Transforming anaerobic granular sludge into a symbiotic system with aerobic granular and floc sludge: Self-assembly and niche differentiation of simultaneous nitrification and denitrification.

Aerobic granular sludge (AGS) facilitates simultaneous nitrification and denitrification (SND) but suffers from protracted start-up and instability. Meanwhile, the synergistic nitrogen removal mechanisms between AGS and its disintegration-derived flocculent sludge (FS) remain unclear. In this study, sieve-enclosed anaerobic granular sludge (AnGS) was inoculated into a sequencing batch reactor (SBR) and transformed into an AGS-FS symbiotic system. This system achieved up to 80.17% total nitrogen (TN) removal and maintained TN removal as high as 78.05% when treating authentic wastewater. Spatial dissolved oxygen (DO) differentiation contributed to the functional differentiation of AGS and FS, favoring denitrification in AGS and nitrification in FS. External carbon was partially sequestered as polyhydroxyalkanoates (PHAs) within AGS, creating a low-carbon environment favorable for nitrification and subsequently supporting endogenous denitrification. Microbial analysis revealed spatially differentiated nitrogen removal potentials. AGS enriched denitrification-associated genera (Diaphorobacter, Denitratisoma, and Hydrogenophaga), accompanied by the enrichment of nirK/S, norB/C, and nosZ, whereas FS enriched Nitrospira and Paracoccus associated with nitrification potential and SND-related functions, accompanied by nxrA/B enrichment. Regarding carbon metabolism, AGS exhibited enrichment of genes involved in the phosphotransacetylase-acetate kinase (PTA-ACK) pathway, tricarboxylic acid (TCA), and glyoxylate cycles, potentially supporting energy metabolism and PHAs storage for denitrification. Conversely, FS was primarily associated with carbon metabolism supporting energy supply for nitrification. This study elucidates the cooperative nitrogen removal mechanisms in AGS-FS symbiosis formed by AnGS inoculation, offering a robust strategy for biological nitrogen removal.