Title: Introduction Text: Electrical resistive heating (ERH) originated in the petroleum industry, where it was developed to heat oil sands and oil shales to enhance oil recovery. In the early 1990s, Pacific Northwest National Laboratory first used ERH to apply an electrical current to heat soils to vaporize organic contaminants for removal by vacuum extraction and subsequent destruction. ERH technology has been applied at several Navy and other remediation sites. However, there are several issues to be carefully considered before use of this technology is planned for a given site. This ERH Web Case Study page reviews the scientific concepts behind the application of ERH and provides lessons learned from two recent ERH case studies. Visual Description: U.S. map showing examples of sites which employed electrical resistive heating treatment technologies.

Title: Contaminants Text: ERH has been demonstrated for the removal of Dense, Nonaqueous Phase Liquids (DNAPLs) and Light, Nonaqueous Phase Liquids (LNAPLs). Common examples of DNAPL include chlorinated solvents such as trichloroethene (TCE) or perchloroethene (PCE). Common examples of LNAPL include diesel fuel or waste oil. Visual Description: Photograph of a contaminant liquid in a beaker.

Title: Technology Description (1 of 2) Text: ERH is a remediation technology that involves passing electrical current through saturated soil, resulting in increased subsurface temperatures usually to the boiling point of water. The soil is heated by the passage of current between the electrodes (not by the electrodes themselves). ERH increases the subsurface temperatures beyond the boiling point of the contaminants, which causes a transition into the vapor phase. The contaminants are then removed through vapor recovery wells. Vendors have managed to isolate the ground surface from the electrical heating occurring in the subsurface zone. Therefore, aboveground operations can continue over the treatment site while ERH is underway. Visual Description: Conceptual diagram of Electrical Resistive Heating shows resistive heating electrodes, residual DNAPL, extraction well, aquitard, water table, surface, power supply, plenum and aboveground treatment system.

Title: Technology Description (2 of 2) Text: In addition to volatilization, contaminants are also degraded by in situ processes. These in situ processes include hydrolysis, hydrous pyrolysis, and biodegradation through reductive dehalogenation. Thermophilic (heat-loving) bacteria can be activated to degrade chlorinated volatile organic compounds (VOCs) at a much higher rate than is typically observed under ambient temperature conditions. In addition to these processes, the elevated temperatures decrease the surface tension between DNAPL and water making it easier for the DNAPL to migrate under a hydraulic gradient. This may necessitate the use of extraction wells and any associated aboveground treatment. Visual Description: Conceptual diagram of Electrical Resistive Heating shows resistive heating electrodes, residual DNAPL, extraction well, aquitard, water table, surface, power supply, plenum and aboveground treatment system.

Title: Hydrolysis Text: Hydrolysis is a chemical substitution reaction in which hydrogen ions in water react with organic molecules, replacing chorine atoms. Oxidizing conditions or available oxygen are not required for hydrolysis.

Title: Hydrous Pyrolysis Text: Hydrous Pyrolysis is the breaking apart of complex molecules into simpler units while dissolved in hot water. It is generally referred to as “hydrous pyrolysis oxidation” (HPO) to differentiate it from hydrolysis. HPO requires oxidizing conditions or dissolved oxygen. HPO is insignificant at room temperatures, but can become important at elevated temperatures.

Title: Electrical Phases (Page 1 of 2) Text: ERH is generally applied to the subsurface in two electrical phases: three-phase and six-phase. The primary advantage of three-phase heating is that it provides the most uniform voltage potential between electrodes, especially in treating irregularly shaped areas, which makes it a more widely accepted application strategy. In this method the current flow and heating pattern is simple, uniform, and regular. The electrodes can be easily mapped over the treatment area. Three-phase heating is naturally balanced electrically, with similar number of electrodes connected to all three phases. Visual Description: Graphic of a three-phase heating pattern.

Title: Electrical Phases (Page 2 of 2) Text: Six-phase heating involves splitting standard line current from three phases to six phases. The electrodes in the method are arranged in a hexagonal six-phase heating array with a neutral electrode sitting in the center. Heating occurs between electrodes which are adjacent to each other and are one different in phase number. Hot spots seem to develop wherever conduction occurs between two electrodes whose phase numbers differ from each other by more than one phase number. Visual Description: Graphic of a six-phase heating pattern.

Title: Design and Implementation Text: "Click" on the different components for more information. Visual Description: Photograph of a ERH site with labels to vapor recovery, PCU, treatment cover, SVE well, thermocouples and electrodes.

