Partial nitrification of wastewaters with high NaCl concentrations by aerobic granules in continuous-flow reactor

•Partial nitrification with aerobic granules was tested under high NaCl stress.•Reactor performance, granular characteristics, microbial community were monitored.•High NaCl led to complete conversion from ammonium to nitrite.•Mechanisms of high-salt tolerance of aerobic granules were discussed. Wastewaters with high salinity are yielded that need sufficient treatment. This study applied aerobic granules to conduct partial nitrification reactions for wastewaters with high NaCl concentrations in a continuous-flow reactor. The present granules revealed partial nitrification performances at nitrite accumulation rate >95% and chemical oxygen demand (COD) removal at >85% at salt concentration up to 50g l−1. High salinity led to compact and tough granules. The granules applied electrogenic ion pump and sodium–calcium exchanger to reduce intracellular Na+ concentration; generated amino acids as osmoprotectants to resist the high osmotic pressure; produced excess extracellular polysaccharides and proteins with secretion of c-di-GMP; revised microbial community with halophilic strains. The present continuous-flow aerobic granule reactor (CFAGR) is a promising process to convert ammonium in highly saline wastewaters to nitrite, which can be applied with a subsequent Anammox process for efficient nitrogen removal.