SRT (Sludge Age) Calculator
Calculate the solids retention time — also called sludge age or mean cell residence time (MCRT) — of an activated sludge process.
Solids retention time
Formula
SRT (days) = (V × MLSS) ÷ [Qw × Xw + (Q − Qw) × Xe]
V = aeration volume, Qw/Xw = waste sludge flow and concentration, Xe = effluent TSS. Units cancel as long as V and Q use the same volume basis.
SRT (days) = (V × MLSS) ÷ [Qw × Xw + (Q − Qw) × Xe]
V = aeration volume, Qw/Xw = waste sludge flow and concentration, Xe = effluent TSS. Units cancel as long as V and Q use the same volume basis.
Typical SRT ranges
| Process | SRT (days) |
|---|---|
| Conventional activated sludge | 5 – 15 |
| Extended aeration | 20 – 30 |
| Nitrification | 8 – 15 (temperature dependent) |
| High-rate / contact stabilization | 2 – 5 |
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