To evaluate the mass transfer coefficients at the interface bulk/biofilm two biofilm systems with autotrophic and heterotrophic microbes were investigated with oxygen microelectrodes over a longterm period (1 year). To determine the transfer coefficient the oxygen profiles were simulated with a mass transfer model and compared with the measured substrate flux. The oxygen profiles at flow velocities >10 cm/s yielded concentration layers of about 100 μm and transfer coefficients > 10−4 m/s. These evaluated transfer coefficients were one magnitude higher than those known from hydrodynamics without biofilm reactions. Thus, the assumption, which determines the transfer coefficient as a quotient of the diffusion coefficient through the thickness of the laminar sublayer is oversimplified.

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