The ultimate BOD of the Susquehanna River just below a sewage outfall from Harrisburg is 50.0 mg/L, and the DO is at the saturation value of 10.0 mg/L. The deoxygenated rate coefficient (kd) is 0.30 day-1, and the reaeration rate constant (kr) is 0.90 day-1. The river is flowing at a speed of 48 miles/day. The only source of BOD is the sewage outfall. Required:
a. Find the critical distance downstream at which DO is a minimum.
b. Find the minimum DO.
c. If a wastewater treatment plant were to be built, what fraction of BOD would need to be removed from the sewage to ensure that downstream DO concentrations stay above 5.0 mg/L