4. (20 points) The volumetric flow to the ACME wastewater treatment plant is 25 MGD .The influent BODS is 200 mg/L and the effluent BODS is 8 mg/L. The plant

is designed to operate with a mean cell retention time of 10 days. The yield coefficient is 0.6 mg biomass/mg substrate, and the biomass decay rate is 0.09/day. The reactor is to operate at a biomass concentration of 2500 mg/L. The settled sludge biomass concentration from the secondary clarifier is 9000 mg/L. Calculate the required volume (in gallons) of the activated sludge reactor.