Gravity thickening increases the solids concentration by allowing the particles to settle to the base of a vessel, producing a concentrated (thickened) solids stream at the vessel base and a supernatant (diluted) stream at the surface.

Sedimentation, or settling, is the simplest of the thickening processes. A gravity sludge thickener has the same design features as a primary sedimentation tank used for wastewater treatment, and most often has cylindrical geometry

A common variant on the gravity thickener design is the picket fence thickener. This technology is fitted with a stirrer comprising a series of relatively closely-spaced vertical arms, and is therefore similar in appearance to a picket fence.

For both designs, the sludge enters the centre of a cylindrical tank and flows out to the periphery. As water flows outwards from the centre, the suspended solids sink to the base of a cylindrical vessel, where they are scraped into the cone-shaped outlet (underflow) by a rotating scraper and removed as the thickened sludge product stream. The diluted supernatant stream reaching the tank wall then flows over a weir at the surface (overflow) and is usually directed back to the inlet of the wastewater treatment works.