SIMULATION OF TCE MIGRATION AND BIODEGRADATION IN A POROUS-MEDIUM UNDER CONDITIONS OF FINITE DEGRADATION CAPACITY

A numerical model has been developed to simulate the biodegradation of trichloroethylene (TCE) by Methylosinus trichosporium OB3b bacteria a s observed in a saturated, open system sand-bed experiment. The sand p ack was inoculated with bacteria in a resting state (without nutrients ) to form a 10 cm thick attached filter along the direction of flow. D egradation occurs through cometabolism of TCE to Cl- and primarily HCO 3- as the major overall carbon product. A steady, 1.5 cm/h saturated f low was established in the sand pack, and was later doped (upstream of the attached filter) with a 13-day, 4 ppm pulse of TCE. Concentration s of TCE were monitored immediately upstream and downstream of the att ached bacterial filter. Accounting for travel time through the test be d, observed downstream concentrations indicated degradation in the fil ter 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 pul se. Measurements of Cl- breakthrough were used to confirm TCE degradat ion quantitatively. The return of TCE in the downstream flow was antic ipated 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-determine Mi chaelis-Menton relationship were used to successfully model the experi mental results in the sand test-bed. Based upon the tube-transfer expe riments, the rate law was adjusted to account for a fixed or limited c apacity of the bacteria in the resting state to degrade TCE. The degra dation capacity of sand-attached M. trichosporium OB3b has been estima ted to be about 0.30 g TCE/g bacteria, and is not expected to be a lea ding limitation in implementing this approach for bioremediation in th e held.