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The Worldwide Leader in Thermal Remediation |
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The Worldwide Leader in Thermal Remediation |
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FREQUENTLY ASKED QUESTIONSGeneral Information Pre-Treatment / Design Considerations During Treatment
1. General1.1 What Is In Situ Thermal Desorption? In Situ Thermal Desorption (ISTD) is an environmentally friendly soil and groundwater cleanup process that is simple and fast. It remediates organic and selected metal contaminants from both soil and groundwater, without having to move or disturb the soil. It can meet and exceed even stringent soil cleanup standards, even for sites with large masses of contaminants such as Non-Aqueous Phase Liquids (NAPLs). 1.2. Why Use This Technology? What Are The Advantages? Some of the advantages are:
1.3. How Does The Thermal Well System Work? Our proprietary electrical heating elements are similar to those in an electric oven or toaster, but much more robust, with a capacity to deliver 330 watts per linear foot (1 kW/m) or more. They hang within pipes installed in the contaminated ground. We can adjust the heat output up or down as needed. The heat radiates out to the walls of the pipes, after which it moves into the surrounding formation primarily by thermal conduction. Arrays of thermal wells are used to treat a larger volume at one time, with a typical spacing between wells being about 7 to 20 ft (2.1 to 6.1 m). As the soil heats up, water and contaminants in the targeted zone are vaporized and boil. The vapors are drawn out of the soil and into heater-vacuum wells by a vacuum unit, and routed into a trailer-mounted vapor processing facility that treats air emissions to meet the required standards. The technology has been field-proven at over 15 sites since 1996. 1.4. What Is Unique About ISTD? Unlike other in-situ thermal remediation technologies, ISTD heats the subsurface by conduction (with simultaneous application of vacuum). Conductive heating is a much more uniform method of heat transport regardless of soil type, stratigraphy, or degree of heterogeneity, relative to other methods of subsurface heating (e.g., steam, electrical resistance heating, radio frequency heating, hot water or hot air injection). This is because the thermal conductivity of the full range of soil types ranges only over a factor of < 10. By contrast, fluid conductivities at a given site often range over several orders of magnitude (a hundred or a thousand-fold). Thus, ISTD can reliably sweep 100% of the target zone, and do so in a highly predictable timeframe (typically 45-90 days). ISTD's use of conductive heating means that the targeted treatment zone in the subsurface can be heated, if desired, past the boiling point of water, to vaporize volatile, semi-volatile, and even non-volatile contaminants wherever they might reside. No other in-situ thermal remediation technology can accomplish this cost-effectively. No fluids are injected during ISTD. Meanwhile, the boundaries of the treatment zone and the air pollution control system are maintained under a net negative pressure throughout the operation period. Thus, opportunities for contaminant mobilization and migration are minimal. 1.5. Where Has This System Been Used? In Situ Thermal Desorption has been field-proven at over 15 sites since 1996. These sites range throughout the continental United States, in every major geographic region. The projects have included government and privately-owned facilities. The sites treated have undergone federal and state agency oversight. 1.6. Is Regulatory Approval Needed Before In Situ Thermal Desorption Can Be Used? As with any remediation project, some level of both state and federal approval is usually required. Experience indicates that the time required is similar to that for other remediation technologies. This depends on a variety of factors including depth of contamination, soil moisture, contaminant types, and the hydrogeology. In general, In Situ Thermal Desorption is cost competitive with alternative processes in many cases, since excavation, hauling, backfilling, and off-site disposal are not required. Also, in many industrial and utility applications, remediation can be completed with minimal disruption to ongoing operations, reducing the overall cost impact. Remediation costs are confirmed after the design has been completed, so that accurate estimates can be given for each site. 1.8. How Much Electrical Power Does ISTD Require? The power required depends largely on how much water is present in the soil, the target treatment temperature, and the rate at which groundwater seeps into the treatment zone. If too much water can seep in, a dewatering system or low permeability barrier may need to be installed to control recharge. 