The aim of the master thesis focuses on the flexographic printing field with the purpose to investigate the efficiency of Fenton process for the treatment of black flexographic dye, as well as to investigate the impact of various process conditions. Fenton process is one of the most efficient oxidation processes used for oxidation/coagulation of water with high content of surfactants and other heavy degradable compounds. It is based on the generation of hydroxyl radicals from hydrogen peroxide with iron ion as catalyst in an acidic medium and environmental conditions. During the experiment, it was found that the catalyst concentration and the pH value largely contribute to the efficiency of the heterogeneous Fenton process. It has been found that a pH decreasing results within increase of decolorization efficiency of the synthetic dye solution.