Oxygenic photogranules in wastewater treatment: Formation mechanisms, engineering optimization and prospects

Oxygenic photogranules (OPG) are a novel class of biogenic aggregates that orchestrate a synergistic division of labor within a single microstructure. Characterized by a photosynthetic outer layer and a heterotrophic core, this unique architecture enables aeration-free operation through in-situ oxygen production, while simultaneously facilitating advanced pollutant control and resource recovery. This review systematically synthesizes the latest advances in OPGs technology, with a focus on elucidating the key microbial processes including initial attachment, light-driven stratification, and metabolic cross-feeding that govern their self-assembly and functional stability. Crucially, optimal operational regimes concerning hydrodynamic shear forces, light strategies, and reactor configurations are systematically evaluated to provide data-driven guidance for system upscaling. The efficacy in removing both conventional pollutants and emerging contaminants is further critically assessed to highlight their remarkable adaptability to various waste streams. Finally, the review outlines the current challenges for scale-up and presents a forward-looking perspective on the opportunities to harness OPGs as a cornerstone for energy-positive, environmentally sustainable wastewater treatment.