Strength Characterization of Full-Scale Aerobic Granular Sludge

For a stable operation the aerobic granular sludge process requires mechanically strong granules in balance with the shear forces in the reactor. Despite a wide general interest in granular stability, the mechanical strength of both anaerobic and aerobic granular sludge received very little attention. In this study, a high-shear method for strength characterization has been evaluated for full-scale aerobic granular sludge (AGS). Abrasion times up to 90 minutes showed a stable abrasion rate coefficient (<i>K</i>), while prolonged periods of abrasion up to 24 hours resulted in a decrease in abrasion rate. Larger granules have higher abrasion rate than smaller granules. No abrasion was observed at low shear rates, indicating a threshold shear rate for abrasion. Lab-scale AGS showed a lower abrasion rate than full-scale AGS. Incubation of full-scale granules in NaCl led to a decrease in abrasion rate at 25 g L^-1 NaCl, but incubation in 50 g L^-1 NaCl led to a further decrease for only half of the tested granular sludge samples.