Decoding the Lifecycle Dynamics of Aerobic Granular Sludge: Quantifying Mechanisms of Stability and Disintegration

Aerobic granular sludge (AGS) offers a compact and energy-efficient alternative to conventional activated sludge systems. Yet, instability under long-term operation remains a critical barrier to its widespread deployment. The mechanisms governing granule formation and disintegration are not fully resolved, largely due to static, reductionist studies that overlook AGS’s temporal dynamics. Here, we present a time-resolved, multiscale framework that tracks AGS evolution through its natural “small-large-small” granule size cycle, treating size as a proxy for developmental stage. This approach captures the dynamic transitions of AGS across multiple growth and decay phases, revealing that stable granule formation requires synergistic coupling between filamentous scaffolds ( Thiothrix abundance, β = 0.843), autotrophic respiration (β = 0.744), and protein-rich extracellular polymeric substances (EPS; PN/PS ratio, β = 0.791). These interactions establish structural cohesion through metabolic-structural feedback. When critical thresholds are not met (e.g., Thiothrix  < 18.7%, PN/PS ratio < 3.5), granulation fails. Disintegration initiates via a cascading failure process, driven by internal cavity expansion (β = 0.876), EPS depletion (LB-EPS < 15.3 mg/g VSS), and reduced viscoelastic resistance (viscosity < 215 Pa·s). These transitions coincide with microbial succession and altered EPS synthesis under oxygen gradients. A strong inverse correlation between granule integrity and mass transfer efficiency (β = -0.753, p < 0.001) exposes a trade-off between structural stability and nutrient diffusion. In parallel with these mechanistic insights into AGS lifecycle transitions, we developed predictive models that link structural traits to pollutant removal performance, offering a theoretical framework for optimizing long-term reactor performance. Together, these mechanistic insights and predictive models provide a unified framework for understanding and controlling AGS lifecycle transitions, marking a significant step forward in the rational design of stable, high-performance wastewater treatment systems.