The influence of internal geometry and angular velocity on the performance of a rotating-disk filtration device is described. This module consists of a disk rotating at speeds up to 2,000 rpm inside a cylindrical housing equipped with a fixed flat membrane. The test fluid was a calcium carbonate suspension (4.7-mu m mean particle diameter), and the membranes were in nylon (0.2-mu m mean pore diameter) and PVDF (0.1 and 0.18 mu m). The shear stress on the stationary membrane in the laminar boundary-layer regime was estimated from a similarity solution, and in the turbulent regime from the friction coefficient for a flat plate. Several inlet and outlet configurations were tested. The permeate flux was independent of the axial gap (disk to membrane), but increased when the radial gap (from disk to housing) was raised from 2 to 5 mm. The inviscid fluid core in the axial gap was observed to rotate at 42% of angular velocity with a smooth disk. This factor rose to 64% when the disk was equipped with small vanes. Fouling was limited to the central part of the membrane at 1,500 rpm and disappeared completely for a disk equipped with vanes. Permeate fluxes were consistently much higher than in classic cross-flow filtration.