There are five stages in the treatment process:
#Fill
#React
#Settle
#Decant
#Idle
First, the inlet valve is opened and the tank is filled, while mixing is provided by mechanical means, but no air is added yet. This stage is also called the anoxic stage. During the second stage, aeration of the mixed liquor is performed by the use of fixed or floating mechanical pumps or by transferring air into fine bubble diffusers fixed to the floor of the tank. No aeration or mixing is provided in the third stage and the settling of suspended solids starts. During the fourth stage the outlet valve opens and the "clean" supernatant liquor exits the tank.