Rapid granulation of aerobic sludge in a continuous-flow reactor with a two-zone sedimentation tank by the addition of dewatered sludge

Abstract Rapid granulation was realized in a continuous-flow reactor with a two-zone sedimentation tank (CFR-TST), and a novel strategy involving the continuous addition of dewatered sludge during the start-up phase to treat the actual domestic sewage with low concentration. The selection pressure produced by the two-zone sedimentation tank can discharge light flocs with poor settleability and retain the fine particles in the reactor. The continuous addition of dewatered sludge in the start-up phase can increase the small particles in the aeration tank by acting as nuclei to accelerate granulation. The experimental results indicated that aerobic granular sludge (AGS) was formed successfully on day 25 (the sludge volume index at 5 min (SVI5) was 37.7 mL/g, the mean particle size was 0.42 mm, and the average settling velocity was 35.2 m/h); after operated continuously for 45 days, the sedimentation capacity and biomass concentration were further enhanced (SVI5 and the mixed liquor suspended solids (MLSS) were stable in the range of 25.8–31.3 mL/g and 4200−4600 mg/L, respectively). Although the dewatered sludge with low biological activity had a slight negative impact on pollutants degradation in the start-up phase, the biological activity of the sludge was enormously upgraded (compared with those of the inoculated dewatered sludge, the SOUR and dehydrogenase activity of the mature AGS increased 3.72 times and 5.01 times on day 84, respectively), and microbes that promote the secretion of EPS and facilitate the degradation of pollutants were enriched during the whole process of granulation. Moreover, the approach developed in this case was simple and eco-friendly for potential full-scale implementation.