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Description  |
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TECHNICAL FIELD
The present invention relates to filters. In particular, it relates to an
apparatus for filtering a broad spectrum of contaminants from a liquid.
DISCLOSURE OF THE INVENTION
Liquids, such as water, may contain many different kinds of contaminants
including, for example, particulates, harmful chemicals, and
microbiological organisms such as pathogenic bacteria, amoeba, flagellate,
and viruses. In a variety of circumstances, these contaminants must be
removed before the liquid can be used. For example, in many medical
applications and in the manufacture of certain electronic components,
extremely pure water is required. As a more common example, any harmful
and observable contaminants must be removed from water before it is
potable, i.e., fit to drink.
Ideally, a filtering apparatus for removing this broad spectrum of
contaminants would comprise a single, small, lightweight, self-contained
device rather than a complex multicomponent and/or multistage system to
remove the various contaminants. Such a device would not only be more
reliable than a complex system, but it would also be far more portable.
Thus, it could be carried into very primitive environments, for instance,
during backpacking, to provide a supply of potable water.
In a preferred design, the filtering apparatus should present a low
resistance to the flow of liquid through the apparatus so that in a remote
environment, where the electricity necessary to drive a pump may be
unavailable, the filtering apparatus may simply be connected between upper
and lower containers of water, for example, between a container of water
hanging from a tree and a container of water resting on the ground. The
filtering apparatus should also have sufficient internal structural
integrity to withstand even greater pressures if, for example, a hand pump
or other source of pressure is available to drive the liquid through the
filtering apparatus. Further, the filtering apparatus should include a
prefilter portion which is able to remove substantial quantities of gross
contaminants without becoming completely clogged so that a considerable
amount of purified liquid can be obtained.
A general object of the present invention is to provide an improved
apparatus for filtering contaminants from a liquid. Specific objects
include providing a filtering apparatus which removes a broad spectrum of
contaminants from a liquid, which is highly portable and reliable, which
presents a low resistance to the flow of liquid through the apparatus, and
which removes the contaminants from a substantial volume of liquid before
becoming clogged.
Accordingly, the present invention may be embodied in a filter assembly
comprising a housing containing a generally cylindrical filter
arrangement. The housing has a liquid inlet and a liquid outlet and
defines a liquid flow path between the inlet and outlet. The filter
arrangement is disposed within the housing in the liquid flow path and
comprises a cylindrically shaped porous structure for removing particulate
contaminants from the liquid, a cylindrically shaped structure containing
a sorbent material (i.e., a material which absorbs or adsorbs certain
substances) for removing chemical contaminants from the liquid, and a
cylindrically shaped microporous structure for removing microbiological
contaminants from the liquid. The structure for removing microbiological
contaminants is disposed downstream from the other two structures. The
filtering apparatus also includes impervious end members mounted to the
ends of the filter arrangement, one of the end members having a central
aperture. These end members direct liquid flow radially through the filter
arrangement.
A filter assembly embodying the present invention may feature, for example,
a cylindrically shaped, particulate removing structure which may be
fashioned from a variety of suitable materials and which has a graded
porosity where the size of the pores increases or decreases along the
radius of the structure.
Further, a filter assembly embodying the present invention may feature a
sorbent containing structure in which the sorbent material is immobilized.
The sorbent material may be immobilized either by a polymeric binder such
as powdered polyethylene, by entrapment in a microfibrous web of various
materials including, for example, polymeric microfibers of polypropylene,
cellulose, or nylon, or by a combination of physically restraining
materials. Preferably, the sorbent containing structure contains a
sufficient quantity of sorbent material and the flow velocity through the
sorbent material is sufficiently low to allow an adequate contact or
resident time between the sorbent material and the chemical contaminants
which are to be sorbed by the sorbent material. Since different chemical
contaminants have different sorbent kinetics and different sorbent
materials have different sorption capacities, the sorbent containing
structure may also be fashioned from a variety of suitable materials
including, for example, activated carbon, activated alumina, molecular
sieve, or ion exchange resins.
A filter assembly embodying the present invention may also feature, for
example, a microbiological contaminant removing structure comprising a
microporous polymeric membrane. Such a membrane may preferably have an
absolute pore rating of 0.2 micron or less and may be fashioned, e.g.,
from nylon or a fluorinated polymer.
