Packed bed reactors that have a series of alternating layers of packing. A
first packing layer 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 of hydrophobic foam. The porous support
elements are randomly packed up to about 10 porous support elements deep.
Further, the packing elements of the first layer include substantially
cylindrical shapes and the porous support elements 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.