Decolorization and biodegradability of dyeing wastewater treated by a TiO₂-sensitized photo-oxidation process

Colour substances in dyeing effluents normally cause certain difficulties in traditional biological treatment processes due to their nonbiodegradable nature. It is necessary to remove colour from dyeing effluents with the help of some physical or chemical treatment processes. This study aims to investigate the colour removal from dyeing wastewater using a TiO₂-sensitized photo-oxidation process and the biodegradability of the products formed in the wastewater. Synthetic dyeing wastewater samples were exposed to near UV radiation at a wavelength of 350 nm in the presence of TiO₂ and aeration. The results show that most dyes used in the experiment can be degraded by the sensitized photo-oxidation successfully. Colour removal from the wastewater was above 95% after 4-6 hours treatment. It was found that there was a relationship between the chemical oxygen demand (COD), total organic carbon (TOC) and biochemical oxygen demand (BOD). While COD and TOC in the wastewater were decreased during the reaction, BOD was found to be increased, which implies that the TiO₂-sensitized photo-oxidation can enhance the biodegradability of the dyeing wastewater. As a process, it might be an effective method to remove colour and to further remove COD after treating dyeing effluent by a conventional biological treatment process.