CEE 577 29 Oct 1997

MID-TERM EXAM

Closed book, 1 sheet of notes allowed.

1. (60%) Crystal Creek emerges from pristine headwaters and runs through an agricultural region before it reaches the city of Metropolis. The BOD of the headwaters is essentially zero, and the dissolved oxygen is at saturation (10.1 mg/L for 15oC). Starting at mile point zero, there is a significant non-point agricultural runoff of BOD amounting to 50 kg/mile/day. At mile point 8.0, the stream leaves the agricultural area, and is met by the Metropolis WWTP outfall. Here a WW flow of 4 cfs is discharged with a BOD of 20 mg/L and a DO of 6 mg/L. Calculate the dissolved oxygen concentration 2 miles downstream of the Metropolis WWTP outfall (T=15oC). Assume the flow is constant at 40 cfs from the headwaters to the WWTP outfall. Assume no significant SOD.

U = 0.150 ft/sec = 2.45 miles/day T = 15oC

DOsat or Cs = 10.1 mg/L (at 15oC) H = 6 ft = 1.83 m

CBOD deoxygenation rate (kd) = 0.8 day-1 (at 15oC) for kd, q =1.047

CBOD settling rate (ks) = 0.080 day-1 (at 15oC) for reareation, q =1.024