Molecular transformation and bioavailability of dissolved organic nitrogen in sewage sludge under hydrothermal treatment

Hydrothermal (HT) treatment has been considered effectively to recover nitrogen-containing organic nutrients from wastewater activated sludge (WAS), and understanding the fate and distribution of dissolved organic nitrogen (DON) during HT process is of great importance to process optimization. Thus, this study aims to investigate molecular transformation pathway of DON during HT process using non-targeted mass difference network analysis and evaluate the bioavailability of DON with chlorella. Results suggested that HT treatment was superior in N-containing organic matter solubilization (DON /TN > 60 %), and DON in WAS underwent extensive transformation processes at various HT temperatures. From room temperature to 70 °C, nucleic acids were solubilized to HT liquor, accomplishing with deamidation (–CONH + H2/H2O). And a representative ribose nucleic acid with a 'PA–pentose (ribose)–uracil (U)' structure was identified. With temperature rising to 120 °C, large amounts of stable protein were solubilized and hydrolyzed into HT liquor. The non-targeted mass difference network analysis suggested that, from 160 °C to 200 °C, aspartyl, serine, histidine, and glutamyl units were representative precursors for a variety of reactions including Maillard reactions, deamidation, deamination and so on. Amino acids were deaminated (–NH) to produce organic acids and ammonia (ammonium) or decarboxylated (–COO) to produce amines and CO2. Dicarbonyl compounds (–C2H2O2) were identified as the most abundant fragments which could either form humic substances or Maillard reaction products. Chlorella growth bioassay indicated that org-N in HT liquor produced at 120 °C showed the highest bioavailability for Chlorella, while that generated in HT liquor above 160 °C inhibited the growth of Chlorella. This study provided molecular insights for DON transformation following HT process and promoted high-quality resource utilization of WAS.