BACTERIA OCCURRING IN SEWAGE 



71 



The number of organisms per c.c. is obtained from the 

 gelatine plate cultures. As pointed out previously, this figure 

 varies enormously, and to give some idea of what may be 

 expected, I have tabulated a few results obtained by various 

 observers from raw sewages and effluents (see Table, p. 70). 



Experiments conducted at Chorley for the Royal Commission 

 upon the average number of organisms present in the sewage 

 during the successive stages of treatment gave the following 

 results : 



Number of 

 Samples. 



Number of 

 Gelatine Plates. 



Average Number of 

 Bacteria per c.c. 



Crude sewage 

 Sewage and precipitate 

 Tank effluent 

 Filter effluent 



124 



93 

 107 

 127 



4,084,827 



1,344,925 

 398,695 



45.755 



The number of liquefying bacteria varied from 20,000 to 

 1,000,000 per c.c. in the crude sewage, and from 470 to 60,000 

 per c.c. in the filter effluent. In examining beds at Leeds worked 

 on the Dibdin principle, it was noticed " that a multiplication 

 of organisms occurs in the sewage during its passage from the 

 entrance to the works on to the beds. The cause of this increase 

 has not yet been ascertained, but the following table suggests 

 that the addition of lime to the crude sewage tends to inhibit 

 the multiplication of organisms " : 



The dilutions employed in these experiments were as follows : 

 I in 1,000,000 for sludge, i in 100,000 and 50,000 for the 

 crude sewages, and i in 10,000, 1,000, 500, and 100 for the 

 filter effluents. The examination of the gelatine plates was 

 made as follows : '* The temperature of incubation was from 

 18° to 20° C. No fixed interval of time was selected for the 

 incubation and counting, but they were carefully watched, and 

 every endeavour made to count the colonies when the maxi- 

 mum number had developed. This was sometimes, but by no 

 means always, rendered difficult by the number of liquefying 

 colonies. The plate was counted by placing it upon a ruled 



