Your Comprehensive Guide to Understanding Thermal Remediation

Thermal remediation is a robust method that has been used to treat some of the world’s most heavily contaminated sites over the last two decades, but you may not be very familiar with it. Thermal remediation consists of the application of energy, in various forms, to heat a contaminated source zone and drive off or degrade subsurface contaminants.

The three thermal remediation technologies are:

  • Thermal Conduction Heating (TCH): applying electricity to heating elements installed in wells within the treatment area, which allows heat to dissipate conductively through the soil
  • Electrical Resistance Heating (ERH): applying electricity to electrodes that are installed in a uniform grid-like pattern in the subsurface and allowing the electricity to pass through the soil from one electrode to another below grade. The current flow between electrodes causes the soil to heat, due to the electrical resistance of the soil
  • Steam Enhanced Extraction (SEE): direct injection of steam into the subsurface via a network of wells with depth specific screened intervals typically 2 feet long. The steam entering the soil initially condenses at the steam injection well (SIW), transferring the energy and heat to the soil. 

We received the following real-life questions about thermal remediation technologies from attendees at our recent webinar, What is Thermal Remediation? The answers were provided by TerraTherm’s Erin Hauber, Senior Technologist and Alyson Fortune, Senior Scientist and will help you gain a better understanding of thermal remediation.

Can you give us a sense of the energy requirements of a thermal treatment system?

Regardless of the technology, the type of contaminant – volatile organic compounds (VOCs) or semi-volatile organic compounds (SVOCs) – dictates how much energy is required. SVOCs require a little bit more energy; we have to operate for longer to boil off a higher percentage of the water that’s stored in the pore volume. In terms of the energy density, between 200 and 245 kilowatt hours per cubic yard (cy) is a good rule of thumb, although it can vary. Higher groundwater velocity will need more energy. Lower cleanup goals means we probably operate a bit longer. But in general, that’s the energy that it takes for common VOC and SVOC sites.

How does thermal remediation work for smaller sites; is it economical or is there a minimum size limit for a treatment area?

Our minimum size is about 1,000 cy. We can go smaller, but cost does come into play because regardless of site size, because you have to mobilize a lot of equipment, and there are fixed costs that can’t be spread out or distributed across that wellfield. We’ve done pilots on the order of 500 to 650 cy, but the sweet spot is anything between 2,500 to 5,000 cy. The average size of our projects is between 10,000 and 20,000 cy.

Is there a volume threshold below which in situ thermal is not cost-effective? For example, I have a site that is 40’ by 40’, with DNAPL from 12 feet bgs to 20 feet bgs and water at 10 feet bgs.

This site is about 650 cy, which as mentioned above, is below the lower end of what we’ve established as cost-effective for thermal remediation. The diagram below shows volume and area on the left versus

cost per cy from four recent TCH projects. The smaller site with a volume of about 2,000 cy was $915 per cy. And then the largest site, about 46,000 cy, the cost was down to $136 per cy. So, that gives a sense for the impact of site size on the cost of a project.

Have you used thermal remediation to treat hard to access areas, such as beneath buildings?

Yes, TerraTherm does have experience with hard to access sites. More and more of the sites we see are below buildings and they have shallow water tables. We can make space for sweeping some of that contamination by putting a plenum on or raising the floor. That way, if there’s an occupied space above, or we need to maintain access to it, there’s both space to sweep those contaminants but also a safety feature to prevent someone from walking on top of a hot slab. Or, if you’re using ERH, there’s some space between an electrified wellfield and where people are walking. Horizontal drilling, whether that is angled or a horizontal directional drilling, lets us access these difficult treatment zones that have occupied spaces above them. We can go up to about 22 degrees from horizontal or do subsurface completions. In the photo below, you can see on the right the treatment zone underneath a building. We had angled borings at different degrees to access that area.

What are the potential offsite impacts to residential areas?

We have a lot of experience in operating sites next to occupied residences without any negative impact. We use real-time monitoring devices, including pressure monitoring points to make sure that we have pneumatic control and that vapors are not approaching nearby residences. This also gives us datapoints to demonstrate to concerned residents that we are not negatively impacting the vapor at the subsurface or their indoor air quality. We use a decibel meter to monitor the noise levels in the area. We also use weather stations noting wind speed and wind direction, photo ionization detectors (PIDs), and dust monitors. We put up a privacy fence if that makes sense. A lot of it is communication; the more coordination you can do up front on the design end, the better. We put the necessary protocols in place to make sure that everyone is satisfied that we’re not impacting the nearby community and their quality of life during the project.  

Have you done closed landfill projects?

We do have experience with landfills, but they are tricky and there are several caveats. The considerations that make landfills different are age and what it contains. A lot of debris will make it difficult to drill the infrastructure, or can make it hard to pass voltage, which could be an impediment to ERH. If the landfill contains a lot of Total Organic Carbon (TOC), that can cause flammable gases. If there’s a lot of gaps and spaces in the landfill, that’s a problem. These issues make it difficult to propagate heat, extract contaminants, and install infrastructure, and impacts our ability to operate with a comfortable level of competence and safety.