BRIEF DESCRIPTION OF THE DRAWINGS
In the accompanying drawings:
FIG. 1 is a sectional side view of an exemplary filtering apparatus
embodying the present invention;
FIG. 2 is a partial sectional end view of the filtering apparatus of FIG. 1
as viewed along lines 2--2; and
FIG. 3 is an illustration of a liquid filtering system incorporating the
filtering apparatus of FIG. 1.
BEST MODE FOR CARRYING OUT THE INVENTION
As shown in FIGS. 1 and 2, an exemplary filter cartridge 10 embodying the
present invention comprises a generally cylindrical housing 11 and a
cylindrical filter arrangement 12 disposed within the housing 11. The
filter cartridge 10 purifies liquids, e.g., water, containing a broad
spectrum of contaminants including, for example, solid particulates
including radioactive isotopes, certain polyvalent dissolved salts, toxic
organic chemcials such as many pesticides, and microbiological
contaminants such as bacteria, amoeba or flagellates.
The housing 11 is preferably fabricated from polypropylene, although it may
be fabricated from any sufficiently rigid material, including other
polymers or sheet metal. In the preferred embodiment, the housing 11 has a
top wall 13, a bottom wall 14 and a cylindrical side wall 15 and comprises
an upper portion 16 and a lower portion 17 joined to one another at a
circumferential joint 20. The top wall 13 of the housing 11 includes an
air-bleed valve 21 and an inlet 22. The inlet 22 comprises a coaxially
projecting cylindrical protrusion 23 with a coaxial hole 24 and a flange
25 which facilitates connecting the inlet 22 to a supply line 26.
Similarly, the bottom wall 14 of the housing 11 has an outlet 30
comprising a coaxially projecting cylindrical protrusion 31 with a coaxial
hole 32 and a flange 33 which facilitates connecting the outlet 30 to a
drain line 34. Alternatively, the cylindrical protrusions 23, 31 of the
inlet 22 and the outlet 30 may have threaded portions rather than flanges
25, 33 to facilitate connection of the supply line 26 and the drain line
34.
The filter arrangement 12 comprises a cylindrical filter assembly 35
disposed between top and bottom end caps 36, 37 which direct liquid
radially through the filter assembly 35. The end caps 36, 37 also lend
axial and radial support to the filter assembly 35. In the preferred
embodiment, both the top and bottom end caps 36, 37 are fabricated from
polypropylene. However, they can be fabricated from any sufficiently
impervious material, including other polymers.
The top end cap 36 comprises a circular disc having a diameter equal to the
inside diameter of the side wall 15. It is preferably disposed coaxially
within the housing 11 a short distance from and parallel to the top wall
13 and is joined to the side wall 15. While the top end cap 36 completely
encloses the top end 40 of the filter assembly 35, it includes peripheral
perforations 41 which allow liquid to flow between the upper space 42,
i.e., the space between the top end cap 36 and the top wall 13 of the
housing 11, and the annular space 43, i.e., the space between the exterior
of the filter assembly 35 and the side wall 15 of the housing 11.
The bottom end cap 37 comprises a circular disc which, in the preferred
embodiment, has a diameter smaller than the inside diameter of the side
wall 15 of the housing 11 but at least equal to the outside diameter of
the filter assembly 35. The bottom end cap 37 is disposed coaxially within
the housing and is joined to the bottom wall 14. Except for a central hole
44 in the bottom end cap 37 which allows liquid to flow between a central
space 45 in the filter assembly 35 and the outlet 30, the bottom end cap
37 completely encloses the bottom end 46 of the filter assembly 35.
In accordance with one aspect of the invention, the filter assembly 35
comprises a cylindrical filter 50 for removing particulate contaminants
and a cylindrical sorbent bed 51 for removing chemical contaminants, both
disposed upstream from a cylindrical filter 52 for removing pathogenic
microbiological contaminants. The cylindrical geometry of the filter
arrangement 12 minimizes the resistance to the flow of liquid through the
filter cartridge 10 compared to comparably sized conventional axial flow
filter cartridges capable of filtering such substances. Consequently, the
filter cartridge 10 of the present invention provides reasonable flow
rates even for a small pressure differential between the inlet 22 and
outlet 30.
