BACTERIA OCCURRING IN SEWAGE ']^ 



are necessary to convert ammonia to nitrate, nitrite being an 

 intermediate state (see Chap. V.). 



Nitrification in filter-beds will be considerably promoted by 

 the organic matter present being greatly reduced in quantity, 

 and the number of organisms producing ammonia should con- 

 sequently diminish if this work has been done at an earlier 

 stage. Thus at present in the working of the Crossness beds 

 the numbers reported per c.c. are : 



% 



Gelatine plates at 20° C. 

 Agar plates at 37° C. ... 



Sewage. 



'6,400,000 

 3,670,000 

 4,100,000 



Effluent. 



1,170,000 

 1,260,000 

 1,630,000 



But the mere counting of the number of bacteria furnishes 

 little information of the character of an effluent, as is shown by 

 the following observations : 



1. Meade Bolton and others have proved that some organisms 

 commonly occurring in water, such as M. aquatilis and 

 B, erythrosponis, can multiply enormously even in sterilized 

 distilled water free from almost every particle of organic matter. 



2. The nitrifying organisms will live in the absence of all 

 organic matter, and will not grow in the ordinary culture media; 

 hence would be entirely omitted in the ordinary counting. 



3. During the purification carbonic acid is produced in con- 

 siderable quantity. This gas is inimical to a large number of 

 bacteria. 



Other Organisms which affect Purification. 



Besides bacteria in sewage, there are generally found many 

 organisms of a higher grade. Water-worms, such as Anguillula 

 and Nais, are stopped or killed in efficient purification ; in fact, 

 one of the causes that calls for an anaerobic stage is that these 

 animals require oxygen, and perish rapidly under the air-free 

 conditions. At the same time, their preliminary agency in 

 consuming and breaking down organic debris is almost cer- 

 tainly of value. Though bacteria are the chief workers in the 

 purification of rivers, it must be borne in mind that fish, rats, 

 and birds act as scavengers, and that the larger flora of the 

 river also plays an important part. Letts remarks on the 

 absorption of nitrogen into the tissues of animals or plants 

 which feed on the sewage: "The bacteria beds at Belfast and 



