Dissolved organic nitrogen depresses the expected outcome of wastewater treatment upgrading on effluent...

Continuously tightening total nitrogen (TN) discharge standards in wastewater treatment plants is a common practice worldwide to mitigate eutrophication. However, given the different bioavailability of effluent dissolved organic nitrogen (DON) and inorganic nitrogen, a great inefficiency of the TN-targeted upgrading might be hidden because of the poor understanding of its impact on effluent eutrophication potential mitigation. Here we show that the tightening TN discharge standards could only considerably promote inorganic nitrogen removal, however, DON concentrations remained constant across different effluent TN levels (p > 0.05, Kruskal-Wallis test). Surprisingly, restricting TN in turn increases the reactivity of DON molecules owing to the accumulation of produced DON by acting on the key biotic and abiotic transformation reactions. The difficulty of removing DON and the increased DON reactivity during wastewater treatment upgrading contribute to the practical elimination effect of effluent eutrophication potential exhibiting lower than expected. This work challenges the rationality of the prevailing pursuit for extreme-low TN discharge, calling for shifting the focus of wastewater treatment upgrading towards the more fundamental eutrophication-targeted perspective.