Which component recycles settled solids from the secondary clarifier back to the aeration tank?

• A. WAS
• B. DO
• C. RAS
• D. Air supply

Answer: (C)

In the activated sludge process, a portion of the settled solids from the secondary clarifier is pumped back to the aeration tank to keep a healthy population of active microorganisms and maintain the proper solids concentration in the aeration basin. This recycling stream is called return activated sludge. Returning solids helps sustain biomass, prevents washout, and promotes good floc formation for effective settling later in the clarifier. The amount returned is balanced by wasting some sludge to control the overall solids level. Dissolved oxygen and the air supply are essential for aerobic digestion, but they don’t move solids back to the aeration tank.