What combination of BOD reduction and wastewater DO increase is required so the Audhumla wastewater in Problem 9–28 does not reduce the DO below 5.00 mg · L−1 anywhere along the Einherjar River? Assume that the cost of BOD reduction is three to five times that of increasing the effluent DO. Because the cost of adding extra DO is high, limit the excess above the minimum amount such that the critical DO falls between 5.00 and 5.25 mg · L−1.
The Audhumla town council has asked that you determine whether the discharge of the town’s wastewater into the Einherjar River will reduce the DO below the state standard of 5.00 mg · L−1 at Gotterdämmerung, 5.79 km downstream, or at any other point downstream. The pertinent data are as follows:
Parameter | Wastewater | Einherjar River |
Flow (in m3 · s−1) | 0.280 | 0.877 |
Ult-BOD at 15°C (mg · L−1) | 6.44 | 7.00 |
DO (in mg · L−1) | 1.00 | 6.00 |
kd at 28°C (in day−1) | N/A | 0.199 |
kr at 28°C (in day−1) | N/A | 0.370 |
Speed (in m · s−1) | N/A | 0.650 |
Temperature (in °C) | 28 | 28 |
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