Note: Descriptions are shown in the official language in which they were submitted.
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MICROORGANISM-REMOVING FILTER MEDIUM HAVING HIGH
ISOELECTRIC MATERIAL AND LOW MELT INDEX BINDER
10 FIELD OF THE INVENTION
The present invention relates to filtration materials, and more particularly,
this
invention relates to a filter medium having enhanced microorganism-removing
properties.
BACKGROUND OF THE INVENTION
The use of home water treatment systems to treat tap water continues to grow
dramatically in the U.S. and abroad, in part because of heightened public
awareness of the
health concerns associated with the consumption of untreated tap water. Of
particular
concern are pathogens, which are microbes that cause disease. They include a
few types of
bacteria, viruses, protozoa, and other organisms. Some pathogens are often
found
in water, frequently as a result of fecal matter from sewage discharges,
leaking
septic tanks, and runoff from animal feedlots into bodies of water
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from which drinking water is taken. Bio-terrorism also poses a significant
threat to
water supplies.
Total Coliforms are a group of closely related bacteria that live in soil and
water as well as the gut of animals. The extent to which total coliforms are
present in
the source water can indicate the general quality of that water and the
likelihood that
the water is fecally contaminated. Specific types of coliforms (i.e., fecal
coliforms or
E. coli) can present serious health risks. The Environmental Protection Agency
(EPA) has set forth minimum standards for acceptance of a device proposed for
use as
a microbiological water purifier. Devices that claim removal of coliforms,
represented by the bacteria E. coli and Klebsiella Terregina, must show a
minimum 6-
log reduction, 99.9999% of organisms removed, from an influent concentration
of
1x107/100 ml.
Cryptosporidium is a single-celled microbe contained in a group generally
known as protozoa. Cryptosporidium may cause a disease, cryptosporidiosis,
when
ingested. Cryptosporidiosis symptoms can range from mild stomach upset to life
threatening disease in those who are immunocompromised (e.g., people with
severely
compromised immune systems). Oocysts are a stage in the life-cycle of some
Cryptosporidium. In this stage, the Cryptosporidium can infect humans and
other
animals. The EPA requires removal of at least 99% of Cryptosporidium from
water
for qualified devices.
Giardia lamblia (commonly referred to as Giardia) are single-celled microbes
contained in a group known as protozoa. When ingested, they can cause a
gastrointestinal disease called giardiasis. Giardiasis is a frequent cause of
diarrhea.
Symptoms may include diarrhea, fatigue, and cramps. Waterborne giardiasis may
occur as a result of disinfection problems or inadequate filtration
procedures. Cysts
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are a stage in the life-cycle of some Giardia. In this stage, the Giardia can
infect
humans and other animals. Devices that claim cyst removal must show a minimum
3
log reduction, 99.9% of cysts removed, from an influent concentration of
1x10'2.
Viruses, including hepatitis A virus, rotaviruses, and Norwalk and other
caliciviruses, are microbes that can cause serious illness. The EPA requires
water
purifiers to ensure a 4 log reduction, 99.99% of viruses removed, from an
influent
concentration of 1x107/L.
Two types of systems exist for the filtration of tap water. One type is
pressurized, such as a faucet-mount system, and typically uses a porous carbon
block
as part of the filtration system. The other type is a low pressure system,
such as a
pitcher filter system, and typically uses activated carbon granules as part of
the
filtration system. However, few filtration materials are able to meet EPA
standards
for more than a few liters of water with filters of a reasonable size.
We have surprisingly found that a synergistic effect occurs when inorganic
materials with high isoelectric points, such as magnesium salts, and activated
carbon
are bound by a low melt index binder. The resulting filter medium is very
effective at
removing microorganisms from large quantities of water, in filters small
enough for
point-of-use systems.
Magnesium salts have been used to remove polar materials from non-polar
liquids by filtration. For example, United States Patent No. 6,338,830 to
Moskovitz
and Kepner describes the use of Group IIA, Group IIIA, Goup IVA, Group VA and
transition metal oxide to remove contaminants from non-aqueous liquid or gas
streams.
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United States Patent Application No. 2002/0050474 to Munson and Roberts
describes the use of magnesium silicate to remove polar impurities from used
cooking
oils.
Magnesium ions have also been used to promote cell survival. For example,
United States Patent No. 6,096,216 to Shanbrom describes the use of magnesium
salts
to preserve biological fluids during filtration through iodinated resin.
United States
Patent Application No. 2002/0053537 to Lucido and Shaffer describes the use of
magnesium as a nutrient to feed microorganisms in a bioreacter.
International Patent Application WO 01/07090 to Hou et al. describes cationic
polymers attached to substrates, including carbon blocks, for removing
microorganisms.
Some prior art filters use biocidal resins and peroxides to kill
microorganisms.
For example, United States Patent No. 4,361,486 to Hou and Webster describes
the
use of magnesium peroxide to oxidize soluble iron and inactivate
microorganisms. A
drawback to such filters is that the biocidal agent as well as the dead
microorganisms
pass through the filter and into the drinking water.
International Patent Application WO 02/076577 to Hughes broadly describes
the use of magnesium compounds in carbon block form to remove microorganisms
from a fluid, and is herein incorporated by reference. The purification
material
disclosed in Application WO 02/076577 removes microorganisms from fluids
through
adsorption to the magnesium compound. However, because the magnesium
containing material only represents a small percentage of the surface area
exposed to
the fluid, the sites to which microorganisms can become adsorbed are few.
