F/M Ratio Calculator (Food to Microorganism)
The F/M ratio compares the organic load coming into the plant (food) with the volatile solids inventory in the aeration basin (microorganisms).
Result
F/M ratio
0.107 lb BOD/lb MLVSS·day
BOD load (food)
1,668 lb/day
MLVSS under aeration
15,638 lb
Your F/M ratio is 0.107: each pound of MLVSS under aeration receives about 0.107 lb of BOD per day. The right range depends on your process type and design, so compare it with your plant's trend.
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The formula
F/M (lb BOD/lb MLVSS/day) = (Q × BOD) ÷ (V × MLVSS)- Q
- Influent flow (MGD)
- BOD
- Influent BOD (mg/L)
- V
- Aeration basin volume (MG)
- MLVSS
- Mixed liquor volatile suspended solids (mg/L)
Good to know
- The 8.34 lb/gal factor appears on both sides of the formula, so it cancels out of the ratio.
- Trend F/M with SVI, DO, SRT and effluent results. The right range depends on your process type and design.
Read the full guide: F/M Ratio in Activated Sludge: How to Calculate It
Frequently asked questions
Why use MLVSS instead of MLSS for F/M?
MLVSS estimates the volatile (mostly biological) part of the solids, so it is a closer stand-in for the microorganisms that eat the BOD.
What happens to the 8.34 factor in F/M?
It appears on both the food side and the microorganism side of the formula, so it cancels out of the ratio.
What is a good F/M ratio?
The right range depends on your process type and design. Trend F/M with SVI, DO, SRT and effluent results rather than judging one reading alone.
Stop running this math by hand
ComplyFlo for wastewater plants can calculate and trend SRT, F/M, HRT, RAS, SVI and removal efficiency from your daily readings, next to your plant dashboard and DMR-ready reports.
See ComplyFlo for wastewater plantsFree checklist
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