CVOC Remediation

Target Temperature: 100°C 

Removal Mechanism: Volatilization/Co-Boiling of the DNAPL and Phase Transfer

Unlike traditional pump-and-treat or SVE systems, thermal remediation methods can directly target DNAPL zones and overcome the mass-transfer limitations associated with the presence of DNAPL in low-permeability soils or fractured bedrock. TerraTherm’s thermal technologies thoroughly heat the impacted zones to co-boil the DNAPLs and volatilize the CVOCs in situ for vapor-phase extraction and above ground treatment. In some cases, partial in situ degradation occurs at elevated temperatures. This approach significantly reduces cleanup timeframes and long-term liability.

This approach can be used to cleanup CVOC DNAPL source zones in 12 to 18 months and achieve low remedial standards such as 0.1 mg/kg in soil or 0.05 mg/L in groundwater.  

Note, lower concentration goals can be achieved (e.g., MCLs); however, thermal remediation is often used to target CVOC source zones with the highest concentrations and most mass, and not the surrounding areas with low to moderate concentrations. Thus, it is often impracticable to treat the source zone to very low concentrations, when mass flux into the source zone will prevent uniformly achieving the low concentration goals or result in future rebound.

Treatment goals, soil type and organic carbon content, presence of co-mingled compounds such as petroleum hydrocarbons, chemical properties of CVOCs, as well as treatment zone heat losses determine how much energy needs to be delivered to the target treatment zone and the duration of treatment. For example, if very low treatment goals are required for PCE (co-boiling point with water of 88.5°C) and the site has high organic content, then a greater energy density and pore volume boil-off, such as 35 to 40% of the water content of the treatment zone may be required. Higher treatment goals in low organic carbon soils for CVOCs with a lower boiling point such as TCE (co-boiling point with water of 73°C), may only require energy to boil off 25 to 30% of the water content.

Target Temperature: 70 to 90°C 

Removal Mechanism: Hydrolysis

For some CVOCs, such as 1,1,1-TCA, DCA, carbon tetrachloride (CT), and dichloromethane (DCM), their hydrolysis degradation rate will increase by several orders of magnitude when heated from ambient temperatures (e.g., 20°C) to 70 to 90°C. For these chemicals, even when present as DNAPL, heating to temperatures between 70 to 90°C, but below the boiling point of water, can be sufficient to reach low remedial goals at a site in 6 to 8 months.  

Target treatment zones are typically heated to 70 to 90°C in 90 days. Once at the target temperature, the power required to maintain temperature is often 1/3 of what is required for initial heating. This approach also has the benefit of not requiring a vapor cover or extraction and treatment system, which greatly reduces costs and site access requirements. For example, the heater wells can be installed below grade allowing normal access and operations in buildings and active work areas or transportation corridors. The combination of low-profile heaters, low power output, low target temperature, and low maintenance power requirements, and no need for a vapor cover or extraction and treatment system, makes this a very sustainable and low-cost approach for treating CVOC DNAPL source zones with amenable contaminants.

Target Temperature: 35 to 40°C 

Removal Mechanism: Biological Degradation

Increasing subsurface temperatures to between 35 and 40°C can increase the biological degradation rate of CVOCs by 2-3 times, resulting in shortening the overall remedial timeframe and reducing the off-site mass flux rate. This approach uses specially designed low-profile/low-power heaters that can be installed in 2-inch diameter pipe and used to gently and uniformly heat the targeted treatment zone to the optimal temperature for biodegradation. The small diameter heaters can be easily installed in direct-push borings, thus saving time and money if coordinated with amendment injections (e.g., EVO or EZVI). 

Target treatment zones are typically heated to between 35°C and 40°C in 90 days. Once at the target temperature, the power required to maintain temperature is often 1/4 of what is required for initial heating. This approach also has the benefit of not requiring a vapor cover or extraction and treatment system, which greatly reduces costs and site access requirements. For example, the heater wells can be installed below grade allowing normal access and operations in buildings and active work areas or transportation corridors.  The combination of low-profile heaters, low power output, low target temperature, and low maintenance power requirements, and no need for a vapor cover or extraction and treatment system, makes this a very sustainable and low-cost approach. 

Our Heating Solutions for CVOCs

Electrical Resistance Heating (ERH)

Ideal for moist, heterogeneous soils (e.g., silty, clayey sands and clays),  ERH can be used to heat subsurface areas to approximately 100°C, boiling DNAPL and volatilizing CVOCs like PCE, TCE, TCA and DCA for vapor-phase extraction. As ERH relies on the flow of current between electrodes placed in and around the target treatment zone to heat the soil and groundwater, ERH requires sufficient soil moisture and electrical conductivity.  For example, performance may be reduced in dry formations unless moisture can be practically and effectively maintained through water addition at the electrodes. In some resistive formations (e.g., fractured granite and dry sand), adding water to the formation to maintain power input is not effective. Soil resistivity should also be incorporated into the design to ensure uniform heating, as the electrical resistivity encountered in common geologies can vary by a factor of 200.

