Rotating disk filters, implemented either with an impermeable disk rotating above a stationary membrane disk or with a stationary baffle next to a rotating membrane disk, are investigated numerically using a commercial computational fluid dynamics package. The fluid is assumed to be Newtonian, incompressible, non-fouling and isothermal. The kappa-epsilon model is used to describe turbulent flow in the vessel surrounding the rotating disk. For given values of the flow rate and RPM, stationary membrane disk filters require less power input. Rotating membrane disk filters produce a higher shear stress on the membrane surface which may reduce fouling but may also result in a reversed flow of permeate due to the 'back pressure' induced by centrifugal force on fluid within the membrane disk. Operating conditions and design parameters should be selected to minimize this 'back pressure' phenomenon and maximize the effective membrane available for filtration. (C)2000 Elsevier Science B.V. All rights reserved.