A waste water stream (flow = 2 m3/sec, ultimate BOD = 90 mg/litre) is joining a small river (flow = 12 m3/s, ultimate BOD = 5 mg/litre). Both water streams get mixed up instantaneously. Cross-sectional area of the river is 50 m2. Assuming the de-oxygenation rate constant, K = 0.25/day the BOD (in mg/litre) of the river water, 10 km downstream of the mixing point is

A waste water stream (flow = 2 m3/sec, ultimate BOD = 90 mg/litre) is

1.	A waste water stream (flow = 2 m3/sec, ultimate BOD = 90 mg/litre) is joining a small river (flow = 12 m3/s, ultimate BOD = 5 mg/litre). Both water streams get mixed up instantaneously. Cross-sectional area of the river is 50 m2. Assuming the de-oxygenation rate constant, K = 0.25/day the BOD (in mg/litre) of the river water, 10 km downstream of the mixing point is
A.	1.68
B.	12.63
C.	15.46
D.	1.37
Answer» D. 1.37

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