In this study, partial nitrifying (PN) aerobic granules were developed in a sequencing batch airlift reactor by controlling the airflow rate and NH4+ loading rate. The PN reactor produced an effluent with a NO2-/NH4+ ratio of approximately one and with an NH4+ conversion rate of 1.22 kg N m(-3) day(-1). More than 95% of the total organic carbon was removed during the process. On the basis of clone library analysis and fluorescence in situ hybridization, ammonia-oxidizing bacteria (AOB) closely related to Nitrosomonas eutropha and putative heterotrophic denitrifiers were mainly present near the surface of the PN aerobic granules. Microelectrode measurements revealed that both NH4+ and NO2- were consumed near the. surface (<200 mu m), whereas no nitrate (NO3-) accumulation was observed throughout the granules. These results indicate that PN by AOB and nitrite denitrification by heterotrophs, but not nitrite oxidation, simultaneously occurred near the surface of the PN aerobic granules. (C) 2013 Elsevier Ltd. All rights reserved.