Experimental design and mathmatical modeling of trichloroethylene cometabolism in a packed bed biofilm reactor by Pseudomonas putida F1

Trichloroethylene (TCE) as a suspected human carcinogen is one of the most common pollutants found in contaminated groundwater. Biofilm reactors have been extensively used in the bioremediation of hazardous organic compounds such as TCE. The biological background of TCE biodegradation is cometabolism. Limitations in the successful removal of TCE include competitive inhibition of primary substrate, TCE product toxicity, and oxygen limitation. In this study, an effective packed-bed bioreactor system was developed for the treatment of aqueous phase TCE by the toluene dioxygenases system of Pseudomonas putida Fl. The reactor was a glass chromatography column 60 cm long and 2.5 cm in diameter. Glass beads (4 mm in diameter) were used to pack the column and support growth of the biofilm. The reactor was operated as a trickling filter with a recycled effluent to maintain low toluene concentrations throughout the column. The aqueous influent contained a minimal salts medium, toluene and TCE in various concentration range. Oxygen supply was directly connected to the column. The gas stream was also recirculated back into the column to prevent the loss of TCE in the vapor phase. Operational conditions that were tested included influent toluene concentration, influent TCE concentration, influent flow rate, and recycle rate. TCE removal efficiencies ranged from 94.5% to 60% with TCE influent concentrations of 0.31 to 1.06 ppm, and influent toluene concentrations of 30 to 53 ppm, respectively.

Modeling of biofilm kinetics is the first step for bioreactor design and process optimization. Although numerous biofilm models have been developed in various situations, very few models are reported for TCE cometabolism, probably due to its biological complexity and computational difficulties. In this study, a relatively comprehensive steady-state biofilm model was proposed for TCE cooxidation by Pseudomonas putida F1 using toluene as a primary substrate. This model coupled diffusion process with substrates competitive inhibition reaction in the biofilm. The Model assumed that TCE degradation was carried out by active biomass only, which was balanced by cell growth and TCE transformation product toxicity effect. Finally, the effective biofilm thickness was calculated. Finite element method was used for the implementation of numerical solutions of non-linear ordinary differential equations. TCE and toluene fluxes under different toluene and TCE bulk concentrations were investigated. The influence of mass transfer, TCE product toxicity on TCE degradation were analyzed. Combining TCE cometabolic biofilm model with dispersion, convection, mass transfer between biofilm phase and liquid phase, and physical equilibrium between gas phase and liquid phase, a bioreactor model was developed for the system of interest. At low TCE influent concentration, model calculation results match experimental data fairly well. Comparing with the experiments, model prediction errors were below 30%. The inverse of TCE removal efficiency was found to be a function of the inverse of transformation capacity in a form of second order polynomial. This enabled the model to predict TCE removal efficiency at high TCE concentration when transformation product toxicity was significant. Similar results were obtained from model simulation by varying influent flow rates, influent TCE concentrations, influent toluene concentrations, and recycle rates as from the experiments, quantitatively or qualitatively. This bioreactor model can be used to design a bioreactor and to optimize operation for TCE cometabolism.