Abstract:
An underwater filtration system or operator which can be floated on and suspended in a river, lake, pond or other water body to filter water from the water body. The underwater filtration operator may be electrically charged with simultaneous introduction of continuous forced air to remove impurities from the water body and increase filtering efficiency and the system includes a split housing having a selected configuration and divided into one or more filtration units, each of which includes a filter medium or mediums for filtering water from the water body. A pump is provided in the interior of the system housing for receiving the filtered water from the filtration units and pumping the filtered water to a collection tank or dispenser, directly to an end user or to a reverse-osmosis water filtration unit for further filtration. This unique operator allows selected individual filtration compartments to be backwashed while other filtration compartments are producing filtered water without the necessity of an external clean water storage tank.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to water filters and more particularly, to an underwater filtration system or operator which can be floated on a water body to filter and disinfect water from the water body for drinking or other purposes. The underwater filtration operator includes a typically split housing having a selected configuration and divided into multiple filtration units which receive water from the water body, each of which filtration units includes at least one, and preferably, two filter elements or medium, for filtering the water. A pump is provided in the housing for pumping the filtered water to a suitable container or dispenser or to a reverse-osmosis filtration unit for further filtration, and a positive electrical charge may be applied to the housing in conjunction with the introduction of forced air into a raw water chamber, to neutralize negatively charged impurities in the raw incoming water flowing through the filter media and improve filtration efficiency. 
     Treatment processes for filtering surface water have remained virtually unchanged for over half a century. Many surface water treatment plants utilize large settling basins, known as clarifiers, to settle out heavy solids from water prior to fine-screening the water, using rapid sand filters. Some of these treatment plants use a mixing chamber clarifier to separate the heavy solids from the water. Such a mixing chamber clarifier requires the introduction of polymers, lime, alum or other types of media into the water to be treated, which media bind particulate impurities in the water and fall with the bound impurities to the bottom of the mixing chamber. The filtered water, substantially devoid of the larger impurities, is then typically processed through horizontal sand filters which remove smaller impurities from the water. The sand filters must be periodically backwashed using large quantities of clean water because they repeatedly become clogged with the smaller particles that were not removed from the water during the clarification process. The fewer the particles removed during the clarification process, the more often the backwashing procedure must be repeated. 
     The foregoing types of surface water treatment plants are associated with many problems due to the nature of their operation. Numerous pumps and an expensive and elaborate intake structure must be installed in the water supplies to conduct the water to the plants for treatment. Another problem involves the disposal of solids that are removed from the water. Formerly, these solids, along with the chlorine, polymers, lime or other particulate binding media, were pumped back into the pre-filtered water from which they were removed. Due to recent environmental legislation, however, it is no longer lawful to discharge the particulate binding media into the pre-filtered water supply, as these materials are not endemic to the water that is being treated. Another problem associated with these filtration systems is that the polymers or other particle binding media introduced into the filtration system are harmful to certain types of boiler water industrial filtration equipment which utilizes reverse osmosis. This increases the cost of boiler water for industrial consumers. Furthermore, disinfectant chemicals introduced into the filtered water do not always kill all parasites found in water sources. Moreover, rapid sand filters cannot remove all of these parasites, some of which remain in the water and present a potentially dangerous health risk. Another problem associated with these surface water treatment plants is the inability to remove harmful chemicals which may contaminate the water supply by agricultural run-off or accidental spills. Accordingly, surface treatment plants can be costly and time-consuming to build and maintain. 
     A number of different types of filters are known in the art for filtering surface water. Patents of interest in this regard include U.S. Pat. No. 4,606,819, issued Aug. 19, 1986, to Colson; U.S. Pat. No. 4,643,836, issued Feb. 17, 1987, to Schmid; U.S. Pat. No. 4,657,672, issued Apr. 14, 1987, to Allen; U.S. Pat. No. 4,950,393, issued Aug. 21, 1990, to Goettl; U.S. Pat. No. 5,160,039, issued Nov. 3, 1992, to Colburn; U.S. Pat. No. 5,549,828, issued Aug. 27, 1996, to Ehrlich; arid U.S. Pat. No. 6,027,639, issued Feb. 22, 2000, to J. Lenhart et al. My U.S. Pat. No. 6,790,345, issued Sep. 14, 2004, details an underwater filtration operator for floating on a water body and producing clarified water from the water body. 
