Direct Blue 71 (C40H23N7Na4O13S4), an azo dye with a high worldwide consumption and providing toxic effluents, can be highly degraded using TiO2 catalyst suspension and irradiation with a UV-C lamp in a circulating upflow photo-reactor with no dead zone. An initial concentration of 50 mg L−1 of dye, within the range of typical concentration in textile wastewaters, was used. The influence of catalyst concentration, pH and temperature were investigated. The results showed that degradation of this dye can be conducted in the both processes of only UV irradiation and UV/TiO2; but with the aim of mineralization, the later process provides significantly better results. Accordingly, a degradation of more than 97% of dye was achieved by applying the optimal operational parameters with 40 mg L−1 of catalyst, natural pH and 45 °C, during 120 min irradiation. A removal of about 50% of COD could also be obtained at the same time. In kinetic investigations, the effect of catalyst particles’ turbidity was taken into account and the rate of degradation of the dye, under mild conditions, was expressed as the sum of the rates of individual photolysis and photocatalysis process branches, with mainly influence of the bulk hydroxyl radicals.
