Membranes are classified into two types based on their structure: asymmetric and symmetric. The former has a denser surface layer, 0.1 micrometers or less in thickness, with ordered micropores, and a bottom layer thickness of 200–250 micrometers; it belongs to surface filtration. The latter is isotropic, lacking a skin layer, with uniform pore size; it belongs to depth filtration.
Industrial applications use asymmetric membranes, with component forms including hollow fiber, plate, plate-and-frame, and tubular types. Membrane-forming materials include cellulose, cellulose acetate, polycarbonate, polyvinyl chloride, polysulfone, polyvinylidene chloride, polyacrylonitrile, modified acrylic polymers, cross-linked polyvinyl alcohol, sulfonated polysulfone, and polysulfonamide. This is a novel chemical unit operation widely used in the separation, concentration, and purification of various biological agents, pharmaceuticals, food industries, blood processing, wastewater treatment, and as a final treatment device in ultrapure water preparation.