Aerobic biodegradation of a mixture of sulfonated azo dyes by a bacterial consortium immobilized in a two-stage sparged packed-bed biofilm reactor

The biodegradation of the sulfonated azo dyes, Acid Orange 7 (AO7) and Acid Red 88 (AR88), by a bacterial consortium isolated from water and soil samples obtained from sites receiving discharges from textile industries, was evaluated. For a better removal of azo dyes and their biodegradation byproducts, an aerobically operated two-stage rectangular packed-bed biofilm reactor (2S-RPBR) was constructed. Because the consortium's metabolic activity is affected by oxygen, the effect of the interstitial air flow rate Q _GI on 2S-RPBR's zonal values of the oxygen mass transfer coefficient k _La was estimated. In the operational conditions probed in the bioreactor, the k _La values varied from 3 to 60h ^-1, which roughly correspond to volumetric oxygen transfer rates, dc _L/dt, ranging from 20 to 375mg O ₂L ^-1h ^-1. Complete biodegradation of azo dyes was attained at loading rates B _V,AZ up to 40mgL ^-1d ^-1. At higher B _V,AZ values (80mgL ^-1d ^-1), dye decolorization and biodegradation of the intermediaries 4-amino-naphthalenesulphonic acid (4-ANS) and 1-amino-2-naphthol (1-A2N) was almost complete. However, a diminution in COD and TOC removal efficiencies was observed in correspondence to the 4-aminobenzenesulfonic acid (4-ABS) accumulation in the bioreactor. Although the oxygen transport rate improved the azo dye mineralization, the results suggest that the removal efficiency of azo dyes was affected by biofilm detachment at relatively high Q _GI and B _V,AZ values. After 225 days of continuous operation of the 2S-RFBR, eight bacterial strains were isolated from the biofilm attached to the porous support. The identified genera were: Arthrobacter, Variovorax, Agrococcus, Sphingomonas, Sphingopyxis, Methylobacterium, Mesorhizobium, and Microbacterium.