PROCESS DESCRIPTION (ISTD)

General Description 

A remarkably robust remediation process, In-Situ Thermal Desorption (ISTD) is simple, fast, and applied in-situ; meaning that one does not have to move or disturb the soil.  The operations are low profile, quiet, and cause little disruption to adjoining communities, commercial centers, or industrial operations.  

Technical Description

ISTD’s primary application uses thermal wells, along with heated extraction wells, which can be placed to virtually any depth in virtually any media.  Heat is applied to soil from a high-temperature surface in contact with the soil, so that radiation and thermal conduction heat transfer are effective near the heater.  As a result, thermal conduction and convection occur in the bulk of the soil volume.  Overall, thermal conduction accounts for over 80% of the heat transfer.  A significant feature of the ISTD process is the creation of a zone of very high temperature (>1000 degrees F) near the heaters, which can oxidize or pyrolize contaminants.  A soil vapor extraction system is used to remove contaminants.  ISTD is essentially a batch treatment process. 

Two ways of applying the ISTD technology

One approach is to accomplish thermal desorption of Semi-Volatile Organic Compounds (SVOCs), using a high-temperature, desiccation approach.  The other primary approach is to accomplish the thermally-enhanced recovery of Volatile Organic Compounds (VOCs), using a low-temperature, non-desiccation approach.  

1. Thermal Desorption of SVOCs -  For high-boiling compounds such as PAHs, PCBs, pesticides, and dioxins, ISTD is used to boil off the soil moisture, dessicate the soil, and volatize the contaminants at temperatures close to their boiling points.  In such cases, control of groundwater recharge into the heated zone may be required.

2. Thermally-Enhanced Recovery of VOCs - It is not necessary to boil off all the water at such sites.  ISTD can be employed to simply raise the soil temperature within most of the treatment volume to the boiling point of water, generating steam in-situ.  This results in steam distillation of the contaminants, similar to steam flooding or electrical resistance heating.  The major differences between this form of ISTD and those technologies are the in-situ thermal destruction that occurs as vapors are drawn into the hot regions in close proximity to heater-vacuum wells, and the enhancement of gas permeability and vapor capture that occurs in such regions.  The result is a lower operating cost and a significant reduction in risk of mobilizing contaminants outside of the treatment zone.  This non-desiccation type of ISTD can be accomplished with much more widely-spaced heaters and simpler off-gas treatment equipment, and thus, is much less expensive than ISTD treatment of high-boiling compounds such as PCBs.   

TerraTherm's 3-Level Heating Approach for Treatment of Manufactured Gas Plant (MGP) Wastes

The flexibility with which TerraTherm can apply heat to a site is particularly applicable to remediation of MGP sites, as well as other sites with heavy NAPLs, including creosote and No. 6 Fuel. Depending on your goals, we can apply heat gently, moderately, or more aggressively - we term these three general approaches Level 1, 2 and 3, respectively. Level 1: Gentle heating can be used to accomplish Thermally-Enhanced Free-Product Recovery, enabling extraction of NAPL such as coal tar that when cold is too viscous to recover. Level 2: Moderate heating, e.g., to near the boiling point of water, can be used to steam-strip VOCs such as benzene and naphthalene from MGP waste, and stabilize the remaining, higher-molecular weight PAHs in an immobile form, consistent with common risk-based solutions. Level 3: Aggressive heating can be used to thoroughly treat the MGP constituents. This approach is identical to that used to describe treatment of SVOCs, above. At some sites it can make sense to apply two of these levels of treatment sequentially. For more information on these options, refer to "Levels of ISTD Treatment: MGP Waste."

Full Scale - Major Unit Process

Ensures Effective Capture of Vapors and Prevents Unwanted Contaminant Mobilization

ISTD employs the following features to ensure that heated vapors are captured and that contaminants are not mobilized outside of the treatment zone:

• The very hot (>100^oC) and dry zone of soil surrounding the heater-vacuum wells results in significant increases in air permeability (2 to 4 orders of magnitude increase) due to the removal of pore water, desiccation cracking, and increases in porosity.  This ensures that steam and contaminant vapors formed at the leading edge of the heat front are pulled back under vacuum through soil of increasing temperatures and collected by the heater-vacuum wells.
• A ring of heater-only wells is installed around the perimeter of the treatment zone (i.e. outside of the area of contamination) thereby creating a heated vapor barrier that prevents condensation of steam and contaminant vapors outside of the treatment zone.  Heaters also extend vertically beneath the lower boundary of the treatment zone for the same reason. 
• Because thermal conduction heating is independent of soil moisture content, soil type, and heterogeneity, ISTD can uniformly heat both saturated and unsaturated regions, thus ensuring the creation of a heated vapor plenum above the target treatment zone.  
• As the soil is heated and dried around a heater-only well, steam and contaminant vapors are conveyed upwards through the hot dry soil around the heater-only well and captured by the overlying vapor plenum (i.e., captured by the heated unsaturated zone and/or surface vapor barrier that is maintained under a negative pressure).

Thermal Blanket Approach

Shell-TerraTherm originally developed the thermal blankets as rectangular, rigid units that were arrayed modularly to treat shallow soil contamination (e.g., PCBs, less than 3' deep). Most of our current work utilizes vertical thermal wells to treat deeper contamination. We have found that it is far more cost-effective to approach sites with shallow contamination using long, horizontal heaters similar to our thermal wells, except that they are laid out horizontally in parallel lines. In a typical installation, for example, to treat soils contaminated with PCBs to a depth of 2 ft, the heaters are installed at a depth of approximately 1.5 ft, thereby positioning the heaters at three-quarters of the treatment depth. The heater elements are placed within stainless steel pipes and laid in each narrow "DitchWitch" or backhoe-excavated trench, after which the trench is backfilled with soil from the excavation. Heaters are spaced at an appropriate distance to ensure uniform heating and efficient duration of the treatment process.  The exact spacing is defined during the design process. We believe this approach to be more cost-effective than earlier ISTD Thermal Blanket designs you may have seen.

After installation of the heaters, the ground surface is covered with a surface seal that has a low permeability to both vapors and rain.  Vapors generated during heating are extracted from beneath the surface seal and conveyed under vacuum from the ISTD well field to the off-gas treatment equipment trailer.  The off-gas treatment system, which is sized to treat the expected vapor and steam flow, consists of a flameless thermal oxidizer, acid gas removal media, and granular activated carbon, a configuration that has been demonstrated to exceed the 6-9s (i.e., 99.9999%) Destruction and Removal Efficiency required under TSCA.