Modeling Biofilms on Gas-Permeable Supports: Flux Limitations

A computer model is used to investigate the microbial uptake of oxygen and a carbon-source substrate for biofilms growing on gas-permeable, hollow-fiber membranes and impermeable solid supports of similar geometry. Substrate and oxygen fluxes are predicted for different biofilm thicknesses as a function of fluid velocity and substrate concentration. Under conditions of oxygen limitation, low water velocities, and moderate to high bulk liquid substrate concentration, the membranes have a clear advantage and outperform solid supports. This improvement in performance stems from the ability of the membrane to deliver high oxygen concentrations (8–20 mg∕L) directly to the biofilm, whereas it is difficult to maintain bulk dissolved oxygen concentrations much above 4 mg∕L in wastewater treatment. The growth of an active biofilm can actually increase the flux of oxygen across the membrane dramatically; however, the presence of a biofilm always reduces the ability of a membrane to oxygenate the surrounding wastewater. This drop in oxygen transfer performance is caused by the fact that the active biofilm consumes oxygen and impedes diffusion of the oxygen into the bulk water. In thick biofilms the oxygen flux can drop to zero so that the external regions of the biofilm and the external wastewater become anaerobic. This may cause some operating problems, but it may also facilitate nitrification-denitrification. Additional aeration of the external wastewater could improve biofilm performance and assist in controlling biofilm growth.