Note: Descriptions are shown in the official language in which they were submitted.
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ANTIINIICROBIAL FILTER CARTRIDGE
:FIELD OF THE INVENTION
The present invention relates generally to filters for the purification of
liquids. In
particular, the invention relater to antimicrobial semipermeable hollow fiber
membranes used
in reverse osmosis, ultrafiltration/nanofiltration and microfiltration.
BAC1KGROUND OF THE INVENTION
In recent years, the public has become increasingly aware of the deteriorating
quality
and quantity of our nation's and the world's fresh water supply. Pollutants,
biological and toxic
waste and other contaminants are being introduced into water supplies at an
ever increasing rate,
making such water supplies unfit for drinking and other necessary uses. For
example, medical
patients with low immunity we now being requested not to drink tap water, and
disease and
illnesses linked to poor quality drinking water have increased dramatically in
recent years. This
problem is especially significa~at outside the United States where water
quality has deteriorated
to an all time low, with the major source of such contamination primarily
being bacterial in
nature.
In many areas of the world potable water is not only contaminated but it is
also scarce.
In these areas people must rely upon expensive purification systems to remove
dissolved solids
from sea water or well water.
Reverse osmosis filtration systems are some of the most common solutions for
improving water quality. Osmosis is the flow or diffusion that takes place
through a
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semipermeable membrane (as in a living cell) typically separating either a
solvent (as water) and
a solution or a dilute solution and a concentrated solution. The semipermeable
membrane
controls the flow of solute from the concentrated solution to the dilute
solution thus bringing
about conditions for equalizing the concentrations of solute on the two sides
of the membrane
to form an equilibrium. In reverse osmosis, pressure is deliberately applied
to the more
concentrated solution causing; the flow of solvent in the opposite direction
through the
membrane, i.e., into the more dilute solution. In this way the liquid can be
separated from
solids and dissolved solids, decreasing the concentration of the solids and
dissolved solids in the
filtered fluid.
The wide spread use of reverse osmosis to produce potable water began in the
early
1960's when Loeb and Sourirajan developed thin-skin cellulose acetate
membranes for use in
reverse osmosis systems. These cellulose acetate membranes provided much
higher salt
rejection (approaching 95%) and solvent flow than previously known reverse
osmosis methods.
Cellulose acetate membranes are also relatively inexpensive and are very
tolerant of chlorine
which is commonly used to eliminate bacteria in water. Since the 1960's the
use of reverse
osmosis has grown dramatically in waste water applications and industrial
desalinization plants
to produce drinking water from brackish and sea waters. More recently
cellulose acetate
membranes have been incorporated into consumer filtration systems to produce
drinking water
at the point of use. Matsuura, T., Synthetic Membranes and Membrane Separation
Processes,
CRC Press, (1994). Although cellulose acetate membranes greatly expanded the
utilization of
reverse osmosis treatment systems, such systems are still restricted by
operational problems.
For example, cellulose acetate membranes biodegrade readily.
Recently, thin film composite polyamide membranes have been developed that
offer
better performance than cellulose acetate membranes. These composite poIyamide
membranes
exhibit salt rejection rates greater than 99.5% at pressures much lower than
the pressures used
for cellulose acetate membrane:.. Additionally, polyamide membranes reject
silica, nitrates, and
organic materials much betteo than cellulose acetate membranes. Because of the
high
performance of composite polyamide membranes, these membranes are used in high
purity or
ultrahigh purity water systems in pharmaceutical and electronics industries.
However, just as
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cellulose acetate membranes exhibit a limiting characteristic (i.e.,
biodegradation) so do
composite polyamide membranes. Composite polyamide membranes are susceptible
to damage
firm chlorine.
As the technology for manufacturing composite polyamide and cellulose acetate
S membranes has progressed, new fields of filtration, called ullrafiltration
(also called
nanofiltration) and microfiltration have been created. Membranes based on
polysulfone,
polycarbonate, polypropylene, polyvinylidene difluoride and nylon have been
developed for
these applications.
For example, membranes used in hyperfiltration remove particles of 1-10
Angstrom
units and include chemical compounds of about 180 to 15,000 molecular weights.
