Tapered aeration

Many modifications for the conventional process have been proposed for improving the system or for treating a specific type of wastewater. These modified processes are extended aeration, contact stabilization, step aeration, tapered aeration, high rate, and pure oxygen process. 

A downcomer must be sufficiently large to transport all of the liquid downflow without choking. If the friction losses in the downcomer and or downcomer entrance are excessive, liquid will back up onto the tray and eventually flood the column. This is termed downcomer choke. The prime design parameter is the downcomer top area, where friction losses are highest- Further down the downcomer, vapor disengages and the aerated liquid downflow is greatly reduced. With sloped downcomers, the downcomer bottom area is normally set at about 1.7 to 2 times less than the area at the top of the downcomer (1,8,9,10,48). This taper is small enough to keep the downcomer top area the prime downcomer choke variable. 

A variety of feeding arrangements may be employed to ensure that particulate matter of appropriate bulk density is presented to the nip of the rolls. For smoothly flowing materials gravity feed may be used with a control device to adjust feed rate (Fig. 5.4). For fine powders which tend to bridge or stick and are of low bulk density, some form of forced feed (such as the tapered screw feeder shown in Fig. 5.5) must be used to de-aerate, pre-corn-... 

Tapered and Step Aeration. In New York City, plants scattered throughout the five boroughs are treating more than 3.8 x 10 (10 gal)... 

If light, fluffy, and aerated powders must be fed, tapered screws as depicted in Figure 226(c) may be applied. Owing to the volume reduction in the taper, a certain deaeration and predensification can be achieved in some cases. 

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