Question

An ideal horizontal flow settling basin is 3 m deep having surface area 900 $m^2$. Water flows at the rate of 8000 $m^3/d$, at water temperature ${20}^{\circ }C(m={10}^{-3}kg/m.sand\rho =1000kg/m^3)$ . Assuming Stoke's law to be valid, the proportion (percentage) of spherical sand particles (0.01 mm in diameter with specific gravity 2.65), that will be removed, is

• A. $67$
• B. $32.5$
• C. $87.5$
• D. $95.5$

Answer 1

The correct option is C $87.5$

Surface overflow rate,

$SOR=\frac{Q}{A}=\frac{8000}{900}$ $=8.889m/d$

Settling velocity,

$V_t=\frac{g{d}^2}{18\mu }(G_s-1){\rho }_w$

$=\frac{9.81\times (0.01\times {10}^{-3})}{18\times {10}^{-3}}(2.65-1)\times 1000$

$=8.993\times {10}^{-5}m/s$

$=8.993\times {10}^{-5}\times 86400m/d$

$=7.770m/d$

$=\%\text{Removal}=\frac{V_t}{SOR}\times 100=\frac{7.770}{8.889}\times 100$

$=87.406\%\simeq 87.5\%$