Abstract:
The water purification cartridge treats water with a halogen liberated from halogenated hydantoinyl resins and polymers, resulting in the safe, complete, and economical purification of non-potable water at low flow rates typically found in gravity feed filtration systems and in systems with low water pressures of less than about 1 psig. The cartridge of the invention purifies non-potable water of bacteria, viruses, and some protozoans without the need for additional power sources or risk to the consumer for exposure to chemicals that have been identified to have harmful, long-term effects. The cartridge described herein enables a gravity fed water purification system by combining the cartridge with conventional filters, such as porous diatomaceous earthen ceramic water filters or activated carbon filters packed either with granular activated carbon or block.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application is a continuation-in-part of application Ser. No. 10/676,730, filed Oct. 1, 2003, expressly incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention is related to water purification and water disinfection in low flow applications, such as gravity feed filtration devices that do not require electricity or power sources to operate.  
       BACKGROUND OF THE INVENTION  
       [0003]     Reliable potable drinking water sources are scarce in developing countries. High mortality rates can be attributed to inadequate water purification. Cholera, typhoid, dysentery, and rotavirus diarrhea are constant problems faced by people in countries who are forced to use contaminated water.  
         [0004]     The lack of safe water supplies perpetuates and aggravates the cycle of poverty and disease. Some efforts have been made to provide clean, potable water in places where power is unavailable. Gravity feed filter systems are normally used to purify non-potable water at the point of use. Gravity feed filter systems are reliable and inexpensive.  
         [0005]     The use of halogens such as chlorine, bromine, and iodine for water purification is well documented and widely used in developing countries. The most common forms for treating water with halogens is to add liquid bleach or a chlorine tablet to a pot of non-potable water and to wait for a prescribed period of time before decanting the water. One of the difficulties of using halogens is in maintaining a proper level of residual halogen for proper microbial control. Additionally, iodine, a commonly used halogen in gravity filters, has lost favor with consumers and health officials because iodine causes permanent and debilitating health problems in children and pregnant women.  
         [0006]     Recently, polymers having heterocyclic N-halamines have been developed that can provide reliable levels of residual chlorine and bromine. One such agent that holds promise is a halogenated polystyrene hydantoin bead, described in U.S. Pat. No. 6,548,054, to Worley et al., incorporated herein by reference in its entirety.  
       SUMMARY OF THE INVENTION  
       [0007]     The present invention is related to a water purification cartridge suitable to be used in gravity fed water purification systems.  
         [0008]     One embodiment of the present invention is related to a gravity fed water purification cartridge. Because the water purification cartridge relies on gravity, the cartridge&#39;s inlet for untreated water is relatively higher than the cartridge&#39;s outlet for treated water, i.e., water at the inlet has more potential energy than water at the outlet of the cartridge. It is to be understood when reading the disclosure that any directional language used is intended to be read in the context of the figures. The water purification cartridge includes an inlet head cap coupled with a prefilter. The inlet head cap has an inlet for untreated water. The water purification cartridge optionally may include a ring member that is in fluid flow communication with the inlet head cap. If provided, the ring member is configured to evenly distribute the untreated water to a purification medium. The water purification cartridge includes a purifier vessel containing the purification medium. The purifier vessel is in fluid flow communication to receive the untreated water from the prefilter, or, if provided, from the ring member. The water purification cartridge includes a bulkhead coupled to the inlet head cap. The bulkhead is configured to divide the untreated water from the treated water. The water purification cartridge includes a dwell chamber coupled to the bulkhead. The dwell chamber is provided downstream from the purifier vessel and is exterior to the purifier vessel. The dwell chamber provides residence time for additional treatment of the water with both the residual halogen supplied by the purification medium as well as halogen atoms attached to microorganisms. The water purification cartridge may also include an outer skin coupled to the bulkhead. This is not required for the proper functioning of the cartridge but such outer skin may be used to reduce post-treatment cross-contamination. When used, the outer skin is exterior to the dwell chamber. The outer skin and the dwell chamber can be configured to provide an annular space therebetween where the treated water can be post treated with additional conditioning medium. The outer skin is configured to discharge the treated water below the inlet of the untreated water.  
