Settling tank 30 m x 3 m, 0.03 mm at 25°C; 0.025 mm at 20°C

For complete removal in an ideal continuous-flow tank, $t=H/v_s$; for smaller particles the removal is $v_s/v_0$ with $v_0=H/t$.

(a) Detention time for 0.03 mm at 25 °C

$d=0.003$ cm, $S_s=2.65$, $\nu_{25}=0.00893$ cm²/s, $H=300$ cm.

$$v_s=\frac{981\times1.65\times0.003^2}{18\times0.00893}=0.0906\ \text{cm/s}\quad(Re=0.030<1)$$ $$t=\frac{300}{0.0906}=3310\ \text{s}=55.2\ \text{min}=0.92\ \text{h}$$

(b) Removal of 0.025 mm particles at 20 °C

Overflow rate of the tank: $v_0=H/t=0.0906$ cm/s. At 20 °C, $\nu=0.01007$ cm²/s, $d=0.0025$ cm:

$$v_s=\frac{981\times1.65\times0.0025^2}{18\times0.01007}=0.0558\ \text{cm/s}\quad(Re=0.014<1)$$ $$\text{Removal}=\frac{0.0558}{0.0906}=0.616\Rightarrow61.6\ \%$$

Answer: detention time = 0.92 h (55 min); removal of 0.025 mm particles at 20 °C = 61.6 %.