In the notes, what does the F/M ratio measure?

• A. Temperature coefficient
• B. Food to microorganism ratio. A measure of food provided to bacteria in an aeration tank in relation to the quantity of microorganisms in the tank. Control by amount of sludge wasted or returned. Ideal .05 lbs BOD/ lbs MLVSS
• C. Oxygen transfer rate
• D. Solids retention time

Answer: (B)

The F/M ratio measures the amount of food (organic matter, typically BOD) available to the microorganisms in the aeration tank relative to the mass of microorganisms present (MLVSS). It shows how heavily the reactor is loaded with substrate. A higher ratio means more food per microorganism, driving faster microbial growth and greater treatment activity, while a lower ratio means less food per organism, resulting in slower growth but a more stable, less active system. Control is achieved by adjusting the biomass in the tank through sludge wasting or returning: wasting reduces biomass and tends to raise F/M, while returning more sludge increases biomass and lowers F/M. The typical target is about 0.05 pounds of BOD per pound of MLVSS.