         [0009]     In another embodiment, the water purification cartridge includes an inlet member, wherein the inlet member provides untreated water in an axial direction. The water purification cartridge may be generally symmetrical about a longitudinal center axis as the cartridge is rotated about the axis. Axial direction means that the general flow or the majority of the flow is in a direction parallel to the axis of rotational symmetry. The water purification cartridge may optionally include a ring member adjacent to the inlet member. If provided, the ring member is configured to distribute the untreated water in a radial direction. Radial direction means that the general flow or the majority of flow is in a direction parallel to a radius line from the center axis of symmetry. If provided, the ring member is in communication with a purifier vessel, otherwise the purifier vessel is in fluid communication with the inlet member. The purifier vessel is configured to treat untreated water to provide treated water. The water purification cartridge includes a bulkhead adjacent to the purifier vessel. The bulkhead is configured to separate untreated water from treated water in the cartridge. The water purification cartridge includes a dwell chamber exterior to the purifier vessel. The dwell chamber is configured to provide treated water flow in an upward axial direction.  
         [0010]     In another embodiment, an alternate sealing mechanism is provided. In this embodiment, the water purification cartridge includes an outer skin of the water purification cartridge attached to a bulkhead. The water purification cartridge includes a purifier vessel attached to the bulkhead. The water purification cartridge includes an inlet head cap that is attached to the bulkhead with a sealing ring that attaches to the bulkhead. The bulkhead divides an area for untreated water and an area for treated water, wherein the passage of water between areas is through the purifier vessel. There is an O-ring that creates a seal between the purifier vessel and the bulkhead, and an O-ring that creates a seal between the inlet head cap and the bulkhead. The inlet head cap passes through the center of the sealing ring and the inlet head cap is held to the bulkhead by screwing the sealing ring to the bulkhead.  
         [0011]     In another embodiment, an alternate sealing mechanism is provided. In this embodiment of the water purification cartridge, the cartridge includes an outer skin of the water purification cartridge attached to a bulkhead portion; a purifier vessel portion attached to the bulkhead portion; and an inlet head cap that is attached to the bulkhead portion. However, the bulkhead portion and the purifier vessel portion form a unitary component of the water purification cartridge. The inlet head cap is attached to the bulkhead in a substantially nonremovable manner.  
         [0012]     In another embodiment, the water purification cartridge includes a purifier vessel to treat untreated water with a purification medium that is selected from at least one from a halogenated polystyrene hydantoin, a halogenated polymeric sulfonamide resin, a halogenated hydantoinyl siloxane, and a halogenated polystyrene triazinedione. The water purification cartridge also includes a dwell chamber after the purifier vessel to provide residence time so that the treated water can contact residual halogen liberated from the purification medium. The halogen can be chlorine or bromine.  
         [0013]     Another embodiment of the present invention is related to a gravity fed water purification system. The water purification system includes a prefilter interior to an untreated water container. The water purification system includes a water purification cartridge in fluid flow communication with the prefilter. The water purification cartridge is interior to a treated water container. The untreated water container and the treated water container are integrally coupled to prevent the entry of untreated water into the treated water container. The cartridge includes a purification medium capable of bonding and releasing a halogen. In use, the water purification system is fed untreated water in the untreated water container. The untreated water flows into the prefilter and through the cartridge. The cartridge treats the untreated water and discharges treated water into the treated water container.  
         [0014]     The water purification cartridge according to the present invention treats water resulting in the safe, complete, and economical purification of non-potable water at low flow rates typically found in gravity feed filtration systems. The cartridge of the invention purifies non-potable water of bacteria, viruses, and some protozoans without the need for additional power sources or risk to the consumer for exposure to chemicals that have been identified to have harmful, long-term effects. The cartridge described herein enables a gravity fed water purification system by combining the cartridge of the invention with conventional filters, such as porous diatomaceous earthen ceramic water filters or activated carbon filters packed either with granular activated carbon or block. Such earthen ceramic filters and activated carbon filters cannot inactivate, remove, or otherwise treat and control many pathogenic microorganisms and viruses such as  Klebsiella terrigena  and poliovirus to levels suitable to meet the United States Environmental Protection Agency (EPA) and/or World Health Organization (WHO) guidelines.  
