60 



WATER SUPPLY 



where most of the sediment settles out. Then the 

 water is further purified by other means. 



Filtration. Suspended matter, and to some extent bac- 

 teria, may be removed from water by filtration. This 

 method does not remove dissolved substances, nor 



to spray the water through the air in fountains. See 

 Figure 104. Aeration helps to remove iron and organic 

 matter and kills some disease-producing organisms. It 

 cannot be used alone, however, as it is not effective for 

 some dangerous impurities. 



Courtesy of St. Louis Department of Public Utility 



FIG. 91. INTERIOR VIEW OF A CITY FILTRATION PLANT 



does it completely take out all living organisms. Water 

 which is carrying disease-producing organisms should 

 be treated with some chemical like chlorine or chloride 

 of lime in addition to being filtered. 



City filter systems consist of beds of sand, often 

 an acre or more in extent. The bottom of the sand bed 

 is covered first with broken stone or large gravel. On 



top of this are placed suc- 

 cessive layers of small 

 gravel, fine gravel, coarse 

 sand, and fine sand. The 

 openings in the sand are so 

 small that only water, dis- 

 solved substances, and some 

 bacteria can pass through. 

 Oxygen from the air pene- 

 trates the sand and kills 

 some of the bacteria. After 

 a time the spaces between 

 the sand particles become 

 clogged with dirt and bac- 

 teria. Then the filter must 

 be cleaned and washed free 

 of its impurities. 



Aeration. This process of 

 purification consists of 

 mixing air with the impure 

 water. It is carried on in 



FIG. 92. A HOME a number of different ways, 



WATER FILTER a common method being 



CRYSTAL 

 TOHCELAIN 

 F1UTERIN& 

 TUBE 



UNTILTERED 

 WATER 



TIVTERED 

 WATER ' 



How is water purified in the home? In the country 

 and in smaller communities it is sometimes difficult to 

 obtain pure water for drinking purposes. Then the 

 water must be purified in the home. Two methods 

 are in common use for purifying water in the home, 

 namely, filtration and boiling. Many types of home 

 filters are on the market today. The most common 

 filtering materials employed in these home filters are 

 sand, charcoal, porous stone, and powdered glass. 

 Domestic filters are dangerous to use because they 

 do not insure absolute purity. If disease-producing 

 organisms are present in your water supply, all water 

 for drinking purposes and water to be used in the 

 preparation of foods should be boiled. Boiling will 

 destroy all disease-producing bacteria present in im- 

 pure water. Boiling is the only sure method which 

 can be used in the home for purifying water. 



How is water purified by distillation? None of the 

 methods of purification mentioned thus far complete- 

 ly removes dissolved substances from water. If water 

 free from dissolved substances is needed, it must be 

 distilled. The process of distillation is carried out by 

 heating water until it vaporizes (evaporates) 

 and then condensing the steam, thus leaving behind 

 all dissolved and suspended solids. This method is 

 too expensive to use for purifying large quantities of 

 water such as are needed by communities. Distilla- 

 tion is used in scientific laboratories to obtain pure 

 water for experimental purposes. It is also used by the 

 United States Navy to convert sea water into drink- 



