Abstract
Trichloroethylene (TCE) is readily mineralized under aerobic condition by cometabolism of non-specific oxygenase produced by toluene-oxidizing microorganisms. The objective of this study was to investigate biodegradation of TCE by a toluene-oxidizing microorganism in an aqueous-phase batch reactor, a bubble column bioreactor and a three phase activated carbon biofilter. The aqueous-phase batch experiments were conducted in which the concentration of TCE was held constant (0.98 mg/l, 1.96 mg/l or 3.93 mg/l) whereas the concentration of toluene was varied. The results showed that biodegradation of TCE was observed when the toluene/TCE concentration ratio was greater than 38.6. In contrast to TCE, nearly 100 % removal efficiency of toluene was observed in these experiments. Moreover, with mixtures of TCE, toluene and benzene, both toluene and benzene were biodegraded completely by the toluene-oxidizing microorganism, but TCE was not biodegraded.The removal efficiency of gaseous TCE in the bubble column bioreactor was above 90 % at a retention time of 1.26 min while inlet concentrations of TCE and toluene were 2.06 g/m3 and 2.33 g/m3, respectively. For the three phase activated carbon biofilter, 70 % removal efficiency of gaseous TCE was obtained at the same operating condition. Thus, the bubble column had a better removal efficiency than the three phase carbon biofilter. At the gas flow rate of 150 g/m3-hr and a low TCE concentration of 0.3 g/m3, the bubble column bioreactor could use a lower toluene concentration to sustain the biomass growth and to maximize the TCE biodegradation than the three phase activated carbon biofilter. In addition, at the same gas flow rate and the toluene concentration of 2.1 g/m3, the removal efficiency of the bubble column bioreactor was 90% above for TCE loadings from 5~53 g/m3-hr, while that of the three phase activated carbon biofilter was 80~90% for TCE loadings from 5~64 g/m3-hr. However, the removal efficiency decreased at high concentrations of TCE or toluene, and the decrease in the bubble column was more dramatic than that in the three phase activated carbon biofilter. As a result, at high concentrations of TCE or toluene the three phase activated carbon biofilter, had a higher removal efficiency than the bubble column bioreactor.