Define F:M using BOD and MLSS

• A. F:M is the ratio of flow rate to mixing energy.
• B. F:M is the ratio of filtration to membrane pore size.
• C. F:M is the food:microorganism ratio where the 'food' is BOD and the microorganisms are the MLSS/MLVSS.
• D. F:M is the ratio of fecal matter to microbes.

Answer: (C)

F:M is the amount of food available per unit of microorganisms in the activated-sludge system. In this context, the “food” is represented by BOD (Biochemical Oxygen Demand) and the microorganisms are represented by the biomass measured as MLSS or MLVSS. So F:M is the ratio of the BOD load (food) to the mass of microorganisms (MLSS/MLVSS) in the reactor, often expressed as daily BOD relative to MLSS/MLVSS. This ratio helps indicate how aggressively the biomass will grow and how much oxygen the system will require: a high F:M means plenty of substrate per biomass, promoting faster growth and higher oxygen demand, while a low F:M means less food per biomass, slowing growth. The other ideas don’t describe this balance between substrate and biomass.