An expanded granular sludge bed (EGSB) reactor was started-up by seeding the anaerobic sludge from a brewery wastewater treatment plant. After 3 months operation, the granular sludge occurred in the reactor and the ammonium nitrogen could be removed continuously. The morphology and inner structure of the granules was observed using scanning electron microscopy (SEM) and the photographs indicated that the anaerobic granular sludge was relatively regular in shape. The outer layer had a rather fluffy and loose structure and was more transparent compared to the centre of the granule. The centre of the granule had a dense structure. The long-term operational results showed that the maximal removal efficiency of NH4+-N was 40% when the influent NH4+-N concentration was 70-250 mg/L; the NO2--N removal efficiency was more than 98% when the influent NO2--N concentration was 70-250 mg/L; and the COD removal efficiency was 84% at the influent concentration of 500 mg/L. This revealed that the reaction of anaerobic ammonium oxidation (ANAMMOX) probably taken place in the EGSB reactor. In order to validate the existence of ANAMMOX reaction and investigate the performance of nitrogen removal by anaerobic granular sludge, the granular sludge taken from EGSB reactor and the activated sludge taken from partial-nitrification reactor was used to treat synthetic wastewater. The experimental results demonstrated that for the activated sludge, the removal efficiency of total nitrogen (TN), NH4+-N and NO2--N was 45.5, 13.4 and 99.9%, respectively; for the anaerobic granular sludge, they were 54.3, 21.7 and 99.9%, respectively. The removal rate of NH4+-N was stable, about 3.17 mgN/(L h). At the beginning of the reaction, the ANAMMOX process was performed mainly through the pathway of interaction between NH4+-N and NO2--N, as time went on, with the exhaustion of NO2--N, the granular sludge could use NO3--N as electron acceptor to oxidize NH4+-N, yielding dinitrogen gas. In this research, the COD removal efficiency was about 90.4%. In the light of estimation of COD consumption, the anaerobic granular sludge could use organic carbon source as electron donor, reduce NO2--N and NO3-N, to carry out the traditional denitrification. The results obtained in this study reveal that in the anaerobic granular sludge reactor the ANAMMOX reaction took place, accordingly, the existence of an anaerobic ammonium oxidation phenomenon in the EGSB reactor could be deduced. (c) 2004 Elsevier Ltd. All rights reserved.