Co-digestion of used oils and urban landfill leachates with sewage sludge and the effect on the biogas production

► Used oil can be a suitable co-substrate for anaerobic digestion processes. ► Used oil as co-substrate doubles the biogas production at laboratory scale. ► The co-digestion process improved the performance of volatile matter removal. ► Total production of biogas per day in the WWTP increased over the period without co-digestion. ► The biogas produced during co-digestion is as efficient as that without co-digestion.