ERH temperatures rise until the boiling point of water is reached (100
C at 1 atm
pressure). The temperature increase in soil-entrapped DNAPL occurs in four stages:
(1) DNAPL and water start to heat up, (2) DNAPL and water co-boil, (3) water heats
up, and (4) water boils (Burghardt and Kueper 2008; Munholland et al. 2016).
Generally, electrode spacing varies between 4.3 and 7.3 m. The closer the
electrodes are to one another, the higher the investment costs, but the shorter the
treatment durations (Beyke and Fleming 2005; Johnson et al. 2009). The vapor or
water extraction wells are either placed in the same unit as the electrodes or placed at
the center of the hexagonal or triangular array (Kingston et al. 2014).
Electrodes are typically placed directly in the core of pure pollution, preferably in
less permeable zones (i.e., the most resistant zones). Moisture is the primary
conductive vehicle; so when using ERH in unsaturated zones, having wetting
systems in place around the electrodes and in the remediation zone becomes
necessary.
Injection variants can be applied: (1) electrodes can be positioned at different
depths in a single well (or electrodes with different alloys and varying conductivity
can be placed in different lithological layers with specific thermal conductivities, to
target the zones to be treated); (2) sheet piles can be used to inject current over a
larger area; (3) electrodes can be placed vertically or horizontally; and (4) power can
be directed to a group of electrodes simultaneously or sequentially, to prioritize
certain treatment zones (Beyke and Fleming 2005; Johnson et al. 2009).
The advantages of ERH are as follows (Beyke and Fleming 2005; Buettner and
Daily 1995; Heron et al. 2005; Johnson et al. 2009; U.S. Army Corps of Engineers
2014; USEPA 2004):
• Even poorly permeable and very uneven zones can be treated since heat propagation is based on soils’ thermal conductivities.
• ERH is very well suited for clay and silt as they are better heat conductors.
• ERH can cause steam bubble flotation due to the formation of bubbles in the
water table, where increased NAPL thickness is observed; this NAPL can then be
recovered by a multiphase extraction system.
• Moisture in the unsaturated zone can be removed to increase soil permeability
particularly in zones with low permeability.
• In situ vapor generation facilitates the transport of volatile contaminants
(by convection).
• ERH allows the treatment of volatile and semi-volatile compounds (semi-volatile
compounds are difficult to treat with simple venting).
The drawbacks of ERH are as follows (Beyke and Fleming 2005; Dablow et al.
2000; Heine and Steckler 1999; Johnson et al. 2009; U.S. Army Corps of Engineers
2014; USEPA 2004):
• Efficiency is impacted by soil moisture and groundwater flow; indeed, dryer soil
reduces the technique’s performance which means that wetting systems have to
be installed; drying soil is generally observed above and near the electrodes.
3 In Situ Thermal Treatments and Enhancements: Theory and Case Study
187
C at 1 atm
pressure). The temperature increase in soil-entrapped DNAPL occurs in four stages:
(1) DNAPL and water start to heat up, (2) DNAPL and water co-boil, (3) water heats
up, and (4) water boils (Burghardt and Kueper 2008; Munholland et al. 2016).
Generally, electrode spacing varies between 4.3 and 7.3 m. The closer the
electrodes are to one another, the higher the investment costs, but the shorter the
treatment durations (Beyke and Fleming 2005; Johnson et al. 2009). The vapor or
water extraction wells are either placed in the same unit as the electrodes or placed at
the center of the hexagonal or triangular array (Kingston et al. 2014).
Electrodes are typically placed directly in the core of pure pollution, preferably in
less permeable zones (i.e., the most resistant zones). Moisture is the primary
conductive vehicle; so when using ERH in unsaturated zones, having wetting
systems in place around the electrodes and in the remediation zone becomes
necessary.
Injection variants can be applied: (1) electrodes can be positioned at different
depths in a single well (or electrodes with different alloys and varying conductivity
can be placed in different lithological layers with specific thermal conductivities, to
target the zones to be treated); (2) sheet piles can be used to inject current over a
larger area; (3) electrodes can be placed vertically or horizontally; and (4) power can
be directed to a group of electrodes simultaneously or sequentially, to prioritize
certain treatment zones (Beyke and Fleming 2005; Johnson et al. 2009).
The advantages of ERH are as follows (Beyke and Fleming 2005; Buettner and
Daily 1995; Heron et al. 2005; Johnson et al. 2009; U.S. Army Corps of Engineers
2014; USEPA 2004):
• Even poorly permeable and very uneven zones can be treated since heat propagation is based on soils’ thermal conductivities.
• ERH is very well suited for clay and silt as they are better heat conductors.
• ERH can cause steam bubble flotation due to the formation of bubbles in the
water table, where increased NAPL thickness is observed; this NAPL can then be
recovered by a multiphase extraction system.
• Moisture in the unsaturated zone can be removed to increase soil permeability
particularly in zones with low permeability.
• In situ vapor generation facilitates the transport of volatile contaminants
(by convection).
• ERH allows the treatment of volatile and semi-volatile compounds (semi-volatile
compounds are difficult to treat with simple venting).
The drawbacks of ERH are as follows (Beyke and Fleming 2005; Dablow et al.
2000; Heine and Steckler 1999; Johnson et al. 2009; U.S. Army Corps of Engineers
2014; USEPA 2004):
• Efficiency is impacted by soil moisture and groundwater flow; indeed, dryer soil
reduces the technique’s performance which means that wetting systems have to
be installed; drying soil is generally observed above and near the electrodes.
3 In Situ Thermal Treatments and Enhancements: Theory and Case Study
187