     SUMMARY OF THE INVENTION 
     These and other objects of the invention are provided in an underwater filtration system or operator and method of filtering water in a water body, which device can be floated on and suspended in a lake, river, pond or other water body to filter water from the water body. The underwater filtration system includes a typically split housing having a selected configuration, typically cylindrical, and enclosing multiple, pie-shaped filtration units, each of which units includes at least one, and preferably two filter elements or medium for filtering water from the water body. A pump is provided in the housing interior for receiving the filtered water from all or selected ones of the filtration units and pumping the filtered water to facilitate back-washing of individual filtration compartments or to a suitable collection facility or dispenser, or to a reverse osmosis filter for further treatment. Selected ones or all of the units may be backwashed at any time to clean the filter element(s) while the remaining units remain on-line in the filtration process. The insulated housing element of the operator may be positively charged with electricity, in conjunction with the introduction of forced air into the raw water chamber, to neutralize negatively charged impurities flowing through the filter media with the raw entry water and enhance the filtration efficiency. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood by reference to the accompanying drawings, wherein: 
         FIG. 1  is a perspective view of a first illustrative embodiment of the underwater filtration system of this invention having a flotation collar for floating the device on a water body; 
         FIG. 2  is a perspective view, with the housing element removed, of the underwater filtration system illustrated in  FIG. 1 ; 
         FIG. 3  is an exploded perspective view of the underwater filtration system illustrated in  FIGS. 1 and 2 , illustrating typical filtration unit cover elements and filtration units of the underwater filtration operator; 
         FIG. 4  is an exploded perspective view of the system illustrated in  FIG. 3 , more particularly illustrating filtration unit screen grid frames and screens in the underwater filtration system; 
         FIG. 5  is a sectional view, taken along section lines  5 - 5  in  FIG. 1 , of the underwater filtration operator; 
         FIG. 6  is a sectional view, taken along section lines  6 - 6  in  FIG. 1 , of the underwater filtration operator; 
         FIG. 7  is a longitudinal sectional view and schematic of the underwater filtration operator illustrated in  FIG. 1 , with the system disposed in filtration sequence; 
         FIG. 8  is a longitudinal sectional view and schematic of the underwater filter operator illustrated in  FIG. 1 , with the system disposed in backwash sequence; 
         FIG. 9  is a perspective view of a three-way diverter element for determining the direction of filtered water flow in the underwater filtration system illustrated in  FIGS. 2-8 ; 
         FIG. 10  is a sectional view, taken along section lines  10 - 10  of the 3-way diverter element illustrated in  FIG. 9 , with the diverter in backwash configuration; and 
         FIG. 11  is a sectional view, also taken along section lines  10 - 10  illustrated in  FIG. 9 , with the diverter in filtering configuration. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring initially to  FIGS. 1-6  of the drawings, an illustrative embodiment of the underwater filtration system or operator of this invention is generally illustrated by reference numeral  1 . The underwater filtration operator  1  is designed for flotation or otherwise positioning in a water body  43  ( FIG. 1 ), such as a river, pond or lake to filter, clarify and selectively disinfect water from the water body  43  for drinking, additional processing or other purposes, as hereinafter described. A housing flotation collar  20 , typically constructed of an expanded foam material such as STYROFOAM (trademark) or other suitable buoyant material, or alternatively, having an inner flotation chamber (not illustrated), may be mounted on the top housing panel  4  to impart buoyancy to the underwater filtration operator  1  in a water body  43 , as illustrated in  FIG. 1  and hereinafter described. In a first preferred embodiment, the underwater filtration operator  1  includes a split housing  2  which, as illustrated in  FIGS. 1 and 2 , can typically be cylindrical, and