Impact of hydrothermal carbonization on heavy metal speciation and risk in waste-activated sludge

Wastewater treatment plants are increasingly transitioning toward resource recovery, with sewage sludge considered a potential feedstock for soil amendments. However, concerns regarding heavy metal content, bioavailability, and long-term environmental risk remain key barriers. This study evaluates hydrochars derived from waste-activated sludge (WAS) via hydrothermal carbonization (HTC), focusing on heavy metal speciation, ecological risk, carbon stability, and regulatory compliance. HTC was performed at three severity levels: low (180 °C/30 min), medium (210 °C/60 min), and high (260 °C, 120 min). Hydrochars were characterized for total metal content, Community Bureau of Reference sequential extraction fractions, ecological risk indices, risk index, and risk assessment code. Results show that metals progressively shifted from labile fractions in WAS to more stable fractions in hydrochar, reducing ecological risks despite increased total metal concentrations. Although cadmium, copper, chromium, and Nickel, to a lesser extent, exceed Swedish or European thresholds, speciation indicated lower bioavailability in hydrochars. Carbon stability data confirms increasing recalcitrance with process severity, supporting long-term metal immobilization. The hydrochars also contain humic-like substances, retain phosphorus, and C/N ratios comparable to mature compost. Overall, medium to high severity hydrochars show potential as low-risk soil amendments, and speciation-based assessments provide a more realistic evaluation of environmental risk than total metal concentrations.