Fed-batch bioreactor strategies for microbial decolorization of azo dye using a Pseudomonas luteola strain

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 (υ_dye) and overall decolorization efficiency (η_dye) 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 η_dye due to higher biomass loading, but lower υ_dye due to lower dye concentration in the bioreactor. With a volumetric flow rate (F) of 25 mL/h, both υ_dye and η_dye increased almost linearly with the increase in the loading rate of dye (F_dye) over the range of 50-200 mg/h, while further increase in F_dye (400 mg/h) gave rise to a decline in and υ_dye and η_dye. As the F was doubled (50 mL/h), the υ_dye and η_dye increased with F_dye only for F_dye < 80 mg/h. The best υ_dye (113.7 mg dye g cell^-1 h^-1) and η_dye (86.3 mg dye L^-1 h^-1) were achieved at F_dye = 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 F_dye = 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.