Display omitted •Direct liquefaction reaction of waste organic matter using Raney Nickel and tetralin.•Biological pretreatment enhances Humin content and the liquefaction process.•Humin fraction as promoter and Humic acids as refractory to liquefaction reaction.•Bio oil heating value close to biopetroleum. The bio-oil synthesis from a mixture of wastes (7wt.% straw, 38wt.% wood, and 45wt.% grass) was carried out by direct liquefaction reaction using Raney Nickel as catalyst and tetralin as solvent. The green wastes were biologically degraded during 3months. Longer the destructuration time; higher the yield into oil is. Biological pretreatment of green wastes promotes the liquefaction process. Among the components of degraded biomass, Humin, the major fraction (60–80wt.%) that was favored by the biological treatment, yields to a bio oil extremely energetic with a HHV close to biopetroleum (40MJkg−1), contrariwise, Fulvic acids (2–12wt.%), the minor fraction is refractory to liquefaction reaction.