The present invention introduces a fiber suspension thickener, particularly adapted for thickening cellulose fiber. The thickener comprises a vat; a hollow filter assembly in the form of a horizontal cylindrical drum disposed within the vat, whereby the drum may be rotated by a motor around its horizontal axis, and whereby the cirumferential walls of the drum are made of a filter material; either a spray nozzle or some other supply conduit that furnishes a fiber suspension into the interior of the vat, so that the drum becomes submerged within a pool of the fiber suspension; a discharge outlet for discharging the fiber suspension on the ascending side of the rotating drum; and a turbulence creating device, that can take many forms, for creating turbulence and mixing the fiber suspension on the ascending side of the rotating drum. The turbulence creating device prevents coarse fibers within the fiber suspension from caking or matting on the exterior of the filter material of the drum, and improves the overall thickening capability of the present invention.