The initial ultimate BOD after mixing of the Bergelmir River is 12.0 mg · L−1. The DO in the Bergelmir River after the wastewater and river have mixed is at saturation. The river temperature is 10◦ C. At 10◦ C, the deoxygenation rate constant (kd) is 0.30 day−1 , and the reaeration rate constant (kr) is 0.40 day−1 . Determine the critical point (tc) and the critical DO. tc = 2.88 days. Critical DO = 7.53 mg · L−1