THE VALUE OF PURE WATER 



73 



EFFECT OF HARDNESS. 



The waters of New England are comparatively soft, although in 

 some instances the ground waters are hard. In the Middle West, on 

 the contrary, most of the surface waters are quite hard, and in some 

 cases the hardness is excessive. The following figures serve to give 

 an idea of the range in the depreciation value of waters due to hard- 

 ness. 



TABLE 7 . 



EFFECT OF FILTRATION. 



Sanitary quality. The following figures show to what extent the 

 sanitary value of a polluted public water supply is increased by an 

 efficient system of filtration : 



Lawrence, Mass. 



Water supply, Merrimack River, filtered by a slow sand filter. 



Population 70,000. 



Water consumption, 40 gallons per capita daily. 



Before filtration the typhoid fever death-rate was 121 per 100,000; since then 



it has been 26. 



Before filtration = 2.7$ (121 20) X W=$6o3- 

 After filtration # = 2.75 (26-2o)X ^=$41. 

 Increase in sanitary value = $693 $41 =$652 per million gallons, or $665,000 



per year, or $9.50 per year per capita. 

 Albany, N. Y. 



Water supply, Hudson River, filtered by sand filter. 



Population, 95,000. 



Water consumption, 165 gallons per capita daily. 



Before nitration the typhoid fever death-rate was 104 per 100,000; since then 



it has been 26. 



Before filtration D = 2. 75(104 20) X}^& 

 After filtration # = 2.75 (26-2o)XH$ 



