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COMPUTATION OF MINIMUM DO (Cont)

EXAMPLE: The BODL in a stream is 3 mg/L and the DO is 9.0 mg/L. The streamflow is 15mgd. A treated sewage effluent with BODL =50mg/L is discharged into the stream at a rate of 5 mgd. The DO of the sewage effluent is 2 mg/L. Assuming that k1 = 0.2, k2 = 0.5, and the saturation DO level is 11 mg/L, determine the minimum DO level in the stream. For a stream velocity of 0.5 ft/s, how far downstream does the minimum DO occur?

SOLUTION: First, it is necessary to compute the diluted BODL and DO using the Mass Balance Equation

BODL = (15 X 3 + 5 X 50)/15 + 5 = 295/20 = 14.8mg/L

DO = (15 X 9 + 5 X 2)/15 + 5 = 145/20 = 7.3 mg/L

Now compute the initial oxygen deficit as

Di = saturation DO Initial DO = 11.0 7.3 = 3.7mg/L

Applying the Streeter-Phelps Model Equations for tc

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