F. W. MOHLMAN 353 



cellulose pass through organic acids of various stages of saturation to carbon dioxide 

 and methane, protein and urea are ammonified, and fats, which are usually present 

 as soaps, are slowly liberated by increased CO2 production, and gradually gasified, 

 although much more slowly than under aerobic conditions. 



This stage of active putrefaction slowly gives way to semi-aerobic conditions due 

 to absorption of oxygen. Colloids are coagulated and deposited, nitrites and nitrates 

 increase, and finally algae and green growths appear. The liquid remains stable dur- 

 ing this stage of decomposition, and may even become supersaturated with dissolved 

 oxygen, due to photosynthesis, but the deposited insoluble solids remain anaerobic 

 for long periods, and when the algae die the sewage may again putrefy. This sequence 

 of biological phenomena may be observed in any small, polluted stream in summer 

 when raw sewage is discharged with little opportunity for dilution or re-aeration, and 

 when th time of flow is several weeks. 



When solids are removed by sedimentation from the sewage before discharge 

 and dilution plus re-aeration are sufficient to maintain dissolved oxygen at all times, 

 the first stage of putrefaction is eliminated and decomposition continues aerobically, 

 at least through the algae stage. Secondary putrefaction may follow, however, if the 

 algae die suddenly. 



The course of oxygen requirement may be shown by results of biochemical oxy- 

 gen-demand determinations, which are carried on under aerobic conditions. A typical 

 oxygen-demand curve for diluted sewage taken from the Chicago Drainage Canal at 

 Lockport is shown in Figure i. This curve, which is similar to Theriault's,' follows the 

 first order (mono-molecular) equation quite well up to ten days, but after eleven or 

 twelve days there is a lag, followed by another rise, which continues for many weeks 

 or months. There is some difference of opinion as to the cause of this secondary rise, 

 but the opinion is now rather general that it is the stage of nitrification, and that 

 carbon oxidation is predominant during the first stage. 



This curve indicates the course of aerobic decomposition of sewage in nature; 

 but when dilution, re-aeration, or time are lacking, artificial processes must be used to 

 accelerate the process and accomplish in hours what requires weeks in nature. Solids 

 must first be removed by sedimentation, stabilized and dried; proteolytic, ammoni- 

 fying, and nitrifying bacteria must be concentrated and their functions accelerated; 

 many re-saturations of dissolved oxygen must be obtained; and, finally, pathogenic 

 bacteria must be destroyed. It is the function of sewage treatment to accomplish 

 these requirements by artificial processes which should be economical, as free from 

 odor as possible, and subject to scientific control. 



PROCESSES or SEWAGE TREATMENT 



Complete treatment of sewage requires biological action, but there are many pre- 

 liminary processes which are purely physical or chemical. Coarse and fine screens are 

 used as strainers, sand and cinders are removed in grit chambers. Settling processes 

 do not depend on bacteria — in fact, they should be as free from bacterial growth as 



'Theriault, E. J.: "The Rate of Deo.xygenation of Polluted Waters," Pub. Health Rep., U.S. 

 Pub. Health Serv., 41, No. 6, 207-17. Feb. 5, 1926. 



