Previous studies showed that iron (Fe2+) increases the granula-tion of anaerobic granular sludge in UASB reactors [17]. The effectsof zero-valent iron (ZVI) on anaerobic granular sludge were alsoinvestigated. The observations are explained by (i) the ZVI reac-tion benefiting the growth of methanogens through acid bufferingand the reduction of the oxidation–reduction potential and (ii)Fe2+leaching from ZVI advancing the granule aggregation [18].However, there is no information on the effects of ZVI on aerobicgranulation.