Identifier: practicalprevent00boyd (find matches)
Title: Practical preventive medicine
Year: 1920 (1920s)
Authors: Boyd, Mark F. (Mark Frederick), 1889-1968
Subjects: Preventive Medicine Public Health
Publisher: Philadelphia and London, W. B. Saunders company
Contributing Library: Columbia University Libraries
Digitizing Sponsor: Open Knowledge Commons

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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 pressure filters are cylindrical steeltanks (Figs. 38, 39). 7. Disinfection of Water.—In this country chlorine is theonly agent extensively employed for this purpose, and is ap-plied either as liquid chlorine or a solution of bleaching powder.Disinfection is applied as a finishing process to water that has
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Pig. 36.—Filter bed No. i, Indianapolis Water Company. This shows work-men cleaning or scraping a filter bed to remove the muddy upper layer of sandcontaining the impurities trom the water. The sand is thrown into piles previousto being removed for washing. been previously filtered or otherwise clarified. The value ofthe process from the standpoint of the destruction of the bac-teria which pass through the filters, including pathogenic andnon-pathogenic germs, has been well demonstrated. From .1to .5 parts per million of available chlorine are commonly em-ployed, requiring the addition of about 5 to 12 pounds of bleachper million gallons of water. Quantities of bleach in excess of25 pounds can be detected by taste. Of recent years the employ-ment of liquid chlorine has been superseding bleach, owing to thelesser cost of the former, as well as its convenience (Fig. 43). The WATER PURIFICATION J39 1

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