Aerobic granulation is a promising technology for wastewater treatment, but problems regarding its formation and stability need to be solved. Divalent metal ions, especially Ca, Mg and Mn, have been demonstrated to play an important role in the process of aerobic granulation. Here, we studied whether iron ions can affect aerobic granulation. Granular sludge formed without iron ion addition (<0.02 mg Fe L) was fluffy and had a finger-type structure and filamentous out-growth. The addition of iron ions to concentrations of 1 and 10 mg Fe L repressed the finger-type structure and filamentous out-growth. The results show that chemical precipitation in the granules with iron ion addition was higher than that in the granules without ferrous addition. The amount of precipitates was higher inside the granules than outside. This study demonstrates that iron ions (Fe/Fe) increase the size and stability of aerobic granular sludge but do not affect the granulation time, which is the time that the first granular sludge is observed. The study shows that aerobic granular sludge technology can be confidently applied to actual wastewater containing a high concentration of iron compounds.

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 408 days. Two reactors were simultaneously operated, one without added ferrous iron (SBR1), and one with 10 mg Fe L of added ferrous iron (SBR2). A total of 1 mg Fe L of added ferrous iron was applied to SBR1 starting from the 191st 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 mg Fe L of added ferrous iron had a relatively adverse effect on the growth of AOB species compared to 1 mg Fe L 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. © 2017 American Institute of Chemical Engineers Biotechnol. Prog., 33:716-725, 2017.