PAPERS OX CHEMISTRY AND PHYSICS 153 



colloidal particles in sewage and the activated sludge 

 particles are, so far as we are able to determine, both 

 negatively charged. Since adsorption of colloids is most 

 effective between oppositely charged particles it should 

 not be applied to the conditions of the activated sludge 

 particles without reservation. Furthermore, adsorption 

 is an almost instantaneous action, while considerable 

 time is required for the activated sludge reaction. 



The biological theory suggests a somewhat different 

 notion of the importance of oxidation in sewage purifi- 

 cation than that ordinarily expressed. When garbage 

 is disposed of by feeding to hogs, only as much oxida- 

 tion takes place as is required to furnish energy for the 

 life processes of the hogs. Final oxidation does not 

 take place until the pork chops are eaten and burned up 

 in the body to furnish human energy. If the analogy of 

 this process to sewage disposal is admitted, oxidation 

 appears as an incidental reaction. Biochemical precipi- 

 tation of colloids would appear to be the important 

 phase of the reaction. 



The Sludge-Digestion Spiral — In nature organic mat- 

 ter is worked over by succeeding generations and races 

 of micro-organisms until a large percentage of it has 

 been broken down to ammonia, nitrates and carbon diox- 

 ide, leaving a relatively small amount of black humus as 

 a residue. The course of this reaction might be repre- 

 sented by a spiral. Suppose at a point A we start out 

 with the dissolved and colloidal organic matter in sew- 

 age. Microbial spores then develop, producing bacteria, 

 molds and sometimes even higher forms whose nourish- 

 ment is drawn from the organic matter, until when we 

 arrive at point B all of the organic matter has been taken 

 up to form the living substances of the growths which de- 

 velop. If these growths continue their life processes 

 they produce a certain amount of carbon dioxide and 

 ammonia, and then eventually die and decompose, bring- 

 ing us to the point A', where we again have dead and 

 more or less liquefied organic matter. The distance 

 from A to A' would represent what some authorities re- 

 fer to as the "wet-burning" or "moist combustion" 

 which has taken place during the first lap around the 



