Efficient nitrification and denitrification of wastewater containing 1,700 mgl(-1) of ammonium-nitrogen was achieved using aerobic granular sludge cultivated at medium-to-high organic loading rates. The cultivated granules were tested in a sequencing batch reactor (SBR) fed with 6.4 or 10.2 kg NH (4) (+) -N m(-3) day(-1), a loading significantly higher than that reported in literature. With alternating 2 h oxic and 2 h anoxic operation (OA) modes, removal rate was 45.5 mg NH (4) (+) -N g(-1) volatile suspended solids(-1) h(-1) at 6.4 kg NH (4) (+) -N m(-3) day(-1) loading and 41.3 +/- 2.0 at 10.2 kg NH (4) (+) -N m(-3) day(-1) loading. Following the 60 days SBR test, granules were intact. The fluorescence in situ hybridization and confocal laser scanning microscopy results indicate that the SBR-OA granules have a distribution with nitrifers outside and heterotrophs outside that can effectively expose functional strains to surrounding substrates at high concentrations with minimal mass transfer limit. This microbial alignment combined with the smooth granule surface achieved nitrification-denitrification of wastewaters containing high-strength ammonium using aerobic granules. Conversely, the SBR continuous aeration mode yielded a distribution with nitrifers outside and heterotrophs inside with an unsatisfactory denitrification rate and floating granules as gas likely accumulated deep in the granules.