Filter Medium

(also septum), a natural or synthetic material or a manufactured article used for filtration. Filter media have a porous structure, that is, a structure permeable to liquids and gases. The filter media used in industry and in laboratory practice should have the following properties: (1) an appropriate porosity (that is, the pores should be of such a size that the particles of the filter cake are trapped in the filter medium); (2) chemical resistance to the filtrate; (3) sufficient mechanical strength; and (4) thermal resistance at the temperature at which the filtration is carried out.

Filter media may be either flexible or inflexible. Flexible filter media include metallic barriers in the form of perforated sheets or screens made of steel, copper, aluminum, nickel, silver, or other materials. Such media are especially suitable for the filtration of chemically aggressive fluids and for filtration at elevated temperatures or large mechanical stresses. Flexible media also may be nonmetallic barriers consisting of cloth or unwoven fibers. Such nonmetallic media may be made of asbestos, glass, cotton, wool, polyvinyl chloride, or Lavsan. Inflexible media include rigid disks, slabs, canisters, and sheets made by molding and sintering powdered ceramics, metals, glass, or synthetic materials. Such media also may consist of beds of unconsolidated particles of stone, coal, charcoal, coke, diatomaceous earth, sand, or clay.

[In English, a distinction is made between thin filter media and thick, or en masse, media. In thin media, such as filter paper or a filter cloth, the pores are smaller than the particles to be removed from the fluid. The pores of thick media, such as a charcoal bed or wood pulp, may be appreciably larger than the particles to be removed.]