MICROBIOLOGY OF SEWAGE 335 



saponification and decomposition. Whether specific enzymes are pres- 

 ent 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 o] 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- 

 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 Bact. urea 

 (Miquel) may be cited as an example. 



The Reduction of Sulphates and Nitrates. The production of sul- 

 phuretted 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 fermentation. 

 Its formation in these cases is analogous to that of ammonia from 

 protein. The amount so produced is small and is usually neutral- 

 ized 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 sul- 

 phate normally present in many sewages. Throughout many parts 

 of the country the water supply contains material quantities of mag- 

 nesium or calcium sulphate, and upon the sea coast the sewage gener- 

 ally receives more or less salt water. 



