the invention provides dual - skinned membranes useful as one way or rectifying membranes which reduce back filtration of solute molecules in dialysis and which improve nutrient supply and product recovery in membrane bioreactors . the membranes are dual - skinned polymeric materials preferably in the form of hollow fibers . the membranes have skins of polymer on the opposite sides with differing permeability to solutes and sieving coefficient characteristics . the skin on each side have pores that are invisible at 10 , 000 times magnification , the microporous structure between said skins contains pores capable of retaining solutes in a molecular weight range of about 5000 to 200000 in an increased concentration between the interior and the exterior skins . improved dialysis devices are formed by using bundles of the hollow fiber membranes as a dialysis means having rectifying properties .