Soil vapor extraction (SVE) systems, modeling and experiments

Abstract

A model for the soil vapor extraction (SVE) process was proposed. The SVE model incorporated a number of improvements on the current state of simulation/prediction models by including unsteady-state concentration profiling in either a radial cylindrical or axial Cartesian coordinate, considering both vaporization and desorption, non-ideal (non-equilibrium) transport and biodegradation. To take into account the heterogeneity of soil, multiple particle domains with different sorption characteristics were proposed. The model thus allowed for flexibility in the characterization of the soil as a combination of sorbents of different properties. The treatment of a mobile gas phase, two liquid phases (aqueous and hydrocarbon) and a multi-domain solid phase, was presented as an adequate representation of the soil environment under SVE. The proposed model was validated by comparing its predictions to published field data from pilot-scale SVE projects and to bench-scale column adsorption and desorption data. The simulation of the data available from both laboratory bench-scale and field pilot-scale was performed with few and in some cases no adjustment of parameters. Verification of the model was also demonstrated by conservation of mass. The model was shown to track or be bounded within certain constraining simplified analytical solutions. Thus confidence in the conservation, accuracy and realism of the simulations was well founded. Data on various VOC's and fumigants were used to show the model applicability to various classes of chemical species. The proposed model was compared with vaporization and ideal transport models. These latter were seen to be too optimistic as predictors of SVE performance. With regard to biodegradation, it was shown that for some contaminants which are attractive candidates for aerobic microbial reaction, such as toluene, the reaction may indeed consume 40% to 60% of the total contaminant and reduce the remediation time by thousands of hours, which is consistent with field observations. It was shown that modification of flow can enhance the biodegradation function over vapor extraction, which is desirable since this reduces handling costs. Finally, a method of predicting gas-phase single-component adsorption isotherms for VOC's on soils and soil constituents was developed from observations on the experimental data.