Problem Solved: MCRT Problem - Wastewater Math

This question comes from the California Wastewater grade 2 certification exam sample questions that can be found here: http://www.swrcb.ca.gov/water_issues/.... The problem reads: At an activated sludge wastewater treatment plant receiving 3.25 MGD, the final effluent suspended solids concentration averages 21.2 mg/L. What would the calculated MCRT value be when the aeration basin carries 2050 mg/L MLSS and wastes 0.0550 MGD. The waste activated sludge has a concentration of 7980 mg/L. The aeration tank has a volume of 1.00 MG and the secondary clarifier has an operational volume of 0.250 MG. a. 2.5 days b. 5.0 days c. 7.5 days d. 15 days e. 42 days http://americanwatercollege.org