Stabilizing photogranule systems under low-light and limited-aeration: From operational strategy to underlying...

Photogranule systems offer a promising route to energy-efficient, low-carbon wastewater treatment by reducing aeration energy demand and achieving carbon fixation. Regulating light-induced oxygen production and hydraulic shear is essential to prevent filamentous proliferation and functional instability. However, mechanisms underlying long-term photogranule integrity under low shear remain unclear. This study evaluated how light and aeration governed granulation and pollutant removal using sequencing-batch reactors treating low-strength wastewater over 100 days. Reactors were operated at three light intensities (240, 120, 60 PPFD) with aeration progressively reduced from 1.0 to 0.2 cm/s. Under optimal conditions (60 PPFD and 0.2 cm/s), the system maintained compact, well-settling granules and achieved high removal efficiencies (86.23 % DOC, 64.47 % TN, 100 % NH₄⁺-N, 92.61 % TP). In contrast, higher light intensities under low aeration led to excessive filamentous growth, poor settleability, and performance deterioration. Mechanistic analysis revealed that under low aeration and illumination, the suppression of filamentous microorganisms and the enrichment of aromatic proteins enhanced granule cohesion. These conditions also promoted the upregulation of photosynthesis-related genes, supporting in-situ oxygen production and nitrification, while minimizing dark respiration and nitrite accumulation. Furthermore, functional genes for nitrogen and phosphorus cycling were also upregulated under these optimal conditions. These findings establish low illumination combined with stepwise aeration reduction as a novel and effective strategy for stabilizing photogranule systems toward low-carbon wastewater treatment.