In a preferred embodiment, the particulate filter 50 and the sorbent bed 51
comprise distinct upstream and downstream regions, respectively, of a
cylindrical, nonwoven, microfibrous filter element 53 composed of a
fibrous mass of the type which is disclosed in the European Patent
Application No. 84309094.5, published under Publication No. 0 148 638 on
July 17, 1985. As disclosed in that application, the fibrous mass
comprises a mass of nonwoven, synthetic polymeric microfibers (e.g.,
polypropylene microfibers) free of fiber-to-fiber bonding and maintained
by mechanical entanglement or intertwining of the microfibers. As further
disclosed this fibrous mass may be fabricated in a graded pore size
configuration, i.e., a configuration having pore sizes which decrease
progressively with decreasing radius of the cylindrical fibrous mass, or a
constant pore size configuration and may have various additives or
combinations of additives, such as activated carbon or ion exchange
resins, interspersed in the fibrous mass.
In a preferred embodiment, the upstream region 50 of the microfibrous
filter element 53 comprises a graded pore size configuration having, for
example, an upstream absolute pore rating in the range from about 50
micrometers to about 150 micrometers and a downsteam absolute pore rating
in the range from about 0.5 micrometer to about 5 micrometers. A graded
pore size configuration is highly effective for removing certain
microorganisms and other fine particulates while delaying the onset of
clogging due to gross contamination components in the influent stream. The
downstream region 51 of the microfibrous filter element 53 may comprise a
microporous fiber matrix having a constant pore size configuration and
containing particles of sorbent material, i.e., material which adsorbs or
absorbs contaminants, such as activated carbon, reticulated water service
resins, activated alumina, molecular sieve, ion exchange resins, and/or
attapulgite clay for removing a broad spectrum of chemical contaminants.
Particles in a wide variety of size ranges may be used, including
particles in the size range of about 50.times.100 U.S. Sieve Series. Since
the sorbent particles are bound, i.e., immobilized, within the fiber
matrix of the filter element 53, settling and channeling of the sorbent
bed due to shock or vibration are minimized or even precluded, enhancing
both the reliability and the durability of the filter cartridge 10.
While the filter assembly 35 of the preferred embodiment of the filter
cartridge 10 comprises distinct upstream and downstream regions 50, 51 of
a single filter element 53, the filter assembly 35 could alternatively
comprise any appropriately configured and suitably fashioned particulate
filter and sorbent bed without departing from the scope of the invention.
For example, the particulate filter and sorbent bed may be coextensive
over the filter element 53 rather than comprised of distinct regions 50,
51. On the other hand, they may comprise entirely independent elements.
Thus, the particulate filter may comprise, for example, a woven mesh of
glass fibers or a pleated, porous nylon membrane while the sorbent bed may
comprise a compressibly-loaded bed of loose sorbent particles or a
binder-immobilized sorbent particle bed as described in European Patent
Application No. 85104933.8, published under Publication No. 0 159 698 on
Oct. 30, 1985, and European Patent Application No. 85305652.1, published
under Publication No. 0172003 on Jan. 19, 1986.
In accordance with European Patent Application No. 85104933.8, the sorbent
particle bed may comprise a bed of binder-immobilized sorbent particles in
which sorbent particles, such as particles of activated carbon, are mixed
with a suitable polymeric binding material, such as powdered polyethylene.
The mixture is then heated and compressed to the solid-liquid transition
stage of the binding material, yielding a binder-immobilized sorbent
particle bed once the mixture cools. A similar process for immobilizing
inorganic sorbent particles is disclosed in European Patent Application
No. 85305652.1.
To remove microbiological contaminants, any suitable microporous polymeric
membrane may be used, including membranes having certain surface charged
properties. Preferably, these membranes have an absolute pore rating in
the range from about 0.02 micrometer to about 0.5 micrometer. The
downstream microbiological filter 52 of the exemplary filter assembly 35
preferably comprises a microporous membrane such as a surface-modified,
hydrophilic, microporous polyamide membrane having an absolute pore rating
of about 0.2 micrometer. This membrane is described in European Patent
Application No. 83300518.4, published under Publication No. 0 090 483 on
Oct. 5, 1983, and is available under the trademark Posidyne from Pall
Corporation. As disclosed in that application, the membrane has a positive
zeta potential in neutral or alkaline liquids, such as water.
Consequently, the membrane is highly effective for removing both
microbiological contaminants and certain ionic contaminants. The
microbiological filter 52 may further comprise upstream and downstream
support layers adjoining both sides of the membrane. The support layers
may be fashioned from any suitable woven or nonwoven polymeric fibrous
material, such as a nonwoven layer of polypropylene or polyester,
terephthalate fibers.
