The 'return sludge ratio' in an activated sludge plant is typically:
Choose the correct answer
100–200% of influent flow
25–50% of influent flow (to maintain MLSS in aeration tank)
5% only
300% always
Correct Answer
B. 25–50% of influent flow (to maintain MLSS in aeration tank)
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Return sludge (RAS) recycles settled sludge from secondary clarifier back to aeration tank to maintain required MLSS (Mixed Liquor Suspended Solids = 2000–4000 mg/L). RAS ratio = Qr/Q (flow of return sludge / influent flow) ≈ 25–50% for conventional ASP. Adequate MLSS ensures sufficient microorganisms for BOD removal.
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