Liquid purification or separation – Filter – Resting on supporting receiver – e.g. – portable
Reexamination Certificate
1998-12-14
2001-05-08
Cintins, Ivars (Department: 1724)
Liquid purification or separation
Filter
Resting on supporting receiver, e.g., portable
C210S483000, C210S502100, C210S506000, C210S510100
Reexamination Certificate
active
06227382
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to water filters and more particularly to a cyst-reduction water filter delivering a substantial water flow rate at low (gravity-assisted) water pressures. Such filters are of particular interest for applications such as gravity flow filter cartridges in water carafes.
Gravity flow filtration units continue to gain in popularity as consumers become concerned about the quality of available drinking water supplies. Carafe gravity flow units effectively address this concern because of their relatively low cost and perceived efficiency in removing unwanted tastes, odors, and harmful pollutants.
Operation of these units is simple and convenient. Water is simply poured into the top of the unit and is allowed to trickle through a replaceable filter cartridge to a treated water reservoir for later use. The filter cartridges are typically packed-bed units containing carbon pellets for the removal of adsorbable/catalyzable constituents such as chlorine and organics, and ion exchange resins for the removal of metal ions such as lead. Bacteriostatic agents such as silver may also be present. Although gaining in acceptance, filter units presently available for use in gravity flow filtration systems suffer from a number of disadvantages. Packed bed systems typically develop preferential flowpaths within the bed, resulting in less than optimal use of the available adsorbents and exchange media and correspondingly decreased purifying effectiveness and service life. They also tend to release fine adsorbent particles into the treated water during initial use.
Even more important to the consumer is the need to maintain adequate water flow rates even as designs for more efficient particle removal are developed. Gravity-assisted water flow rates through the filters are quite limited even at maximum acceptable filter cartridge sizes. Increasing the particle removal efficiency of the cartridges would likely involve more tightly packed filtration media or special membranes, both of which would be expected to further reduce gravity flow rates. Similarly, commercially available membrane filters capable of effective cyst removal do not generally provide rapid filtration. Thus, given present commercial flow rate requirements, no practical way to achieve the removal or reduction in concentration of such fine biological impurities as cyst particles (<4 micrometers in diameter) in a gravity flow filtration environment has yet been identified.
Fluid filtration elements comprising plugged honeycomb filters are well known in the art. U.S. Pat. No. 4,329,162 and published European patent application EP 0745416, for example, describe the use of such filter elements for the removal of particulates from vapor, water and other fluid streams. However, up to the present, no adaptation of such filters has been identified or considered which would offer the capability of removing very fine particulate biological contaminants such as cysts from a water stream while still offering acceptable water filtration rates under the low pressure (gravity feed) conditions found in water carafes.
SUMMARY OF THE INVENTION
The present invention comprises a water carafe incorporating a water filter element that is both effective for use as a cyst reduction filter and that provides water flow rates which, under gravity feed only, equal or exceed present commercially available filter cartridges. The filter element is a compact microporous ceramic filter body, optionally incorporating an active water treatment coating, which provides a very large effective filter area. For purposes of the present description the term “microporous” refers to pores on the order of microns in size. The large filter area is provided by the channeled honeycomb configuration of the filter, which offers high geometric surface area in a small unit. High flow rates are due largely to the relatively short water flowpath through the active filter material.
In a first aspect, then, the invention includes a cyst-reduction water filter which incorporates a cellular ceramic honeycomb filter element of a selectively plugged channel configuration. The filter element imparts cyst reduction performance, by which is meant that it can exhibit at least 99.95% removal of 3-4 &mgr;m particles when tested in accordance with NSF Standard 53, Drinking Water Treatment Units—Health Effects (September 1997).
All of the water flowpaths through the filter element traverse the porous channel walls of the honeycomb, and the walls can be relatively thin without loss of cyst-reduction characteristics. Consequently, the filter can provide a primed water flux of at least about 0.3 ml/min/cm
3
of filter element volume at very low inlet water pressures, i.e. water pressures typical of the water head pressures attained through gravity feed in commercial water carafes. By primed water flux is meant the water flux after the filter element has been water-primed, i.e., infiltrated with water to an extent sufficient to remove trapped air from the pore microstructure of the channel walls. Water fluxes for unprimed filter elements, or primed filter elements which have been re-exposed to air, are much lower and not indicative of the performance of the filter in use.
The honeycomb structure of the cyst-reduction filter element is typically formed of a plurality of channels separated by porous channel walls, the channels traversing the structure from a filter inlet end to a filter outlet end and including a first plurality of channels open only at the inlet end and a second plurality of channels open only at the outlet end. This selective plugging compels all raw water being filtered to traverse the channel walls of the honeycomb.
To achieve the required combination of gravity flow and efficient small particle reduction, the porous channel walls of the filter will exhibit 30-70% by volume of open porosity, with the pores being characterized by a median pore diameter in the range of about 2-8 micrometers as determined by mercury porosimetry. Further, the distribution of pore diameters about the median as determined by this method will be such that pores over 10 microns in diameter will comprise not more than about 10% of the open pore volume of the channel walls. This pore size distribution will permit the attainment of primed water flux levels of at least about 0.3 ml/min/cm
3
under a test water pressure of 0.3 psig which is typical of the gravity feed environment of water carafes.
In yet another aspect the invention includes a gravity-flow water carafe incorporating a cellular ceramic honeycomb water filter element of a selectively plugged channel configuration as above described. Thus the porous walls of the filter will have a median pore diameter of 2-8 microns, not more than 10% of pores over 10 microns in diameter, and a primed water flux of at least about 0.3 ml/min/cm
3
under a water pressure of 0.3 psig. For purposes of convenience and design flexibility, the filter element will maintain these characteristics while having an overall filter element volume not exceeding about 150 cm
3
.
The gravity-flow water carafe of the invention will have a design such that the cellular ceramic honeycomb water filter element will remain submerged in water during repeated cycles of carafe use, regardless of whether the raw water reservoir is full or empty. This is desirable to avoid exposure of the filter element to ambient air after priming in a manner which will re-admit air into the pore structure of the filter and thereby necessitate re-priming.
In a further aspect the invention includes a method for achieving effective and rapid particle (cyst) reduction in a raw water feedstream under water pressures as low as found in gravity fed systems. By effective particle reduction is meant the removal of at least 99.95% of 3-4 micrometer (diameter) particles from a raw water feedstream comprising such particles in a concentration of up to 5×10
4
particles/ml. By rapid particle reduction is meant that treated water flux from the filter
Cutler Willard A.
Gadkaree Kishor P.
Tao Tinghong
Cintins Ivars
Corning Incorporated
Sterre Kees van der
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