is typically shaped by a pair of half cylinders  2   a,  joined at diametrically-opposed cylinder flanges  2   b,  having flange bolt holes  2   d  ( FIG. 2 ) for receiving flange bolts  2   c,  illustrated in  FIG. 1  and cooperating nuts (not illustrated). The housing  2  is closed at the top by an oversized top housing panel  4  and an oversized bottom housing  5  ( FIG. 2 ) closes the housing  2  at the bottom. The housing  2  encloses multiple filtration units  8  ( FIG. 2 ), which are pie-shaped and separated from each other in the housing  2  by partitions  6  ( FIGS. 3 and 4 ), which extend inwardly from the housing  2  to a pump housing  23  and between the top housing panel  4  and the bottom housing panel  5 . While the embodiment of the underwater filtration operator  1  illustrated in the drawings typically includes eight filtration units  8 , it is understood that any number of filtration units  8  can be contained in a housing  2  of any desired size and shape. As illustrated in  FIGS. 7 and 8 , each filtration unit  8  is fitted with an air vent opening  4   b  and typically includes an outermost, annular raw water chamber  17 , defined by the housing  2  and each respective filtration unit cover  8   a,  which closes the corresponding filtration units  8  ( FIG. 2 ). Each of the filtration unit covers  8   a  has a filtration screen  7  on the top thereof for receiving raw water from the raw water chamber  17  ( FIG. 6 ) as hereinafter further described. Each of the filtration screen covers  8   a  is also mounted on a corresponding pie-shaped screen grid frame  12  at the bottom thereof and to a top flange  3  mounted on the top housing panel  4  and mounted to the bottom housing panel  5  ( FIG. 5 ), using bolts (not illustrated). As hereinafter further described, the filtration units  8  each contain a selected top filter medium  13 , such as coal or charcoal, for example, and a selected bottom filter medium  13   a,  such as, for example, sand. It will be appreciated by those skilled in the art that the filtration units  8  can be any desired size to contain any desired volume of any selected filter medium, including sand, charcoal, or the like, in non-exclusive particular, depending on the degree of filtration desired. Each of the filtration units  8  is designed to contain the selected particulate top filter medium  13  and/or the bottom filter medium  13   a,  in selected quantities, respectively. 
     As illustrated in  FIGS. 3 and 4 , each of the pie-shaped screen grid frames  12  forms the bottom of a filtration unit  8  and typically includes a sieve screen  11 , sandwiched between a pair of expanded metal screens  10  ( FIG. 4 ). The screen openings of each sieve screen  11  are smaller in size than the particles of the top filter medium  13  or bottom filter medium  13   a,  respectively, to prevent inadvertent movement of the top filter medium  13  and bottom filter medium  13   a  downwardly, beyond the confines of the respective filtration units  8 . As further illustrated in  FIGS. 7 and 8 , an air space  47  is defined between the bottom surface of the top housing panel  4  and the top filter medium  13  in the respective filtration units  8 , to compensate for media expansion during the filter medium backwash cycles. As illustrated in  FIGS. 1-5 ,  6  and  7 , access caps  4   a  are typically provided to close corresponding openings (not illustrated) for accessing the respective filtration units  8  as necessary and changing or adding to the top filter medium  13  and/or the bottom filter medium  13   a,  respectively. The access caps  4   a  are typically threaded onto correspondingly-threaded receiving rings (not illustrated) provided on the top housing panel  4 . 
     Referring again to  FIGS. 5-7  of the drawings, each filtration unit  8  rests on a screen grid frame  12  and lies adjacent to a vertical pump chamber  16 , defined by a pump housing  23 , typically provided in the center of the housing  2 . The pump housing  23  is typically characterized by a pump housing pipe  22  of selected size which extends upwardly from a pump housing bottom cover plate  22   a.  