F/M compares the daily BOD load with the biomass under aeration. Conventional plants often run about 0.05 to 0.4 lb BOD/lb MLVSS/d.
F/M Ratio
Formula
F/M = (Flow, MGD x BOD, mg/L x 8.34) / (Aeration volume, MG x MLVSS, mg/L x 8.34)
Inputs used
- Flow to aeration (MGD)
- Aeration influent BOD (or CBOD) (mg/L)
- Aeration basin volume (MG)
- MLVSS (mg/L)
Worked example (White River WRRF, fictional)
- F = 8.0 x 150 x 8.34 = 10,008 lb/d
- M = 3.6 x 2,340 x 8.34 = 70,256 lb
- F/M = 10,008 / 70,256 = 0.142
Tips
- Use aeration influent BOD (primary effluent).
- Use MLVSS, not MLSS, unless your plant reports F/M on MLSS (state which).
- F/M and SRT move in opposite directions.
Results are estimates for operator use. Check units and inputs, and confirm decisions with plant data and procedures.