Microbial community dynamics and mechanistic insights into rapid ammonia nitrogen removal via Acinetobacter harbinensis HITLi7^T enhanced activated carbon

The biological enhanced activated carbon (BEAC) system, utilizing the bioaugmentation with Acinetobacter harbinensis HITLi7^T, demonstrated remarkable performance in removing ammonia nitrogen (NH₄⁺-N) from aquatic environments. However, the mechanism through which HITLi7^T bioaugmentation influenced microbial communities remains incompletely understood. Therefore, a comparative analysis of startup speed and NH₄⁺-N removal efficiency was conducted between biological activated carbon (BAC) and BEAC bench-scale systems, with an in-depth examination of microbial dynamics and mechanisms within BEAC. Within 16 days post bioaugmentation, the biomass of BEAC (BEAC_1 and BEAC_2) was 4.75/9.07 times that of BAC, with NH₄⁺-N removal efficiencies 29.36%/28.68% higher. 16–135 days, there was no significant difference in biomass among the groups. Specifically, from day 17–90, the NH₄⁺-N removal efficiency of BEAC was still 25.80%/25.37% higher than that of BAC. After 90 days, the effluent NH₄⁺-N levels from BEAC were more stable. In BEAC, the abundance of HITLi7^T decreased while the abundance of Nitrosomonas increased, exhibiting a characteristic of "species compensation". Subsequently, Candidatus_Nitrotoga and Nitrospira became enriched. After 16 days, the total abundance of Nitrosomonas, Candidatus_Nitrotoga, and Nitrospira in BEAC was 3.21%–5.66% and 3.68%–10.07% higher than that in BAC. The microbial community in BEAC was more influenced by diffusion limitation, especially for Nitrosomonas and Nitrospira. Within the microbial co-occurrence network, while Nitrosomonas and Nitrospira lacked a direct correlation with HITLi7^T, the presence of intermediates might potentially hinder the diffusion of Nitrosomonas and Nitrospira. This study deepened our understanding of how HITLi7^T bioaugmentation affects microbial community structure, dynamics, and co-occurrence patterns.