Title: - Text: Vapor recovery (VR) involves conventional vapor extraction techniques utilizing shallow wells installed either vertically or horizontally. Once steam and volatile contaminants have been collected by the VR system, the steam is condensed and the vapor is cooled to ambient temperatures. Conventional vapor treatment techniques are used to adsorb or destroy the contaminants. Vapor recovery wells are sometimes co-located in the same boreholes as the electrodes. Visual Description: Photograph of a ERH site with labels to vapor recovery, PCU, treatment cover, SVE well, thermocouples and electrodes.

Title: - Text: A Power Control Unit (PCU) is a skid-mounted electrical control system which includes a transformer and other electrical equipment to supply electric currents to pre-constructed electrodes. A transformer is used to convert electrical power down to six separate electrical phases and to adjust the utility voltage to the appropriate level for subsurface heating. The PCU is capable of isolating a transformer to force ERH current to flow only between the electrodes. The PCU is designed to operate in such a way that the electricity will not be transferred away from the ERH field to outside the treatment area. It is generally recommended that an emergency power switch is installed at or near the main PCU unit to shut-off the system in case of any failure. Visual Description: Photograph of a ERH site with labels to vapor recovery, PCU, treatment cover, SVE well, thermocouples and electrodes.

Title: - Text: A treatment cover or a site cap (plenum) is generally installed on the site to ensure vacuum coverage to prevent fugitive emissions. This site cap typically consists of horizontal wells, gravel, insulating material, and a final cover of a black and UV-resistant polyethylene liner. Visual Description: Photograph of a ERH site with labels to vapor recovery, PCU, treatment cover, SVE well, thermocouples and electrodes.

Title: - Text: Electrodes are usually constructed of galvanized steel pipes that are installed in the subsurface in a similar manner as the groundwater monitoring wells. The electrodes are installed in direct contact with the formation in the aquifer and within the impacted area in a grid-type formation. Horizontal spacing between the electrodes typically ranges from 8 to 16 feet. The vertical depth of the treatment achieved can be controlled by varying the depth of the electrodes and the size of the PCU (or the voltage applied to the subsurface). Visual Description: Photograph of a ERH site with labels to vapor recovery, PCU, treatment cover, SVE well, thermocouples and electrodes.

Title: - Text: A bundle of thermocouple wires are used to measure temperature in the aquifer. Typically, thermocouple wires are bundled with uniform distance to be placed at various depths. Visual Description: Photograph of a ERH site with labels to vapor recovery, PCU, treatment cover, SVE well, thermocouples and electrodes.

Title: - Text: Soil vapor extraction (SVE) wells are used to extract volatized vapor resulting from the heating of the vadose and saturated zone and some groundwater together with other liquids within the treatment area. Visual Description: Photograph of a ERH site with labels to vapor recovery, PCU, treatment cover, SVE well, thermocouples and electrodes.

Title: Aboveground Treatment System (1 of 2) Text: The continuous treatment of extracted vapor and liquids before discharge is typically conducted above ground. During ERH operation, the extracted steam from the vapor recovery extraction wells is processed in a liquid/vapor separator (knock-out tank) where any residual contaminants are settled in the tank by gravity. Water, produced by condensation of the steam, is then placed in another storage tank for further processing. This condensed water is recirculated within the storage tank through spray nozzles to reduce the levels of VOCs. Depending on the preferences of the site owner and regulators, the water can either be 1) reinjected to rewet the soil around the electrodes to increase electrical conductivity or 2) pumped through a series of granular activated carbon tanks before discharge. Visual Description: Photograph of aboveground treatment system with labels to baker tank, air treatment stack, heat exchange, knockout tank, control trailer and PCU.

Title: Aboveground Treatment System (2 of 2) Text: Following the larger knock-out tank, the recovered vapor passes through a condensation unit that consists of a heat exchanger, cooling tower, and a smaller knock-out tank. The heat exchanger further condenses any recovered steam and then the condensed water is pumped into the storage tank. The vapor exiting the heat exchanger goes through a smaller knock-out tank to collect condensate from the cooling process. The extracted vapor from the ground is then run through the off-gas treatment system by a vacuum blower at the end of the treatment train. The vapor discharge of the blower is sent to a catalytic oxidizer to treat the remaining VOCs in the residual vapor before being discharged to the atmosphere. If chlorinated VOCs are treated, the catalytic oxidizer is equipped with a hydrochloric acid scrubber. Visual Description: Cross sectional graphic diagram of above ground treatment system shows ERH system, SVE system, treatment system and cap.

Title: Operations (1 of 2) Text: There are a number of key factors that affect operations at a given site including: Actual heating profile during the startup of the system within the treatment area and the treatment interval of the contaminants. Actual time required to heat and boil the treatment zone. Actual SVE performance in all the main geologic units of the treatment zone. The amount of VOCs extracted versus degraded in situ. These factors are not mentioned in the order of priority. In practice, they are managed as the field data suggest with guidance from modeling calculations.

