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.
This calculator also appears on ActivatedSludge.com, our consulting site. Results are for training and operator use; always follow your plant’s procedures.