A novel aerobic denitrification reactor, aerobic denitrification using nitrifying and anoxic ammonium-oxidizing (anammox) bacteria immobilized on gel carriers in a single stage (AIGES), was developed. Two types of gel carriers, a nitritation gel carrier and an anammox gel carrier, were installed in single reactor, and the denitrification performance of simultaneous nitritation and anammox was evaluated. The denitrification performance increased gradually with increased aeration rate, reaching a denitrification rate of 1.4 kg N m(-3) d(-1) 2 weeks after the nitritation and anammox gel carriers were mixed. A high average denitrification efficiency of 82% was confirmed. Stable aerobic denitrification performance was observed for more than half a year. In the startup period of AIGES operation, ammonia-oxidizing bacteria were shown by fluorescence in situ hybridization analysis to grow on the surface layer of anammox gel cubes. These results indicated that anammox gel carriers promptly adapted to an aerobic environment by altering the microbial ecosystem. (C) 2013 Elsevier Ltd. All rights reserved.