Korstaverken near Sundsvall in Sweden produces electricity and district heating for about 80 000 citizens. Household and commercial wastes are used as fuel in their cogeneration plant. The produced fly ash is treated in an ash washing facility to reduce leaching of hazardous substances. Leaching of chromium and molybdenum from the ash residue exceeds although the limit values for non-hazardous landfills. The aim of this project has been to identify factors that influence the leaching of chromium and molybdenum and how a reduction can be achieved. Reduced mobility can be reached by addition of pH and redox reducing agents, or by carbonation. The results show that there are a number of measures that can generate a washed ash that meets the requirements of non-hazardous landfills. In order to design the optimal full scale solution from a practical and economical perspective, the identified alternatives must be evaluated.