Background The concrete industry uses large amounts of fly ash from coal-fired power plants as a partial substitute for cement. Using ash reduces the need for its disposal in landfills. Furthermore, it reduces the consumption of fossil fuels due to partial substitution of of cement in concrete production. This saves natural resources and reduces CO2 emissions, in line with the EU’s Sixth Environmental Action Programme and the Environmental Technologies Action Plan. Bio ash, which is currently disposed of in landfills, can also be used for concrete production, with the same environmental benefits as fly ash. However, full-scale bio-ash concrete production remains a very small segment of the industry. A lack of documentation of environmental impacts and the technical properties of resulting concrete products, and the need for structured dissemination of existing know-how, are some of the factors impeding its more widespread use. Objectives The project aimed to prove the technological, economic and environmental viability of the use of ash from wastewater sludge incineration for the production of concrete. A number of demonstration projects were planned, and results and expertise were planned be collected in a handbook with guidelines for the use and quality control of bio ash in concrete. It was expected that by the end of the project at least 50% (5 000 tonnes) of the Copenhagen area’s total annual generation of bio ash would be used for concrete production. Results The project has reached its objectives and found an innovative, contemporary solution to a large and increasing trans-European waste problem. The solution involved an upgrading of the bio ash from a waste material that is currently landfill deposited to a raw material for concrete production. The use of bio ash for concrete production follows the successful use of fly ash from coal-fired power plants. As a result, the project focused on the technical characterisation of bio ash compared with the fly ash. During the LIFE project period 1 900 t of bio ash was reused in concrete. This was less than expected due to the finding that ash from fluidised bed furnace was directly applicable whereas ash from multiple hearth furnace needed milling prior to the use in concrete. The project results re-confirmed that a maximum 50% of the fly ash can be replaced by bio ash, without a reduction in the workability and strength of concrete. At the same time, it was concluded that for visible structures the maximum use of bio ash would need to be limited due to its reddish colour, which could be problematic if concrete without bio ash was used in the same part. Studies showed that the maximum amount of bio ash, which can be used in order to avoid the reddish colour of the concrete, is estimated to be 5-10 kg per m3 of concrete when using iron precipitated bio ash and approximately 40 kg/m3 when using aluminium precipitated bio ash, as the colour is significantly lighter, when using the light aluminium bio ash instead of the red (iron precipitated) bio ash. Technically the light bio ash is at least just as good as the red bio ash. However the use of aluminium precipitated bio ash is not always economically feasible as the aluminium is normally more expensive than iron. The project has proved that bio ash quality fulfils the European standard for coal fly ash (EN-450-1) and can be used as a partial substitute of fly ash in concrete mix designs. Bio ash could, however, not reduce the amount of cement. An example of a “Product Standard” is integrated into the guideline for use of bio ash concrete. The heavy metals leaching tests showed that the use of bio ash for concrete production has very limited environmental impact and there is not significant difference in this respect between concrete with or without bio ash. Comparison of bio ash concrete and reference concrete (with fly ash only) specimens has shown that different concrete mixes had overall similar properties regarding durability and strength development – the only major difference is bio ash concrete has a higher setting time. This demonstration project represents a significant step forward for the dissemination of know-how on bio ash use in the production of concrete. The project published a guideline/handbook in English and Danish on implementation and use of bio ash in concrete. Also a comprehensive website containing all the project results was launched.