MICROBIOLOGY OF SEWAGE. 217 



able proportion of the nitrogen of sewage is present initially as urea. 

 Owing to the ease and rapidity with which the reaction takes place, 

 however, no special effort is necessary to bring it about in sewage 

 treatment and it therefore receives brief attention in discussions of the 

 chemistry of sewage. The change to ammonia takes place in the small 

 sewers of the system and it is difficult and generally impossible to detect 

 the presence of urea in sewage. It has even been suggested that certain 

 enzymes present in fecal matter are instrumental in bringing about this 

 change and that the bacteria are only indirectly concerned. It is known, 

 however, that a large number of bacteria of general occurrence have the 

 power to produce this fermentation. Of these the Bad. urea (Miquel) 

 may be cited as an example. 



The Reduction of Sulphates and Nitrates. The production of 

 sulphuretted hydrogen during the anaerobic decomposition of sewage 

 is commonly noted. This substance may arise in at least two 

 ways. Sulphur, being a constituent of most protein substances, is split 

 off from the molecule in this form during certain types of fermen 

 tation. Its formation in these cases is analogous to that of ammonia 

 from protein. The amount so produced is small and is usually 

 neutralized and precipitated by the small amounts of iron and other 

 metals always present in sewage. There is therefore no liberation of the 

 gas itself and it is often said that sulphuretted hydrogen is not formed 

 normally in a septic tank. This conclusion is readily disproved by a 

 simple test of the black residue found at the bottom of such tanks. 



A second and more important source of this substance is the sulphate 

 normally present in many sewages. Throughout many parts of the 

 country the water supply contains material quantities of magnesium or 

 calcium sulphate, and upon the sea coast the sewage generally receives 

 more or less salt water. 



In these cases the reduction of sulphates to sulphuretted hydrogen is 

 not only of interest bacteriologically but probably exerts an influence upon 

 all the reactions that are going on simultaneously. In fact this example 

 serves excellently to illustrate the great complexity of these anaerobic 

 reactions and the mutual interdependence of each upon all the others. 

 Sulphates, under anaerobic conditions, are a source of oxygen and it is 

 upon oxygen that the course of all these reactions depends. Therefore 

 the presence of sulphates and the possibility of their yielding oxygen may 



