Managing carbon dioxide mass transfer in photobioreactors for enhancing microalgal biomass productivity.

Mitigating greenhouse-gas emissions, of which fossil-derived carbon dioxide (CO 2) is the dominant component, is becoming increasingly imperative. One of the tools for lowering the demand for fossil carbon is cultivation of microalgae, which are fast-growing photosynthetic microorganisms that utilize sunlight for energy and CO 2 as their carbon source. In addition, microalgae can provide feedstock to replace fossil sources, particularly for transportation fuels. In open and closed microalgal cultivating systems (also called open ponds and photobioreactors, respectively), CO 2 can be sparged into the culture medium through a gas distributor; CO 2 molecules diffuse through the gas-liquid interface and dissolve into the culture medium, from which they can be taken up for the biosynthesis of microalgal cells. Due to the modest solubility of CO 2 in water, optimal design and operating variables (e.g., inlet gas flow rate, sparger characteristics, CO 2 concentration in the inlet gas, and the height of a PBR or sump) are required to increase the CO 2 mass transfer rate into the medium and, consequently, CO 2 uptake and biomass productivity. The concepts and phenomena discussed in this work apply to photobioreactors and open ponds that are sparged with CO 2. This review systematically evaluates how the key design and operating variables affect bubble behavior and the rate of CO 2 delivery into the medium. The review also addresses advanced strategies that are being employed to increase the rate of CO 2 transfer, but with lower costs than with sparging. [Display omitted] • CO 2 management is critical during microalgal cultivation in photobioreactors (PBRs). • Inlet CO 2 concentration and gas flow rate affect CO 2 mass transfer. • Sparger design and sparging style determine effective CO 2 delivery to the system. • Carbon utilization efficiency (CUE) is dictated by the pH management in PBRs. • CUE can be improved through membrane carbonation or by using carbonic anhydrases. [ABSTRACT FROM AUTHOR]