Effects on the Physicochemical Properties of Hydrochar Originating from Deep Eutectic Solvent (Urea and ZnCl₂)-Assisted Hydrothermal Carbonization of Sewage Sludge

Abstract

Deep eutectic solvents (DESs) (ZnCl 2 and urea) have been used to solubilize organic matter from sewage sludge (SS), followed by subsequent hydrothermal carbonization (HTC) to obtain low-nitrogen-content hydrochar. The nitrogen content in hydrochar obtained after DES addition decreased to 1.93 from 3.15% (no DES) at 210 C. DES can notably dissolve proteins and lipids during HTC of SS. HTC of polysaccharides was enhanced, increasing the degree of carbonization. The key role of DES in SS during HTC was the dissolution of proteins, promoting carbonization of polysaccharides, Maillard reactions, deamination, and decarboxylation of proteins. ZnCl 2 was probably converted into -Zn(OH)C1 and ZnO during HTC. Results pointed to relevant enhancements when DES was added, useful for organic waste valorization such as SS, food waste, poultry manure, and related waste feedstock.