1.9. Isn't The Power Itself Excessively Expensive? No, the power cost is not that high, and represents only a fraction of the overall cost. For treatment of volatile organic compound (VOC) sites, where as a rule of thumb, less than ~0.5 pore volumes of water need to be heated and boiled off, electrical costs run between $10 and $20 per cubic yard ($13 and $26 per m3) of soil treated. For treatment of semi-volatile organic compound (SVOC) sites, where generally all the water needs to be boiled off, the cost of power tends to range from $20 to $40/cubic yard ($26 to $52 per m3) of soil treated. Factors include electricity rates, soil moisture, and the geometry of the treatment zone. 1.10. What Are The Risks Of Using ISTD? Working in the subsurface can be uncertain regardless of the in-situ technology. The In Situ Thermal Desorption process is quite robust, but at each project site we attempt to avoid overdesigning the system by sizing it to the mass estimated to be in the ground, based on pre-existing site data. If unexpectedly high contaminant mass is encountered, additional time and/or costs may be entailed to address it. Similarly, we design the system to address the estimated water content in the ground; if much greater amounts of water are encountered than expected based on pre-existing data, the time and/or cost to address it can be significant. This is true for other in-situ thermal technologies as well. Other potential risks, such as electrical hazards, are dealt with
using sound work practices and experienced personnel. To date, TerraTherm
has had no Lost-Time Injuries or OSHA Recordables. 2. Pre-Treatment / Design Considerations2.1. What Pollutants Can Be Treated This Way? In Situ Thermal Desorption will treat just about any organic compound, including:
This technology can also collect and capture some low boiling point metals, such as mercury, arsenic, and certain organic forms of cadmium and lead. 2.2. Which Contaminants Cannot Be Cleaned With In Situ Thermal Desorption?
2.3. How Clean Does It Get, And What Contaminants Are Left Behind? Usually, the low residual organic contaminant concentrations left behind are significantly lower than typical state and federal cleanup levels. In comparison with other technologies, contaminant destruction and removal by In Situ Thermal Desorption is very complete. For groundwater VOC sites, concentrations are reduced to less than 1 mg/kg and 0.1 mg/L in soil and groundwater, respectively. Where desired, the system may be operated long enough to achieve Maximum Concentration Limit (MCL) concentrations in the groundwater, and non-detect in soils. For SVOC treatment in soils, non-detect concentrations have been achieved by treating at 570-660 °F (300-350 °C) for a period of several weeks. Basically, the ISTD system can be designed for the desired remedial efficiency, as demanded by the local regulations. 2.4. Can ISTD Meet MCL In The Source Zone? No site to date have been treated with this objective. However, results from 3 recent CVOC sites indicate that this is a realistic target for contaminants such as TCE, PCE, and BTEX. If necessary, a polishing period during cool-down can be used to ensure that all groundwater samples meet the criteria. However, meeting MCLs in the source zone requires that the source is carefully delineated, and no NAPL exists around the perimeter of the target zone. If contamination existed outside the treatment zone, the vacuum and water flow would continue to pull in trace amounts of contaminants, and prevent complete cleanup in the treatment volume. Where the characterization is sufficient, MCL can be reached in the source zone. 2.5. How Deep Can Thermal Wells Operate? Thermal wells can be used to treat contaminants to theoretically hundreds of feet or deeper, as well as under structures and roads. The deepest full-scale application to date is to 105 ft (32 m). However, thermal conduction heaters are also used for thermally enhanced oil recovery at depths of more than 1,000 ft (300 m). 