GATE 2021 CE (CE1) – Question 62
A secondary clarifier receives 9600 m³/day at solids concentration 3000 mg/L, thickens to 12000 mg/L and is designed for solids flux 3.2 kg/(m²·h). Find thickening surface area in m² (integer).
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Correct answer: 375 to 375
Explanation
Influent solids concentration is 3 kg/m³, giving solids load 9600×3=28800 kg/day=1200 kg/h. Divide by the specified design solids flux: $A=1200/3.2=\mathbf{375}$ m². The target underflow concentration describes thickening but does not change the incoming solids load.