Practical preventive medicine . apid Sand Filtration.—-This is accomplished by two typesof filters, either pressure or gravity, according to the mannerin which the head of water is secured upon the filter. Theoperation of each is based upon the same principle. Thismethod requires only a small installation, with a low initialcost, but a high cost of maintenance. They are very efficient WATER PURIFICATION J37 in the removal of color and turbidity, and slightly less efficient(95 per cent, to 99 per cent.) and less uniform in the degree ofbacterial removal secured as compared with slow sand niters
Practical preventive medicine . apid Sand Filtration.—-This is accomplished by two typesof filters, either pressure or gravity, according to the mannerin which the head of water is secured upon the filter. Theoperation of each is based upon the same principle. Thismethod requires only a small installation, with a low initialcost, but a high cost of maintenance. They are very efficient WATER PURIFICATION J37 in the removal of color and turbidity, and slightly less efficient(95 per cent, to 99 per cent.) and less uniform in the degree ofbacterial removal secured as compared with slow sand is effected by the formation of a thin film of theprecipitated coagulant upon the surface of the sand throughwhich the water passes, either by gravity or pressure. Sincethe film of precipitate held back by the sand is constantlyincreasing, the amount of water which passes through is soonfinally reduced below efficient limits. When this occurs, usuallyevery 6 to 12 hours, the filter is disconnected and the precipi-. FiG. 35.—Filter bed No. 6, Indianapolis Water Company. This illustrationshows the smooth, clean surface of the sand in a filter bed which is ready forwater to be admitted. The water covers the sand to a depth of about four feet. tate removed by reversing the flow of water, and agitating thesand, either by means of compressed air or mechanical rakes(Figs. 39, 42). The dirty wash water is wasted. When thesand is cleaned, the reverse flow is stopped, the sand settlesinto place over the gravel, and the coagulant containing wateris again sent through the filter. These filters are operated atthe rate of ioo to 150 million gallons per acre per day, hencethe small installations. Sedimentation to remove some of the precipitated coagulant 138 PRACTICAL PREVENTIVE MEDICINE is usually used as a preliminary process, in order to prevent thetoo rapid clogging of the filters. The gravity filters are open vats or reservoirs or wood or con-crete (Fig. 41), while the pr
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Keywords: ., bookcentury1900, bookdecade1920, booksubjectprevent, bookyear1920
