A settling tank in a water treatment plant is designed for a surface overflow rate of $30 \dfrac{m^3}{\text{day} \cdot m^2}$. Assume specific gravity of sediment particles $=2.65$, density of water $(\rho) =1000 \: kg/m^3$, dynamic viscosity of water $(\mu)=0.001 \: N.s/m^2$, and $\textit{stokes’ lawis}$ valid. The approximate minimum size of particles that would be completely removed is
1. $0.01$ mm
2. $0.02$ mm
3. $0.03$ mm
4. $0.04$ mm