Problem 4: A stream with a flow of 5.0 m/s receives a wastewater discharge. The river contains 4.5 a mg/L of ultimate BOD and 10.3 mg/L of dissolved oxygen upstream from the wastewater flow, and is at a temperature of 8.5C. The wastewater is at a temperature of 15C and contains 52 mg/L of ultimate BOD and 1 mg/L dissolved oxygen. The BOD decay rate coefficient, kd, is 0.180 d-1 at 20C, and the river reaeration coefficient, kr, is 0.290 d 1 at 20C. Temperature correction coefficients (0) for ka and kr are 1.047 and 1.03, respectively. Use the temperature correction formula kt = k2007-20. a) Determine the dissolved oxygen, BOD and temperature just downstream of the discharge, assuming a wastewater flow rate of 30,000 m/d. b) What is the saturated dissolved oxygen concentration in the river downstream of the mixing zone under these conditions? c) Under the conditions given in the problem statement and in part (a) above, determine the downstream travel time, t, where the dissolved oxygen concentration passes through the minimum