For 100°C treatment approaches, vapor extraction wells, either co-located with the electrodes, or separate vertical or horizontal wells are required to maintain pneumatic control during treatment. At some sites with sufficiently high groundwater flux rates, multiphase extraction wells may be used to remove vapor and water to provide both pneumatic and hydraulic control during heating.

ERH can also be used to gently and uniformly heat sites to 35 to 40°C for thermally enhanced biodegradation or 70 to 90°C for thermally enhanced hydrolysis. 

Steam Enhanced Extraction (SEE)

Injects steam into the subsurface to co-boil DNAPL and strip CVOCs from saturated soils, or fractured rock with sufficient permeability to prevent excess groundwater flow and cooling. Generally, SEE is best suited for lithologies with an effective hydraulic conductivity of 1 x 10-3 cm/s or higher (e.g., sand and/or gravel formations). When properly designed, SEE can inject high rates of energy and quickly and uniformly heat and treat CVOC sites. If site conditions are amenable and subsurface permeabilities high enough, SEE is often the most cost-effective way of heating and treating CVOC DNAPL source zones due to the wide spacings that can be used between the steam injection wells, the high energy input rates, and typically low cost of the fuel for producing the steam paired with local availability of boilers. 

Because steam is injected into the subsurface under pressure at temperatures >100°C, it is not a good candidate for gently heating sites to temperatures <100°C for thermally enhanced biodegradation or hydrolysis.  

In addition, because the steam is injected under pressure and can condense as water, an aggressive network of multiphase extraction wells is required to extract both vapors and liquids (water and NAPL) to maintain pneumatic and hydraulic control during treatment.

SEE can be combined with ERH or TCH to treat sites with both low and high permeability zones that have low and high groundwater flux. SEE is often paired with ERH or TCH when groundwater flux within a portion of the target treatment zone exceeds 1 ft/day. 

Thermal Conduction Heating (TCH)

Effective in all soil types, regardless of moisture content (wet and dry, above and below the water table) and especially effective in fractured rock. TCH relies on thermal conduction of energy from a heater into the surrounding soil or rock. TCH provides highly uniform and predictable heating because the thermal conductivity of most sites only varies by a factor of 2 to 3.  

TCH can be used to uniformly heat soil and rock to the full range of treatment temperatures required for the various CVOC treatment approaches: 35 to 40°C for thermally enhanced biodegradation, 70 to 90°C for thermally enhanced hydrolysis, and 100°C for volatilization and co-boiling of DNAPL. Importantly, at some sites with low permeability soil (e.g., silts and clays), the soil immediately surrounding the heaters (e.g., 6 inches) will dry out, which can provide beneficial pathways for volatilized CVOCs to migrate from deeper soils to the vadose zone where they can be effectively captured and removed for treatment.

For 100°C treatment approaches, vapor extraction wells, either co-located with the heaters, or separate vertical or horizontal wells are required to maintain pneumatic control during treatment.  At some sites with sufficiently high groundwater flow rates or free phase NAPL present, multiphase extraction wells may be used to remove vapor, NAPL, and water to provide both pneumatic and hydraulic control during heating and recover mobile NAPL. 

TerraTherm’s engineers customize every remediation approach based on site-specific characteristics and our client’s remediation objectives. We assess:

• Soil type and permeability
  
  
• Contaminant depth and distribution
  
  
• Contaminant type and presence of DNAPL and co-contaminants
  
  
• Starting contaminant mass
  
  
• Site access limitations and surrounding land use
  
  
• Regulatory targets and timeline constraints
  
  

For sites with CVOCs like PCE, TCE, TCA or DCA, TCH or ERH are both very good options for most geologies. Selecting one or the other depends on site specific conditions such as soil electrical resistivity, soil moisture, depth of treatment, and need to treat bedrock. SEE is a very good option for permeable sites with high groundwater flux, i.e., >1 ft/day, and we will often combine SEE with TCH or ERH to effectively heat and treat sites that have both low and high permeability zones.  

Ready to Address CVOCs at Your Site?

Our team has successfully remediated some of the most complex chlorinated solvent sites in the U.S. Using field-proven thermal technologies and data-driven design, we help site owners and consultants achieve regulatory closure faster.

Talk to a Thermal Remediation Expert

 Explore Our SVOC & PAH Remediation Technologies:

• Electrical Resistance Heating (ERH)
  
  
• Steam Enhanced Extraction (SEE)
  
  
• Thermal Conduction Heating (TCH)