Microbial and hydrodynamic properties of aerobic granules in a sequencing batch reactor treating landfill leachate

A sequencing batch reactor (SBR) seeded with activated sludge was established for landfill leachate treatment. Small bio-aggregates began to appear after 40-d operation, and gradually changed to mature aerobic granules, with a mean size of 0.36–0.60 mm. Their sludge volume index at 5 min (SVI5 min), mixed liquor volatile suspended solids (MLVSS), and wet density were around 35 ml/g, 3.4 g/L, and 1.062 g/cm3, respectively. The settling velocities of the granules in distilled water ranged from 0.3 to 1.3 cm/s, which were faster than those in landfill leachate with a salt content of 1.4% (w/v), and also slightly faster than those predicted by Stokes’ law for porous but impermeable particles. Microbial community evolution during the granulation process and stages under different nitrogen loading rates (NLRs) were monitored and analyzed by polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE), cloning, and sequencing of 16S ribosomal RNA (rRNA) fragments. Results revealed that some primary and dominant communities in inoculating activated sludge died out gradually; while a few common bacteria, inhabiting soils, municipal wastewater, or activated sludge systems, dominated in the SBR system throughout. In addition, some other dominant species, associated with the aerobic granulation process, were thought to play a significant role in the formation and growth of aerobic granular sludge. During the stable operation time under low NLR, a few species were present in abundance, and may have been responsible for the high organic removal efficiency at this time.