Potential of a liquid foam-bed photobioreactor for microalgae cultivation

The liquid foam-bed photobioreactor is a novel photobioreactor for microalgae cultivation. A mathematical model was developed to evaluate its potential, and to optimize the design and operation of a large-scale unit. This model describes light limited microalgal growth in a rising foam column in a foam-bed photobioreactor, which is continuously operated at constant biomass density. The microalgae-containing liquid is recirculated from the bottom of the reactor and dispersed equally on the top of the foam column, in order to ensure homogenous microalgae distribution and a wet and stable foam. The model combines calculations of liquid fraction gradient, light penetration, microalgal growth, and gas transfer in the foam-bed. The liquid fraction and light model was experimentally validated. The areal productivity of a 5 cm deep foam-bed photobioreactor operated at 30 g L−1 microalgae and 1500 μmol photons m−2 s−1 was estimated to be 67.7 g m−2 d−1. This productivity is slightly lower compared to what is achievable in flat panels, which is related to light scattering in the foam-bed. Nevertheless, the advantages of the foam-bed photobioreactor, such as high gas transfer rate and high biomass densities, were confirmed with the simulations. In addition, it was calculated that a CO2 uptake efficiency of 97% can be obtained ensuring minimal CO2 loss. These benefits result in reduced gas supply requirement and reduced energy required for downstream processing. The total energy required for the production and separation of 1 g biomass in liquid foam-beds is only 8.5% of what is required in flat panels with suspended biomass. These results highlight the potential of foam-bed photobioreactors for large scale application for microalgae production.