Filter Media

FILTER MEDIA

The choice of filter medium for a particular operation demands considerable experience. In clarification, high filtration rates and the retention of fine particles are opposing requirements. Permeability and retentive capacity can be deter-mined and used to guide small-scale experiments with the materials to be fil-tered, facilities for which are often made available by filter manufacturers. Other relevant factors are the contamination of the filtrate by the medium and associated housing, the adsorption of materials from solutions, and, where necessary, the ability of the medium to withstand repeated sterilization.

In cake filtration, the medium must oppose excessive penetration and promote the formation of a junction with the cake of high permeability. The medium should also give free discharge of cake after washing and dewatering.

Rigid Media

Rigid media may be either loose or fixed. The former is exemplified by the deposition of a filter aid on a suitable support. Filtration characteristics are gov-erned mainly by particle size, size distribution, and shape in a manner described earlier. These factors may be varied for different filtering requirements.

Fixed media vary from perforated metals used for coarse straining to the removal of very fine particles with a sintered aggregate of metal, ceramic, plastic, or glass powder. The size, size distribution, and shape of the powder particles together with the sintering conditions control the size and distribution of the pores in the final product. The permeability may be expressed in terms of the constant given in equation (1). Alternatively, the medium may be charac-terized by air permeability. The maximum pore size, which is important in the selection of filters for sterilization, may be determined by measuring the pres-sure difference required to blow a bubble of air through the medium while it supports a column of liquid with a known surface tension. Full details of methods used for the measurement of air permeability and maximum pore diameter are given in British Standard BS 1752:1963.

Flexible Media

Flexible media may be woven or unwoven. Filter media woven from cotton, wool, synthetic and regenerated fibers, glass, and metal fibers are used as septa in cake filtration. Cotton is most widely used, while nylon is predominant among synthetic fibers. Terylene is a useful medium for acid filtration. Pene-tration and cake discharge are influenced by twisting and plying of fibers and by the adoption of various weaves such as duck and twill. The choice of a particular cloth often depends on the chemical nature of the slurry.

Nonwoven media occur in the form of felts and compressed cellulose pulps and are used for clarification by depth filtration. A disadvantage, unless carefully prepared, is the loss of fibrous material from the downstream side of the filter. The application of sheet media has already been discussed. High wet strength is conferred on paper sheets by resin impregnation. An alternative manufacture employs asbestos fibers supported in a cellulose framework.