302 DISTRIBUTION OF BACTERIA 



action of the nitrifying bacteria which develop in the inter- 

 stices of the filter between the flushings when the material 

 is filled with air. 



Sewage. By sewage is meant the liquid waste from laun- 

 dry, kitchen, and water closet. It contains organic matter 

 and enormous numbers of bacteria. Winslow found 712,000 

 per cc. in winter and 11,487,500 per cc. in summer. Many 

 different species occur. Three groups are of particular inter- 

 est. One group is composed of the putrefactive bacteria, 

 B. vulgaris, etc. Another group is composed of those bac- 

 teria which are characteristic of sewage, such forms as B. 

 coli, B. cloacae, B. welchii, and Str. pyogenes are included. 

 The third group, and from a public health standpoint the 

 most important, is the disease-producing bacteria, as B. 

 typhosus, Msp. comma, and B. dysenteriae. 



The Purification of Sewage. The savage and barbarian 

 are little concerned with this problem. Their refuse is 

 thrown in a pile, and when it becomes offensive they move 

 on. Civilized man moves the refuse. In the case of single 

 dwellings and small communities this has been done by bury- 

 ing or putting it into a cesspool. In larger communities it 

 has been washed into an adjacent body of water. As the 

 population increases it is found that these methods are un- 

 satisfactory for the care of the growing volume of sewage. 

 Recently bacteriological methods of sewage purification have 

 been developed. In order to purify sewage its organic matter 

 must be destroyed, and the pathogenic bacteria which it may 



