Low Temperature Thermal Technology: When to Use It and Why to Choose It

Thermal remediation is one of the leading approaches to cleaning up contaminant source zones, however, it has a reputation for being expensive. Traditional thermal remediation where the targeted treatment zone is heated to temperatures of 100C or greater, requires a sophisticated extraction and treatment system capable of handling contaminants, 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 and require substantial infrastructure (insulated vapor cover, extraction wells and manifold piping and extraction and treatment equipment), 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 remediation can be achieved using the same technologies utilized for 100C remediation, namely, thermal conduction heating (TCH) and electrical resistance heating (ERH).  For TCH, heaters are installed in the subsurface and heat is transferred radially from the heater through thermal conduction to the soil and for ERH, electrodes are installed in the subsurface and energy transferred to the soil by joule or resistive heating as electrical current is passed between the electrodes. Unlike traditional 100C TCH or ERH, 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 mass transfer mechanisms (i.e., contaminant availability) and naturally occurring biological and abiotic degradation mechanisms of targeted contaminants. Because most 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. To enhance biological degradation the subsurface is heated to between 30 and 40C, the temperature where microbes capable of degrading common COCs like chlorinated VOCs or petroleum hydrocarbons thrive (i.e., mesophiles). At temperatures much above 40C the enzymes responsible for degrading the COCs breakdown and the microbes go dormant or die off. Whereas, to enhance abiotic degradation mechanisms such as hydrolysis or chemical oxidation, temperatures of 70 to 90C are typically targeted. The higher temperatures result in more energetic molecular collisions and increased reaction rates (i.e., decreased contaminant half-lives). 

For low temperature thermal remediation, heaters or electrodes are installed on a regular pattern throughout and around the targeted treatment zone. The spacing between the heaters/electrodes depends on the target temperature, depth, and interval of treatment, rate of groundwater flux (if below the water table), and project schedule. Spacings of 12 to 18 feet between the heaters/electrodes are typical. The power input rates are substantially lower than with typical 100C thermal so as to gently and uniformly heat the targeted zone without generating significant amounts of steam and COC vapors.  As targeted subsurface temperatures are well below 100°C (i.e., the boiling point of water), 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 temperature to between 20 and 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 and brominated 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 chlorinated brominated 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 100C 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 when groundwater is present without compromising its effectiveness.

Why should you choose low temperature thermal technology?

There are several reasons to choose low temperature thermal remediation over traditional thermal or other remediation technologies. Perhaps the best reason is cost. Low temperature thermal heaters/electrodes are low-cost, while still robust. For example, TerraTherm’s patent pending low temperature heaters can be installed using less expensive direct push methods of drilling. Once installed, the heaters/electrodes 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 without excess generation of steam and COC vapors. Importantly, constant, 24/7 heating ultimately increases the uniformity, predictability, and effectiveness of our low temperature thermal projects, thus reducing the operating timeline and energy usage and decreasing the overall project cost.

Low temperature thermal remediation 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), incorporating low temperature thermal technology may also increase the reaction rates of the injectants being added, much like with naturally occurring 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 upcoming webinar titled "Understanding the Science and Strategy Behind Low Temperature Thermal Remediation.“ If you have any follow on questions or would like to talk with one of our experts, please contact us.