Integration of microalgae Cultivation as a bioregeneration step in a closed loop carbonate-based CO2 absorption process

The rising atmospheric CO2 levels, major cause in the global climate crisis, led to the development of various carbon sequestration techniques. However, their industrial application is still hampered by the high energy demand ad difficult CO2 storage. The Bicarbonate Algae Production System (BICCAPS) allows for the integration of a bicarbonate-rich solution from a CO2 chemical absorption process with microalgal growth, to both regenerate the absorption solvent and obtain valuable commodities. In this work, the alkaliphilic cyanobacteria Spirulina sp. was cultivated in harsh alkaliphilic environment to investigate the feasibility of algae cultivation integration into carbonate-based carbon capture. Initially, the effects of EPS accumulation on biomass growth and CO2 absorption capacity were investigated through repeated batch experiments. Results showed that EPS accumulation reduced specific growth rate after each run. However, CO2 absorption capacity of culture medium was not negatively impacted by the accumulation of secondary metabolites. The feasibility of growing Spirulina species in continuous mode at sodium bicarbonate concentrations like those obtained from an industrial absorption process and to completely remove air sparging has been tested. A maximum biomass concentration of 0.67±0.04 g L-1 was reached at more than 6 g L-1 of inorganic carbon. Further, no significant differences in growth performance were found when air sparging was removed, confirming the tolerance of this species for BICCAPS-like environments. The feasibility of recycling medium for Spirulina cultivation at high bicarbonate concentrations has been tested alongside the effect of EPS accumulation on microalgal growth and CO2 absorption. Results have shown that cultivation was possible up to a recycle ratio R=85% with a biomass concentration of 0.72±0.01 g L-1 and no evident difference in the time needed to reach the equilibrium or in the ionic species concentration was found. Lastly, possible effects of a different cation on microalgal growth were tested using medium containing potassium bicarbonate. No significant difference with sodium bicarbonate cultures was found. Moreover, since many industrial carbon capture processes utilize molecules as promoters to enhance sequestration, the effects of some promoters on microalgal culture have been tested through batch experiments.