The Probert Encyclopaedia of Science & Technology

FILTRATION

Filtration is the process of freeing a liquid from solid matter suspended in it by causing it to pass through some pervious substance or substances which catch and retain the solid matter, The materials of which a filter is composed must have pores or interstices sufficiently coarse to allow the passage of the liquid, and yet sufficiently fine to prevent the passage of any solid particles. On a small scale unsized paper is often used; on a large scale various kinds of stone, sand, gravel, powdered glass, clay, charcoal, coke, etc, are traditionally employed. In domestic filters the simplest forms are those in which the water passes down by gravity through the filtering medium to a reservoir below. Lateral and ascending filters are not uncommon.

Filtration can be hastened by applying suction or pressure. The latter method is used in the Pasteur-Chamberland filter, consisting of a hollow cylinder of porous porcelain, which can be attached to an ordinary water-tap, the water being thus forced through the pores. The filters at water-works are large tanks or beds, filled with layers of large stones, pebbles, coarse gravel, fine gravel, coarse sand and fine sand - the fine sand being at the top. Other materials are sometimes utilized, such as furnace cinders or clinkers, shells or shell-sand, and so forth. The water in the reservoir, collected from springs, streams, and rain, is allowed to deposit its suspended matter in settling-tanks, and then it is run into the filters. By percolation the rest of the mineral matter is removed, and the water then flows into the pipes which are to convey it to the locality where it is to be used. Filtration can only remove substances mechanically suspended, for dissolved substances distillation is necessary.
Research Filtration