Practical preventive medicine . sent, by their exposure to an unfavorable en-vironment. We have already called attention to the fact thatthe survival of pathogenic micro-organisms in water is but for WATER PURIFICATION *33 a short period. They are exposed to the direct action of thesuns rays and they are devoured by predatory organisms, ofwhich the infusoria are probably the most important. Onemonths storage under these circumstances will result in theirdestruction. The greatest drawback to the employment of thismethod is the fact that sufficient space in the shape of a reservoiris frequently
Practical preventive medicine . sent, by their exposure to an unfavorable en-vironment. We have already called attention to the fact thatthe survival of pathogenic micro-organisms in water is but for WATER PURIFICATION *33 a short period. They are exposed to the direct action of thesuns rays and they are devoured by predatory organisms, ofwhich the infusoria are probably the most important. Onemonths storage under these circumstances will result in theirdestruction. The greatest drawback to the employment of thismethod is the fact that sufficient space in the shape of a reservoiris frequently not available so as to permit a city to store a monthssupply, or else a sufficient reserve of water is not available topermit its aging for a month before consumption (Fig. 31). 3. Sedimentation.—The lowering of the velocity of a stream,such as that effected when it enters a lake or reservoir, permitsthe fine suspended particles to settle to the bottom. The finerthe particles, the lower must be the velocity and the longer the. Fig. 31.—Lake Cheesman dam and reservoir. Denver Union Water Com-pany. A large impounding and storage reservoir. (Hazen: Clean Water andHow to Get It, John Wiley and Sons.) quiescent period before their removal is effected. This methodin itself is used chiefly as a means of removing turbidity, andis of but slight value in the removal of bacteria unless theperiod of quiescence approximates the conditions of open tanks or reservoirs are employed for maybe employed in conjunction with the processof coagulation. 4. Coagulation is used in conjunction with sedimentation oras a process of treatment preliminary to rapid sand is a necessary measure to remove the fine turbidity, parti-cularly that produced by colloidal clay, which would notcompletely settle out no matter how long the period of sedi-mentation. Coagulation consists in the employment of cer- 134 PRACTICAL PREVENTIVE MEDICINE tain chemicals, s
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