MICROBIOLOGY OF SEWAGE 287 



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 be- 

 come 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 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 constantly re-inoculate the' 

 liquid above (Fig. 118). 



The Destruction of Sewage Bacteria 



By Biological Processes. — Reference has already been made 

 to the e£fect of biological processes of purification upon pathogenic 

 bacteria. What was stated in regard to the pathogens is equally true 

 of the sewage bacteria as a whole. Their destruction is due to time and 

 an environment unfavorable to growth, rather than to any specific 

 cause. Further evidence of these facts may now be given. Bacteria 

 as a whole do pass even the fine-grained filters in large numbers. 

 Careful analyses of their types show them to be a haphazard mixture 

 from the original sewage flora with Uttle or no observable selection. 

 Houston pointed out the relative abundance of the streptococci, sup- 

 posedly delicate organisms, and found on the whole that the relative 

 abundance of the different kinds of bacteria seemed to be much the 

 same in the effluent as in the crude sewage. 



On the whole we may conclude that the biological processes remove 

 bacteria not by any specific antagonistic action but by delaying their 

 passage and permitting the natural decrease that occurs when multi- 

 plication is prevented. The more efficient the mechanism of the 

 filter in producing this delay the more complete will be the removal. 



By ChemicAl Processes. — A much more reliable and economical 

 method for bacterial destruction is now available in chemical disin- 

 fection of sewage effluents. The writer's studies at Boston, Baltimore 

 and elsewhere have shown that the application of hypochlorite of 

 calcium in amounts depending upon the character of the effluent, and 



