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-fib er membranes and impermeable solid supports of similar geometry. Substrate and oxygen fluxes are predicted for different biofilm thicknesses as a func tion of fluid velocity and substrate concentration. Under conditions of oxy gen limitation, low water velocities, and moderate to high bulk liquid subs trate concentration, the membranes have a clear advantage and outperform so lid 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 concent rations much above 4 mg/L in wastewater treatment. The growth of an active biofilm can actually increase the flux of oxygen across the membrane dramat ically: however, the presence of a biofilm always reduces the ability of a membrane to oxygenate the surrounding wastewater. This drop in oxygen trans fer performance is caused by the fact that the active biofilm consumes oxyg en and impedes diffusion of the oxygen into the bulk water. In thick biofil ms the oxygen flux can drop to zero so that the external regions of the bio film and the external wastewater become anaerobic. This may cause some oper ating problems, but it may also facilitate nitrification-denitrification. A dditional aeration of the external wastewater could improve biofilm perform ance and assist in controlling biofilm growth.