Define Food-to-Microorganism (F/M) ratio and explain how it influences reactor performance.

• A. F/M ratio is the rate of substrate supply per unit mass of microorganisms; it governs growth rate, sludge age, and overall treatment efficiency.
• B. F/M ratio is the mass of microbes per unit of substrate.
• C. F/M ratio measures the oxygen demand of the system.
• D. F/M ratio is the ratio of solids to liquids in the reactor.

Answer: (A)

The main idea is that the F/M ratio represents how much substrate is available to each unit of microbial biomass. It’s the rate of food supply per unit mass of microorganisms, usually expressed as COD or BOD per day per unit of MLSS/VSS. This ratio directly controls how fast the microbes grow, how long the solids stay in the system (sludge age or SRT), and how effectively the reactor treats the wastewater.

When the F/M ratio is high, the microbes have plenty of substrate relative to their mass, so they grow quickly. You’ll see rapid removal of readily degradable organics, but biomass production increases, which means more waste sludge to maintain the desired SRT and more oxygen and energy are needed to support the heightened activity. If the F/M is kept high, the system can become less stable and settleability issues may arise unless sludge wasting is adjusted.

If the F/M ratio is low, growth slows and the sludge ages longer. This tends to stabilize the process, supports slower-growing organisms like nitrifiers, and can improve treatment stability, but the rate of organics removal drops and some biodegradable material may pass through if the feed is not matched to the system’s capacity.

The other ideas described in the choices don’t define F/M correctly: it isn’t simply the mass of microbes per unit substrate, nor is it a direct measure of oxygen demand or the solids-to-liquids ratio. The correct definition centers on substrate supply per unit biomass and its impact on growth, sludge age, and overall treatment performance.