How light and aggregate size shape oxygen profiles in photogranules

The redox structure of photogranules emerges from the balance between light-driven oxygen production and microbial respiration, but the spatial coupling of these processes remains poorly quantified. Here, we investigated how light intensity, organic carbon supply, and granule size shape oxygen gradients in photogranules. Oxygen microprofiles were measured in photogranules (3.2 to 5 mm diameter) exposed to light intensities of 0 to 170 µmol_PAR·m^-2·s^-1 and acetate concentrations of 0, 30, and 500 mg_COD·L^-1. Granules originated from acetate- and methane-fed bioreactors. Light penetration was quantified in parallel. A one-dimensional reaction-diffusion model was applied to estimate oxygen production, consumption, and transport. Light penetration was limited to less than 700 µm and followed the Beer-Lambert relationship, independent of granule size. All granules developed anoxic cores at light intensities below 50 µmol_PAR·m^-2·s^-1 and COD concentrations of 30 mg_COD·L^-1. The model reproduced measured oxygen profiles when O₂ production in the model was confined to the granule surface, overlapping with heterotrophic respiration, while heterotrophic activity extended deeper into the granule interior. Granules cultivated under different carbon regimes showed up to a four-fold difference in activity, indicating a legacy effect of growth conditions and reactor operation. Simulations further showed that the bulk oxygen concentration shifts the balance between oxic and anoxic zones. These results identify light intensity and granule size as key operational parameters to control redox zonation. Managing illumination and size distribution may support concurrent aerobic and anaerobic processes within individual granules, enhancing treatment efficiency and functional flexibility in wastewater systems.