336 MICROBIOLOGY OF WATER AND SEWAGE 



In these cases the reduction of sulphates to sulphuretted hydro- 

 gen 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 complex- 

 ity 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 alter the course of the other 

 reactions involved. The products of the protein hydrolysis for ex- 

 ample may be profoundly modified by the presence of this additional 

 source of oxygen. 



The effect upon the bacteria themselves is also to be considered 

 as a factor quite distinct from the purely chemical effect just de- 

 scribed. It has frequently been observed, and in fact would be ex- 

 pected, that the products of anaerobic putrefaction are themselves 

 detrimental to the activity of the organism producing the changes 

 in question. The nature of sulphuretted hydrogen makes it appear 

 quite probable that we are dealing here with a toxic substance that 

 would at least inhibit the activities of certain bacteria and in this way 

 further modify the final result. 



The same might be said of almost all the reactions with which we 

 have to deal but this example is cited as a typical one. 



It is known in practice that the presence of sulphates in a sewage 

 does lead to a distinct type of anaerobic change which is characterized 

 by the marked blackening of the sewage, the formation of secondary 

 reaction products which precipitate after the removal of the suspended 

 matter of the sewage, the evolution of hydrogen sulphide, an excessive 

 amount of mineral or non-volatile residue in the sludge and the forma- 

 tion of free sulphur upon subsequent aeration of the sewage. 



Here again, as in the other types of reaction, it is useless for the pres- 

 ent to attempt to ascribe this reaction to any particular species. Sp. 

 desulphuricans and B. sulphur eus have been isolated. A non-liquefy- 

 ing anaerobic bacillus, which reduced sulphates strongly, was isolated 

 from Boston sewage in the writer's laboratory by G. R. Spaulding. 

 Others have been described and there is undoubtedly a large group of 

 organisms capable of bringing about the reaction. 



Just as the reduction of nitrates is a function performed by many, 



