Performance and adaptation mechanisms of Anammox granular sludge under salinity stress: Role of EPS, microbial community and functional genes

Conventional biological treatment methods are often less efficient in treating anaerobic digestion liquor from food waste due to its high salinity and high ammonia nitrogen content. Anammox could effectively treat high-salinity wastewater after adaption, and deep understanding the adaptive mechanisms under varying salinity is key for process optimization. In this study, a salt-tolerant Anammox granular sludge (AGS) system was comprehensively investigated across varying salinity levels (0–25 g/L NaCl). The investigation focused on the interplay among nitrogen removal performance, specific Anammox activity (SAA), biomass dynamics, extracellular polymeric substances (EPS), key enzymes, functional genes and microbial community succession. A stable nitrogen removal performance (>86 %) of 15 g/L after acclimation was achieved, with peak SAA (6.73 mg N/h/g VSS) at 5 g/L and severe inhibition occurred at 25 g/L NaCl. These results represent one of the highest performances reported in current studies under similar high-salinity conditions. Declined SAA at 10–15 g/L NaCl was compensated by increased microbial biomass, maintaining system efficiency. Significant increases in EPS, especially tightly-bound (T-EPS) and protein/humic components was observed, which enhanced structural protection. Enrichment of salt-tolerant AnAOB (Candidatus Brocadia, Candidatus Kuenenia) and heterotrophic denitrifiers, coupled with dynamic regulation of key nitrogen cycling genes (hzo, hzs, narG, nirS) and enzyme activities (Hao, Nar, Nir, Amo) attributed to the adaptation to high salinity. At 25 g/L, process blockage occurred due to imbalance like increased Amo activity (causing nitrite accumulation) and decreased Nir activity (hindering nitrite removal). These findings confirm system resilience up to 15 g/L NaCl and offer valuable insights for applying Anammox AGS to treat high-salinity wastewaters.