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A waste water stream (flow = 2m³/s, ultimate BOD = 90 mg/l) is joining a small river (flow = 12 m³/s, ultimate BOD = 5 mg/ l). Both water streams get mixed up instantaneously. Crosssectional area of the river is 50 m². Assuming the deoxygenation rate constant, k = 0.25.day, the BOD (in mg/l) of the river water, 10 km downstream of the mixing point is
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- 1.68
- 12.63
- 15.46
- 1.37
Correct Option: C
| BOD of mixture, Y = | = 17.143 g/m³ | |
| 2 × 12 |
KD = 0.434 × k1
= 0.434 × 25 = 0.1085
| Velocity of stream = | = 0.28 m/s | |
| 50 |
| Time taken = | = 9.921 hours | |
| 0.28 m/s |
Yt = Yo (1 – 10–kt)
= 17.143 (1 – 10–0.1085 × 9.921) = 15.7 mg/l
