MICROBIOLOGY OF SEWAGE. 221 



Life in Septic Tanks and Filters. With the introduction of the septic 

 tank at Exeter, England, in 1893, the question of the fate of pathogenic 

 bacteria in such a tank was raised. It was even suggested that bacteria, 

 such as the typhoid organism, might multiply in the tank. The question 

 was investigated by Professor Sims Woodhead, who concluded that no 

 organisms capable of setting up morbid changes in animals were dis- 

 charged from the tank. This negative evidence however has little weight 

 in the light of more recent experiments. Pickard introduced an emulsion 

 of typhoid bacteria into this same tank and noted only a gradual decrease. 

 After fourteen days he was able to detect i per cent of the initial number. 

 He also reported a removal of 90 per cent of the typhoid organisms in- 

 troduced into a contact filter. These data must be interpreted in the light 

 of two established facts. The typhoid organism tends to die at a rapid 

 but diminishing rate under any but the most favorable conditions. This 

 results in a rapid decrease at first, with a prolonged survival of a few 

 individuals. This process takes place in sewers, in streams, and, in 

 fact, under most artificial conditions. The second fact of importance 

 is the difficulty of recovering the typhoid organism under experimental 

 conditions like those described. 



A thorough study of the bacteriology of sewage and of filter effluents 

 led Houston to conclude that the biological processes at work in a filter 

 or tank were not strongly inimical, if hostile at all, to the vitality of path- 

 ogenic germs. 



A conservative study of all the evidence bearing upon this important 

 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 multipli- 

 cation. 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 suspended 

 materials through the body of a filter. Even fine sand offers little strain- 

 ing 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. 



