Transformation characteristics of polycyclic aromatic hydrocarbons during hydrothermal liquefaction of sewage sludge

Abstract Transformation characteristics of polycyclic aromatic hydrocarbons (PAHs) during hydrothermal liquefaction (HTL) of sewage sludge are explored in this study. PAHs are generated in the range of 7.3–13.8 mg/kg during HTL of sludge. The increase of reaction time (0–60 min) favors the formation of PAHs; however, the reaction temperature and solid–liquid ratio exhibit a critical value (300 °C and 0.1 g/mL, respectively), at which, the amounts of PAHs synthesized are the lowest. Most PAHs (> 89%) are distributed in bio-oil. Compared with those of raw sludge, the total/leachable contents and toxic equivalent quantity (TEQ) of PAHs in biochar are all reduced, suggesting a lower environmental risk for biochar in view of PAH hazard. Notably, the total contents of PAHs in bio-oil reach 55.0–106.6 mg/kg, with high TEQ values of 6.1–8.6 mg/kg. The liquefaction process should be further improved to reduce the content and toxicity of PAHs in bio-oil.