Simulation of TCE migration and biodegradation in a porous medium under conditions of finite degradation capacity

Abstract A numerical model has been developed to simulate the biodegradation of trichloroethylene (TCE) by Methylosinus trichosporium OB3b bacteria as observed in a saturated, open system sand-bed experiment. The sand pack was inoculated with bacteria in a resting state (without nutrients) to form a 10 cm thick attached filter along the direction of flow. Degradation occurs through cometabolism of TCE to Cl− and primarily HCO3− as the major overall carbon product. A steady, 1·5 cm/h saturated flow was established in the sand pack, and was later doped (upstream of the attached filter) with a 13-day, 4 ppm pulse of TCE. Concentrations of TCE were monitored immediately upstream and downstream of the attached bacterial filter. Accounting for travel time through the test bed, observed downstream concentrations indicated degradation in the filter was complete for 0·5 days, substantially complete for another 1·5 days, and increasingly limited over the next 11 days, after which the outflow concentrations returned to zero, indicating the end of the pulse. Measurements of Cl− breakthrough were used to confirm TCE degradation quantitatively. The return of TCE in the downstream flow was anticipated from repetitive exposure, cyclic transfer experiments in sealed tubes which showed a limited capacity of the bacteria in the resting state to degrade TCE. Simulations based upon a previously-determined Michaelis-Menton relationship were used to successfully model the experimental results in the sand test-bed. Based upon the tube-transfer experiments, the rate law was adjusted to account for a fixed or limited capacity of the bacteria in the resting state to degrade TCE. The degradation capacity of sand-attached M. trichosporium OB3b has been estimated to be about 0·30 g TCE/g bacteria, and is not expected to be a leading limitation in implementing this approach for bioremediation in the field.