MLSS and F:M Ratio Calculator
Activated-sludge process loading: food-to-microorganism (F:M) ratio, mean cell residence time (SRT / sludge age), and volumetric BOD loading. Operating ranges and red flags for plant performance.
Defaults: 2 MGD plant, conventional activated sludge. F:M target = 0.2 to 0.5; SRT target = 4 to 15 days for BOD-only; 15+ days if nitrifying.
F:M target ranges (Metcalf & Eddy)
- Conventional activated sludge: 0.2 to 0.5 /day. Standard target.
- Extended aeration / oxidation ditch: 0.05 to 0.15 /day. Long detention, low loading, very stable effluent.
- High-rate / step-feed: 0.4 to 1.5 /day. Higher loading, more sensitive to upsets.
- Contact stabilization: 0.2 to 0.6 /day in contact tank only.
F:M outside the design range is the first thing to check when a plant struggles. Too high (over 0.6): washout risk, poor settleability (filamentous bulking), high effluent BOD. Too low (under 0.1): pin floc, nitrification of ammonia even when not designed for it, dispersed turbid effluent.
SRT and the nitrification threshold
SRT (sludge age) controls which microbial populations are sustained. Below ~4 days, only fast-growing heterotrophs persist and BOD is removed but ammonia passes through. Above ~7 to 10 days at 15°C, nitrifying bacteria can compete and ammonia is oxidized to nitrate. Modern plants requiring ammonia limits target 12 to 25 days SRT depending on temperature.
SRT also drives the trade-off between sludge production and oxygen demand. Long SRT minimizes WAS output but increases endogenous oxygen demand and energy use. Short SRT produces more sludge to dewater but uses less aeration energy per pound BOD removed.
Volumetric loading sanity check
Volumetric BOD loading (lb BOD/day per 1000 ft³, or kg BOD/m³/d in SI) is a quick design check. Conventional plants run 30 to 50 lb/d/1000 ft³ (0.5 to 0.8 kg/m³/d). Extended aeration is 10 to 25 (0.16 to 0.4 kg/m³/d). High-rate plants exceed 100 (1.6+ kg/m³/d) but are rare in modern POTWs.
Wasting strategy
The SRT formula assumes wasting from the return-activated-sludge (RAS) line at concentration Xw. If wasting from the aeration basin directly, Xw = X (basin MLSS) and you can waste much more volume to hit the same SRT. Hydraulic wasting is operator-friendly but uses more pump volume. Most plants waste from RAS to minimize WAS handling.
Reference: Metcalf & Eddy / AECOM (2014). Wastewater Engineering: Treatment and Resource Recovery, 5th ed., McGraw-Hill, ch. 8. Davis, M.L. (2010). Introduction to Environmental Engineering, 4th ed., ch. 9.