A centrifugal separator (20) for removing particulate matter from a fluid includes a base (30) defining inlet (41), an outer shell (31) assembled to the base (30), a stationary centertube (34) assembled to the base (30), and a rotor assembly (20a) which is mounted onto the centertube (34) and positioned within the outer shell (31). The rotor assembly (20a) is bearingly mounted to the centertube (34) for rotation about the centertube (34). The rotor assembly (20a) includes a rotor shell (26), a cone-stack assembly (24), and a retention-medium sleeve (21) positioned between the cone-stack assembly (24) and the rotor shell (24) and the rotor shell (26) for receiving and retaining particulate matter which is centrifugally from the fluid being "processed" by the centrifugal separator (20). A water-absorbent material is included as part of the retention-medium sleeve (21), whose preferred material is a high-void medium of at lest 70 per cent. An additional retention-medium member formed into a coil-like shape is positioned between the cone-stack assembly (24) and the base (30). The base (30) includes a pair of tangential flow jet nozzles (50,51) which are used for driving the rotor assembly (20a).