a modular biofiltration support system is quickly erectable in virtually any size and shape using plastic modules that are interconnected at the site . included in the system are bed platform modules which assemble together to form a bed platform to support the weight of the biofilter medium within the area defined by the frame , and a multiplicity of bed support legs which are assembled into the bed platform modules to hold the bed platform at a raised position above the ground , defining a plenum for entering gases , between the ground and the bed platform . in one embodiment , at least some of the legs include air directing vanes which can be oriented in positions to cause generally even distribution of the gases within the plenum . the bed platform modules have openings to direct the gases to flow upwardly through the biofilter medium . the periphery of the plenum within the bed platform preferably is substantially closed , and this can be by taller peripheral modules in one embodiment , or by angled edge platform modules in another embodiment , that form a slope from the bed platform down to the ground or base .