Enhancement of the removal and settling performance for aerobic granular sludge under hypersaline stress

The aerobic granular sludge (AGS) dominated by halophilic microorganisms, was successfully cultivated in a lab-scale sequencing batch reactor (SBR) under varying salinity levels (from 0% to 6% (w/v)). Removal performance of AGS improved with the increase of salinity and increased up to 42.86 mg g−1 VSS h−1 at 6% salinity. Increased salinity resulted in better settling performance of AGS in terms of the sludge volume index (SVI), which was initially 148.80 mL/g at 0% salinity and gradually decreased to 59.1 mL/g at 6% salinity. The increase of salinity stimulated bacteria to secret excessive extracellular polymeric substances (EPS), with its highest production of 725.5 mg/(g·VSS) at 5% salinity. The total protein (PN) exhibited highly positive correlation with the total EPS (R = 0.951), indicating that selective secretion of some functional PN played a key constituent in resisting the external osmotic pressure and improving sludge performance. Salinicola, accounted for up to 91% relative abundance at 6% salinity, showed the high positive correlation (R = 0.953) with salinity. The enrichment of such halophilic or halotolerant microbial community assured both stable and improved removal performance in the AGS system. The enrichment of salt response pathways and altered metabolic processes for salt-tolerant bacteria indicated that the microbial community formed special metabolic pattern under long-term hypersaline stress to maintain favourable cellular activity and removal performance.