The water framework directive has given new emphasis to the protection of the ecological status of receiving waters, as well as their chemical status. This emphasis on ecological quality is the principal driver for the TREES project, which is focused on both the toxicity of steel industry effluents and the reduction of this toxicity by innovative treatment techniques. Traditionally, the impact of iron and steel industry streams on the aquatic environment has been evaluated by comparing the concentration of the pollutant in the receiving water with environmental quality standards defined by current legislation. This method, unfortunately, does not provide information on how the effluent affects the biodiversity of the receiving waters. That is achieved only through ecotoxicological tests. The selection of the most suitable tests for assessing the ecotoxicity of steel works" effluents and understanding their effects on the aquatic environment has been analysed in great detail. Recently,...