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Remediation Technologies Screening Matrix, Version 4.0 4.20 Solidification/Stabilization
(Ex Situ Soil Remediation Technology)
  Description Synonyms Applicability Limitations Site Information Points of Contact
Data Needs Performance Cost References Vendor Info. Health & Safety
Table of Contents
Technology>>Soil, Sediment, Bedrock and Sludge

>>3.5 Ex Situ Physical/Chemical Treatment (assuming excavation)

      >>4.20 Solidification/Stabilization
Introduction>> Contaminants are physically bound or enclosed within a stabilized mass (solidification), or chemical reactions are induced between the stabilizing agent and contaminants to reduce their mobility (stabilization).


Figure 4-20: Typical Ex Situ Solidification/ stabilization Process Flow Diagram  

As for in situ solidification/stabilization (S/S) (see Technology Profile No. 4.9), ex situ S/S contaminants are physically bound or enclosed within a stabilized mass (solidification), or chemical reactions are induced between the stabilizing agent and contaminants to reduce their mobility (stabilization). Ex situ S/S, however, typically requires disposal of the resultant materials. Under CERCLA, material can be replaced on site.

There are many innovations in the stabilization and solidification technology. Most of the innovations are modifications of proven processes and are directed to encapsulation or immobilizing the harmful constituents and involve processing of the waste or contaminated soil. Nine distinct innovative processes or groups of processes include: (1) bituminization, (2) emulsified asphalt, (3) modified sulfur cement, (4) polyethylene extrusion, (5) pozzolan/Portland cement, (6) radioactive waste solidification, (7) sludge stabilization, (8)soluble phosphates, and (9) vitrification/molten glass.

Typical ex situ S/S is a short- to medium-term technology.


In the bituminization process, wastes are embedded in molten bitumen and encapsulated when the bitumen cools. The process combines heated bitumen and a concentrate of the waste material, usually in slurry form, in a heated extruder containing screws that mix the bitumen and waste. Water is evaporated from the mixture to about 0.5% moisture. The final product is a homogenous mixture of extruded solids and bitumen.

Emulsified Asphalt

Asphalt emulsions are very fine droplets of asphalt dispersed in water that are stabilized by chemical emulsifying agents. The emulsions are available as either cationic or anionic emulsions. The emulsified asphalt process involves adding emulsified asphalts having the appropriate charge to hydrophilic liquid or semiliquid wastes at ambient temperature. After mixing, the emulsion breaks, the water in the waste is released, and the organic phase forms a continuous matrix of hydrophobic asphalt around the waste solids. In some cases, additional neutralizing agents, such as lime or gypsum, may be required. After given sufficient time to set and cure, the resulting solid asphalt has the waste uniformly distributed throughout it and is impermeable to water.

Modified Sulfur Cement

Modified sulfur cement is a commercially-available thermoplastic material. It is easily melted (127 to 149 C (260 to 300 F)) and then mixed with the waste to form a homogenous molten slurry which is discharged to suitable containers for cooling, storage, and disposal. A variety of common mixing devices, such as, paddle mixers and pug mills, can be used. The relatively low temperatures used limit emissions of sulfur dioxide and hydrogen sulfide to allowable threshold values.

Polyethylene Extrusion

The polyethylene extrusion process involves the mixing of polyethylene binders and dry waste materials using a heated cylinder containing a mixing/transport screw. The heated, homogenous mixture exits the cylinder through an output die into a mold, where it cools and solidifies. Polyethylene’s properties produce a very stable, solidified product. The process has been tested on nitrate salt wastes at plant-scale, thereby establishing its viability, and on various other wastes at the bench and pilot scale.

Pozzolan/Portland Cement

Pozzolan/Portland cement process consists primarily of silicates from pozzolanic-based materials like fly ash, kiln dust, pumice, or blast furnace slag and cement-based materials like Portland cement. These materials chemically react with water to form a solid cementious matrix which improves the handling and physical characteristics of the waste. They also raise the pH of the water which may help precipatate and immobilize some heavy metal contaminants. Pozzolanic and cement-based binding agents are typically appropriate for inorganic contaminants. The effectiveness of this binding agent with organic contaminants varies.

Radioactive Waste Solidification

In radioactive waste solidification (Grouting/Other) treatment, solidification additives are used to form a uniform and stable matrix to encapsulate radioactive waste materials. Assemblies include pumps for liquids or slurries, conveyors for sludges or solids, storage silos, weigh feeders, piping, mixers and disposal or storage.

Sludge Stabilization

The sludge stabilization process is the addition of a reagent, either slags or cementitious materials, to sludge to transform the material so that the hazardous constituents are in their least mobile or toxic form. Sludges which leach heavy metals or other contaminants are often stabilized to immobilize the hazardous constituents.

