Photosynthetic Biogas Upgrading Using Microalgal–Bacterial Consortia: Fundamentals, Process Optimization and Challenges

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.