         [0015]     The cartridge of the invention can be provided with a purification medium of polystyrene hydantoin and halogenated polystyrene hydantoin resins, heterocyclic N-halamine polymers, and hydantoinyl siloxanes as described in U.S. Pat. No. 6,548,054, filed Sep. 6, 2001; U.S. Pat. No. 5,490,983, filed Jul. 28, 1994; and U.S. application Ser. No. 10/400,165, filed Mar. 24, 2003, all expressly incorporated herein by reference in their entirety. Such resins and polymers can bind and release a halogen, such as chlorine or bromine. A suitable form of the polystyrene hydantoin and halogenated polystyrene hydantoin used in the present invention is a spherical bead of about 50 microns to about 700 microns in diameter. This polymer form lends itself to application in a packed bed configuration. The cartridge according to the invention can be used with other halogen binding media used for water purification, such as halogenated derivatives of poly(styrene-vinyl dibenzene)sulfonamide resins, polystyrene triazinediones, hydantoinyl siloxanes, and derivatives of polystyrene hydantoin. In general, polymeric compounds including N-halamine moieties, can be used in the filter cartridge according to the present invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:  
         [0017]      FIG. 1  is a schematic illustration showing the location of a cartridge relative to a prefilter in a gravity fed water purification application according to the present invention;  
         [0018]      FIG. 2  is an illustration of an exploded cross section of the cartridge according to the present invention;  
         [0019]      FIG. 3  is an illustration of a cross section of the cartridge according to the present invention;  
         [0020]      FIG. 4  is an illustration of a cross section of another embodiment of a cartridge according to the present invention;  
         [0021]      FIG. 5  is an illustration of a cross section of yet another embodiment of a cartridge according to the present invention;  
         [0022]      FIG. 6  is a graph plotting the log reduction value as a function of water flow from the water purification cartridge according to the present invention; and  
         [0023]      FIG. 7  is a graph plotting the log reduction value as a function of water flow from the water purification cartridge according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0024]     Referring to  FIG. 1 , a gravity fed water purification system  100  comprises a feed water container  102  (untreated water container), and a purified water container  104  (treated water container). The entire water purification system  100  may be supplied as a unit and can be located at the point of use, such as in a dwelling. Untreated water desired to be purified can be loaded into the feed water container  102 . The feed water container  102  contains a prefilter  106  in the interior thereof. The prefilter can be of conventional design that may include ceramic filters, bag filters, and/or carbon filters. The prefilter may initially remove particulates of specific size, and/or remove any odors or color, and any non-soluble particulates. The prefilter  106  is connected to a cartridge  108  that contains the water purification medium, such as a polymer having heterocyclic N-halamine moieties. The feed water container  102  is attached to the purified water container  104  in an integral manner to prevent untreated water from bypassing the prefilter  106  and cartridge  108 . Cartridge  108  is interior to the purified water container  104 .  
         [0025]     Untreated water fed to the feed water container  102  travels through the prefilter  106  and into the cartridge  108 , where the untreated water contacts the purification medium and is thereby treated. The treated water exits the cartridge  108  and is collected in the purified water container  104 . The purified water can be dispensed through faucet  110 . The orientation of the water purification system is such to take advantage of gravity. The force of gravity is the force that drives the water through the prefilter  106  and the cartridge  108 . The shape and size of the feed water container  102  and purified water container  104  can be of any dimensions to accommodate the design capacity, pressure, and flow rate through the prefilter  106  and cartridge  108 .  
         [0026]     Referring now to  FIGS. 2 and 3 , the cartridge  108  comprises an inlet head cap  112 , a compression ring  124 , a bulkhead  134 , a purifier vessel  132 , a dwell chamber  142 , an outer skin  116 , gasket  130 , interposed between the inlet head cap  112  and bulkhead  134 ; and gasket  150 , interposed between the purifier vessel  132  and bulkhead  134 .  
         [0027]     The inlet head cap  112  comprises a neck  152  that can attach to a variety of outlet nozzles typically found on prefilters. The inlet head cap  112  is attached to the prefilter  106  in an integral manner via the neck  152 . It will be apparent to those skilled in the art from a reading of the disclosure that attaching the inlet head cap  112  to the prefilter  106  can be achieved in a variety of ways. Specifically, in the embodiment described herein, the inlet head cap  112  is attached to the prefilter  106  via a threaded connection  118  provided on neck  152 .  
         [0028]     The inlet head cap  112  is designed to have an axial fluid flow path leading from the threaded inlet  118  to a compression ring  124 . A pair of cylindrical walls extend downward from the inlet head cap, thusly forming the neck portion  152  and axial fluid flow path. The walls of the neck come together to create a flattened lower surface that during use, will press against the upper surface of the compression ring  124 . The compression ring  124  has a cylindrical sidewall with a plurality of holes  128 . The water flow path is diverted by the compression ring  124  from the generally axial flow produced by the inlet head cap  112  to a generally radial flow at the compression ring  124 . Directly below the compression ring  124 , a purifier vessel  132  that contains the purification medium, is provided.  
         [0029]     The purifier vessel  132  is designed with a generally open upper end to receive untreated water flowing through the compression ring  124 , and a generally closed lower end, that is capable of discharging the treated water. The open end has a horizontal flange  136  that is formed perpendicular to the cylindrical sidewall of the purifier vessel  132 . A plurality of pegs  140  are spaced along the upper circular surface of the horizontal flange  136 . The pegs  140  define a plurality of spaces between adjacent pegs to allow untreated water to flow therethrough and into the purifier vessel  132 . The upper ends of the pegs  140  define the perimeter of a circle, wherein the ends of the pegs  140  are configured to mate with a groove  126  formed underneath the compression ring upper surface. The lower end of the purifier vessel  132  has a plurality of holes  156  generally distributed in a radial pattern to distribute the treated water from the lower end of the purifier vessel. The water leaving the purifier vessel is distributed in a generally radial direction after travelling generally axially through the main portion of the purifier vessel.  
