376 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1956 



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 Figure 5. — Simplex aerator. 



ported by and rotates about a horizontal shaft, extending along and 

 adjacent to one of the walls of an aeration tank at the surface of the 

 sewage. A longitudinal vertical baffle, reaching from the surface of 

 the sewage to a point near the bottom of the tank, is provided below 

 the paddle wheel and about 18 inches away from the wall supporting 

 the aerator. As the paddle wheel rotates, it not only breaks up the 

 surface of the sewage, thus exposing it to the atmosphere, but it also 

 imparts a spiral motion to the flow of sewage in the tank. Sewage is 

 drawn upward between the above-mentioned baffle and tank wall from 

 the bottom of the tank. The sewage then flows horizontally across 

 the surface of the tank in contact with the air ; and after reaching the 

 opposite tank wall, the flow is directed downward to the tank bottom. 

 Aeration tanks provided with this type of aerator have been installed 

 at Collingswood, N. J., and Fort Atkinson, Wis. 



Diffused-air aeration. — Various types of compressors are used to 

 supply air to aeration tanks. During the early history of the activated- 

 sludge process single-stage piston compressors were used. At the 

 present time positive-pressure blowers or centrifugal compressors are 

 generally adopted. The advantage of positive-pressure blowers is that 



