Abstract: Apparatus is described for purifying waste water or other feed fluid, which includes a stack of membrane packs wherein each pack contains a conduit sheet, a pair of support sheets of filter paper or the like, each lying against a face of the conduit sheet, and a pair of membranes each lying over a filter paper sheet. The pack has exit holes, and the filter paper extends up to the exit hole walls and has a region thereabout which is impregnated with an adhesive which seals the filter paper against the inflow of concentrate while also holding the filter paper to the conduit sheet and membranes. The packs are spaced apart by inner and outer gaskets, the radially inner gaskets having central holes forming a feed pipe for carrying the feed fluid to all the packs.