Abstract:
A composite water filter for reducing the cyst content for drinking water utilizing a non-woven microfiber layer prefilter wrapped over a porous carbon block filter to retain at least 99.95% of cyst-size particles while retaining a relatively low pressure drop and high flow rate.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to filters for the purification of drinking water and, more particularly, to the use of a pre-filter media with a carbon block filter element to remove cysts, permit the use of a less dense carbon block, and to improve flow rate and filter life.  
           [0002]    Historically, carbon block filters, comprising carbon particles bonded under pressure, have provided a filtration media that has performed a multitude of tasks in the water treatment industry. Carbon block is used to reduce heavy metals, chlorine, volatile organic compounds, sediment, cryptosporidium, giardia and other protozoan cysts, and improve taste and odor. The block structure must be dense and composed of very small particles to remove cysts and therefore the resultant pressure drop across the block is very high for a given water flow rate. In addition, these high density blocks will tend to filter out all types of particulate matter present in the water with high filtration efficiency. This results in premature plugging of the block pores and more frequent filter changes.  
         SUMMARY OF THE INVENTION  
         [0003]    An improved dual stage filtration carbon block is disclosed that allows for reduced pressure drop while still maintaining the filtration efficiency to remove cysts. Recently, a class of filtration media has emerged that improves the filtration efficiency of particulate matter. This media is typically melt blown fiber or glass fiber deposited on or between spun bonded papers.  
           [0004]    The present invention utilizes this media as a pre-filtration wrap around a carbon block. The resultant filter possesses properties unlike that of present production carbon block in that it filters and retains cysts and other small particles on the outer wrap and utilizes the carbon block inner core as the chemical filter. Current production blocks sometimes are constructed with a pre-filter wrap; however, the wrap that is utilized is only used for course sediment removal and to cover the block for aesthetic reasons. This invention utilizes a wrap specifically formulated for fine particle removal. The result of this invention is a true dual stage filter where the density of the carbon block and resultant pressure drop of the carbon block filter can be much lower than a block that is currently designed to remove cysts.  
           [0005]    In a presently preferred embodiment, a high efficiency, low pressure drop water purification filter particularly adapted for cyst reduction includes an inner porous carbon block element that is made of bonded carbon particles having a nominal size range of about 40 microns to 600 microns and a block density in the range of about 0.3 gm/cm 3  to 0.75 gm/cm 3 ; and outer wrap utilizing a non-woven fiber layer that encloses the carbon block and is capable of retaining at least 99.95% of particles as small as 3 microns; and wherein the fiber layer is supported on the carbon block to prevent collapse and is sealed at the interface between the layer and the block at both axially opposite ends and along opposed enclosing edges of the layer.  
           [0006]    The non-woven fiber layer may comprise glass fibers or melt blown plastic fibers of, for example, polypropylene. Preferably, the non-woven fiber layer is provided with a highly porous backing layer or layers to provide support and protection for the non-woven layer. The backing layer may comprise a spun bonded paper layer and the interface of the non-woven fiber layer with the carbon block may also be covered with a highly porous backing layer or the same or similar spun bonded paper layer. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    [0007]FIG. 1 is a vertical sectional view through a water purification filter made in accordance with the present invention.  
         [0008]    [0008]FIG. 2 is a graph showing net pressure drop versus flow rate comparing prior art filters with a filter of the present invention.  
         [0009]    [0009]FIG. 3 is a graph showing flow rate versus inlet pressure and comparing the performance of prior art filters with filters of the present invention.  
         [0010]    [0010]FIG. 4 is a graph showing the cyst reduction performance of prior art filters versus filters of the present invention.  