         [0030]     The underside of the purifier vessel horizontal flange  136  is in contact with the gasket  150 , which rests on a depressed surface  138  formed in the center of the bulkhead  134 . Gasket  150  is a compressible gasket having a hardness of about 20 to about 80 units measured with a Shore A durometer. Gasket  150  is made from a non-leaching material, such as ethylene propylene diene monomer (EPDM), which is suitable for potable water applications. The main body of the purifier vessel  132  passes through a center hole provided in the bulkhead  134 . The bulkhead  134  is circular and can have a gradual slope from the outermost perimeter to the center hole. The untreated water from the compression ring  124  flows through the compression ring holes  128  and is distributed on the upper side of the bulkhead  134 . The untreated water thus collects evenly around the perimeter of the open end of the purifier vessel  132 . The water is channeled into the purifier vessel  132  through the spaces between the pegs  140 . The purifier vessel  132  may be tapered as illustrated, or may be of a constant diameter. The untreated water contacts the purification medium within the purifier vessel  132 . The treated water exits the purifier vessel  132  at the outlet holes  156  in a generally radial manner. After passing through the purifier vessel  132 , the treated water flows into the dwell chamber  142 . The dwell chamber  142  is provided for additional treatment residence time with the residual halogen species. Suitable minimum residence times are about 30 seconds to about 5 minutes. The dwell chamber  142  is a container with a closed lower end, and outlet holes  144  provided at an upper location on the vertical walls of the dwell chamber  142 . The treated water accumulates in the dwell chamber  142  as the level builds upward in the dwell chamber  142  in a generally axial manner. The water is allowed to reach the uppermost portion of the dwell chamber  142 , which is directly beneath the bulkhead  134 . Dwell chamber holes  144  allow the water to exit the dwell chamber  142  from an upper location thereof in a generally radial direction. The dwell chamber outlet holes  144  are large enough not to impede flow and create additional pressure drop.  
         [0031]     The dwell chamber  142  is provided within the interior of the outer skin  116 , and exterior to the purifier vessel  132 . The outside diameter of the dwell chamber  142  is smaller than the inside diameter of the outer skin  116 , so that an annular space is formed between the dwell chamber  142  and the outer skin  116 . Both the dwell chamber  142  and the outer skin  116  can be tapered at an angle to match the taper of the purifier vessel  132 . Alternatively, the purifier vessel  132 , the dwell chamber  142 , and the outer skin  116 , can be provided as components having a constant diameter. The treated water generally flows downward in an axial direction in the annular space between the dwell chamber  142  and the outer skin  116 . The water is discharged from the outer skin  116  in a generally radial direction through holes  146  in the lower portion of the outer skin  116 . The treated water exiting the outer skin  116  is collected in the purified water container  104 , shown in  FIG. 1 .  
         [0032]     The bulkhead  134  can have horizontal circular grooves on its underside that can mate with a horizontal circular ring on the dwell chamber  142 , or on the outer skin  116 , to attach the dwell chamber  142  and outer skin  116  to the bulkhead  134 . The dwell chamber  142  and outer skin  116  can be attached to the bulkhead  134  with adhesives, sonic welding, snap together fasteners, or other suitable means. The outer skin  116  is exterior to the dwell chamber  142 . The outer skin  116  is sealed at the upper end to the bulkhead  134 , and has outlet holes  146  located at a lower portion thereof. Treated water exits the outer skin  116  in a generally radial direction from holes  146 , after having passed through the annular section between the dwell chamber  142  and outer skin  116 .  
         [0033]     Bulkhead  134  separates the untreated water above the bulkhead  134  in the inlet head cap  112  from treated water below the bulkhead  134 . The inlet head cap  112  has threads  148  located at the interior side of the lower end of the inlet head cap  112 . The threads  148  mate with corresponding threads  154  on the bulkhead  134 . Gasket  130  is provided at the union of the inlet head cap  112  with the bulkhead  134 . Gasket  130  is generally incompressible so as to allow slippage of the inlet head cap  112  and the bulkhead  134  for ease of taking apart. As the inlet head cap  112  is screwed to the bulkhead  134 , the flattened surface  120  of the neck portion  152  presses against the compression ring  124 . In turn, the compression ring  124  presses down on the purifier vessel  132  that abuts the bulkhead  134 , and compresses the gasket  150  against the bulkhead  134 , thus sealing the untreated water side of the bulkhead  134  from the treated water side.  
