Activated Sludge · Sludge Age and Food-to-Microorganism Ratio
What Is the Food-to-Microorganism Ratio?
The food-to-microorganism ratio, or F/M, compares the organic load entering biological treatment with the mass of microorganisms available to treat it. BOD is commonly used for food and MLVSS for microorganism mass.
To find F/M, divide the daily BOD load coming in by the mass of MLVSS in the aeration basin. Use the plant's approved sampling location and calculation boundary.
A higher F/M means more measured food per unit of measured biomass. A lower F/M means less food per unit of biomass. Neither condition is automatically good or bad without treatment goals and plant history.
F/M changes when flow, BOD concentration, basin volume in service, or MLVSS changes. Operators should identify which part changed before selecting a response.
Key point: F/M equals BOD load in pounds per day divided by MLVSS in pounds. More load with the same biomass means a higher F/M.
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Activated Sludge Process Control for Wastewater Operators
This page comes from the course. It covers aeration, settling tests, solids inventory, return and waste flows, sludge age, microscopy, process upsets and daily records, one step at a time, with short knowledge checks and a certificate of completion.
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