Background The increasing amount of waste generated by the olive oil sector presents an environmental problem. In particular, inadequate management and uncontrolled disposal have affected water bodies and nearby communities, raising health concerns. Landfill has been the usual disposal method for solid and liquid wastes from the processing of waste vegetable oils into biodiesel fuel, but these contain a high percentage of organic compounds. Therefore, current methods for landfill disposal do not comply with the European Landfill Directive (1999/31/EC). However, the residues from the pre-treatment process, as well as the by-products of the refining process, can be used either as substrate for biogas production or burned in thermo-electrical turbines, rather than being sent to landfill. Objectives The objective of the LIFE project VALUVOIL was to demonstrate an environmentally- and economically-feasible method of improving the anaerobic digestion of residues and by-products generated from the processing of used vegetable oil. The project adopted a life-cycle approach, taking into account energy savings, transportation expenses, carbon dioxide emissions, water consumption, and all the other known environmental and economic parameters. Anaerobic digestion can transform residues from the pre-treatment process, and by-products from the refining process, into biogas for direct application in thermoelectric energy generation to power the refining facility. For added value, and to complete the zero-waste cycle of the whole process, the project also aimed to valorise organic residue into compounds for application as biostimulants and biofertilisers. Results The VALUVOIL project achieved its main objectives, by demonstrating the viability of the improved anaerobic digestion process for biofuel production and the potential uses of the organic by-products for stimulating plant growth and as compost for improving soils. Early in the project, the chemical and physical properties of the residues and the by-products generated at the pre-treatment and refining stages of waste vegetable oil processing were characterized and evaluated in terms of the anaerobic digestion process. Optimal conditions for the production of biogas were determined for two different approaches using continuously-stirred tank reactors: one-phase (5 L reactor) or two-phase (1 L reactor for hydrolysis and acidification connected to a 5 L reactor for methanogenesis). Different scaling-up assays were performed using both approaches in the laboratory, with different substrates. The two-phase reactor was found to be a feasible, environmentally-friendly and sustainable treatment system for vegetable oil waste, with pig manure as a co-substrate. This alternative system produced less organic sludge, and is comparable with the conventional physiochemical treatment system that produces large quantities of hazardous sludge. The digestates generated in the anaerobic digestion process, at laboratory level and in a prototype system, were tested in germination and microcosm experiments to determine their biofertilizer and biostimulant effects on plants and soils. Gasification assays were also carried out on digestates in the laboratory and in a pilot plant, to optimize working conditions and combinations with other raw materials. A versatile, technically viable and economically feasible prototype of 1 100 L was designed and constructed. It was installed in a mobile container for in-situ demonstrations. After optimizing the working conditions, two different experiments at industrial level were carried out. In these, biogas production was obtained at levels anticipated from laboratory studies, with 65% methane content; organic material was eliminated from waste sludge, as shown by reduced chemical oxygen demand and total dissolved solids; and materials with biofertilizer and biostimulant activity were obtained with potential applications in agriculture. There is a large potential market for the implementation of this innovative technology, with around a billion tons of waste vegetable oil generated each year in Europe. The VALUVOIL project system offers several advantages, through biofuel production, bio-products of use to the farming sector, and dramatic reductions in hazardous sludge requiring further treatment and safe disposal in landfill sites. It also reduces the costs associated to the treatment of oil contaminated water (2,4€/l at sewage treatment plants) and energy consumption (saving around 484 kWe/year).