A membrane-based biofilm photobioreactor for enhanced algal growth rates

Biofilm-based algal processes are increasingly used for wastewater treatment, carbon capture, and production of biofuels and other valuable products. They provide high cell densities, are more robust, and are easier to harvest and concentrate than suspended algae. However, algal biofilms are more likely to experience carbon limitation, O₂ inhibition, and pH limitations, especially when thick and exposed to high light intensities. To address these limitations, we studied a novel photobioreactor based on CO₂-supplying hollow-fiber membranes, where the algal biofilms grow directly on the membranes. We used modelling and experiments to study our membrane biofilm photobioreactor (MB-PBR) system and to compare it to a control with atmospheric CO₂ and bicarbonate supplied in the bulk liquid.

Mathematical models of the MB-PBR and the control were developed using COMSOL Multiphysics®. The models included phototrophic growth, diffusion of gases (CO₂, O₂, N₂) across the membrane, nutrient diffusion from the bulk liquid, pH-dependent carbonate speciation, and light attenuation. Experimentally, we compared the MB-PBR and control using bench-scale photobioreactors with hollow-fiber membranes attached to them, 10% BG-11 media and white light from an LED lamp. The MB-PBR membranes were supplied with 5% CO₂ and 95% N₂.  The control system had sealed membranes, to prevent gas exchange.  We measured the biomass dry weight gravimetrically and the biofilm growth rates by daily measurement of the thicknesses using optical coherence tomography (OCT).

Both modeling and experiments suggested that MB-PBR biofilms grow significantly faster than the control. Using our model, we studied the effect of light intensity, pH, buffer concentration and light and oxygen inhibition on MB-PBR behavior. Growth was inhibited by excessively high levels of light and O₂. By providing CO₂ through the membrane, the carbon limitation was minimized, O₂ was stripped from the biofilm, and pH shifts were attenuated. These results suggest the MB-PBR may provide a more efficient platform for algal biofilm processes.