The contact stabilization process has an F/M ratio in which range?

• A. 0.1 to 0.3
• B. 0.2 to 0.6
• C. 0.4 to 0.8
• D. 0.6 to 1.0

Answer: (B)

The F/M ratio shows how much food (organic substrate) is available for the microorganisms for each unit mass of active biomass in the system. For contact stabilization, you want a moderate loading so the microbes have enough substrate to stay active but not so much that they overgrow and cause problems in settling or sludge production. The typical operating range is 0.2 to 0.6 per day (often expressed as pounds of BOD5 per day per pound of MLSS). When the ratio sits in this range, COD/BOD removal tends to be efficient and the solids stay stable. If the ratio is too low, substrate becomes limiting and treatment efficiency drops; if it’s too high, excessive substrate drives rapid biomass growth, leading to more sludge and potential settling and aeration issues.