Step Aeration| Activated Sludge Process | Wastewater Treatment Plant

 Tapered Aeration and Step Feed 

(Occasionally Described as Step Aeration)

In a conventional activated sludge system, the wastewater is aerated in a circular or square basin in which the mixed liquor is constantly agitated as the air is applied. The objective is to maintain a completely mixed environment wherein the concentration of BOD5, SS, and dissolved oxygen are constant throughout the basin. With this type of reactor, which is closely modeled as an ideal completely mixed reactor, the effluent concentration of BOD5, SS, and dissolved oxygen are essentially equal to the concentrations at any point within the reactor. Thus, the reactor has to be large enough to maintain suitable internal concentrations for effective discharge parameters to be met at the outlet from the secondary sedimentation basin that follows.

With a rectangular aeration basin, however, things change. A rectangular basin typically does not act as a completely mixed reactor, but rather one more closely resembling a modified plug flow reactor wherein the flow comes in at one end, undergoes chemical and biological change as it moves through the reactor, and then leaves at the back end with much different concentrations than those with which it entered the reactor. Consequently, the concentrations of BOD5 and SS concentrations are constantly declining as the wastewater flows through the reactor and the dissolved oxygen concentration tends to constantly increase.

The reason for using a rectangular basin is to reduce the volume of air required to maintain adequate dissolved oxygen concentration throughout the basin. The usual, or “conventional,” way to do this is through a schematic called “tapered aeration.” With tapered aeration, the volume of air applied is reduced as the wastewater moves through the tank—the most air is applied where the BOD5 is the highest and less air is used where the BOD5 is less.

A variation of that concept is based on the idea that adding influent to the aeration basin at various points along the basin's edge, thereby keeping the BOD5 concentration fairly constant, would allow for the use of a constant air supply, which is easier to implement, while still producing the same results as the tapered aeration concept. This variation is “step feed,” which really means “step loading” of the reactor.

Step feed concept splits the influent to the tank and applies it at three or four different places along the edge of the reactor.