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MLSS / F:M Ratio Calculator

Calculate activated sludge food-to-microorganism ratio and biomass loading.

m³/day (1 m³/day = 1 KLD)
mg/L entering the biological aeration stage
Total active aeration volume in m³
Mixed Liquor Suspended Solids, mg/L
Used to estimate MLVSS when direct MLVSS is unavailable.
Advanced Settings
If entered, measured MLVSS overrides the MLVSS/MLSS ratio.
Food-to-Microorganism Ratio
–
kg BOD₅ / kg MLVSS·day
Estimated / Measured MLVSS
Daily BOD Load
MLVSS Biomass Inventory
Hydraulic Retention Time
About F:M Ratio

The food-to-microorganism ratio represents the organic loading applied to the biological solids inventory in an activated sludge aeration system. It is commonly expressed as kilograms of BOD₅ applied per kilogram of MLVSS per day.

MLVSS is generally a better representation of the biological/volatile fraction of mixed liquor than MLSS. When laboratory MLVSS is available, enter it directly. Otherwise, this calculator estimates MLVSS from the user-selected MLVSS/MLSS ratio.

F:M should not be interpreted in isolation. Appropriate operating conditions depend on the activated sludge process configuration, wastewater characteristics, solids retention time, temperature, dissolved oxygen, settleability and treatment objectives. Plant-specific design and operating criteria should take precedence over generic ranges.

Reference basis: US EPA activated sludge process-control literature, which expresses F/M as organic substrate loading relative to aeration basin volatile suspended solids (MLVSS). Results are intended for engineering screening and operational calculations.