Biodegradation of Acid Orange 7 and its auto-oxidative decolorization product in membrane-aerated biofilm reactor

Abstract

Many azo dyes could be decolorized anaerobically to aromatic amines with a hydroxy group in ortho-position to an amino group. These aromatic amines can easily auto-oxidize, resulting in the generation of more recalcitrant compounds. The biodegradation of Acid Orange 7 (AO 7) and its auto-oxidative decolorization product was explored in a membrane-aerated biofilm reactor (MABR) for the first time. It was found that oxygen supply pressure had a remarkable impact on the MABR performance. Under the optimal oxygen pressure 0.03 MPa, AO 7 decolorization efficiency could reach 98% in 6 h and the effluent kept colorless and stable with over 80% COD removal. However, in the conventional anaerobic–aerobic (A/O) process only 57.8% COD was removed with yellow effluent. Denaturing gradient gel electrophoresis and 16S rDNA gene sequencing analysis revealed diverse bacteria capable of decolorizing or tolerating azo compounds in the anaerobic sludge of MABR. Shewanella sp. XB used for MABR bioaugmentation could persist dominantly in both biofilm and suspended sludge, which was beneficial for the AO 7 decolorization and aerobic biodegradation of the produced aromatic amines. These results indicated that MABR has potential applications in the biodegradation of the auto-oxidative aromatic amines.