CHAPTER XXII. 

 PURIFICATION OF WATER SUPPLIES. 



Necessity of Treating the Waters. - - The process by which the 

 pure waters falling upon the ground as rain gradually become 

 mineralized in their passage through the soils and rocks has been 

 described in the preceding chapter. The surface waters, supplied, 

 as they are, chiefly from seepage from the ground, carry all the 

 mineral matter dissolved by the ground waters, in addition to 

 which there is soon added a certain amount of organic matter. 

 Swampy waters often take on a deep brown color from the decay- 

 ing leaves and wood, algae frequently develop until the water is 

 obnoxious both in odor and taste, while bacteria may multiply 

 until the water is dangerous to health. 



When such conditions arise, some method of reducing or re- 

 moving the objectionable matter becomes imperative. Most of 

 the substances, both mineral and organic, which give rise to the 

 undesirable qualities may, fortunately, be removed or their effect 

 neutralized by certain comparatively simple and relatively inex- 

 pensive methods of treatment. Several of the more common of 

 such methods are described below. 



Color. By the color of a water is meant the appearance of 

 the clear liquid. It should be distinguished from turbidity, or 

 the appearance due to the presence of clay or other suspended 

 matter. The color is usually the result of contact with the de- 

 caying vegetable matter of swamps, etc., and varies from a faint 

 yellowish tinge to a deep amber or dark brown. . 



Water is decolorized to a certain extent when stored in reser- 

 voirs exposed to sunlight, which process is helped by aeration, but 

 it is practically impossible to entirely remove the discoloration. 



The discoloration of the waters of a spring can usually be pre- 

 vented by cleaning the spring and carefully removing all leaves 



1 68 



