The anaerobic ammonium oxidation (anammox) process has attracted considerable attention in recent years as an alternative to conventional nitrogen removal technologies. In this study, an innovative hybrid reactor combining fluidized and fixed beds for anammox treatment was developed. The fluidized bed was mechanically stirred and the gaseous product could be rapidly released from the anammox sludge to prevent washout of the sludge caused by floatation. The fixed bed comprising a non-woven biomass carrier could efficiently catch sludge to reduce washout. During the operation, nitrogen loading rates to the reactor were increased to 27.3 kg N/m(3)/d, with total nitrogen removal efficiencies of 75%. The biomass concentration in the fluidized bed reached 26-g VSS/L Anammox granules were observed in the reactors. with settling velocities and sludge volumetric index of 27.3 +/- 6.5 m/h and 23 mL/g, respectively. Quantification of extracellular polymeric substances revealed the anammox granules contained a significant amount of extracellular proteins. (C) 2011 Elsevier Ltd. All rights reserved.