Microbial synergy achieving simultaneous nitrification-denitrification (SND) in salt-tolerant aerobic granular sludge

The adaptation of microbial community is vital for the potential use of aerobic granular sludge (AGS) as a technology for high-salinity wastewater. Although AGS has dense microbial aggregates and excellent settling ability, it requires a long adaptation period and may become unstable when exposed to salt stress. This study investigated the adaptation process of microbial community in salt-tolerant AGS (S-AGS) reactors to identify core microbes and their synergistic action during the AGS adaptation period. Compared to the control groups (R1, municipal sludge; R2, industrial sludge), the S-AGS (R3, mixed sludge) was quickly formed within 30 days and maintained with good structural stability (integrity coefficient > 0.95, particle size > 1200 μm) and high removal efficiency to COD (>90 %) and TN (>78 %). The core community of S-AGS in R3 was dominated by Thauera spp. and Pseudofulvimonas spp., while Rhodobacter spp. and Flavobacterium spp. were significantly inhibited during high salt stress. Co-occurrence network analysis revealed that Thauera spp. and Pseudofulvimonas spp. were the essential modules for S-AGS. The RT-qPCR results indicated that a higher level of nitrogen metabolism-related genes (nxrB, napA) ensured the functional potential of simultaneous nitrification-denitrification (SND). Furthermore, the results of microbial community analysis indicated that microbial synergy via enhancing ion transport and tryptophan secretion played a vital role in resisting the salt stress of AGS. This study provided a deeper insight into the stable development of S-AGS through microbial synergy, thereby providing practical implications for the saline wastewater treatment.