This work explores the hydrothermal upgrading (HTU) of sewage-sludge-derived pyrolysis bio-oil as a mild upgrading strategy aiming at reducing its oxygen and nitrogen content and improving its stability and compatibility with downstream processing in refineries. The use of zerovalent metals (ZVMs), such as Fe and Zn, as heterogeneous in-situ hydrogen producers together with a commercial sulfided Co/Mo hydrotreating catalyst, was investigated in HTU. The tests were conducted in a 10 mL volume reactor at 330 °C and with a reaction time of 3 h. Raw and upgraded oils were analyzed via thermogravimetric analysis, elemental analysis, GC-MS analysis and calorimetric analysis. The upgraded bio-oils exhibited relative reductions of O/C and N/C molar ratios up to 40% and 55%, respectively, while the combined effect of ZVMs and catalyst led to a maximum increase in H content of 29.4% in the upgraded oil using Zn and catalyst, with respect to the original bio-oil. After upgrading, the biocrude recovery ranged between 61.6% and 67.5%, due mainly to the migration of organic compounds into the aqueous phase. Nevertheless, the higher heating value of the oil increased from 33.4 MJ/kg up to 37.9 MJ/kg, resulting in an overall energy recovery between 69% and 76% for all the upgrading tests. The upgraded oil also showed enhanced concentration of aliphatic hydrocarbons and lower concentration of fatty acids, respect to raw bio-oil, associated with an increased bio-oil stability and compatibility with petrol-derived fractions for subsequent coprocessing strategies. © 2026 American Chemical Society

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