The Metropolitan Wastewater Treatment plant in St. Paul (180 MGD; BOD5 = 220 mg/l, DO = 3 ppm) discharges into the Mississippi River (flow = 18,000 ft3/s; mean depth = 14 ft; mean width = 0.3 miles). The dissolved oxygen in the river is at the saturation concentration of 9.0 ppm and has an ultimate BOD of 2.0 mg/L. The deoxygenation rate for BOD is 0.23 day-1 and the re-aeration rate is 0.15 day-1. a. Calculate the ultimate BOD in the Wastewater discharge. (322 ppm) b. Calculate the ultimate BOD in the river at the point where the discharge and the river meet. What is the value of the initial deficit to be used in the DO sag curve? (0.09 ppm) c. Graph DO (in ppm) as a function of distance in miles. Print out the graph and attach it to the homework. d. What is the distance where the concentration of DO would be a minimum? (feel free to just label the graph) (70 miles) e. Calculate the minimum DO at the critical time (feel free to label the graph) (5.87 ppm) f. What is the minimum DO concentration if kr is 0.1 day-1 instead of 0.15 day- 1? (To answer you can either print out a new graph or solve the equation to find the answer.) (5.33 ppm) g. Name one assumption that you made to solve this problem. What would happen if this assumption is not true? (note: You can use the givens as assumptions since in the real world you would need to question these givens.)
The Metropolitan Wastewater Treatment plant in St. Paul (180 MGD; BOD5 = 220 mg/l, DO = 3 ppm) discharges into the Mississippi River (flow = 18,000 ft3/s; mean depth = 14 ft; mean width = 0.3 miles). The dissolved oxygen in the river is at the saturation concentration of 9.0 ppm and has an ultimate BOD of 2.0 mg/L. The deoxygenation rate for BOD is 0.23 day-1 and the re-aeration rate is 0.15 day-1. a. Calculate the ultimate BOD in the Wastewater discharge. (322 ppm) b. Calculate the ultimate BOD in the river at the point where the discharge and the river meet. What is the value of the initial deficit to be used in the DO sag curve? (0.09 ppm) c. Graph DO (in ppm) as a function of distance in miles. Print out the graph and attach it to the homework. d. What is the distance where the concentration of DO would be a minimum? (feel free to just label the graph) (70 miles) e. Calculate the minimum DO at the critical time (feel free to label the graph) (5.87 ppm) f. What is the minimum DO concentration if kr is 0.1 day-1 instead of 0.15 day- 1? (To answer you can either print out a new graph or solve the equation to find the answer.) (5.33 ppm) g. Name one assumption that you made to solve this problem. What would happen if this assumption is not true? (note: You can use the givens as assumptions since in the real world you would need to question these givens.)
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