This paper evaluates the potential of mesophilic anaerobic digestion (AD) in the treatment of used oils and landfill leachates through co-digestion of the sludge produced in a wastewater treatment plant (WWTP). Biomethane potential (BMP) tests were performed on two different co-substrates and on WWTP sludge. The biogas production per kg of fresh matter of the sludge generated in a WWTP was only 6.1 NI/kg due to its low volatile solids concentration. Biogas production of landfill leachates per kg of fresh matter was found to be low (12.4 NI/kg). The biogas production per kg of fresh matter for the used oil over a period of 47 days (not completely degraded) was 970.6 NI/kg. Used oil was selected as co-substrate according to these results. Anaerobic co-digestion of sludge and used oil was conducted in a pilot plant fed in semi-continuous regime in the mesophilic range (38 degrees C) to obtain their adequate mixture in order to ensure the correct function of the process. The optimum oil percentage in the mixture was 19% (volatile solids basis). Once the appropriate mixture was obtained the co-digestion process was carried out in the digester of the Pobla de Farnals WWTP. The co-digestion process improved the performance of volatile matter removal (showing an 11% increase over the period without co-digestion) as well as the biogas production (a 23.5% increase over the period without co-digestion) without disturbing the anaerobic digestion process. The total production of biogas per day in the WWTP increased over the period without co-digestion. (C) 2013 Elsevier Ltd. All rights reserved.