Thus, the
efficiency of the filter is limited, in that many microorganisms are not
captured but
merely pass through the filter. In addition, the adsorption sites quickly fill
up, making
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adsorption difficult if not impossible and/or resulting in clogging of the
filter pores
ultimately resulting in a short filter life. For example, Application WO
02/076577
only discloses the ability to remove microorganisms from 500 ml of water.
Moreover, the filter disclosed in Application WO 02/076577 is very large, with
an
outer diameter of 2.5 inches, an inner diameter of 1.25 inches, and length of
9.8
inches, making it unsuitable for many point-of-use purposes and in portable
devices.
United States Patent Nos. 4,753,728 and 5,017,318 to Vanderbilt et al.
describe a filter constructed of powdered activated carbon bound by an ultra
high
molecular weight polyethylene binder, but which is only capable of capturing
insignificant quantities of microorganisms.
United States Patent Application No. US 2003/0038084 to Mitchell et al.
describes a filter composed of carbon particles heated in an oven in an
atmosphere of
ammonia that purportedly removes microorganisms through a combination of
capturing fimbriae and surface polymers of the microorganisms in pores on the
surface of the particular carbon particle, by adsorption and size exclusion.
What is needed is a more efficient filter medium capable of removing
microorganisms to EPA standards from substantially larger quantities of water
per
unit filter medium than was heretofore possible.
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SUMMARY OF THE INVENTION
The present invention solves the problems described above by providing a
filter medium capable of removing a large percentage of microorganisms from a
fluid
such as water. The filter medium includes particles of activated carbon (e.g.,
granular
activated carbon (GAC), powdered activated carbon, etc.). The filter medium
also has
particles of a substantially insoluble inorganic material having an
isoelectric point
greater than the pH of the fluid being filtered, typically above about 7.0 pH,
preferably greater than 9.0 pH, and even more preferably greater than 10.0 pH.
A
binder binds the particles of activated carbon and particles of inorganic
material. The
binder has a melt index of less than about 1 gram per 10 minutes as determined
by
ASTM D1238 at 190 degrees C. and 15 kilograms load, such that the binder will
become tacky at elevated temperatures without becoming sufficiently liquid to
substantially wet the particles of activated carbon and inorganic material.
When
water at a pH less than the isoelectric point of the inorganic material is
passed through
the filter, the high-isoelectric-point inorganic material imparts a positive
charge on the
filter surface, thereby attracting and adsorbing negatively charged
microorganisms to
the filter surface by electrostatic forces.
In one embodiment, the inorganic material is present in an amount ranging
from about 25 weight percent to about 45 weight percent of the total weight of
the
filter medium. In another embodiment, the inorganic material is present in an
amount
ranging from about 40 weight percent to about 50 weight percent of the total
weight
of the filter medium. A preferred inorganic material is a magnesium compound
such
as magnesium hydroxide or magnesium oxide.
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The binder can be an ultra high molecular weight polymer having a molecular
weight greater than about 4 million. For example, the binder can be ultra high
molecular weight polyethylene. In one embodiment, the melt index of the binder
is
less than about 0.1 grams per 10 minutes as determined by ASTM D 1238 at 190
degrees C. and 15 kilograms load. In one embodiment, the binder is present in
an
amount ranging from about 20 weight percent to about 35 weight percent of the
total
weight of the filter medium.
Another embodiment implements inorganic particles of differing particle size
distributions to optimize structural integrity of the filter medium as well as
contaminant removal. Particularly, the material with the larger particle size
distribution results in a filter with greater structural integrity whereas the
material
with the smaller particle size distribution provides greater contaminant
removal.
Note that additional adsorptive and/or binding materials may be added other
than the activated carbon, inorganic material, and binder to alter the
properties of the
filter.
To further improve the antimicrobial effectiveness of the filter, and to
extend
the life of the filter, an antimicrobial material can be incorporated into the
filter to
prevent biofilm growth. The use of a biocidal material in combination with the
high
isoelectric point material provides a trap-and-kill mechanism for
microorganism
removal. Illustrative antimicrobial materials are silver-containing materials.
One type
of silver-containing material implements soluble silver material to kill
microbes by
entering the water surrounding the filter medium via ion exchange. Another
type of
silver-containing material kills microorganisms on contact but does not elute
or leach
into solution. Particularly useful are materials comprising a polymeric matrix
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impregnated with a water-insoluble antimicrobial compound such as a silver
halide (e.g.,
silver iodide).
In one embodiment, 75 grams of the filter medium performs a greater than 1 x
104
plaque forming units/milliliter reduction of viruses in 100 gallons of water
passing through
the filter medium.
The mean pore size of pores formed by the particles and binder is can be
between 0.01
micron and 10 microns, and preferably between 0.1 micron and 1 microns.
The filter medium can be shaped into any desired form, such as in the form of
a block
or sheet. For example, the filter medium can be cylindrically shaped with an
outer diameter of
less than about 4 inches and a maximum length between ends of the filter
medium of less than
about 3 inches.
The filter medium can be formed by mixing particles of activated carbon,
particles of
inorganic material, and the binder. The mixture is heated such that the binder
becomes tacky
without becoming sufficiently liquid to substantially wet the particles of
activated carbon and
inorganic material.
The filter medium is adaptable for use in a filtration device having a
housing. The
filtration device may be of the type adapted to be mounted to a water source,
a pitcher, a
bottle, water dispenser, etc. A pump can be coupled to the housing for
controlling the flow of
the fluid through the filter medium.
