Abstract: The stack assembly (FIG. 9) is formed by identical plates (P) of rectangular shape alternating with semi-permeable membranes (12). A face of each plate is formed with a plurality of transversely extending flow-conveying beds, each of which has a through-aperture at each of its opposite ends. The membrane is affixed to the other face of the plate by means of plug members to provide a modular unit, some of the plug members in the stack assembly being "open" (13a) and the other being "blind" (13b). Hydraulic connections in "series", in "parallel" and in "series/parallel" are obtainable by properly distributing the open and blind plug members.