PAPERS ON CHEMISTRY AND PHYSICS 155 



of the microscopic life, namely, by blowing air into the 

 sewage, we will waste considerable effort if we allow the 

 process to proceed very many laps around the spiral. 

 The most efficient process would seem to be one which 

 removes the sludge as near the point B as possible; that 

 is, as soon as a luxuriant growth has been developed and 

 before any of this growth begins to die and decompose. 

 This fact is the fundamental distinction between tank- 

 and-filter treatment and activated-sludge treatment, the 

 object of the former being to go round the spiral ap- 

 proaching the center as nearly as possible, that is with 

 the maximum of wet-burning, while the latter should go 

 only to the point B. 



There is also a mechanical difference between the ac- 

 tivated-sludge process and the tank-and-filter system. 

 Hering has shown that the amount of purification is pro- 

 portional to the surface exposed. Since it is necessary 

 to use rather large stone in niters to avoid ponding or 

 clogging, it is obvious that the ratio of surface to volume, 

 that is the efficiency per cubic foot, is comparatively low, 

 while in the activated-sludge process the floe surface is 

 relatively enormous, being in the neighborhood of 500 

 sq. ft. of surface area per cubic foot of aeration tank 

 volume. 



It will not be possible to discuss all of the chemical 

 changes which take place, but I want to consider briefly 

 the nitrogenous compounds and the changes which they 

 undergo. In this field we find that the chemistry of soils 

 and fertilizers and the chemistry of sewage have gone 

 hand in hand. Chemists in one line apply freely the. ad- 

 vances made in the other. Although the chemistry of 

 nitrogen and the changes which it undergoes in nature 

 is one of the oldest subjects of chemical investigation we 

 find that there is much misinformation in the literature 

 and much still to be learned. 



"When bacterial enzymes attack the organic matter of 

 sewage, progressive hydrolysis takes place with the for- 

 mation of ammonia. This is known as ammonification 

 and is brought about by a variety of organisms. 



