GATE Civil Engineering (CE) 2014 Shift 2 Solved Paper

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