portion  85  to the second engagement portion  84 . The shaped slot  80  is positioned axially and circumferentially on the plunger  70  such that the upstream tab  53  enters the bayonet open end  98   b  and just begins to be rotatably engaged within the bayonet slot  98   a  when the tab  96  is in the second engagement position. Thus, the upstream tab  53  prevents the downstream housing  90  from moving axially relative to the upstream housing  51  by virtue of being constrained within the bayonet slot  98   a . The downstream housing  90  is only permitted to rotate about the central axis of the filter cartridge  30  between the positions where the upstream tab  53  engages the bayonet closed end  99   b  and where the tab  96  engages the third engagement portion  85 . This corresponds to a total rotation of just less than one half of the circumference of the filter cartridge  30 , in this embodiment. 
     Further rotation of the downstream housing  90  causes the tab  96  to move from the second engagement portion  84  to the first engagement portion  82 . In this position, the filter cartridge  30  is in a first flow configuration, as will be further described hereinafter. 
     Referring additionally to  FIG. 5A , the plunger lower end  78  is equipped with a sealable port  71 , comprising a port wall  77  that protrudes from the plunger lower end  78  and is equipped at its lower end with an outwardly radially extending port lip  74 . A plurality of port ribs  73  are provided on an interior of the port wall  77  and protrude radially inwardly to suspend a closed port end  72  from their lower edges. The space between the port ribs  73  is open, allowing a port aperture  75  to be formed between the port end  72  and the port wall  77 . Treated water exiting the membrane  112  ( FIG. 9A ) into a treated water chamber  82  ( FIG. 9A ) formed between a lower end of the membrane housing  60  and the plunger lower end  78  enters the sealable port  71  through port opening  76  and passes between the port ribs  73  to exit the plunger  70  through the port apertures  75 . 
     Returning to  FIG. 6A  and  FIG. 7 , the sealable port  71  is situated within a port receiving portion  93  of the downstream housing  90  when the downstream housing  90  is assembled with the upstream housing  51 , as previously described. The shaped slot  80  is positioned axially on the plunger  70  such that, when the tab  96  is engaged within the first engagement portion  81  of the shaped slot  80 , the radially outwardly extending port lip  74  is within the port receiving portion  93  and sealingly engaged with a radially inwardly extending sealing lip  94 . The port wall  77  protrudes by a distance such that the port end  72  is spaced apart from a port seal  95  formed in the port receiving portion  93 . This permits treated water passing through the port aperture  75  to exit through the downstream flow opening  92 . The downstream flow opening  92  may be provided within an optional fitting, such as downstream hose fitting  91 , as shown, or may simply allow treated water to exit the downstream housing  90  through the downstream receiving portion  34 . This is referred to as a first flow configuration of the water filter cartridge  30 . 
     Returning briefly to  FIG. 7 , it should be noted that the downstream hose fitting  91 , when provided, may be used to connect the downstream reservoir  40  via downstream reservoir hoses  47 , as an alternative to threaded connections via the downstream threaded portion  37 . 
     The first flow configuration of the filter cartridge  30  is further illustrated with reference to  FIG. 9A  and  FIG. 9B . Referring to  FIG. 9B , the tab  96  is situated at a first engagement portion  81  of the shaped slot  80 . An incoming flow  110  passes into the upstream housing  51  as previously described and is forced to enter the membrane  112 . A membrane spacer  120  is illustrated at an upstream end of the membrane  112  to center the membrane within the filter. A filtration flow  111  passes through the membrane and is treated once it exits the membrane  112  into treated water chamber  82 . Thereafter, a treated flow  113  exits the treated water chamber  82  via the sealable port  71  passing outwardly therefrom through port apertures  75 . The treated flow  113  then passes between the port end  72  and port seal  95  to exit the filter cartridge  30  through downstream flow opening  92 . The first flow configuration may therefore be referred to as the “open” position. 
     The treated flow  113  is prevented from leaking out of the downstream housing  90  by sealing engagement of the port lip  74  with the sealing lip  94 . Similarly, the incoming flow  110  is prevented from leaking past the membrane  112  by an O-ring disposed within the inner O-ring recess  64 . In addition, leakage past the membrane cartridge  60  is prevented by virtue of O-rings located within outer O-ring recesses  62 . A seal may also be provided to prevent leakage between the plunger  70  and the upstream housing  51  at least when the filter cartridge  30  is in the first flow configuration. It is worthwhile noting that, while in the first flow configuration, the downstream housing recess  97  substantially overlaps with the upstream housing recess  52  so that the downstream housing  90  abuts the upstream housing  51 . 
     A second flow configuration of the filter cartridge  30  is illustrated with reference to  FIG. 10A  and  FIG. 10B . Referring to  FIG. 10B , the tab  96  moves from the first engagement portion  81  (illustrated in  FIG. 9B ) to the second engagement portion  84  by rotation of the downstream housing  90 . The downstream housing  90  is rotated in an unlocking direction  100  ( FIG. 8A ), which is a counterclockwise direction in this embodiment when viewed from the downstream end. Since the downstream housing  90  is prevented from moving axially relative to the upstream housing  51  by engagement of the upstream tabs  53  within the bayonet slot  98   a , movement of the tab  96  along the slot  80  causes the plunger  70  to move axially relative to the downstream housing  90 . Axial movement of the plunger  70  relative to the upstream housing  51  is permitted by virtue of the plunger tabs  79  moving along the plunger slots  59 . The amount of axial movement provided is defined by the length of the first engagement portion  81 . This length is selected such that movement of the tab  96  to the second engagement portion  84  causes the port end  72  to move into the downstream flow opening  92 . This in turn causes the port seal  95  to deform and seal the port aperture  75 , thus preventing a flow of treated water from exiting the filter cartridge  30 . Therefore, when the tab  96  is within the second engagement portion  84 , the filter cartridge  30  is in a second flow configuration wherein the flow of treated water from the filter cartridge  30  is prevented. The second flow configuration may be referred to as the “closed” position. 
