Azo dye remediation in periodic discontinuous batch mode operation: Evaluation of metabolic shifts of the biocatalyst under aerobic, anaerobic and anoxic conditions

• Switching between aerobic and anaerobic condition facilitated good dye degradation.
• Bio-electro kinetics supported the shifts in biocatalyst metabolic activities.
• Facultative bacteria under limited oxygen availability function in enhanced dye removal.
• Dehydrogenase and azo reductase correlated well with the metabolic function.

Shifts in microbial metabolism with the function of aerobic, anaerobic and anoxic microenvironments were evaluated in periodic discontinuous batch mode operation in response to the treatment of azo dye (C.I.Acid Black 10B) bearing wastewater. Anoxic operation depicted higher dye removal efficiency (95.18%) followed by anaerobic (93.67%) and aerobic (68.77%) operations. Switching between aerobic and anaerobic metabolic functions during anoxic operation facilitated the reduction of dye to its intermediates (anaerobic) followed by their mineralization (aerobic). Function of dye as a terminal electron acceptor (TEA) along with the limited oxygen available might be the reason for the observed higher dye removal under anoxic operation. Azo reductase activity showed marginal variation with the function of anoxic and anaerobic conditions indicating the similar capabilities of the biocatalyst. However, dehydrogenase activity showed significant variation with the function of operating microenvironment indicating the differences in the delivery of reducing powers from the substrate metabolism towards dye removal. The bio-electro kinetics showed well correlation with the redox reactions, enzyme activities and dye removal. Microbial inventory analysis documented the involvement of organisms with anaerobic and facultative metabolic functions having heterotrophic and autotrophic nutrition modes. Aerobic operation documented the dominance of facultative (83%) followed by obligative aerobic (17%) bacteria while anoxic and anaerobic operations facilitated the distribution of facultative (67% and 83%, respectively) and obligative anaerobic (33% and 17%, respectively) bacteria.

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