Mechanistic Insights into the Phenomenon of Ammonia-Only Removal in Sulfate-Rich Environments

In sulfate-rich environments, the mechanisms of ammonia nitrogen removal and the role of potential electron acceptors remain unclear. To investigate this, an upflow spiral bed reactor (USBR) operated for 173 days using batch experiments and microbial analysis. The reactor maintained stable ammonia removal, while sulfate levels stayed nearly unchanged, indicating sulfate was not the primary electron acceptor. Batch results showed that trace dissolved oxygen (0.1~0.2 mg/L) and reactive oxygen species (ROS) caused partial nitrification. The resulting nitrite interacted with anaerobic ammonium oxidation (Anammox) to remove nitrogen. Bicarbonate (HCO3−) served only as an inorganic carbon source; when its concentration exceeded 1000 mg/L, it inhibited ammonia removal and was consumed internally, confirming it was not an electron acceptor. Microbial analysis revealed Proteobacteria and Chloroflexi supported short-range nitrification, while Planctomycetota (Candidatus Brocadia) facilitated Anammox. Sulfate-reducing bacteria decreased significantly, consistent with the absence of sulfate reduction. Functional prediction indicated enrichment of nitrogen metabolism genes but limited sulfur metabolism. This study uncovers a new pathway for ammonia nitrogen removal in sulfate-rich environments. Unlike traditional sulfate-dependent ammonium oxidation (SRAO), the process observed occurred without sulfate reduction and was instead driven by a micro-oxygen/ROS-induced ammonia oxidation–Anammox coupling mechanism. These results broaden the current understanding of nitrogen transformation in sulfate-rich wastewater systems.