         [0034]     During operation, the compression ring  124  uniformly transfers the compressive force from the inlet head cap  112  to the underlying purifier vessel  132 . The compression ring  124  uniformly redistributes the water flow into the inlet of the purifier vessel  132  in a generally radial manner. The compression ring  124  also can provide a uniform water level to evenly distribute the untreated water equally to all sides of the purifier vessel  132 . The water flows into the purifier vessel  132  in the spaces between adjacent pegs  140 .  
         [0035]     The compressive forces ensure that the purifier vessel  132  is properly sealed for use as a water purifier without bypassing untreated water. The compression ring  124  seals to the purifier vessel  132  by transferring the compressive force created by fastening the inlet head cap  112  to the bulkhead  134  via a gasketed threaded system comprising a mated thread  148  and gasket  130 . The compressive force is transferred to the compression ring  124  from the inlet head cap  112  via the neck  152  to the purifier vessel&#39;s pegs  140 , thus pushing down on the purifier vessel  132 . The compressive force is transferred from the purifier vessel&#39;s  132  horizontal flange  136  to the bulkhead&#39;s seat portion  138 . The result is a watertight seal that is easily disassembled by hand.  
         [0036]     The segmented pegs  140  that extend from the top of the flange  136  of the purifier vessel  132  serve two purposes. The first purpose is to transfer the compressive force from the compression ring  124  down to the flange  136 . The second purpose is to provide the end user with a means to easily remove and insert the purifier vessel  132 , when required. Removal of the purifier vessel  132  can allow recharging polystyrene hydantoin and/or replacing of the entire purifier vessel  132  and/or quickly and easily inserting the purifier vessel  132  into the cartridge during initial manufacture. The compressible gasket  150  that underlies the flanged portion of the purifier vessel  132  and bulkhead  134  is of an appropriate durometer and thickness to allow for adequate compression and sealing of the purifier vessel  132  to the bulkhead  134  so as to prevent any water from bypassing the purifier vessel  132 . The shape of the purifier vessel  132  has a geometry that allows for even plug flow through the purification medium bed within the purifier vessel  132 . In one embodiment, the aspect ratio, defined as the ratio of the length to the largest inner diameter dimension of the purifier vessel  132  is greater than or equal to 3. Higher aspect ratios may be used, but at ratios above 4.5, the pressure across the bed purification medium bed can increase to a point where it may impede water flow and reduce the performance of the cartridge. The slight taper from inlet to outlet of the purifier vessel  132  can be used to improve manufacturability, but is not required. The purifier vessel walls are impermeable so that water cannot permeate into the other portions of the cartridge prior to traveling through the biocidal purification medium bed within the purifier vessel  132 . A purification medium bed with an aspect ratio equal to or greater than 3 ensures that the water will flow through the purification medium bed within the purifier vessel  132  efficiently and in a plug flow fashion. Retaining elements (not shown) of a permeable material, such as nonwoven mesh, or monofilament filter cloth, can be attached to the inlet end of the purifier vessel  132  and the outlet end of the purifier vessel  132 , that are capable of holding particle sizes on the order of about 50 to about 750 microns in diameter. The outlet end of the purifier vessel  132  can have a nonwoven batt that overlies the inside of the outlet holes of the purifier vessel  132  to retain the purification medium within the purifier vessel  132 . The batt is porous enough not to impede flow from the medium, while fine enough to retain the purification medium within the purifier vessel  132 . In one embodiment of the cartridge, the purifier vessel  132  can hold about 5 to about 100 grams of a purification medium.  
         [0037]     Post-purification treatment of the treated water after exiting the purifier vessel  132  can take place in the dwell chamber  142  and outer skin  116 . The residence time spent within the dwell chamber provides an opportunity to post treat the treated water. Water level rises in the dwell chamber  142 , then flows radially out of the dwell chamber  142  and down through the annular space created between the dwell chamber  142  and the outer skin  116 . The annular space can be filled with a variety of water treatment media for additional post-purification treatment of water. The annular space can provide a means to customize the cartridge for the specific local needs of the water to be treated. The volume of the annular space can be adjusted either by increasing the diameter and/or length of the dwell chamber  142  or outer skin  116 . The annular space can be filled with media that is capable of removing heavy metals (e.g., KDF 55, iron sulfate, chitosan treated iron granules), residual organics and halogens (i.e., granular activated carbon) and/or to add mineralization for taste. The annular space provides a means to post-treat the treated water in an economical and compact way.  
