The Supply and Uses of Water 219 



Sedimentation takes place in still water, the very fine sus- 

 pended matter, both inorganic and organic, settling to the 

 bottom. Coloring matter is generally removed, and hard water 

 is greatly modified for the better. The most important fact is 

 that water of most dangerous sort can be purified by still storage. 



It has long been known that water becomes safe and pure 

 if stored long enough in casks or stone jars. It does not matter 

 how polluted water may be at the beginning. At certain 

 stages of the process the water may be vilely odorous; but 

 eventually it becomes odorless, tasteless, and safe. From the 

 old so-called practice of letting water purify itself, came the 

 suggestion of storage reservoirs. It is now known, however, 

 that water in storage is purified through the action of bacteria. 

 These minute plants may flourish in the water for a while, but 

 after consuming all the food materials in the water, they perish 

 and settle to the bottom as sediment. Careful investigations 

 have shown that many kinds of bacteria are destroyed and 

 other kinds greatly reduced in numbers by the storage of water 

 for two or three weeks. The germs that cause such diseases as 

 cholera and typhoid fever are greatly weakened, and they and 

 other dangerous germs are destroyed when the water is stored 

 for a longer period. This fact alone is sufficient to justify 

 storage in still reservoirs wherever the water supply is exposed 

 to contamination. 



Another advantage of still storage which should be mentioned 

 is that it gives opportunity to test water previous to filtration 

 or distribution. If an outbreak of disease that may be water- 

 borne should occur, all sources of supply may be tested at once 

 and the suspected source of supply shut off. Storage also 

 reduces the work of the filter and increases its effectiveness. 

 Filtering devices may fail accidentally to remove all impurities ; 

 storage prevents such accidents. 



There is a disadvantage in the still storage of water ; for the 

 water may be contaminated while in storage. Exposed water 

 catches dust, pollen, spores, and minute seeds from plants, and 



