Abstract: A filtration apparatus is provided, of the type that includes a stack of rapidly rotating membrane packs (32, FIG. 2) and a stack of stationary separator elements (36A, 36B) interleaved with the membrane packs to leave thin gaps (40A, 40B) between them, which obtains the advantages of both series-connected and parallel-connected systems. A feed conduit (82) connects the radially outer ends (110) of the gaps, to carry feed fluid into and out of each gap. The rapidly rotating membrane packs cause radially outward flow (94, FIG. 5) near their surfaces, which causes radially inward flow (102) near the surfaces of the stationary elements, to cause fluid flow radially inwardly and then outwardly through each gap. The system is operated so particles (111, FIG. 6) of the feed fluid can build up on the rotating pack surfaces (92) only to a thickness that is much less than one-half the thickness (116) of the gap, before commencing any procedure to clean the membrane pack surfaces.