         [0011]    [0011]FIG. 5 is a graph showing flow rate versus total flow volume in prior art filters and filters of the present invention operating to remove cysts.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0012]    Referring to FIG. 1, a filter  10  made in accordance with the present invention includes a porous carbon block  11  surrounded by an outer wrap  12  which act together to provide a filter for high cyst removal capability and relatively low pressure drop. The carbon block  11  and outer wrap  12  are enclosed together in an upper end cap  13  and a lower end cap  14  so that radial flow of water to be filtered from the outside of the filter  10  to the inside of the carbon block  11  must pass first through the outer wrap  12  without by-pass. The filter  10  is typically sealed in the end caps  13  and  14  with a polyolefin hot melt adhesive  24 . Similarly, the edges of the outer wrap  12  which includes a non-woven fiber layer  19 , as will be described in greater detail, must be sealed with a butt joint or overlapping joint where the opposed edges of the outer wrap  12  meet. The sealed joint may utilize an adhesive or may comprise a heat seal. The filter  10  is of a type adapted to be placed in a hollow housing or sump (not shown) and enclosed with a housing cap (also not shown), the housing cap providing an inlet to the outside of the filter for untreated water and an outlet from the hollow interior  15  of the carbon block for filtered water all in a manner well known in the art.  
         [0013]    The carbon block  11  is made from fine particulate carbon mixed with a suitable binder, such as polyethylene, and formed under heat and pressure into a solid porous block. Variations in the particle size and the formation conditions result in carbon blocks of varying density and porosity. In the porous blocks preferred for use in the present invention, the carbon particles are preferably in a size range of about 40 microns to about 600 microns and the formed carbon block has a density in the range of about 0.3 gm/cm 3  to about 0.75 gm/cm 3 . Carbon blocks made in accordance with the foregoing specifications are generally considered to be unsuitable for cyst removal. However, the actual density of carbon blocks useful in this invention will vary quite widely depending on the type of the carbon (i.e. the density and size of the carbon particles) and other materials used in the construction of the block. The current applicable standard for cyst removal in a filter for domestic drinking water use requires removal of more than 99.95% of cysts, based on the nominal particle size as small as 3 microns. However, these carbon blocks are desirable nevertheless for their ability to remove other contaminants such as heavy metals, chlorine, VOCs and sediment while exhibiting a desirable low pressure drop.  
         [0014]    In accordance with the present invention, a low density carbon block  11  is combined with an outer wrap  12  utilizing a non-woven fiber layer  19  that is capable of filtering and retaining at least 99.95% of particles as small as 3 microns. The combination of a pre-filter for cyst removal utilizing a non-woven fiber layer  19  as part of the outer wrap  12  and a low density carbon block  11  provides a unique combination that permits cyst removal at relatively low pressure drop and without premature clogging of the filter.  
         [0015]    One particularly suitable non-woven fiber layer  19  is a micro-glass material made by Lydall, Inc. and sold under the trademark LYPORE. This material comprises a dense mat of extremely fine glass fibers (with a nominal diameter of about 1 μm) laid down in a mat having a thickness of 24 mils (0.6 mm) to provide a mean pore size of 2 microns. The fibers are held in the mat with an adhesive binder, such as EVA.  
         [0016]    The outer wrap  12  may alternately include a non-woven fiber layer  19  comprising melt blown plastic fibers. Such a material may comprise, for example, polypropylene fibers with a nominal diameter of about 3 μm. The other physical properties of the non-woven plastic fiber layer are similar to those of the non-woven glass fiber layer. Both the non-woven glass fiber and non-woven plastic fiber layers  19  are typically laid on a backing layer  16  comprising a highly porous spun bonded paper. It is preferable to provide a similar backing layer  16  to the other side of the non-woven fiber layer  19  to protect the interface of the wrap  12  with the carbon block  11 .  
         [0017]    Referring now to FIGS. 2-5, the performance of filters  10  made in accordance with the present invention and utilizing either a glass fiber outer wrap or a fine melt blown plastic outer wrap  12  are compared with (1) similar carbon blocks with no wrap or a coarse fiber wrap, and (2) with high density blocks (suitable for cyst removal, but having a high pressure drop) having a coarse melt blown wrap or no wrap whatever. In each of the graphs of FIGS. 2-5, the various plots are numbered consistently to show a high density block with no wrap  17 , a high density block with a coarse wrap (the wrap per se not capable of retaining cysts)  18 , a low density block  20  with no outer wrap, a low density block  21  with a coarse outer wrap (the same wrap as filter  18 ), a low density block  22  of the present invention using a non-woven glass fiber layer  19  in the outer wrap  12 , and a low density block  23  of the present invention having a fine melt blown micro-fiber layer  19  in the outer wrap  12 .  