     Referring to  FIG. 8A ,  FIG. 8B  and  FIG. 8C , a sequence of operations performed in the back-washing of the filter cartridge  30  is illustrated. Back-washing may be performed as frequently or as infrequently as desired by users of the filter cartridge  30 , based upon the cleanliness of the water being treated and the acceptable rate of flow through the filter cartridge  30 . In  FIG. 8A , the filter cartridge  30  is shown in the first flow configuration with the downstream housing  90  abutting the upstream housing  51 . Rotation of the downstream housing  90  about the central axis of the filter cartridge  30  in an unlocking direction  100  (counterclockwise when viewed from the downstream end), causes the upstream tabs  53  to disengage from the bayonet open end  98   b  when the tab  96  is in the third engagement position  85 , as illustrated in  FIG. 11   b . This allows the downstream housing  92  to move axially relative to the upstream housing  51  in the extension direction  101 , as illustrated in  FIG. 8B . 
     Referring to  FIG. 11A , axial movement of the downstream housing  90  in the extension direction  101  causes the plunger  72  to also move axially relative to the housing  51 , by virtue of the interaction between the tab  96  and the third engagement portion  85  of the slot  80 . This axial movement is permitted by translation of the plunger tabs  79  along the plunger slots  59  and causes enlargement of the treated water chamber  82 . This causes a negative pressure to form, drawing incoming flow  110  through the membrane  112  to create the treated flow  113 . The treated flow  113  accumulates, as indicated by the level of treated water  83  within the treated water chamber  82 . The treated water  83  is prevented from exiting through the aperture  75  by the port seal  95 , which remains engaged with the aperture  75  as previously described with reference to the second flow configuration. Thus,  FIG. 11A  illustrates a third flow configuration of the filter cartridge  30 . The third flow configuration may be referred to as the “pre-back-wash” position. 
     Returning to  FIG. 8C , back-washing is conducted by moving the downstream housing  90  in the plunging direction  102 . Referring to  FIG. 12A , movement of the downstream housing  90  in the plunging direction  102  causes a decrease in volume of the treated water chamber  82  and pressurizes treated water  83 . This pressure forces a back-wash flow  115  to move through the membrane  112  in a direction opposite to the normal filtration direction and dislodges accumulated debris therefrom. The debris contaminated water is expelled from the upstream housing  51  as indicated by contaminated flow  116 . The contaminated flow  116  may be returned to the upstream reservoir  20 ; alternatively, the upstream reservoir  20  may be removed prior to back-washing and a separate contaminated water reservoir (not shown) may be provided. Thus,  FIG. 12A  illustrates a fourth flow configuration of the filter cartridge  30 . The fourth flow configuration may be referred to as the “back-wash” position. While four flow configurations are described, it is contemplated that fewer, or additional, flow configurations may be incorporated with the filter cartridge  30 . For instance, the “closed” position may be omitted, if desired, since the flow valve is also closed during the back-wash position. 
     Although it is often sufficient to perform a single back-wash cycle in order to restore an acceptable flow rate through the filter cartridge  30 , any number of successive pre-back-wash and back-wash cycles may be performed. 
     Referring to  FIG. 12B , since the tab  96  remains engaged in the third engagement portion  85  of the slot  80 , the upstream tab  53  is positioned outside the bayonet open end  98 B. When the downstream housing  90  is reinserted within the upstream housing  51 , rotation of the downstream housing  90  (in a clockwise direction when viewed from the downstream end) such that the tab  96  moves to the second engagement portion  84  (illustrated in  FIG. 10 b   ) causes the upstream tab  53  to re-engage the bayonet slot  98   a . This prevents axial withdrawal of the downstream housing  90  from the upstream housing  51 . Further rotation of the downstream housing  90  such that the tab  96  reaches the first engagement portion  81  (illustrated in  FIG. 9A ) restores the filter cartridge  30  to the first flow configuration and allows it to be used to purify water once again. 
     The pumping action of filter cartridge  30  during the back-wash operation can serve three purposes: (1) back-wash the membrane; (2) start the siphon; and (3) clear out any bubbles that cause an airlock condition. 
     First, the pumping action allows the user to clean the filter cartridge  130  through back-washing techniques. Specifically, as discussed above with respect to the first water filter cartridge  30 , when the top section is pulled up, a negative pressure is created inside the cylinder, causing water to be drawn in from upstream, through the filter membrane and into the cavity  191  in the cylinder  190 . When the top section is pushed down, that same water is expelled back the way it came, which flushes away any debris that may be trapped in the membrane. This process, called back-washing, keeps the filter from gradually becoming clogged. The back-wash water exits through the top of the filter, where it can be discarded. 
     Second, when the filter is configured to receive water by siphoning the water from an upstream container through a hose, the pumping action also offers an additional feature of priming the filter cartridge and hose  130  (i.e., starting the siphon). A siphon is created when a tube is positioned in an inverted U shape, and liquid is caused to flow uphill, above the surface of the upstream reservoir, without pumps, powered by the fall of the liquid as it flows down the tube under the pull of gravity, and is discharged at a level lower than the surface of the upstream reservoir it came from. For example, when the upstream end of the filter cartridge  130  is attached to a water source, the user may unlock and back-wash the filter assembly, thereby drawing water into the membrane bundle  187  and expelling any air in the filter cartridge  130  and membrane bundle  187 . 
