Optimization of ultrasound-assisted liquefaction of solid digestate to produce bio-oil: Energy study and characterization

•Ultrasound (US)-assisted liquefaction of solid digestate from industrial waste is optimized.•US-assisted liquefaction increases high calorific value of residues from biogas plant.•US-assisted direct liquefaction is an energy saving process to provide bio-oil.•Bio-oil production as precursor of fuel and polymers through biorefinery is proposed.