Landfills may also contain siloxane compounds. Siloxanes are found in a lot of personal care and other types of products. Once, we were on a project site that had siloxanes in the subsurface and we didn’t know it. Upon heating, the siloxanes were extracted in the vapor and went through our thermal oxidizer, which caused it to form silicon dioxide. In other words, the siloxanes became sand, which suddenly built up and caused our oxidizer to shut down.

Thermal remediation can potentially be a good fit for a decades-old landfill that’s predominantly full of soil. We have treated many pits where a contaminant had been dumped and then soil was backfilled. So we’ll look at landfill projects on a case by case basis, with many caveats.

What are thermal decomposition byproducts of concern with chlorinated VOCs?

There are known breakdown pathways and byproducts for common contaminants. For example, when starting with trichloroethylene (TCE), it breaks down to cis-1,2-dichloroethene and then eventually down to vinyl chloride. We see this routinely, and we monitor the process. If the contaminant of concern is very unique, we’re not familiar with it, and there’s no literature about what the breakdown pathway and products will be, we do treatability studies.

For the vapor recovery phase, is your system permitted by the SoCal Air Quality Management Districts for various locations? Or do you need to get a site-specific permit?

Permitting is typically site-specific because we design the treatment system for that site and its needs. We have certainly done plenty of work in California and in various air quality management districts in California. Each site will have its own air permit application, requirements, testing and monitoring requirements. Typically, we monitor the stack for VOCs using a handheld PID or sometimes a flame ionization detector (FID). We sometimes take analytical samples periodically, or there might be formal source tests involved. We will have a discharge permit of so many tons per year, which is way more than anything we come close to discharging because thermal remediation is a fast technology. We’re operating over six to ten months, period, so we don’t normally come close to the discharge limits.

How long will there be elevated volatilization of VOCs after heating is turned off?

Depending on the site and how quickly groundwater flux or air exchange is cooling down the site, it’s going to be hot for a while. Depending on the type of site, we build in a cool down phase anywhere from seven to 21 days after heating is turned off for continued vapor extraction. After we’ve met our endpoints, whatever those are, the site will be warm for a while, possibly even years, which can be to the benefit of the site. Increased bio activity can continue to attenuate the contaminants of concern, especially down in the plume.

We design the heating zone to envelop the treatment zone. Our extraction strategy is such that we are keeping all the vapors within that treatment zone and we’re maintaining pneumatic hydraulic control. In the above ground vapor treatment system, we use insulation, heat trays, and duct heaters to knock out that condensate above grade.

How do you avoid condensation vapors in cooler zones?

We design the heating zone to envelop the treatment zone. Our extraction strategy is such that we are keeping all the vapors within that treatment zone and we’re maintaining pneumatic hydraulic control. In the above ground vapor treatment system, we use insulation, heat trays, and duct heaters to knock out that condensate above grade.

Have changes to soil properties been studied post-thermal remediation; for example, do tight soils become more permeable?

In typical applications where we’re heating to 100° C, we’re not desiccating the subsurface, so we’re not changing the moisture content. Let’s say we boil off 30% of that pore volume, then water re-infiltrates or can continue to backfill. In most situations, we’re not changing the soil properties. If we’re treating and heating a peat stone, something with a high organic carbon, it can lead to settlement as some of that peat is transformed into other products. So there are special considerations where the soil properties may change, but largely, we don’t see subsidence as an issue on our sites. There are some bench scale study graphics that show higher permeability, because we are creating some micro fractures, which is one of the mechanisms for recovering some vapors. But we don’t see much change on a macro scale. Of course, we’re talking about in situ treatment. For an ex situ high temperature application, the soil properties may indeed change because we are drying out the soil completely. It’s not a concern because the soil is already ex situ; there are no issues with settlement or future use on the site.

Of the three methods, which can be effectively employed for the Vadose (unsaturated) zone versus the saturated zone?

All three technologies, ERH, TCH, and SEE, work in both the Vadose zone and the saturated zone. With ERH, we make sure that we maintain a certain minimum moisture content because that’s what’s going to help maintain the voltage and that passing of the current between electrodes. We manage that hydration with drip lines.

Have other questions about thermal remediation, or want to learn more about the three technologies? You can watch the complete webinar on-demand: What is Thermal Remediation? Or contact us to schedule a time to chat.

About the Authors

Erin Hauber, Senior Technologist

Erin Hauber has over 15 years of experience as a remediation engineer in the environmental industry, focusing on development of remedial treatment strategies with an emphasis on in situ thermal remediation and injection-based technologies.

As Senior Technologist at TerraTherm, Erin supports site evaluation to determine if thermal is appropriate and if so, the technology and approach best suited to cost effectively meet a client’s remedial goals. As technical director for ongoing projects, Erin is responsible for supporting the subsurface design and monitoring operational data to ensure project goals are met on time and on budget.

Alyson Fortune, Senior Scientist.

Alyson Fortune joined TerraTherm in 2012 as a Senior Scientist with over 15 years of experience in the environmental industry, including experience with source testing, analytical laboratory testing, and environmental consulting. Alyson is the data quality/operations and compliance monitoring discipline lead for the Engineering department. In this role, she is responsible for developing and implementing sampling and analytical plans and quality assurance project plans (SAP/QAPPs), managing thermal remediation treatability studies and project laboratory interactions, conducting data quality reviews on laboratory data, maintaining complex field equipment monitoring systems (e.g. FTIR), and performing other data management functions.


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