Answer Happy

Q4 A city discharges 25 million gallons (US) per day (mgd) of domestic sewage into a stream with a typical flow rate of 250 cubic feet per second (cfs). The velocity of the stream is approximately 3 miles per hour. The temperature of the sewage is 20 °C, while that of the stream is 15 °C. The 20 °C BOD of the sewage is 180 mg/L, while that of the stream is 1.0 mg/L. The sewage contains no dissolved oxygen, but the stream is 90 percent of its saturated dissolved oxygen concentration. k1 is estimated to be 0.197 per day while k, is 0.587 per day. [40 marks] Note: ✓ Saturation dissolved oxygen concentration at 15 °C is 10.2 mg/L and after mixing (at 15.7 °C) is 10.1 mg/L. ✓ 1 Cubic Feet Per Second (ft3/s)=2446575.55 Liters Per Day (L/d) ✓ 1 million gallons per day= 3791911.8 Liters Per Day (L/d) ✓ 1mile=1.61 km (a) By the process of applying the critical time, deficit and dissolved oxygen obtained from the Streeter-Phelps model calculate, how far downstream (km) is the lowest DO, and what is the DO at that point? (b) Determine the dissolved oxygen concentration at 1000 km from the point source.