WATER 15 



the departure from the normal in that vicinity that is significant. Sewage, 

 which contains a very high content of salt, will raise the chlorine content 

 very quickly if a little of it gets into the water. 



Although sedimentation, flocculation with chemicals, and filtration 

 through layers of sand and gravel I'emove nuich suspended matter and 

 hence lessen the bacterial content of water, these methods alone cannot 

 be relied upon for purification of a public water supply. The agent most 

 commonly employed to destroy bacteria in water is liquid chlorine. 

 Penfield and Gushing state that 85 per cent of the public water supplies 

 of the United States are sterilized with chlorine. According to these 

 authors the average typhoid death rate in 77 important cities of the 

 United States has decreased from 20.54 deaths per 100,000 in 1910 (when 

 the use of chlorine was initiated) to only 0.76 in 1937. Better water 

 supplies, and the use of sulfa drugs, antibiotics, and other therapeutic 

 measures, have decreased the typhoid death rate to 0.2 per 100,000 in 1948. 



Hardness of water 



From the household and industrial viewpoint, the hardness of water is 

 an economic question that involves the cost of large quantities of soap and 

 water-softening materials. The hardness of water is due to the presence 

 of bicarbonates, sulfates, chlorides, and silicates of calcium, magnesium, 

 and iron. These salts form insoluble precipitates with soap and therefore 

 give what is called hardness to the water. New types of cleaning agents, 

 called "synthetic detergents" (p. 87) , have been developed in recent 

 years. They form soluble calcium and magnesium soaps and hence can 

 be used in hard water. 



Hardness of water is spoken of as being temporary or permanent de- 

 pending upon whether it is due to bicarbonates or other salts. If the 

 hardness is due to bicarbonates, heating or boiling the water will, to a large 

 extent, precipitate the calcium or magnesium bicarbonates as insoluble 

 carbonates. On the other hand, boiling has no effect upon water that 

 contains sulfates or other salts of calcium and magnesium. Such waters 

 are said to be permanently hard. Many different methods for softening 

 hard water can be used such as boiling, or addition of lime, washing soda, 

 phosphates, or other precipitants. In all cases a precipitation of the 

 calcium and magnesium is the end to be desired. The action of these 

 precipitants may be represented by the following equations: 



Heat : 



Ca(HC03)o = CaCOa + HoO + COo 



• Lime : 



Ca(HC03)o + Ca(0H)2 = 2CaC03 + 2H2O 



