A lamella for a clarifier formed by a rectangular or square plate, the upper edge and the lower edge of which run parallel to a transverse direction, the two remaining edges running parallel to a longitudinal direction, the plate being provided with longitudinal folds curved in the longitudinal direction and running from the upper edge to the lower thereof; the plate being curved longitudinally, so that the area closer to the lower edge of the plate is less inclined relative to the horizontal than the area closer to the upper edge.