340 MICROBIOLOGY OF WATER AND SEWAGE 



A conservative study of all the evidence bearing upon this impor- 

 tant question including the vitality and fate of certain non-pathogenic 

 species, such as B. coli, leads to the conclusion that the removal of 

 pathogenic bacteria in purification methods is due to two allied causes, 

 the efficiency of which can be approximately determined. There is 

 first the time element and the known rapid decrease in the numbers of 

 certain bacteria such as B. typhosus when placed under conditions that 

 preclude multiplication. The rate of decrease varies but is roughly 

 about 50 per cent in twenty-four hours. 



The second factor, acting in reality in conjunction with the first, 

 is the mechanical hindrance that is offered to the free passage of sus- 

 pended materials through the body of a filter. Even fine sand offers 

 little straining action as such, since the open channels are thousands 

 of times as big as the bacterial cell, but surface tension phenomena 

 tend to make all solid material adhere to the medium and thus its 

 passage is delayed. This action is prominent although of less impor- 

 tance in coarse-grained filters. Actual experiments by the writer have 

 indicated that while the liquid may pass through a trickling filter 

 in half an hour, small suspended particles such as ultramarine and B. 

 prodigiosus cells require an average of over twenty-four hours. In 

 this way the actual time of passage is greatly delayed even when coarse 

 broken stone is the filter medium, and the times that are now known 

 to be necessary for the passage are ample in themselves to account for 

 the reductions that have been noted. 



It may therefore be stated as a conservative view of the efficiency 

 of purification processes in the removal of pathogenic bacteria, that 

 there are no strongly inimical processes at work in the tanks or filters, 

 and that the rate of decrease is not materially greater than would be 

 observed in the same period of time under the conditions of a running 

 stream. 



THE CULTIVATION or SEWAGE BACTERIA 



There are two general methods employed for the cultivation of 

 those bacteria which are of assistance in sewage purification. They 

 may be cultivated in so-called filters of sand or coarser material, or 

 in specially constructed tanks such as the septic or the hydrolytic tank. 

 In the former case the bacterial growth occurs upon the special medium 

 provided, the sand or stone; in the latter, it takes place in the liquid 



