In environmental biotechnology applications for wastewater treatment, bacterial-based bioprocesses are mostly implemented; on the contrary, the application of fungal-based bioprocesses, is still challenging under non-sterile conditions. In a previous lab-scale study, we showed that when specific tannins are used as the sole carbon source, fungi can play a key role in the microbial community, under non-sterile conditions and in the long term. In a previous study, an engineered ecosystem, based on fungal tannin biodegradation, was successfully tested in a lab-scale bioreactor under non-sterile conditions. In the present study, a kinetic and stoichiometric characterisation of the biomass developed therein was performed through the application of respirometric techniques applied to the biomass collected from the above-mentioned reactor. To this aim, a respirometric set-up was specifically adapted to obtain valuable information from tannin-degrading fungal biofilms. A mathematical model was also developed and applied to describe both the respirometric profiles and the experimental data collected from the lab-scale tests performed in the bioreactor. The microbial growth was described through a Monod-type kinetic equation as a first approach. Substrate inhibition, decay rate and tannin hydrolysis process were included to better describe the behaviour of immobilised biomass selected in the tannin-degrading bioreactor. The model was implemented in AQUASIM using the specific tool Biofilm Compartment to simulate the attached fungal biofilm. Biofilm features and transport parameters were either measured or assumed from the literature. Key kinetic and stoichiometric unknown parameters were successfully estimated, overcoming critical steps for scaling-up a novel fungal-based technology for tannins biodegradation.