Ultrafiltration filters particles of 30 to 1,100 Angstrom units that includes
macromolecules of
molecular weight of 10,000 to 2.50,000. Microfiltration which is mainly used
to remove bacteria
from solutions covers the range of 500 Angstrom to 20,000 Angstroms or 0.05 to
2 microns.
(Lonsdale, H.K. "The Crrowth of Membrane Technology" Journal of Membrane
Science, 10,
p.80-81 (1982)). Unfortunately, these great strides in filtration have come at
a cost, primarily
in the form of bacteria contamination of filters and increased back pressures.
Bacteria contained in irdluent water may be arrested by reverse osmosis
filters. In such
a filter bacteria accumulate on the surface of the semipermeable membranes.
Bacteria multiply
every 30-60 minutes. Their growth is logarithmic and a single bacterial cell
will result in 16
million bacteria in 24 hours. The explosive growth of bacteria results in
fouling of the
membrane which reduces the f low of water through the membrane and can
adversely affect the
filtering properties of the membrane. For example, bacteria build-up typically
has an adverse
affect on salt rejection in a reverse osmosis membrane. (Wes Byrne, Reverse
Osmosis, Chapter
9 - Biological Fouling). Fouled membranes require higher operating pressures
which in turn
increases operating costs.
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In addition to reducing water quality and pressure, bacteria fouled membranes
are
diffcult to clean. As a result of bacterial growth on the membrane, a
gelatinous biofilm is
formed on the upstream surface of the membrane which is very difficult to
remove, except
through use of strong chemical oxidants that damage the membrane. The biofilm
protects the
bacteria from the normal clearang and sanitizing procedures and leads to a
break through of
bacteria across the membrane. This phenomena is not completely understood,
since the pores
of most reverse osmosis and ul.trafiltration membranes are at least 2 to 4
orders of magnitude
smaller than the bacterial cells, One possible explanation is that the
bacterial cells exist in a
dynamic state with continuous'. morphological changes occurring throughout the
population.
These bacteria then get more opportunities and time to find their way to an
accommodating
pathway through the membrane. Typically, bacteria are detected on the
downstream side of the
membrane in 48 to 72 hours. 'I7ze downstream side of the membrane becomes
discolored or
black over time as the bacteria colonize on the downstream side of the
membrane and form a
biofilm that is di~cult to rerrtove. Such biological fouling can also lead to
formation of
1 S localized extremes in pH that c;an damage the membrane.
The filter cartridges described in U.S. Patent 5,762,797; Application Serial
No.
08/877,080 and Application Serial No. 60/090,966 provide solutions to the
problems created
by bacteria buildup in reverse osmosis filters. By incorporating antimicrobial
agents within
various structures within the filter, water filters may be produced that are
capable of removing
and eliminating practically all r;nicroorganisms that may be present in the
influent.
However, these filters, especially those with smaller pore sizes, create
substantial back
pressures in water delivery sys~:ems. In many countries the water pressure in
municipal water
lines is less than 60 psi. In such countries 0.1 to 0.45 micron rated filters,
such as those
described in U.S. Patent 5,762.,797, result in flow rates too low for
practical operation. To
address this problem the continuation application, Serial No. 08/877,080,
taught among other
things, the use of a filter cartrid;~e with semipermeable membranes having a
nominal pore size
of 0.75 microns. Increasing thc; nominal pore size increases the flow of the
water through the
filter cartridge without increasing back-pressures.
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Unfortunately, increasing the nominal pore size of a filter also compromises
the filter's
ability to retain and deactivate bacteria. For example, some bacteria may slip
through pores of
0.75 microns. In theory, it is preferable to approach a nominal pore size of
0.1 micron, because
as the nominal pore size decre~~ses, the higher the log reduction of bacteria
and the better the
performance of the filter cartridge as a bactericidal device.
Perhaps the primary factor limiting flow of water through the above described
filters is
the total surface area of the mernbrane through which water is able to pass or
more specifically,
the lack of surface area. When a semipermeable membrane is in the form of a
flat sheet, as is
typically utilized in a microfiltration filter cartridge, the maximum surface
area is limited to the
circumference of the plastic or activated carbon core over which it is
wrapped. One method to
increase surface area is to pleat; the filter medium as is done in purely
mechanical membrane
filters, such as automobile oil filters. In the microfiltration context this
solution is difficult to
implement.
