Integrating fermentation with denitrification into anammox system for stable nitrogen removal at low carbon to nitrogen ratio

Integrating fermentation process into anammox system is a prospective strategy to enhance nitrogen removal efficiency (NRE) and facilitate advanced recovery of organic matter from wastewater. In this study, the feasibility of achieving collaboration between fermentation bacteria and denitrifying bacteria (DNB) in a mainstream anammox system was verified at low influent carbon to nitrogen ratio (C/N, 3.0 → 0). The results indicated a stable NRE (81.8 %–85.2 %) when the C/N was decreased from 3.0 to 1.0. At C/N = 0, the NRE initially declined to 46.8 % and then recovered to 61.2 %, with simultaneous removal of NO₃⁻-N and NH₄⁺-N, suggesting the occurrence of simultaneous anammox, denitrification, and fermentation. A high abundance of hydrolytic fermentation bacteria was observed in the system via microbial community analysis, such as Calorithrix (approximately 13.3 %), while partial denitrification bacteria, including Thauera (2.6 % → 4.8 %), progressively enriched. Candidatus_Brocadia, the sole identified genus of anammox bacteria, increased from 0.6 % to 2.2 %. Microbial correlation network analysis further highlighted a strong positive correlation between fermentation bacteria and DNB, such as norank_f__Saprospiraceae and Thauera. These bacteria promoted partial denitrification/anammox, thereby supporting stable nitrogen removal under low C/N. Their interactions can reduce the reliance of nitrogen removal on organic matter and facilitate the recovery of organic matter from wastewater.