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CMPS&F - Environment Australia
Appropriate technologies for the treatment of scheduled wastes
Review Report Number 4 - November 1997


17. IN SITU VITRIFICATION


 

17.1 Technology Description

17.1.1 General

In Situ Vitrification (ISV) is a commercially available mobile, thermal treatment process that involves the electric melting of contaminated soils, sludges, or other earthen materials, wastes and debris for the purposes of permanently destroying, removing, and/or immobilising hazardous and radioactive contaminants. The ISV process was developed by Battelle, Pacific Northwest National Laboratory, for the US Department of Energy. The ISV technology has been licensed to Geosafe Corporation and Geosafe has used ISV to successfully treat a number of Superfund sites in the US The process is widely applicable to all soil types and all classes of contaminants including organics, heavy metals and radionuclides. The ISV process is available in Australia through Geosafe Australia Pty. Ltd. The ISV process has been selected for use at the Maralinga site in South Australia to treat burial pits containing soil and debris contaminated with plutonium and uranium as well as lead, barium, and beryllium (Thompson, 1997).

The ISV process is a batch process that involves forming a pool of molten soil at the surface of a treatment zone between an array of four electrodes. The molten soil serves as the heating element of the process wherein electrical energy is converted to heat via joule heating as it passes through the molten soil. ISV melt temperatures typically range between 1,500-2,000C. Continued application of energy results in the melt pool growing deeper and wider until the desired volume has been treated. When electrical power is shut off, the molten mass solidifies into a vitreous monolith with unequalled physical, chemical, and weathering properties compared to alternative solidification/stabilisation technologies. Individual melts up to 7 m deep and 15 m in diameter are formed during commercial operations. Large volumes of contaminated material requiring more than one batch melt are treated by making a series of adjacent melts resulting in the formation of one massive continuous monolith. The process is operated on an around the clock basis and can achieve treatment rates of up to 150 tonnes per day. The process is illustrated in Figure 17.1.

The ISV process can be configured in several different ways to treat soils and wastes. Figure 17.2 illustrates the three primary treatment configurations. The three treatment modes are as follows:

Figure 17.1
In Situ Vitrification Process Sequence
Figure 17.1

Figure 17.2
Primary Treatment Configurations

Figure 17.2

ISV results in a 25-50% volume reduction for most soils, and up to a 75% volume reduction for sludges and wastes that de-water and/or decompose during processing. The volume reduction results in a subsidence volume above the vitreous monolith. In most applications, the subsidence volume is filled with clean soil and the monolith is left in the ground since it is no longer hazardous. Sites treated by ISV are normally capable of future use without restriction associated with the vitreous monolith. However, if site requirements dictate that the monoliths be removed, the vitrified monoliths can be fractured and removed with conventional heavy equipment.

17.1.2 Contaminant Destruction and Containment Performance

The ISV process destroys organics by pyrolysis within the soil closely adjacent to the melt. No organics remain in the melt or the vitreous monolith due to the high temperatures involved in treatment. A broad range of organic contaminants have been successfully treated in full-scale commercial operations including solvents, pesticides, herbicides, and chlorinated compounds such as pentachlorophenol, hexachlorobenzene, PCBs and dioxin. Organic species in the surrounding soil vaporise and move towards the melt where they are destroyed and removed due to a concentration-based diffusion mechanism. The typical destruction and removal efficiencies (DRE) for all organic species are greater than 99.9999%. This DRE includes the percentage destroyed by the melt (typically >99.9%) and the percentage destroyed and/or removed from the off-gas stream by the off-gas treatment equipment.

High concentrations of organic contaminants, 10-20 wt%, can be treated by the process with excisting equipment. Organic concentrations in excess of 20 wt% can be treated with modified equipment.

Heavy metals and radionuclides are subject to physical and chemical incorporation into the vitreous product during ISV processing, which results in permanent immobilisation. Most species of metals and radionuclides are stable as oxides in the melt, are uniformly distributed through the melt and are immobilised in the vitreous product. The percentages of these species that are normally retained in the melt are summarised in Table 17.1.

