Modeling of the cometabolic biodegradation of trichloroethylene by toluene oxidizing bacteria in a biofilm system

Because of its intensive use in industry, trichloroethylene (TCE) is one of the most widespread contaminants in soil and groundwater. The aerobic biodegradation of TCE depends on the supplement of a primary carbon source, of which toluene appears to be the most efficient/practicable. For this reason, the cometabolic biodegradation of TCE was investigated in a continuously fed biofilm reactor with a mixed culture of toluene degraders. The interaction phenomena between toluene and TCE were studied and modeled in order to develop a kinetic model for the design of treatment processes. TCE degradation ([TCE] = 40-135 mg/L) was dependent upon the presence of toluene; however, if the latter was supplied at concentrations above 1 mg/L, TCE degradation was strongly inhibited. Similarly, TCE inhibits toluene degradation ([TCE] < 50 mu g/L). A simple kinetic model which incorporates competitive inhibition between toluene a nd TCE, as well as the activation effect from toluene, was developed. A fair agreement between modeled and experimental data was found. However, the kinetic model was not able to predict the ICE removal in the absence of toluene (resting cells) or at very low toluene concentrations (i.e., below 0.1 mg/L). Parameter estimation yielded a maximum TCE degradation rate, k(x(TCE)), of 0.38 +/- 0.11 gTCE g(x) day(-1) and a half-saturation constant for TCE, K-S(TCE), of 0.17 +/- 0.1 mg/L. Furthermore, the model calculations suggested that the active biomass (toluene degraders) accumulated at the top of the biofilm in an active layer of ca. 120 mu m. Finally, sensitivity analyses defined the model's uncertainties to be +/-30-35% for TCE. The calibrated model is able to predict fairly well the removal of TCE for concentrations ranging from 0 to 5 mg/L.