Solidification/stabilization (S/S) reduces the mobility of hazardous substances and contaminants in the environment through both physical and chemical means. Unlike other remedial technologies, S/S seeks to trap or immobilize contaminants within their 'host' medium (i.e., the soil, sand, and/or building materials that contain them), instead of removing them through chemical or physical treatment. Leachability testing is typically performed to measure the immobilization of contaminants. S/S techniques can be used alone or combined with other treatment and disposal methods to yield a product or material suitable for land disposal or, in other cases, that can be applied to beneficial use. These techniques have been used as both final and interim remedial measures.
Auger/caisson systems and injector head systems are techniques used in soil S/S. They apply S/S agents to soils to trap or immobilize contaminants.
Bottom barriers are horizontal subsurface barriers that prevent vertical migration by providing a floor of impermeable material beneath the waste. The installation of a grout injection bottom barrier involves directional drilling with forced grout injection. Implementation of this technology is highly dependent on the physical properties of soil.
In Situ Vitrification (ISV)
In situ vitrification (ISV) is another in situ S/S process which uses an electric current to melt soil or other earthen materials at extremely high temperatures (1,600 to 2,000 °C or 2,900 to 3,650 °F) and thereby immobilize most inorganics and destroy organic pollutants by pyrolysis. Inorganic pollutants are incorporated within the vitrified glass and crystalline mass. Water vapor and organic pyrolysis combustion products are captured in a hood, which draws the contaminants into an off-gas treatment system that removes particulates and other pollutants from the gas. The vitrification product is a chemically stable, leach-resistant, glass and crystalline material similar to obsidian or basalt rock. The process destroys and/or removes organic materials. Radionuclides and heavy metals are retained within the molten soil.
The timeframe for in situ S/S is short- to medium-term, while in situ ISV process is typically short-term.
The target contaminant group for in situ S/S is generally inorganics (including radionuclides).
The Auger/Caisson and Reagent/Injector Head Systems have limited effectiveness against SVOCs and pesticides and no expected effectiveness against VOCs; however, systems designed to be more effective in treating organics are being developed and tested.
The ISV process can destroy or remove organics and immobilize most inorganics in contaminated soils, sludge, or other earthen materials. The process has been tested on a broad range of VOCs and SVOCs, other organics including dioxins and PCBs, and on most priority pollutant metals and radionuclides.
Factors that may limit the applicability and effectiveness of the in situ S/S include:
Depth of contaminants may limit some types of application processes.
Future usage of the site may 'weather' the materials and affect ability to maintain immobilization of contaminants.
Some processes result in a significant increase in volume (up to double the original volume).
Certain wastes are incompatible with variations of this process. Treatability studies are generally required.
Reagent delivery and effective mixing are more difficult than for ex situ applications.
Like all in situ treatments, confirmatory sampling can be more difficult than for ex situ treatments.
The solidified material may hinder future site use.
Processing of contamination below the water table may require dewatering.
Data needs include particle size, Atterberg limits, moisture content, metal concentrations, sulfate content, organic content, density, permeability, unconfined compressive strength, leachability, pH, and microstructure analysis. For ISV, a minimum alkali content in soil (sodium and potassium oxides) of 1.4 wt% is necessary to form glass. The composition of most soils is well within the range of processability.
Auger/Caisson and Reagent/Injector Head Systems processes are well demonstrated, can be applied to the most common site and waste types, require conventional materials handling equipment, and are available competitively from a number of vendors. Most reagents and additives are also widely available and relatively inexpensive industrial commodities.
Auger/Caisson and Reagent/Injector Head Systems processes have demonstrated the capability to reduce the mobility of contaminated waste by greater than 95%. The effects, over the long term, of weathering (e.g., freeze-thaw cycles, acid precipitation, and wind erosion), ground water infiltration, and physical disturbance associated with uncontrolled future land use can significantly affect the integrity of the stabilized mass and contaminant mobility in ways that cannot be predicted by laboratory tests.
There have been few, if any, commercial applications of ISV. The ISV process has been operated for test and demonstration purposes at the pilot scale and at full scale at the following sites: (1) Geosafe Corporation's test site, (2) DOE's Hanford Nuclear Reservation, (3) DOE's Oak Ridge National Laboratory, and (4) DOE's Idaho National Engineering Laboratory. More than 170 tests at various scales have been performed on a broad range of waste types in soils and sludge. A demonstration will take place at the Parsons/ETM site in Grand Ledge, Michigan, where the process is currently operating.
Process depths up to 6 meters (19 ft) have been achieved in relatively homogeneous soils. The achievable depth is limited under certain heterogeneous conditions.
Costs for Auger/Caisson and Reagent/Injector Head Systems processes vary widely according to materials or reagents used, their availability, project size, and chemical nature of contaminants (e.g., types and concentration levels for shallow applications). The in situ soil mixing/auger techniques average $50 to $80 per cubic meter ($40 to $60 per cubic yard) for the shallow applications and $190 to $330 per cubic meter ($150 to $250 per cubic yard) for the deeper applications.
The shallow soil mixing technique processes 36 to 72 metric tons (40 to 80 tons) per hour on average, and the deep soil mixing technique averages 18 to 45 metric tons (20 to 50 tons) per hour.
The major factor driving the selection process beyond basic waste compatibility is the availability of suitable reagents. Auger/Caisson and Reagent/Injector Head Systems processes require that potentially large volumes of bulk reagents and additives be transported to project sites. Transportation costs can dominate project economics and can quickly become uneconomical in cases where local or regional material sources are unavailable.
The cost for grout injection varies depending on site-specific conditions. Costs for drilling can range from $50 to $150/ft and grouting from $50 to $75/ft, not including mobilization, wash disposal, or adverse site condition expenses.
For ISV, average costs for treatability tests (all types) are $25K plus analytical fees; for PCBs and dioxins, the cost is $30K plus analytical. Equipment mobilization and demobilization costs are $200K to $300K combined. Vitrification operation cost varies with electricity costs, quantity of water, and depth of process.