The objective of this work was to evaluate the physical characteristics of the thermophilic granular aerobic sludge used in the treatment of bleached kraft pulp mill effluents. Four sequential batch reactors (SBRs) were operated with cycles of 12 hours. Reactor (R1-FSR) with flocculent sludge was used as control. The other three reactors (R2-GSR, R3-GSR+100, and R4-GSR+200) were operated with granular aerobic sludge. Concentrations 100 and 200 mg. L-1 of calcium were applied in the R3-GSR+100 and R4-GSR+200, respectively. The pH was maintained in the neutral range in all reactors. The experimental plan was carried out for 490-day period, in 5 phases at different temperatures of 35 °C to 55 ºC. All SBRs showed COD removal efficiency above 60% in all temperature ranges. The granule average diameter in the R2-GSR, R3-GSR+100, and R4-GSR+200 ranged from 5 to 8 mm. The reactor R3-GS+100 showed better stability due to the addition of 100 mg. L-1 of calcium. The granular sludge sedimentation velocity was 40 m.h-¹, which was eight times higher than the flocculent sludge. Thermophilic treatment (55 °C) using aerobic granular sludge proved to be a promising way for treating bleached kraft pulp mill effluent without a prior cooling process.
