The present invention relates to an apparatus and a method for treating pulp. The apparatus comprises at least one water-permeable surface rotating around a shaft, onto which surface a layer is formed of pulp, means for feeding the pulp being treated into the apparatus, means for discharging the treated pulp from the apparatus and means for removing filtrates from the apparatus. A characterizing feature is that the apparatus is provided with at least one inside structure for forming at least two treatment sections in the apparatus in such a way that each section is connected to at least means for feeding one pulp so that the pulps being treated in the sections originate from essentially different treatment stages, to means for discharging the pulp so that the treated pulps are discharged from the apparatus separately from each other, and to means for removing filtrates so that the filtrates are removed from the apparatus separately from each other.