Hydrothermal Carbonization of Sewage Sludge under Supercritical Pressure Conditions for the Upgrade of Process Water

Hydrothermal carbonization (HTC) experiments were carried out with sewage sludge under very high-pressure conditions. The feedstock was treated at 200, 220 and 240 °C, while pressure was maintained at 250 bar. The solid product was an energy-rich hydrochar, with higher heating values (HHV) from 23.6 to 30.8 MJ/kg. The process water showed a high organic load, with Chemical Oxygen Demand (COD) concentrations of 9,423.2-15,307 mg/L. Major volatile fatty acids (VFAs) were also present at high concentrations, with formic acid ranging from 429 to 439.1 mg/L. The fatty acid methyl ester (FAME) measurements were particularly noteworthy, showing substantial production despite the relatively low HTC temperature window. Methyl stearate exceeded 1400 mg/L, methyl oleate reached more than 1354 mg/L, and methyl linoleate exceeded 3000 mg/L, indicating that high pressure promoted the formation or stabilization of lipid-derived compounds in the aqueous product.