2.6. Given Their Relatively Close Spacing, Are Thermal Wells Expensive To Install? Most of the thermal wells are little more than pipes installed in the ground with electric heater elements installed inside them - these "heater-only" wells are much less expensive to install than typical groundwater monitoring wells. Production rates for their installation range from 200 to 400 ft/day (60 to 120 m/d), depending on the nature of the subsurface. "Heater-vacuum" wells, which are much fewer in number within an ISTD well field, are somewhat more expensive because the heaters are typically installed within a vacuum extraction well screen. Production rates for installation of heater-vacuum wells range from 100 to 200 ft/day (30 to 60 m/d). Simpler vapor collection wells are also available, and can cut costs further. 2.7. When Is A Site Too Small For In-Situ Thermal? Sites of less than about 1,000 cubic yards (750 m3) are seldom cost-effectively treated by this method. At some sites, even several thousand cubic yards tends to be too small. Governing factors include site geometry, type of contaminant, emission standards, and operational requirements. As the site volume increases, the base costs of design, mobilization, installation, and operation can be borne by a larger treatment volume, reducing unit costs accordingly. The deeper the problem, the more attractive the ISTD solution. 2.8. When Is A Site Too Big For In-Situ Thermal? Sites of greater than about 1,000,000 cubic yards (750,000 m3) are usually not realistically addressed by this means. However, given that the unit costs diminish as the scale of the project increases, it would be advisable to consult us for a site-specific evaluation. For large sites, unit costs may be on the order of $40-70 per cubic yard ($70-90/m3). 2.9. What If Contamination Is Really Shallow? Contaminants extending no more than 3-6 feet (1-2 m) in depth may be difficult to treat cost-effectively in-situ. In such cases, it may be more cost-effective to consolidate the material and treat it in aboveground piles using TerraTherm's In-Pile Thermal Desorption (IPTD) process, described elsewhere in the website. 2.10. Can The Entire Thermal Well field Be Installed Below Ground Surface, To Permit Existing Facility Activities To Continue? Yes, it is possible to install the wellheads, electrical cable, collection piping, and instrumentation in below-ground vaults and utility corridors, at additional cost. A period of time would be required for their installation, when sections of the facility will be temporarily cordoned off. 2.11. Does Soil Moisture Affect Performance? With runoff controls to limit recharge rates from areas upgradient, and site management to control groundwater influx if necessary, In Situ Thermal Desorption can be used to treat saturated as well as unsaturated sites. Thus, high soil moisture does not necessarily preclude the use of these technologies. However, the energy cost to complete in situ thermal remediation of any type rises with the amount of water that must be vaporized during treatment. 2.12. Can This Process Be Used Around Buried Utilities? Many types of buried utilities, such as concrete stormwater lines and steel water lines can be left in place and/or protected during heating through appropriate placement of heaters and insulation. Some utilities (e.g., gas lines, PVC pipe) may need to be rerouted or decommissioned. 2.13. Can This Process Be Used Adjacent To Foundations? Yes. Since the heat front drops off sharply adjacent to the heated
zone (typically within 5-7 ft), experience has shown that heating
adjacent to foundations typically has no effect on the foundations.
Measures can be taken to further protect structures if necessary. 3. During Treatment3.1. How Long Does In Situ Thermal Desorption Take? Treatment periods depend on a number of factors: depth of contamination, soil moisture, contaminant types, and so forth. In general, treatment times are greatly accelerated relative to conventional methods, and can require as few as a couple of months, and more commonly 3-5 months. Some sites may take as long as a year or two, but this is still fast compared to the pace of conventional operations over large, deep areas. 