The characteristics of a novel bioreactor system developed for the simultaneous treatment of toxic hexavalent chromium (chromate) and high-strength organic pollution by introducing a chromate-resistant and chromate-reducing bacterium Enterobacter cloacae strain HO-1 (HO-1, hereafter) were studied to optimize its operating condition. Based upon the growth and chromate-reducing kinetics of HO-1, a mathematical model to simulate the change of chromate removal rate per unit volume of bioreactor and that per unit cell mass under the fed-batch operation was proposed. The effects of repeated cultivation of HO-1 with ORP-controlled aeration and the cumulative chromate reduction per unit cell mass on the microbial activity were investigated to represent the kinetic expression of chromate reduction. By using the mathematical model developed through the present research, the relationship between the operating conditions and the chromate reduction rate in the bioreactor was simulated and the model was verified by comparing it with the observed data. Thus the optimal operating conditions to give the higher reduction rate and the longer operating interval with and without aerated cultivation of HO-1 are extensively discussed. This bioreactor can be used for the treatment of chromate wastewater provided that some high concentration organic wastes are available. The rate of chromate reduction in the bioreactor is as high as 10 - 15 g-Cr6+/m3h. Chromium hydroxide, products of biological chromate reduction, can be recycled as a green pigment.

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