F. W. MOHLMAN 355 



that any extension of time even for the hquid sewage permits the progress of prote- 

 olysis and ammonification in preparation for oxidizing processes. In sewages contain- 

 ing much carbohydrate, storage in tanks may permit the development of too much 

 acid for satisfactory final treatment. For example, Levine and Soppeland' have found 

 that dairy wastes containing lactose develop acidities sufficient to prevent proteoly- 

 sis. The optimum reaction for proteolysis was neutral or slightly alkaline (pH 7.0- 

 7.5). Acidities up to pH 6.4 produced no appreciable inhibition under aerobic condi- 

 tions, but it was felt that under less favorable anaerobic conditions this acidity would 

 have been detrimental. Proteolysis was retarded by higher acidities and frequently 

 stopped if the reaction reached pH 5.0-5.5. 



General bacteriological surveys have been made from time to time of cultures 

 isolate! from sewage-treatment plants. In 1904 Clark and Gage^ studied the reac- 

 tions of some thirty colonies isolated from the Lawrence (Mass.) sewage and effluents 

 from septic tanks and intermittent sand filters. Their observations dealt mostly with 

 the nitrogen relations. They found that bacterial growth produced ammonia from 

 organic matter, reduced nitrates to nitrites to ammonia and elementary nitrogen, 

 liberated nitrogen from solutions of organic matter, either with or without the pres- 

 ence of nitrates, and fixed atmospheric nitrogen. Many sewage bacteria also pro- 

 duced the lower oxides of nitrogen as reduction products of nitrates, oxides which 

 appeared to play an important part in the further decomposition of the organic mat- 

 ter in solution, either through katalytic action or by direct chemical action. They 

 noted a compound intermediate between nitrites and nitrates, but apparently did 

 not note an additional reaction, which has since been of interest in activated-sludge 

 studies, namely, the synthesis of insoluble protein from nitrites and ammonia, as dis- 

 cussed by Buswell and Neave.^ 



A year's study of the Plainfield (N.J.) sewage treatment plant was made by Dr. 

 Margaret Hotchkiss,'' in 1924. This plant consists of a Riensch-Wurl screen, Imhofif 

 tanks, sprinkling filters, and secondary settling tanks. She found that the organisms 

 most important numerically throughout the plant were the nitrate reducers, the hy- 

 drogen sulphide producers (from protein), and the albumen digesters. Nitrifying and 

 sulphur-oxidizing bacteria occurred throughout the plant and were consistently found 

 even in the digestion chamber of the Imhoff tank. Nitrifying organisms increased 

 in the filter bed although they never became numerically predominant. Hydrogen 

 sulphide producing organisms were practically eliminated in the trickling filter. 



DISPOSAL OF SLUDGE 



The most troublesome and difficult feature of sewage treatment is the disposal 

 of the sludge produced in settling tanks. Fresh sludge is greasy, colloidal, and odor- 



' Levine, M., and Soppeland, L.: Proteolysis by Bacteria from Creamery Wastes, Engin. Exper. 

 Sta., Iowa State College, Ames, Iowa, Bull. S2. Oct. 13, 1926. 



= Gage, S. DeM.: "The Bacteriolysis of Peptones and Nitrates," /. Am. Chem. Sac, 17, 327. 

 1905. 



3 Buswell, A. M., and Neave, S. L.: Bio-chemistry of the Activated Sludge Process, III. State Water 

 Surv., Bull. 18, pp. 68-81. 1922. 



^ Hotchkiss, M.: "A Survey of the Bacteriological Flora of a Sewage Treatment Plant," /. Bad., 

 9, 437-54- Sept., 1924. 



