Abstract: An apparatus for treating pulp includes a pressure-proof outer casing for a suspension to be treated. A shaft is rotatably arranged in the outer casing and a number of disks are arranged substantially radially on the shaft within the casing. The disks are formed by a number of sectors each having two opposite filter surfaces for allowing liquid from the suspension to be drained therethrough and a liquid compartment between the filter surfaces for the drained liquid or filtrate, so that a substantially annular filter surface is formed. The liquid compartment is in communication with a liquid discharge member. The apparatus further includes members for dividing the annular filter surfaces into two or more portions. The members are stationary relative to the casing, so that the members are used for separating different treatment stages of fiber suspension from one another.