Spatial DO gradients engineering in a SBR with asymmetric aeration and controlled perforated baffling to enhance...

Simultaneous nitrification-denitrification and phosphorus removal (SNDPR) has received increasing attention as an efficient and cost-effective biological wastewater treatment process for mitigating eutrophication. Numerous bio-based enhancement strategies, including biofilm carriers, aerobic granular sludge, and short-cut nitrogen pathways, have been developed to improve SNDPR performance; however, physical-based engineering approaches, which can further exploit limited reactor volume to establish spatial dissolved oxygen (DO) gradients as a stable supplementary strategy, have been largely underestimated. In this study, we present a novel physical microenvironment engineering strategy that integrates asymmetric aeration and controlled perforated baffling to establish spatial DO gradients in a single sequencing batch reactor, targeting enhanced SNDPR, particularly endogenous denitrification. During long-term operation, a chemical oxygen demand removal efficiency of 83.0% and a total inorganic nitrogen removal efficiency of 91.1% were achieved under a low carbon/nitrogen ratio of 4.1, with effluent phosphorus concentration reaching as low as 0.4 mg/L and excellent simultaneous nitrification-denitrification efficiency of 81.4%. Notably, the spatial DO gradients promoted biomass granulation and selectively enriched denitrifying glycogen-accumulating organisms (DGAOs). The sludge volume index decreased to 42.2 mL/g, with an average particle diameter of 363.16 μm, and DGAOs (mainly Candidatus_Competibacter) became dominant with a relative abundance of 47.4%. Benefiting from the micro-niches differentiated by the spatial DO gradients, a positive feedback loop was established between DGAOs and biomass granulation, while ensuring a highly efficient synergetic functional consortium within the SNDPR system. Finally, scale-up considerations were proposed, indicating the practical applicability and future potential of this strategy for enhanced nutrient removal and energy reduction.