Table 17.1.
Percentages of Radionuclides and Heavy Metals Retained in the Melt
Non-volatiles (Pu, Sr, Ba, Cr)
Semi-volatiles (Co, Cs, Pb, Cd, As)
Volatiles (Hg)
99.99% - > 99.9999%
90% - 99.99%
0% - 90%

The ISV process is capable of treating high concentrations of both organic and inorganic wastes simultaneously, including all classes of organics, heavy metals and radionuclides. ISV is also distinguished by its ability to tolerate significant amounts of debris within the treatment zone. Organic debris materials such as plastic, wood and paper behave as other organics during ISV processing; they are destroyed primarily by pyrolysis. Inorganic debris materials such as steel and concrete are incorporated in the melt and the resulting vitreous product. Types of debris previously processed by ISV in commercial operations include: scrap metal, steel drums, concrete, asphalt, rock, wood, plastic, paper, protective clothing, HEPA filters, activated carbon filters, automobile tyres, and general construction demolition debris. Very large amounts of debris can be accommodated by the process; depending on the type of debris, debris loadings to 50 weight percent or more can be treated in any single melt.

17.1.3 Characteristics of the Vitrified Product

The vitreous product generally consists of high concentrations of silica (50-80%) and low levels of alkali oxides (1-5%), which results in an extremely durable and highly leach resistant product. The vitrified product far surpasses relevant TCLP criteria. The product is typically 10 times stronger than concrete in both tension and compression and is unaffected by freeze/thaw or wet/dry cycles. Long-term durability test results indicate that the vitreous product is typically 5 to 100 times more durable than borosilicate glasses used to immobilise high level nuclear wastes. Although the ISV product often consists of crystalline phases intermixed with the vitreous phases, the crystalline phases formed in ISV products are not detrimental to the product’s durability. In many cases, the crystallinity results in a localised increase in the silica content of the vitreous phase, thus increasing the product’s durability.

17.1.4 Process Equipment Description

The equipment primarily consists of a transformer to convert three phase electrical power into two phase power for soil melting, an off-gas treatment system, and a steel containment hood that is positioned over the melt to capture off-gases. A schematic of the process is shown in Figure 17.3. The ISV process equipment is all trailer mounted except for the off-gas hood, which is transported to the site and then assembled. Only two personnel are required to operate the equipment.

A stainless steel hood is employed to collect off-gases that evolve from the treatment zone. The hood is normally maintained at a slight vacuum to collect the off-gases. The off-gases are then piped to the off-gas treatment system. Two hoods have been employed for some commercial projects for increased efficiency. In a two-hood operation, one hood is used during a melt operation while the second hood undergoes preparations for the subsequent melt.

The off-gases are processed by an off-gas treatment system designed to ensure all emissions fully satisfy regulatory limits. The standard off-gas treatment system consists of quenching, two stages of high efficiency scrubbing, de-watering, heating and one or two stages of high efficiency particulate air filtration. Thermal oxidation can be used as a final polishing for sites involving high concentrations of organic contaminants, if desired. In addition, other off-gas treatment steps can be added as necessary to meet site specific treatment requirements.

Figure 17.3
In Situ Vitrification Process Equipment Schematic
Figure 17.3

The electrical power transformer provides two phase alternating current at the appropriate voltage and amperage to the electrodes. The transformer is fitted with multiple taps to accommodate changes in the resistivity of the melt.

Other major components include the process control station, a back-up off-gas treatment system and a diesel powered generator.

17.1.5 Utility and Service Requirements

The ISV process typically requires 0.7 to 0.8 kWh per kg of material vitrified, which is efficient compared to other thermal treatment processes. The typical power demand for a full-sized melt is 2.5 to 3.5 MW. Normal utility requirements for applications in Australia would be 11 kV three phase electrical power which can be supplied either from the utility grid or from diesel powered generators.

The thermal oxidiser, if used, typically requires 3 Mbtu/hr of fuel. A source of potable water is also required to support process operations.

17.1.6 Safety and Environmental Considerations

ISV is relatively safe and represents a low risk to the environment as demonstrated by successful commercial operations in the US and in Japan. Factors that enhance the safety of the process are:

For the in situ mode of treatment, sites must be characterised to ensure that there are no high integrity sealed containers, such as drums, and that there are no other structures present where liquids can accumulate and become trapped. Sealed containers and other trapped liquids become pressurised upon heating and can result in sudden gas releases through the melt. This can result in melt bubbling and splatter inside the steel containment hood and overheating of the hood. Preconditioning steps have been successfully used to eliminate this concern. Staging of materials for the staged in situ or stationary batch mode of treatment eliminates this concern altogether.

The ISV operation could be enclosed inside a building to provide secondary containment. This has not been necessary for operations in the US

17.2 Example Applications

Commercial US operations completed to date have involved the treatment of a wide range of organic and inorganic contaminants and extensive amounts of debris. For each of the projects completed to date the treatment configuration used was the staged in situ mode. For each site, contaminated soils, debris, and other wastes from multiple locations was consolidated into a single on-site location for treatment. The following are summaries of some of the commercial projects relevant to the treatment of scheduled wastes that have been completed to date.

