Experiences with solar application of photocatalytic oxidation for dye removal from a model textile industry wastewater

Aqueous solutions of the azo dye "Acid Orange 7" simulating dye-containing textile industry wastewaters have been irradiated by sunlight during circulation through a 2 cm deep channel conducted as a meander in a 2.5 X 1 m double-skin sheet reactor made of plexiglasB in batch experiments after addition of different titanium dioxide concentrations (1 to 10 gll). The experiments have been performed in a region with relatively low solar irradiation (HamburgIGermany). Depending on titanium dioxide concentration, different portions of the dye have been mineralized as recorded by TOC analyses. With the highest investigated titanium dioxide concentration, specific TOC removals (g TOCkWh photons passing the reactor surface) were increasing linearly with increasing TOC concentrations of the aqueous solutions. These data were used for developing a simple design method (concerning demand of irradiation time and area) for photocatalytic oxidation sequencing batch reactors eliminating the investigated azo dye and were compared to results derived from irradiation experiments of aqueous Acid Orange 7 solutions utilizing a UV-A lamp. In some experiments, decolorization of the solutions had been complete. The results have shown that dyes can be appropriately removed from aqueous solutions by this process. No addition of chemicals is required, because the catalyst can easily be removed by sedimentation and reused. The photocatalytic oxidation process can also be applied in a "low-tech" version (even avoiding pumps) and is thus proposed for treating textile dyeing wastewaters in very poor countries.