A Pseudomonas luteola strain possessing azoreductase activity was utilized to decolorize a reactive azo dye (C. I. Reactive Red 22) with fed-batch processes consisting of an aerobic cell growth stage and an anaerobic fed-batch decolorization stage. The fed-batch decolorization was conducted with different agitation and aeration rates, initial culture volumes, dye loading strategies, and yeast extract to dye (Y/D) ratios, and the effect of those operation parameters on azo dye decolorization was evaluated. Dissolved oxygen strongly inhibited the azo reduction activity; thus aeration should be avoided during decolorization but slight agitation (around 50 rpm) was needed. With the periodical feeding strategy, the specific decolorization rate (υdye) and overall decolorization efficiency (ηdye) tended to increase with increasing feeding concentrations of dye, whereas substrate inhibition seems to arise when the feeding concentration exceeded 600 mg dye/L. In the continuous feeding mode, higher initial culture volume resulted in better ηdye due to higher biomass loading, but lower υdye due to lower dye concentration in the bioreactor. With a volumetric flow rate (F) of 25 mL/h, both υdye and ηdye increased almost linearly with the increase in the loading rate of dye (Fdye) over the range of 50-200 mg/h, while further increase in Fdye (400 mg/h) gave rise to a decline in and υdye and ηdye. As the F was doubled (50 mL/h), the υdye and ηdye increased with Fdye only for Fdye < 80 mg/h. The best υdye (113.7 mg dye g cell-1 h-1) and ηdye (86.3 mg dye L-1 h-1) were achieved at Fdye = 200 mg/h and F = 25 mL/h. The yield coefficient representing the relation between dye decolorized and yeast extract consumed was estimated as 0.8 g/g. With Fdye = 75 mg/h, the Y/D ratio should be higher than 0.5 to ensure sufficient supply of yeast extract for stable fed-batch operations. However, performance of the fed-batch decolorization process was not appreciably improved by raising the Y/D ratio from 0.5 to 1.875 but was more sensitive to the changes in the dye loading rate.
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