160 ELEMENTS OF GENERAL SCIENCE 



methods. 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 

 very small proportions, are fatal to the bacteria but are not 

 harmful to people. 



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 

 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. 



176. The problem of sewage disposal. Enough has been said 

 in the preceding chapter to indicate that the disposal of city 

 wastes is an important matter. Much of the waste is in solid 

 form (garbage, ashes, etc.) and the proper disposal of this is 

 important and difficult. We shall here concern ourselves only 

 with that which is washed away in running water the sew- 

 age. That there will be a great volume of this is evident, for 

 most of the water that is pumped into the city eventually 

 finds its way into the sewers. In addition most of the water 

 from rains is turned into the sewers by the catch basins in 

 the gutters. 



177. Sewers. The simplest way to get rid of waste water 

 and sewage is to allow it to run off over the surface of the 

 ground in gutters. Obviously there are objections to this prac- 

 tice, but it is common in oriental cities, and several American 

 cities have such systems. Indeed, open sewers may be found 

 in neglected parts of many of our cities. An open sewer is 

 always a source of danger and should be a matter of concern 

 to every citizen. 



