MICROBIOLOGY OF SEWAGE. 213 



as individuals. A study of these sewage types, therefore, is a study of 

 the chemical changes induced in the medium by the activities of one or 

 the other group of bacteria. 



TYPES OF SEWAGE BACTERIA. 



According to the general character of the changes which they bring 

 about, sewage bacteria are divided into two large groups, the anaerobic 

 or putrefactive bacteria, and the oxidizing bacteria. In regard to the 

 former, no attention is paid to the fine distinctions that have been made 

 in recent years in connection with the definition of putrefaction. In 

 sewage chemistry putrefaction is that change which takes pi ace naturally 

 in sewage after anaerobic conditions have become established. It in- 

 volves the reduction of urea, the hydrolysis of protein and of cellulose, the 

 emulsification of fats, the reduction of nitrates and sulphates and possibly 

 of phosphates, and those other changes which are characterized by the 

 withdrawal of oxygen and the hydrolysis of complex molecules. These 

 changes are always noted in sewage under anaerobic conditions and the 

 terms putrefactive and anaerobic change are for the present purposes 

 practically synonymous. 



The oxidizing reactions on the other hand might be classed under 

 the general heading of aerobic reactions, except that they constitute only 

 a small portion of the group of reactions which take place normally under 

 aerobic conditions. They are distinguished by the fact that oxygen is 

 added to the molecule, the product always containing more oxygen than 

 the initial substance. Carbon dioxide, water and nitrates are produced, 

 in distinction from methane, hydrogen and ammonia, which characterize 

 the anaerobic reactions. A third type, possessing objective rather than 

 subjective functions, in sewage, is made up of pathogenic and other 

 harmful bacteria. These play no part in our theories of purification and 

 the proof of their presence is generally lacking. For the protection of the 

 public health, it is assumed that they are always present in sewage, and 

 our procedure in sewage disposal is modified throughout in accordance 

 with this assumption. 



With these definitions in mind we may proceed to a more detailed 

 study of the bacterial types themselves. 



PUTREFACTIVE AND ANAEROBIC BACTERIA. Putrefaction or anaerobic 

 fermentation involves the withdrawal of oxygen from one molecule or 

 part of a molecule and the subsequent oxidation of another molecule or 



