Into which two main locations can settled solids be returned or wasted?

• A. Sludge digester and aeration basin
• B. RAS and WAS
• C. Sludge blanket and thickener
• D. Final clarifier and gravity belt filter

Answer: (B)

In an activated sludge system, settled solids are managed by two main return/waste paths: returning a portion of the settled solids back to the aeration basin (return activated sludge) to maintain the needed biomass and treatment efficiency, and removing excess solids from the system to waste (waste activated sludge) to control sludge age and system performance. The return line helps keep the mixed liquor suspended solids (MLSS) at the right level, while the waste line removes excess solids so the process doesn’t become overloaded. The other components mentioned are treatment stages or equipment, not the two primary flows used to either return settled solids to the reactor or send them out of the system.