GATE 2020 CE (CE1) – Question 35
A river carries 1000 MLD with BOD5 = 5 mg/L and DO = 8 mg/L. Saturation DO is 10 mg/L. Wastewater enters at 100 MLD with BOD5 = 200 mg/L and DO = 2 mg/L. Assume complete mixing. Find immediate DO deficit (mg/L), to two decimals.
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Correct answer: 2.45 to 2.65
Explanation
Mixed DO is $[1000(8)+100(2)]/1100=7.454545$ mg/L. Immediate deficit is $10-7.454545=\mathbf{2.55}$ mg/L. BOD controls subsequent depletion, not instantaneous mixing.