From wasted sludge to valuable biochar by low temperature hydrothermal carbonization treatment: Insight into the surface characteristics

Abstract

In this study, the hydrothermal carbonization (HTC) technology at low temperature was applied to convert the wasted sludge into biochar, by establishing the correlation between the operation conditions and biochar characteristics. The contents of C, H, O, N and S, and the values of (O + N)/C and O/C in biochar were negatively correlated to the temperature, retention time and initial pH conditions. This implies that increasing temperature and retention time are beneficial for sludge dewatering and reduction. The maximum high heating value of biochar at 15.21 MJ/kg was achieved, which is similar to the cellulosic biochar and lignite surface known as the primary renewable fuel. The typical macromolecules such as proteins and lipids still existed on the biochar surface after low temperature HTC treatment. Decreasing the initial pH enforced the protonation effect and structural stability of the organic macromolecules in sludge, resulting in a diverse morphology with more compact microspheres on the biochar surface. Particularly, the catalytic ability to generate the carbon-centered persistent radicals was enhanced with g-factor at 2.01138–1.98428. In terms of environmental safety, the detected concentrations of heavy metals (Zn, Pb, Cu and Al) during the secondary dissolution of biochar were below 0.25 mg/L, indicating the low leaching risk for the land application. In addition, some humic acid and fulvic acid analogues were also detected in the secondary dissolution liquid, which can facilitate the redox reaction in soil microenvironment. In sum, this study confirmed that the low temperature HTC is a feasible method for sludge treatment and resource recovery.