The treatment of synthetic wastewater containing azo dyes present in textile industry wastewater was carried out by biological method (anaerobic microbial) in Linnaeus University in Sweden and chemical treatment method (catalytic wet peroxide oxidatin, CWPO) in Ege University in Turkey. Display omitted ► High residence time, yeast extract, microorganism were effective on dye removal. ► The highest removal obtained by microbial process was approximately 89%. ► In CWPO, optimum activated carbon, H 2O 2 loading, temperature, pH were determined. ► The highest removal obtained by CWPO was approximately 93%. ► Dye removal was accomplished by 60% adsorption and by 40% oxidation. The treatment of synthetic wastewater containing azo dyes found in textile industry wastewater was carried out by anaerobic biological method and chemical oxidation. The main target of this study was to compare different treatment methods and to evaluate the effect of different parameters on treatment effectiveness. In the microbial process, the results have shown that increasing the residence time, the amount of yeast extract and the addition of microorganisms originally growing on forest residues had positive effects on the dye removal. In the catalytic wet peroxide oxidation process, CWPO, the reaction conditions were optimized at 0.5 g/L activated carbon loading with 2 mL H 2O 2/300 mL solution (35 wt%), at 80 °C, in 2 h with pH = 3. At the optimum conditions, approximately 93% of the dye was removed. At these optimized conditions, the CWPO process was tested with real textile industry wastewater. The percentage of dye removal with this wastewater was 50%. The adsorption effect of the activated carbon was also investigated. At pH = 7, the removal by just adsorption was around 15%. But in acidic conditions (pH = 3) and at higher temperatures the adsorption effect of activated carbon increased. Adsorption and oxidation performances were compatible at 80 °C, however, at lower temperatures the adsorption effect was more considerable than the oxidation. It can be concluded that, generally the decolorization was accomplished by 60% adsorption and by 40% oxidation.