         [0038]     Additionally, the bulkhead  134  provides a means for the dwell chamber  142  and outer skin  116  to be properly aligned and attached to the remainder of the cartridge elements. The bulkhead  134 , through its center opening, provides a means to align the purification vessel  134  properly with the dwell chamber  142  and outer skin  116 . The gradually sloping surface of the bulkhead  134  to the central hole results in a water level that is uniformly and substantially the same height all around the entrance to the purifier vessel  132 . The bulkhead  134  provides users with access to the compression ring area without having to remove the entire cartridge from the system. The inlet head cap  112  can be attached to the bulkhead  134  through the use of a frictional seal. The sealing gasket  130  located at the union of the inlet head cap  112  with the bulkhead  134  is made from a material that prevents binding of the bulkhead  134  to the inlet head cap  112  and creates a leak-proof seal, thereby preventing untreated water from bypassing the purifier vessel  132 . In one embodiment, the gasket  130  can be made from polyurethane.  
         [0039]     The reaction of viruses with halogenated polystyrene hydantoin is substantially irreversible and requires time to demonstrate a certain level of microbial efficacy. In the case of bacteria, however, the inactivation could be reversed if inadequate time (i.e., volume) is used. For the cartridge herein described, the medium free volume of the dwell chamber  142  is about 300 cubic centimeters. This volume is suitable for the inactivation of both viruses and bacterium to meet efficacy levels of 4-log and 6-log reductions respectively recommended by the EPA and WHO.  
         [0040]     It should be noted that the contact time in the dwell chamber  142  of the present invention is not analogous to conventional holding tanks and conventional dwell chambers. Holding tanks contain non-potable water that passes through a high residual biocidal halogen such as iodine resin or chlorine tablets during which time the water acquires a concentration of biocides suitable for purification, typically on the order of 14 ppm iodine, 14 ppm bromine or 9 ppm free chlorine. The mechanism for purification then takes place within the holding tanks in which the halogen attaches and inactivates or lyses organisms. Typical times in such holding tanks are on the order of 30 minutes to 70 minutes. Using the chlorinated polystyrene hydantoin beads described in U.S. Pat. No. 6,548,054, the residual halogen concentration leaving the purification vessel  132  and residing in dwell chamber  142 , is on the order of 0.1 ppm to 0.5 ppm by weight free chlorine. Even at holding tank sizes of 70 minutes, such low residual free chlorine concentrations are inadequate to achieve 4-log inactivation of halogen resistant viruses such as poliovirus. However, use of chlorinated polystyrene hydantoin beads achieves 4-log inactivation even at the low levels of residual chlorine, such as less than 1 ppm.  
         [0041]     Additionally, because of the low flow rate normally achieved in gravity feed devices, the flow character through the dwell chamber  142  is quiescent and laminar. The water surface level is horizontal thereby achieving uniform filling of the dwell chamber  142 . The majority of the pressure generated from the height of the water is consumed when the water passes through the prefilter. The remaining pressure is sufficient for water to flow through the purification vessel  132 , dwell chamber  142 , annular space and exit at the outlet of the outer skin  116 . It should be appreciated that volumes and shapes of these components can be varied to achieve different flow rates and different inactivation levels.  
         [0042]     Without the proper design of the dwell chamber  142 , the annulus could not be used to house additional treatment media. However, the cartridge  108  does not require additional media in order to purify non-potable water to purification standards, such as those specified by the EPA and WHO. In addition to the annulus, additional post-treatment media can be added at a bottom interior location in the outer skin  116 . After passing through the annulus and the outer skin  116 , the treated, purified water exits the cartridge  108  from the bottom of the outer skin  116  through the holes  146  in a generally radial direction.  
         [0043]     The outer skin  116  further serves to provide an aesthetically pleasing shape to the overall cartridge  108  and to prevent cross-contamination of the treated water during handling. A common problem with many gravity filtration and purification devices is that when handled by the user, the surfaces on which treated waters flow can be easily contaminated by contaminated hands, thus rendering the cartridge useless at purifying water. It should also be noted that the outer skin  116  of the cartridge  108  serves several functions. Firstly, the outer skin  116  provides a means for the end user to handle the cartridge  108  while removing the purification vessel  132  from the inlet head cap  112 . Secondly, the outer skin  116  ensures that none of the treated water or treatment surface of the dwell chamber  142  will be contaminated during routine handling of the cartridge  108 . Cross-contamination is a chronic problem with many water filter and treatment devices and it is highly recommended by the WHO to avoid and minimize cross-contamination as the treated water would be rendered unsuitable for drinking.  
