Revealing dissolved organic nitrogen transformation during the oxic process in 50 full-scale sewage treatment plants:...

Wastewater-derived dissolved organic nitrogen (DON), posing high eutrophication and disinfection byproduct formation risks, warrants special focus. However, despite the oxic process being the core biological unit of inorganic nitrogen removal, the full understanding of its impact on DON is highly constrained owing to the restricted research scope. Herein, a large-scale study encompassing 50 sewage treatment plants across China with diverse treatment processes was conducted to explore the transformation patterns of DON during the oxic process. Surprisingly, only 17 sewage treatment plants show decreasing DON concentrations (-2.15 to -0.02 mg/L), while up to 33 plants show an increase in DON (0.01 to 1.55 mg/L). Molecular analysis revealed distinct differences between these two groups: the relative abundance of produced highly unsaturated DON was significantly higher in the oxic processes with increasing DON concentrations (30.77%) than in those with decreasing concentrations (22.12%; p < 0.01), implying that highly unsaturated DON generated from microbes contributed to the increase in DON concentration. Subsequently, statistical evidence (Mantel test and Procrustes analysis, p < 0.05) linked this molecular shift to microbial activities, specifically a community shift toward DON-producing taxa and the enhanced pyruvate metabolism. Given that per milligram of DON can stimulate the production of nearly ten times more chlorophyll a than inorganic nitrogen, indicating a higher eutrophication potential, this study greatly challenges the widely recognized beneficial role of the oxic process in nitrogen removal with the finding of the unexpected increase in DON concentration.