AbstractThe treatment of fully and partly weathered olive mill wastewaters by means of ozonation was investigated, with emphasis on the effect of treatment conditions (organic loading, treatment time, ozone concentration) on the extent of chemical oxygen demand (COD) and total phenol degradation, colour removal as well as ecotoxicity to the marine bacterium Vibrio fischeri. In general, ozonation was capable of substantially reducing the concentration of phenols as well as decolourising the effluent after relatively short treatment times, with the extent of phenol and colour removal exceeding 80% in most cases. The fast and selective degradation of phenols was ascribed to the direct electrophilic attack by molecular ozone (ozonolysis). Conversely, complete mineralisation proved difficult; at the conditions employed in this study, COD removal varied from as little as 10% to about 60%. Both effluents were strongly ecotoxic owing to the presence of phenols and other organics in substantial concentrations; toxicity decreased partly following considerable degradation of the organic matter. Copyright © 2006 Society of Chemical Industry