Sosath (1999) used a laboratory-scale two-step process with an anaerobic step and a subsequent aerobic step. In both reactors, bacteria were immobilized on the surface of rotating discs. He used a mixture of yeast extract and acetate as an auxiliary sub strate. In step 1, 66% decolorization was observed without remarkable consumption of the auxiliary substrate DOC, which was removed nearly completely in step 2, down to the remaining DOC of the dye. Obviously, the metabolites of decolorization are not biodegradable in this system. The mineralization of RB 5 seems only to be possible by chemical oxidation or a combination of chemical oxidation and biodegradation. Krull et al. (1998) treated a highly concentrated dyehouse effluent by using combination of biological and chemical methods. Breithaupt et al. (2001a) were successful in using a recycle reaction system consisting of an aerobic biological rotation disc reactor and an ozone batch reactor with automatic process control. Nearly 88% of the DOC could be removed. Approximately 66% of the ozone which would be needed for a 90% chemical oxidation could be saved (Wiesmann 2002).
An oxidation step with H2O2/UV and subsequent aerobic biodegradation operating in recycle mode was suitable to reduce the RB 5 DOC of 450 mg L-1 to about 33% by chemical oxidation and to reduce it by a further 33% by biological oxidation (Mohey El-Dein 2002). Of more practical interest is the treatment of more realistic dyehouse effluent, a mixture of RB 5 (18% DOC), a fixer (e.g. Akrofil PGM, 75%) and a surfactant (7%). Because of the biodegradability of these latter compounds, the most effective reactor system would consist of a first aerobic biological stage, followed by an ultrafiltration membrane to hold back microorganisms, an ozone reactor and an aerobic rotating disc reactor to mineralize products of chemical oxidation. The DOC could be reduced down to about 18% (a = 82%), including the remaining molecules of all three model components (Rapp and Wiesmann 2003; Rapp 2005).
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