The embodiments described herein have particular applicability for countering
a
bioterrorism act, by enabling removal of potentially life-threatening
microbials introduced
into a water supply by a terrorist.
Accordingly, in one aspect, the present invention resides in a filter medium
for use in
removing microorganisms, comprising: particles of activated carbon; particles
of a insoluble
inorganic material having an isoelectric point greater than a pH of a fluid
being filtered; a
binder for binding the particles of activated carbon and particles of
inorganic material;
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wherein the insoluable inorganic material is admixed together with the
particles of activated
carbon; and wherein a first portion of the particles of inorganic material
have a first mean
particle size distribution and a second portion of the particles of inorganic
material have a
second mean particle size distribution, the first mean particle size
distribution being larger
than the second mean particle size distribution, and wherein the filter medium
is formed in a
block or sheet and wherein the binder has a melt index of less than 1 gram per
10 minutes as
determined by ASTM D1238 at 190 degrees C. and 15 kilograms load.
In another aspect, the present invention resides in a filter medium for use in
removing
microorganisms, comprising: particles of activated carbon; particles of
magnesium hydroxide
present at 40% to 50% by weight; and a binder for binding the particles of
activated carbon
and inorganic material, wherein magnesium hydroxide is admixed together with
the particles
of activated carbon and wherein, the binder has a melt index of less than 1
gram per 10
minutes as determined by ASTM D1238 at 190 degrees C. and 15 kilograms load,
and
wherein the filter medium is formed in a block or sheet.
In yet a further aspect, the present invention resides in a filter medium for
use in
removing microorganisms, comprising: particles of activated carbon; particles
of a insoluble
inorganic material having an isoelectric point greater than a pH of a fluid
being filtered; a
binder for binding the particles of activated carbon and particles of
inorganic material; an
antimicrobial material, and wherein the filter medium is formed from a block
or sheet and
wherein the binder has a melt index of less than I gram per 10 minutes as
determined by
ASTM D1238 at 190 degrees C. and 15 kilograms load.
BRIEF DESCRIPTION OF THE DRAWINGS
For a fuller understanding of the nature and advantages of the present
invention, as
well as the preferred mode of use, reference should be made to the following
detailed
description read in conjunction with the accompanying drawings.
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Figure 1 is a chart showing the microorganism-removing capabilities of filter
media
having varying the concentration of magnesium hydroxide.
Figure 2 illustrates a block of the filter medium in cylindrical form.
Figure 3 illustrates the filter medium in the form of a sheet.
Figure 4 is a chart showing the removal of PRD-1 and MS-2 (representing
rotavirus
and poliovirus, respectively) and Klebsiella Terregina (representing bacteria)
for 120+
gallons.
Figure 5 is a table showing the results of seven experiments testing the
removal of
bacteriophage MS-2 by cylindrically shaped filter media with varying
concentrations of
carbon, binder and magnesium hydroxide.
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BEST MODES FOR CARRYING OUT THE INVENTION
The following description includes the best embodiments presently
contemplated for carrying out the present invention. This description is made
for the
purpose of illustrating the general principles of the present invention and is
not meant
to limit the inventive concepts claimed herein.
The present invention provides a filter medium capable of removing
microorganisms (bacteria, viruses, cysts, etc.) from large quantities of
water, and in
compliance with the EPA standards mentioned above. The inventors have
surprisingly found that a synergistic effect occurs when inorganic materials
with high
isoelectric points and activated carbon are bound by a low melt index binder.
While not wishing to be bound by any particular theory, it is believed that
the
high-isoelectric-point inorganic material tends to adhere to the surface of
the binder
and possibly the carbon as well, and acts as an adsorption-enhancing material
that
imparts a positive charge on the surface of the pores of the filter medium
when in the
presence of a fluid having a pH lower than the isoelectric point of the
inorganic
material. Most microorganisms of concern have a negative surface charge. For
example, most types of bacteria have a membrane layer of phospholipids that
give the
bacteria a negative charge. When negatively charged viruses and bacteria pass
through the pores of the filter medium, they are attracted to the positively
charged
surface of the filter medium and become adsorbed to the surface by
electrostatic
interactions. Because most, if not all, of the surfaces of the pores
themselves become
charged, the filter medium has more charged sites with which to adsorb
microorganisms, as well as an overall increase in electrostatic forces. Thus,
the filter
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medium is able to remove substantially more microorganisms per unit weight of
filter
medium and/or per unit volume of filter medium than was heretofore possible.
As mentioned above, the adsorption-enhancing inorganic material has a high
isoelectric point. A high isoelectric point is preferred, because if a fluid
having the
same or higher pH as the inorganic material is introduced to the filter
medium, the
microorganisms adsorbed to the filter medium will become detached and exit the
filter. Thus, the "high isoelectric point" of the inorganic material is above
a threshold
pH suitable for the desired use. In other words, the isoelectric point of the
inorganic
material should be higher than the pH of the fluid being filtered.
Additionally, the
greater the isoelectric point of the inorganic material, the greater the
surface charge of
the filter at neutral pH. One skilled in the art will understand that the pH
of the fluid
to be filtered can be readily determined. For example, if the fluid is water
with a pH
varying from 6.9 to 7.1, the isoelectric point of the inorganic material
should be
higher than 7.1. For commercial filters, a higher isoelectric point may be
required.
