352 



BACTERIA IN THE TREATMENT OF SEWAGE 



is generally saturated with dissolved oxygen, containing 9.2 parts per million at 20° C. 

 As bacterial growth progresses in the sewage the dissolved oxygen decreases and dis- 

 appears in a few hours. This initial aerobic period is negligible in comparison with the 

 subsequent oxygen requirements of the sewage, which may range from 100 to several 

 thousand parts per million. Anaerobic species soon predominate, although it has been 

 shown that a comparatively small percentage of the total bacteria are obligate an- 

 aerobes. Facultative forms are usually found, and even oxidizing and nitrifying types 



TABLE I 



Seasonal Changes in Numbers of Bacteria Added to Streams 

 BY Sewered Populations 



City 



Chicago, III.: 



Summer 



Winter 



Year 



Peoria, III.: 



Summer 



Winter 



Year 



Cincinnati, O.: 



Summer 



Winter 



Year 



Louisville, Ky.: 



Summer 



Winter 



Year 



Averages : 



Summer 



Winter 



Year 



Des Plaines Treatment 



Works, Sanitary Distmct 



OF Chicago: 



Year 



Billions of Bacteria per Capita per Day 



Gelatin 

 20° C. 



A?ar 

 37° C 



Bad. coli. 



24 , 800 



4,260 



15,300 



15,770 



2,130 



1 1 , 050 



14, TOO 



2,590 



II ,770 



13,560 

 7,360 



I o , 840 



17,070 



4,080 



12,240 



612 



25 , 650 



847 



25,640 



7,650 



19,310 



18,310 



1 ,000 

 12,260 



15,840 

 907 



7,960 

 21,360 



2 ,600 

 I 2 , 680 



416 



428 



42 



230 



231 

 141 

 187 



583 

 119 



358 



2gi 



193 

 222 



383 

 124 

 249 



106 



survive, under completely anaerobic conditions, and later develop and complete the 

 work of stabihzation. 



The unstable organic matter in sewage is suitable for the predominant develop- 

 ment of anaerobic or reducing reactions. The food material is rich and complete, in- 

 cluding carbohydrates, proteins, fats, cellulose, and mineral salts. Normal sewage 

 has an optimum pH for bacterial growth, and is usually buffered sufficiently by bi- 

 carbonates to permit the inception of vigorous growth. Nevertheless, the limitation 

 of growth and delay of stabilization in the solids is to a large extent caused by change 

 in pH to a more acid reaction. 



If no artificial means are taken to prolong the initial aerobic period by absorption 

 of oxygen, anaerobic conditions develop, characterized by production of hydrogen 

 sulphide, indol, mercaptans, and other foul-smelling compounds. Carbohydrates and 



