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Microporous Filters


Depth FilterMicroporous filters can be classified in three categories: depth, surface and screen. Depth filters are matted fibers or materials compressed to form a matrix that retains particles by random adsorption or entrapment. Surface filters are made from multiple layers of media. When fluid passes through the filter, particles larger than the spaces within the filter matrix are retained, accumulating primarily on the surface of the filter. Screen filters (also called membrane filters) are inherently uniform structures which, like a sieve, retain all particles larger than the precisely controlled pore size on their surface.

Benefits & Limitations
Benefits
  • Screen filters are absolute filters that remove all particles and microorganisms greater than their pore size.
  • Efficient operation throughout their lifetime, unless they are damaged.
  • Maintenance is limited to replacement.
Limitations
  • They will clog when the surface is covered by contaminants. Therefore, they should be used as a last purification step, as a type of insurance.
  • Will not remove dissolved inorganics, organics or pyrogens.
  • Not regenerable.

Screen FilterThe distinction between filters is important because the three serve very different functions. Depth filters are usually used as prefilters because they are an economical way to remove ≥ 98 % of suspended solids and protect elements downstream from fouling or clogging. They owe their high capacity to the fact that contaminants are trapped and retained within the whole filter depth. Surface filters remove 99.99 % of suspended solids and may be used either as prefilters or clarifying filters. Screen (microporous membrane) filters are 100 % efficient at retaining contaminants larger than their pore size. These filters are placed at the furthest possible point in a system to remove the last remaining traces of resin fragments, carbon fines, colloidal particles and microorganisms. For example, 0.22 μm MilliporeSigma membrane filters, which retain all bacteria, are routinely used to sterilize intravenous solutions, serums and antibiotics.

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