During the acclimation phase (days 1–12), the seed sludgedeveloped into large flocs. The mean diameters of the flocs andgranules increased rapidly during the acclimation phase. After 7days of operation, small granules were firstly observed in R2, andthen gradually became much denser and bigger. In R1, no granuleswere found until day 11.The multiplication phase lasted from day 13 to the day of com-plete granulation. On day 19, the majority of granules in R2 had anuneven surface and soft texture and an average diameter less than0.5 mm. After 26 days of operation, a number of granules appearedwhile the irregular granules became stable and were smoother andround shaped with a solid surface in R2. The color changed frombrown to yellow in R2 after 26 days. Scanning electron microscopyrevealed that there was no difference in the composition and mor-phology of the aerobic granulation in the two reactors. All granulesin the two reactors had a dense and compact bacterial structure,which implied that the addition of ZVI had no effect on the micro-bial morphology. The multiplication phase was characterized by asignificant increase in the concentration of MLVSSs in the reactors.After complete granulation, the concentrations of MLVSSs in bothreactors exceeded 2700 mg L−1. After complete full granulation, abiomass wasting strategy was implemented to maintain an MLVSSconcentration in the SBARs of 2700 mg L−1with an SRT of 20 days.The granulation was considered to be complete when a clearshape of granules was observed and the ratio of SVI30/SVI5of thegranulation sludge sample exceeded 90% [28]. It took about 43 daysfor R2 to reach full granulation, at which time the ratio of SVI30/SVI5was 90.7% (Fig. 1). It took about 64 days for R1 to reach full granu-lation, at which time the ratio of SVI30/SVI5was 90.3%. The processof granulation in reactor R1 was similar to that in R2. However, thegranulation was slower in R1 than in R2. This clearly shows that ZVIargumentation reduced the start-up time of aerobic granulation.The COD and NH4+-N removal efficiencies during the granula-tion process are shown in Fig. 2. There were fluctuations of CODand NH4+-N removal efficiencies in the first few days because ofthe washing out of the sludge and the inadequate adaptation ofmicroorganisms. For the remainder of the experiment, the reac-tors efficiently removed COD and NH4+-N. However, better CODand NH4+-N removal efficiencies were found for R2 (94.6% ± 1.0%for COD and 95.1% ± 1.0% for NH4+-N), than for R1 (93.4% ± 1.1% forCOD and 93.8% ± 0.9% for NH4+-N). This suggests that the COD andNH4+-N removal efficiencies in R2 were slightly improved with theaugmentation of ZVI.