Dynamic Underground Stripping and Hydrous Pyrolysis Oxidation
Dynamic Underground Stripping and Hydrous Pyrolysis Oxidation (DUS/HPO) combines several technologies to remediate soil and groundwater contaminated with fuel and other organic compounds. It is very similar to some thermal soil cleanup technologies, except that it also treats groundwater and the saturated matrix. DUS/HPO injection and extraction wells are installed so that their screened sections are in both saturated and unsaturated zones. Steam is injected at the periphery of a contaminated area to heat permeable subsurface areas, vaporize volatile compounds bound to the soil, and drive contaminants to centrally located vacuum extraction wells. Electrical heating is used on less permeable clays to vaporize contaminants and drive them into the steam zone. DUS/HPO also uses an underground imaging system called Electrical Resistance Tomography (ERT) that delineates heated areas to monitor cleanup and process control. Hydrous Pyrolysis/Oxidation (HPO) is added to this process. This adds oxygen in parallel with steam. When injection is halted, the steam condenses and contaminated groundwater returns to the heated zone, where it mixes with oxygen-rich condensed steam. This enhances natural biodegradation of certain materials by providing nutrients to microorganisms that thrive at high temperatures (called thermophiles).
Limitations and Concerns
Microorganisms that are destroyed by steam can foul the system. Small particles that are pumped to the surface can also clog the system. High temperatures increase carbonates and silicates in the extracted liquids, potentially fouling the treatment units.
The success of the DUS/HPO process is dependent upon boiling the subsurface environment. Consequently, the process uses a large amount of energy.
Aboveground treatment systems must be located so they donÕt interfere with access to the subsurface treatment zone, in case additional injection, extraction, heating, or monitoring wells need to be installed.
Steam adds significant amounts of water to the subsurface. Precautions must be taken to avoid mobilizing contaminants past the capture zones. In one demonstration, downgradient capture wells equipped with in-situ bio-filters were being considered.
Air emissions from both aboveground treatment units and steam-generating equipment are of concern.
DUS/HPO is not applicable at depths less than five feet. It has been used at depths up to approximately 120 feet.
There has been some concern that DUS/HPO will sterilize the subsurface so that microorganisms will not attack the contaminants. At Lawrence Livermore National Laboratory (LLNL), DUS/HPO was found to be compatible with efforts to bioremediate residual contamination following steam injection. After application of DUS/HPO at LLNL, viable microbial populations continued to degrade gasoline at the site at temperatures above 158 degrees F.
Treated soils can remain at elevated temperatures for years after cleanup. This could affect site reuse plans or other remedies for the same geographic unit. At Lawrence Livermore National Laboratory, 10 years after this technology was first demonstrated in the early 1990s, the groundwater temperature is still approximately 100 degrees F. This has changed the underground ecosystem, and there is a continuing concern that indigenous bacteria will not reestablish themselves. Also, little is known about the thermophiles. Soil venting can accelerate the cooling process.
In one demonstration (Beale AFB), because of displacement/mixing of the steam-injection, it was difficult to evaluate whether the process actually degraded the VOCs, or reductions in concentrations were caused by dilution.
DUS/HPO has been successfully demonstrated to remediate fuel hydrocarbons bound in the saturated and unsaturated soil matrix. Laboratory tests have been successful for a variety of volatile and semi-volatile compounds including diesel fuel and both light nonaqueous phase liquids (LNAPLs) and dense nonaqueous phase liquids (DNAPLs). A recent demonstration in California found DUS/HPO to degrade wood preservatives and polychlorinated biphenyls (PCBs). The minimum depth for application of DUS/HPO is approximately 5 feet.
Technology Development Status
DUS/HPO is being field tested at several sites. Additional data on long-term routine operating experience with DUS/HPO is needed to better plan future applications.
Other Resources and Demonstrations
See http://www.clu-in.org/download/contaminantfocus/dnapl/Treatment_Technologies/HPO-DUS-2005_ER-0014.pdf for cost and performance analysis of a demonstration at Beale Air Force Base in California.