Photosynthetic Biogas Upgrading Using Microalgal-Bacterial Consortia: Fundamentals, Process Optimization and...

Biogas is a key renewable energy vector that can support the transition toward a net-zero carbon economy. Its direct use as a natural gas substitute is limited because it must be upgraded to meet CH₄ purity specifications required for injection into the gas grid or for use as a vehicle fuel. This review summarizes current progress in photosynthetic biogas upgrading, an emerging biotechnology based on the symbiotic action of microalgal-bacterial consortia capable of supporting gas purification with nutrient recovery in a single integrated process. This biotechnology relies on two stages: an absorption unit that enables gas-liquid mass transfer of the biogas pollutants, and a photobioreactor in which CO₂ and other contaminants are removed. Optimal system performance is strongly influenced by the liquid to gas (L/G) ratio, with values between 0.5 and 1.0, typically balancing effective CO₂ removal and limited CH₄ dilution. High-alkalinity nutrient media (1.5-2.5 gIC L^-1) and pH > 9 remain essential to sustain the chemical gradients driving CO₂ mass transfer. Robust microalgae/cyanobacteria such as Chlorella vulgaris and Pseudanabaena sp. frequently dominate these systems. Recent efforts in the biostimulation of photosynthesis are presented based on their potential to enhance biomass productivity and CO₂ removal, which could decrease the footprint of the process and facilitate its large-scale adoption for biomethane production.