Mean Cell Residence Time (Sludge Age)

Sludge age is the average time a bacterium stays in the plant before it is wasted, and it is the master control variable of activated sludge: pick the SRT and the MLSS, the F/M and the effluent quality all follow. Take the mass of solids in the aeration basin and divide by the mass leaving in the waste stream each day. A 4800 m³ basin at 2500 mg/L holds 12,000 kg; wasting 120 m³/d at 8000 mg/L removes 960 kg/d, so the SRT is 12.5 days.

The number that matters is the minimum SRT required to keep an organism in the system: anything that cannot double faster than it is wasted washes out. Heterotrophs managing BOD need only 2–4 days, but the nitrifiers that convert ammonia to nitrate are slow and temperature-sensitive, needing perhaps 5 days at 20 °C and 15–20 days at 10 °C, which is why nitrification fails every winter in plants that never adjusted their wasting. This simplified form ignores the solids that leave over the clarifier weir; the rigorous version divides by $Q_w X_w + Q_e X_e$, and on a plant with a poorly settling sludge losing 30 mg/L to the effluent, that second term is not small. Waste continuously and in small increments if you can: a plant that dumps its whole week's wasting on Friday afternoon is running a different SRT every day of the week.