Soluble Phosphates

The soluble phosphates process involves the addition of various forms of phosphate and alkali for control of pH as well as for formation of complex metal molecules of low-solubility to immobilize (insolubilize) the metals over a wide pH range. Unlike most other stabilization processes, soluble phosphate processes do not convert the waste into a hardened, monolithic mass. One application of soluble phosphates and lime is in stabilizing fly ash by immobilizing the lead and cadmium in the ash.

Vitrification/Molten Glass

Vitrification, or molten glass, processes are solidification methods that employ heat up to 1,200 C to melt and convert waste materials into glass or other glass and crystalline products. The high temperatures destroy any organic constituents with very few byproducts. Materials, such as heavy metals and radionuclides, are actually incorporated into the glass structure which is, generally, a relatively strong, durable material that is resistant to leaching. In addition to solids, the waste materials can be liquids, wet or dry sludges, or combustible materials. Borosilicate and soda-lime are the principal glass formers and provide the basic matrix of the vitrified product.

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M13 (Vitrification)
N11 (Solidification/Stabilization)
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The target contaminant group for ex situ S/S is inorganics, including radionuclides. Most S/S technologies have limited effectiveness against organics and pesticides, except vitrification which destroys most organic contaminants.

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Factors that may limit the applicability and effectiveness of the process include:
  • Environmental conditions may affect the long-term immobilization of contaminants.
  • Some processes result in a significant increase in volume (up to double the original volume).
  • Certain wastes are incompatible with different processes. Treatability studies are generally required.
  • Organics are generally not immobilized.
  • Long-term effectiveness has not been demonstrated for many contaminant/process combinations.

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Data Needs:

A detailed discussion of these data elements is provided in Subsection 2.2.1 (Data Requirements for Soil, Sediment, and Sludge). Soil parameters that must be determined include particle size, Atterberg limits, moisture content, metal concentrations, sulfate content, organic content, density, permeability, unconfined compressive strength, leachability, microstructure analysis, and physical and chemical durability.

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Performance Data:

The performance of ex situ S/S is dependent on the type of S/S process used.

DOE has demonstrated the Polyethylene Encapsulation of Radionuclides and Heavy Metals (PERM) process at the bench scale. The process is a waste treatment and stabilization technology for high-level mixed waste. Specific targeted contaminants include radionuclides (e.g., cesium, strontium, and cobalt), and toxic metals (e.g., chromium, lead, and cadmium). Scale-up from bench-scale tests has demonstrated the feasibility to process waste at approximately 2,000 lb/hr. The scale-up feasibility tests have successfully demonstrated the potential to encapsulate at least 60% by weight nitrate salt in polyethylene. Polyethylene waste forms have been demonstrated to exceed Nuclear Regulatory Commission, EPA, and Department of Transportation waste form criteria. Waste forms containing up to several thousand ppm of toxic-metal contaminants have passed the EPA's TCLP.

DOE also demonstrated the arc melter vitrification process, which is capable of melting soil and metals, pyrolizing or oxidizing residual organics, melting structural metals from melted slag (silica and metal oxides), and partitioning transuranic (TRU) waste into slag phase. Durability tests with the resultant slag showed an approximately order of magnitude reduction in leachability when compared with high-level borosilicate glass.

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The key cost driver information and cost analysis was developed in 2006 using the Remedial Action Cost Engineering and Requirements (RACER) software.

Key Cost Drivers 

        Type of Waste

o       Moisture content in the sludge drives up costs compared to solid

o       Contaminant concentration and type determine the amount of reagents added to the waste to attain the required treatment standards

        Size of the mobile s/s system

o       Choosing the correct size mobile s/s system to adequately handle the throughput of waste volume

Cost Analysis

The following table represents estimated costs (by common unit of measure) to apply solidification/stabilization technology at sites of varying size and complexity.   A more detailed cost estimate table which includes specific site characteristics and significant cost elements that contributed to the final costs can be viewed by clicking on the link below.





Scenario A

Scenario B

Scenario C

Scenario D

Small Site

Large Site

























Detailed Cost Estimate

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Treatment Technologies for Site Cleanup: Annual Status Report (ASR), Tenth Edition, EPA 542-R-01-004

Innovative Remediation Technologies:  Field Scale Demonstration Project in North America, 2nd Edition

Remediation Technology Cost Compendium - Year 2000

Abstracts of Remediation Case Studies, Volume 4,  June, 2000, EPA 542-R-00-006

Guide to Documenting and Managing Cost and Performance Information for Remediation Projects - Revised Version, October, 1998, EPA 542-B-98-007

Battelle Memorial Institute, 1995. ReOpt. V3.1, by Battelle Memorial Institute for DOE under Contract DE/AC06/76RLO 1830.