In short, a need exists fir a reverse osmosis water filter that is capable of
retaining and
eliminating bacteria and allowi~lg sufficient fluid flow and water pressure to
be of practical use
in water systems around the world.
OBJECTS OF THE INVENTION
It is the principal object; of this invention to provide a water filter that
achieves a high
level of separation of water soluble contaminants.
It is also an object of this invention to provide a water filter that resists
fouling due to
bacterial growth.
It is another object of this invention to provide a microfiltration filter
capable of
increased fluid flow.
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It is a fiuther object of this invention to provide a microfiltration filter
that may be
effectively utilized in low pressure water systems.
SUMMARY OF THE INVENTION
The present invention is~ directed toward an antimicrobial filter cartridge
which utilizes
a bundle of semipermeable hollow fibers centrally located within a housing.
The housing has
a two chambers separated by a barrier through which the hollow fibers extend.
The hollow
fibers are enclosed in one chamber and open in the other.
The chamber housing the enclosed hollow fibers also contains a microporous
filter
medium, such as a melt blown polymer web or a tightly wound yarn, that sun
ounds the hollow
fibers. This chamber also receives the fluid to be filtered. The barrier
between the two
chambers forces the water through the microporous filter medium where solid
contaminates are
removed. The water is also forced through the walls of the semipermeable
hollow fibers which
work to remove various dissolved solids finm the water.
The water that enters tl;~e hollow fibers flows within the hollow fiber and
through the
barner where it is then discharged into the other chamber of the housing firm
where it flows out
of the housing and to its end u.~e:
The various components of the antimicrobial filter, such as the hollow fibers
and the
microporous filter medium, may be treated with an antimicrobial agent to
eliminate any
microorganisms, such as bacteria, that may be filtered from the water.
BRIF;F DESCRIPTION OF THE DRAWINGS
The foregoing and other objects will become more readily apparent by referring
to the
following detailed description rind the appended drawings in which:
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FIG. 1 is a cross-sectional view of one embodiment of an antimicrobial filter
in
accordance with the invention.
FIG. 2 is a cross-sectic~nal_ view of a second emLodirmPnt of an
ant;.mic._v.biat filter in
accordance with the invention.
DETAILED DESCRIPTION
4 ,
In the following description, like reference numerals designate like yr
corresponding
parts throughout the several figures. It is to be also understood that such
terms as "front", "rear",
"side", "up", and "down" are used for purposes of locating one element
relative to another and
are not to be construed as limiting terms:' Further, it should be understood
that the illustirations
are for the purpose of describing preferred embodiments of the invention, and
thus are not
intended to limit the invention in any manner.
One aspect of the present invention is an improvement upon.the bactericidal
filters
described in U.S: PatentNo. 5,762;797 (the '797 patent) and U.S. Application
08/877,080 (the
'080 application). In general terms, a filter cartridge is piovided.that is
similar to the catiridges
disclosed ;n the '797 patent arid the '080 application excxpt that the flat
semipemaeable
membranes utilised in those carrsidges is replaced with hollow fiber
membranes. The hollow
fiber membranes utilised in this application can be used with or without
treatment with an
antimicrobial agent, such as Microban~ Additive B, but the use of such an
antimicxobial agent
is preferred. A description of hollow fiber membranes and their method of
manufaatm~e may
be found in U.S. Patent No. 5,762,798 to Wenthold et al.
In order to aid in the understanding of this application a brief introduction
to hollow
fiber membranes is necessary. A microporous hollow fiber is a polymeric tube
having an
outside diameaer less than or equal to 2 mm and whose wall functions as a
semipermeable
membrane. Tl~se microporous hollow fibers can be created with controlled
porosity starting
7
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from as low as 0.05 micron to slightly less than 1 micron using techniques
that are familiar to
those well versed in the art. See Cabasso, "Hollow Fiber Membranes", Kirk-
Othmer
Encyclopedia & Chemical Technology, 3rd Ed., John Wiley & Sons, 12:492-517
(1984).
