The Hydrochar Characters of Municipal Sewage Sludge Under Different Hydrothermal Temperatures and Durations

TLDR

Municipal sewage sludge volume requires innovative management, and hydrothermal carbonation offers potential advantages for producing hydrochar as a soil conditioner. The study aims to investigate hydrochar properties from municipal sewage sludge via HTC at 190 °C and 260 °C for 1–24 h, and to assess the future value and risk of applying the charred sludge to soil. HTC was performed on municipal sewage sludge at 190–260 °C for 1–24 h to produce hydrochars. HTC of sewage sludge at 190–260 °C for 1–24 h reduced N, O, and H contents by >54.6 %, 37.9 %, and 10.0 % respectively, slightly altered C, converted fatty acids to alkenes, olefins and aromatics, increased aromaticity and hydrophobicity, expanded pore structure (up to 17.30 m² g⁻¹ surface area and 0.83 cm³ g⁻¹ volume), lowered N, P, K availability, and suppressed heavy‑metal activity despite their accumulation.

Abstract

Innovative measure is a urgent requirement for managing the huge volume of municipal sewage sludge. The hydrothermal carbonation (HTC) shows some potential advantages for using hydrochar as a soil conditioner. The aim of this work was to investigate the properties of hydrochars, by means of the HTC of municipal sewage sludge under different temperatures (190 and 260°C) and reaction hours (1, 6, 12, 18 and 24 h). The HTC led to the decreases of N, O and H contents by more than 54.6, 37.9 and 10.0%, respectively, and slight changes of C content. The Py-GC-MS analysis showed that a large proportion of fatty acids, in particular hexadecanoic acid, transferred into alkenes, olefins and aromatic compounds. The 13C-NMR and fourier transform infrared spectra (FTIR) confirmed the transformation and changes in chemical structure in which hydrochar contained lower oxygen-containing organic C of O-alkyl, carboxylic and carbonyl C and aliphaticity, but higher aromatic C and aromaticity. The rich hydrophobic functions induced in high positive charges in the charred sludge. The HTC facilitated the pore structure development, proved by higher specific surface area and specific pore volume, with a maximum of 17.30 and 0.83 cm3 g−1, respectively. The availabilities of N, P and K markedly reduced during HTC treatment. The activities of most heavy metals were depressed though they accumulated in the hydrochar. Further work is required to investigate the values and risk of the charred sludge amended to soil.