For example, one test protocol of the EPA requires washing the filter with
water
having a pH of 9Ø The filter media disclosed in the EXAMPLES section below
do
not lose microorganisms under these conditions.
The term "particles" as used herein can refer to particles of any shape, as
well
as short pieces of strands or fibers, hollow or porous particles, etc. The
term "fluid"
includes aqueous fluids such as water, or gases and mixtures of gases.
One method of making the filter medium is by mixing and heating particles of
activated carbon, particles of inorganic material, and particles of binder in
a mold of
the desired shape, then compressing the mixture to encourage binding and to
adjust
the pore size. An illustrative range of compression is between about 1 percent
to
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about 30 percent reduction of the volume of the filter medium. This method is
described in more particularity in the EXAMPLES section below, but is
presented
briefly here to provide a context for the following description.
The pore size of the filter medium is important, as it is desirable to place
the
microorganism in close proximity to the adsorbent surface of the filter
medium. In
general, the smaller the pore size, the more readily the microorganisms become
adsorbed to the surface of the filter medium. This is because as pore size
decreases,
the microorganisms come into closer proximity to the adsorptive surface as
they pass
with the fluid through the pores of the filter medium. The pore size can be
made
small enough to physically filter out more oocysts (e.g., cryptosporidium) and
cysts
(e.g., Giardia muris and Giardia lamblia) than required by the EPA standards
discussed above. An exemplary range of mean pore sizes is 0.01 to 10 microns.
In
one embodiment, the mean pore size of the filter medium is within the range
0.1 to I
microns.
The term "low melt index binder" preferably refers to binders that have very
low to virtually no melt index (melt flow rate), meaning that when heated the
binders
will become tacky at elevated temperatures without becoming sufficiently
liquid to
significantly wet the surfaces of the carbon particles and the particles of
inorganic
material, i.e., will not flow. The use of a low melt index binder in the
present
invention maximizes the effectiveness of the inorganic material. Because the
binder
becomes tacky rather than fluid, the activated carbon and inorganic material
adhere to
the surface of the binder rather than becoming encased in the binder during
formation
of the filter medium into its final shape. This maximizes the exposed surface
area of
the activated carbon and inorganic material, and thus their effectiveness.
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The melt flow rate or melt index is determined by ASTM D1238 or DIN
53735 at 190 degrees C. and 15 kilograms. The amount of material that flows
through the die should be less than about I gram/I 0 minutes, in one
embodiment is
less than 0.5 grams/10 minutes and in another embodiment is less than 0.1
gram/10
minutes. The most preferred binder is an ultra high molecular weight, high
density
polyethylene. The high molecular weight gives rise to the restricted flow
properties
of the melted material which is so important to this aspect of the invention.
The
following table shows a comparison of selected properties of the ultra high
molecular
weight, high density polyethylene with other types of polyethylene binders.
Table 1 - Binders for carbon blocks
Melt Temp. C Melt index* Extrudable
LDPEa 102-110 5-70 yes
HDPE b 134 10.5 no
VHMWPEc 135 1.8 no
UHMWPEd 135 <0.1 no
The melt index of a material is measured at 190 C with a 15 Kg weight and the
units are in
grams/10 minutes.
a) Low Density Polyethylene
a) High Density Polyethylene
c) Very High Molecular Weight Polyethylene (VHMW PE)
d) Ultra High Molecular Weight Polyethylene (UHMW PE)
One ultra high molecular weight, high density polyethylene has a density of
0.930
grams per cubic centimeter and a melt index of less than 0.1 grams per ten
minutes as
determined by ASTM D1238 at 190/15. It has a vicat softening point of
approximately 80 degrees centigrade and a crystalline melting point of about
135
degrees C. (The Vicat softening point, measured by ASTM D 1525 (ISO 306)
procedures, is the temperature at which a flattened needle of 1 mm2 cross
section, and
under a specified constant load, penetrates a specimen of the plastic to a
depth of I
mm. It is useful as a rough comparative guide to a resin's resistance to
elevated
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temperatures.) Another ultra high molecular weight, high density polyethylene
has a
density of 0.935 grams per cubic centimeter and a melt index of less than 0.1
grams
per ten minutes as determined by DIN 53735 at 190/15. It has a vicat softening
point
of approximately 74 degrees centigrade and a crystalline melting range of 135
to 138
degrees C. Such polyethylenes have a molecular weight higher than 4 million,
and
typically from about 4 to about 6 million.
The temperature at which the binder used becomes sufficiently tacky to adhere
to the carbon particles may vary depending on the specific polymer used. With
the
high molecular weight, high density polyethylene, the binder and carbon
particles can
be processed at a temperature of from about 175 degrees C. to about 205
degrees C.
for about 2 hours.
The percentage of binder used to bind the activated carbon and inorganic
material can be in the range of about 10 to about 40 weight percent, in
another
embodiment in the range of about 20 to about 35 weight percent, and in yet
another
embodiment about 25 to about 30 percent by weight based on the total weight of
the
filter medium. These ranges provide enough binder to hold the particles of
carbon
and inorganic material together, while not blocking the surface pores of the
carbon
particles.
The binder can be utilized in particulate or powder form so that it can be
uniformly mixed and dispersed with the carbon particles. The use of polymer
binders
allows one to bind the particles of carbon and inorganic material together
without
excessively wetting the particles when melted and thereby effectively
occluding much
of the surface area of the particles.
