A settling tank in a water treatment plant is designed for

A settling tank in a water treatment plant is designed for a surface overflow rate of 30(m³/day.m²) . Assume specific gravity of sediment particles = 2.65, density of water (ρ) = 1000 kg/m³, dynamic viscosity of water (m) = 0.001 N.s/m², and Stokes law is valid. The approximate minimum size of particles that would be completely removed is

To calculate minimum size of particles equating settling velocity to overflow rate, we get
V_o = V_s

V_o =	(G_sl)&gaamma;_wd²
	18μ

Now V_o =	30 m³	=	30	m
Now V_o =	day.²	=	24 × 60 × 60	s

⇒	30	=	(2.65 - 1) × 9.81 × 10³ × d²
	24 × 60 × 60		18 × 0.001

Solving we get d = 1.965 × 10^–5m ≈ 0.02 mm