Hollow fiber membranes are made with many types of synthetic polymers such as
acrylonitrile, polysulfone, polyethersulfones, aromatic polyamides,
polyimides, polyamide-
imides, and polyvinylidene fluoride. The preparation of membranes for diverse
applications is
extensively described in the patent and technical literature, some of the
relevant patents being,
Klein et al U.S. Patent No. 4,051,300 and Wenthold et al U.S. Patent No.
5,762,798. Also see
"Hollow Fiber Membranes", Kirk-Othmer Encyclopedia of Chemical Technology, 3d
Ed., John
Wiley & Sons 12:492-517 (1984)s Preferably,
the hollow fibers utilized in the invention are treated with an antimicrobial
agent. Preferably,
the antimicrobial agent is selected from the group consisting, of 2,4,4'-
trichloro-2'hydroxy
diphenol ether and 5-chloro-2-phenol(2,3-dichlorophenoxy). The antimicrobial
agent is present
in a concentration from about 500 ppm to about 20,000 ppm by weight, and
preferably from
about 2,500 ppm to about 20,000 ppm by weight based upon the weight of the
polysulfone and
polyvinylidene fluoride polymer. The antimicrobial agent is incorporated into
hollow fibers by
adding it to the "dope" solution used to form the follow fibers. A wide
variety of hollow fiber
membranes may be made depending on their, applications which include, reverse
osmosis,
ultrafiltration, microfiltration, etc. Although the concepts of the present
invention apply equally
to all three of these areas, this discussion is directed primarily to the area
of microfiltration.
By using bundles of these microporous hollow fibers as a membrane instead of a
flat
sheet microporous membrane, it is possible to increase the available filter
surface area within
a filter cartridge of the same dimension by several orders of magnitude. For
example, in the
conventional 10 inch filter cacti idge design described in the '797 patent and
the '080 application,
the surface area of the flat sheet membrane is approximately 0.04 mi. In a
filter cartridge of the
same basic design using hollow fibers, it is possible to achieve a microporous
membrane surface
area of between 60 to 160 m2 or more depending on the diameter of the hollow
fibers utilized.
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Availability of such a large surface area results in higher flow rates, lower
back pressures and
the ability to use lower pore diameters resulting in higher bacterial log
reduction.
'The prP~rt impro~~ement l:: deSlgn ~f 13a~~.terlC:dal ~.ItSi' C
ui'~~°w°, S'ua~, h Fu u~'loSc
described in the '797 patent and the '080 application consists of substituting
a microporous
hollow fiber membrane for a flat sheet microporous membrane. This substitution
allows the use
of membranes with much finer pore diameters without compromising flow rates or
creating
unacceptable back pressure.
FIG. 1 illustrates a preferred embodiment of an antimicrobial filter cartridge
10
constructed in accordance with the present invention. The antimicrobial filter
cartridge 10
includes a plurality of semipezmeable hollow fibers 12 centrally located
within housing 14
which has a first chamber 16 and a second chamber 18 separated by a barrier 20
through which
hollow fibers 12 extend.- The second chamber 18 is in fluid communication with
the source of
the fluid to be filtered through fluid inlet 24. For purposes of this
discussiozt water will be used
as the fluid to be filtered. The first chamber 16 acts as a temporary
repository of filtered water.
I S The semipenneable hollow fibers 12 tnay be made of any of the types of
synthetic
polymers discussed above including acrylonitrile, polysulfone,
polyethersulfones, aroniatic polyamides, polyimides, polyamide-imides, and
polyvinylidene
fluoride. Preferably the hollow fibers 12 also incorporate an antimicrobial
agent. Preferably,
the antimicrobial agent used to, treat the hollow fibers, and any other
component of the filter, is
practically insoluble in the water passing through and over the filter
cartridge, and is safe, non-
toxic, non-carcinogenic, non-sensitizing to human and animal skin and does not
accumulate in
the human body when ingested. Generally, therefore, the antimicrobiai is a
broad spectrum
antirnicrobial agent, i.e., it is equally effective against the majority of
harmful bacteria
encountered in water. For example, an antimicrobial agent such as 2,4,4'-
trichloro-2'-
hydroxydiphenol ether, or 5-chloro-2phenol (2,4 dichlorophenoxy) commonly sold
under the
trademark MICROBAN~B, by Microban Products Co., Huntersville, North Carolina,
typically
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will be used. However, it will be understood various other anixmicrobial
agents that are safe
non-toxic and substantially insoluble in water can be used in the present
invention.
