TECHNOLOGY FOCUS: SATURATED WATER INFUSION PROCESS
Saturated water infusion (SWI) process equipment from inVentures Technologies, Inc. has been used for removing both LNAPLs and DNAPLS. The equipment can be enclosed and secured in a trailer-mounted system. The carbon dioxide enhances the saturated water infusion process since carbon dioxide is highly soluble in water, and the bubbles of carbon dioxide will volatilize liquid droplets of contaminants (such as PCE, TCE, and petroleum hydrocarbons) into the bubbles of carbon dioxide. A two phase extraction system is used to remove the carbon dioxide and contaminant vapors from the vadose zone as long as the mass of TCE in vapors and groundwater continues to decline.
The saturated water infusion process is an example of a cost-effective method for extraction and infusion for rapid removal of non-aqueous phase liquids (NAPLs) of hydrocarbons and chlorinated solvents. The SWI process is especially effective with NAPLs that are trapped in pore spaces as ganglia. The removal of free product from pore spaces was modeled recently. In the study, Soltrol, a nonvolatile hydrocarbon, was modeled by Leif Nelson below (Figure 1).
FIGURE 1 – Conceptual model of NAPL ganglia removal by SWI by Leif Nelson (Jacobs et al, 2008).
SWI allowed for controlled mobilization of the NAPL ganglia into the water table for collection using dual phase extraction. SWI technology relies on water which is supersaturated with carbon dioxide, a highly soluble gas, in providing a mass transfer system. Carbon dioxide saturated water is injected under high pressure into the former tank pit where the carbon dioxide bubbles nucleate at the targeted area of the aquifer. The rising carbon dioxide bubbles contact with the submerged NAPL the saturated zone and cause volatilization of the free product into the vapor phase and mobilization of NAPL trapped in the pores.
Several extraction wells and dozens of small-diameter reinjection probe-rod ports were used to recirculate the carbon dioxide saturated water and provide a closely-spaced delivery and extraction system. The carbon dioxide is distributed by flowing water resulting in effective carbon dioxide distribution followed by heterogeneous bubble nucleation and continuous growth of gas bubbles in situ. A gas saturation front developed which expanded laterally and vertically towards the water table in the former underground tank pit. The NAPL mobilizes to soil gas and is extracted with a dual phase extraction system.
FIGURE 2 - Direct pore-scale evidence of volatilization by the SWI process of a drop of TCE
(photo-micrograph courtesy of inVentures Technologies, Inc.)
The trapped TCE droplet in a pore space opening is shown above (see FIGURE 2). Using the saturated water infusion technology, the A carbon dioxide bubble nucleates (above) by mass transfer from the injected supersaturated aqueous phase carbon dioxide into water. Upon contact with the carbon dioxide bubble, the NAPL, in this case, a drop of TCE, spontaneously spreads over water and the volatile components of the NAPL are readily transferred into the carbon dioxide bubble. The carbon dioxide bubble migrates to the top of the groundwater surface, where it is extracted using dual phase extraction and the TCE is treated using above ground methods, such as a thermal oxidizer.
At the point the SWI process has removed as much of the DNAPL and TCE as possible, the SWI process is changed from using dissolved carbon dioxide to dissolved hydrogen. Dissolved hydrogen enhances anaerobic treatment in the source remediation management zone (RMZ 1) and the residual remediation management zone (RMZ 2). Using the infused hydrogen to maintain a highly reducing environment, the addition of fermentable oils from JRW Bioremediation LLC (WilClear®, Accelerite®, and LactOil®) provide the short and long-term (up to 24 months) carbon substrate for anaerobic degradation of TCE. Recent studies in New Mexico with tetrachloroethylene (PCE) have shown that chlorinated solvent degradation rates can be increased by 50% when the fermentable oil is enhanced with solubilized hydrogen gas (Sheldon et al., 2008). If the microbial counts for anaerobic microbes are low based on the anaerobic biofeasibility studies, dehalogenating bacteria can be added to the subsurface to enhance the TCE degradation process.
REFERENCES
Jacobs, J., and Brewer, R., 2009, Workshop 7: RBCA Grows Up: Introduction to Environmental Hazard Evaluation and Advanced Approaches for Site Investigation, The Nineteenth Annual AEHS Meeting and West Coast Conference on Soils, Sediments, and Water, March 12, 2009, Mission Valley Marriott, San Diego, California.
Jacobs, J., Nelson, L., and Begley, J., 2008, Two Rapid Enhanced Flushing NAPL Recovery Methods, Sixth International Conference on Remediation of Chlorinated and Recalcitrant Compounds, Battelle Memorial Conference, Monterey, California, May 19-22, Abstracts.
Sheldon, J., Fogel, S. and Begley, J.F., 2008, Results of Field Testing Hydrogen Gas Infusion for PCE Bioremediation, Sixth International Conference on Remediation of Chlorinated and Recalcitrant Compounds, Battelle Memorial Conference, Monterey, California, May 19-22, Abstracts.
Jim Jacobs, P.G., C.H.G., is a hydrogeologist with 25+ years of experience. He has a bachelors and graduate degree in geology and is a Fulbright scholar in environmental science/engineering with three awards. He has co-authored two books and more than 100 articles. He is active in the Groundwater Resources Association, American Institute of Professional Geologists, The California Council of Geoscience Organizations and the Consultants-Owners-Regulators-Envirovendors (CORE) Foundation. He can be reached at (tel: 415-381-5195) or
jimjacobs@ebsinfo.com
