A membrane filtration device, for withdrawing permeate from a liquid substrate, includes a first solid body (41u); a second solid body (41b); and a plurality of hollow fiber membranes (42) potted in the solid bodies such that the peripheries of the membranes are bounded by an elongated rectangular shape and such that the membranes have open ends flush with or protruding from a permeate discharging face of one or both solid bodies. A permeate collection means (43u, 43b) is arranged to collect permeate in fluid communication with the permeate-discharging face of each solid body (41u, 41b) that has a permeate-discharging face. The membranes (42), the solid bodies (41u, 41b) and the permeate collection means (43u, 43b) together form a submersible assembly. The first solid body (41u) is upper and disposed in vertically spaced-apart relationship above the second solid body (41b). The opposed faces of the solid bodies (41u, 41b) are separated by a vertical distance and the membranes (42) have a length from 0.1% to less than 5% greater than the vertical distance between the opposed faces of the solid bodies (41u, 41b).