Investigating dynamic processes in a porous substrate biofilm photobioreactor — A modeling approach

In the present study, a one dimensional kinetic model was developed for a porous substrate biofilm photobioreactor (PSBR), a biofilm bioreactor for the production of microalgae and other biotechnological applications. Light transfer was modeled with a radiative transfer equation (RTE), considering absorption, scattering and biomass pigment adaptation. Dissolved chemical species were modeled using mass balance equations (partial differential equations), with terms describing diffusion, convection, biomass growth, biomass consumption, and chemical conversions. pH was modeled as a state variable, and a novel approach of modeling biomass increase in a phototrophic biofilm was introduced. The model was solved using numerical methods and model parameters were acquired either from literature or from experimental work carried out in the framework of this study. The proposed mode was applied to simulated gradients of light, dissolved oxygen concentration, pH, etc. in a PSBR biofilm. The simulated results were compared with experimental data acquired in previous studies. Our results show that the proposed model can accurately predict light intensity distribution in the modeled PSBR biofilm, provided the optical properties of the biomass were measured experimentally. The prediction of the concentration of dissolved oxygen and pH profiles also reflected experimental data with high fidelity. The results also strongly suggest that facilitated CO2 transfer due to the presence of extracellular carbonic anhydrase in the biofilm matrix plays an important role in DIC transport in the modeled PSBR biofilm at low gas phase CO2 concentration; and the pH gradient along the depth gradient of the modeled biofilm was mainly caused by the uptake of dissolved inorganic carbon. In addition, the simulation can predict biomass growth in the modeled PSBR with minimal error, thus, with minor modifications, the model can also be applied to predict biomass growth in larger-scale PSBRs. •A comprehensive model was proposed to describe dynamic processes in a porous substrate biofilm photobioreactor.•Comparison of simulated results and experimentally derived data were carried out along the depth gradient of the biofilm.•The model predicted distributions of light, dissolved oxygen, pH and biomass growth with a high fidelity.•Results strongly suggest a vital role of facilitated CO2 transfer in inorganic carbon transfer at low CO2 concentrations.•With minor modifications, the proposed model can be