Deep eutectic solvent-assisted hydrothermal carbonization of sewage sludge: Process optimization using response surface methodology and physicochemical characterization of the hydrochar

Abstract

Integrating resource recovery with a waste management approach has been considered a promising strategy for advancing sustainability. Sewage sludge (SS), an unavoidable byproduct of wastewater treatment, has attracted research interest due to its significant potential for resource recovery. This study presents a sustainable strategy for valorising SS through a deep eutectic solvent (DES) catalyzed hydrothermal carbonization (HTC) process to obtain carbon-rich material. Here, the residue sludge (RS) obtained after removing extracellular polymeric substances (EPS) from SS is used as a precursor for the HTC process to produce RS-derived hydrochar (RSHC) as a potential biobased carbonaceous material. The DES was prepared with arginine and butyric acid as hydrogen-bond acceptor and hydrogen-bond donor, respectively. Additionally, the DES-assisted HTC process was optimized for improved carbon content in RSHC using response surface methodology, with variables including temperature (160-280 °C), time (45-150 min), and DES concentration (10-30 % v/v). The total carbon content of the RSHC significantly increased by 21.9 % at the optimum HTC condition of ∼ (206 °C, 91 min, and 21% v/v). The higher heating value of the RSHC also increased from 11.6 ± 0.35 to 13.13 ± 0.51 MJ/kg, indicating improved RSHC quality. The RSHC were characterized for structural, morphological, and thermal properties. The reaction mechanism, thermo-kinetics, and carbon and material balance were investigated to elucidate the physicochemical transformations of RS components during DES-assisted HTC. Overall, this study will provide a promising direction for the sustainable valorisation of RS into carbon-rich material for future applications, aligning with the circular economy concept.