68 



SEWAGE AND ITS PURIFICATION 



By cultivation in this material large numbers of bacteria are 

 eliminated, and those capable of thriving are mainly of intes- 

 tinal origin. After one or two days' incubation at blood heat, 

 if organisms of the coli group are present, the fluid will have 

 become turbid, the blue will be reddened and later may be 

 bleached, and the fermentation tube will contain gas ; the 

 culture may be then plated and pure subcultures prepared. 

 The great delicacy of this test in the bacteriological examination 

 of drinking-waters has considerably modified proposed limits 

 for the frequency of B. coli. 



A convenient method of testing the bacterial efficiency of a 

 process is to add a portion of the liquid to sterile sewage, 

 obtained by means of a Pasteur filter, and to analyze it after a 

 certain time. Thus, Dr. Sims Woodhead, in November, 1896, 

 isolated at Exeter five distinct species of bacteria from the 

 crude sewage and three from the tank effluent, and found that 

 these were practically the only ones which could grow freely in 

 the sewage. He filtered samples of the tank effluent through a 

 Pasteur-Chamberland filter into sterile flasks and tubes, and 

 inoculated them in duplicate with cultures of the various 

 organisms separated by the plate culture method. After six 

 days' growth at the ordinary temperature, I determined for 

 him the nitrate and nitrite, with the following results : 



It is evident from these experiments that the sewage contained 

 organisms (No. 6) which reduced nitrate to nitrite, and others 

 (No. 5) which oxidized nitrites to nitrates, so that under prac- 

 tically the same conditions two different changes can take place. 



