Solute diffusion through porous membranes and its application to membrane bioreactors

The performance of many membrane bioreactors can be significantly affected by the transport characteristics of the semi-permeable membranes used in these devices. We have obtained data for the diffusion of a variety of metabolites and nutrients, as well as model larger molecular weight compounds, through both isotropic and anisotropic flat sheet membranes. The mass transfer coefficients for relatively small solutes in the isotropic membranes could be evaluated directly from the free solution solute diffusion coefficients with appropriate corrections for the membrane porosity and tortuosity. In contrast, the mass transfer coefficients through the anisotropic membranes were more than an order of magnitude smaller than those evaluated based simply on the porosity and tortuosity effects. The bulk of the resistance to mass transport for the smaller molecular weight solutes through these anisotropic membranes was actually due to the porous Tyvek support upon which the flat sheet membranes were cast, even though this porous support structure had a very large characteristic pore size. Steric hindrance effects in the skin layer of the smaller molecular weight cut-off anisotropic ultrafiltration membranes were observed for solutes with molecular weights greater than a few thousand. These results can have important implications for the design and optimization of membrane bioreactors for a variety of different applications.