     Furthermore, to optimize the effectiveness of the priming operation, it may be advantageous to configure the stroke volume of the pumping motion (i.e., the change in internal volume when you pump the plunger up and down) to be greater than the internal volume of the upstream hose. That is, if the hose volume is greater than or equal to the stroke volume, pumping the filter cartridge  130  will simply pull in air from the hose and expel that same air back to the hose. However, if the hose volume is less than the stroke volume, each upstroke will first draw in the air from the hose and then draw in some of the water from the upstream container, and each downstroke will expel the air first and then some of the water. Accordingly, when a smaller hose volume is used, the filter and hose will fill with water as the user pumps, instead of simply pumping the same air back and forth, to and from the hose. For example, when the stroke volume is 40 mL and the internal volume of the hose is 22 mL, each stroke of the pump will add 18 mL of water to the filter/hose assembly. 
     Third, the pumping action allows the clearing out of any air bubbles that could otherwise create an airlock condition, which can reduce flow rate and/or prevent flow completely. Indeed, an airlock condition is a relatively common problem among water filters and typically occurs when there is surface tension at the air-water interface that can resist the flow of water, effectively blocking the filter. Airlock is particularly problematic for intermittently-used filters because they are frequently filled and emptied of water, providing many opportunities for air to be trapped in the wrong place. Fortunately, the above-described pumping action clears out any bubbles in the filter cartridge  130  and/or membrane bundle  187 , thereby alleviating the airlock problem. 
     Regarding embodiments that make use of a hollow-fiber membrane filter, it is common in the industry to use a hydrophilic (water-attracting) membrane material, because the more hydrophilic the material is, the less pressure it takes to drive water through the membrane. However, to improve effectiveness of the priming feature and the airlock-clearing feature, it may be advantageous to include a minority of hydrophobic (water-repelling) fibers in the membrane bundle. Hydrophobic fibers create a low-resistance path for air to pass through the membrane bundle. Thus, by including some hydrophobic fibers, both air and water are able to pass forward and backward through the membrane. In some embodiments it may be preferable for 5-15% of the fibers to be hydrophobic fibers. More specifically, it may be preferable for 10% of the fibers to be hydrophobic fibers. This may facilitate the expulsion of air from the filter, making it easier to start the siphon and clear away air bubbles causing airlock. 
     It should be noted that, although the membrane  112  is described illustratively in this embodiment, other types of back-washable filtration media may be used without departing from the function of the filter as described in connection with this embodiment. 
     Turning now to  FIG. 13 , a second embodiment of a water filter cartridge  130  is shown. As will be discussed in greater detail below, the filter cartridge  130  generally comprises an upstream housing  151 , a downstream housing  190 , end caps  131   a ,  131   b , membrane cartridge  160 , valve cap  171 , and a water treatment material, such as a hollow-fiber membrane filter bundle  187 . Like the filter cartridge  30  of the first embodiment, the filter cartridge  130  may be cylindrical in shape and equipped with threaded  132  and/or hose  133  fittings. Similarly, the upstream housing  151 , downstream housing  190 , end caps  131   a ,  131   b , membrane cartridge  160 , and valve cap  171  may also be constructed from a substantially rigid non-porous plastic material. Examples of potentially suitable plastics that could be used include polypropylene (PP), various copolymers, polycarbonate (PC), acrylonitrile butadiene styrene (ABS), a polycarbonate/acrylonitrile butadiene styrene blend (PC/ABS), high-density polyethylene (HDPE), low-density polyethylene (LDPE), polyester, and copolyester. Because the basic functionality of the filter cartridge  130  of the second embodiment is similar to the filter cartridge  30  of the first embodiment, for brevity, common elements and features will not be discussed at length because they may obscure the invention with unnecessary detail. 
       FIGS. 14A through 14D  illustrate four views of the end cap  131 , which may be fused, or otherwise adhered, at each end of the filter cartridge  130 . Specifically,  FIG. 14A  illustrates a side view of an end cap of the second water filter,  FIG. 14B  illustrates a cross-sectional view of the end cap of the second water filter,  FIG. 14C  illustrates a top plan view of an end cap of the second water filter, and  FIG. 14D  illustrates a bottom plan view of the end cap of the second water filter. The end cap  131  is generally circular and sized to substantially conform to an end of the filter cartridge  130 . The threaded  132  and/or hose  133  fittings may be integrated with, or otherwise attached to, each end cap  131  and configured to mate with, for example, standard bottle threads (e.g., SP-410) or flexible tubing. The hose fitting  133  may be configured to couple with flexible tubing having, for example, an internal diameter between ⅛″ and 1″, or more preferably, between ⅛″ and ½″, or most preferably, between 3/16″ and ¼″. However, it is anticipated that the diameter may be adjusted to meet a particular need. Such fittings allow the filter cartridge  130  to be used with a variety of containers, including pop bottles, “soft bottle” bags (a rugged bag that has a threaded opening like a bottle), buckets, jerry cans, etc. 
     At the center of each end cap  131  is an opening  186  through which the fluid may pass. The opening  186  may be configured such that the fluid is directed through the center of the threaded  132  and/or hose  133  fittings so that the fluid may be ultimately directed to, or from, a container or hose. As illustrated, the end cap  131  may comprise a plurality of ribs  177  along its outer circumference and one or more standoff protrusions  181  on the inner surface. The plurality of ribs  177  may be used to secure a second component, such as the valve cap  171  or membrane cartridge  160 , while the stated standoff protrusions  181  maintain a gap between the end cap  131  and the second component, thereby reducing risk of fluid blockage. Further, as illustrated in  FIGS. 29-29 , one or more anti-seal notches may be incorporated into the end cap  131  to prevent the downstream vessel (i.e., a flange) from sealing to the end cap surface and allow air to escape through the threads, thus avoiding pressurizing the downstream vessel. Finally, one or more radial protrusions  185  may be provided around the opening  186  on the inner surface of the end cap  131 . As will be discussed below, the one or more radial protrusions  185  may be used to secure a third component, such as, for example, a spring  183 . 
