F. W. MOHLMAN 357 



Two factors, rightly controlled, may greatly decrease this period of digestion. 

 A proper mixture of ripened sludge with fresh solids greatly modifies the initial acid 

 stage; also lime may be added to neutralize the acids and keep the pH from 7.0 to 7.4. 

 Rudolfs has found that by proper admixture of ripe sludge the digestion period may 

 be cut to fifty days, and more recently that by addition of lime, controlled by pH 

 determinations, it may be possible to shorten the period to thirty or thirty-five 

 days. Fair and Baity at Harvard also confirm Rudolfs' work. Stroganoff, Bach, and 

 Sierp had also considered the desirability of neutralizing excessive acidity by means 

 of hme, but had not worked out the proper relations or control. 



Much work still remains to be done on the bacteria responsible for the various 

 stages of digestion. Carbohydrate fermenters are active in the initial stages, but an- 

 aerobic spore-formers are probably of most importance in later stages. Greer' found 

 that CI. wclchil was present up to 10,000,000 per gram in Imhoff sludge at the Cal- 

 umet Sewage Treatment Plants of the Sanitary District of Chicago. 



OXIDATION, NITRIFICATION, AND DENITRIFICATION 



Complete treatment of sewage is accomplished by artificial devices which foster 

 the growth of liquefying and oxidizing bacteria. Given these growths, it is necessary 

 to bring all of the sewage into intimate contact with them. In intermittent sand fil- 

 tration and broad irrigation, there is also a physical straining action, but other proc- 

 esses now in use are not designed to accomplish clarification other than by sedimen- 

 tation. In all processes it is essential to provide extensive surfaces to which the bac- 

 teria may attach themselves; after growths are established it is necessary either to 

 pass the sewage over these growths, as is done in sand filters, trickling filters, and 

 contact beds, or to mix the growths intimately throughout the sewage, as is done in 

 the activated-sludge process. Continuous presence of dissolved oxygen is also essen- 

 tial. The only other requirement is a setthng tank for removal of solids before dis- 

 charge of the effluent. 



In all nitrifying devices, organic nitrogen must first be liquefied and ammonified 

 before it can be nitrified. The progress of nitrification, as measured by chemical 

 analyses, is then dependent on the velocity of opposing reactions, nitrification versus 

 denitrification. Winogradsky^ first demonstrated that in pure culture organic matter 

 inhibits growth of nitrite- and nitrate-forming bacteria, but that in nature nitrifica- 

 tion can occur in the presence of organic matter by symbiotic growth with B. ranio- 

 sus, a common soil bacterium of the subtilis group. Boullanger and MassoP showed 

 that when an ammoniacal solution, inoculated with both forms of the nitrifying bac- 

 teria, was aUowed to undergo complete oxidation in contact with clinker and then 

 withdrawn, a further supply of the ammoniacal solution poured on the same clinker 

 did not then interfere with the nitrate-formers, but the oxidation proceeded at once 

 as far as nitrate, and only traces of nitrite could be found. In this case a growth of 



' Greer, F. E.: "Anaerobes in Sewage," /. Am. Pub. Health Assoc, 16, 500-501. May, 1926. 



* Winogradsky, S., and Omeliansky, V.: "The Influence of Organic Matter on the Work of 

 Nitrifying Bacteria," Centralbl.f. Bakteriol., 2, 329-440. 1899. 



3 Boullanger, E., and Massol, L.: "Etudes sur les Microbes Nitrificateurs," Ann. de I'Inst. 

 Pasteur, 17, 492. 1903; 18, 181. 1904. 



