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Ultra-Pure Water Production
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Introduction Argonide has developed and patented a new filter media, NanoCeram®,
whose active component is an alumina (AlOOH) fiber two nanometers in diameter. The nano alumina fiber is highly electropositive, and will attract and
retain particles, no matter how small. The nano fibers are dispersed throughout a microglass fiber matrix resulting in a media with 2 micron average
pore size and with water flux typically of that pore size. However, the media functions as if it were a 0.03 micron pore size filter. A single
layer 0.8 mm thick retains greater than 99% of 0.03 µm monodisperse latex spheres or 0.025 µm size MS2 virus, justifying an Absolute rating of 0.03 µm.
It can be pleated to produce a high surface area cartridge (“Superfilter”) with a high filtration efficiency, and with a dirt holding capacity tens or
twenty times greater than microglass, meltblown or membrane filters. |
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Characteristics of NanoCeram media -
The predominant mode of filter liquids is mechanical processes such as sieving,
interception, impaction and diffusion. Wound string depth filters are typically rated down to about 1 micron, with filtration efficiencies
ranging up to only about 95%. Pleated microglass or polymeric filter media and microporous and ultraporous membrane are better suited for
filtering particles smaller than about 1 micron. Absolute 0.2 µm membrane filters are capable of retaining all types of bacteria with very
high retention (>99.9999%) but they are transparent to much smaller particles such as most virus.
Electrokinetic adsorption is used for filtering particles from both air
(“electrets”) and water. Most colloidal particles in water are negatively charged as a result of differences in electrical potential
between the water and the particle phases. This charge is due to an unequal distribution of ions over the particle surface and the surrounding
solution. Asbestos fiber filters (which are electropositive) had been used for more than a century until it was found to be a health
hazard. So far there has been little success in finding an asbestos substitute. Membranes have been modified to provide some
electropositive functionality but their flow resistance is very high and because they are surface filters, they are prone to clogging.
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NanoCeram® media consists of
nano alumina (boehmite) fibers 2
nanometers in diameter (Figure 1) that
are distributed onto a microglass fiber
scaffolding and formed into a non-woven
media. Boehmite (AlOOH) is an
ingredient in over the counter analgesic
medicines. |
Fig.1- TEM Micrograph
of Alumina Nanofibers
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The nano alumina is attached to the
microglass rather than forming
agglomerates that would clog the
structure. Cellulose and polymeric
fibers are added to strengthen the media
and increase its flexibility so that it
can be pleated. The resulting media
has a maximum pore size of seven microns
(as measured by bubble pressure), with an
average of two µm.
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Table 1 shows zeta potential values
for NanoCeram® media with a thickness between 1.5 and 2 microns, comparing MS2 virus (~25 nm size)
adsorption as a function of nano alumina content. The media becomes highly electropositive when the nano
alumina content exceeds about 15 weight percent, and is then capable of adsorbing > 99.9999% of virus as well as
larger particles.
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Nano-Ceram® content, wt-%
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True zeta potential
(z
true) , mV |
Surface
conductance
ls
, nS |
MS2 % removal |
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0 |
-35 |
0.92 |
8 |
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5 |
-12 |
0.06 |
29 |
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10 |
7 |
0.10 |
94 |
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15 |
23 |
0.55 |
>99.9999 |
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25 |
32 |
0.67 |
>99.9999 |
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40 |
29 |
0.42 |
>99.9999 |
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50 |
23 |
0.3 |
>99.9999 |
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Table 1 - Zeta potential and Specific Surface
Conductance of NanoCeram®
filters |
Retention of Micron and Sub-micron Particles
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The media can be wound
around a mandrel to produce a multilayer depth filter or can be pleated. The pleated
construction has greater area and therefore allows greater flux than the depth filter.
We assembled a pleated filter 2” (63 mm) diameter X 4.5” (114 mm) high, consisting of a
single layer of filter media (0.8 mm thick) that had a filter area of 800 cm2. We
challenged it with AC fine test dust (1 µm) starting at 15 NTU (Nephelometric Turbidity
Units) and at a flow of 1 liter/min. |
Fig.1-
Spent pleated cartridge
filter
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After passing 810
liters, the dust level was increased to 90 NTU for another 150 liters and then to about
872 NTU for the last 57 liters. At this point the mixture was very muddy. The
turbidometer (sensitivity 0.01 NTU) detected breakthrough only after clearing the last 20
liters at 872 NTU (total 980 liters). At this point filter efficiency was >99.999%.
During the test the cartridge retained 71 grams of dust particles or 89 mg/cm2 of filter
area. Figure 2 shows the spent cartridge. |
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A similar test was run on a pleated cartridge 2.5” (63 mm) diameter X 4” (127) mm high.
