Dynamic yet functionally convergent microbial communities enable robust ammonia removal during long-term operation of a granular sludge bioreactor.

The study investigates the long-term operation and optimization of mature leachate treatment, emphasizing ammonia removal amid seasonal influent variations. A granular sludge bioreactor was operated for over 600 days to assess its feasibility, stability, and response to seasonal fluctuations in leachate quality. Key performance parameters, including ammonia (NH4+-N), nitrite (NO2--N), and nitrate (NO3--N) concentrations, were monitored and optimized to enhance nitrogen reduction. Despite significant fluctuations in influent quality, the bioreactor consistently achieved high ammonia removal efficiency (>98.8%) during stable operation, while total nitrogen (TN) and COD removals averaged only 11.8% and 26.0%, respectively, constrained by limited biodegradable organic carbon. Optimization strategies, including adjustment of the aeration phase and hydraulic retention time (HRT), were pivotal in maintaining stable performance. Sludge characterization indicated stable sludge volume index (SVI) values, suggesting good settleability and minimal bulking. Microbial analysis revealed ecological succession from early Thauera dominance to enrichment of canonical nitrifiers (Nitrosospira and Nitrobacter). Although community composition shifted over time, nitrifying populations converged toward a stable ammonia-oxidizing group under carbon-limited conditions. Functional redundancy within the nitrifying guild sustained process resilience despite seasonal influent variability. This study provides valuable insights into the scalability and sustainability of granular sludge reactor (GSR) technology for mature landfill leachate treatment, highlighting the crucial role of microbial community dynamics in long-term process robustness.