Argonide Corporation

Particle removal efficiency of Nano alumina fiber filters

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Courtesy of Argonide Corporation

A 2 nanometer alumina fiber is combined with a microglass fiber to produce a nonwoven filter. Its pore size is 2 microns, yet it is functionally rated at 0.03 microns. Data are presented showing dirt holding capacity and filtration efficiency for particles from 0.001 to 7 microns. The filters have high retention for micron size silica dust, bacteria, virus, DNA/RNA, tannin and latex spheres.

Introduction and Background
Clean water is crucial for drinking and in many industrial processes. The contaminants in fresh water include suspended particulates such as clay, silt, finely divided natural organic matter, inorganic matter, plankton and microscopic organisms. Waterborne pathogens have caused significant disease outbreaks of acute gastroenteritis in the United States and continue to pose a significant problem. The causes are usually traced to various viruses, bacteria, or protozoa. Pathogens are generally attached to larger particles and effective filtration of turbidity has been correlated with low bacterial counts and low incidences of viral disease.

Industrial streams have an even greater diversity of particles than naturally occurring water. They may include colloidal metal oxides, silica, free metal particles, dispersed polymers, dyes and chemical by-products. Filtration is the preferred method of removing them from water.

Fibrous or depth filters, generally non-woven, remove particles principally by the mechanisms of interception and impaction. The removal efficiency is optimized when the fiber and particles have the same diameter, but fibers much below 0.25 microns diameter are not commercially available. Moreover, non-woven media prepared from sub-micron fibers have a high pressure drop and tend to clog prematurely.

Typically, wound fiber filters have a pore size rating down to about 1 micron, with filtration efficiencies ranging up to only about 95 percent. Pleated microglass or polymeric filter media are better suited for filtering particles smaller than about 1 micron, and some of these filters are rated as Absolute (>99%) filters.

Polymeric membranes are surface filters and sieve particles by passing the fluid through various pore size openings. Microporous (1.0 -0.1μm), ultraporous (0.1- .01 μm) nanoporous (5-0.5 nm) and reverse osmosis (<1 nm) membranes have a high pressure drop, requiring large filter areas and high pumping force. Smaller systems use back up reservoirs to compensate for low flow. Membranes are also prone to fouling and require frequent back-washing.

Reverse osmosis membranes, with the smallest of pores, are most susceptible to fouling and require prefilters to remove sub-micron particles and extend their life.

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