228 



Microbes and You 



in Germany of the cholera epidemic of 1892. Hamburg, using the 

 raw water from the Elbe River, reported hundreds of new cases of 

 cholera each day, while Altona, adjacent to Hamburg, drew its 

 water from the same source but subjected it to filtration prior to 

 use. Altona reported few or no cases of cholera each day. 



SLOW SAND FILTER 

 Small communities may resort to slow sand filtration in purifying 

 their water supply. These filters may be several acres in size. 



Water 



Fine Sand 



Coarse Sand V•^yv/.^••^.^V;v■:■•.■:*V•!:^V.^^^'^•^A.^\Vv^^^ 





Gravel 



Tiles 

 Cement 



DDDDDD 



Elizabeth 

 tice-Hall, 



Fig. 36. Cross section of a sand filter. (C. E. Turner, 

 McHose, Effective Living, Copyright 1941, 1945, 1950 by Prer 

 Inc., New York, p. 346. Reprinted by permission of the pubhsher.) 



After laying a concrete base, perforated drainage tile is placed on 

 the floor and covered with coarse gravel, on top of which is added 

 fine gravel. Finally, from two to four feet of graded sand is 

 placed on top of this. Without previous treatment, the water is 

 allowed to flow through the filter bed by natural gravity flow. 

 The sand becomes covered with a colloidal flocculent mass called 

 a Schmutzdecke which aids in the filtration process. As the layer 