         [0044]     Ideally, the purification medium used in the purifier vessel  132  should be stable at ambient temperature within the cartridge  108 . The purification medium should be insoluble in water so that it cannot be consumed by persons drinking the treated water. The purification medium should control and/or inactivate a wide variety of pathogenic microorganisms. The purification medium should not leach hazardous and/or harmful chemicals into the water. The purification medium should provide residual free halogen species at a low level so as not to impart an undesirable odor, taste, or produce subsequent reaction by-products, such as trihalomethanes. The purification medium should be effective under a broad range of water pH and temperatures. The purification medium should provide biocidal effects for relatively long periods of use requiring simple and easy operation for the user. The purification medium should be regenerable and rechargeable as needed with commonly available sources so as to increase the cost effectiveness of the cartridge. The purification medium may need to be periodically recharged. The halogen depleted polystyrene hydantoin can be halogenated with either chorine or bromine. As a result, a variety of sources of free chlorine, such as sodium hypochlorite or calcium hypochlorite according to the methods described by U.S. Pat. No. 6,548,054 can be used to rechlorinate the polystyrene hydantoin.  
         [0045]     To fully take advantage of the performance of the halogenated polystyrene hydantoin beds, the cartridge  108  must allow for the end user to recharge the medium. In one embodiment, the end user can remove the purification vessel  132  from the inlet head cap  112 . Once removed, the purifier vessel  132  that contains the halogen-depleted polystyrene hydantoin can be quickly recharged in situ according to U.S. Pat. No. 6,548,054. Once recharged, the purifier vessel  132  can be reinserted within the bulkhead  134  and sealed for leak-proof operation.  
         [0046]      FIG. 4  is a cross section of another embodiment of a water purification cartridge  208  of the present invention. The cartridge  208  is of a similar design as cartridge  108  however with modifications for a different sealing mechanism. Numbered items between  100  and  199  in  FIG. 4  correspond in the basic general function to the items of  FIGS. 2 and 3  that are similarly numbered. In this embodiment, the cartridge  208  includes a bulkhead  226  that is configured to fit with the outer skin  116  and the dwell chamber  142 . To this end, the bulkhead  226  includes circumferential grooves  230 ,  232  on the underside of the bulkhead  226  to mate with the upper circumferential edge of the outer skin  116  and the dwell chamber  142 . The bulkhead  226  has a flat annular portion  234 . The grooves  230  and  232  for mating with the outer skin  116  and dwell chamber  142  are disposed on the underside of the flat annular portion  234 . The bulkhead  226  includes a cylindrical wall  236  rising from the annular portion  234  that includes exterior screw threads  228  to engage with a sealing ring  210 . The sealing ring  210  has corresponding threads. An inlet head cap  220  is configured to fit snuggly inside of the cylindrical wall  236 , so that the lower edge of the inlet head cap  220  rests on the bulkhead annular portion  234  on the inside of the cylindrical wall  236 . The sealing ring  210  has a hole in the center large enough so that the inlet head cap  220  will fit within the hole. The sealing ring  210  has a lip  238  projecting inward that abuts against a ledge  240  formed on the inlet head cap  220 . A rubber or elastomeric O-ring  214  can be provided between the inside wall of the lower interior inlet head cap  220  and a small upwardly projecting lip  242  on the bulkhead  226  that projects upward from the annular portion  234 . The lip  242  is part of a second cylindrical wall  224  that extends above and below the bulkhead annular portion  234 , which is then followed by a second flat annular portion  248 . The O-ring  214  can be attached to the inlet head cap  220 . The lip  242  on the bulkhead  226  is interior to the cylindrical wall  236  with screw threads  228 . By screwing the sealing ring  210  to the bulkhead  226 , the O-ring  214  seats to the bulkhead sealing surface  216  holding the inlet head cap  220  and the purifier vessel  132  in place in the cartridge  208 . The O-ring  214  seals the inlet head cap  220  to the bulkhead  234 . The upper portion of the purifier vessel  132  has a horizontally disposed annular portion  244  and a cylindrical wall  246  rising therefrom. A second O-ring  218  is provided in a groove of the cylindrical wall  246  of the purifier vessel  132 . The bulkhead  226  is provided with a center hole through which the main body of the purifier vessel  132  can fit through and extend into the dwell chamber  142 . However, the purifier vessel  132  is restrained from passing all the way through the bulkhead  226  by the horizontally disposed annular portion  244  of the purifier vessel  132  abutting against the second flat annular portion  248  on the bulkhead  226 . The hole through the center of the bulkhead  226  is surrounded by the second flat annular portion  248  which then rises to form the wall  224  and lip  242  to which the O-ring  214  seats. The O-ring  218  abuts against this rise in the bulkhead wall  224 , thus sealing the purifier vessel  132  to the bulkhead  226 . To aide in removal of the purifier vessel  132 , a handle feature  212  is located at the top of the purifier vessel  132  that allows an end-user to easily grab the purifier vessel  132  and remove it and securely reinsert it into the bulkhead  226 .  
