356 BACTERIA IN THE TREATMENT OF SEWAGE 



ous, usually containing only about 5 per cent solids. When removed from settling 

 tanks it will not dry on sand beds, wiU not filter in filter presses, and remains a nui- 

 sance for an indefinite period. When retained in septic tanks, as formerly practiced, 

 it becomes partially digested to a less objectionable condition, but meanwhile is foul- 

 ing the effluent and producing odors. Imhoff sludge digests in the lower compartment 

 of the tank, fairly well separated from the upper settling compartment. When diges- 

 tion is satisfactory the sludge has very little odor, is full of gas bubbles, contains 

 from 12 to 18 per cent soHds, and dries well on sand beds. When digestion is not sat- 

 isfactory, foaming and expulsion of solids occur, the sludge is high in moisture, will 

 not dry, and has a bad odor. Difficulties in the digestion of sludge have led to various 

 studies of sludge digestion during the past ten years. The process is purely biological, 

 and when proceeding normally and under control it is the most satisfactory and eco- 

 nomical way of disposing of sewage solids. 



Scientific work on sludge digestion has been in progress in Stroganoff's' labora- 

 tory in Moscow, in the Emscher district in Germany under the direction of Bach and 

 Sierp,^ and at Harvard University under Fair^; but by far the most extensive work 

 has been done by Rudolfs^ and his colleagues at the New Jersey Sewage Experiment 

 Station, New Brunswick (N. J.). 



The characteristic feature of the first stage of sludge digestion is the production 

 of increasing amounts of CO2 and organic acids from easily available carbohydrates 

 (sugars, soluble starches, and cellulose). It was formerly believed that cellulose was 

 very resistant to bacterial attack, but Heukelekians has recently shown that it is di- 

 gested comparatively rapidly. Approximately 80 per cent was digested in the first 

 three weeks, while the digestion of the nitrogenous material takes several months. 

 The CO2 and acids produced during the first two weeks of digestion of fresh solids 

 at 20° C. may lower the pH from 7.0 to 5.5 or less, the acidity depending upon the 

 temperature and the organic matter in the sludge. After this initial acidity the basic 

 products of digestion slowly neutralize the acid, the pH rises to 6.5-7.0, and prote- 

 olysis becomes active. Gas formation, which increases considerably during the first 

 stage, decreases during this stage, but the percentage of methane increases. Follow- 

 ing this stage of proteolysis and digestion of acids comes the final period of intensive 

 digestion of more resistant materials, the more complex proteins, fats, lignins, and 

 soaps. The gas volume and percentage of methane increase, pH increases to 7.0- 

 7.5, and finally after from six to nine months' storage the sludge is completely digested 

 and ready for drying. 



' Stroganoff, S. N.: Anaerobic Decomposition of Sewage Sludge, Slh Rep. Sew. Research Commis. 

 of Moscow, p. 85. 1926. (German abstract.) 



^ Bach, H., and Sierp, F.: "Untersuchungen zur Frage der Sumpfgasbildung aus Abwas- 

 serklarschlamm," Cenlralbl.f. Bakteriol., Abt. II, 60,318-28. 1924. 



3 Fair, G. M.: "Sludge Digestion — Reaction and Control," /. Boston Soc. Civ. Engin., 14, No. 

 2, 82-130. Feb., 1927. 



■t Rudolfs, W. M.: 4th Rep. Sew. Substa. New Jersey Agric. Exper. Sta., New Brunswick, N.J, 

 1925; 5//i Rep. Sew. Substa. New Jersey Agric. Exper. Sta., Neiv Brunswick, N.J. 1926. 



5 Heukelekian, H.: "Decomposition of Cellulose in Fresh Sewage Solids," Indust. 6* Engin. 

 Client., 19, 928-30. Aug., 1927. 



