An apparatus for insertion into a housing structure having a housing shell, the apparatus comprising: a potted bundle positionable within said housing structure, said potted bundle including a fiber bundle of hollow fiber membranes (64a, 64b, 64c) and a pair of tubesheets (46a, 46b); wherein each of said pair of tubesheets (46a, 46b) includes a respective face seal groove and two respective radial seal grooves (70a, 70b, 70c, 70d), wherein each of said grooves (68, 70a, 70b, 70c, 70d) is configured to receive an O-ring for sealing with said housing structure; and an axial support assembly positioned around said potted bundle radially outward from and over said fiber bundle and radially inward from said housing shell when the apparatus is inserted into the housing structure, the axial support assembly providing rigidity to said fiber bundle sufficient to prevent collapse of said fiber bundle during normal operation, wherein the axial support assembly includes two supportive elements (76a, 76b) configured to fit over the fiber bundle of the potted bundle to form a supported fiber bundle assembly; wherein each supportive element (76a, 76b) includes a respective flow-through component (80a, 80b) and one respective tubesheet coupling component (82a, 82b, 82c, 82d), each being a strip of solid plate, positioned at each end of the flow-through component (80a, 80b); wherein each supportive element (76a, 76b) extends only between the radial seal grooves (70b, 70d) of the tubesheets (46a, 46b); and wherein each of said tubesheets (46a, 46b) includes a support coupling groove (74a, 74b) along an axial edge of the tubesheets (46a, 46b) and is configured to engage a respective tubesheet coupling component (82a, 82b, 82c, 82d) of the axial support assembly so that neither the respective tubesheet coupling component (82a, 82b, 82c, 82d) nor the corresponding flow-through component (80a, 80b) of the axial support assembly protrude out of the groove in a radial direction. a potted bundle positionable within said housing structure, said potted bundle including a fiber bundle of hollow fiber membranes (64a, 64b, 64c) and a pair of tubesheets (46a, 46b); wherein each of said pair of tubesheets (46a, 46b) includes a respective face seal groove and two respective radial seal grooves (70a, 70b, 70c, 70d), wherein each of said grooves (68, 70a, 70b, 70c, 70d) is configured to receive an O-ring for sealing with said housing structure; and an axial support assembly positioned around said potted bundle radially outward from and over said fiber bundle and radially inward from said housing shell when the apparatus is inserted into the housing structure, the axial support assembly providing rigidity to said fiber bundle sufficient to prevent collapse of said fiber bundle during normal operation, wherein the axial support assembly includes two supportive elements (76a, 76b) configured to fit over the fiber bundle of the potted bundle to form a supported fiber bundle assembly; wherein each supportive element (76a, 76b) includes a respective flow-through component (80a, 80b) and one respective tubesheet coupling component (82a, 82b, 82c, 82d), each being a strip of solid plate, positioned at each end of the flow-through component (80a, 80b); wherein each supportive element (76a, 76b) extends only between the radial seal grooves (70b, 70d) of the tubesheets (46a, 46b); and wherein each of said tubesheets (46a, 46b) includes a support coupling groove (74a, 74b) along an axial edge of the tubesheets (46a, 46b) and is configured to engage a respective tubesheet coupling component (82a, 82b, 82c, 82d) of the axial support assembly so that neither the respective tubesheet coupling component (82a, 82b, 82c, 82d) nor the corresponding flow-through component (80a, 80b) of the axial support assembly protrude out of the groove in a radial direction.