Surface Overflow Rate (SOR) of a primary settling tank (discrete settling) is $20000\:litre/m^2$ per day. Kinematic viscosity of water in the tank is $1.01\times10^{-2}\:cm^2/s$. Specific gravity of the settling particles is $2.64$. Acceleration due to gravity is $9.81\:m/s^2$. The minimum diameter (in $\mu m$, round off  to one decimal place) of the particles that will be removed with $80\%$ efficiency in the tank, is  ______.