Sludge Retention Time (SRT, also called sludge age or mean cell residence time) in the activated sludge process is calculated as:
Choose the correct answer
Volume of aeration tank / daily influent flow
Mass of MLSS in aeration tank / mass of SS leaving system per day
BOD of influent / BOD of effluent
HRT x recycle ratio
Correct Answer
B. Mass of MLSS in aeration tank / mass of SS leaving system per day
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SRT = (V x MLSS) / (Qw x MLVSS_waste + Qe x MLVSS_eff), where V = tank volume, Qw = waste sludge flow, Qe = effluent flow. Higher SRT = more complete BOD removal, more nitrification, lower sludge yield but older sludge. Typical SRT: 5-15 days for BOD removal.
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