How do salt-tolerant granular sludge cultivated under different conditions carry out nitrogen and phosphorus removal?

Aerobic granular sludge (AGS) technology had distinctive advantages and broad application prospects in the treatment of salinity wastewater. However, there was a lack of in-depth exploration of the nitrogen and phosphorus removal process of salt-tolerant AGS, which has restricted the technology development. Nitrogen and phosphorus removal process and mechanism of AGS at different salinity levels obtained by different acclimation methods were investigated in this study. The distribution of functional bacteria in salt-tolerant AGS was revealed: ammonia oxidizing bacteria (AOB) was mainly concentrated in the outer area, while nitrite oxidizing bacteria (NOB) occupied the entire area. Besides, simultaneous nitrification and denitrification (SND) was found in granules at 0 and 1 % salinity; adsorption and partial nitrification and denitrification (PND) were respectively demonstrated in granules at 3 % salinity achieved through "one-step method" and "step-up method". Additionally, Heterotrophic nitrification led by Zoogloea, Enterobacter, Thauera, and Rhodobacter was the main pathway for ammonium nitrogen (NH₄⁺-N) removal in all reactors. Extracellular polymeric substance (EPS) adsorption and biological phosphorus removal (BPR) based on phosphorus accumulating organisms (PAOs) and denitrifying phosphate accumulating organisms (DPAOs) were the main phosphorus removal methods for AGS under salinity conditions. And denitrification phosphorus removal pathway was found under non-salinity and 1 % salinity conditions, which was led by Enterobacter, Thauera, Zoogloea and Rhodobacter. Thauera was the only functional microorganisms associated with BPR present in granules at 3 % salinity. These findings could provide more technical support for the treatment of actual salinity wastewater and was conducive to the application and development of AGS technology.