A wastewater treatment plant discharges 1.0 m3/s of effluent having an ultimate BOD of 40 mg/L into a stream flowing at 10.0 m3/s. Just upstream from the discharge point, the stream has an ultimate BOD of 3.0 mg/L. The deoxygenation constant kd is estimated at 0.22/day.

a. Assuming complete and instantaneous mixing, find the ultimate BOD of the mixture of waste and river just downstream from the outfall.

b. Assuming a constant cross-sectional area for the stream equal to 55 m2, what BOD would you expect to final at a point 10,000 m downstream.