An illustrative mean particle size of the binder is in the range from about 80
microns to about 140 microns. Note, however, that the mean particle size of
the
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binder used is not critical and can be made larger or smaller based on the
desired
properties of the filter medium. For example, smaller particle size can be
used to
make the pore size smaller with a resultant increase in contaminants captured
and
reduction in flow rate.
The mean particle size of the carbon used is not critical and can be made
larger or smaller based on the desired properties of the filter medium. For
example,
smaller particle size carbon can be used to make the pore size smaller with a
resultant
increase in contaminants captured and reduction in flow rate.
The percentage of carbon in the filter medium can be in the range of about 30
to about 50 weight percent, in another embodimment in the range of about 37.5
to
about 45 weight percent, and in another embodiment from about 40 to about 45
percent by weight based on the total weight of the filter medium.
As mentioned above, we have surprisingly found that a synergistic effect
occurs when substantially insoluble inorganic materials with high isoelectric
points
and activated carbon are bound by a low melt index binder. This effect is
clearly
illustrated in Table 2 below comparing filters prepared with the same amount
of
magnesium hydroxide and binders of different melt indexes.
Table 2 - Removal of bacteriophage MS-2 by faucet mount filters with
binders with different melt indexes (MIs)
Unit ID 74-A 74-C 77-C 78-F 168-C2 171-Al
Binder EVA EVA VHMW PE VHMW PE UHMW PE UHMW PE
MI 5-70 5-70 1.8 1.8 <0.1 <0.1
0 2.525 1.039 4.396 >4.697 >6.099 >6.099
30(25%) 1.358 0.245 2.412 2.869 >6.035 >6.035
60 50% 0.933 0.243 2.132 1.832 >6.054 >6.054
0
Da y2
0 stagnation NA NA NA NA >6.088 >6.088
90(75%) 0.939 0.228 0.609 2.983 >5.906 >5.906
120 100% 0.431 0.208 0.169 0.81 >5.937 >5.937
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Illustrative inorganic materials have an isoelectric point above 7 pH,
preferably above
9 pH, and ideally above 10 pH. The percentage of inorganic material in the
filter
medium can be in the range of about 10 to about 50 weight percent, in another
embodiment in the range of about 25 to about 45 weight percent, and in yet
another
embodiment from about 28 to about 40 percent by weight based on the total
weight of
the filter medium.
The following table lists several insoluble inorganic materials that can be
implemented in the filter medium of the present invention. Note that the
inorganic
materials can be added to the filter medium individually or in combination
with each
other.
Table 3 - Insoluble inorganic materials
Compound Isoelectric point
Magnesium hydroxide, Mg(OH)2 10.5
Magnesium oxide, MgO 12.5
Titanium dioxide, Ti02 6.6-8.9
Zirconium dioxide, Zr02 6.7-7.4
Aluminum oxide, A1203 6.8-9.2
Barium oxide, BaO 13.3
Calcium oxide, CaO 12.9
Cesium oxide, Ce203 9.8
Iron (II) oxide, FeO 11.8
Iron (III) oxide, Fe203 9.3
Zirconium oxide, Zr02 11.3
Hydroxyapatite, Ca5(HPO4)30H <6.9
Chromium oxide, Cr203 9.2
Cobalt oxide, Co304 8-9
Chrysotile asbestos, M 3Si2O5(OH)4 >7
The preferred inorganic materials are magnesium compounds with an
isoelectric point above 9 pH, and more preferably above 10 pH, such as
magnesium
hydroxide and magnesium oxide. The most preferred material is magnesium
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hydroxide, as it is non-toxic to humans and exhibits superior adsorptive
properties. A
preferred mean particle size for magnesium hydroxide is in the range of about
5 to
about 14 microns, but again, larger or smaller particle sizes can be used.
When selecting a particle size for the inorganic material, several things
should
be considered. Smaller particles provide a greater surface to volume ratio,
and
consequently exhibit increased effectiveness at microbe removal. However, the
use of
smaller particle sizes results in an increased pressure drop, and
consequential lower
flow-through rate. On the other hand, larger particle sizes allow greater
structural
integrity.
It has surprisingly been found that by mixing inorganic particles of differing
particle size distributions, the structural integrity of the filter medium as
well as
contaminant removal can be optimized. Particularly, the material with the
larger
particle size distribution results in a filter with greater structural
integrity whereas the
material with the smaller particle size distribution provides greater
contaminant
removal.
In generally, about 85% or more of the inorganic particles should have a mean
particle size distribution that is at least twice as large as the mean
particle size
distribution of the remaining (15% or less) inorganic particles. In one
embodiment
implementing magnesium hydroxide, a first portion of the magnesium hydroxide
particles have a particle size distribution of greater than about 10 m while
about 5%
(+/- 2.5%) by weight of the magnesium hydroxide particles have a particle size
distribution of less than about 10 m. In one example, some of the magnesium
hydroxide particles have a size distribution of about 12 - 15 m, while about
5% (+/-
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2.5%) by weight of the magnesium hydroxide particles have a particle size
distribution of about 4-5 m.
The following table illustrates how varying the concentration of magnesium
hydroxide in the filter medium affects the microorganism-removing capacity of
the
filter medium. This experiment used carbon blocks tested with faucet mount
system
at 60 PSI. As shown, even small amounts of magnesium hydroxide remove
microorganisms. Increasing the amount of magnesium hydroxide in the filter
medium
improves its microorganism-removing properties. Note also that the log
reduction
flattens out at about a 30 weight percent concentration of magnesium hydroxide
in
this particular configuration.
