224 



MICROBIOLOGY OF WATER AND SEWAGE. 



material, are later "shed" from the stones appearing again in the effluent 

 as humus or stable organic matter. 



ANAEROBIC TANKS. The cultivation of bacteria in anaerobic tanks is 

 not quite as simple a matter as that which has just been described. The 

 sewage is allowed to flow slowly through the tank and after some time, 

 from a few days to a month or more, a normal and constant flora will 

 have become resident there. This flora will soon have become so well 

 established that the incoming sewage laden with a flora of its own mingles 

 with a liquid in which the established flora is so greatly in excess that the 



FIG. 65. Sketch of septic tank. (Original.) 



former in large measure gives way to the latter. In this way, while the 

 sewage itself moves onward and is gone within a few hours, the flora is 

 constant and persistent. A further aid in preserving this constant flora 

 is the sludge at the bottom, in which the bacteria lodge and multiply and 

 from which they are carried upward by the ever moving eddies and con- 

 stantly re-inoculate the liquid above (Fig. 65). 



THE DESTRUCTION OF SEWAGE BACTERIA. 



BY BIOLOGICAL PROCESSES. Reference has already been made to the 

 effect of biological processes of purification upon pathogenic bacteria. 

 What was stated in regard to the pathogens is equally true of the sewage 



