Graduate Thesis Or Dissertation
 

Modeling cometabolic transformation of a CAH mixture by a butane utilizing culture

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  • The goal of this research was to mathematically simulate the ability of bioaugmented microorganisms to aerobically cometabolize a mixture of chlorinated aliphatic hydrocarbon (CAH) compounds during in-situ treatment. Parameter values measured from laboratory experiments were applied to the transport model with biotransformation processes included. In laboratory microcosm studies, a butane-grown, enriched culture was inoculated in soil and groundwater microcosms and exposed to butane and several repeated additions of 1,1,1-trichloroethane (TCA), 1,1-dichioroethylene (DCE), and 1,1-dichloroethane (DCA) at aqueous concentrations of 200 μg/L, 100 μg/L, and 200 μg/L, respectively. Microcosms containing the bioaugmented culture showed 1,1-DCE to be rapidly transformed, followed by slower transformation of 1,1-DCA and 1,1,1-TCA. After most of the butane had been consumed, transformation of these latter CAHs increased, indicating strong inhibition by butane. With repeat biostimulations, butane utilization and CAH transformation accelerated, showing the increase in cell mass. These trends occurred in two sets of microcosm triplicates. No stimulation was observed in controls containing only the microorganisms indigenous to Moffett Field, confirming that activity seen in the bioaugmented microcosms was a result of the introduced culture's activity. Batch reactor results were simulated using differential equations accounting for Michaelis-Menten kinetics, transformation product toxicity, substrate inhibition, butane utilization, and CAH transformation. The equations were solved simultaneously by Runge-Kutta numerical integration with parameter values adjusted to match the microcosm data. Having defined the parameter values from laboratory studies, the biotransformation model was combined with 1-D advective-dispersive transport to simulate behavior of the culture and the substrates within an aquifer. The model was used to simulate the results of field studies where the butane-utilizing culture was injected into a 7 m subsurface test site and exposed to alternating pulses of oxygen and butane, along with the contaminant mixture studied in the microcosms. Monitoring wells spaced at 1 m, 2.2 m, and 4 m from the injection well allowed temporal and spatial changes in substrate concentrations to be determined. Model simulations of the field demonstration were performed to determine how well the biotransformation/solute transport model predicted actual field observations. To model the influences of solute transport, simulations were run and compared to breakthrough test data (prior to bioaugmentation) to determine the values for advection, dispersion, and sorption. The simulations showed that flow ranged from 1.0 to 1.5 m³/day (average linear velocity of 2.0 m/day). Dispersion was estimated as 0.31 m²/day. Sediment sorption partitioning coefficients for 1,1-DCE, 1,1-DCA, and 1,1,1-TCA were determined to be approximately 0.69, 0.50, and 0.50 L/kg, respectively. It was more difficult to determine an appropriate value of the mass transfer rate coefficient for non-equilibrium sorption, so simulations were run to compare equilibrium and non-equilibrium cases. Results indicated that non-equilibrium (with mass transfer rate coefficient of approximately 0.2 day⁻¹) better simulated the field data. Using these transport parameters and the biotransformation values determined from the laboratory experiments, simulations of the field data showed that the model was capable of simulating the effects of transformation rates, butane inhibition, and 1,1-DCE product toxicity. Simulations for varying pulsing cycles and durations provided possible improvements for future field demonstrations. Overall, this work proved that there is good potential in extrapolating laboratory based kinetics to simulate biotransformation at a field scale. Although the complexity of such systems makes modeling difficult, such simulations are useful in understanding and interpreting field data.
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  • File scanned at 300 ppi (Monochrome, 256 Grayscale) using Capture Perfect 3.0.82 on a Canon DR-9080C in PDF format. CVista PdfCompressor 4.0 was used for pdf compression and textual OCR.
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