Bio-crude production and inorganics elements distribution during the hydrothermal liquefaction of tannery sludge

Tannery sludge (TS) is the major waste in leather industry. The wastewater treatment of tanning plants produces large amounts of sludge contaminated with chromium, which disposal is a hard issue to deal. In this context, hydrothermal liquefaction (HTL) is an innovative thermochemical process to convert the this contaminated biomass into bio-fuels. In this work, a tannery sludge was chosen as substrate to perform the HTL process. The HTL tests were carried out in a 500 mL batch reactor for different reaction times (0–10 min) and temperatures (250°C, 275°C, 300°C and 350°C). Results proved that under the best operating conditions (350ºC and 10 min) it is possible to obtain a bio-crude yield (on dry and ash-free basis) of 29.5% with an associated energy recovery of 44.1% (higher heating value of 36.1 MJ/kg). ICP-MS analysis of the three HTL products (bio-crude, aqueous phase and solid residue), showed that the inorganic elements are mainly concentrated in the solid residue. Finally, UV-vis analysis of Cr(VI) showed that it is present in reaction products in its trivalent form, thus demonstrated that HTL performed in this study avoids the undesired oxidation of Cr(III) to Cr(VI).