Apparatus for washing and dewatering pulp comprising rotatable press rolls including permeable outer surfaces and a casing comprising a vat in which the rolls are installed are disclosed. A feed for feeding the pulp through the gap between the vat and the outer surfaces of the press rolls is provided and the vat includes a vat segment providing at least one compartment between the vat segment and the casing, the at least one compartment extending along the longitudinal extension of one of the first and second rotatable press rolls and a pressure device to maintain the pressure in the at least one compartment corresponding to the pressure in the gap.