The present invention provides a flow-control support for a distributor plate in a composite pressure vessel. The distributor plate includes a thermoplastic polymeric disk having a top side, a bottom side, a perimeter edge and a central opening and a plurality of fluid flow passages through the disk. The flow-control support is disposed between the bottom side of the disk and an inner side of an end of a thermoplastic liner assembly. The flow-control support includes a spirally fluted upper surface that is adapted to direct fluid flowing from a supply pipe disposed in the central opening of a distributor plate that is supported by the flow control support such that the fluid swirls in a space between the bottom side of the distributor plate and an inner side of an end portion of a water treatment vessel. The swirling water insures that fluid is distributed to substantially all of the fluid flow passages through the disk during backwashing operations.