Biomass, such as wood and straw, is currently used in EU as a renewable energy source for energy production and this application is expected to rise in coming years. Combined heat and power installations produce fly ash, which is considered hazardous waste. The fly ash management issue should be addressed before biomass combustion is considered a truly sustainable technology. The electrodialytic process is a remediation technique able to assist the management of fly ash. For this work, straw and co-combustion of wood ash are briefly characterized and their electrodialytic treatment is carried out under acidic conditions.Straw ashes presented high removal rates for Ca, Cu, Cr and particularly for Cd, which has been reduced to a level bellow the established by Danish regulations. Acidification also induced a high dissolution and a lower pH of the ash. Fly ash from co-combustion of wood presents similarities with wood ash alone. However, further characterization should be carried out before any comparison regarding applicable legislation. Under acidic conditions, the electrodialytic treatment was not effective for the co-combustion wood ash. The heavy metals appeared in the least soluble fraction of this ash and lowering the pH of such an alkaline material does not mean sustainability and may hinder its reuse.