SEWAGE TREATMENT — KEEFER 375 



sludge, called mixed liquor, flows to sedimentation tanks. The clari- 

 fied effluent from these tanks can be discharged into a watercourse, 

 and the settled solids, called activated sludge, is also removed. The 

 major portion is added to the sewage flowing to the aeration tanks, 

 and the remainder is treated and disposed of in a variety of ways. 



The activated-sludge process has several advantages. It is free 

 from odors and flies, and the necessary treatment units occupy less 

 space and are less costly to construct than trickling filters and humus 

 tanks. The disadvantages of the process are that it is costly to operate, 

 and it is frequently adversely affected by industrial wastes in the 

 sewage. 



Authorities differ as to how the process functions. The various 

 reactions involved have been stated to be biological, biochemical, 

 physiochemical, base-exchange and enzymic. There seems to be no 

 question that the bacteria and Protozoa, which are present in vast 

 numbers in the activated sludge, play a major role in the transforma- 

 tion of the nitrogenous and carbonaceous substances in the sewage into 

 simpler and more stable compounds. The three requirements of the 

 process are biologically active sludge, an ample supply of air, and an 

 intimate mixing of the sludge and the sewage for a sufficient time. 



METHODS OF AERATION 



Three methods of introducing air into sewage, to which activated 

 sludge has been added, are used. These are (1) mechanical aeration, 

 (2) diff used-air aeration, and (3) a combination of these two methods. 

 Mechanical aeration consists of providing mechanical means of intro- 

 ducing air from the atmosphere at the surface of the sewage-sludge 

 mixture flowing through aeration tanks. Diffused-air aeration in- 

 volves blowing compressed air through nozzles, perforated pipes, or 

 porous diffusers at some distance below the surface of the mixed 

 liquor. 



Mechanical aeration. — Many different types of mechanical aeration 

 have been perfected. Only two will be described. One of these, called 

 a Simplex aerator (text fig. 5), was developed in Bury, England. 

 This aerator consists of a steel cylinder, which is placed in a vertical 

 position on the center line of a relatively deep tank, with the bottom 

 a few inches above the tank floor. Attached to the top of the cylinder 

 at the surface of the sewage there is a rotating cone with steel vanes, 

 driven by an electric motor. As the cone rotates, it draws the mixed 

 liquid up the cylinder and throws it out over the surface of the sewage 

 in a spray. Oxygen is absorbed from the air by the spray and the 

 agitated surface of the sewage. Simplex aerators have been provided 

 at Princeton, 111., and Dunsmuir, Calif. 



A second type of aerator (fig. 6) embodies the use of a paddle wheel 

 about 30 inches in diameter in the form of latticework, which is sup- 



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