3.2. How Do You Know When Cleanup Is Complete? First, remediation cleanup levels and cleanup times can be predicted accurately by computer simulation before the job starts. After that, monitoring systems and thermocouple probes in the soil are used to evaluate progress throughout the treatment zone. Experience has shown that there is a strong correlation between the computer predictions and actual results, and typically, pre- and post-treatment soil samples are used to confirm the adequacy of remediation. If desired, confirmatory samples are taken before the system is shut down, and the site can be declared clean before demobilization of the equipment. 3.3. What About Air Emissions Using This Technology? Air emissions are treated using off-the-shelf conventional treatment components, such as a thermal oxidizer and granular activated carbon filter, designed on a site-specific basis to address each of the constituents as cost-effectively as possible. Experience shows that any contaminant emissions are quite low, and that we consistently perform much better than established standards. The principal substances released to the air are carbon dioxide and water. 3.4. Does ISTD Create Dioxins And Furans? ISTD is quite different from ex-situ thermal desorption or incineration. With these aboveground thermal technologies, the soil or sludge being treated is exposed to high temperatures only briefly - typically for seconds or minutes. Thus, there can be cool spots where the soil does not get fully treated and where compounds such as dioxins and furans can sometimes be created. By contrast, with ISTD the entire treatment zone is heated to target temperatures for days, at a minimum. Most (> 95-99%) of the organic contaminants are destroyed in-situ. Not only are dioxins and furans not created, treatability and field data indicate they too are destroyed, typically to below background levels. Dioxins and furans that are extracted are treated in the air pollution control system. For more information, please refer to the paper "In-Situ Thermal Destruction (ISTD) Performance Relative to Dioxins." 3.5. What If The System Leaks? The treatment zone is completely covered by a vapor barrier at the ground surface. Also, the last process unit in the vapor treatment facility is a vacuum blower that draws vapors through the rest of the system. Thus, the boundaries of the treatment system are maintained at sub-atmospheric pressures, and any leakage would be from the outside inward. Contaminated vapors thus cannot escape to the atmosphere before passing through the vapor treatment system. 3.6. Will DNAPL Be Mobilized To Greater Depth? First, no fluids are injected during In Situ Thermal Desorption. Studies of chemical properties during conductive heating show that the heating process favors upward, not downward movement. With heating, DNAPL swells slightly, which negates the slight reduction in interfacial tension. It can be shown that the pool thickness that can be maintained in a stable form above a capillary barrier actually increases with heating, i.e., the tendency for downward mobilization diminishes. Moreover, volatilization causes upward movement of vapors due to their buoyancy. Proper placement of heater-only and heater-vacuum wells ensures that chemicals are captured close to points of potential mobilization, and when combined with appropriate placement of insulation and vapor seals, condensation in cool spots and fugitive emissions are prevented. Finally, experience shows no evidence of downward mobilization at In Situ Thermal Desorption sites. 3.7. How Is Contaminant Spreading Avoided? It is avoided through the proper placement of the surface insulation/vapor barrier, and the thermal well field. All locations targeted for treatment are thereby heated to the desired temperature, while the boundaries of the treatment zone are maintained at a negative pressure. Thus contaminant movement is toward the vacuum collection points, and condensation of contaminants in cool zones is prevented. 3.8. Does This Process Create Dust? Not at all. Since the soil is not disturbed during treatment, there is no excavating or hauling. Also, since there is no earth moving equipment or soil transport, there is no dust created, other than during drilling and heater installation activities. Simple dust control measures are used. 3.9. What Happens To Halogens From PCBs And Similar Materials? When PCBs and other halogenated hydrocarbons are destroyed, acids like
hydrochloric acid (HCl) are produced. These acids are stabilized rapidly
by precipitation with natural soil elements, principally iron. For
example, HCl and iron will form FeCl3, which is harmless and very stable.
