The Return Activated Sludge (RAS) rate should be between what percent and what percent of the incoming sewage to the aeration tank?

• A. 15% to 75%
• B. 30% to 150%
• C. 5% to 50%
• D. 50% to 200%

Answer: (B)

Return activated sludge is the portion of settled sludge that is recycled from the secondary clarifier back to the aeration basin. Expressed as a percentage of the flow entering the aeration tank, this recycle rate is adjusted to keep enough biomass and a stable mixed liquor suspended solids (MLSS) for effective biological treatment. If the return rate is too low, the biomass isn’t replenished adequately, and treatment performance can drop. If it’s too high, you can overload the aeration tank or the clarifier downstream, leading to poor settling and process instability. The range that supports a stable, well-seeded system in most plants is about 30% to 150% of the influent flow to the aeration tank, with many plants operating near 100%. The other ranges either under-represent the necessary biomass return or push beyond practical operational limits for the clarifier and aeration system.