A sedimentation device for separating a material mix, preferably plastic flakes, including a sedimentation basin configured to receive the material mix and a liquid, for example water; an inlet configured to introduce the material mix and/or the liquid into the sedimentation basin; an outlet configured to remove sediment from the sedimentation basin; at least one stilling plate within the sedimentation basin configured as an ultrasonic oscillating element. The invention moreover relates to a method for removing sediment from a sedimentation device.