IN THE VICTORIAN ERA 13 



ing the necessary conditions under which the 

 maximum amount of work is obtainable from 

 these novel bacterial labourers. 



Two different classes of bacteria are required to 

 carry on the purification of sewage : those which 

 flourish in the absence of air and are known as 

 anaerobic bacteria, and those to which the presence 

 of air is essential for the exercise of their func- 

 tions, the latter being therefore called aerobic 

 bacteria. 



The work of the anaerobic labourers consists in 

 breaking down the complex organic compounds 

 present in sewage, whilst the completion of the 

 process of purification is left to the aerobic varie- 

 ties. In the ordinary course of nature both these 

 processes are going on side by side, but it has been 

 found advisable to separate these two different 

 classes of bacteria as far as possible, and allot 

 distinct premises to the anaerobic and aerobic 

 varieties respectively engaged in the purification 

 of sewage, for by so doing experience has shown 

 that the work is not only more expeditiously, but 

 also more efficiently, carried out 



Now the anaerobic bacteria are supplied along 

 with the sewage, and the retention of their services 

 offers practically no difficulty as long as an ample 

 allowance of space and time is given them in which 

 to carry on their labours. The aerobic bacteria, 



