90 



WATER AND OTHER BEVERAGES 



waters, it is extremely unsafe, since exposure to the sun 

 and air will not purify it from any contaminating drain- 

 age which may run into it. To be made safe, it must be 

 purified by some such efficient means as sand filtration. 



Protection of water supply. Various methods of 

 protecting and purifying water have been adopted by 

 cities and towns. If a natural watershed can be pro- 

 tected so that the water which falls upon it and drains 

 off is uncontaminated by human life, it is usually safe 

 for consumption. In mountain regions, such water sup- 

 plies are readily secured without undue expense. Not 

 infrequently, water is thus obtained for cities at great 

 distances from the watershed. 



Sand filtration. The problem of obtaining pure 

 water for flat, thickly settled regions has been solved 



Biver 



FIG. 57. Diagram showing arrangement of a filter plant. (Egbert.) 



by the discovery that if the water from lakes and rivers 

 is filtered slowly and intermittently through beds of 

 sand several feet thick, the living organisms are removed 

 from it and the water rendered pure. 1 Many cities, 2 

 therefore, have constructed large filtering beds to insure 



1 This process of purification is due to the action of harmless 

 microbes in the soil, which form a thin film upon the sand grains. 

 They have the power of destroying both the organic substances and 

 the microbes in the water as they pass through it in the process of 

 filtering. Since the soil microbes require air, the passage of the 

 water through the sand has to be intermittent, in order to permit 

 air to enter the sand. 



2 As Philadelphia, Lawrence and Berlin. 



