Aerobic granular sludge (AGS) and conventional activated sludge (CAS) are two different biological wastewater treatment processes. AGS consists of self-immobilised microorganisms that are transformed into spherical biofilms, whereas CAS has floccular sludge of lower density. In this study, we investigated the treatment performance and microbiome dynamics of two full-scale AGS reactors and a parallel CAS system at a municipal WWTP in Sweden. Both systems produced low effluent concentrations, with some fluctuations in phosphate and nitrate mainly due to variations in organic substrate availability. The microbial diversity was slightly higher in the AGS, with different dynamics in the microbiome over time. Seasonal periodicity was observed in both sludge types, with a larger shift in the CAS microbiome compared to the AGS. Groups important for reactor function, such as ammonia-oxidising bacteria (AOB), nitrite-oxidising bacteria (NOB), polyphosphate-accumulating organisms (PAOs) and glycogen-accumulating organisms (GAOs), followed similar trends in both systems, with higher relative abundances of PAOs and GAOs in the AGS. However, microbial composition and dynamics differed between the two systems at the genus level. For instance, among PAOs, Tetrasphaera was more prevalent in the AGS, while Dechloromonas was more common in the CAS. Among NOB, Ca. Nitrotoga had a higher relative abundance in the AGS, while Nitrospira was the main nitrifier in the CAS. Furthermore, network analysis revealed the clustering of the various genera within the guilds to modules with different temporal patterns, suggesting functional redundancy in both AGS and CAS.

Aerobic granular sludge (AGS) plants have gained growing interest and application due to their low energy demand, small footprint, and low operational costs. However, the fulfilment of strict discharge limits for nitrogen and phosphorus, vast seasonal temperature variations, and large peaks in influent flows may pose challenges to the implementation of AGS. Moreover, the knowledge about microbial community assembly and process performance under varying environmental conditions in full-scale reactors is still limited. In this study, the first implementation of the AGS process in the Nordic countries was assessed. In two full-scale AGS reactors with different seeding sludges, the start-up was associated with rapid changes in microbial community composition in both, but only successful granulation in one. As a consequence, the non-granulated reactor was eventually reseeded with biomass from the better granulated reactor. This resulted in a convergence of the microbial communities in the two reactors with the maintenance of stable sludge concentrations (6-8 g L⁻¹) with large granules (50-80% with diameter >2 mm) and fast settling of biomass (SVI₃₀/SVI₁₀ of 0.9-1). Immigration from the influent wastewater was a minor factor affecting the microbial community once the granules had formed, while the seasonal variations in environmental factors were identified as important. Key guilds of AOB (Nitrosomonas), NOB (mainly Ca. Nitrotoga), PAOs (mainly Tetrasphaera), and GAOs (mainly Ca. Competibacter) varied considerably in abundance throughout the study period. After 15 months, stable organic matter, nitrogen, and phosphorus removal were attained with low effluent concentrations. During the start-up, the BOD₇/N ratio, influent flow, and temperature were important factors influencing the performance of the AGS.