The filter assembly 35 also comprises a cylindrical perforated core 54
disposed coaxially within the microbiological filter 52 for radially
supporting the particulate filter 50, the sorbent bed 51, and the
microbiological filter 52. The perforated core 54 is preferably fabricated
from polypropylene, although it can be fabricated from many sufficiently
rigid materials, including other polymers or sheet metal. In alternative
embodiments, the filter assembly may further include a perforated cage
disposed upstream from the particulate contaminant filter to support and
protect the particulate contaminant filter and/or a midstream perforated
core disposed between the sorbent bed and the microbiological filter to
support the sorbent bed.
The filter cartridge 10 may be fabricated and assembled according to
several known techniques. For example, the upper and lower portions 16, 17
of the housing 11, the top and bottom end caps 36, 37, and the perforated
core 54 may be formed by injection molding, a well known technique for
fabricating polymeric structures. The microfibrous filter element 53 and
the microporous polyamide membrane may be fabricated according to the
disclosures in the previously referenced European Patent Application Nos.
84309094.5 and 83300518.4, respectively. The membrane may then be disposed
between the upstream and downstream support layers to form a composite and
the composite may then be pleated and arranged in a cylindrical
configuration in a conventional manner to form the microbiological filter
52. The microbiological filter 52 may then be disposed within the
microfibrous filter element 53 and the perforated core 54 may be disposed
within the pleated membrane 52, yielding the filter assembly 35.
The top and bottom ends 40, 46 of the filter assembly 35 may then be joined
to the top and bottom end caps 36, 37, respectively, for example, by
spinbonding, a well known technique for joining polymeric structures, or
by heat-melt sealing, a technique disclosed in Pall et al, U.S. Pat. No.
3,457,339 yielding the filter arrangement 12. The bottom end cap 37 of the
filter arrangement 12 may then be spunbound to the lower portion 17 of the
housing 11, and the upper portion 16 of the housing 11 may be spunbound to
both the lower portion 17 of the housing 11 and the periphery of the top
end cap 36 without closing off the peripheral perforations 41.
In one preferred mode of operation, the filter cartridge 10 is connected
between an upper container 55 which contains a contaminated liquid, such
as water, and a lower container 56 which stores purified water, as shown
in FIG. 3. The water is fed by gravity from the upper container 55, along
the supply line 26, and through the filter cartridge 10 where it is
purified. The purified water is then fed along the drain line 34 and into
the lower container 56. The cylindrical geometry of the filter arrangement
12 minimizes the resistance to the flow of water and, therefore, permits a
reasonable flow rate, e.g., 500-2,000 ml/minute, using a small gravity
head, e.g., a 27 inch water column. Alternatively, a pump may be installed
in the supply or drain line 26, 34.
More specifically, the water flows from the upper container 55, down the
supply line 26, through the coaxial hole 24 of the inlet 22, and into the
upper space 42 of the filter cartridge 10. The water then flows through
the peripheral perforations 41 in the top end cap 36 and into the annular
space 43. To insure that the upper space 42 and annular space 43 fill with
water and that all air escapes from the filter cartridge 10, the air-bleed
valve 21 is opened until significant amounts of water begin escaping the
filter cartridge 10 through the valve 21 and is then closed.
From the annular space 43 the water flows radially inwardly through the
microfibrous filter element 53, passing first through the graded pore size
upstream region 50, where particulates and certain organisms are removed,
and next through the sorbent-containing downstream region 51, where
chemical contaminants are removed. The water then flows radially inwardly
through the pleated membrane 52, where microbiological contaminants and
certain ionic contaminants are removed. From the pleated membrane 52, the
purified water flows radially inwardly through the perforated core 54 and
into the central space 45 of the filter assembly 35. From the central
space 45, the purified water flows axially through the central hole 44 in
the bottom end cap 37, through the coaxial hole 32 of the outlet 30, along
the drain line 26 and into the storage reservoir 56.
Although the present invention has been described in terms of a particular
embodiment, it is not limited to this embodiment. Alternative embodiments
and modifications which would still be encompassed by the invention may be
made by those skilled in the art, particularly in light of the foregoing
teachings. Therefore, the following claims are intended to cover any
alternative embodiments, modifications or equivalents which may be
included within the spirit and scope of the invention as defined by the
claims.
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