A municipal waste-water treatment plant discharges 18,925 m³/d of treated wastewater to a stream. The effluent discharge has a BOD5 of 30 mg/L with a k₁ of 0.23/d at 20 ⁰C. The temperature of the effluent is 27^oC, and the dissolved oxygen is 2.0 mg/L. The stream just above the point of wastewater discharge flows at 0.65 m³/s, has a BOD5 of 5.0 mg/L and is 90% saturated with oxygen. The temperature of the stream is 23 ⁰C. After mixing, the stream and wastewater flow at a velocity of 0.5 m/s and the reaeration constant is 0.45/d at 20 ⁰C.
I. What is the oxygen level of the stream at a distance of 50 km downstream of discharge?
II. What is the critical oxygen level in the stream?
III. How far downstream will (ii) occur?
IV. State clearly all assumptions made.
V. Sketch the DO profile in this stretch of the stream