Dense Nonaqueous Phase Liquids (DNAPLs)Chromium VI
Dense Nonaqueous Phase Liquids (DNAPLs)
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Treatment Technologies
In Situ Flushing:
Cosolvent/Alcohol Flooding, Surfactant Flushing
This page identifies general resources that contain detailed information on the design and implementation of in situ flushing technologies. Information on applications of in situ flushing technology specific to a chemical class can be found in the class subsections listed to the right. More resources on flushing technologies for a wide range of contaminants can be found in the In Situ Flushing pages of Technology Focus.
Soil flushing involves flooding a source area with an appropriate solution to remove the contaminant from the soil. Water or liquid solution is injected or infiltrated into the area of contamination. The contaminants are mobilized by solubilization, formation of emulsions, or a chemical reaction with the flushing solutions. After passing through the contamination zone, the contaminant-bearing fluid is collected by a groundwater extraction system and brought to the surface for disposal, recirculation, or on-site treatment and reinjection. Application of soil flushing is based on delivery of the flushing fluid, control of the fluid flow, and fluid recovery.
Flushing solutions may be water, acidic aqueous solutions, basic solutions, chelating or complexing agents, reducing agents, cosolvents, or surfactants. For example, water-soluble (hydrophilic) or water-mobile constituents can be flushed with water, and different metals can be removed with acidic solutions, basic solutions, and/or chelating, complexing, and reducing agents. Cosolvents are usually miscible and are effective for some organics, and surfactants can assist in the removal of hydrophobic organics (U.S. EPA 1991).
The techniques most frequently employed in soil flushing are surfactant and cosolvent flooding for fuels and chlorinated solvents. Many types of surfactants (cationic, anionic, nonionic) are available, but anionic or nonionic surfactants are generally used because their negative or neutral charge reduces the possibility of their sorption to negatively charged clay particles. They also are generally less toxic than cationic surfactants. Surfactant/cosolvent flushing has been shown to be effective for several DNAPL types, including spent degreasing solvents (TCE and TCA), dry cleaning solvents (PCE), heavy fuel oils, and coal tar/creosote. Laboratory work has also demonstrated applicability to PCB-containing mineral oils (ITRC 2003).
Surfactants are commonly constructed with hydrophobic and hydrophilic chemical components, meaning that one end of the molecule is attracted to oil (or organic compounds) and the other to water. Surfactants chosen primarily to increase the contaminant (generally a NAPL) solubility are used in a solubilization flood. Surfactants chosen to produce ultra-low interfacial tensions are employed in a mobilization flood (Kueper et al. 1997). Mobilization flooding should only be considered when there is a high degree of certainty that the solution can be recovered, such as with a competent bedrock or capillary barrier underlying the treatment zone.
A typical surfactant solution also may contain additives, such as electrolytes and a cosolvent. In addition to being effective with the target contaminant, the surfactant solution should be compatible with the site-specific soil, soil pore water, and groundwater (if applicable). A cosolvent, such as isopropanol, can be used to improve the surfactant solubility in solution and provide the surfactant/contaminant solution with an acceptable viscosity. A side effect of adding chemicals to the surfactant solution is that they need to be treated along with the contaminant at the recovery end (NAVFAC 2002).
Cosolvents, usually alcohols, are chemicals that dissolve in both water and NAPL. In an alcohol flood, the alcohol may partition into both the NAPL and water phases. Partitioning affects the viscosity, density, solubility, and interfacial tension of the NAPL (Kueper et al. 1997). The physical properties of the NAPL vary with the amount of alcohol available for interaction, and whether the alcohol preferentially dissolves into the NAPL or into the water. Complete miscibility is achievable and results in a pumpable solution that, depending upon the density of the NAPL and the proportions of alcohol and water in the solution, may be more or less dense than water.