     With reference to  FIGS. 15A through 15C , three views of the upstream housing  151  are illustrated. Specifically,  FIGS. 15A and 15C  illustrate side and top views while  FIG. 15B  illustrates a cross-sectional view of the upstream housing  151 . As illustrated in  FIGS. 15B and 15C , the inside surface of the upstream housing  151  may be provided with one or more tabs  180  configured to engage with the downstream housing  190 &#39;s shaped slot  178 , which functions as a guide channel. 
     To form the top cap assembly  176 , adhesive, for example, may be placed around the inside circumference of the upstream end of the upstream housing  151 . That is, on the end opposite the end having the one or more tabs  180  that engage the downstream housing  190 , which is identified as surface C in  FIG. 15B . A first end cap  131   a  may then be inserted into the upstream end of the upstream housing  151  containing the adhesive, thus bonding surface C of the upstream housing  151  with surface F of the first end cap  131   a  to create a water-tight seal. 
     The membrane cartridge  160  will be discussed with reference to  FIGS. 16A through 16C . Specifically,  FIG. 16A  illustrates a side view of a membrane cartridge  160 , while  FIG. 16B  illustrates a cross-sectional view and  FIG. 16C  illustrates a top plan view of the same. The membrane cartridge  160  houses the membrane bundle  187 , or an equivalent thereof. Indeed, the filter cartridge  130  may employ any suitable water treatment technology to treat contaminated or potentially contaminated water so that it is potable (i.e., safe to drink In some embodiments, membranes, such as microporous membranes, or ultrafiltration membranes, are used in filter cartridge  130 . Microfiltration membranes may refer to membranes having pores in the range of 0.1 to 10 microns. Ultrafiltration membranes may refer to membranes having pores in the range of 0.001 to 0.1 microns. In some embodiments, activated carbon is used in filter cartridge  130 . In some embodiments, a chemical water treatment technology, such as chlorination, is used in filter cartridge  130 . In some embodiments, a radiative water treatment technology, such as ultraviolet light, is used in the filter cartridge  130 . In some embodiments, a combination of multiple water treatment technologies is used in filter cartridge  130 . 
     The membrane bundle  187  may be potted inside the membrane cartridge  160  with potting resin at the open, upper end of the membrane cartridge. The membrane bundle  187  may occupy the entire inner cavity  161  of the membrane cartridge  160 , or a substantial portion thereof. The potting resin forms a water-tight seal between the membrane bundle  187  and the wall of the membrane cartridge  160 , thereby directing substantially all fluid through the membrane bundle  187 . By directing fluid through the membrane bundle  187 , as opposed to around it, filtering efficiency is increased. To reduce friction, one or more friction-reducing ribs  182  may be provided lengthwise along the membrane cartridge  160 &#39;s outer surface. Openings between the supporting ribs  179  allow water to exit the membrane cartridge  160  at the downstream end. 
     To create a water-tight seal between the membrane cartridge  160  and downstream housing  190 , an O-ring  163  (e.g., an AS568A-319 O-ring) may be placed into the radial gland  162  at the bottom of the membrane cartridge  160 . As will be discussed in greater detail below, the membrane cartridge  160  further comprises an actuating protrusion  164  at the downstream end that may be used to actuate a flow valve. The actuating protrusion  164  may be adhered to, or integrated with, the membrane cartridge  160 . 
     Details of the valve cap  171  will be discussed with reference to  FIGS. 17A through 17D .  FIGS. 17A through 17C  illustrate top, bottom and side views of the valve cap  171 , respectively, while  FIG. 17D  illustrate a cross-sectional view of the valve cap  171 . With reference to  FIG. 17D , a plunger ball  172  is placed into the valve cap  171  in the center annulus  173  defined by the radial protrusions  174  to provide a flow valve. The plunger ball  172  may be fabricated from, for example, plastic, rubber, and/or metal. A spring  183  may then be inserted into the center annulus  173  and placed on top of the plunger ball  172 . The radial protrusions  174  guide the plunger ball  172  such that the plunger ball  172 &#39;s motion can only be in the vertical direction (i.e., Direction X). Indeed, as noted above, the second end cap  131   b  may contain one or more radial protrusions  185  around the opening  186  on the inside surface that hold the spring  183  aligned in the axial position and prevent the spring  183  from blocking the opening, which would restrict the flow of water through the center opening  188 . 
     To form the spring-and-ball valve assembly  175 , adhesive may be placed on the inside of a second end cap  131   b  at surface B. The combination of the valve cap  171 , spring  183  and plunger ball  172  may then be inserted into the second end cap  131   b , thus bonding surface B of the second end cap  131   b  with surface D of the valve cap  171  to create a water-tight seal. Once assembled, the spring  183  generates a force that presses the plunger ball  172  into the radial valve seat  184  on the inside surface of the valve cap  171  when the filter is in the “off” position to create a water-tight seal. When the filter is in the “on” position, the actuating protrusion  164  on the end of the membrane cartridge  160  depresses or displaces the plunger ball  172  from the valve seat  184  by compressing the spring  183 , thus allowing water to flow through the spring-and-ball valve assembly  175 . 
     The downstream housing  190  will now be described with reference to  FIGS. 18A through 18D . Specifically,  FIGS. 18A and 18B  illustrate side and cross-sectional views, respectively, of the downstream housing  190  of the second water filter. As illustrated, the upstream end of the downstream housing  190  is configured to engage the top cap assembly  176 , while the bottom end is configured to be adhered or fused with the spring-and-ball valve assembly  175 . 
