Question

A city of 200,000 people deposits 37 cfs of sewage having a BOD of 28.0 mg/L and 1.8 mg/L of DO into a river that has a flow rate of 250 cfs and a flow speed of 1.2 ft/s. Just upstream of the release point, the river has a BOD of 3.6 mg/L and a DO of 7.6 mg/L. The saturation value of DO is 8.5 mg/L. The deoxygenation coefficient (kd) is 0.61/d, and the reaeration coefficient (kr) is 0.76/d. Assume complete and instantaneous mixing of the sewage and river. What is the minimum DO downstream?