Thermal technology is one of the leading technologies in the remediation world—but it has a reputation for being expensive. Traditional thermal remediation requires a sophisticated extraction and treatment system capable of handling the COCs, air, steam and water removed from the subsurface. These systems consume a substantial amount of energy, usually in the form of electricity or natural gas, which makes it expensive when compared to monitored natural attenuation (MNA) or injection technologies. But what if there was another, less expensive thermal option?
Today’s blog post will present a different approach to thermal remediation – Low Temperature Thermal – and answer the three main questions surrounding the technology: how does it work; when would you use it; and why should you choose it?
How does low temperature thermal work?
Low temperature thermal technology utilizes the same concepts as thermal conduction heating (TCH), where heaters are installed in the subsurface and heat is transferred radially from the heater through thermal conduction to the soil. Unlike traditional TCH, low temperature thermal aims to gently heat the target treatment volume to temperatures between 30 and 90°C, below the boiling point of water. By heating within this temperature interval, low temperature thermal aims to enhance naturally occurring biological and abiotic degradation mechanisms of targeted contaminants. Because chemical reactions are temperature dependent, gently increasing the temperature in the subsurface will increase the biotic and abiotic mechanistic reaction rates, which will, in turn, lead to a more rapid removal of contaminants and a shortened remediation timeline.
Small diameter heaters are installed using direct push methods on a regular pattern throughout and around the targeted treatment zone. The spacing between the heaters depends on the target temperature, depth, and interval of treatment, rate of groundwater flux (if below the water table), and project schedule. Heater spacing of 12 to 18 feet are typical. Heat efficiently and uniformly moves out radially around the heaters by thermal conduction (i.e., transfer of energy from the heater directly to the soil). As targeted subsurface temperatures are well below 100°C, separate extraction wells and extraction and treatment of steam, contaminant vapors, air, and groundwater are not required.
When might low temperature thermal be used?
Low temperature thermal may be a good fit for projects where aerobic and anaerobic biodegradation and/or abiotic pathways, such as hydrolysis, are the remediation pathway of interest. These degradation pathways may be of interest when the targeted contaminants are:
- Petroleum hydrocarbons: Increasing the treatment volume temperature to between 30 and 40°C can enhance the aerobic biodegradation reaction rates of petroleum hydrocarbons by three to four times, which will reduce the biologically mediated cleanup timeline by a similar factor.
- Chlorinated volatile organic carbon (CVOCs): Gently increasing the treatment volume to 20-30°C will encourage the anaerobic biodegradation pathways of CVOCs, particularly chlorinated ethenes, increasing the anaerobic biodegradation reaction rate and effectively reducing the time it takes to reach site closure measures.
- Chlorinated ethanes: Increasing the temperature within the treatment volume to between 60 and 90°C can substantially increase the abiotic pathway (hydrolysis, for example) reaction rate of select chlorinated ethanes. For instance, carbon tetrachloride has a hydrolysis half-life of ~20,000 years at 15°C. By increasing the temperature to 90°C, the hydrolysis half-life is reduced to ~100 days.
Additionally, low temperature thermal technology may be a good fit for a site that is targeting those contaminants mentioned above that also has above ground space and use limitations. Because low temperature thermal technology utilizes a degradation pathway, vapors are not generated as is the case with traditional thermal; therefore, no extensive above ground process treatment system and piping is necessary, and the overall footprint of the treatment is relatively small. The wellfield can be installed in an unobtrusive manner in the background, allowing for a site or facility to continue to operate while the treatment is ongoing. Finally, low temperature thermal technology can work for sites with challenging lithology or groundwater present without compromising its effectiveness.
Why should you choose low temperature thermal technology?
There are several reasons to choose TerraTherm’s Low Temperature Thermal Technology over traditional thermal or other remediation technologies. Perhaps the best reason is cost. Low temperature thermal heaters are a low-cost, while still robust, heater. They can be installed, along with their well materials, using less expensive direct push methods of drilling. Once installed, the heaters can be operated unobtrusively 24/7, and the heat output can be individually adjusted, which allows for a faster and more uniform heat-up of the treatment volume. This constant, 24/7 heating of TerraTherm’s Low Temperature Thermal electric heaters ultimately increases the uniformity, predictability, and effectiveness of low temperature thermal, thus reducing the operating timeline and energy usage and decreasing the overall project cost.
Low temperature thermal technology can also be considered a sustainable technology due to its reduced power usage and ability to use alternative, renewable energy sources, such as solar or wind, to power the heaters. Additionally, because the heaters, electrical cables, and other equipment used during low temperature thermal treatment are reusable, the overall carbon footprint of low temperature treatment is significantly less than traditional thermal treatment.
For projects where amendments are added to the subsurface using remediation technologies like in situ chemical oxidation (ISCO) and in situ chemical reduction (ISCR), incorporating low temperature thermal technology may also increase the reaction rates of the injectants being added, much like with biological or abiotic degradation. Increasing those reaction rates can help increase the efficiency of injection remediation and reduce cleanup times, leading to reduced overall project costs.
While low temperature thermal remediation will not work in every situation or for every remediation goal, when abiotic and biological degradation is the targeted mechanism of removal, either natural or with amendments, low temperature thermal should be considered as a potential remediation technology. As described above, low temperature thermal provides many advantages and may just be the best choice for a project that aims to reduce remediation timelines while lowering cost and taking a big step toward more sustainable remediation.
If you’d like a deeper dive into low temperature thermal, check out our archived webinar titled Low Temperature Thermal: The Sustainable Approach. There we dove into the technology, provided examples of situations where low temperature thermal may be a good fit, and provided a cost analysis of low temperature versus traditional thermal technologies.
Hillary Easter is one of our chemists and is managing sampling logistics and laboratory analytical data for several thermal remediation projects. Hillary is also serving as a Data Manager on several thermal projects and oversees the operational data and mass energy balance associated with thermal remediation projects. Before joining TerraTherm in 2018, Hillary worked for…