removed by the jets of suspension from the spray member 7 when the circumferential wall 2 has passed the jets of suspension from the spray member 7 there is once more created on the circumferential wall 2 a layer of fibre pulp which in turn is easily removed by the jets of suspension from the spray member 8 thus the descending side of the circumferential wall 2 in the pool of coarse fraction is substantially free from a thick tight layer of fibre pulp with the result that the pool of coarse fraction is efficiently dewatered through the circumferential wall 2 at the sinking side of the latter on the rising side of the circumferential wall 2 a thick layer of fibre pulp is created which follows the circumferential wall out of the pool of coarse fraction to the removal device 15 this removes the fibre pulp from the circumferential wall 2 and transfers the fibre pulp to the trough 16 whereafter the conveyor screw 17 discharges the fibre pulp from the drum filter the part of the circumferential wall 2 which has just been freed from fibre pulp by the removal device 15 is cleansed by means of the spray member 1 whereafter the.