Cartridge Membrane Filters
Cartridge membrane filters or trap filters are classified as surface filters where the membrane is a uniform continuous size. The cartridge membrane filter is often pleated and encased in a plastic cartridge that is usually inserted into a single housing, in order to handle a meaningful flow. Cartridge membrane filter and housing are shown in Figure 15.4. They are available in many different pore sizes.
Membrane filters, given the mechanisms of retention, work as either depth filters or surface (screen) filters, and in some cases has aspects of both. However, membrane filters are predominately used in the final filtration step because they assure the most reliable retention of particles and specific beer-spoiling microorganisms. Retention mainly takes place on the membrane surface by the sieving affect.
Multiple-stage cartridge filtration is generally practiced to limit particle loading on the final stage. The first stage should be designed for high solids capacity with a rating of between 10 and 20 µm used as a "trap filter" for kieselguhr or PVPP leakage. A membrane filter with this micron rating provides protection to the downstream filters.
Cartridge membrane filters are effective for sterile filtration, or microbiological stabilization of beer after it has undergone depth filtration. For brewery use a pore size of 0.45 µm is necessary to retain most potential spoilage organisms, yeast, and inert particulate matter on the surface.
Advantages and Disadvantages
Cartridge membrane filters have the advantage of providing good pressure stability against momentary leakage of microbes from pressure surges, while depth filters (e.g., diatomaceous filtration) are significantly more susceptible to surges.
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