The flow rate of the Spokane River is about 5,000 cubic feet per second (CFS). The Spokane River has an ultimate BOD value of 5 mg/L and DO of 6 mg/L. The Spokane Wastewater Treatment Plant (WWTP) discharges 50 million gallons per day (MGD) of treated wastewater with an ultimate BOD of 30 mg/L and DO equal to 8 mg/L.

What will be an ideal response?


a. What is the BOD [mg/L] immediately downstream of the discharge?





b. What is the DO [mg/L] immediately downstream of the discharge?





c. If the temperature for the mixed water at the point of discharge is 20ºC, what is the initial oxygen deficit in mg/L?





d. If the deoxygenation constant is 0.4 day–1 and the re-aeration rate is 0.8 day–1, what is the critical time [days]?





e. What would the minimum dissolved oxygen concentration in the Spokane River be in mg/L?

The WWTP actually improves water quality.







f. Plot the oxygen sag curve for the river.

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