iiS 



SEWAGE AND ITS PURIFICATION 



It is curious that the percentage of the heat evolved in the 

 products is in each case nearly the same : 



These enzyme reactions follovv^ the ordinary chemical lav^ of 

 going in the direction of an evolution of heat. They occur 

 at atmospheric temperature, and it has been pointed out by 

 Van t'Hoff that the lower the temperature the more nearly will 

 Berthelot's law of maximum work be obeyed. 



The Second Stage, or Semi-aerobic Breaking-down 

 OF THE Intermediate Dissolved Bodies, 



is not generally distinguished sufficiently from the first, nor ^ 

 allowed adequate time to develop. It occurs in the upper layers ^ 

 of bacterial filters, as requiring little oxygen, and results 

 generally in the production of nitrites, the conditions being 

 favourable to the growth of B. nitrosonwnas. In this stage the 

 amido-compounds, fatty acids, and dissolved residues of hydro- 

 lysis undergo a further resolution. 



Nitrosifi cation, or the production of nitrites, and secondarily 

 of nitrogen and its lower oxides, by partial oxidation, should 

 normally occur in the second stage of bacterial purification. 

 Wherever we find a final filter acting badly, either from deficient 

 aeration or other cause, the fault is at once indicated by the 

 appearance of a high proportion of nitrites, as nitrosification is 

 not nearly so delicate a process or so difficult to initiate or 

 control as nitrification^ or the production of nitrates, which it 

 would naturally precede. For example, P. F. Richter isolated 

 a coccus of medium size, which in twenty minutes produced a 

 very intense nitrite reaction in fresh urine, and in addition 

 reduced nitrate to nitrite — a retrograde change which I have 

 already remarked as common to many bacteria, and character- 

 istic of crude attempts to introduce nitrification before the 

 sewage is properly hydrolysed and prepared. Nitrosification 

 proceeds most rapidly in the presence of diffused light and of 