Bricka, R.M., et al., 1988. An Evaluation of Stabilization/Solidification of Fluidized Bed Incineration Ash (K048 and K051), USAE-WES Technical Report EL-88-24.

California Base Closure Environmental Committee (CBCEC), 1994. Treatment Technologies Applications Matrix for Base Closure Activities, Revision 1, Technology Matching Process Action Team, November, 1994.

DOE, 1993. "Technology Name: Polyethylene Encapsulation", Technology Information Profile (Rev. 2) for ProTech, DOE ProTech Database, TTP Reference No. BH-321201.

DOE, 1995. Technology Catalogue, Second Edition, Office of Environmental Management and Office of Technology Development, DOE/EM-0235.

EPA, 1989. Chemfix Technologies, Inc. Chemical Fixation/Stabilization, EPA RREL, Technology Evaluation Vol. I, EPA/540/5-89/011a, PB91-127696; and Technology Evaluation Vol. II, EPA/540/5-89/011b, PB90-274127.

EPA, 1989. Harcon Solidification, EPA RREL, series includes Technology Evaluation Vol. I, EPA/540/5-89/001a, PB89-158810; Technology Evaluation Vol. II, EPA/540/5-89/001b, PB89-158828; Applications Analysis, EPA/540/A5-89/001; and Technology Demonstration Summary, EPA/540/S5-89/001.

EPA, 1989. Solidtech, Inc. Solidification, EPA RREL, series includes Technology Evaluation Vol. I, EPA/540/5S-89/005a; Technology Evaluation Vol. II, EPA/540/5S-89/005b, PB90-191768; Applications Analysis, EPA/540/A5-89/005; Technology Demonstration Summary, EPA/540/S5-89/005; and Demonstration Bulletin, EPA/540/M5-89/005.

EPA, 1989. Stabilization/Solidification of CERCLA and RCRA Wastes - Physical Tests, Chemical Testing Procedures, Technology Screening and Field Activities, EPA, ORD, Washington, DC, EPA/625/6-89/022.

EPA, 1992. Silicate Technology Corporation Solidification/Stabilization of Organic/Inorganic Contaminants, EPA RREL, Demonstration Bulletin, EPA/540/MR-92/010; Applications Analysis, EPA/540/AR-92/010, PB93-172948.

EPA, 1993. Solidification/Stabilization and Its Application to Waste Materials, Technical Resource Document, EPA, ORD, Washington, DC, EPA/530/R-93/012.

EPA, 1993. Solidification/Stabilization of Organics and Inorganics, Engineering Bulletin, EPA, ORD, Cincinnati, OH, EPA/540/S-92/015.

EPA, 1994. Innovative Site Remediation Technology: Solidification/Stabilization, Vol. 4, EPA OSWER 542/B-94/001.

EPA, 1997. Best Management Practices (BMPs) for Soil Treatment Technologies: Suggested Operational Guidelines to Prevent Cross-media Transfer of Contaminants During Clean-UP Activities, EPA OSWER, EPA/530/R-97/007.

EPA, 1997. Technology Alternatives for the Remediation of Soils Contaminated with As, Cd, Cr, Hg, and Pb, Engineering Bulletin, EPA540/R-97/008.

Federal Remediation Technologies Roundtable, 1997. Remediation Case Studies: Soil Vapor Extraction and Other In Situ Technologies, EPA/542/R-97/009.

Federal Remediation Technologies Roundtable, 1998. Remediation Case Studies: Debris and Surface Cleaning Technologies, and Other Miscellaneous Technologies, EPA/542/R-98/017.

USAEC, 1997. "Plasma Arc Technology Evaluation" in Innovative Technology Demonstration, Evaluation and Transfer Activities, FY 96 Annual Report, Report No. SFIM-AEC-ET-CR-97013, pp. 107-110.

Wittle, J.K.,, 1995. Graphite Electrode DC Arc Technology Program for Buried Waste Treatment, Electro-Pyrolysis, Inc. Wayne, Penn.

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Site Information:

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Points of Contact:

General FRTR Agency Contacts

Technology Specific Web Sites:

Government Web Sites

Non Government Web Sites

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Vendor Information:

A list of vendors offering Ex Situ Physical/Chemical Soil Treatment is available from EPA REACH IT which combines information from three established EPA databases, the Vendor Information System for Innovative Treatment Technologies (VISITT), the Vendor Field Analytical and Characterization Technologies System (Vendor FACTS), and the Innovative Treatment Technologies (ITT), to give users access to comprehensive information about treatment and characterization technologies and their applications.

Government Disclaimer

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Health and Safety:

Hazard Analysis

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