Convective mass transport in heterogeneous biofilms, consisting of cell clusters and voids, was investigated using oxygen microelectrodes. Oxygen concentration profiles were measured and contour plots constructed at different (average) flow velocities (U(avg)). The profiles were used to determine the thickness of the mass transfer boundary layer (δ(h)) above the voids and the cell clusters. The δ(h) above the biofilm was inversely related to flow, as expected, and decreased exponentially with increasing flow velocity. However, the δ(h) above the voids decreased more rapidly than the δ(h) above the cell clusters resulting in two distinct situations; at low flow velocities the oxygen contours were parallel to the substratum but at high velocities were parallel to the irregular biofilm surface. It was concluded that at low flow velocities the biofilm could be modeled one-dimensionally, with fluxes perpendicular to the substratum and the exchange area being equal to the substratum area, but at higher velocities biofilm voids facilitate mass transport and a more complex, three-dimensional model would be more appropriate. In this latter case fluxes are multidirectional, and the exchange area is equal to that of the convoluted biofilm surface.
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