Problem 4. The ultimate BOD of a river just below a sewage outfall is 50.0 mg/L and the DO is at the saturation value of 10.0 mg/L. The deoxygenation rate

coefficient is 0.30 day and the reaeration rate coefficient, k, is 0.90 day. The river is flowing at 48.0 miles/day. The only source of BOD is this outfall. A. Find the critical distance downstream at which DO is a minimum. B. Find the minimum DO. C. If a wastewater treatment plant is to be built to treat the sewage, what fraction of the BOD would have to be removed to assume a minimum of 5.0 mg/L downstream?