The
purpose of these web pages is to provide a context for evaluating treatment
alternatives by providing an overview of the types of compounds typically
encountered and liberated by soil vapor extraction (SVE) and from the treatment
processes applied to SVE emissions.
Contaminants and Vapor Streams Treated by SVE
Common
contaminants treated by SVE are those that readily vaporize at the ambient
temperature of the soil and atmosphere.
These include gasoline components and solvents such as benzene,
dimethylbenzene (xylenes), ethylbenzene, and toluene constituting the BTEX
compounds; dichloromethane, dichloroethane (DCA), trichloroethane (TCA or
commonly referred to as methyl chloroform), dichloroethene (DCE or commonly
referred to as dichloroethylene), trichloroethene (TCE or commonly referred to
as trichloroethylene), tetrachloroethene (commonly referred to as PERC or
perchloroethylene) constituting common chlorinated solvents; and Freon 113 (a
mixed chlorinated and fluorinated molecule).
The BTEX compounds from an SVE operation are commonly treated by various
oxidation processes, including those that can be biologically mediated, e.g., a
gasoline station leak. The
biodegradation of chlorinated compounds is more complicated than simple
hydrocarbons such as the BTEX compounds; hence physico-chemical treatment
systems are more commonly used for chlorinated solvents. Selection of a specific treatment process
has to be determined on a case-by-case basis, and depends upon many factors,
including the presence of other contaminants.
The number of viable treatment alternatives to SVE for contaminants in
the vadose zone is currently limited.
The
presence of other components in the SVE vapor stream, e.g., high concentrations
of water vapor, may make certain treatment processes less economic. For example a high water content in the air
stream, i.e., high relative humidity, typically will reduce the effectiveness
of carbon adsorption. Similarly, the
lower the concentration of the pollutant in the stream, the more costly the
removal of each unit of mass, e.g., cost per pound removed. The cost per unit of mass removed increases
rapidly as concentration decreases for all treatment systems. Typically for SVE processes, the
concentrations are initially higher and decrease with time.
Treatment Effectiveness - Destruction Removal Efficiency (DRE)
The
destruction and removal efficiency (DRE) of a pollutant is the term commonly
used as a measure of the effectiveness of a treatment technology in reducing
emissions to the air. The
concentrations of compounds encountered in SVE operations commonly ranges from
about 100 ppmv to 1000 ppmv in the gas. Thus a DRE of 90% would result in outlet
concentrations of 10 ppmv to 100 ppmv while a DRE of
99.9% would result in outlet concentrations of 1 ppmv to 10 ppmv. A larger DRE corresponds to greater of
removal of the compound from a contaminated stream and less discharge to the
atmosphere. In order to obtain emission
levels within an order of magnitude of typical background air concentrations,
DRE's of at least 90 to 99% or greater are commonly required.
All
chemical processes, whether natural metabolic activity of plants or animals or
human industrial activity, generate byproducts (or residuals) that are not
necessarily the desired end-product.
When SVE is applied for purposes of cleaning a contaminated soil the
control technologies applied generate a "byproduct" or
"residuals" stream. For
example carbon adsorption yields a contaminated carbon that needs to be
properly disposed of or treated, a biological treatment process typically
yields some form of wastewater or biomass (possibly live and dead
microorganisms) that may need to be discharged to a domestic sewer system for
treatment. Thermal oxidizers and
catalytic oxidizers besides producing the complete products of combustion
carbon dioxide and water (and hydrochloric acid in the case of a chlorinated
solvent contaminant), may produce trace amounts of other chemicals referred to
as products of incomplete combustion (PICs).
If biodegradation is incomplete, some of the pollutant molecules may be
dissolved in low concentrations in the wastewater stream. For example, in the biodegradation of MTBE
in groundwater, it has been observed that tertiary butyl alcohol may be formed
as an incomplete biodegradation byproduct.
A small amount of the original compound may also pass through the treatment
process and be discharged. Thus emission
of some byproduct is unavoidable and never zero. The possibility for exposure and the dose that might be received
should be a consideration when selecting among alternatives. The reader should keep in mind that there is
no single treatment technology solution that "fits" all situations.
A
potential byproduct of the treatment of chlorinated solvents are the class of
compounds known as polychlorinated dibenzodioxins (PCDD) and polychlorinated
dibenzofurans (PCDF). Ambient
measurements of these compounds are sparse, but levels appear to be decreasing
worldwide since about the mid-1970's.
In the mid-70's[1],
the 24-hr average concentrations of these compounds in Riverside, CA may have
been in the range 0.5 to 1.0 pg/m3 and during the late 1980's[2]
in San Bernardino, CA in the range of 0.1 to 0.5 pg/m3, expressed in
terms of toxic equivalents (TEQ). The
California Air Resources has embarked on a new two-year monitoring program at
sites within the San Francisco Bay Area and South Coast Air Basin beginning 2002. Interested readers are directed to the
following site for additional information Consumer Information:
Dioxins - What ARB Is Doing. For
more detail on recent tests of thermal and catalytic oxidizers, follow this link.
The
USEPA maintains a website where the public can access information about dioxins
in the environment and their effects on health. You may follow the link to that site National Center for
Environmental Assessment - Dioxin and Related Compounds or download a
Questions and Answers PDF file prepared by the USEPA by clicking here http://www.epa.gov/ncea/pdfs/dioxin/dioxin
questions and answers.pdf.
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[1] Leeman, W. et al. (2000). "Past and Present Contribution of Leaded Gasoline Combustion to the Global Dioxin Budget." Organohalogen Compounds, V46:338-341.
[2] California Air Resources Board (1990), "Ambient Concentrations of PCDDs/PCDFs (Polychlorinated Dibenzodioxins/Dibenzofurans) in the South Coast Air Basin."