Multiple diffuser pipes  27  are located in the respective diffuser pipe chambers  25  and project from corresponding 3-way diverters  28  located in the pump chamber  16 , into the respective filtration units  8  beneath screen grid frame  12  to establish water communication between the pump chamber  16  and the filtration units  8 . A water pump  24 , the purpose of which will be hereinafter described, is provided in the bottom of the pump chamber  16 . The oversized bottom housing panel  5  extends radially outwardly from the pump housing  23  and typically terminates beyond each filtration unit  8 , to define, between the respective filtration unit covers  8   a  and the inside of the housing  2 , a housing cover flange  29  connected to the bottom housing panel  5 , and having intake openings  29   a.  The intake openings  29   a  establish communication between the outside of the housing  2  and the raw water entering the underwater filtration operator  1  through the water openings  29   a  ( FIG. 6 ). The multiple intake openings  29   a  are positioned to receive raw water into the annular raw water chamber  17 . The pump housing  23  extends upwardly through the top housing panel  4  of the housing  2  and is typically closed by a removable top plate  30 . A filtered water discharge pipe  26 , provided in fluid communication with the discharge of the water pump  24 , extends upwardly from the water pump  24 , through the pump chamber  16  and through an air-sealed opening (not illustrated) provided in the plate  30 , to a filtered water discharge storage facility or the like (not illustrated). A filtered water discharge valve  18  ( FIG. 2 ) and a pressure gauge  14  is typically provided in the filtered water discharge pipe  26 . An air introduction line  31  extends to an air manifold  36  and is connected at the other end to a blower  32  or a source of compressed air (not illustrated), for selectively introducing pressurized air from the air manifold  36 , into the spaced-apart, radially-oriented, peripheral air pipes  33 , having air pipe openings  33  ( FIG. 2 ) for disinfectant and combining small particles during the electrical charging process hereinafter described. An air line  36   a  also projects from the air manifold  36  into the housing  2  and an air line valve  36   b  is provided in the air line  36 a. An auxiliary line  40  extends from the air line  36   a  and includes an auxiliary line valve  41  therein for adding selected chemicals such as disinfectants, to the system or for venting air from pump chamber  16 . An auxiliary air line  37  further extends to the air manifold  36  for possible introduction of chemical disinfectants into the annular raw water chamber  17 , typically through an auxiliary air valve  38 , provided in the auxiliary air line  37  ( FIGS. 7 and 8 ), as hereinafter described. 
     Referring again to  FIGS. 7 and 8  of the drawings, multiple filtered water receiving pipes  42  extend from the water manifold  35  through openings (not illustrated) provided in the top plate  30 , and into the pump housing  23  and the pump chamber  16 , where they connect to the corresponding respective three-way diverters  28  in the pump chamber  16 . Each of the filter water receiving pipes  42  is fitted with a receiving pipe valve  39  for controlling the flow of water to and from the water manifold  35  as it is pumped through the respective filtered water receiving pipes  42  in the filtering and backwash cycles illustrated in  FIGS. 7 and 8 , respectively, as hereinafter described. 
     As illustrated in  FIGS. 9-11  of the drawings, each of the three-way diverters  28 , located in the pump chamber  16 , is typically characterized by a diverter housing  44 , provided with a horizontal pipe fitting  45  and a vertical pipe fitting  46 , each of which interfaces with a housing interior  44   a.  A diverter  48  is pivotally attached to the diverter housing  44  in the housing interior  44   a  by means of a diverter pin  48   a,  as further illustrated in  FIGS. 10 and 11  and the diverter pin  48   a  is able to swing from a first position illustrated in  FIG. 10  to the second position illustrated in  FIG. 11 , responsive to water flow through the housing interior  44   a,  as hereinafter further described. Housing plates  49  serve to close the open sides of the diverter housing  44 , typically using plate bolts  49   a,  as further illustrated in  FIGS. 9-11 . A flow opening  50  is provided in the top portion of the diverter housing  44  for accommodating a flow of water flowing from the housing interior  44   a  responsive to the position of the diverter  40   a  illustrated in  FIG. 11 , as hereinafter further described. A small bypass opening  52  is also provided in the housing  44  and communicates with the housing interior  44   a  ( FIG. 11 ). 
