Simultaneous Nitrification–Endogenous Denitrification with Phosphorus Removal Sustained by Aeration-Free Filamentous Microalgal–Bacterial Granular Sludge for Low C/N Wastewater Treatment

The innovative and carbon-neutral phototrophic simultaneous nitrification–endogenous denitrification with phosphorus removal (P-SNDPR) process presents significant advantages for low C/N wastewater treatment. Effective implementation of SNDPR relies on establishing anoxic zones during light-aerobic stages, which poses challenges in suspended sludge systems. Microalgal–bacterial granular sludge (MBGS) may address these challenges due to its biological and chemical stratified microniches, although the granulation, structure, and performance of MBGS under low C/N and aeration-free conditions remain unreported. This study successfully cultivated filamentous MBGS with a distinct oxygen gradient and microbial stratification within the granular landscape. Filamentous cyanobacteria (f_Leptolyngbyaceae) and functional genera responsible for nutrient removal and granulation (Acinetobacter, Candidatus Competibacter, Zoogloea, Flavobacterium, and Hydrogenophaga) were enriched. The MBGS had high removal efficiencies for COD (92.9 ± 1.2%), NH₄–N (95.0 ± 0.7%), TN (83.8 ± 1.4%), and PO₄–P (96.8 ± 0.9%) with an aerobic SND efficiency of 77.8 ± 2.7%. Typical cycle and batch tests demonstrated that the MBGS achieved efficient SNDPR. Mass balance calculations indicated that most removed carbon (76.0%) and phosphorus (97.6%) were converted to biomass. The MBGS-SNDPR process established a synergistic pathway for carbon, nitrogen, and phosphorus metabolism, suggesting potential applications in sustainable wastewater treatment.