Molecular insights into the bioavailability and transformation of dissolved organic phosphorus and nitrogen in a wastewater treatment plant

Dissolved organic phosphorus (DOP) and dissolved organic nitrogen (DON) have high potential bioavailability, making them key factors in the water eutrophication process. For wastewater treatment plants (WWTPs), one of the challenges in alleviating eutrophication in receiving waters is reducing levels of DOP and DON, which are harder to remove than inorganic phosphate and dissolved inorganic nitrogen. In this study, for the first time we simultaneously investigated the bioavailability and transformation of DOP and DON in a WWTP at the molecular level, via Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and paired mass distance (PMD)-based network analysis. Results showed that the removal of DOP and DON in the biological treatment contributed 83.3 % and 100.3 %, respectively, to the total removal in the WWTP. Meanwhile, the double bond equivalence values of DOP and DON markedly increased from 8.93 to 13.36 and from 7.57 to 11.72 during biological treatment, respectively, whereas slight changes were observed during advanced treatment, indicating that the biological treatment played a dominant role in reducing the concentration and bioavailability of DOP and DON. PMD-based network analysis indicated that the variation in the bioavailability of DOP and DON in the biological treatment was primarily associated with the transformation from low unsaturation lipid- and protein-like species to highly unsaturated lignin-like species. Our findings enhance the understanding of the bioavailability and transformation of DOP and DON in WWTPs, thereby offering guidance for optimizing biological nutrient removal and designing targeted advanced treatment strategies during WWTP upgrading. Synopsis: This work simultaneously investigated the bioavailability and transformation of DOP and DON in a WWTP at the molecular level for the first time. • DOP and DON are primarily removed during the biological treatment. • FT-ICR-MS simultaneously revealed DOP and DON molecular characteristics in a WWTP. • Biological treatment effectively reduces the bioavailability of DOP and DON. • The mineralization of DOP and DON is closely linked to lignin-like species formation.