GATE 2020 CE (CE1) – Question 62
A primary discrete-settling tank has surface overflow rate 20000 L/(m²·day). Water kinematic viscosity is 0.0101 cm²/s; particle specific gravity is 2.64 and g = 9.81 m/s². Find minimum particle diameter removed with 80% efficiency (μm), to one decimal.
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Correct answer: 14 to 15
Explanation
For ideal discrete settling, $v_s=\eta\,SOR=0.8(20/86400)$ m/s. Convert viscosity to $1.01\times10^{-6}$ m²/s. Stokes’ law gives $d=\sqrt{18\nu v_s/[(S-1)g]}$, yielding **14.5 μm**.