Stoke's Law : Settling velocity of a discrete particle depends upon specific gravity and diameter of the particle, gravitational acceleration (g) and kinematic viscosity of fluid (water in sedimentation treatment). Stoke's law relation is as follows Settling velocity, Vs =$\frac{g}{18}$($s_s$-1)$\frac{d^2}{v}$,

Where,

g = Gravitational acceleration,

$s_s$ = Specific gravity of the particle,

v = Kinematic viscosity of water in centistokes (dependent on temperature of water).

Stoke's law is applicable to all discrete particles ranging in size 1$0^{-5}$cm to 1$0^{-1}$cm. This is the requisite range of particles which is significant in water treatment.

Limitations of Stoke's Law

• All particles, as assumed, may not be discrete.

• Temperature of water may not be consistant.

• The liquid (water) may not be at rest (quiescent).

• The shape and size of the particles may not be truly spherical.

• The law is valid only when Reynold's number $R_e$ < 1.