The activated sludge (bio mass) collected in the final clarifier and returned to the secondary treatment process to mix with the incoming wastewater is:

• A. Mixed Liquor Suspended Solids (MLSS)
• B. Waste Activated Sludge (WAS)
• C. Mixed Liquor Volatile Suspended Solids (MLVSS)
• D. Return Activated Sludge (RAS)

Answer: (D)

Return Activated Sludge is the portion of settled biomass that is pumped from the final clarifier back into the aeration or secondary treatment process to mix with incoming wastewater. This return helps maintain an adequate concentration of active microorganisms in the system, preserving biological treatment efficiency and proper solids retention time.

MLSS is the overall solids concentration in the mixed liquor, including both living biomass and inert material, not the action of returning sludge. MLVSS represents the organic (volatile) portion of MLSS, indicating the active biomass, but again it describes a measurement, not the recirculation itself. Waste Activated Sludge is the portion of sludge purposely removed from the system to control MLSS, not the material returned to the treatment process.