Background Selective collection of Urban Solid Waste (USW) is an established practice in several countries; in which organic matter, glass, plastics and paper are managed as separate residues with their own channels of recovery and treatment. Yet, there is still a substantial amount of waste - the ‘rest fraction’ - that is not sorted at source and that needs to be managed. A significant portion of this solid rest fraction goes to landfill or for incineration, although it contains a high percentage of organic matter. However, waste treatment facilities exist that use different types of mechanical and biological processes to recover or reuse as much organic residue as possible before disposing to landfill or incineration. Spain, and especially Catalonia, has developed eco-parks for this purpose. The efficiency values of these facilities are good, but there is still room for improvement. For instance, urban wastewater is typically dealt with as a totally separate waste stream and treated in different treatment plants. Objectives The overall objective of the URWASTECH project was to demonstrate a more efficient and sustainable treatment of the rest fraction from sorted Urban Solid Waste (USW). It aimed to integrate the treatment of this fraction with wastewater management, to create a highly innovative pilot treatment plant for the processing of urban waste. This used a hydro-mechanical separation technique and a biological treatment processes, including anaerobic digestion, membrane biological treatment and wetland technologies. Using wastewater in the proposed USW treatment plant avoids the need for clean water consumption. The project also aimed to improve the efficiency of the hydro-mechanical separation, so as to reduce the percentage of undesirable non-organic materials entering the anaerobic digester. Results The URWASTECH project successfully designed, constructed and tested a plant that is able to treat from the USW rest fraction with waste water from leachate treatment and recover energy in the form of biogas. This prototype was designed and built in a waste treatment plant managed by SECOMSA GESTIÓ, located in Botarell, Baix Camp (Tarragona), Spain. It was able to treat 500 l/day of waste and consisted of two main units: a hydro-mechanical unit, comprising a mixing tank and a flotation-decantation tank, and a biological unit, including an anaerobic digester and a wetland system in the surrounding eco-park. The economic and environmental assessment of the project concluded that the technology is more environmentally friendly and economically feasible than the current treatment procedures, with a reasonable overall cost-benefit balance due to the generation of energy from the biogas produced. During the construction of the pilot plant, the hydro-mechanical unit presented the most problems and it could be further optimised. Some of the suggestions for this are to use different geometries of the flotation/decantation tanks or to shred the urban waste before entering the tank. Also, the cleaner the waste is, the easier the process becomes. The project team expects that a plant built at industrial scale would be able to treat 125 m3 of waste per year. According to this, it would avoid 44 kg of CO2 eq. per day: 16 tonnes of CO2 eq. per year. The Global warming potential of the process has therefore been significantly reduced by the URWASTECH intervention. The use of clean water was decreased, compared to current procedures, through the use of wastewater (leachate) which as a waste needs to undergo costly treatment. In addition, the valorisation of the alternative energy, in the form of biogas, and the valuable residues produced, also reduced to a high degree the environmental impact of the pilot plant. The improvement in the recovery of USW could bring savings on treatment and disposal costs when scaled up, a benefit made possible by the return of recyclable materials into the market. The innovation aspect of the project resides in the procedure rather than the technology itself, and the challenge of treating a waste in which all the materials that are usually recycled have been removed (the ‘rest fraction’ of urban waste). The approach URWASTECH contributes to compliance with the Waste Directive by making use of wastewater, which is usually considered as a separate waste stream. As the process can be done with wastewater, the procedure would also contribute to improving the quality of the water discharged and therefore help to better implement the Water Framework Directive. Specifically, the URWASTECH approach is a tool to achieve the target set up in the 1999/31/CE, which establishes a reduction of biodegradable urban waste from 75% in the year 2006 to 35% in the year 2016. Further information on the project can be found in the project's layman report and After-LIFE Communication Plan (see "Read more" section).