F/M Ratio (Food-to-Microorganism)
F/M = BOD Loading (lbs/day) ÷ MLSS (lbs). Typical conventional range: 0.2–0.5.
The F/M ratio compares the food (organic load) entering the system to the mass of microorganisms (MLSS) available to consume it.
Formula: F/M = [BOD, mg/L × Flow, MGD × 8.34] ÷ [MLSS, mg/L × Aeration Volume, MG × 8.34], which simplifies to (BOD × Flow) ÷ (MLSS × Volume) since the 8.34 factor cancels.
Worked example: influent BOD = 220 mg/L, flow = 4.0 MGD, MLSS = 2,200 mg/L, aeration volume = 1.6 MG. F/M = (220 × 4.0) ÷ (2,200 × 1.6) = 880 ÷ 3,520 = 0.25 lb BOD/lb MLVSS/day — solidly in the conventional range.
Conventional activated sludge: 0.2–0.5 lb BOD/lb MLVSS/day. Extended aeration: 0.05–0.15. High-rate systems can run above 1.0. Oxidation ditches typically run 0.05–0.3.
A low F/M means microorganisms are starved and will over-oxidize, producing a dark, pin-flock sludge that shears easily and can pass through the clarifier as light, powdery carryover.
A high F/M means microorganisms are overloaded, often producing a light, filamentous, poorly settling floc with visible foaming.
F/M should always be evaluated against MLVSS (volatile, i.e. biologically active solids), not raw MLSS, at plants carrying significant inert or chemically precipitated solids — using total MLSS in that case understates the true food-to-organism ratio.
Use F/M alongside SVI: a healthy F/M with poor settling usually points to a filament issue rather than a loading issue; a swinging F/M with stable settling usually means the biology has enough buffer capacity to absorb normal load variation.
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