With so many options for remediation out there, can In Situ Thermal Remediation (ISTR) provide a solution for your site? And how do you know which thermal remediation approach is best?
ISTR has three major approaches: Thermal Conduction Heating (TCH), Electrical Resistance Heating (ERH), and Steam Enhanced Extraction (SEE). These methods, alone or in combination, cover just about every possible scenario, so you can find a solution that’s right for your particular site needs.
In TCH, electric heater elements installed in wells within the treatment area are used to heat the soil via thermal conduction. With ERH an electrical current is applied into the subsurface between sets of electrodes and the resistivity of the soil generates heat. SEE involves directly injecting steam into the subsurface through screened wells.
We received these real-life questions about when to use thermal remediation and which ISTR approach makes sense from attendees at our webinars, Introduction to In Situ Thermal Remediation Part 1 and Part 2. The answers were provided by Steffen Griepke, Vice President of Technology, and Nikole Huard, Senior Project Engineer.
Under what circumstances is thermal treatment more cost-effective than other remediation technologies?
High Contaminant Mass
A number of conditions make ISTR the most cost-effective approach, high contaminant mass being one of them. If your site is too deep and can’t be excavated cost-effectively, that’s also a sweet spot for ISTR.
When the site is located under existing infrastructure – buildings, roads, utility lines, anything that can’t be temporarily removed – thermal is often selected simply because we can do it in situ, underneath existing structures. We can even install the wellfield piping and electrical connections below grade or angled from the outside, if we need to treat under an operational manufacturing facility, for example.
Speed to Completion
If schedule is the driver – for example, if the client is a developer who can’t do anything with the site until environmental concerns are removed – that’s also a sweet spot for thermal technologies because the certainty of getting to the clean-up goals within a specific timeframe is high.
Are there any depth limitations to applying thermal technologies?
In-situ thermal heating technologies were originally developed for use in the oil industry and have been employed to heat soil and rock to thousands of feet deep, so in general, depth is not an issue for ISTR applications. TerraTherm’s experience includes a TCH deployment at a bedrock site down to 170 feet and SEE on sites as deep as 250 feet below grade.
The key is reaching the bottom of the contamination. ERH, however, is not typically a good solution for deep sites (over 100 ft deep) like these because of the number of stacked electrodes required. Long electrodes come with a risk of heating up the site differently at depth, so we typically stack electrodes for a deeper site, which may require larger bore holes, all adding to the cost. The required infrastructure in the ground makes up a large segment of the total cost for thermal remedies. You may be surprised to know that power is typically only 10 to 15% of the total cost of a thermal remediation project.
What soil matrix is most conducive to treatment by thermal technologies?
Thermal technologies work with most geologies.
TCH and ERH are best for low permeable sites. Steam is best for high permeable geologies, especially when we are looking to battle high groundwater flux, which cools the site.
Sometimes TCH and ERH can be used in high permeable settings; even saturated sand and gravel can be a perfect fit, as long the groundwater is not moving too fast.
Our rule of thumb is that if the groundwater flux is more than one foot per day, then neither TCH nor ERH alone will be enough to heat the site, without groundwater flow control measures. We can add more energy using ERH as compared to TCH, so ERH is a little more robust when it comes to sites where there’s a higher flow velocity of water. But if the groundwater flux is too high, then the best and most cost-effective approach will be SEE. Steam easily offsets the heat losses caused by rapidly moving water. Also, when we inject steam, we create a dual-phased system, combining the steam, the vapors, and the cold water, and that also blocks off the water.
When we get into bedrock settings with a fairly high porosity (sandstone, mudstone, limestone), ERH and TCH can both be good options. However, TCH is the best choice for low permeable bedrock types, such as granite, with porosity below two to four percent.
For saturated zone treatment, under what conditions does thermal conduction heating have an advantage over electrical resistance heating?
In saturated zones, both TCH and ERH are good options. It often comes down to efficiency, which is typically related to the homogeneity of the geology.
If the site has layers or pockets with different soil types and resistivity, ERH won’t be as effective because some layers or pockets will heat up faster than others. TCH would be the better and more cost-effective approach, as heating is much more uniform, regardless of soil type. ERH is a good choice when the soil is homogenous and the site is shallow.