17.2.1 Pesticides, Herbicides, Mercury-Contaminated Soils

The first large-scale commercial application of the ISV technology was completed at the Parsons Chemical/ETM Enterprises Superfund site in Michigan State in 1994. This project involved treating 4,400 tonnes of pesticide and mercury-contaminated soil and debris. Low levels of lead, arsenic and dioxin were also present in the contaminated soil. Debris present in the treatment zone included concrete, steel, plastic, used filters from the ISV off-gas treatment system, and automobile tyres. The project also involved the US EPA SITE Program demonstration test for the ISV technology. The staged in situ configuration mode was used and involved the consolidation and staging of contaminated soil from various on-site locations and from near-by affected off-site areas. The contaminated soil was staged to a depth of approximately 5 m in a treatment cell. The treatment cell arrangement also included employment of a ground water intercept and diversion system to prevent intrusion of seasonal perched water into the treatment zone. The treatment volume was processed by eight ISV melts settings, producing a single contiguous vitreous monolith. Post-operational evaluations confirmed the following:

17.2.2 PCB-Contaminated Soils

The ISV process has been used successfully to treat PCB-contaminated soils and debris. One such project involved a privately owned Superfund site in EPA Region 10, Washington State. The maximum PCB concentration in the soil was 17,860 ppm. This project involved the staged in situ mode of treatment. Debris included drums, concrete (8 wt%) and asphalt (11 wt%). The project was used as an official demonstration for the US EPA under the provisions of the National Toxic Substances Control Act (TSCA). The contaminated soil and debris were collected from around the site and staged into a treatment cell to a depth of approximately 5 m. The volume was processed by five melts to create one contiguous monolith. The project was completed successfully and resulted in the US EPA granting Geosafe a National TSCA Operating Permit for the ISV process. This allows treatment of PCB-contaminated soils of up to 17,860 ppm by the ISV process anywhere in the US The process satisfied all performance criteria established for the project as follows:

17.2.3 Dioxin, Hexachlorobenzene and Pentachlorophenol-Contaminated Soils

The ISV process was successfully used to treat 5,400 tonnes of soil and debris at a privately-owned Superfund Site in Salt Lake City, Utah. The site was a former industrial facility associated with the packaging and distribution of acids, caustics, organic solvents, pesticides, herbicides, fertilisers and other agricultural chemicals. Prior chemical process operations at the site involved transfer of contaminated liquids into a concrete evaporation pond that was filled with layers of earthen materials.

The ISV process was used to treat the materials contained in the evaporation pond in situ along with additional contaminated soils and debris that had been collected from around the site and consolidated into the pond. Consequently, this treatment configuration was a combination of the in situ mode and the staged in situ mode. In addition to the soil and debris, an oily liquid waste contaminated with dioxin was blended with soil and staged in the pond for treatment. The primary types of debris added to the pond area for treatment included drums, plastic, scrap metal, wood and investigation-derived wastes including protective clothing and sampling equipment. Also treated were secondary wastes resulting from the ISV process such as used filters. Treatment of secondary wastes was accomplished by burying the wastes in the treatment zone before initiating melts in those areas. ISV operations at the site required 37 melts to form one contiguous monolith. A ground water diversion system was employed because the ground water was normally within 300 mm of the ground surface. The operation was fully successful. Principal results include the following:

17.2.4 Applications in Japan

A joint venture company has been formed and a stationary batch type of ISV plant has been constructed in Ube City to evaluate and demonstrate the potential applications of ISV to Japanese site remediation and waste treatment needs. The treatment system is being employed to test and demonstrate the process on a range of waste materials including coal ash, incinerator ash, sewage sludge, asbestos, various types of debris, and contaminated soil. Several melting campaigns have been successfully completed on materials such as asbestos, sludges, and ash.

One series of tests involved the treatment of waste agricultural chemicals. The waste chemicals included high concentrations of hexachlorobenzene (HCB), DDT, parathion, arsenic and lead. The waste chemicals were mixed with soil and the mixture treated by the ISV process. The pre-treatment concentrations of HCB in the soils were 13.2% in one test and 32.8% in a second test. The tests were fully successful and resulted in melt destruction efficiencies of between 99.97% and 99.98% indicating that the complete process including the off-gas treatment system could be expected to achieve a destruction/removal efficiency for HCB well in excess of 99.9999%.

17.3 Considerations in the Application of the Technology

The ISV process can be used to treat all soil types (sands, silts, clays, etc.) provided there is a sufficient concentration of glass formers (silica and alumina) and alkali oxides (>1 wt%) to provide adequate electrical conductivity in the molten soil. Most soils and other earthen material meet this criteria and can be processed without modification. If necessary, an additive can be used to allow treatment of otherwise unacceptable media.

