High light and carbon dioxide optimize surface productivity in a Twin-Layer biofilm photobioreactor

Immobilized cultivation of microalgae recently received increasing attention avoiding some of the major drawbacks of suspension cultivation. Twin-Layer photobioreactors (TL-PBRs), a type of porous substrate bioreactor (PSBR), have proven to be an efficient cultivation system for microalgal biofilms. However, the TL-PBR had not yet been optimized with respect to light and CO2. The effect of light intensity up to 1486 μmol m− 2 s− 1 as well as supplementary CO2 (0.12-5%) was investigated using the green alga Halochlorella rubescens as a model. With a combination of 1023 μmol photons m− 2 s− 1 and a CO2 level of 3% a surface productivity of 31.2 g dry matter m− 2 d− 1 was obtained. Along with a standing crop of 205 g m− 2 growth area (1011 μmol photons m− 2 s− 1 at atmospheric CO2 level) these values represent the highest biomass productivity and total biomass yield yet reported for a PSBR. This work, furthermore, demonstrates the successful transfer of laboratory scale data to a small prototype-scale greenhouse photobioreactor. With respect to an optimal Twin-Layer photobioreactor design, results suggest that the highest footprint productivities (~ 50 g m− 2 d− 1 without supplementary CO2) can be obtained at high light dilution rates. In contrast, a scenario using high irradiances and supplementary CO2 might be favourable from an economic point of view.