Aerobic Granular Sludge (AGS) is an innovative and efficient biotechnology for wastewater treatment that has been successfully applied on full-scale worldwide. Full-scale municipal AGS systems typically contain both granular sludge (granules) and flocculent sludge (flocs). Studies on the different roles of granules and flocs remain limited. In this study, a laboratory-scale AGS reactor fed with complex synthetic wastewater was operated to simulate full-scale AGS systems and to study the different functional roles of granules and flocs. The laboratory reactor achieved a coexistence of granules and flocs with a floc mass fraction of 17 %. The activities of different size fractions were evaluated using batch experiments and compared for carbon, nitrogen, and phosphorus removal: flocs (FL; <0.2 mm), small granules (SG; 0.2∼1.0 mm), medium granules (MG; 1.0∼2.0 mm), and large granules (LG; >2.0 mm). During feeding, large granules and medium granules exhibited more substrate uptake than small granules and flocs due to preferential substrate access. For aerobic conversion, flocs and small granules showed higher biomass-specific nitrification rates, while medium granules and large granules showed higher phosphorus uptake and denitrification capacity. Furthermore, large granules and medium granules showed stronger mass transfer limitation of oxygen, which limits their nitrification capability. Microbial community analysis using metagenomics and metaproteomics was performed across size fractions, and distinct communities in granules and flocs were shown. Granules showed a high abundance of Candidatus Accumulibacter (polyphosphate-accumulating organisms, PAOs) and Candidatus Competibacter (glycogen-accumulating organisms, GAOs). Flocs showed a high abundance of Nitrosomonas (ammonium-oxidizing bacteria, AOB) and Tetrasphaera (fermentative PAOs) and a low abundance of Ca. Accumulibacter. The distribution of microbial activities and microbial community over sludge size fractions in the laboratory reactor is similar to full-scale AGS systems, indicating that this laboratory setup can simulate full-scale systems and can be used for future research. Overall, this study highlights the importance of maintaining a good balance between different granule sizes and flocs to optimize nutrient removal.

Enhanced biological phosphate removal and aerobic sludge granulation are commonly studied with fatty acids as substrate. Fermentative substrates such as glucose have received limited attention. In this work, glucose conversion by aerobic granular sludge and its impact on phosphate removal was studied. Long-term stable phosphate removal and successful granulation were achieved. Glucose was rapidly taken up (273 mg/gVSS/h) at the start of the anaerobic phase, while phosphate was released during the full anaerobic phase. Some lactate was produced during glucose consumption, which was anaerobically consumed once glucose was depleted. The phosphate release appeared to be directly proportional to the uptake of lactate. The ratio of phosphorus released to glucose carbon taken up over the full anaerobic phase was 0.25 Pmol/Cmol. Along with glucose and lactate uptake in the anaerobic phase, poly‑hydroxy-alkanoates and glycogen storage were observed. There was a linear correlation between glucose consumption and lactate formation. While lactate accounted for approximately 89 % of the observed products in the bulk liquid, minor quantities of formate (5 %), propionate (4 %), and acetate (3 %) were also detected (mass fraction). Formate was not consumed anaerobically. Quantitative fluorescence in-situ hybridization (qFISH) revealed that polyphosphate accumulating organisms (PAO) accounted for 61 ± 15 % of the total biovolume. Metagenome evaluation of the biomass indicated a high abundance of Micropruina and Ca. Accumulibacter in the system, which was in accordance with the microscopic observations and the protein mass fraction from metaproteome analysis. Anaerobic conversions were evaluated based on theoretical ATP balances to provide the substrate distribution amongst the dominant genera. This research shows that aerobic granular sludge technology can be applied to glucose-containing effluents and that glucose is a suitable substrate for achieving phosphate removal. The results also show that for fermentable substrates a microbial community consisting of fermentative organisms and PAO develop.