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 process presents significant advantages for low C/N wastewater treatment. Key to implementing SNDPR during light/aerobic phases is the establishment of anoxic zones, a challenge in suspended sludge systems. Microalgal-bacterial granular sludge (MBGS) may address this issue due to its biological and chemical stratified micro-niches, 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 distinct oxygen gradients and diverse functional microorganisms. Genera such as Candidatus Accumulibacter, Dechloromonas, and Candidatus Competibacter were enriched, with functional genes related to ammonia oxidation, polyphosphate synthesis, and PHB metabolism being more abundant. The MBGS based SNDPR process demonstrated high removal efficiencies for COD , NH4N , TN , and PO4P with an overall SND efficiency of 77.8. During the light phase, the DO concentration was maintained between 0.59 to 0.65 mg/L, creating a low-oxygen environment for achieving SNDPR. Mass balance calculations indicated that a significant portion of removed carbon (78.0%) and phosphorus (97.6%) was converted to biomass. This study provides compelling evidence that MBGS can effectively treat low C/N wastewater without mechanical aeration, suggesting potential applications in energy-efficient wastewater treatment facilities.