In the one-dimensional mixed-culture biofilm model, volume expansion due to microbial growth leads to a displacement of biomass towards the biofilm surface, where solids (cells and particles) can be detached and attached. In this model there are no suspended solids in the pore volume of the biofilm. In a growing biofilm, this description makes a significant penetration of solids from the bulk fluid into the biofilm impossible and it leads to a fast elimination of attached inert particles from the biofilm. Recent experiments with fluorescent microbeads used as solid tracers are in disagreement with this description. The inert tracer particles added to the bulk fluid penetrate the biofilm quite fast and they show a residence time in the film that is much larger than that of the bacteria. For this reason the processes “transport of solids suspended in the biofilm pore volume” and “attachment to” and “detachment from the solid matrix in the biofilm interior” were added to the mixed-culture biofilm model. These processes were also implemented in the biofilm reactor compartment of the simulation program AQUASIM which is used here to demonstrate that the new processes make a qualitative simulation of the observed behavior possible. Therefore, the mixed-culture biofilm model with these processes added can be used as a tool for further research on the mechanisms which determine the development of the biofilm solid matrix and on transport of solids in the biofilm pore volume.

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