Background Limited reserves of natural gas and oil are causing prices to rise almost continuously and the link between these fuels and climate change is now largely accepted. Nevertheless, they still account for most of the fuel for heating in the EU. Alternatives that are ecologically and economically feasible are vital. An innovative and potential alternative source of green energy is wastewater in the sewage system. The heat produced by wastewater can be harnessed and transferred to buildings by placing heat exchangers directly inside the sewer. The city of Arnhem is an Urban Heat Island (UHI); it has a metropolitan area which is currently about 7°C warmer than its surrounding rural areas. This effect is largely caused by waste heat generated by energy usage and the use of materials in urban development which retain heat. Arnhem has the stated ambition to become CO2 neutral. Objectives The main objective of the project is to demonstrate a new SEWage Energy EXchange system (SEWEEX) to transfer heat from the sewage system to urban buildings. It expects to show important environmental and financial benefits compared to other energy sources. The SEWEEX technology will be used to exchange energy between wastewater and a minimum of 50 000 m2 of residential space (around 500 households), 38,000 m2 of offices and shops, and 33,000 m2 of public buildings. The technology will harness around 5 000 MWh of green energy per year, which will be used to heat the buildings during winter. During warm summers, the system will channel heat away from the urban area, reducing the UHI effect by 10%. SEWEEX aims to reduce CO2 emissions, as well as fine dust or nitrogen oxides (NOx) emissions, by 78% compared to the use of fossil fuels. The remaining emissions will come from the energy required for the heat exchangers themselves. Significantly though, this can be generated outside the city, delivering zero emissions and improved air quality in the urban centre. SEWEEX also expects to demonstrate that the energy potential from sewage wastewater is greater than from solar, wood and biomass combined and that it is not subject to the same variations in strength that can affect wind and solar power. Expected results: Demonstration of the SEWEEX system for 120 000 m2 of building space; Reduction of CO2, fine dust and NOx emissions by 78% compared to fossil fuels; A 10% reduction in the UHI effect; Superior performance to other green energy sources.