Settling tank 30 m x 3 m; velocity for 0.02 mm

For complete removal in an ideal horizontal-flow tank, a particle must settle through depth $H$ while the water flows through length $L$:

$$v_h=v_s\,\frac{L}{H}$$

Step 1: Settling velocity of the 0.02 mm particle

$d=0.002$ cm, $S_s=2.65$, $\nu=0.01$ cm²/s:

$$v_s=\frac{981\times1.65\times0.002^2}{18\times0.01}=0.03597\ \text{cm/s}=0.3597\ \text{mm/s}=21.58\ \text{mm/min}\quad(Re=0.007<1)$$

Step 2: Velocity of flow

$L=30$ m, $H=3$ m, $L/H=10$:

$$v_h=21.58\times10=215.8\ \text{mm/min}$$

(= 0.216 m/min, which is below the usual maximum of 0.3 m/min.)

Answer: velocity of flow $\approx216$ mm/min.