         [0047]      FIG. 5  is a cross section of another embodiment of a water purification cartridge of the present invention. Numbered items  100  through  199  in  FIG. 5  correspond in general function to items similarly numbered in  FIGS. 2, 3  and  4 . The cartridge  308  is of a similar design as cartridges  108  and  208 . However, in this embodiment, the purifier vessel portion and bulkhead portion are combined into an unitary member collectively represented by reference numeral  326 . Furthermore, the inlet head cap  320  has been attached to the bulkhead/purifier vessel  326  in a substantially nonremovable manner. The inlet head cap  320  can be welded, glued, or otherwise held with fasteners, making the removal of the inlet head cap  320  more difficult than in previously described embodiments. In this embodiment, the purification media contained in the purifier vessel portion of the integral member  326  can be rehalogenated, or, alternatively, the biocidal activity can be sustained by the use of a tablet, liquid droplets, or liquid injection that releases free chlorine or bromine into water ahead of the purifier vessel.  
         [0048]     The materials of manufacture for the components of the cartridges are of chlorine-resistant polymers and/or plastics, such as polypropylene or polycarbonate. The water purification cartridges according to the present invention have the capacity to hold about 5 to about 100 grams of a purification medium. The water purification cartridges are provided with a dwell chamber that is capable of providing a residence time of at least about 30 seconds to about 5 minutes. With these parameters, the water purification cartridges according to the present invention are capable of maintaining a log reduction value for poliovirus of at least about 4, and can treat a total of at least 700 liters of water or more, and preferably at least 1040 liters of water or more.  
         [0049]     While the preferred embodiment of the invention has been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.  
       EXAMPLE 1  
     Determination of Log Reduction Value of a Water Purification Cartridge Having a Halogenated Polystyrene Hydantoin Polymer as the Purification Medium  
       [0050]     A water purification cartridge of the embodiment described in  FIGS. 1 through 3  was tested to confirm microbiological efficacy. The cartridge was installed under a commercially available gravity feed ceramic drinking water systems as shown in  FIG. 1  (Doulton Berkefeld) and filled with 20 grams of HPSH polymer (having 14% Cl+ wt/wt). The system was challenged with feed water having the characteristics of 1,500 mg/L total dissolved solids as sea salts, 10 mg/L of total organic carbon as humic acid, a pH of 9.0 adjusted using NaOH and HCl and a temperature of 4° C. The system was allowed to run daily at 10 liters per day at an average flow rate of 15 ml/min using chlorine-free tap water and challenged periodically using the above challenge water to measure poliovirus log reduction as determined using the methods described in the USEPA  Guide Standard and Protocol for Testing Microbiological Purifiers: Report of Task Force,  1987. At approximately 7-day use intervals, the purification media was recharged in accordance with methods described by U.S. Pat. No. 6,548,054 for the polystyrene hydantoin. The test continued for 700 L during which it was found that the cartridge was able to maintain a 4-LRV poliovirus biocidal activity. The poliovirus LRV for the system is plotted as a function of total water through the system in  FIG. 6 .  
       EXAMPLE 2  
     Determination of Log Reduction Value of a Water Purification Cartridge Having a Halogenated Polystyrene Hydantoin Polymer as the Purification Medium  
       [0051]     A water purification cartridge of the embodiment described in  FIG. 4  was tested to confirm microbiological efficacy. The cartridge was installed under a commercially available 5-gallon bucket that contained a commercially available gravity feed ceramic prefilter (Doulton Berkefeld Imperial Sterasyl) as shown in  FIG. 1  and filled with 20 grams of HPSH polymer (having 15% Br+ wt/wt). The system was challenged with feed water having the characteristics of 150 mg/L total dissolved solids as sea salts, 0.5 mg/L of total organic carbon as humic acid, a pH of 7 adjusted using NaOH and HCl and a temperature of 4° C. The system was allowed to run daily at 20 liters per day at an average flow rate of 25 ml/min using chlorine-free tap water and challenged periodically using the above challenge water to measure poliovirus log reduction as determined using the methods described in the USEPA  Guide Standard and Protocol for Testing Microbiological Purifiers: Report of Task Force,  1987. The test continued for 1,040 L during which it was found that the cartridge was able to maintain a 4-LRV poliovirus biocidal activity. The poliovirus LRV for the system is plotted as a function of total water through the system in  FIG. 7 .