The results of numerical simulations for a growing biological film are presented to justify the use of steady-state biofilm models for approximating the behavior of both unlimited and shear-limited biofilms. For an unlimited biofilm we show that although the total biofilm thickness may continue to increase over time, the active biofilm volume will reach a constant value. We also show that the profile of active microorganisms within the biofilm will become constant with respect to the biofilm/fluid interface and simply move outward as the biofilm thickness increases. For a shear-limited biofilm we similarly show that once a "limiting" thickness has been reached the active biofilm volume, substrate consumption, and profile of active microorganisms within the biofilm will also be independent of the biofilm thickness.

For a packed-bed biofilm reactor two reactor models are proposed. One model is for the limiting case of a biofilm with a constant biofilm thickness in which diffusion within the biofilm is shown to be negligible. The second model assumes that the thickness of the biofilm is limited by the concentration of substrate within the biofilm. The analytical solutions for these reactor models are shown to agree very well with the numerical solutions to the exact differential equations.

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Due to the surface charge inherent on all microorganism cell surfaces, the consumption of charged molecules by these microorganisms will be affected by the electrostatic interactions between the substrate and the cell surface. This article derives an equation that shows that these electrostatic interactions result in a change in the Monod half-velocity constant. The resulting rate of consumption is affected and can be either higher or lower than the rate of consumption of an uncharged substrate molecule.