What is the term for solids returned from the clarifier back to the aeration tank?

• A. WAS
• B. MLSS
• C. Returned Activated Sludge (RAS)
• D. Sludge age

Answer: (C)

In an activated sludge system, a portion of the settled solids from the secondary clarifier is pumped back into the aeration tank to maintain an active biomass and keep the mixed liquor suspended solids (MLSS) at the right level. This return flow is called Returned Activated Sludge (RAS). It ensures there’s enough microorganisms in the aeration basin to treat the incoming wastewater efficiently. The material removed from the system to control biomass is Waste Activated Sludge (WAS). MLSS is the concentration of suspended solids in the mixed liquor, and sludge age refers to how long the solids stay in the system. So the solids returned to the aeration tank are called Returned Activated Sludge.