Definitions

# Settling

[set-ling, -l-ing]
Settling is the process by which particulates settle to the bottom of a liquid and form a sediment.

Settling velocity or fall velocity or terminal velocity (ws) of a (sediment) particle is the rate at which the sediment settles in still fluid. It is diagnostic of grain size, but is also sensitive to the shape (roundness and sphericity) and density of the grains as well as to the viscosity and density of the fluid. It integrates all of these into a key transport parameter.

For dilute suspensions, Stokes' Law predicts the settling velocity of small spheres in fluid, either air or water. Stokes' Law finds many applications in the natural sciences, and is given by:

$w=frac\left\{2\left(rho_p-rho_f\right)gr^2\right\}\left\{9mu\right\}$

where w is the settling velocity, ρ is density (the subscripts p and f indicate particle and fluid respectively), g is the acceleration due to gravity, r is the radius of the particle and μ is the dynamic viscosity of the fluid.

Stokes' law applies when the Reynolds number is less than 0.1.

Settleable solids are the particulates that settle out of a still fluid. Settleable solids can be quantified for a suspension using an Imhoff tank or cone.

Settling tanks are used for separating solids and/or oil from another liquid. In food processing, the vegetable is crushed and placed inside of a settling tank with water. The oil floats the top of the water then is collected. In water and waste water treatment a flocculant is often added prior to settling to form larger particles that settle out quickly in a settling tank leaving the water with a lower turbidity.

## Winemaking

In winemaking, the French term for this process is débourbage. This step usually occurs in white wine production before the start of fermentation.