332 MICROBIOLOGY OF WATER AND SEWAGE 



PUTREFACTIVE AND ANAEROBIC BACTERIA. Putrefaction or anae- 

 robic fermentation involves the withdrawal of oxygen from one molecule 

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

 or part of the same molecule. The energy released in this process is 

 utilized in the vital functions of the organism. This action is neither 

 oxidation nor reduction, or more strictly, they are both taking place 

 simultaneously. 



A good example of such a process is the fermentation of urea. The 

 reaction takes place as follows: 



CO(NH 2 ) 2 + 2 H 2 = (NH 4 ) 2 C0 3 . - 



Carbon is oxidized at the expense of hydrogen, a process which, by itself, 

 is endothermic, that is, requires heat or energy for its maintenance. 

 But the heat of formation of the final product is greater than that of the 

 initial substances and the energy thus liberated becomes available for 

 use by the bacteria. It is in this way that hydrolytic changes of 

 this character play the same role in anaerobic reactions that is played 

 by direct oxidation under aerobic conditions. 



The Liquefaction of Protein. One of the most clearly defined and 

 useful types of bacterial activity to be seen in the various sewage 

 disposal processes is that which we term liquefaction. This term is 

 used to denote broadly all those changes by which solid and insoluble 

 organic matter is converted into a soluble condition. The particular 

 process known as protein liquefaction is in the main analogous to gas- 

 tric digestion. Its one characteristic is the increased solubility of the 

 product. The practical importance of protein liquefaction in sewage 

 disposal is very great and the value of the liquefying bacteria corre- 

 spondingly high. Nevertheless, aside from our knowledge of analogous 

 processes in digestion and in bacterial putrefaction of albuminous sub- 

 stances, we know almost nothing of the chemistry or the bacteriology 

 of this process. An enormous variety of bacteria are included in this 

 group. The whole process is doubtless the result of a very complicated 

 symbiosis in which various sub-groups of bacteria carry out the initial re- 

 action, from which point other groups carry it through successive stages. 

 Absence of one or another of these groups or of some important species of 

 any group doubtless accounts for the diverse results that are recorded. 

 It is well known that the activities within a septic tank, for example, 



