Abstract: Liquid-containing pulp material, especially sulphate pulp containing black liquor, is deposited as a mat on a moving filter and subjected to displacement washing with a liquid such as clean water supplied to one side of the pulp mat and displacing the black liquor, which emerges in substantially undiluted form from the opposite side of the mat. The filter is a screen moving over a supporting floor, said floor and screen together forming a "resistance floor" providing a substantial flow resistance to the liquid. A layer of displacement or washing liquid (e.g., water) is maintained over the pulp mat covering the surface thereof. This layer may have a depth or height of e.g. 10 to 100 cm, thus protecting the surface of the mat against the effect of supply means for displacement liquid and providing a pressure on the upper side of the pulp mat assisting in overcoming the flow resistance of the pulp and the floor.