The remaining acidic vapors are scrubbed aboveground with wet or dry
scrubbing media. Experience shows that acid gas emissions are typically
very low. 4. Post-Treatment Considerations4.1. Does This Process Prevent Revegetation After Treatment Is Complete? No. Immediately after treatment by In Situ Thermal Desorption, the soil is sterile, but experience shows that recovery will be rapid. After the soil is disked, fertilized, and seeded following normal revegetation practices, regrowth during the first growing season after treatment should be as good as with other soil. One example in upstate New York even showed that weeds, moss, and other vegetation can naturally cover a treated area within one growing season without any fertilization or other intervention. At another site in Indiana, large trees growing adjacent to a treatment zone were unharmed by the heating process. Ask to see the before and after photos. 4.2. Does In Situ Thermal Desorption Sterilize The Soil And Prevent Natural Attenuation? Source zones heated to temperatures at and above the boiling point of
water will be sterilized, but upon cooling will undergo repopulation by
indigenous microorganisms. Microbiota residing in locations in the
downgradient dissolved plume (i.e., outside the target treatment zone)
may see mildly elevated temperatures, which are likely to promote rather
than hinder their growth and attenuative capacity. Thus, natural
attenuation can be enhanced by heating within the source zone. Actually,
TerraTherm is working on combinations of ISTD and bioremediation, since
these technologies can be combined for improved efficiency and reduced
cost. 5. Technology Comparison5.1. How Does ISTD Compare With Other In-Situ Remediation Technologies? A major reason for the unsurpassed effectiveness of TerraTherm's ISTD technology is its application of heat to the soil using thermal conduction. During conductive heating, heat generated by simple electrical heating elements moves out through the soil and waste material in a highly predictable fashion, regardless of how heterogeneous the soil is, or its permeability. This is in sharp contrast to the movement of a fluid through the soil, which is the basis for nearly all other in-situ remediation technologies (e.g., groundwater pump-and-treat, soil vapor extraction, air sparging, steam injection, solvent and surfactant injection, or chemical oxidant injection). Rates of fluid flow can vary over many orders of magnitude from one place within the soil to another, depending on how permeable the soil is, and on the degree of heterogeneity. Fluid-based technologies thus tend to bypass some contaminated zones, leading to poor efficiency, diffusion-limited mass transport, and a very protracted duration of remediation (Baker et al. 1999). By contrast, the thermal conductivity of a wide range of soil types varies over less than a factor of plus or minus two. Thus, thermal conduction heating is a very uniform and predictable process (Stegemeier and Vinegar 2001). ISTD has a wider applicability than other in-situ thermal technologies. For example, the process of electrical resistivity heating, also known as Six-Phase Heating or joule heating, relies on the flow of electrical current through soil. Electrical conductivity can vary over two orders of magnitude. Since electrical current ceases to flow in soils once water has boiled off, moreover, electrical resistivity heating cannot heat the soil above the boiling point of water. Thus, it is not applicable to treatment of high boiling-point compounds such as pesticides, PCBs, and PAHs to stringent soil cleanup levels (Baker and Bierschenk, 2001). Similarly, steam injection in the shallow subsurface is limited to heating approximately to the boiling point of water. This is because superheating of steam requires maintaining pressures much higher than atmospheric pressure. In shallow unconfined soils, however, injection of superheated steam simply results in breakthrough of the steam to the surface along a path of least resistance as soon as overburden pressures are exceeded. TerraTherm's patented thermal conduction heating method is not subject to these limitations. For treatment of DNAPLs composed of low-boiling chlorinated solvents such as TCE and PCE, heating of the entire soil volume above the boiling point of water is not required. With such compounds, most of the desired heat and mass transfer can be obtained by steam distillation of these compounds at their eutectic points, which are somewhat below the boiling point of water. Like steam injection and electrical resistivity heating methods, ISTD can accomplish this aim. For such applications, ISTD wells can be spaced more widely (e.g., 15-25 ft, or 5-7.5 m) than for high boiling compounds, which have much higher target temperatures. Unlike steam injection or electrical resistivity heating methods, however, ISTD can still accomplish a high degree of in-situ destruction in the proximity of heater-vacuum wells, significantly reducing the aboveground treatment requirements and costs. ISTD can also achieve 100% sweep of the targeted zones, even in heterogeneous formations and fractured bedrock. In addition, ISTD results in the greatly increased permeability of clays in the proximity of heater-vacuum wells, thereby improving vapor capture in predominately low permeability formations. The Portland, IN project was a full-scale demonstration of ISTD's capability to treat DNAPLs in sand and clay (Vinegar et al., 1999). TerraTherm more recently completed an 11,200 cy (8,600 m3) chlorinated solvent project in Ohio at a thermal treatment cost of less than $130/cy ($170/m3).
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