Packed bed reactors (100) that have a series of alternating layers of packing . A first packing layer (120) has open body packing elements randomly packed in a layer up to about 10 packing elements deep; and a second packing layer has porous support elements (110) of hydrophobic foam. The porous support elements (110) are randomly packed up to about 10 porous support elements deep. Further, the packing elements of the first layer (120) include substantially cylindrical shapes and the porous support elements (110) of the second layer include substantially rectilinear shapes. In some embodiments, the second layer supports microbial organisms in a biological reactor, and the first layer of open body packing provides fluid flow spaces to facilitate fluid flow and distribution, mass transfer, and to minimize channeling.