Return Activated Sludge Rate

The aeration basin only holds solids because the clarifier keeps sending them back. Write a solids balance on the basin — return flow times return concentration equals the combined flow times the mixed liquor concentration — and the required return rate falls straight out. A 4 MGD plant holding 2500 mg/L MLSS on a 8000 mg/L return must pump 4 × 2500/(8000 − 2500) = 1.82 MGD of RAS, which is 45% of the influent flow. Typical returns run 25–75% for conventional plants and up to 100% or more for extended aeration.

What makes this equation subtle is that X_r is not a free choice — it is whatever the clarifier can actually thicken to, which depends on the sludge settleability and on how fast you pull. Pump harder and you get more flow at a lower concentration; pump gently and the blanket rises until solids go over the weir. The two classic failures sit at either end: too little return and the blanket climbs into the effluent launder, too much return and you hydraulically overload the clarifier with your own recycle and wash the floc out anyway. Many operators skip the arithmetic entirely and set RAS by the settleometer — return roughly the percentage of the cylinder the sludge occupies at 30 minutes — which is a surprisingly good approximation of exactly this balance.