2l6 MICROBIOLOGY OF WATER AND SEWAGE. 



series of side reactions produced during the fermentation of the ni- 

 trogenous material rather than a definite reaction upon which the met- 

 abolism of any single species depends. This view is strengthened by the 

 general observations that this fermentation is in most cases due directly 

 to enzymes. Viewed in this light it is easy to understand the difficulty 

 that has surrounded the isolation of definite cellulose fermenting organ- 

 isms. Many have been described, chief of which are B. butyricus or B. 

 amylobacter, B. omelianski, Sp. mgula, and others. 



The Saponification of Fats. A third great group of type reactions 

 occurring under anaerobic conditions is the saponification or splitting of 

 fat. Our knowledge of this process is even less definite than of the 

 cellulose fermentations. It is a fact that there does take place in sewage a 

 gradual saponification and emulsification by which the fat loses its 

 identity and mingles with the liquid. This effect is most noticeable in the 

 case of long sewers in which considerable velocities are maintained. In 

 quiescent tanks there is a tendency for the fats to rise to the surface and 

 thus become removed from the influence of this action. Thus in small 

 installations enormously heavy scums form upon the tanks and analysis 

 shows a considerable percentage of fat in this material. In larger systems 

 on the other hand there is less and less evidence of fatty material as such. 

 It is true that there is a deposit upon the walls and tops of such sewers 

 and that small floating objects, like matches, rolling along such a wall will 

 accumulate layers of grease and become eventually the familiar "grease- 

 balls" found in the discharge, but in the main the fatty material has be- 

 come well disintegrated before the outlet is reached. 



In this case also as in that previously discussed it is not believed that 

 the action is* a direct result of the activity of any particular organism. 

 The proteolytic changes are accompanied by the freeing of alkaline 

 products, ammonia and amines, which leads to some saponification, and 

 which, in turn, leads to a further emulsification. Whether specific 

 enzymes are present which assist in this final process or not has never 

 been determined. It is significant to note, however, that where sewages 

 are slightly acid, unaltered fats are much more abundant, even though the 

 acidity is insufficient to prevent vigorous putrefactive changes in the 

 sewage itself. 



The Fermentation of Urea. The fermentation of urea has already 

 been referred to as a typical and simple case of anaerobic decomposition. 

 This reaction has great significance in sewage chemistry since a consider- 