Hollow fibers formed of polyvinylidene fluoride (pvd) containing Microban~ B
have
been tested and found to exhibit excellent antimicrobial properties as shown
in the test results
below.
MICR.OBIOLOGICAL TEST REPORT
Kirby Bauer
Test Organism: Syaphylococcus aureus ATCC 6538
Escherichia coli ATCC 25922
Sample Material: PVDF
Sample Size: Variable
Growth Medium: Mueller-Hinton Agar
Test Conditions: Incubated at 37° ~ 2°C for 18-24 hours
Results (Zone
Size)
Sample Identification S. aureus E. coli
7196-OCP-TP-1 (1.92) Microban in l9mm l2mm
dope
7197-OCP-TP-1 (0.98) Microban in l7mm l Omm
dope
7198-OCP-TP-1 (0.48) Microban in l4mm 9mm
dope
Interpretation of Results
NZ - No Zone of inl~ubition surrounding the sample
NI - No Inhibition of Growth Under the Sample
I - Inhibition of Growth Under the Sample (If Observable)
mm - Zone of Inhibition Reported in Millimeters
The hollow fibers 12 should be arranged such that an open end extends from the
barrier 20 into the first chamber 16 while a closed end extends from the
barrier 20 into the
second chamber 18. Such an ~~rrangement may be accomplished by enclosing one
end of a
single hollow fiber 12 and extending that end of the hollow fiber 12 into the
second chamber
18. A similar arrangement ma.y be accomplished by bending a hollow fiber 12
that has both
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ends open. This second possit~ility is illustrated in FIG. 1. In FIG. 1 the
hollow fibers 12 are
long open-ended tubes that ane bent such that the two ends are approximately
parallel and
approximately equidistant from a midpoint. The hollow fibers 12 must not be
bent so as to
compromise their structural integrity. The hollow fibers 12 are bundled
together by the barrier
20 and placed centrally within housing 14 such that the open end portions of
the hollow fibers
12 extend into the first chamber 16.
Preferably the barner 20 is formed of a thermoset or thermoplastic polymer
such as
polyurethane or an epoxy. Th.e barrier 20 which encloses a portion of the
hollow fibers 20
may be manufactured external to the housing. It is anticipated that in most
instances the
barrier 20 enclosing hollow fibers 12 will be formed in a mold external to the
housing so that
other elements may be more easily secured through placement within the barrier
20 as will
be discussed below.
Continuing with FIG. :l, surrounding the bundle of hollow fibers 12 is a
microporous
filter medium. In FIG. 1 the ~nicroporous filter medium is a melt-blown
polymer web 22.
The polymer may be selected from the group consisting of nylon, polypropylene,
cellulose
acetate, rayon, lyocell, acrylic., polyester, polyethylene and mixtures
thereof. In a preferred
embodiment polypropylene fibers are impregnated with Microban~ B during
extrusion and
blown into a continuous web having an effective pore size of 5 microns. The
concentration
of the antimicrobial agent in the fibers generally is between 50 to 20,000
ppm, preferably
between 1000 ppm to 5000 ppm.
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The melt blown web 2~; may be held in place by making it of sufficient
thickness such
that the sides of housing 14 ke;ep its position secure. In fact it is
important to surround the
hollow fibers 12 and all the empty volume near them with antimicrobial fibers
because the
objective is to force all bacteri;~ mechanically withheld by the microporous
filter medium to
come into contact with an antimicrobial surface so that the bacteria may be
deactivated.
Alternatively and as illustrated in FIG. I the melt blown web 22 may be
secured by setting
one end of the web within barrier ZO thereby making the barner 20, the bundle
of hollow
fibers 12 and the melt blown web 22 a single unit within housing 14.
