A novel osmotic anammox (OsAMX) system coupling nitritation-anammox with forward osmosis (FO) has been developed for removal of reverse-fluxed ammonium when using NH4HCO3 as a draw solute. In this study, long-term performance and microbial community structure were investigated. The nitritation-anammox reactor maintained an ammonium concentration of 7.0 +/- 5.0 mg N L-1 (DO = 0.9 +/- 0.2 mg O-2 L-1), while the FO achieved a water flux of 2.3 +/- 0.4 LMH (0.5 M NH4HCO3 draw). The low water flux was obtained likely due to concentration polarization, reverse salt flux (RSF) and membrane fouling. Sequencing analyses reveled that Candidatus Jettenia was the dominant anammox genus, while Candidatus Brocadia was most abundant in biofilm. The shift of anammox bacterial population indicated possible higher tolerance of Ca. Brocadia for DO or elevated RSF. These results encourage further investigation of OsAMX system optimization, membrane fouling migration strategies, and application with actual wastewater. (C) 2016 Elsevier Ltd. All rights reserved.