Impact of fall ammonia fluctuations on winter nitrification in moving bed biofilm reactors

This pilot-scale study investigated nitrifying moving bed biofilm reactors (MBBRs) in a post-lagoon treatment setup over two years to evaluate the impact of seasonal ammonia fluctuations on winter nitrification. In Year 2, reactors without fall ammonia starvation achieved significantly higher winter ammonia removal (97.2 ± 1.5 %) and surface area ammonia removal rates (SARR) (0.69 ± 0.06 g N/m²·d) compared to Year 1 (63.7 ± 2.5 % ammonia removal, SARR of 0.35 ± 0.04 g N/m²·d), demonstrating the critical role of fall ammonia availability for winter nitrification. Biofilms in Year 2 were thinner and denser, with higher biomass concentrations, potentially supporting more active biomass and improved substrate uptake. Seasonal shifts and diversity loss were observed within the biofilm microbial community, and nitrifiers were identified as Nitrosomonadaceae and Nitrospiraceae. Moreover, linear relationships were explored between winter ammonia removals and two ratios: (1) days with influent ammonia levels ≤ 5 mg N/L to days with temperatures above 5 °C, and (2) average ammonia concentration during fall to peak winter ammonia concentration. The modeling results indicated that winter ammonia removal performance could be enhanced by minimizing low-ammonia periods in the fall and maximizing pre-winter ammonia concentration. Overall, this study not only provided a deeper understanding of the year-round nitrifying MBBR process but also highlighted the importance of maintaining adequate substrate levels during fall to ensure sufficient biomass accumulation and activity for robust winter nitrification performance. These findings are essential for enhancing wastewater treatment performance in cold climates and offer practical guidance for optimizing biofilm-based nitrification systems.