Why is Returned Activated Sludge (RAS) returned to the aeration basin?

• A. To maintain MLSS and SRT.
• B. To increase BOD in the influent.
• C. To discharge solids.
• D. To disinfect solids.

Answer: (A)

In an activated sludge system, you need enough active biomass in the aeration basin to break down the organic matter. Returning settled sludge from the secondary clarifier back to the aeration basin (RAS) keeps both the suspended solids concentration (MLSS) and the age of the sludge (SRT) within design ranges. By recycling part of the biomass, the aeration tank isn’t starved of microbes and the sludge isn’t washed out with the effluent, so the treatment process remains stable and effective.

This isn’t about increasing BOD in the influent—BOD is the organic load entering the plant, and RAS doesn’t add to it. It isn’t about discharging solids, which is what waste sludge would do, and it isn’t about disinfecting solids. The primary purpose of returning sludge is to maintain the right amount and age of biomass in the aeration basin.