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CONCRETE TECHNOLOGY20 CPI %u2013 Concrete Plant International %u2013 4 | 2025 www.cpi-worldwide.comIn most EU countries, the effective disposal of sewage sludge from wastewater treatment plants remains a persistent challenge and a significant cost burden, partly due to incomplete regulatory frameworks. Currently, approximately 27% of sewage sludge in the EU undergoes thermal treatment %u2013 a figure that is steadily increasing. The principal residue of this process is sewage sludge ash (SSA), produced at an estimated rate of 0.7 million tons per year. Although the overall quantity of SSA is considerably lower than that of municipal waste or biomass ash, its high phosphorus content (5%u201310 wt%), a critical raw material on the EU list, renders SSA as a valuable resource. Projections for the next 20 years suggest that SSA production could double or even quadruple because of increased thermal treatment of sewage sludge, while biomass ash production is also expected to double due to coal replacement by biomass. To address these changes, several established industrial-scale treatment methods are currently employed. Chemical treatments (e.g., storage/aging and water washing), physical methods (e.g., grinding, sieving, density separation, magnetic separation, and eddy current separation), and thermal processes are used to reduce water-soluble salts and to recover components such as glass, magnetic metals, and non-magnetic metals. In such cases, sludge may be repurposed as a fuel or raw material for hydrogen production. The growing interest in hydrogen as a cleaner alternative fuel is driven by its potential to replace fossil fuels, thereby contributing to the decarbonization of key sectors, including public transport, manufacturing, and energy-intensive industries, while the resulting ash can serve as a new raw material in cement and concrete production. Additionally, direct utilization of sewage sludge within the cement industry represents another promising avenue for valorisation.The use of biomass energy, especially from wood, has surged in recent years as nations increasingly adopt renewable energy sources, leading to a significant rise in biomass ash production, estimated at approximately 480 million tonnes worldwide. Although comprehensive statistics on wood biomass ash production in the EU are not available, data suggest that around 2.9 million tonnes are produced annually in countries such as Austria, Denmark, Germany, Italy, the Netherlands, and Sweden [7]. Within the EU, an estimated 70% of this ash is deposited in landfills, while the remaining 30% is repurposed as a soil additive in agriculture or used for other applications. The ash%u2019s composition is largely dependent on the type of wood biomass used, with calcium, phosphorus, potassium, and silica as its main constituents [8]. In Croatia, there are 42 wood biomass power plants, primarily situated in the east, north, and northwest, as well as in the Lika and Gorski Kotar regions [9]. These facilities produce various types of ash during energy generation, including fly ash, mixed ash, and bottom ash, amounting to roughly 30,000 tonnes annually [10]. Currently, the majority of these ashes is disposed of, with only a limited fraction being used in agricultural practices. Figures 1(a) and 1(b) illustrate the collection methods of ash at energy plants, while Figures 2(a)%u2013(c) highlight the variations in ash type based on its source and collection location.Within the scope of the AshCycle project, the investigation focuses on the innovative use of wood biomass ash (WBA) Fig. 1: Wood biomass power plants in Croatia and ash collection methods: (a) an illustration of the ash collection process at a wood biomass energy plant; (b) storage of biomass ash in jumbo bags.a) b)