Table 4 - Removal of bacteriophage MS-2 by faucet mount filters with different
concentrations of magnesium hydroxide
60 gal
C 3.3% magnesium hydroxide 1.7 log
reduction
B 15% magnesium hydroxide 2.4
D 30% magnesium hydroxide 3.8
F 40% magnesium hydroxide 3.8
Control - no magnesium 0.7
hydroxide
Experiments have found that amounts of inorganic material at the upper end of
the range (e.g., 40-50 weight percent inorganic material) provide the longest
filter life,
with respect to microbe removal. Data from long-term studies show that
increasing
the magnesium hydroxide content to 50% by weight extends the life of the
filter to
beyond 4 months with greater than 4 log reduction of viruses and without a
significant
reduction in virus removal properties.
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Figure 1 is a chart 100 showing how varying the concentration of magnesium
hydroxide in the filter medium affects the microorganism-removing capacity of
the
filter. This experiment used carbon blocks tested with faucet mount system at
60 PSI.
The influent liquid was water containing a 4.3 log (lx 1043 PFU/ml)
concentration of
MS-2, where PFU = Plaque Forming Units. PFUs represent an estimate of the
concentration of a bacteriophage solution, determined by mixing the
bacteriophage
with a solution of susceptible bacteria, plating, incubating, and counting the
number
of plaques present on the bacterial lawn, with each plaque representing a
viable
bacteriophage. For example, if a phage stock solution has 1010 PFU/ml, it
means that
every ml of this stock has 1010 phage particles which can form plaques.
As shown in Figure 1, at 20 weight percent magnesium hydroxide and above,
a 6 log reduction of MS-2 is still achieved after 120 gallons of contaminated
water
introduced to the filter. For filters with no magnesium hydroxide and 10
weight
percent magnesium hydroxide, the effectiveness of the filter medium is
inversely
proportional to the volume of water filtered.
Table 4 (below) shows a relative comparison of the microorganism-removing
capabilities of materials having different isoelectric points. As shown, the
materials
with higher isoelectric points removed significantly more MS-2 than materials
with
lower isoelectric points. The experiment was conducted by swirling the given
amounts of the compounds in water spiked with approximately 9.0 x 105 PFU/ml
of
bacteriophage MS-2 for 5 minutes then filtered through a 0.45 micron syringe
filter,
previously treated with 5 ml of 1.5% beef extract solution (pH 7.2, 0.05 M
glycine).
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Table 5 - Isoelectric point and MS-2 removal
Mineral (grams tested) Isoelectric point (pH) Removal (Log reduction)
Mg(OH)2 (2.0) 10.5 4.9
Magnesium silicate (5.0) -3 2.7
M (2.0 12.5 3.68
MgCO3 (Magnetite) 5.5 N/A*
A1203 (2.0) 9.4 2.71
AlO(OH) (Boehemite) (5.0) 9.7 4.67
A12Si2O5 OH 4 (Kaolinite) 1.5-3.5 N/A
a-Fe203 (hematite) 7.5 N/A
FeO(OH) (2.0) N/A 2.49
Cr2O3 9.2 N/A
Sn02 4.5 N/A
(CaF)Ca4(PO4)3 (Apatite) (5.5) 4-6 3.08
TiO2 (2.1 6.6-8.9 0.96
TiO(OH) (2.0) N/A 1.93
ZrO2 11.3 N/A
Sepiolite (2.0) N/A 2.22
Saponite (2.0) N/A 0.125
* Indicates data not gathered.
The following table shows the results of a batch study comparing the
effectiveness of magnesium hydroxide to that of magnesium silicate. The
experiment
was conducted using 20 ml of water that was contaminated with MS-2
bacteriophage.
1, 3, or 5 grams of the material was added (as indicated below) to the
contaminated
water. The mixture was then mixed and the magnesium compound and filtered off
to
determine how much virus the material removed. As shown, magnesium hydroxide
works significantly better than magnesium silicate, primarily because
magnesium
hydroxide has a higher isoelectric point and superior adsorptive properties.
Table 6 - Removal of bacteriophage MS-2 by powdered magnesium silicate and
magnesium hydroxide
MS-2 log 10 removal
I magnesium silicate 2.0
5 magnesium silicate 2.7
1 magnesium hydroxide 2.9
3 magnesium hydroxide 3.8
5 g magnesium hydroxide 4.4
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As mentioned above, magnesium oxide can also be used. Magnesium oxide
hydrates in the presence of water, forming a magnesium hydroxide surface
layer.
However, starting with magnesium hydroxide as the adsorptive material results
in
better performance, in part because of the length of time for magnesium oxide
to
completely hydrate, and also because magnesium oxide is slightly soluble in
water
and so can wash out of the filter.
Additional materials having high isoelectric points include titanium dioxide,
iron oxides, aluminum oxides, barium oxides, calcium phosphate and alumina-
coated
silica. Other magnesium-containing minerals include: antigorite,
clinochrysotile,
lizardite, orthochrysotile and parachrysotile, clinochore, hectabrite,
vermiculite,
ripidolite, saponite, and sepiolite.
The filter medium can be created in virtually any desired shape. Figure 2
illustrates a block 200 of the filter medium in cylindrical form, and which is
particularly adapted to faucet mount systems such as the system found in U.S.
