In the secondary clarifiers, quiescent conditions are maintained so that up to 99% of suspended solids are removed.
Similar to the primary clarifiers, skimmers skim any scum off the water surface, while blades scrape the solids from the bottom of the tank. To maintain an adequate population of bugs in the aeration basins, a portion of the solids are returned to the secondary treatment process. These solids are called "return activated sludge." The remainder of solids, called "waste activated sludge," is sent to the digesters for processing.
Blowers provide the aeration necessary to maintain aerobic conditions in the aeration basins.