Synthetic dyeing wastewater consisting of the three different commercial dyes with different strengths of COD (about 900 and 3000 mg/l respectively) was initially treated by an aerobic biological process, Intermittently Decanted Extended Aeration reactor (IDEA), for BOD removal, and then continuously treated by a TiO2 sensitised photoreactor for a further COD removal and decolorization. The catalysed photooxidation process can degrade those non-biodegradable organic substances in the effluent treated by the IDEA process and also decolorize the effluent completely. It is also found that some nonbiodegradable organic substances can be converted to biodegradable forms by the sensitized photo-oxidation reaction. A bio-photoreactor system was designed to combine this photocatalytic reactor with the IDEA reactor for the treatment of dyeing wastewater. The performance of this combined bio-photoreactor system with and without recirculation was investigated and compared. The system with recycled water has similar efficiency for decolorization and COD removal to that without recirculation, but has a high capacity to eliminate the effects caused by a shock loading, and also the system can treat dyeing wastewater with a higher organic concentration.

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