Before implementing surfactant and/or cosolvent flushing, laboratory and bench-scale treatability testing should be done to ensure the selection of one or more agents best suited for the contaminant and the site-specific soil and geochemical conditions. Modeling of subsurface conditions is commonly done to identify the best delivery system. Flushing is most efficient in relatively homogeneous and permeable (K ≥ 10-3 cm/sec) soil (NAVFAC 2002). Heterogeneous soil reduces the efficiency of the flood sweep and may prevent optimum contact between the agent(s) and the target contaminant. Flushing of relatively homogeneous but lower permeability (10-4 to 10-5 cm/sec) units is possible, but it requires a high induced gradient to move the agent, while greatly increasing the remediation time (NAVFAC 2002).
Other soil factors that may adversely affect efficiency are high cation exchange capacity, high buffering capacity, high organic soil content, and pH. Land disposal restrictions and underground injection control regulations also may limit selection of the flushing solution. At a former drycleaner, ethanol was substituted for isopropanol because of regulatory concern about the toxicity and persistence of isopropanol. Most states allow in situ flushing of saturated or unsaturated soil, with a permit, if the aquifer in the area is already contaminated. When applying for a permit, all chemicals involved, including unreacted compounds and impurities, must be listed (NAVFAC 2002).
Due to its use for decades in oil field applications, soil flushing is considered a mature technology; however, it has found limited application in the environmental arena. ITRC (2003) estimates the cost of surfactant/cosolvent flushing of a DNAPL source zone to range between $65 and $200 per cubic yard, whereas the NAVFAC website gives cost estimates of $100 to $300 per cubic yard for flushing. The variability stems from the waste type and the quantity to be treated. The NAVFAC figures do not include design and engineering costs, which can be considerable. Cost per cubic yard can be misleading, and the cost per gallon recovered or destroyed should also be evaluated. Overall costs can be lowered if the flushing agent is recovered and recycled (U.S. EPA 2006).
Technology Advantages
- Elimination of the need to excavate, handle, and transport large quantities of the original contaminant;
- Enhancement to pump and treat may speed site remediation and closure;
- Applicable to a wide range of contaminants in both vadose and saturated zones; and,
- May be used in conjunction with other technologies or in stages for complex cases (Roote 1997).
Technology Limitations
- Lengthy remediation times due to the slow rate of diffusion processes in the liquid phase);
- Potential for spreading contaminants beyond the capture zone, laterally or vertically, if the extraction system is not properly designed or constructed, hydraulic control is not maintained, or a groundwater discharge zone captures flow from the treatment zone;
- Limited regulatory acceptance due to the potential for spreading contaminants, and concern with introducing flushing solutions into the subsurface that may remain in residual quantities;
- Uncertainties involved in prediction of performance and duration to achieve clean up goals;
- Limitations on effectiveness resulting from man-made obstructions, such as pipes/utilities, especially at underground storage tank sites;
- Geologic setting conditions, e.g., low permeability, high clay or organic content, high degree of heterogeneity or secondary permeability, and close proximity to sensitive recharge areas or potable aquifers;
- Selection of flushing solutions that reduce effective soil porosity by adhering to soil, accelerating biogrowth, or causing precipitation or other reactions with ambient soil or groundwater; and,
- Inability to separate flushing additive from elutriate, resulting in high consumption and prohibitive expense of flushing additive (Roote 1997).
Additional research is needed to advance good sweep efficiency, to optimize the implementation of surfactant/cosolvent technologies in karst and fractured bedrock formations, to evaluate the combination of flushing technologies with other source zone and/or plume remedial technologies, and to evaluate the long-term impact of the mass removal on post-flushing water flooding and natural attenuation.
Primary Source: U.S. EPA. 2006. 
In Situ Treatment Technologies for Contaminated Soil: Engineering Forum Issue Paper. EPA 542-F-06-013.
ITRC. 2003. Technical and Regulatory Guidance for Surfactant/Cosolvent Flushing of DNAPL Source Zones.
Kueper, B. et al. 1997. Technology Practices Manual for Surfactants and Cosolvents (TR-97-2). Advanced Applied Technology Demonstration Facility Program, Rice University.
NAVFAC. 2002. Surfactant-Enhanced Aquifer Remediation (SEAR) Design Manual, NFESC Technical Report TR-2206-ENV.
Roote, D.S. 1997. In Situ Flushing: Technology Overview Report. Ground-Water Remediation Technologies Analysis Center, TO-97-02.
