Question

1. Wastewater of 30ft³/s with a biochemical oxygen demand (BOD) of 55mg/l, and dissolved oxygen (DO) of 3.3mg/l enters a river where the flow is 125ft³/s with an ultimate BOD of 7.0 mg/l, and DO of 6.8mg/l. The downstream has an average velocity of 0.8ft/s.

i           Calculate the minimum DO level and its distance in miles downstream using the oxygen sag equation. Assume k₁ = 0.046 per day and k₂ = 0.304 per day. The saturation DO = 8.5mg/l. (1 mile = 5280ft)

ii          At what distance downstream would the DO level of the river be equal to 6.10 mg/l?

iii      How much should the wastewater DO be increased to maintain a minimum DO of 4.8mg/l anywhere along the river. Assuming that the BOD of the wastewater and the river flow remains the same?

Answer 1