How effective is in situ thermal remediation in extreme cold, such as Arctic sites?
The challenge in cold climates is that the subsurface starts out colder than normal, so it may take longer to achieve the desired treatment temperatures, but it can absolutely be done. In many cases, we use insulated vapor covers at the surface to help mitigate any heat losses. On the treatment side, we winterize the equipment, using heat trace and insulation or heated enclosures, sometimes even tent-like structures. For example, we might house our liquid treatment equipment inside a container or tent to keep wind and ambient conditions off the treatment equipment. The goal is to prevent freezing and keep the liquids and vapors running. We work all year round and have worked around the globe in both extreme heat and extreme cold. The key is to design your system components for the environment in which it will operate.
What is the best ISTR technology for a very high groundwater flow aquifer?
In most cases, we would use SEE in this situation. TCH and ERH aren’t usually good candidates because they have a hard time overcoming the loss of heat due to the groundwater flow (see the answer above about high permeable settings and the speed of groundwater flux). In some cases, we choose to actively pump groundwater or to install some sort of a physical barrier such as a sheet pile wall that helps control groundwater flow into the treatment zone, so that it doesn’t impact our heating operations.
How does the size of the initial mass of contaminant impact the design of an ISTR system?
We use the total contaminant mass to select the best and most cost-effective treatment system. A low mass site – less than ten thousand pounds – calls for an activated carbon system. If the contaminant mass is between ten and forty to fifty thousand pounds, a steam regenerated carbon system may be the way to go. For high mass sites, greater than fifty thousand pounds of contaminants, we’re typically looking at a thermal oxidizer or accelerator, where the cost is not a direct factor of the amount of mass pulled out of the ground. High mass sites with chlorinated solvent contamination require the use of an acid-gas scrubber after the thermal oxidizer to prevent release of hydrochloric acid to the atmosphere.
What is hot soil sampling and why is it important?
Before we shut down an ISTR system, we need to know if we’ve met the treatment goals. We use hot soil sampling to monitor the performance. We collect soil samples in situ, cap and cool them at the surface, then send them to a lab. When the results come back, we know if we’ve met the goals.
We prefer to evaluate sites based on soil concentrations rather than vapor or groundwater concentrations because vapor and liquid can move, and with differences in equilibrium when conducting thermal remedies, liquid and vapor may not tell the full story of what remains at the site.
When is heating the soil with an ISTR system not the right option for chemical contamination remediation?
For some sites that are lower in contaminant mass, or lower concentrations, such as a dilute groundwater plume scenario, ISTR is not usually cost-effective. You’ll spend a lot more money per unit of treatment area, compared to less aggressive remediation methods.
We’re also hesitant to use a thermal approach in high organic geology conditions, such as peat layers, particularly around structures or below buildings. We’d be concerned about degradation of that material and possible consolidation or shifting of soil conditions within the treatment zone, which obviously can create issues if you’re operating below a building.
Some chemical contaminants require high temperature treatment and if they are located below the water table, that’s another situation where ISTR (alone) may not be the answer. From a technical standpoint, if it’s not feasible to totally dewater a site to get to the higher target temperature, it may pose a challenge for ISTR.
What is the average operational timeframe for thermal remedies?
Anywhere from 8 -12 months. Of course, this can vary depending on the application.
Prior to start-up, the construction period can take 2 – 3 months depending on the size of the site, sometimes longer for very large sites.
It generally takes 4 – 6 months from the time we turn on the heat until when we have confirmed that the treatment has been successful. The larger the site, the more we spread out the wells to save on infrastructure, and the longer it takes to remediate.
Once we confirm remediation is complete, we have to de-mobilize the site, remove the equipment and restore the site. That’s probably another 2 – 3 months.
So in total, 8 to 12 months on site for a typical site. Very large sites can take up to 18 months due to the long construction and demobilization periods.
Have other questions or want to learn more about thermal remediation?
Nikole Huard is a Chemical Engineer and is one of the lead Project Engineers at TerraTherm. During the 7 years she has worked at TerraTherm, she has been designing thermal systems, managing operational data from the field, writing technical work plans and reports, collecting field samples, and coordinating engineering and field efforts during the design,…