What is the term for solids wasted from the treatment system?

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

Answer: (A)

Waste Activated Sludge is the portion of biomass that is intentionally removed from the activated sludge system to control the amount and age of sludge in the wastewater treatment process. By wasting this material, operators keep the mixed-liquor suspended solids (MLSS) at the target level and maintain proper sludge age for effective treatment. Returned Activated Sludge, on the other hand, is the portion of settled solids that is recycled back to the aeration basin to maintain a sufficient biomass. MLSS is simply the concentration of solids in the mixed liquor, not the waste itself. A sludge blanket is the dense layer of solids that forms in the clarifier, not the material being wasted.