Granulation is a gradual process that makes flocculent sludge granular through the simultaneous densification and selection of aggregates via sedimentation. The damage to the granule structure over time in a bioreactor operation is one of the most severe barriers to the practical application of the process. The addition of metal ions may increase aggregation rates and granular structure stability. Four sequential batch reactors fed with pulp mill effluent were operated and monitored. Three reactors contained aerobic granular sludge and one contained flocculent sludge. One granular sludge SBR received the addition of 100 mg∙L-1 of Ca2+, the second 200 mg∙L-1 of Ca2+, and the third received no intentional addition of calcium. The fourth SBR was operated with conventional flocculent sludge. The efficiency of the organic matter removal and the effect of calcium on the morphological characteristics of the granules formed were evaluated. The removal efficiency of the COD and the BOD was similar among all SBR, i.e., 60% and 90%, respectively. The addition of calcium did not interfere with granule size. The addition of 100 mg∙L-1 of Ca2+ increased the uniformity and the mechanical strength of the granules. It also increased approximately 36% of the settling velocity of the granules.