     Referring now to  FIGS. 1-4 ,  7  and  8 - 11  of the drawings, in typical operation of the underwater filtration operator  1 , the filtered water discharge tube  26  is connected to a suitable water collection container or dispenser (not illustrated) and the air introduction tube  31  is connected to a blower  32  or alternative source of pressurized air (not illustrated). Appropriate electrical connections (not illustrated) are also made to facilitate operation of the water pump  24 . The housing  2  is placed in the water body  43  such that the housing  2  initially floats on the water body  43  due to the buoyancy of the housing flotation collar  20 , as illustrated in  FIG. 1 . Accordingly, the bottom of the housing  2  is normally suspended just beneath the surface of the water body  43 , as illustrated in  FIG. 1 , typically by means of buoyancy imparted to the housing  2  by means of the housing flotation collar  20 . As the housing  2  floats on the water body  43 , raw water from the water body  43  is drawn first into the annular raw water chamber  17  adjacent to each filtration unit  8 , through the respective raw water intake openings  29   a  located in the bottom housing panel  5  or cover flange  29  ( FIGS. 2-4  and  6 ) of the housing  2 , by operation of the water pump  24 . The raw water then flows through the respective filtration screens  7  ( FIG. 2 ) in the corresponding filtration unit covers  8   a  and downwardly, through the top filter medium  13  and the bottom filter medium  13   a,  and finally as filtrate, through the respective diffuser pipe openings  27   a  in the diffuser pipes  27  extending into the corresponding filtration unit  8 . From the diffuser pipes  27 , the filtered water flows into the horizontal pipe fitting  45  and the housing interior  44   a  of the respective 3-way diverters  28 , where it is directed upwardly through the open flow opening  50 , into the pump chamber  16 , by operation of the pivoting diverter  48  ( FIG. 11 ). The water pump  24  pumps the filtered water in the pump chamber  16  upwardly through the filtered water discharge tube  26  into the water manifold  35  and through the open filtered water discharge valve  18 , and finally, into a filtered water collection tank or dispenser to an additional treatment facility such as a reverse-osmosis unit (not illustrated). Accordingly, operation of the water pump  24  facilitates a continuous flow of the water from the water body  43  into the annular raw water chamber  17 , through the respective filtration screens  7  in the corresponding filtration unit covers  8   a  of the filtration units  8  and through the top filter medium  13 , the bottom filter medium  13   a,  the diffuser pipes  27  and the corresponding diverters  48 , into the pump water chamber  16 . In conjunction with the pumping process, the blower  32  is operated simultaneously therewith and air is caused to flow into the air manifold  36 , through the opened air pipe valves  34  and the peripheral air pipes  33 , having air pipe openings  33   a  and into the annular raw water chambers  17  ( FIG. 7 ). 
     If all of the filtration units  8  are to be backwashed simultaneously, the filtered water discharge valve  18  is closed, the receiving pipe valves  39  opened and filtered water is pumped from an external source through the water manifold  35  and into the filtered water receiving pipes  42 , as illustrated in  FIG. 8 , using an auxiliary water intake line (not illustrated) connected to the water manifold  35  and an external source of water. If only selected ones of the filtration units  8  are to be backwashed, the corresponding receiving pipe valves  39  are opened and the filtered water discharge valve  18  remains open. Selective backwash with filtered water production is thereby achieved. In both cases, as the water flows in reverse through the 3-way diverters  28  ( FIG. 10 ) and the diffuser pipe openings  27   a  and top filter medium  13  and the bottom filter medium  13   a,  the top filter medium  13  and the bottom filter medium  13   a  are cleared of both large and small particulate impurities, as well as some bacteria and microorganisms. It will be appreciated by those skilled in the art that as the water flows through the selected filtration units  8  in this reverse, backwash mode, the water flowing into the respective filtration units  8  helps to “fluidize” the top filter medium  13  and bottom filter medium  13   a,  to expand the media into the respective air spaces  47  and effect a more complete cleansing of the medium. Furthermore, chlorine or other disinfectant chemicals can be introduced into the pre-filtered water through the auxiliary air line  37  and the air pipe openings  33   a  of the peripheral air pipes  33  by opening the auxiliary air line valve  38  in selected raw water chambers  17 , to kill bacteria, algae and other microorganisms and ensure filtered water containing few or no live bacteria, algae or microorganisms which may otherwise evade the filtering process. The top filter medium  13  and the bottom filter medium  13   a  can be added to and removed from the respective-filtration units  8 , respectively, and replaced with fresh or alternative filter medium, as deemed necessary, by accessing these pie-shaped chambers through a corresponding access opening (not illustrated) communicating with the air spaces  47  and provided in the top housing panel  4 , after removing the respective access caps  4   a,  as described above. 
     As further illustrated in  FIGS. 7 and 8 , the underwater filtration operator  1  typically includes a split, vertically-flanged housing  2 , designed as illustrated in  FIGS. 1-6  and fitted with an extended top housing panel  4  that projects beyond the curved outer surface of the housing  2 . In a preferred embodiment the housing  2  is positively charged with electricity by means of a positive lead  70  that connects to battery charger or battery  69 , and a negative lead  71 , connected to the top housing panel  4 , for reasons more particularly hereinafter set forth. Insulation (not illustrated) is provided on the housing  2 , to electrically isolate and insulate the housing  2  from the remainder of the underwater filtration operator  1 . More specifically, in a preferred embodiment of the invention the cylindrically-shaped housing  2  is characterized by a pair of semi-cylindrically-shaped plates  2   a,  each having longitudinal cylinder flanges  2   b,  with spaced-apart flange openings  3  ( FIG. 2 ) for bolting together using flange bolts  2   c  and nuts (not illustrated) to enclose the respective filtration units  8 , as illustrated. 