U.S. EPA. 1991. Engineering Bulletin: In Situ Soil Flushing. EPA 540-2-91-021.
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General Resources |
Abstracts
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AATDF Technology Practices Manual for Surfactants and Cosolvents
Advanced Applied Technology Demonstration Facility for Environmental Technology (AATDF) Program, TR-97-2, 1997
In Situ Flushing Site Profiles
EPA has developed a searchable database that contains information about ongoing and completed applications of in situ flushing technologies to treat chlorinated solvents, petroleum products, metals, explosives, and PCBs in groundwater and soil. The project profiles provide summary information about each application, including site information, contaminants and media treated, technology design and operation, cost information, and performance results, as well as points of contact and references.
In Situ Flushing: Technology Overview Report
D.S. Roote.
Ground-Water Remediation Technologies Analysis Center (GWRTAC). TO-97-02, 24 pp, 1997
INDOT Guidance Document for In-Situ Soil Flushing
L.S. Lee, X. Zhai, and J. Lee.
SPR-2335; FHWA/IN/JTRP-2006/28, 49 pp, Jan 2007
Contains information ranging from the basic chemistry and mechanisms of cosolvent and surfactant flushing to the key factors that need to be considered during the selection, design, and implementation of this technology. Also provides information on several categories of contaminants subject to in situ flushing. This text should be used as a general guidance rather than as a design manual.
NAPL Removal: Surfactants, Foams, and Microemulsions
C.H. Ward.
CRC Press/Lewis Publishers, Boca Raton, FL. ISBN: 1566704677, 592 pp, 2000
Surfactant-Enhanced Aquifer Remediation (SEAR) Design Manual
Naval Facilities Engineering Service Center. NFESC Technical Report TR-2206-ENV, 110 pp, 2002
Surfactant-Enhanced Aquifer Remediation (SEAR) Implementation Manual
Naval Facilities Engineering Service Center. NFESC Technical Report TR-2219-ENV54, 54 pp, 2003
Surfactant-Enhanced DNAPL Remediation: Surfactant Selection, Hydraulic Efficiency, and Economic Factors
D.A. Sabatini, R.C. Knox, J.H. Harwell.
EPA 600-S-96-002, 15 pp, 1996
Surfactant Enhanced DNAPL Removal: ESTCP Cost and Performance Report
Environmental Security Technology Certification Program (ESTCP), 216 pp, 2001
Surfactants/Cosolvents: Technology Evaluation Report
C.T. Jafvert.
Ground-Water Remediation Technologies Analysis Center, TE-96-02, 50 pp, 1996
Technical and Regulatory Guidance for Surfactant/Cosolvent Flushing of DNAPL Source Zones
Interstate Technology & Regulatory Council (ITRC). DNAPLs-3, 151 pp, 2003
Modeling Field-Scale Cosolvent Flooding for DNAPL Source Zone Remediation
H. Liang and R.W. Falta. Journal of Contaminant Hydrology, Vol 96 Nos 1-4, p 1-16, 29 Sep 2007
When a 3-D, compositional, multiphase flow simulator was used to model a field-scale test of PCE DNAPL removal by cosolvent flooding, the effectiveness of DNAPL source zone remediation was affected mainly by characteristics of the spatial heterogeneity of porous media and the variable (and unknown) DNAPL distribution. The inherent uncertainty in the DNAPL distribution at real field sites means that some form of calibration of the initial contaminant distribution will almost always be required to match contaminant effluent breakthrough curves.
Simultaneous Optimization of Dense Non-Aqueous Phase Liquid (DNAPL) Source and Contaminant Plume Remediation
A. Mayer and K.L. Endres.
Journal of Contaminant Hydrology, Vol 91 Nos 3-4, p 288-311, 2007
A framework developed for simultaneous, optimal design of ground-water contaminant source removal and plume remediation strategies allows for varying degrees of effort and cost to be dedicated to source removal versus plume remediation. High and low estimates of capital and operating costs for chemical flushing removal of the source are considered.
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Last updated on Thursday, July 24, 2008