Lab-scale experiments were conducted to investigate the effects of ferrous iron on nutrient removal performance and variations in the microbial community inside aerobic granular sludge for 408days. Two reactors were simultaneously operated, one without added ferrous iron (SBR1), and one with 10mg Fe2+L-1 of added ferrous iron (SBR2). A total of 1 mg Fe2+L-1 of added ferrous iron was applied to SBR1 starting from the 191 st day to observe the resulting variations in the nutrient removal performance and the microbial community. The results show that ammonia-oxidizing bacteria (AOB) could not oxidize ammonia due to a lack of iron compounds, but they could survive in the aerobic granular sludge. Limited ferrous iron addition encouraged nitrification. Enhanced biological phosphorus removal (EBPR) from both reactors could not be maintained regardless of the amount of ferrous iron that was applied. EBPR was established in both reactors when the concentration of mixed liquor suspended solid (MLSS) and the percentage of Accumulibacteria increased. A total of 10 mgFe(2+)L(-1) of added ferrous iron had a relatively adverse effect on the growth of AOB species compared to 1 mgFe(2+)L(-1) of added ferrous iron, but it encouraged the growth of Nitrospira sp. and Accumulibacteria, which requires further study. It could be said that the compact and stable structure of aerobic granular sludge preserved AOB and NOB from Fe-deficient conditions, and wash-out during the disintegration period. (c) 2017 American Institute of Chemical Engineers Biotechnol.