The sedimentation rate of a particle is its theoretical downward speed in clear, standing water. A particle will only settle if:
- In a longitudinal flow, the length/height ratio of a tank is greater than the water velocity/sedimentation velocity ratio.
- In a vertical ascending flow, the velocity of rising water is below the settling speed limit.
The settling follows what is known as Stokes’ law. This law shows that the falling velocity of a particle is proportional to the square of the particle size and the density difference between the particle and the liquid.
Therefore, the increase in particle diameter significantly increases sedimentation. This is why flocculation is a widely used technique in water treatment, whether for sanitation or for drinking water production.
There are physical forces that also play a role in settling such as viscosity and particle size.