Abstract: The apparatus comprises inside a rotating drum, two conveying screws nested inside one another and a partition passing through the drum axis to form two series of succeeding cells inside which the solids and the liquid move in counter-current, two liquid channels being so arranged as to start from each cell to end in another cell, the one channel extending along the drum axis while the other channel lies cross-wise thereto. The cross-wise channel increases the liquid portion separated from the solids by recovering liquid which would otherwise flow back into the solids as portions of the partition pass the horizontal. The channel is obtained by extending one wall of a basket provided in each cell beyond the axis of the drum.