Transport and biodegradation of toxic organics in biofilms

Abstract This paper summarizes our studies on transport and biodegradation of azo dyes in aerobic biofilms. Three lab-scale rotating drum biofilm reactors (RDBRs) were used to culture the biofilms. synthetic, municipal-type wastewater, spiked with several azo dyes, was continuously fed into the RDBRs. The study has revealed that Acid Orange 7 (AO7) and Acid Orange 8 (AO8) can be degraded aerobically in an efficient and effective manner under properly controlled operating conditions. The other dyes tested, however, are essentially non-degradable in an aerobic reactor. Microelectrodes and a microslicing technique have been used to elucidate changes in the structure of biofilms with depth and to determine the influence of these changes on mass transport and transformation processes. The results show that biofilms are non-homogeneous and highly stratified. Accordingly, biofilm is usually characterized by aerobic and anaerobic zones, with the anaerobic part used to break the azo bond and the aerobic surface layer to achieve the mineralization of anaerobically recalcitrant intermediates. Results concerning the inhibition effect of azo dyes on biofilm COD removal and respiration rate are also presented in this paper.