Distinct roles of granules and flocs in aerobic granular sludge processes

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.