Liquefaction bio-oil from sewage sludge using different organic solvent (methanol, ethanol or acetone) at temperature of 260–380 °C was characterized in terms of its elemental and chemical composition, and was further upgraded by microemulsion technology. The bio-oil obtained had high carbon content of 67.60–73.58% and was mainly composed of esters when methanol or ethanol was used as liquefaction solvent, or esters, ketones, alcohols and phenols when acetone was used, meaning its great potential to be used as fuel. Diesel extraction procedure found that ethyl esters were the least diesel-solvable compounds in diesel among the main components of the liquefaction bio-oil, while methyl esters were extracted completely. The bio-oil obtained with methanol as solvent, compared to acetone or ethanol, and the bio-oil obtained at higher liquefaction temperature was easier to be upgraded by water/diesel microemulsion in terms of Span 80 consumption. In addition, the stability mechanism of the component of liquefaction bio-oil in the water/diesel/bio-oil microemulsion was proposed. The diesel-insolvable fractions, e.g. part of ethyl esters, were probably solubilized in the water phase or inserted between surfactants; while the diesel-solvable fractions, e.g. methyl esters, part of ethyl esters, ketones, alcohols and phenols, were probably dissolved in the diesel phase.
