Multiple-perspective research on the spatial distribution of full-size aerobic granular sludge in wastewater treatment

With the further application of aerobic granular sludge (AGS), the granules specific morphological and structural assessment is more essential. Full-size AGS was evaluated from multiple-perspective of mass transfer performance, spatial structure and specific morphology. The results demonstrated that AGS with 2–3 mm particle size had the best performance in all aspects. Effective diffusion coefficient (De) values of ammonia nitrogen and oxygen for 2–3 mm AGS was 0.945 × 10−9 m2/s and 0.678 × 10−9 m2/s respectively, which was higher than other particle size. Pore distribution and specific surface area revealed the differences in full-size AGS. Excellent development of 2–3 mm AGS on mesopores and macropores provided a pathway for the diffusion of NH4+-N and O2. Its specific surface area was significantly higher than other granules, reaching 151.293 m2/g, which contributed to the adsorption of the substrate. Digital holographic microscopy (DHM) images were applied firstly to observe full-size AGS without staining, indicating the 2–3 mm AGS had the highest bacterial activity. Confocal laser scanning microscopy (CLSM) images showed a good distribution of extracellular polymeric substances (EPS) and cells in AGS. By optimizing the proportion of 2–3 mm AGS in all particles, the efficiency and stability of the AGS system is expected to be greatly improved.