Title: Operations (2 of 2) Text: The ERH system is typically equipped with a control system to automate operations including the system startup, monitoring, and notification of alarm conditions. However, it is strongly recommended for on-site personnel to be present for the duration of the initial heating stage. A flow chart of the typical operations sequence is shown here. Prior to the system initiation, the SVE system is activated to establish negative pressure over the treatment region and to capture all the vapors that are produced during the initial heating of the soil. Once the system goes into full operational mode, the total power applied, energy delivered, electrical voltage, currents, and soil temperatures are measured frequently using the computer-based control and data acquisition system. The heating pattern and rates in the subsurface are determined by measuring the temperatures from thermocouple bundles and the pressure on wellheads installed in the treatment area. As the liquid/vapor is extracted, the liquid is treated and stored in a storage tank for reinjection or disposal. The extracted vapor containing the volatilized contaminants is treated before release into the atmosphere. The aboveground treatment system must be inspected and monitored periodically throughout the system operation. In addition, a proper air monitoring program must be in place as part of the ERH system operation. Soil and groundwater sampling is conducted periodically to monitor the performance of the ERH system. Visual Description: A flow chart of a typical operations sequence is illustrated.

Title: Performance Assessment (1 of 2) Text: Following is the suggested list of performance assessment objectives: Determine changes in concentration, mass, and/or mass flux in the constituents of concern (COCs). Determine changes in the aquifer water quality in and downgradient of the ERH treatment area. Determine fate of the COCs in and around the treatment area. Verify system operation requirements and maintenance. Visual Description: Photographs of engineers conducting field measurements and lab analysis to support performance assessment objectives.

Title: Performance Assessment (2 of 2) Text: These performance objectives can be measured by conducting some or all of the following monitoring activities: Groundwater monitoring before and after ERH treatment for the contaminant concentrations in and around the treatment area. Soil core sampling before and after ERH treatment for estimating the reduction in the concentration of the contaminants due to application of the technology. Groundwater monitoring for geochemistry (inorganic analytes) to evaluate changes in the aquifer water quality. Vapor monitoring at a breathing level and on the surface of the treatment area to measure fugitive emissions to the atmosphere. Vapor monitoring of the discharge from the aboveground vapor treatment system before release to the atmosphere. Temperature monitoring in and near the treatment area to determine the heating efficacy. Regular visual inspections and frequent system checks to maintain proper operations. Post-treatment sampling and analysis to evaluate the cleanup efficiencies.

Title: Optimization Text: Efforts can be undertaken to optimize the ERH application at a site. Some general recommended optimization actions are: Establish and maintain desired subsurface temperature (i.e. boiling points of the contaminants) within the treatment area to enhance the removal of the contaminant source. Establish and maintain control of contaminant migration through groundwater, vapors, and air emissions. Provide the system with higher power inputs during system startup. Use a lower power input when the system is in full-scale operations and the desired subsurface temperatures are met. A lesser amount of energy is needed to maintain the constant temperature during full-scale operations. Once the mass removal rate becomes constant, the SVE system can be shut off. This improves the cost-efficiency of the whole system. Provide a system for near-real-time data delivery, performance and compliance monitoring, and project communications.

Title: Cost Efficiency Text:

Title: Advantages Text: In situ application, which minimizes aboveground soil and groundwater disposal requirements. Aboveground use of the site can continue during ERH system operation. Uses commercially available equipment. Relatively short-term field application compared to a conventional pump-and-treat system. Source reduction results in an improvement in downgradient aquifer quality. Applicability in heterogeneous aquifer and in tighter soils. Recent evidence contains some indication that contaminant biodegradation rates may be enhanced at elevated temperatures.

Title: Limitations Text: High capital and O&M costs may limit the cost effectiveness of ERH at sites with large source areas. Typical ERH costs range from $55 to $400 per cubic yard with a median cost of $120 per cubic yard. Not all types of contaminants are amenable to ERH treatment. In addition, some contaminants, such as certain heavy metals, if present, could be mobilized by heating. Aquifer heterogeneities can make the application more difficult, necessitating more complex application schemes, greater amounts of heat (energy), and/or longer application times. The limitation is not related to the ability of the method to provide heat to the subsurface, but with its ability to transport and capture the contaminant vapors in an efficient manner. Some sites may require greater hydraulic control to minimize the spread of contaminants especially in the very permeable aquifer conditions. This may necessitate the use of extraction wells and any associated aboveground treatment. Aquifer heating is generally accomplished relatively quickly in a week or at the most in a month, but it takes the subsurface almost a year to return back to ambient temperatures for sampling to begin. Power requirements may be high at sites where groundwater flow velocity is relatively high.