286 BACTERIOLOGICAL AND ENZYME CHEMISTRY 



however, worthy of suggestion whether in the case of small 

 manufacturing premises, where much waste cellulose matter 

 accumulates, e.g., packing paper, extracted plants from drug 

 manufactories, waste hops from breweries, etc., it might not 

 be economical to produce gas in this way in small tanks, 

 maintained at the optimum temperature by waste steam. 



Ammoniacal fermentation. — As already mentioned, the 

 greater part of the nitrogen of sewage is present as urea, and 

 the ammoniacal fermentation readily sets in ; in many cases 

 indeed it may be almost complete before the sewage reaches 

 the purification works. A great deal of discussion as to the 

 necessity or otherwise of preKminary anaerobic treatment 

 of sewage, has been confused by the failure clearly to dis- 

 tinguish between absolutely fresh sewage and sewage which 

 has passed, it may be, through several miles of sewers. The 

 consideration of the necessary conditions for nitrification, 

 which found its place in Chapter XIII, shows the necessity 

 for a preliminary conversion of urea, and allied substances 

 such as amino acids, into ammonia before nitrification takes 

 place. The author has found beyond question that if 

 absolutely fresh sewage is to be put upon a filter, considerably 

 more filter space is requisite to convert the nitrogen into 

 nitrates, than if time is first allowed, e.g., by retention in 

 tanks, for ammoniacal fermentation to take place. It is, 

 however, rarely, in the case of an ordinary town sewage works, 

 that sewage is met with in such a fresh condition ; as already 

 stated, ammoniacal fermentation nearly completes itself in 

 the sewers, or at any rate during a comparatively short tank 

 treatment. It is doubtful whether urea would ever be found 

 in an ordinary sample of town sewage. 



Apart from the urea, however, the other nitrogenous 

 constituents of the sewage have to be considered ; these are 

 of a very complex character. Broadly speaking, all the 

 various decomposition products of albumin will be represented 

 in some form or other, together with actual undigested portions 