         [0018]    The high density blocks  17  and  18  are outside the ranges of particle size and block density set forth above, whereas, the low density blocks  20 - 23  are all within those ranges.  
         [0019]    Referring specifically to FIG. 2, the pressure drops through the high density blocks  17  and  18  are seen to be much higher than pressure drops across any of the low density blocks  20 - 23 . The addition of a coarse melt blown outer wrap (not capable of retaining cysts) does not significantly increase the pressure drop of either the high density or low density blocks. The addition of the non-woven glass fiber layer  19  to the low density block  22  and the addition of the non-woven meltblown fiber layer  19  to the low density block  23  increases the pressure drop slightly, but the pressure drops remain significantly less than pressure drop across the high density blocks  17  and  18 .  
         [0020]    Referring to FIG. 3, both high density blocks  17  and  18  produce less than 2 gpm flow of water at 30 psi inlet pressure. The low density blocks  20  and  21  with no wrap and with a coarse wrap, respectively, produced over 8 gpm flow at 30 psi inlet pressure. The low density blocks  22  and  23 , respectively, having the non-woven layers  19  of glass fibers and melt blown fibers of the present invention produced between 5 and 7 gpm at 30 psi pressure. These flow rates are only slightly lower than for the low density blocks with no wrap or coarse wrap, but substantially higher than either of the high density blocks  17  and  18 .  
         [0021]    [0021]FIG. 4 shows the results of the cyst reduction tests for the same six filter blocks tested in the preceding FIGS. 2 and 3. For these tests, surrogate cysts comprising three micron latex microspheres were utilized. Both high density blocks  17  and  18  passed the cyst removal requirement of more than 99.95% removal, however, these dense blocks are specifically constructed for cyst removal as indicated above. The low density blocks  20  and  21  having, respectively, no wrap or a coarse wrap failed completely the cyst removal test. These blocks exhibited extremely poor performance that continued to degrade through the course of the tests from an initial reduction of 60%-75% to as low as 0%. By comparison, the low density block  22  with the fine non-woven glass fiber layer  19  and the low density block  23  with the fine melt blown non-woven plastic fiber layer  19  both passed the cyst removal test requirement of greater than 99.95% removal for all four sample points. These results show a very significant increase in performance over identical low density blocks  20  and  21  having no wrap or a very coarse melt blown wrap. These tests also show that a low density block  22  or  23  with an appropriate non-woven fiber layer  19  will provide essentially the same cyst removal performance as the high density blocks  17  and  18  but with a much lower pressure drop as shown in FIG. 2.  
         [0022]    Referring now to FIG. 5, tests were run to compare the flow rate through the various filters with total filtered volume to determine how rapidly the filters plugged when operating to remove cysts. Both high density blocks  17  and  18  began with relatively low flow rates of 1-2 gpm and very quickly plugged to drop to a flow rate of 0.5 gpm after less than 200 gallons total flow. The low density block  20  with no outer wrap also plugged very quickly at less than 200 gallons total flow even though it began at a much higher flow rate of greater than 8 gpm. The low density block  21  with a coarse outer wrap also had a high initial flow rate of greater than 8 gpm, and performed best of all the filters tested and was able to process 700 gallons before plugging. This filter, however, has no cyst removal capability. Both low density blocks  22  and  23  utilizing the fine non-woven glass fiber layer or the fine meltblown non-woven plastic fiber layer of the present invention exhibited initial flow rates between 5 and 6.5 gpm and plugged at slightly more than 300 and 450 gallons total flow, respectively. Both of these filters  20  and  23 , with cyst removal capability, performed much better than the high density cyst removal filters  17  and  18  which plugged at about only half the total flow.