FILTER AND FILTRATION (Pr. 'Hire, OUblet of Fr., OF. feutre, II. fell ro, felt, from OHG. fib, Ger. AS., Eng. felt ; connected with OHG. fat:, Ger. rah, fold, Lat. p4iere, to drive). The original and oftentimes the sole object of filtration is the removal of suspend ed matter from liquids by the mechanical Tiro mess of straining. Recently science has shown that in some of its applications filtration is far more than a straining process, particularly in water and sewage purification, where, by the agency of bacteria, organic matter, both in solu tion and suspension. is removed or transformed, thus preventing or delaying the clogging of the filter by the latter. The end to he attained in filtration may lie the securing or the recovery of matter suspended in a liquid. or the clarifica tion or other purification of the liquid itself. The flitcr may he either a vessel of porous material, such as carbon in sonic form or baked clay, or it may be a vessel containing it granular or fibrous material, supported on a perforated bot tom. Granular filtering materials may be sand, crushed quartz, powdered or crushed glass. bone or wood charcoal, crushed coke or cinders, or other substances more or less similar. Fibrous filtering, material may be cotton, wool, or asbes tos, either in the form of cloths or otherwise. and like substances. Both experience and theory show that in many instances the material to be filtered out adheres to the surface of the individual grains or fibres of the filtering material. often
forming a layer or membrane to the surface of the walls of filter vessels. In such eases ad hesion and agglutination assist in the process, and the reduction of the size of the pores of the medium. whether through straining or adhesion, makes it more difficult for the sus pended matters to pass through. Another con tributing cause is the tortuous passage through the filtering material, the several particles of the latter serving as so many dams or barriers to the progress of the suspended matters, adhesion and agglutination also aiding in this process. The speed or capacity of filters, and also their effi ciency or thoroughness, may be aided in some cases by the use of a coagulant, which serves to brimg the suspended matters together in flakes or clots which are readily retained by the filter ing material. The burden upon filters, or their tendency to clog, may he lessened in some eases by previous sedimentation. When filters de teriorate so they give a filtrate deficient in qual ity or quantity, they may be either discarded or cleaned, according to their nature. Sometimes they max be washed by reversing the flow of liquid through them. See SEWAGE DISPOSAL; WATER PURIFICATION.