Denitrifying glycogen-accumulating organism enrichment in algal-bacterial granular sludge: rapid granulation and...

Algal-bacterial granular sludge (ABGS) is considered a promising wastewater treatment approach with potential sustainability advantages, yet its practical application is hindered by slow granulation and early-stage structural instability. To address these limitations, ABGS was rapidly constructed using denitrifying glycogen-accumulating organism (DGAO)-enriched seed sludge, and the effects of DGAO enrichment on granule formation and nutrient removal were systematically evaluated. The DGAO-enriched system exhibited clear advantages in granulation and stability, producing larger granules (685.9 μm), higher biomass (12,369 mg/L), and better settleability (SVI₃₀ = 74.0 mL/g·SS) than the non-enriched control (520.5 μm, 6873 mg/L, and 113.6 mL/g·SS, respectively). Extracellular polymeric substance analyses indicated enhanced polymer secretion by DGAOs under illumination, promoting the formation of crystalline cores and facilitating algal enrichment on the granule surface. X-ray diffraction and fluorescence in situ hybridization confirmed the formation of a stable "algae-outside/bacteria-inside" structure in the DGAO-enriched system, enabling efficient nitrogen and phosphorus removal via coupled partial nitrification, endogenous denitrification, and phosphorus removal. In contrast, the non-enriched system formed looser aggregates with limited nutrient removal. Microbial analysis revealed Candidatus_Competibacter as the dominant genus, with higher abundance in the enriched system (38.08%), with suppression of nitrite-oxidizing bacteria and enhanced functional microbial activity. Consequently, stable nitrogen and phosphorus removal efficiencies of 99% and 93% were achieved in Phase II. Overall, this study suggests that DGAO enrichment may be a promising strategy for promoting ABGS granulation and structural stratification, and was associated with stable nitrogen and phosphorus removal under illuminated conditions.