Hydrothermal treatment of septic sludge: Revealing temperature-sensitive dissolved organic matter and potential toxicity relationships in the hydrothermal liquid

Abstract

Hydrothermal treatment of septic sludge can result in the transfer of significant amounts of dissolved organic matter (DOM) into the hydrothermal liquid (HL). However, there is a lack of research exploring the relationship between temperature-sensitive fractions of DOM in HL and ecological risks. In this study, spectroscopic techniques combining two-dimensional correlation spectroscopy (2D-COS), self-organizing maps (SOM) and structural equation modeling (SEM), respectively, were employed to investigate temperature-sensitive DOM and its potential correlation with phytotoxicity at five process temperatures (180–340 °C). The findings revealed that DOM content peaked at 260 °C, measuring 7625 mg·CL−1. At peak levels, the concentrations of chemical oxygen demand, ammonium nitrogen, total nitrogen, and total phosphorus in the HL reached 16900 mg L−1, 34.8 mg L−1, 1920 mg L−1 and 756 mg L−1, respectively. Results from EEM-PARAFAC-SOM indicated that temperature significantly influences the variations in fluorescent components within DOM. Additionally, 2D-COS analysis identified conjugated structures and critical turning points at 220 °C and 300 °C. Notably, the -CO-NH- functional group, which is closely associated with aromatic protein II, exhibited the highest sensitivity to temperature changes. Wheat seed germination experiments revealed that the DOM sample at 180 °C exhibited the most pronounced inhibition of wheat root length, while demonstrating the least effect on germination. In contrast, seed growth was most severely impaired at 340 °C. SEM analysis revealed the influence of temperature—both direct and indirect—on the properties of DOM, identifying aromatic protein I as the primary determinant limiting seed germination. This research provides valuable insights for the management and utilization of HL.