     Referring again to  FIGS. 7 and 8 , the positive lead  70  of the battery or a battery charger  69  can be attached to the housing  2 , while the negative lead  71  of the battery or the battery charger  69  is attached to the top housing panel  4  to facilitate applying an electrical potential to the top filter medium  13  and the bottom filter medium  13   a.  Accordingly, since raw water flowing from the water body  43  into the interior of the underwater filtration operator  1  through the raw water intake openings  29   a  contains negatively charged particles as impurities, these particles are neutralized as they contact the positively charged surface of the housing  2  and are therefore more effectively and efficiently filtered through the top filter medium  13  and the bottom filter medium  13   a.  The introduction of forced air from the air blower  32  through the peripheral air pipes  33 , and the air pipe openings  33   a,  into selected raw water chambers  17  aids the charging process of the particles entering the selected raw water chambers  17 . Consequently, creating a positive electrical charge on the incoming particles of sand, grit, clay and the like in the raw intake water, prevents these particles from being repelled by each other and facilitates a more effective filtration and coalescing of the particles together in the top filter medium  13  and the bottom filter medium  13   a.  The result of the more efficient filtration is exceptionally clear water which enters the pump chamber  16  and is pumped by means of the water pump  24  from the pump chamber  16  of the water filtration system  1 , to storage, use or to an additional water treatment filter unit, such as a reverse osmosis unit, as desired. 
     It will be appreciated by those skilled in the art that as heretofore described, the underwater filtration operator  1  of this invention can be selectively operated in a backwash cycle without the use of an external clear water storage tank to remove impurities, filtered from the water, from the top filter medium  13  and the bottom filter medium  13   a,  respectively, by reversing the direction of water flow through the respective filtration units  8  individually or in any desired combination. This is accomplished as described in detail above by opening the respective receiving pipe valves  39  of the respective filtered water receiving pipes  42  that serve the filtration units  8  to be backwashed and pumping filtered water from the pump chamber  16 , through the filtered water receiving pipes  42 . This action, illustrated in  FIG. 8 , forces the filtered water in the pump chamber  16  upwardly through the bottom filter medium  13   a  and the top filter medium  13  in the respective filtration units  8  and into the raw water chamber  17 . The reverse flow of water through the bottom filter medium  13   a  and the top filter medium  13 , respectively, of each filtration unit  8 , as described above, removes all or most of the filtered particles and some microorganisms from the top filter medium  13  and the bottom filter medium  13   a  of each filtration unit  8 , and directs these impurities back into the water body  43 . Furthermore, when the water is forced through the bottom filter medium  13   a  and the top filter medium  13 , the air space  47  in each of the filtration units  8  enables the top filter medium  13  and the bottom filter medium  13   a  to expand and fill the entire volume of the respective inner filtration units  8 . Accordingly, the top filter medium  13  and bottom filter medium  13   a  become fluid in the filtration units  8 , respectively, and this facilitates a thorough cleansing of the top filter medium  13  and bottom filter medium  13   a.    
     Referring again to  FIG. 1  of the drawings, it will be appreciated by those skilled in the art that the housing flotation collar  20  is particularly suitable for suspending the housing  2  beneath the surface of the water body  43  under circumstances in which the level of the water body  43  is subject to fluctuation. Alternatively, it is understood that the housing  2  can be positioned beneath the surface of the water body  43  by securing the housing  2  to a dock, barge, piling or the like (not illustrated). It will be further appreciated by those skilled in the art that the controls for the various valves, both air and water, may be provided in a land-based control panel (not illustrated) for convenient, expedient and/or automated operation of the underwater filtration operator  1 . Furthermore, it is also understood that any source of direct electric current, in addition to a battery charger can be used to supply the desired potential across the filter media. 
     While the preferred embodiments of the invention have been described above, it will be recognized and understood that various modifications can be made in the invention and the appended claims are intended to cover all such modifications which may fall within the spirit and scope of the invention.