Food to Microorganism (F/M) Ratio Calculator
Model activated sludge organic food loading: calculate the F/M ratio in lbs BOD/day per lb MLVSS, diagnose floc settleability, and prevent bulking.
Influent Flow & Organic BOD Load
Aeration Basin & Active Biomass
F/M Loading Ratio
Volumetric & Hydraulic Metrics
Settleability & Bulking Diagnosis
Recommended Tools & Equipment
Tested hardware and components for high reliability
Frequently Asked Questions
What is the Food-to-Microorganism (F/M) ratio and why is it important?
The F/M ratio is a fundamental operational parameter that quantifies the balance between incoming organic food (pounds of BOD applied per day) and the active biological workforce (pounds of MLVSS in the aeration tank). It directly controls bacterial growth kinetics, floc formation, and effluent quality.
What is the difference between MLSS and MLVSS in the F/M formula?
Mixed Liquor Suspended Solids (MLSS) is the total weight of all suspended particles in the aeration tank, including inert sand, silt, and dead cell debris. Mixed Liquor Volatile Suspended Solids (MLVSS) is the organic, burnable portion (typically 70% to 80% of MLSS) that represents the living, active bacterial culture.
What problems occur if the F/M ratio is too high?
When F/M exceeds ~0.5 to 0.6, bacteria enter the "log-growth" phase where they have unlimited food. Instead of clumping together into heavy flocs, they remain dispersed single cells that will not settle in clarifiers, causing high turbidity and effluent permit violations.
What problems occur if the F/M ratio is too low?
When F/M drops below ~0.10, bacteria enter "endogenous decay" and starve. Flocs break apart into tiny, light "pin-point floc" that floats across clarifier weirs. Starved conditions also favor filamentous organisms like Microthrix parvicella that create viscous, chocolate-brown foam on aeration tanks.