one question: A wastewater treatment plant need in 3 hours

 Problem: A wastewater treatment plant discharges to a receiving stream. After mixing of the wastewater effluent and the receiving stream, the following data apply: o COD5= 6 mg/L o BOD5 = 8 mg/L o BOD ult= 10 mg/L o Wastewater Temperature: 24◦C o Upstream Temperature: 18◦C o Downstream Temperature: 20◦C Reaeration constant (Base e at 20◦C)= 0.40 1/d Deoxygenation Constant (Base e at 20◦C)= 0.23 1/d Reaeration Temperature Coefficient = 1.024 Deoxygenation Temperature Constant = 1.047 Dissolved Oxygen = 6 mg/L Stream Velocity = 1 fps Assuming the time of travel required to achieve the maximum dissolved oxygen deficit in the stream is 3 days, what is the minimum dissolved oxygen concentration (mg/L) in the river? [Answer: 6.2]