A separation process used to remove liquid from a suspension of particles in a solution, usually under the influence of gravity. The settling of the particles gives rise to an increase in the concentration of particles in a solution as a sludge or slurry in which clear liquid above overflows and is removed. In a thickener, which consists of wide tank with a slightly conical base, a revolving rake moves the sludge towards the centre of the base for removal. Compare clarification.