The granule size distributions of aerobic granulation on day 85in R1 and R2 in triplicate are shown in Fig. 3. The volume percent-age of aerobic granules in the two reactors with size in the range1.5–2.5 mm was about 50%; i.e., there was little difference in thevolume percentage between the two reactors. Moreover, 66.4% ofgranules in R1 were in the size range 1–2.5 mm, and 6.6% of gran-ules were larger than 3 mm. In contrast, 76.8% of granules in R2fell in the size range 1.5–3 mm while 13.1% of granules were largerthan 3 mm. Similar results were obtained in other studies [16,29].It can be inferred that ZVI augmentation promotes aerobic gran-ulation, resulting in a larger diameter of granules. The result alsoimplies that the microbial community structure differed betweenthe two reactors. The granule microstructure and microbial diver-sity are strongly dependent on the composition and loading rateof the carbon source available via the selection and enrichment ofdifferent bacterial species [12]. In this research, the influent of thetwo reactors had the same composition and loading rate of the car-bon source (1200 mg L−1COD). The difference between the reactors