MICROORGANISMS IN WATER. 



205 



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

 purify a suspiciously contaminated water by freeing it from organisms 

 injurious to health. There are several methods of accomplishing 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 







FIG. 58. Section of a sand filter. 



proper filtration of Elbe water saved the town of Altona from an epidemic 

 of cholera which devastated Hamburg as a result of drinking unfiltered 

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

 reservoirs, where the effect of sedimentation and storage reduces con- 

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

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

 in Fig. 58. This filtration removes from 97 to 99.5 per cent of the 

 microorganisms. 