     Specifically, as illustrated in  FIG. 18A , the downstream housing  190  is equipped with one or more shaped slots  178  configured to engage with the one or more corresponding tabs  180  on the top cap assembly  176 . The one or more shaped slots  178  are positioned axially and circumferentially on the downstream housing  190 . An enlarged view of a shaped slot  178  is provided in  FIG. 18C . The shaped slot  178  enables the user to select between: (1) a first engagement position  178   a  (i.e., “on position”), which permits flow through the membrane and filter assembly; (2) a second engagement position  178   b  (i.e., “off position”), which prevents flow and seals the “clean end” of the filter assembly for storage; and (3) a third engagement position  178   c  (i.e., “unlocked position”), which allows the top cap assembly  176  to disengage from the downstream housing  190  and allows the user to pump the top cap assembly  176  and membrane cartridge to back-wash the membrane bundle  187 . 
     During final assembly, the membrane cartridge  160  may be inserted into the widest end (i.e., downstream end) of the downstream housing  190 , until the upstream end of the membrane cartridge  160  protrudes from the downstream housing  190  by, for example, two inches from the narrow end (i.e., upstream end) of the downstream housing  190 , which enables the user to adhere the top cap assembly  176 . A lubricant, such as silicone, polytetrafluoroethylene (e.g., Teflon) or another water- and food-safe lubricant, may be placed inside the downstream housing  190  at a location several centimeters from the downstream end, prior to assembly, to help facilitate assembly and overall function of the filter. 
     Next, adhesive may be placed on the outside circumference of the membrane cartridge  160  at surface A. The top cap assembly  176  may then be inserted onto the membrane cartridge  160 , thus bonding the two assemblies together and creating a water-tight seal. Specifically, the inner surface B of the end cap  131  may be bonded with the outer circumference surface A of the upstream end of the membrane cartridge  160 . As noted above, the one or more standoff protrusions  181  may be provided on the inner surface of the end cap  131  to maintain a gap between membrane cartridge  160  and end cap  131 . More specifically, the one or more standoff protrusions  181  prevent the membrane cartridge  160  from mating against the flat surface of the end cap  131   a , which would prevent water from entering the membrane bundle  187 . Furthermore, as noted above with reference to  FIG. 16 b   , an O-ring  163  may be placed into the radial gland  162  at the bottom of the membrane cartridge  160  to create a water-tight seal between the membrane cartridge  160  and downstream housing  190 . 
     Finally, adhesive may be placed around the inside circumference of the wide end of the downstream housing  190  at surface E. The spring-and-ball valve assembly  175  may then be inserted into the downstream housing  190 , thus bonding surface F of the second end cap  131   b  with surface E of the downstream housing  190  to create a water-tight seal. The top cap assembly  176  is then depressed toward the downstream housing  190  and rotated until the tabs  180  on the inside surface of the top cap assembly  176  engage with the shaped slot  178  on the downstream housing  190 . At this point, the filter cartridge  130  assembly is now functional and difficult to disassemble without causing damage. 
     To provide an overview, the second embodiment may be illustrated by the following Example. This Example is provided to aid in the understanding of the invention and is not to be construed as a limitation thereof. Referring to  FIGS. 19A-21A , a sequence of operations performed in the back-washing of the filter cartridge  130  is illustrated. As discussed above, back-washing may be performed as frequently or as infrequently as desired by users of the filter cartridge  130 , based upon the cleanliness of the water being treated and the acceptable rate of flow through the filter cartridge  130 . 
     Turning specifically to  FIG. 19A , a perspective view of the second filter cartridge  130  is shown in a first position (i.e., an open position). The filter cartridge  130  is shown in the first flow configuration with the downstream housing  190  abutting the upstream housing  151 . In the first position, the filter cartridge  130  is “on,” meaning that water can enter from the top, flow through the porous membrane inside, and exit at the bottom via the “clean end.” The normal direction of flow during filtration is from top to bottom, whereby the top end of the filter is the “dirty end” and the bottom is the “clean end.”  FIG. 19B  illustrates the first engagement position of the shaped slot  178  and the relative location of the top cap assembly  176 &#39;s tab  180  while in the first flow configuration. Finally,  FIG. 19C  illustrates a side view, while  FIG. 19D  illustrates a cross-sectional view of the second water filter in an open position. Turning now to  FIG. 19D , note that the actuating protrusion  164  provides a downward force against the plunger ball  172  such that the plunger ball  172  is pushed away from the valve seat  184 , thereby actuating the flow valve by breaking the water-tight seal and allowing fluid to freely flow between the valve cap  171  and membrane cartridge  160 . 
       FIG. 20A  illustrates a perspective view of the second filter cartridge  130  in a second position (i.e., a closed position). In the second position the filter is “off,” meaning the flow of water is stopped. Rotation of the downstream housing  190  about the central axis of the filter cartridge  130  in an off direction (as indicated by the arrow), causes the upstream housing  151  to slightly disengage from the downstream housing  190 . This allows the spring-loaded plunger ball  172  to be seated in the valve cap  171 , thus creating a water-tight seal, thereby blocking the flow of water through the valve cap  171 .  FIG. 20B  illustrates the second engagement position of the shaped slot  178  and the relative location of the top cap assembly  176 &#39;s tab  180  while in the second flow configuration.  FIG. 20C  illustrates a side view, while  FIG. 20D  illustrates a cross-sectional view of the second water filter in a closed position. Turning now to  FIG. 20D , note that the actuating protrusion  164  has been retracted, thus removing the downward force from the plunger ball  172 , thereby allowing the spring  183 &#39;s upward force to push the plunger ball  172  toward the valve seat  184 , thereby closing the flow valve by creating a water-tight seal and prohibiting fluid from flowing between the valve cap  171  and membrane cartridge  160 . 
