Background Wastewater treatment plants (WWTP) often rely on carbon-based fuels to power the aeration systems involved in purifying the water. Opportunities have been identified to improve the effectiveness and efficiency of both the water purification and WWTP power supply processes. Objectives The GREENLYSIS project aimed to build a pilot WWTP plant to design and demonstrate a new technology for separating water into hydrogen and oxygen using electrolysis. The full-scale pilot plant incorporated wastewater pre-treatment; water purification; an electrolysis unit; oxygen storage; hydrogen storage; a pilot biological reactor fed with oxygen from the electrolysis stage; a photovoltaic, wind and thermal solar energy system, and an energy management system. The aim was to use oxygen released from the water during electrolysis in the purification of the waste stream, and the hydrogen produced by the electrolysis to power the treatment plant. Key goals involved reducing WWTP energy inputs and identifying a viable alternative to carbon-fuelled WWTP systems. Results The GREENLYSIS project gathered different innovative technologies and combined them to construct a prototype plant, able to produce hydrogen and oxygen using wastewater coming from an urban wastewater treatment plant (WWTP). The core process was the electrolysis and the entire plant was fed with renewable energy. The data obtained demonstrated the technical viability of the entire pilot plant to produce hydrogen and oxygen using only renewable energy. GREENLYSIS designed, constructed and operated its pilot plant in the wastewater treatment plant (WWTP) of Montornès del Vallès (Barcelona, Spain). The plant comprised a water pre-treatment system, an electrolysis step, and a biological reactor to use the oxygen produced. The system was powered entirely by a renewable energy system (solar and wind), an energy storage unit comprising several batteries, and an energy management system that enabled the energy generated to adapt to energy demand. The project was technically challenging and innovative because it involved the combination of several novel technologies, coupled to a system of energy production totally independent of the national grid. The project team showed that WWTP effluent could be used for the electrolysis needed in a first pre-treatment to remove solids and turbidity in wastewater. Oxygen was produced (with purity higher than 95%) from the WWTP effluent electrolysis, and tests showed that potential energy savings could be obtained from the use of the oxygen in an industrial WWTP (for secondary wastewater treatment). Hydrogen was also produced (purity higher than 99%) and has been used in a combustion engine to run a vehicle (motorbike). The project designed and implemented a renewable energy system (photovoltaic and thermal solar panels, and a wind turbine) for off-grid energy production, which largely met the energy requirements of the plant. Recent technologies of known efficiency were tested and adjusted to the project’s requirements. These included membrane distillation of WWTP effluent using thermal solar energy to produce deionised water, and a refrigeration system for photovoltaic solar panels. The project developed its own automatic energy manager system to optimise the management of energy generation with respect to demand, and for battery storage and discharge. The operation of the pilot plant helped identify the main technical and financial constraints that could limit the application of the technology in the short-term, and helped define some solutions. Guidelines for the future implementation and reproduction of the project system at industrial scale were developed, and recommendations for improvements made, which are available to anyone interested in the project. Environmental benefits derive mainly from the production of hydrogen from wastewater and renewable energy. More energy is produced than used in the process (as the hydrogen and oxygen generated can be used in the WWTP). The greenhouse gas (GHG) emissions of the whole system are very low, with the carbon footprint estimated to be only 35kg CO2eq/year. The wider environmental benefits will only become obvious if the technology is applied on a large scale, which is unlikely in the near future. The overall system was found to be technically feasible, but not economically viable at this point. However, parts of the project system are likely to be transferable in the short term, especially the renewable energy system and the energy manager system. For example, an isolated renewable electric grid based on that of the project is under construction in Aguas de Murcia (Spain) to power a pump installed in a well, while the beneficiary SAFT has used knowledge obtained during the project to develop an improved battery charger system. The project generated technical and management tools that will help in the future implementation of EU legislation regarding wastewater treatment. The knowledge and experience gained will also be valuable for other uses and applications. Project beneficiary CETaqua, for example, gained considerable know-how on the purification of wastewater to generate hydrogen for energy, via electrolysis using renewable energy. The outputs of the project are of notable value at EU level in the eco-innovation fields of water treatment and membrane technology. Further information on the project can be found in the project's layman report and After-LIFE Communication Plan (see "Read more" section).