Treatment of dyehouse liquors in a biological sequencing batch reactor with recursive chemical oxidation

A new developed sequencing batch process for the purification of residual water containing concentrated azo dye was investigated. Within a treatment cycle the biological anoxic decolorization, followed by an aerobic mineralization of organic metabolites in combination with the biodegradability-achieving partial oxidation with ozone are carried out sequentially. The split flow can be destructively purified to 90% with respect to the parameter DOC. It was decolorized to an extent of 98% and the toxicity measured by the bioluminescence test decreased up to 99%. With an unspecific facultative anaerobic bacterial mixed culture anoxic decolorization of the residual liquor (20 g(dye)/L) without addition of an external auxiliary substrate was observed. In the first phase of the treatment cycle, the azo dye-molecules are cleft at the azo bond by biochemical reduction which leads to the corresponding sulfonated aromatic amines. In the following aerobic phase the cleft products were mineralized by the same microorganisms in the same reactor. Because of the recalcitrant and respectively toxic character of a part of the remaining metabolites, further aerobic mineralization was initialized by partial oxidation with ozone. The recursive ozonization in a recircled stream with biological post-treatment of the transformed substances led to an increased reaction selectivity and lower consumption of ozone. The results have shown that the chosen sequencing batch reactor with the ozonization bypass is suitable for an effective treatment of high concentrated dyehouse liquors.