Structure and stability of aerobic granules cultivated under different shear force in sequencing batch reactors

The cultivation of stable aerobic granules as well as granular structure and stability in sequencing batch reactors under different shear force were investigated in this study. Four column sequencing batch reactors (R1–R4) were operated under various shear force, in terms of superficial upflow air velocity of 0.8, 1.6, 2.4, and 3.2 cm s⁻¹, respectively. Aerobic granules were formed in all reactors in the experiment. It was found that the magnitude of shear force has an important impact on the granule stability. At shear force of 2.4 and 3.2 cm s⁻¹, granules can maintain a robust structure and have the potential of long-term operation. Granules developed in low shear force (R1, 0.8 cm s⁻¹ and R2, 1.6 cm s⁻¹) deteriorated to large-sized filamentous granules with irregular shape, loose structure and resulted in poor performance and operation instability. Granules cultivated under high shear force (R3, 2.4 cm s⁻¹ and R4, 3.2 cm s⁻¹) stabilized to clear outer morphology, dense and compact structure, and with good performance in 120 days operation. Fractal dimension (D_f) represents the internal structure of granules and can be used as an important indicator to describe the structure and stability of granules. Due to the combined effects of shear force and growth force, the mature granules developed in R3 and R4 also displayed certain differences in granular structure and characteristics.