Handbook of Solvents - George Wypych - ChemTech - Ventech!

1626 Barry J. Spargo, James G. Mueller
were combined to create an average DNAPL solubility of 620,000 mg/L (compared with a
TCE solubility of 1,100 mg/L in natural groundwater). The solution was injected over a period
of time, in an amount equal to 2.4 pore volumes in the test portion of the aquifer.
DNAPL removal was 99 percent (estimated) and surfactant recovery at the extraction wells
was 94 percent. Based on these results, a full-scale system was designed and implemented.
23.2.3.3 Foam-enhanced product recovery
The use of foams to remove heavy immiscible fluids such as DNAPL from soil was developed
by the petroleum industry for crude oil production. Subsequently, The Gas Research
Institute developed the use of foams to release and mobilized DNAPL contaminants in the
subsurface. Coupled with in situ or ex situ bioremediation, foam-enhanced product recovery
can, potentially, transport CHC contaminants upward in the groundwater, thus reducing
the potential for driving the contamination to previously non-impacted areas.
The use of foam for CHC DNAPL is currently viewed as experimental. The delivery
of the foam, its sweep front, the foam stability, its ability to release CHC DNAPLs in the
subsurface, and the resultant biodegradability of residuals can potentially be aided through
the proper selection of foaming agents and nutrients. In theory, the technology can tailor the
foam system to aerobic or anaerobic subsurface environments, depending on the selection
of the carrier gas. This allows adequate biodegradation for the particular CHC in the
foam-pollutant system. For example, DCE can be biodegraded aerobically, whereas PCE
needs to be degraded anaerobically.
23.2.3.4 Thermal desorption - Six Phase Heating
Six-Phase Heating TM (SPH) is a polyphase electrical technology that uses in situ resistive
heating and steam stripping to achieve subsurface remediation. The technology was developed
by Battelle’s Pacific Northwest Laboratories for the U.S. Department of Energy to enhance
the removal of volatile contaminants from low-permeability soils. The technology is
also capable of enhancing the removal of DNAPLs from saturated zones.
SPH uses conventional utility transformers to convert three-phase electricity from
standard power lines into six electrical phases. These electrical phases are then delivered
throughout the treatment zone by steel pipe electrodes inserted vertically using standard
drilling techniques. Because the SPH electrodes are electrically out of phase with each
other, electricity flows from each electrode to the adjacent out-of-phase electrodes. In situ
heating is caused by resistance of the subsurface to this current movement. In this manner, a
volume of subsurface surrounded by electrodes is saturated with electrical current moving
between the electrodes and heated. By increasing subsurface temperatures to the boiling
point of water, SPH speeds the removal of contaminants such as CHCs via three primary
mechanisms: increased volatilization, steam stripping, and enhanced residual mobility toward
extraction wells via viscosity reduction.
Once subsurface soil and groundwater reach the boiling point of water, the in situ production
of steam begins. Through preferential heating, SPH creates steam from within silt
and clay stringers and lenses. As this steam moves towards the surface, it strips contaminants
such as CHCs from both groundwater and soil matrix. Released steam can act as a carrier
gas, sweeping CHC out of the subsurface and to extraction wells. However, it can also
cause constituent migration and CHC displacement if the steam is allowed to condense prior
to extraction.