The contact stabilization process should have a F/M ratio of __ to __ and a contact tank detention time of __ to ___ hrs.

• A. 0.2 to 0.6; 0.75 to 2
• B. 0.3 to 0.7; 1.0 to 3
• C. 0.1 to 0.4; 0.5 to 1
• D. 0.6 to 1.0; 2.5 to 5

Answer: (A)

In a contact stabilization setup, you want the wastewater to have a moderate amount of food relative to the microorganisms and to spend only a short time in the contact tank. The F/M ratio measures how much organic matter (food) is available per unit of active biomass (microorganisms). A moderate range, about 0.2 to 0.6 pounds of BOD per pound of MLSS per day, keeps the microorganisms actively digesting without becoming overloaded or starving. If the F/M is too high, growth and oxygen demand can surge, hurting settleability and process control; if it’s too low, the system wastes potential treatment capacity.

The contact tank should provide enough time for good mixing and contact with the biomass but not so long that solids start aging or the system becomes inefficient. A detention time of roughly 0.75 to 2 hours fits this goal, giving enough contact for organic removal while keeping the process flow through the plant efficient. The stabilization tank then handles the longer digestion and aging steps.

So, the combination of a moderate F/M ratio and a short contact-tank detention time aligns with typical design practice for contact stabilization.