WATER SUPPLY AND SEWAGE DISPOSAL 165 



Many modern cities have installed filtering plants in which the 

 bacteria are removed from the water by passing it through beds 

 of sand. Other cities kill the bacteria by the action of certain 

 chemicals which, when added to the water in proper propor- 

 tions, are fatal to the bacteria but are not harmful to people. 



1906 

 1909 



191H10 



1912 6 



FIG. 82. Effect of purifying the water supply 



Piagram showing the typhoid death rate per hundred thousand population in Pitts- 

 burgh, Pa., during a term of years. A filtration plant was installed in 1908 



Storage reservoirs in the form of natural or artificial lakes 

 are common accessories of water systems. .Usually the prin- 

 cipal purpose of a reservoir is to accumulate a supply of 

 water -when it is plentiful, in order that there may be no 

 scarcity during a dry season or in case of unusual demands 



FIG. 83. Types of water faucets 



The faucet a must be closed by hand ; 6 is self-closing by water pressure ; c is 

 self-closing by pressure of a wire spring 



of any kind. At the same time the quiet waters of the reser- 

 voir offer a favorable place for the finer sediments, including 

 bacteria, to settle to the bottom. The water is thus purified 

 while it is being held in storage, and in some water systems 

 this is the most important means of purification used. 



