Model for Effective Diffusivities in Aerobic Biofilms

Reliable estimates of effective diffusivities in biofilms are required to calculate the combined diffusion and biotransformation of soluble substrates. For aerobic biofilms, effective diffusivities have been previously correlated empirically to the biofilm dry densities. Biofilms consist primarily of microbial cells, insoluble extracellular polymeric substances (EPS), and interstitial water. Biofilms are therefore a natural, heterogeneous, multiphasic material. Effective diffusive properties of such materials, both biological and nonbiological, have historically been correlated to the phase-volume fractions and diffusive properties of the individual phases. This paper presents a conceptually similar model for effective diffusivities in aerobic biofilms. The model predicts separate effects for increasing concentrations of microbial cells and insoluble EPS. Predicted effective diffusivities are expressed as a fraction of the corresponding dilute aqueous values.