Patent
No. 6,123,837 to Wadsworth et al. and to pitcher systems such as the system
found in
U.S. Patent No. Des. 398,184 to Silverberg et al., each of which are herein
incorporated by reference. A standard-sized cylindrical filter block for point-
of-use
systems is about 4 inches in length or less between the ends 202, 204 of the
block
200, and has an outer diameter (OD) of less than about 4 inches and an inner
diameter
(ID) of less than about one inch. One embodiment is less than about 3 inches
in
length and has an outer diameter of less than about 2.5 inches and an inner
diameter
of less than about 0.5 inch.
Figure 3 illustrates the filter medium in the form of a sheet 300. The sheet
300
can then be placed in a housing and a fluid such as water passed therethrough.
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The filter medium can be used in a wide variety of applications. As
mentioned above, one use to which it is particularly adaptable is for
pressurized and
gravity-flow applications such as faucet-mount filters and pitcher filters, as
well as in
water dispensers (e.g., standalone water coolers), such as the type having a
bottle
receiving portion for receiving a bottle of water, a filtration portion for
filtering water
received from the bottle, and a filtered water outlet (e.g., spigot). Other
applications
are use in granular filters, high volume "under-the-sink" or commercial-type
filters,
and refrigerator filters. Yet another application is use in air filtration
systems.
The filter medium can also be made for/used in portable applications, such as
for use in filters for camping, bottles with filters, emergency kits, etc. The
filter
medium is also useful in med-evac systems, allowing filtration of water in the
field to
rehydrate soldiers. In portable uses, the filter medium can be formed in a
block
smaller than the cylindrical block disclosed above for 5, 15, 30 gallons, etc.
The filter medium would also be particularly effective at purifying water
contaminated by an act of bio-terrorism. For example, the faucet-mount system
could
allow users to continue to use a contaminated public water supply until fresh
water
were made available. Similarly, portable versions (pitchers, bottles, bags,
etc. with
the filter medium attached) can be stored in homes and businesses, stored in
emergency kits, carried in automobiles, etc. Further, such portable versions
can be
made available and/or distributed to people rather quickly in response to a
bio-
terrorism attack.
A hand-pump, foot-pump, battery-pump, solar-powered pump, etc. may be
coupled to any of the embodiments described herein to pressurize the influent
water
and/or reduce pressure in the effluent stream to draw water through the filter
medium.
EXAMPLES
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Example I
Following is an example of a preferred procedure for forming a porous block
of filter medium. Granular activated carbon with a mean particle size (outer
diameter)
of about 100 microns is mixed with particles of an ultra high molecular weight
polyethylene binder (and/or other binder) having a mean particle size in the
range of
about 120 to 140 microns, a melt index of less than 1, and a melting
temperature of
about 135 C. Particles of magnesium hydroxide (and/or other inorganic
material) are
blended into the mixture of carbon and binder. The preferred particle size of
the
magnesium hydroxide is in the range of about 5 to about 14 microns. See Table
6 and
Figure 5 for illustrative compositions of the mixture. The mixture of
magnesium
hydroxide, carbon, and binder are thoroughly mixed in a blender or other
suitable
mixing device for a period of time sufficient to create a substantially
uniform
dispersion of materials in the mixture. The blended mixture is heated and
compressed
in a stainless steel mold having the desired shape. The material in the mold
is heated
to about 473 degrees F (245 C.) for about 40 minutes. The heating makes the
binder
sticky so that it binds the magnesium hydroxide and carbon particles into a
porous
block. The magnesium hydroxide may also adhere to the carbon.
Formation of the filter medium by extrusion is also possible, though is not
desirable for the preferred embodiments as the preferred materials require
higher
heating temperatures, which require a longer extruder heating zone resulting
in a very
high backpressure on the extruded block. As a result, these extruded blocks
have high
pressure drops and low flow-through rates.
Example 2
Following is an example of a preferred procedure for forming a porous
cylindrically-shaped block of filter medium. Particles of magnesium hydroxide,
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activated carbon, and binder are blended into a mixture. The blended mixture
is
heated and compressed in a stainless steel mold having the desired shape. The
material in the mold is heated to about 473 degrees F (245 C.) for about 40
minutes.
The ends of the block can be capped using any suitable adhesive, such as
polymeric glue. The block can then be placed in a housing that directs
influent water
to an outer periphery of the block so that the water passes through the block
into the
center chamber of the block and is then expelled through one of the end caps
as
filtered water. Note that the flow through the filter may also be reversed.
Figure 4 is a chart 400 showing the reduction of microorganisms using a filter
medium such as the one described in Example 2 with about 30 weight percent
magnesium hydroxide, about 25 weight percent ultra high molecular weight
polyethylene binder, and about 45 weight percent activated carbon and about a
pressure drop of 5.2 atm across the filter medium. As shown, a greater than 4
log
reduction of PRD-1 and MS-2 (representing rotavirus and poliovirus,
respectively) is
achievable for 120+ gallons of water. A greater than 7 log reduction of
Klebsiella
Terregina (representing bacteria) is also achievable for 120+ gallons.
Examples 3-5
The following table illustrates the results of experiments testing the removal
of
bacteriophage MS-2 by cylindrical filters with different concentrations of
carbon,
binder and magnesium hydroxide. The influent liquid for the 30 and 90 gallon
runs
was water containing a 4.3 log (1x1043 PFU/ml) concentration of MS-2, where
PFU =
Plaque Forming Units. The influent liquid for the 120 gallon runs was water
containing a 4.5 log (1x1045 PFU/ml) concentration of MS-2. The filter blocks
themselves were about 75 g carbon blocks of the composition shown with
dimensions
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of about 2.94 inches in length, about 1.84 inch outer diameter and about 0.5
inch inner
diameter tested on a faucet mount system at 60 PSI.
