Aerobic granular sludge (AGS) technology holds great promise of becoming the standard for biological wastewater treatment due to its lower energy consumption, small footprint, and high removal efficiency of nutrients compared to the conventional activated sludge processes. Different-sized aggregates have been shown to harbor a different microbial community composition. The central question is do full-scale AGS wastewater treatment plants (WWTPs) select for core microbial communities across different aggregate sizes and how these selected organisms differ between the different-sized aggregates. This study analyzed samples from nine geographically distributed full-scale AGS WWTPs that consistently perform well in terms of chemical oxygen demand (COD) and nutrient (N and P) removal. The main results showed that site-specific conditions highly influence microbial composition in smaller aggregates (< 1 mm), while larger granules form stable communities independent of WWTP location. Notably, all aggregates contained a small subset of 128–139 core OTUs that were both prevalent and abundant across all sizes. These core OTUs include key functional groups such as fermenters, aerobic heterotrophs, polyphosphate-accumulating organisms (PAOs), glycogen-accumulating organisms (GAOs), and nitrifiers, which play a crucial role in COD and nutrient removal. Additionally, an enrichment pattern was observed, with aerobic heterotrophs dominating in flocs, PAOs in small granules, and GAOs and nitrifiers in large granules. This study offers valuable insights into the core microbiome of different-sized aggregates in full-scale AGS WWTPs and highlights their potential role in overall system performance.

Currently, the most cost-effective and efficient method for phosphorus (P) removal from wastewater is enhanced biological P removal (EPBR) via polyphosphate-accumulating organisms (PAOs). This study integrates a literature review with genomic analysis to uncover the phylogenetic and metabolic diversity of the relevant PAOs for wastewater treatment. The findings highlight significant differences in the metabolic capabilities of PAOs relevant to wastewater treatment. Notably, Candidatus Dechloromonas and Candidatus Accumulibacter can synthesize polyhydroxyalkanoates, possess specific enzymes for ATP production from polyphosphate, and have electrochemical transporters for acetate and C4-dicarboxylates. In contrast, Tetrasphaera, Candidatus Phosphoribacter, Knoellia, and Phycicoccus possess PolyP-glucokinase and electrochemical transporters for sugars/amino acids. Additionally, this review explores various detection methods for polyphosphate and PAOs in activated sludge wastewater treatment plants. Notably, FISH-Raman spectroscopy emerges as one of the most advanced detection techniques. Overall, this review provides critical insights into PAO research, underscoring the need for enhanced strategies in biological phosphorus removal.