Application of fluidized-bed biofilm reactor in the biodegradation of polychlorinated biphenyls

• Main Author: Borja, Josephine Quintillan
• Format: text
• Language: English
• Published: Animo Repository 2006
• Subjects: Fluidized reactors; Biofilms; Polychlorinated biphenyls; Chemical Engineering
• Online Access: https://animorepository.dlsu.edu.ph/etd_doctoral/129; https://animorepository.dlsu.edu.ph/context/etd_doctoral/article/1128/viewcontent/CDTG004109_P.pdf
• Institution: De La Salle University

The biodegradation study was conducted using biofilm technology in a three-phase fluidized-bed reactor under anaerobic and aerobic conditions, and combining the anaerobic and aerobic processes in sequential mode. Aerobic biofilm was developed using microorganisms isolated from a PCB-contaminated soil with biphenyl as sole carbon source. The anaerobic biofilm was developed on the aerobic biofilm, which served as conditioning layer and facilitated attachment of the anaerobic sludge from a brewery wastewater treatment facility. The aerobic biofilm formed was 80um thick and was dominated by Pseudomonas sp. and Flavobacterium sp. On the other hand, the combined aerobic and anaerobic biofilms was 170um thick. The anaerobic biofilm was used to degrade Aroclor 1260 with an initial concentration of 10ppm. The hexa-to octachlorobiphenyls were dechlorinated through the removal of doubly flanked para and meta chlorines. The heptachlorobiphenyl was 19.99% dechlorinated through the removal of unflanked ortho chlorine, while the hexachlorobiphenyl was dechlorinated through the removal of singly flanked ortho chlorine. Pentachlorobiphenyls accumulated in the medium as the higher PCBs were dechlorinated. Peak profiles from Gas Chromatograph-Electron Capture Detector (GC-ECD) revealed new peaks not present initially but these were not identified by the Gas Chromatograph-Mass Spectrometer (GC-MS). These peaks may be congeners with chlorine atoms lower than five. The aerobic biofilm was acclimatized to PCBs by gradual exposure to PCBs. Acclimatization by alternate feeding of biofilm with biphenyl and PCBs initially caused physiological stress on the biofilm but regained catabolic activity as acclimatization progressed. In the acclimatization method where the initial PCB concentration was gradually increased, there were no observed negative effects on the biofilm but the extent of PCB degradation decreased with increase in initial PCB concentration. For the same acclimatization period, the two methods exhibited the same decrease in pH of the medium but the second method yielded slightly higher PCB degradation. In the process using solely aerobic process, PCB biodegradation rate increased from one batch to another reaching 95 + 2.01 % in five days after conducting ten batch runs. The kinetics of PCB degradation can be modeled by a kinetics that shifted from second to third order as the degradation rate increased. Analysis of the data using Table Curve for Windows (TCWIN) revealed a 3.4 order of reaction with a specific rate of 0.0050 ppm-h. Oxidative dechlorination also occurred with 79% of the chlorines removed. Peak profiles from GC-ECD showed the presence of peaks not present initially, however, these peaks were not identified in the GC-MS. These are believed to be metabolites of PCB degradation. In the coupled anaerobic-aerobic process, the anaerobic portion exhibited the same dechlorination pattern as in the purely anaerobic process. The highly chlorinated biphenyls were dechlorinated in the anaerobic portion of the process causing the accumulation of lower chlorinated biphenyls. The formation of low chlorinated biphenyls in the anaerobic process enhanced the biodegradation of PCBs in the aerobic process. Overall, the combined process achieved 98% degradation in ten days compared to the 95% degradation achieved in ten weeks using solely aerobic process. A reaction mechanism for the biodegradation of 2,2',3,3',4,4'5,5'- octachlorobiphenyl was proposed based on the results of the sequential anaerobic-aerobic process. Dechlorination was based on the pattern obtained in the anaerobic process where chlorine was removed from the para and meta positions. It is assumed that the dioxygenase attack of the aerobic microorganisms on the dechlorinated compound is at the position with no chlorine substitution. This results in the formation of hydroxylated chlorobiphenyl which is cleaved resulting in the formation of chlorobenzoic acid.