Chemodiversity transformation of organic matters in a full scale MBR-NF wastewater reclamation plant

Membrane bioreactor (MBR) and nanofiltration (NF) process has been attractive in wastewater reclamation, and was set as the target process in this study. Dissolved organic matter (DOM) and trace organic contaminants (TrOCs), closely associated with water safety, are noteworthy pollutants. Though the general DOM characteristics and TrOCs removal in MBR-NF reclamation process have been reported in lab-/pilot-scale experiment, the molecular characteristics of DOM revealed by high resolution mass spectrometry, and the correlation between DOM and TrOCs have been rarely studied in full-scale MBR-NF wastewater reclamation plant. In this work, biological and NF processes contributed significantly to the removal of DOM and TrOCs, while MBR filtration contributed slightly. Spectroscopic analyses revealed that DOM with higher aromaticity and lower molecular weight were more recalcitrant along the treatment. Aromatic protein-like substances were preferentially removed comparing to humic-like substances. Fourier transform ion cyclotron resonance mass spectrometry was applied to investigate DOM transformation at molecular level. DOM molecules with higher H/C and lower O/C, especially the aliphatics and peptides, were readily biodegraded into higher‑oxygenate, highly unsaturated, and aromatic compounds. The generated species mainly included condensed aromatics, polyphenols, and highly unsaturated compounds. Filtration in MBR tended to reject higher oxygenated molecules. NF effectively removed most of the DOM molecules, especially higher oxygenated molecules with low H, N and S. The residual TrOCs in the NF effluent, including sulfamethoxazole, ofloxacin, and bisphenol A, still displayed above medium environmental risk. Significant correlations were found among organic compounds, spectral indices, and peptides molecules. Positive correlation between most of the TrOCs and several DOM parameters implied that they were synchronously removed in biological and membrane filtration processes. SUVA and FI might be potential indexes in monitoring the performance of MBR-NF process in both DOM and TrOC removal. These findings would expand the understanding of DOM and TrOCs behavior in wastewater reclamation process and simplify an in-depth system monitoring.