       FIG. 21A  illustrates a perspective view of the second filter cartridge  130  in a third position (i.e., an unlocked position). Rotation of the downstream housing  190  about the central axis of the filter cartridge  130  in an unlocking direction (as indicated by the arrow) causes the upstream housing  151  to disengage from the downstream housing  190 . This allows the downstream housing  190  to move axially relative to the upstream housing  151  in the extension direction  101 , as illustrated in  FIG. 21A .  FIG. 21B  illustrates the third engagement position of the shaped slot  178  and the relative location of the top cap assembly  176 &#39;s tab  180  while in the third flow configuration.  FIG. 21C  illustrates a side view, while  FIG. 21D  illustrates a cross-sectional view of the second water filter in an unlocked position. Turning now to  FIG. 21D , note that, as in the second position, the actuating protrusion  164  remains retracted and the water-tight seal is maintained (i.e., the flow valve is closed). 
     However, unlike the second position, the filter cartridge  130  is “unlocked” in the third position, thereby allowing the top section to be pumped up  102  and down  101  relative to the bottom section by the user, as indicated by the arrows. As discussed in relation to the filter cartridge  30  of the first embodiment, the pumping action of filter cartridge  130  can serve the purposes of: (1) back-washing the membrane; (2) starting the siphon; and (3) clearing out any bubbles that cause airlock. 
     It should be noted that, although the membrane is described illustratively in this embodiment, other types of back-washable filtration media may be used without departing from the function of the filter as described in connection with this embodiment. Moreover, while three flow configurations are described, it is contemplated that fewer, or additional, flow configurations may be incorporated with the filter cartridge  130 . For instance, the second position may be omitted, if desired, since the valve is also closed during the third position. 
       FIGS. 22-27  illustrate filter cartridges according to a plurality of additional embodiments. Because the basic functionality of the filter cartridge of the plurality of additional embodiments are similar to the filter cartridges  30 , and  130  of the first and second embodiments, for brevity, common elements and features will not be discussed at length because they may obscure the invention with unnecessary detail. Similarly, as discussed above, the pumping action can serve the purposes of: (1) back-washing the membrane; (2) starting the siphon; and (3) clearing out any bubbles that cause airlock. 
     Turning now to  FIG. 22 , a filter cartridge  230  of a third embodiment may be controlled by positioning a ring, optionally constructed from multiple parts for ease of manufacture, which can be positioned to selectively hold or release an upper assembly  201  and a lower assembly  202 . The upper assembly  201  generally comprises the upstream housing  251  and end cap  231 , while the lower assembly  202  generally comprises the downstream housing  290  and end cap  231 . Those skilled in the art will recognize that while one such ring is pictured, there are multiple configurations that achieve comparable functionality. When the ring is in a first position, the upper assembly  201  is held and the lower assembly  202  is released. This allows the lower assembly  202  to extend, thus unplugging an exit valve so water can pass through the filter via the filter membrane  287  within the membrane cartridge  260 . When the ring is in a second position, the upper assembly  201  is released and the lower assembly  202  is held. This holds the exit valve shut while allowing the upper assembly  201  to extend and retract, which constitutes a multi-functional pumping motion to back-wash the membrane  287 , start a siphon, or clear any bubbles causing airlock. The closed exit valve in this position ensures that the extension and retraction of the upper assembly causes flow back and forth through the filter&#39;s inlet, while no flow occurs through the filter&#39;s outlet. When the ring is in a third position, both the upper assembly and the lower assembly are held. This holds the filter is in a closed or “off” position, so no water can flow, and no pumping can take place. 
     Turning now to  FIG. 23 , a filter cartridge  330  of a fourth embodiment may be controlled by positioning an upper assembly  301 , the upper assembly  301  having various tabs that can selectively hold the upper assembly  301  in various positions or release the upper assembly  301  to extend and retract. The upper assembly  301  generally comprises the upstream housing  351  and end cap  331 , while the lower assembly  302  generally comprises the downstream housing  390  and end cap  331 . The filter cartridge  330  also has a two-directional flow-controlled valve. The valve has a spring (which may be built-in or a separate component) that tends to return it to a resting position in which it does not block flow, unless the flow in either direction is strong enough to deflect the valve such that it blocks flow. Those skilled in the art will recognize that while one such valve is pictured, there are multiple configurations that achieve comparable functionality. The filter also has an annular interference valve that is optionally opened or closed by a plug extending from the upper assembly  301 . When the upper assembly  301  is in a first position, the plug is positioned to not block the annular interference valve so water can pass through the filter. When the upper assembly  301  is in a second position, the plug is positioned to block the annular interference valve so that water cannot pass through the filter. When the upper assembly  301  is in a third position, the upper assembly  301  is released so it can extend and retract, which constitutes a multi-functional pumping motion to back-wash the membrane  387  within the membrane cartridge  360 , start a siphon, or clear any bubbles causing airlock. The extension and retraction of the upper assembly  301  generate sufficient pressure to activate the flow-controlled valve, such that the valve is closed during both extension and retraction of the upper assembly  301 . This ensures that the extension and retraction of the upper assembly  301  causes flow back and forth through the filter&#39;s inlet, while no flow occurs through the filter&#39;s outlet. 
