MICROBIOLOGY OF SKWAGE 275 



bacteriology. If they have any functions in the bacterial changes of 

 sewage, they receive attention as members of a corresponding tj^e, not 

 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 place natur- 

 ally in sewage after anaerobic conditions have become established. It 

 involves 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 pur- 

 poses 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 oxy- 

 gen is added to the molecule, the product always containing more oxy- 

 gen 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 t)rpe, possessing objec- 

 tive rather than subjective functions, in sewage, is made up of patho- 

 genic 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 modi- 

 fied throughout in accordance with this assumption. 



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

 study of the bacterial types themselves. 



