Addressing Complex Remediation Sites
With the onset of improved technology and understanding of remediation processes, sites of increasing complexity and difficulty are now coming into the realm of possibility as viable remediation sites. These sites can be successfully remediated through innovative remedial design where multiple in situ remediation technologies are integrated to optimize treatment, increase overall project efficacy, and minimize project costs.
These combined approaches may take the form of combining multiple in situ thermal remediation (ISTR) technologies or combining ISTR with other in situ remediation technologies such as in situ bioremediation (ISB) or in situ chemical oxidation (ISCO) to take advantage of thermally enhanced reactions and remediation mechanisms.
What is a Complex Site?
The environmental industry has reached a stage of maturity where a great fraction of the sites that have yet to achieve regulatory closure are considered “complex”. Various remedies may have been implemented at these sites, without reaching the desired remedial goals for many reasons. As an industry, we have learned that there is no “silver bullet” remedy to address complex sites; often, a combination of remedies is required to achieve remediation goals.
Some factors that may contribute to making a site complex include:
- Multiple contaminated zones
- Hard-to-treat geologies such as bedrock or thick clays
- Hydrogeological conditions limiting or preventing the use of certain remediation strategies
- Source area too deep for excavation or soil mixing
- Hard-to-treat chemicals and mixed waste
- Access limitations
Can you Combine Multiple In Situ Thermal Remediation Technologies?
Often, one ISTR technology stands out as a superior fit. However, on many sites, choosing between technologies can be a close call, and in some cases, site conditions or project goals dictate that one ISTR technology alone won’t get the job done. By not limiting an ISTR solution to a singular technology, it is possible to exploit the technical advantages of multiple different ISTR technologies to generate creative combined ISTR system designs. These designs offer effective solutions for more complex sites.
In today’s marketplace, the three main ISTR technologies are Thermal Conduction Heating (TCH), Electrical Resistance Heating (ERH) and Steam Enhanced Extraction (SEE). Combined thermal remedies come in a variety of combinations of these technologies, including:
- TCH – SEE
- ERH – SEE
- ERH – TCH
- ERH – TCH – SEE
Both TCH and ERH can be combined with SEE to effectively treat sites that exhibit a low porosity, low hydraulic conductivity interval (TCH or ERH) and a highly transmissive aquifer featuring groundwater flow rates from 1 ft/day to tens of ft/day. On sites where portions of the treatment zone are best accessed by angled or horizontal installations, TCH heaters can be easily placed on angles or horizontally and can be combined with ERH electrodes that require sub-surface installation.
By addressing site and project complexities using the correct combination of thermal remediation technologies, it is possible to effectively generate a workable solution to otherwise difficult project features. These unique combined thermal remedies will allow you to efficiently achieve your project goals in a shorter time frame and with a lower overall cost.
Can you Combine Thermal with other In Situ Remediation Technologies?
Often, the best solution for a site is to combine technologies spatially and/or temporally. Choosing the best technology at the right time and place for each zone or stage of the project is more cost-effective and efficient than introducing one technology too late in the project.
Can you Combine In Situ Remediation and In Situ Bioremediation?
During traditional applications of ISTR, temperatures of 100°C or higher are reached throughout a treatment volume. While these high temperatures would seemingly limit biodegradation and inhibit microbial populations, surmounting evidence over the past few decades suggests that not only do ISTR projects not “sterilize” microbial populations from a site, many contaminant degrading populations survive, recolonize and flourish at ISTR sites as temperatures begin to cool to more modest conditions.
Additionally, more moderate heating (i.e., 30 to 40°C) can effectively accelerate dissolution/desorption rates and enhance the naturally-occurring biotic degradation at a site.
For petroleum hydrocarbons, BTEX biodegradation has been shown to triple from 10 to 20°C, and petroleum hydrocarbon biodegradation rates can peak between 30 and 40°C.
For chlorinated solvents, many studies indicate that optimal temperatures for maximum bacteria population growth and biodegradation rates peak at around 30 to 35°C. Up to approximately 35°C, dechlorination rates are expected to double with every 10°C increase in subsurface temperature due to:
- Population growth
- Electron availability (release from organic material)
- Metabolic rates/degradation rate
It is possible to take advantage of the residual heat energy in the subsurface of an ISTR project to enhance bioremediation efforts. These combined ISTR-ISB projects take on three archetypal forms:
- Bio-Polishing – Utilize residual heat energy from completed ISTR areas to “polish” off source area contamination through enhanced biodegradation.
- Low-Temp Heat Enhanced Bio Application – Deploy an low temperature ISTR system with the operational strategy of achieving 30 to 40° temperatures (depending on target contaminants) throughout the subsurface, maximizing hydrolysis and biodegradation reaction rates while increasing free product extraction (if it exists).
- ISTR Source and Heat Enhanced Biodegradation to Diffuse Downgradient Plume – Deploy an ISTR system with the operational strategy of achieving 100°C temperatures in the source area, allowing warm water to move downgradient to aid in the biodegradation of dissolved phase diffuse plume areas.
Can you Combine In Situ Thermal Remediation and In Situ Chemical Oxidation?
Exposing a solution of sodium persulfate to elevated temperatures causes enhanced activation. The rate of persulfate activation increases with temperature. Thermal activation is thought to proceed where heat decomposes persulfate into two sulfate radicals responsible for the in situ destruction of HVOC contaminants in groundwater and sorbed to the soil matrix.
During ISTR, contaminant removal rates increase until a peak rate is achieved. This peak rate is defined by the energy requirements for boiling and vaporization of groundwater and the chemical properties of the given COCs present in the treatment volume. After the peak mass removal rate is achieved, removal rates often decrease along a first-order (exponential) decay curve until reaching an asymptotic lower limit.
Historical data from sites completed by TerraTherm shows that the average energy requirement per unit of contaminant mass removed is approximately 7- 10 times higher during post-asymptote operations, compared to during pre-asymptote operations.
Continuing ISTR system operations once the asymptote recovery rate has been reached is neither economically nor environmentally responsible. As continued mass removal is negligible, energy consumption per unit of contaminant mass removed become astronomically high, and the cost per unit mass removed increases significantly. Turning off the power to an ISTR project once these conditions are met and implementing an alternative polishing strategy in the treatment area will take advantage of thermal activation while offering the potential for project cost savings.
Complex sites require smarter and more up-to-date remediation strategies, often in the form of combined remedy approaches. With a greater understanding of these sites and the types of ISTR technologies that can be combined to deliver successful remediation, you’ll be better prepared for your next project.
If you’re considering ISTR technology or simply want to learn more, contact our team of thermal remediation experts today.