     Turning now to  FIG. 24 , a filter cartridge  430  of a fifth embodiment may be controlled by positioning an upper assembly  401 , the upper assembly  401  having various tabs that can selectively hold the upper assembly  401  in various positions or release the upper assembly  401  from the lower assembly  402  to extend and retract. The filter also has a two-directional flow-controlled valve. The valve is attached to a tension member that can selectively hold the valve in a neutral position, so as to not block the flow of water leaving the filter. The flow-controlled valve may be configured to block all flow in the reverse direction, so that the user can never accidentally drive water through the valve in the reverse direction. This is beneficial because it reduces the risk of accidental misuse of the filter. Those skilled in the art will recognize that while one such valve is pictured, there are multiple configurations that achieve comparable functionality. When the upper assembly  401  is in a first, fully extended, position, the tension member is engaged to hold the valve in the neutral position, so as to not block the flow of water leaving the filter. In the first position water can pass through the filter. When the upper assembly  401  is in a second, fully retracted, position, the tension member is not engaged to hold the valve in the neutral position, so the flow-controlled valve is free to block the flow of water leaving the filter. In the second position water cannot pass through the filter. When the upper assembly  401  is in a third position, the upper assembly  401  is released so it can extend and retract, which constitutes a multi-functional pumping motion to back-wash the membrane  487  within the membrane cartridge  460 , start a siphon, or clear any bubbles causing airlock. The extension and retraction of the upper assembly  401  generate sufficient pressure to activate the flow-controlled valve, such that the valve is closed during both extension and retraction of the upper assembly  401 . This ensures that the extension and retraction of the upper assembly  401  causes flow back and forth through the filter&#39;s inlet, while no flow occurs through the filter&#39;s outlet. 
     Turning now to  FIG. 25 , a filter cartridge  530  of a sixth embodiment may be controlled by positioning an upper assembly  501 , the upper assembly  501  having various tabs that can selectively hold the upper assembly  501  in various positions or release the upper assembly  501  from the lower assembly  502  to extend and retract. The filter also has a two-directional flow-controlled valve. The valve is attached to a tension member that can selectively hold the valve in a neutral position, so as to not block the flow of water leaving the filter. The flow-controlled valve may be configured to block all flow in the reverse direction, so that the user can never accidentally drive water through the valve in the reverse direction. This is beneficial because it reduces the risk of accidental misuse of the filter. Those skilled in the art will recognize that while one such valve is pictured, there are multiple configurations that achieve comparable functionality. When the upper assembly  501  is in a first, fully retracted, position, the tension member is engaged to hold the valve in the neutral position, so as to not block the flow of water leaving the filter. In the first position water can pass through the filter. When the upper assembly  501  is in a second, fully extended, position, the tension member is not engaged to hold the valve in the neutral position, so the flow-controlled valve is free to block the flow of water leaving the filter. In the second position water cannot pass through the filter. When the upper assembly  501  is in a third position, the upper assembly  501  is released so it can extend and retract, which constitutes a multi-functional pumping motion to back-wash the membrane  587  within the membrane cartridge  560 , start a siphon, or clear any bubbles causing airlock. The extension and retraction of the upper assembly  501  generate sufficient pressure to activate the flow-controlled valve, such that the valve is closed during both extension and the retraction of the upper assembly  501 . This ensures that the extension and retraction of the upper assembly  501  causes flow back and forth through the filter&#39;s inlet, while no flow occurs through the filter&#39;s outlet. 
     Turning now to  FIG. 26 , a filter cartridge  630  of a seventh embodiment may be controlled by positioning an upper assembly  601 , the upper assembly  601  having various tabs that can selectively hold the upper assembly  601  in various positions or release the upper assembly  601  from the lower assembly  602  to extend and retract. The filter also has a valve. A spring holds the valve against its seat, except when the valve is engaged by the upper assembly  601 , overcoming the spring force and opening the valve. The flow-controlled valve may be configured to block all flow in the reverse direction, so that the user can never accidentally drive water through the valve in the reverse direction. This is beneficial because it reduces the risk of accidental misuse of the filter. Those skilled in the art will recognize that while one such valve is pictured, there are multiple configurations that achieve comparable functionality. When the upper assembly  601  is in a first, fully retracted, position, the valve is engaged by the upper assembly  601 , thus opening the valve, so as not to block the flow of water leaving the filter. In the first position water can pass through the filter. When the upper assembly  601  is in a second position, the valve is not engaged by the upper assembly  601 , so the valve is held against its seat by the spring force. In the second position water cannot pass through the filter. When the upper assembly  601  is in a third position, the upper assembly  601  is released so it can extend and retract, which constitutes a multi-functional pumping motion to back-wash the membrane  687  within the membrane cartridge  660 , start a siphon, or clear any bubbles causing airlock. During the extension and retraction of the upper assembly  601 , the valve is held against its seat by the spring force, such that the valve is closed during both extension and retraction of the upper assembly  601 . This ensures that the extension and retraction of the upper assembly  601  causes flow back and forth through the filter&#39;s inlet, while no flow occurs through the filter&#39;s outlet. 
     Turning now to  FIG. 27 , a filter cartridge  730  of an eighth embodiment may be controlled by positioning an upper assembly  701 , the upper assembly  701  having tabs that can selectively hold the upper assembly  701  in various positions or release the upper assembly  701  from the lower assembly  702  to extend and retract. The filter also has an annular interference valve which can be selectively engaged by the upper assembly  701 . The plug of the annular interference valve has tabs that move within a shaped slot or channel such that rotating the plug of the annular interference valve causes the assembly to selectively block or not block the flow of water. When the upper assembly  701  is in a first position, the plug of the valve is engaged by the upper assembly  701  to not block the flow of water leaving the filter. In the first position water can pass through the filter. When the upper assembly  701  is in a second position, the plug of the valve is engaged by the upper assembly  701  to block the flow of water leaving the filter. In the second position water cannot pass through the filter. When the upper assembly  701  is in a third position, the upper assembly  701  is released so it can extend and retract, which constitutes a multi-functional pumping motion to back-wash the membrane  787  within the membrane cartridge  760 , start a siphon, or clear any bubbles causing airlock. During the extension and retraction of the upper assembly  701 , the plug of the valve is held such that the flow of water is blocked. This ensures that the extension and retraction of the upper assembly  701  causes flow back and forth through the filter&#39;s inlet, while no flow occurs through the filter&#39;s outlet. 
