A wastewater treatment plant discharges 2.19 m3/sec into a stream with a flowrate of 10 cubic meters per second. The ultimate BOD concentration in the treated wastewater is 30 mg/L, and 3 mg/L in the river. DO in the river is 6.5 mg/L and the DO in the treated wastewater is 4 mg/L. Saturation for DO at the ambient temperature is 8.2 mg/L. At the current temperature the deoxygenation coefficient, kd, is 0.61 day 1, while the reaeration coefficient kr = 0.76 day 1. The rivers flow velocity is 0.41 meters per second. How long does it take to reach the critical time in the river? a. 35.4 hours
b. 9.6 hours
c. 1.0 days
d. 1.7 days