An activated sludge system (sketched below) is operating at equilibrium with the following information. Wastewater related data: flow rate $= 500 \: m^3/hour$, influent BOD $=150 \: mg/L$, effluent BOD $=10 \: mg/L$. Aeration tank related data: hydraulic retention time $=8$ hours, mean-cell-residence time $=240 $ hours, volume $=4000 \: m^3$, mixed liquor suspended solids $=2000 \: mg/L$.
The food-to-biomass (F/M) ratio (in kg BOD per kg biomass per day) for the aeration tank is
- $0.015$
- $0.210$
- $0.225$
- $0.240$