BACKGROUNDAerobic granular sludge (AGS) technology has been widely investigated for wastewater treatment. The morphological and structural characteristics of granules and nitrogen pathways have not been fully clarified using real domestic wastewater. To address this issue, microbial community distribution, diversity and functional pathways towards nitrogen conversions in a sequencing batch reactor were investigated. RESULTSGranules (d approximate to 300 mu m) were formed with low-strength domestic wastewater (COD influent < 200 mg CODsoluble L-1). COD and ammonium effluent concentrations (55 mgCOD L-1, 14 mgNH(4)(-)-N L-1) achieved national treatment standards. Nitrite accumulation was observed (16.5 mgNO(2)(-)-N L-1) and N2O emissions (2.8% of nitrogen load) were detected. Granules were composed of protozoa and bacteria, with an outer layer dominated by ammonium-oxidizing bacteria (AOB) and deeper layers with nitrite-oxidizing bacteria (NOB) and denitrifiers. Partial nitrification, denitrification via nitrite, nitrifier denitrification, complete and incomplete denitrification pathways were assessed by qPCR. CONCLUSIONSAGS process showed the establishment of partial nitrification in a low-strength real wastewater condition. The granular system showed N2O emission in a lower range than others known partial nitrification processes. In order to achieve nitrogen removal saving costs in terms of energy and space, engineering wastewater designers can consider AGS as an attractive technology. (c) 2016 Society of Chemical Industry