Circular zero-residue process using microalgae for efficient water decontamination, biofuel production, and carbon dioxide fixation

Microalgae can be easily integrated into an effective and scalable zero-carbon-emission circular process that exploits their CO2 fixation and wastewater remediation capabilities while simultaneously using their residues as an energy and biofuel source to power global sustainable development. Herein, the use of glycogen-rich Arthrospira platensis microalgae for efficient water decontamination, biofuel production, and CO2 fixation is explored. The circular process is initiated by efficient bioremediation of heavy metals (removal >99%) during microalgal growth, followed by bioethanol production, pellet production, and mineralization of persistent organic pollutants from wastewater, and is accompanied by CO2 fixation and/or oxygen production. Microalgal biomass generated during heavy metal bioremediation is used to produce bioethanol via simultaneous saccharification and fermentation (0.4 L per kg of dried microalgal biomass). The residual biomass obtained after bioethanol production is dried and compressed into pellets for its use as a fuel in biomass boilers (calorific power = 20.6 ± 0.2 MJ kg−1). The iron-rich ashes produced during pellet combustion are subsequently used as heterogeneous re-usable Fenton-like catalysts for the photo-Fenton degradation (mineralization >99%) of persistent organic pollutants. Finally, the low-activity ashes are incorporated into an ash-based medium for the efficient cultivation of microalgae. For each proposed step, product characterization and evaluation were conducted to optimize and enhance the process performance while minimizing negative effects on the environment.