NOVELTY - A modular support system comprises polygonal bed platform modules (12) having perforations (26) to allow gas to pass through a bed platform, bed support legs (30) having upper ends to form a plenum (28), a peripheral closure, a series of flow distribution platform modules (32) comprising bed platform modules and a flow distribution channel space, and a gas inlet (18) opening into the plenum and communicating with the flow distribution channel space.    USE - A modular support system for a bed of filter or compost material.    ADVANTAGE - The system is capable of adjusting for proper air distribution after erection of the modular support structure, so that gas flow distribution can be tuned to maximize even distribution.    DETAILED DESCRIPTION - A modular support system comprises:    (A) polygonal bed platform modules having planar top surfaces and shaped to be arranged side-by-side contiguously to form a bed platform covering a selected area, at least some of the bed platform modules having perforations to allow gas to pass through the bed platform;    (B) bed support legs having upper ends connected to bottom sides of the bed platform modules, and of length to support and elevate the bed platform modules above a base surface on which the support system rests to form a plenum under the bed platform modules;    (C) a peripheral closure along the periphery of the bed platform and generally closing the plenum;    (D) a series of flow distribution platform modules comprising bed platform modules and arranged contiguously in the bed platform, defining in the plenum a flow distribution channel space beneath the series of flow distribution platform modules; and    (E) a gas inlet opening into the plenum and communicating with the flow distribution channel space, where the flow distribution platform modules have slots in vertical planes near left and right sides of the modules, and including slide gates to be assembled down into the slots and to hang down into the plenum to close off the respective sides of the flow distribution channel space to a desired degree at each side of each flow distribution platform module.    DESCRIPTION OF DRAWING(S) - The drawing is a perspective view, partially broken away, of the support modules.    Platform modules (12)    Gas inlet (18)    Perforations (26)    Plenum (28)    Bed support legs (30)    Flow distribution platform modules (32)