Abstract: A thickening apparatus comprises a cylindrical vessel, an axially extending delivery conduit having an outlet in the upper third part of the vessel, a mixer feeding sludge and a flocculating agent to the delivery conduit whereby the sludge and flocculating agent mixture is delivered into the vessel through the delivery conduit outlet, a collecting basin discharging liquid removed from the mixture peripherally of the vessel at the upper part thereof, a vertical rotary shaft extending through the vessel, vanes mounted on the rotary shaft, counter-vanes arranged in the vessel to cooperate with the vanes, the vanes and counter-vanes extending in opposite inclined planes and the vanes forming a discontinuous helix extending from the upper to the lower part of the vessel, the pitch of the helix diminishing towards the lower part, and a discharge conduit for the thickened sludge in the lower part of the vessel.