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Evidence of Biodegradation of Reactive Red Textile Azo Dye in an Anoxic/Aerobic Bioremediation System

The biodegradation of reactive red (RR) textile azo dye was investigated. Three bacterial strains, Enterobacter cloacae, Pseudomonas sp. and Bacillus sp. were used to decolourize and/or degrade the dye. A small bench-scale suspended bed bioreactor was used to study the capacity of these three bacterial strains to decolourize the dye solutions added as 5 successive supplements. The degradation of azo dyes was judged by the formation of aromatic amines (first product of dye biodegradation). All bacterial strains tested in this study produced aromatic amines under anoxic conditions. Additional evidence for biodegradation of RR textile azo dye was determined using 3 bioassay methods. Six test strains contributing to soil fertility were grown in spent media obtained from RR biodegradation. The growth of each test strain on biodegradation products was as high as the growth on the same specific media for each strain. This is evidence for the removal of toxicity in the biodegradation products. Another bioassay method used the germination of plant seeds to test dye phytotoxicity. The biodegradation of RR removed the phytotoxic effects of the dye on wheat (Triticum aestivum Giza ‘167’) and berseem clover (Trifolium alexandrinum L.) seed germination. The chemical oxygen demand (COD), which indicates the organic load in the medium, decreased from 2048 to 599 ppm under aerobic conditions. The bacterial growth after biodegradation of RR increased indicating the breakdown of organics. A continued decrease in the COD value indicates a steady biodegradation of the anoxic biodegradation products as the growth conditions turned aerobic. This study shows that the biodegradation of a toxic textile azo dye, RR, is possible by using potent bacterial strains in a sequential anoxic/aerobic bioremediation system. _