Industrial Water Purification
7 August 2006, News release from M-Tech Diagnostics Ltd.
Argonide has developed and patented a new filter media, NanoCeram(R), whose active component is an alumina (AlOOH) fibre two nanometers in diameter.
The nano alumina fibre 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.
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 fibre 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.
|NanoCeram(R) media consists of nano alumina (boehmite) fibres 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 Nanofibres
|The nano alumina is attached to the microglass rather than forming agglomerates that would clog the structure. Cellulose and polymeric fibres 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.|
Table 1 (below,) shows zeta potential values for NanoCeram(R) 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.
|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 litre/min.||Fig.1- Spent pleated cartridge filter||After passing 810 litres, the dust level was increased to 90 NTU for another 150 litres and then to about 872 NTU for the last 57 litres. 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.|
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  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. 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(R) 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(R) is higher than the microglass - 60 and 120 ml/min/cm2 respectively at pressures of 0.5 and 1 bar.
NanoCeram® filters are capable of filtering nano size particles with high permeability. They have been challenged with a number of different nano size 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(R) 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.
|The experiment below demonstrates the effectiveness of NanoCeram(R) for filtering ~ 3 nm particles.||Figure 6 - Dye Penetration Through Millipore Membrane|
(c) 2004 Argonide Corporation. All rights reserved.
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