MICROORGANISMS IN WATEI^ 



267 



luted area. Consequently expensive installation must be made, in order 

 to purify a suspiciously contaminated water by freeing it from organ- 

 isms injurious to health. There are several methods of accomplish- 

 ing such purification; and these will be briefly mentioned. 



Sedimentation and Filtration.— This method of purifying water 

 has been used for nearly a hundred years; but the great impetus given to 

 this hygienic measure was due to Koch, who showed in 1893 that the 



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Fig. III. — Section of a sand filter. 



proper filtration of Elbe water saved the town of Altona from an epi- 

 demic of cholera which devastated Hamburg as a result of drinking un- 

 filtered water. In this system of purification, the water is first stored in 

 large reservoirs, where the effect of sedimentation and storage reduces 

 considerably the number of bacteria. From the reservoir, the water is 

 filtered through sand, gravel, and pebbles, etc., arranged as shown 

 in Fig. III. This filtration removes from 97 to 99.5 per cent of. the 

 microorganisms. 



The action of the fiter bed is due to the mechanical obstruction of 

 impurities, to oxidation of the organic matter, and to nitrification due 