In operation water enters antimicrobial filter cartridge 10 second chamber 18
through
fluid inlet 24 and is prohibited from flowing into the first chamber 16 by the
barrier 20. The
barner 20 along with the walls of the second chamber 18 force the water into
contact with the
melt blown web 22 and the semipermeable walls of the hollow fibers 12 where
the fluid is
filtered and any retained microorganisms, such as bacteria, are eliminated by
coming into
contact with the antimicrobial agent.
The water that passes through the semipermeable walls of the hollow f hers 12
and
into the annular space within each hollow fiber 12 then exits the hollow fiber
12 into the first
chamber 16 and is discharged out of fluid outlet 26. The filtering capability
of such a filter
should meet that of the filters described in the '797 patent and the '080
application while
operating at higher flow rates and reduced back pressure.
FIG. 2 illustrates another preferred embodiment of the antimicrobial filter
according
to the invention that is very similar to FIG. I . However in this embodiment
the melt blown
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web 22 is replaced by a wrapping of yarn 28. The yarn can be made of cotton,
nylon,
polypropylene, cellulose acetate, rayon, lyocell, acrylic, polyester,
polyethylene or any
mixture thereof. In a preferred embodiment shown in FIG. 2, the yam 28 is 0.60
cotton count
(cc) yarn. The yarn 28 contains polypropylene fiber between 0.3 denier per
filament (dpf) to
10 dpf, the preferable range based on cost and performance being 1.5 dpf to .
6 dpf. The
polypropylene fiber is cut into 2 inch staple, then opened and carded and
friction spun into a
0.60 cc yarn. The polypropylene fiber is impregnated with an antimicrobial
agent, such as
Microban~ Additive B during extrusion. The concentration of the antimicrobial
agent in the
fibers generally is between 50~ to 20,000 ppm, preferably between 1000 ppm to
5000 ppm.
The 0.60 cc yarn is tightly wound around the hollow fibers 12 in a spiral
pattern to cover the
bundle of hollow fibers 12 completely and to give an effective pore size of 1-
5 m. The yam
28 may also be wrapped in a criss-cross pattern as is well described in the
'797 patent and the
'080 application.
Due to the fragile nature of the hollow fibers 12, it is recommended that the
yarn 28
be wrapped around a ridged g~~ide 30 which is set in the barrier 20 and which
surrounds and
is in very close proximity to th~~ bundle of hollow fibers 12. The guide 30
may simply consist
of two or more poles situated ~~t the edge of the bundle of hollow fibers 12
as shown in FIG.
2 or it may be a perforated cylindrical object that completely encloses the
bundle of hollow
fibers 12.
In an additional embodiment of the antimicrobial filter according to the
invention, it
is possible to have an activated carbon core working in conjunction with the
semipermeable
hollow fibers 12. For example, the hollow fibers 12 could be situated within a
core of
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granulated activated carbon which is in turn surrounded by a microporous
filter medium.
Likewise the hollow fibers 12 could surround a core of activated carbon. If
activated carbon
is utilized it is preferable that it be treated with an antimicrobial agent as
is thoroughly
discussed in the '797 patent arid the '080 application.
The invention has been described in detail, with reference to certain
preferred
embodiments, in order to enable the reader to practice the invention without
undue
experimentation. However, a person having ordinary skill in the art will
readily recognize that
many ofthe components and parameters may be varied or modified to a certain
extent without
departing from the scope and spirit of the invention. Furthermore, titles,
headings, or the like
are provided to enhance the reader's comprehension of this document, and
should not be read
as limiting the scope of the present invention. Accordingly, the intellectual
property rights
to the invention are defined only by the following claims and reasonable
extensions and
equivalents thereof.
SUMMARY OF THF: ACHIEVEMENTS OF THE OBJECTS OF THE
INVENTION
From the foregoing, it is readily apparent that I have invented an
antimicrobial filter
cartridge that achieves a high level of separation of water contaminants while
simultaneously
resisting fouling due to bacterial growth. Furthermore, the design of the
antimicrobial filter
cartridge according to the invention provides a microfiltration filter
cartridge capable of
increased fluid flow and that may be effectively utilized in low pressure
water systems.
14
