So you’ve done your homework and feel that thermal may be a good fit for your site. Now it’s time to determine the thermal technology or combination of technologies that will best accomplish cleanup goals. There are three predominant thermal technologies in the industry: Steam enhanced extraction (SEE); resistive heating in the form of electrical resistive heating (ERH); and thermal conduction heating (TCH), which combined with vacuum extraction is known as in-situ thermal desorption (ISTD). When selecting a thermal technology the following key items need to be understood:
Boiling points, vapor pressures, and water solubility of the contaminants:
- For volatile organic compounds (VOCs), the typical target treatment is the boiling point of water, or 100°C, although if the remedial goal is more relaxed, a lower target temperature may suffice.
- For Polycyclic Aromatic Hydrocarbons (PAHs), Polychlorinated Biphenyls (PCBs), Dioxins, and other high boiling Semi-Volatile Organic Compounds (SVOCs), the target treatment temperature is usually around 300°C.
- Lower-boiling SVOCs, such as naphthalene and chlorobenzenes can call for target treatment temperatures of between 100°C and 200°C, depending on how stringent the remedial goals are.
- If the only goal is to enhance recovery of a heavy NAPL such as coal tar or No. 6 Fuel Oil, a target temperature of 50-75°C may be sufficient to greatly reduce the NAPL viscosity.
- Unusual compounds may require a treatability study to determine effective treatment temperature.
The ability for compounds to thermally degrade:
- Ethanes can thermally degrade by hydrolysis at temperatures below the boiling point of water.
- Higher-boiling SVOCs can thermally decompose within the heated soil, at temperatures above 300 to 400C°.
- Lab treatability studies can also help determine thermal degradability for unusual compounds.
What are the clean-up criteria?:
- If the clean-up criteria are stringent then a more robust thermal technology may be required (ISTD), or a combination of technologies may be necessary (SEE and ISTD, or SEE and ERH).
- If clean-up criteria are less stringent then a suitable thermal technology or combination of technologies may be best suited (SEE, ERH or ISTD).
How does the setting and groundwater hydraulics of the site affect the need for groundwater controls?:
- Typically, treatment at 100°C can be accomplished below the water table without dewatering. Some measures to prevent groundwater flow from cooling the treatment zone may be needed, such as placement of additional heaters or multiphase extraction to flatten hydraulic gradients.
- If the target treatment temperature below the water table is above the boiling point of water, dewatering by one or more methods will be necessary.
What if the contaminated zone extends into bedrock?
- Typically thermal conduction heating (ISTD) is the best option for treating sites with DNAPL in weathered and/or competent rock.
- Use of steam around the perimeter of the targeted bedrock zone may be helpful if the groundwater flux is unpredictable or too high to otherwise effectively treat the site.
- Typically, bedrock can only be treated to 100°C because eliminating the groundwater can be tricky with unpredictable fracture locations and networks; and therefore, higher boilers are difficult to treat in bedrock below the water table, or may call for installation of a freeze wall.
The overall stratigraphy of the treatment area:
- Some thermal technologies are more sensitive to geology than others.
- SEE requires a relatively high hydraulic conductivity, i.e., it is not suited for lower-permeability settings.
- ERH require the presence of moisture and are most effective in silts and clays since they are less resistive. They are effective in many geologies provided groundwater flux is controlled.
- TCH/ISTD can be applied universally in saturated or unsaturated zones. It is effective in any geology and combinations of geologies provided groundwater flux is controlled.
Armed with all of this information, there still may be a concern on your part related to making an initial decision about thermal for your site. Have no fear, TerraTherm is here! We’d like to help you understand more about thermal and why or why not thermal would be effective at your site.
If you would like to learn more about our process please don’t hesitate to ask. We would be happy to set up a meeting, our tailor a webinar to your companies’ exact specifications.
If you have a site, please fill out a feasibility study by completing one of our site questionnaires, or call us at (978) 730-1200.