Table 7 - Removal of bacteriophage MS-2 by cylindrical filters with different
concentrations of magnesium hydroxide
MS-2 log 10 removal
30 gal 90 gal 120 gal
A 37.5% magnesium > 4.3 > 4.3 > 4.5
hydroxide
37.5% Activated Carbon
25% Binder
B 30% magnesium hydroxide > 4.3 > 4.3 > 4.5
45% Activated Carbon
25% Binder
C 28% magnesium hydroxide > 4.3 > 4.3 > 4.5
42% Activated Carbon
30% Binder
Control - no magnesium 1.4 0.2 0.1
hydroxide
Examples 6-12
Figure 5 is a table 500 showing the results of seven experiments testing the
removal of bacteriophage MS-2 by cylindrical filters with varying
concentrations of
carbon, binder and magnesium hydroxide. The influent liquid was water
containing a
4.755 log (1x104755 PFU/ml) concentration of MS-2, where PFU = Plaque Forming
Units. The filter blocks themselves were about 75 g carbon blocks of the
composition
and compression shown with dimensions of about 2.94 inches in length, about
1.84
inch outer diameter and about 0.5 inch inner diameter tested on a faucet mount
system.
One particular parameter of interest is the concentration of inorganic cation
(of
high isoelectric point) times the pressure drop. As shown in Figure 5, the
performance of the filters depends on the amount of Mg and the flow rate (or
pressure
drop) of the filter. The pressure drop shown in Figure 5 is measured in pounds
per
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square inch (psi). The inorganic cation is calculated as the percentage of
Mg(OH)2 in
the filter times the ratio of Mg to Mg(OH)2. A sample calculation for Filter A
follows:
[0.375 g Mg(OH)2 / g filter] x [0.417 g Mg / g Mg(OH)2] x [4.15
lb/in2]
= 0.649 [g Mg = lb] / [g filter = in2] Equation I
The pressure drop measurements can be taken by measuring the resistance of
air flow through the filter at a given pressure, and more particularly, by
measuring the
differential pressure of air flow through the filter with the tared pressure
being the
pressure of the air flow without a filter.
As described herein, the filter medium meets EPA standards for viruses (4 log
reduction (99.99%) required for viruses). In fact, the filter medium can
achieve near
100% microorganism removal at over 120 gallons. In comparative
experimentation,
the filter disclosed in International Patent Application WO 02/076577 to
Hughes was
only effective up to about 1.33 gallons; at 30 gallons, very poor results were
obtained.
Thus, the virus-removing properties of the filter medium disclosed herein can
process
nearly 100 times the volume, and thus may have almost 100 times the life, as
other
systems.
Using inorganic materials with high isoelectric points such as magnesium
hydroxide is instrumental in the removal of the microorganisms from water, but
does
not effectively kill the trapped organisms. In other words, the microbes
remain viable
on the surface of the filter medium. If the filter is used infrequently, or
for an
extended time period, the trapped microorganisms may form a biofilm over the
active
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filter surfaces, reducing the performance of the filter over time by covering
up the
active sites. To further improve the antimicrobial effectiveness of the
filter, and to
extend the life of the filter, an antimicrobial material can be incorporated
into the
filter to prevent biofilm growth. The use of a biocidal material in
combination with
the high isoelectric point material provides a trap-and-kill mechanism for
microorganism removal.
Illustrative antimicrobial materials are silver-containing materials. One type
of
silver-containing material implements soluble silver material to kill microbes
by
entering the water surrounding the filter medium via ion exchange.
An effective amount of the silver-containing material is mixed with the other
materials and formed into the filter media as described above. In general, the
silver-
containing material is added at about 1-10% by weight based on the total
weight of all
materials. Experimentation has shown that the addition of about 1.5-5.0% by
weight
of silver-impregnated zirconium phosphate significantly extends the life of
the filter.
An illustrative mean particle size of the silver-containing material is
between
about 1 m-200 m or larger, and in another embodiment is between about 75 m-
I00 m. Note that the particle size is not as critical here, as the overall
mass of silver-
containing material present is the important parameter.
Another type of silver-containing material kills microorganisms on contact but
does not elute or leach into solution. Particularly useful are materials
comprising a
polymeric matrix impregnated with a water-insoluble antimicrobial compound
such as
a silver halide (e.g., silver iodide or silver chloride). Illustrative
polymeric materials
are positively charged polymers containing quaternary ammonium compounds. The
silver is transferred directly to a microorganism contacting the material,
causing a
toxic accumulation that results in cell death. These materials provide
sustained
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antimicrobial action in spite of frequent washing. One such material is
SURFACINE,
available from Lonza Inc., 17-17 Route 208, Fair Lawn, NJ 07410.
An effective amount of particles coated with the non-eluting silver-containing
material is mixed with the other materials and formed into the filter medium
as
described above. Note that the substrate carrying the non-eluting silver-
containing
material can be the activated carbon particles. In general, the non-eluting
silver-
containing material is added at about 0.1-10% by weight based on the total
weight of
all materials in the filter medium.
While various embodiments have been described above, it should be
understood that they have been presented by way of example only, and not
limitation.
Thus, the breadth and scope of a preferred embodiment should not be limited by
any
of the above-described exemplary embodiments, but should be defined only in
accordance with the following claims and their equivalents.
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