GATE 2021 CE (CE2) – Question 61

Water and Waste Water Quality and Treatment · Primary, secondary and tertiary treatment, effluent standards · 2 marks · Numerical answer

ASP receives 7500 m³/day, influent ultimate BOD 200 mg/L and target effluent BOD 20 mg/L. Reactor MLVSS is 3000 mg/L and underflow 12000 mg/L. HRT=0.2 day and solids-retention time=10 days. Find sludge-wastage underflow in m³/day to one decimal.

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Correct answer: 37 to 38

Explanation

Reactor volume V=Qθ=7500(0.2)=1500 m³. Ignoring effluent suspended solids, $\theta_c=VX/(Q_wX_r)$. Thus $Q_w=1500(3000)/[10(12000)]=\mathbf{37.5}$ m³/day. The BOD values are not required because the solids inventory and residence time are given directly.