MICROBIOLOGY OF SEWAGE. 225 



bacteria as a whole. Their destruction is due to time and an environment 

 unfavorable to growth, rather than to any specific cause. Further evi- 

 dence 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 

 little or no observable selection. Houston pointed out the relative 

 abundance of the streptococci, supposedly 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 multiplica- 

 tion 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 disinfection 

 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 ranging from 

 one to five parts per million of available chlorine (25 to r25 pounds of 

 bleaching powder per million gallons), will produce a bacterial removal 

 amounting to 98 or 99 per cent. This disinfectant is the most efficient of 

 the known germicides, cost being considered. By this means it is possi- 

 ble to practically eliminate the bacteria, good and bad, from an effluent 

 and it is no longer necessary nor desirable to seek high bacterial removals in 

 the purification process proper. By thus dividing the work of purifica- 

 tion into its component parts each part can be carried out at a maximum 

 of efficiency and economy. 