Sludges and soils that have a high moisture content (>70 wt%), including soils below the water table, have been successfully treated with the process. De-watering or water diversion techniques have been used during commercial ISV operations to facilitate the treatment of sites where the water recharge rate is excessive (>1x10-4 cm/s). Fully water-saturated soils and wastes can be treated with the process if the recharge rate is controlled.

Treatment depths are limited to approximately 7 m from grade level for the in situ treatment mode. Other ISV configuration options exist for sites requiring greater treatment depths including restaging the materials.

Site requirements include an area for treatment that has been cleared of active buried utilities and has sufficient space for the equipment. The process normally requires a set-up area for equipment that is a minimum of 30 m in length and 12 m in width directly adjacent to the area to be treated. However, other more compact equipment arrangements are possible if space is limited.

For an in situ application where there is uncertainty about what might be buried in a site, certain preconditioning steps have been developed to ensure that no sealed drums or other high integrity containers remain in the volume to be treated. It is not desirable to treat a site containing sealed drums or other large, high integrity containers. Sites containing such containers have been successfully treated in commercial projects by using a simple and cost effective preconditioning step to disrupt any sealed containers. For applications involving staged in situ or stationary batch modes of treatment, sealed containers are breached during the staging process and the contents mixed with the soil to be treated.

17.4 Experience and Availability in Australia

The ISV process is represented in Australia and a full-scale plant is under construction. Construction of the plant will be completed in 1997. The plant will then be used for treatment of a number of burial pits at the Maralinga site containing soil and debris contaminated with plutonium, uranium and various heavy metals.

The ISV process has been used to successfully treat a number of Superfund sites in the US and an operation in Japan has been established. In addition to the full-scale site remediations, several hundred tests and demonstrations have been conducted at all scales to assess a wide range of applications.

17.5 Summary

(a) Proponents (in Australia)

Geosafe Australia Pty. Ltd. (Adelaide)

(b) Wastes Applicable

ISV is applicable to all classes of contaminants (organics, heavy metals and radionuclides) and is applicable to mixtures of these contaminants.

ISV has been demonstrated to be effective in commercial full-scale operations at treating a wide range of volatile and semi-volatile organic compounds including PCBs, hexachlorobenzene, dioxin, pesticides, herbicides, and solvents.

ISV is primarily a technology for treating contaminated soils, sludges or other earthen media although it can be used to treat liquid and non-soil wastes by mixing the wastes with soil.

ISV has a high tolerance for debris and can accommodate a wide range of debris types without the need for handling or size reduction. Types of debris previously treated in full-scale commercial operations include: concrete, rock, asphalt, drums, scrap steel, plastic, wood, tyres, etc.).

(c) Contaminants Applicable

All scheduled compounds, nearly all other organics compounds, heavy metals and radionuclides.

(d) Timing for Commercialisation in Australia

A full-scale treatment plant is under construction. Treatment operations will commence at Maralinga in early 1998. It is not clear when the plant may be available for use on other treatment projects.

(e) Cost (example only)

Extrapolating established US costs results in a cost of $500 to $750 per tonne for contaminated soil, however this would need to be confirmed under Australian conditions.

Liquids and non-soil wastes would be mixed with soil. Treatment costs for liquid and non-soil wastes would depend on the soil mixture ratios.

Establishment costs are expected to be significant for the ISV process and therefore it is likely to be relatively expensive for smaller projects.

(f) Safety/Environmental Risk

ISV is relatively safe and represents a low risk to the environment as demonstrated by successful commercial operations in the US and in Japan. Off-gases are collected for treatment, organic contaminants are destroyed in the melt and heavy metals are contained within the vitreous product.

(g) Non-technical Impediments

None. The ISV process has received favourable public and regulatory support in the US and Japan. Geosafe note that in the case of the Maralinga project they have worked with the Department of Primary Industry and Energy in consulting with affected parties, including aboriginal groups. Geosafe note that no issues that may compromise implementation of the technology have been identified.

(h) Preferred Mode of Implementation

The mobile process is designed for on-site treatment. The three treatment configurations of in situ, staged in situ, and stationary batch provide the site owner with several configurations to meet site specific needs.

(i) Limitations

ISV requires either soil or some other earthen material to serve as the treatment media (melt). Liquids or other non-soil wastes can be mixed with soil for treatment.

Chapter 16 - Thermal Desorption Systems Chapter 18 - Solar Detoxification