Abstract: Apparatus is provided for treatment of a fluid by use of a membrane tube wherein a volume displacement rod is positioned longitudinally through the membrane tube to provide desired filtration conditions for removal of a permeate from the fluid, and an improved support arrangement for the rod is provided that eliminates the use of a spider or similar support structure that constitutes a fluid flow obstruction. In the disclosed embodiment, the remote ends of the rod are seated in sockets located in bends in the walls of the inlet and outlet conduits associated with the membrane tube.