Calculators / F/M Ratio
F/M Ratio calculator
F/M compares the BOD you feed the aeration basin each day with the pounds of microorganisms (MLVSS) already under aeration. Conventional activated sludge often runs around 0.2 to 0.5, but use your plant's and your study manual's ranges.
F/M = (Flow MGD × BOD mg/L × 8.34) ÷ (Aeration volume MG × MLVSS mg/L × 8.34)
Worked example
An aeration basin holds 1.2 MG. Flow is 3 MGD with a primary effluent BOD of 150 mg/L. MLVSS is 2,500 mg/L. What is the F/M ratio?
- Food = 3 × 150 × 8.34 = 3,753 lbs BOD/day
- Microorganisms = 1.2 × 2,500 × 8.34 = 25,020 lbs MLVSS
- F/M = 3,753 ÷ 25,020 = 0.15
Answer: 0.15
Common mistakes
- Using MLSS when the question gives MLVSS (or the reverse). Read which one the question asks for.
- Using influent BOD when the question gives primary effluent BOD to aeration.
Check your exam's reference sheet for its exact constants. Some use 3.14 instead of 3.1416, which changes answers in the last digit.