     As illustrated in  FIGS. 28 a  through 28 d   , it may be advantageous in certain embodiments to provide one or more surface protrusions  280  on an outer surface of the housings  890 ,  851 . The surface protrusions  280  prevent the filter cartridge  830  from easily rolling when placed on a flat surface, thereby reducing the risk of damage that can result from the filter cartridge  830  rolling off of a table or counter top. The surface protrusions  280  may be used in conjunction with any of the above-described embodiments. For example, the one or more surface protrusions  280  may be placed on the downstream housing  890  (as illustrated), the upstream housing  851 , the end caps, or a combination thereof. In fact, plural surface protrusions  280  may be placed along the entire circumference of the filter cartridge  830 , or a portion thereof. While oval surface protrusions  280  are illustrated, it is anticipated that other shapes may be used to facilitate a particular design or aesthetic need. For example, each surface protrusion  280  may be in the shape of an alphanumeric character, which would collectively spell out a brand name, message, capacity, etc. The protrusions may even be square, circular or domed in shape (or any other ornamental shape, i.e., a water droplet). 
     As illustrated in  FIG. 28 e   , by placing the protrusions on the backside of the filter, the filter will come to a rest such that the product trademark, wordmark, images and/or function position indicators are facing toward the user, thereby making the features more readily visible. As used herein, backside of the filter refers to the surface of the filter opposite from the surface ornamentation that displays, for example, the product trademark, wordmark, images and/or function position indicators. Alternately, the protrusions may be placed on the front of the filtration device and also be incorporated into the surface ornamentation. When two protrusions are placed at the same axial location but are separated radially by several degrees such that both protrusions are able to contact the hard surface simultaneously, the features serve as a stable base that prevents the filter from rolling when placed on a surface; by placing one or more additional pairs of protruding features at the opposite end of the filter, the resistance to rolling increases dramatically. 
     Alternatively, in lieu of surface protrusions  280 , the filter cartridge  830  may employ an outer housing or other component (e.g., the end caps), having a non-circular cross-section, such as an oval or polygonal cross-section. For example, the outer housing&#39;s cross-section may be triangular, square, pentagonal, hexagonal, heptagonal, octagonal, etc. However, to facilitate the above described functions, such as the twisting and pumping action (e.g., back-flushing), the inner components, including the cartridge membrane and other inner components, may remain substantially circular, thereby allowing them to function (e.g., rotate) as needed. 
     As illustrated in  FIGS. 29 a  and 29 b   , one or more anti-seal notches  2902  may be incorporated into the end cap  2900 . The design of the filtration device allows the user the option to use a bottle (such as a plastic soda bottle) as the downstream vessel for collecting filtered water. Many plastic bottles contain flanges, plastic rings and other features that, when tightened securely against the outer face of the end cap, create an air-tight seal that prevents air from escaping the downstream vessel. As filtered water enters the vessel, an equal volume of air must be allowed to leave the vessel or the vessel will begin to pressurize; as the pressure in the vessel increases, the flow rate decreases and eventually stops. By placing one or more small notches or grooves around the outside edge of the threaded portion of the end cap a pathway is created that allows air traveling out of the vessel and through the threads of the end cap to bypass the flange, ring or other feature that would otherwise create a seal. These anti-seal grooves  2902  thus eliminate the possibility of pressuring the downstream vessel regardless of the vessel&#39;s design. 
       FIGS. 30 a -30 b    illustrate an embodiment of a water filter having a second guide channel design. As discussed above, the downstream housing of the filtration device contains a channel/groove that engages with tabs located on the inside surface of the top cap. When the tabs are first engaged with the channel, the top cap and downstream housing are secured together such that there is no free movement in the axial direction. This position is referred to as ‘OFF’ as the membrane cartridge is not able to actuate the flow valve. When the top cap is rotated clockwise with respect to the downstream housing, the channel causes the top cap/membrane cartridge to move axially within the downstream housing, eventually activating the flow valve, which then allows water to flow through the device. This is referred to as the ‘ON’ position. It is beneficial to provide tactile feedback to the user when they are moving back and forth between the two positions, and when disengaging the tabs from the channel so that the filter may be back-washed (i.e., cleaned). By creating a deliberate direction change in the channel shape, the motion of the top cap is momentarily and noticeably interrupted, providing tactile feedback to the user to indicate the filter has been placed into the ‘OFF’ position. 
     The double-swept shape of the channel provides a secondary function that resolves a potential issue experienced with continuous incline channel shape (i.e., similar to a bottle thread). When the filter is placed into the ‘ON’ position, the spring inside of the valve exerts an upward force on the membrane cartridge and thus the top cap. If the tabs were engaged with an angled surface, the top cap would be able to rotate and move up in the channel due to the force, thus causing the valve to seal. By placing flat surfaces at both the ‘ON’ and ‘OFF’ positions, we prevent any axial force from causing the top cap to rotate out of the desired position. 
     A number of exemplary aspects and embodiments have been discussed above, and those of skill in the art will recognize certain modifications, permutations, additions and sub-combinations thereof. It is therefore intended that features introduced here are interpreted to include all such modifications, permutations, additions and sub-combinations as are within their true spirit and scope. Moreover, while the embodiments have been discussed in terms of filtering water, the above described embodiments may be applied to other fluids in addition to water.