Inhibitory Substrate Utilization by Steady‐State Biofilms

Numerical simulations were performed that examined the utilization of inhibitory substrates by steady‐state biofilms. Based on kinetic and energetic constraints, the growth of a monolayer of cells into a steady‐state biofilm requires that substrate concentrations be between Smin and Smax. The continued exposure of a monolayer or dispersed cells to substrate concentrations less than Smin or greater than Smax will not lead to the development of a self‐perpetuating biological treatment process. However, once developed, a steady‐state biofilm can tolerate bulk substrate concentrations greater than Smax, because the inner layers of the biofilm are exposed to substrate concentrations less than Smax due to mass transport resistance and substrate utilization in the outer layers of the biofilm. In such situations, maximal rates of substrate utilization and microbial growth occur in the middle or at the inner surface of the steady‐state biofilm. The simulations also indicated that there are two sets of steady‐state biofilms: one associated with Smin (stable steady‐state solutions) and the other associated with Smax (unstable steady‐state solutions).