This cartridge was challenged by a continuous stream of 250 NTU fine test dust at 5.5
liters/min (1.5 GPM). At 90 minutes (535 liters) it had filtered out 119 g of dust
while maintaining <0.01 NTU in the effluent at which point the test was terminated without
breakthrough. While there are many claims about high dirt holding capacity there are
little published data to compare against. A recent paper by C. Shields [1] is very
useful in that it compared various media with respect to filtration efficiency, flow and
dirt holding capacity. The media he tested compared submicron microglass, meltblown
and membrane media with pore size ratings in the range of 0.2 µm, 0.5 µm, and 1 µm.
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Each media was tested for
filtration efficiency for monodisperse latex beads of 0.4 µm, 0.8 µm and 3.0 µm.
Only the 0.2 microglass and 0.2 membrane media achieved “100%” retention of the smallest
(0.4 µm) latex spheres. In contrast, a single layer (0.8 mm thick) of NanoCeram®
retains greater than 99% of 0.03 µm latex particles or 99.7% retention of MS2 virus (25
nm). Shields clean water flux data was respectively 17, 21 and 25 ml/min/cm2
for the 0.2 µm, 0.5 µm and 1.0 µm microglass media, values which were significantly
greater than meltblown or membranes for each of the pore size ratings tested. In
comparison, the permeability through 0.8 mm thick NanoCeram® is higher than the
microglass - 60 and 120 ml/min/cm2 respectively at pressures of 0.5 and 1 bar.
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Fig. 3 – Dirt Holding Capacity of Media as a Function of Pore
Size Ratings
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Figure 3
shows Shields data for the dirt holding capacity (DHC) of AC fine test dust by the
different media. The DHC of the NanoCeram® cartridge shown in Figure 2 is
compared to the Shields data. The data are shown on semi-log coordinates since the
relative DHC of NanoCeram® is so great that the data for meltblown and
membranes would not be discernable. It’s DHC of 574 mg/in2 is almost
twenty times greater than the microglass media if compared at a pore size rating of 1 µm
and far greater than that if compared to smaller pore size ratings of the microglass or if
compared to meltblown or membrane media at any of the ratings. NanoCeram®
filters have also been challenged with cysts (~4-6 µm) and bacteria (E coli and Klebsiella
teragina, Kt), ~ 0.5 to 1 µm. A 1.5 mm thick layer was capable of 5 LRV (Log
retention value) of Cryptosporidium, a cyst that is responsible for a number of drinking
water contamination incidents. No data were taken on thinner NanoCeram layers.
Kt bacteria clearance of a 0.8 µm layer was measured at 6 LRV. |
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NanoCeram® filters are
capable of filtering nano size particles with high permeability. They have
been challenged with a number of different nanosize particles including MS2 virus
(25 nm), latex spheres (30 nm), humic acid and a blue dye available from Epsom, with
a particle size of ~3 nm. Figure 4 shows test data with 30 nm latex spheres, where
the effluent is monitored using a turbidometer that has a detection limit of 0.01
NTU. The data show that the capacity of NanoCeram® is directly proportional to
the ratio of nano alumina fibers in the filter. The capacity is also directly
proportional to the filter thickness (number of layers). Computations showed
that a 25 mm. diameter filter approximately 1 mm thick was able to absorb 4·1013
particles before spheres were detected in the effluent. We were then able to
calculate that the filter absorbed 8·1012 particles/cm2 before appearing in the
effluent at breakpoint. The very high retentivity in a narrow zone suggests a
very sharp adsorption zone. |
Figure 4
– Filter capacity as a function of NanoCeram®
content
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Fig. 5
shows experimental data on MS2 virus
adsorption versus filter thickness for two different flow conditions.
The dashed lines on the figure are projected virus capacity based on a
model we developed projecting filter performance as a function of
flowrate, filter thickness and particle concentration. The model
has proved to be very useful for projecting performance of filters with
larger area and thickness. A 25 nm pore size membrane
(Millipore VS) was placed within a filter holder and by using a syringe,
the diluted Epsom particulate dye solution was forced through the
membrane. The effluent was as blue as the original solution (See
Figure 5), and the membrane showed no discoloration. The experiment
was repeated with a 25 mm. nano alumina filter disc, 1.5 mm thick.
In this case, the backpressure was not significant and the nano dye
particles were removed to produce a water white fluid. Examination
of the filter showed a brilliant blue on the influent side and a barely
pale blue on the effluent face. |
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Figure 5 –
MS2 Retention as a Function
of Filter Thickness |
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