Abstract: In removing impurities, such as printing inks, binders, fiber fragments and pigments, from a fibrous suspension of waste paper containing water, the fibrous suspension is directed downwardly over an inclined sieve for removing the impurities through the sieve. In its length direction between its upper and lower ends, the sieve is divided into a number of individual sections. A triangularly shaped groove-like trough is located between the lower end of one section and the upper end of the next downwardly adjacent section. The trough extends transversely of the length of the sieve and is shaped to provide a swirling movement to the suspension flowing into the trough. At the outlet from the trough a flow-stemming device is provided for affording the uniform level of the suspension as it flows over the next section of the sieve.