A novel bioreactor system was developed for tbe simultaneous treatment of toxic and mutagenic hexavalent chromium (chromate) and high strengtb organic pollutants by introducing chromate-resistant and chromatereducing bacteria Enterobacter cloacae strain HO1. Growth and chromate reducing characteristics of E. cloacae HO1 were extensively investigated to setup the bioreactor system and to clarify the optimal operating conditions of it. To enrich active E. cloacae HO1 in the bioreactor, the aerated cultivation with oxidation-reduction potential (ORP) control was successfully applied. Chromate and organic substrates in the artificial wastewater and in the metal plating wastewater were satisfactorily removed in the bioreactor. The reduced chromate was removed by precipitation as chromium hydoxide from the wastewater. The rate of chromate reduction was as high as 500 mg-chromate/g-microbes as particulate organic carbon and 10-60 gchromate/m3.h in the bioreactor.

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