The combined biological and chemical treatment of highly concentrated reactive azo dye-containing residual dyehouse liquors with recalcitrant compounds was investigated in a sequencing batch reactor (SBR). The plant consists of a batch reactor in which the anoxic and aerobic phases are carried out by sequenced steps. Water-soluble reactive dyes were reductively cleft and decolorized by a facultative anaerobic bacterial mixed culture under anoxic conditions. Complete decolorization was observed up to concentrations of nearly 20 g dye/L without addition of an external auxiliar substrate. Mineralization of the cleavage products occurs with the same bacterial mixed culture in the same reactor under aerobic conditions. The biomass used for the anoxic treatment is grown in this aerobic phase by the use of split flows with readily biodegradable compounds. Because of the recalcitrant toxic character of some remaining substances, further aerobic mineralization was initiated by partial oxidation with ozone. Partial ozonization in a circulated stream with biological post-treatment of the transformed substances led to an increased reaction selectivity, to a better biological degradation and not least to a lower consumption of ozone. Due to this purification procedure involving highly concentrated residual dyehouse liquors a total decolorization and an overvall degradation of nearly 90% in DOC was achieved.

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