Remediation of Contaminated Sites – In Situ Thermal Remediation

Remediation of Contaminated Sites – In Situ Thermal Remediation

By Dr. Yashpal Singh

Chairman, The Wealthy Waste School India

In-situ treatment of the source of a soil or ground water plume is being accomplished using mass transfer and recovery methods e.g. Soil vapor extraction, air sparging, surfactant/co solvent flushing or destruction methods like bio remediation, oxidation/reduction sometimes enhanced by enabling techniques like soil fracturing and soil heating. For the treatment of distal regions of ground water plumes, natural bio geo-chemical attenuation and permeable reactive barrier technologies are used.

The Physical properties of fluid contaminants and their interaction with soils control their movement in the subsurface and their recovery. Most of these properties are temperature dependent. Thermally enhanced extraction is an innovative combination of ground water extraction, soil vapour extraction, steam injection, hot air injection and control techniques for accelerated and complete remediation

In situ thermal remediation processes can be used to enhance the recovery of organic contaminants specially gasoline and diesel and chlorinated solvents. The main mechanism is an increase in vapour pressure with temperature which allows these chemicals to be removed in the vapour phase. Enhanced solubility and decreased adsorption at higher temperature also contribute to the recovery of these chemicals. Temperature induced viscosity reduction, increased relative permeability and decreased capillary forces also play an important role. Residual contaminants can be degraded by soil microbes which can survive high temperatures. Thermo physical reactions can partially or fully oxidise some organic contaminants.

In situ thermal technology options are advantageous over other technologies. The main advantage of in situ remediation is that it allows ground water to be treated without being brought up to the surface. This results in cost savings. Also, diverse contaminants can be treated at the same time. However, thermal technologies may be difficult to apply near occupied or active sites, may require more sophisticated design and operation, may enhance the migration of contaminants to uncontaminated areas and the post remediation high temperatures may persist for years or months adversely effecting the soil ecology. Cleaning soil and ground water with thermal methods may take a few months or several years.

The most commonly used insitu thermal technologies include steam-based heating (steam enhanced extraction, conductive heating (insitu thermal desorption), electrical resistance heating (six or three phase heating), radio frequency heating and insitu soil mixing combined with steam and hot air injection. Heat in all these technologies is relied upon to enhance removal and treatment of contaminant vapours and liquids from the subsurface. Heating may decrease contaminant liquid viscosity, decrease interfacial tension, increase bio-degradation rates, increase solubility or increase volatility.

Steam Enhanced Extraction

Non aqueous phase chemicals are a serious issue when contaminating soil or ground water. They can continue to contaminate ground water for centuries. Steam enhanced extraction can play a very important role in their removals. In SEE steam is injected to the subsurface to heat the target zone of contamination to the boiling point while actively pumping water, NAPL and vapours from other wells. It involves installation of a network of steam injection and extraction of hot fluids points from onsite separation and treatment. The source area is surrounded by steam injection wells installed in cleaner matrices. The extraction wells are located in high concentration areas each surrounded by 5 to 6 wells.

 It has been satisfactorily employed for the recovery of TCE, NAPL, Gasoline and Creosote from soil and the removal of their soluble fractions from ground water.

At one site, dense non aqueous phase liquids (DNAPL) consisting mainly (70%) trichloro ethene had leached from trenches used for dispersal of wastes from solvent recovery units and accumulated 50 feet below the surface in the contaminated zone. Prior to experimenting with Thermally enhanced extraction, 26, 200 gallons of DNAPL had been recovered with a pump and treat system. The TEE process included 11 days of thermal injection followed by 12 days of ambient air injection accompanied by ground water extraction and soil vapour extraction in all the phases. The TEE process removed an estimated 908 gallons of DNAPL in a 3 weeks period. The average contaminant concentrations in the channel soil were reduced by 98% and the average concentration in soil above the aquitard (Zone of DNAPL accumulation) by 50%.

In another experiment where steam injection was tried over NAPL consisting mainly of weathered fuels and oils and chlorinated solvents, steam injection was found very effective. A reduction of 95% was observed in such compounds in steam swept areas and the final ground water concentrations in the test cell were close to clean up levels.

Electric Resistance heating

Electrical Resistance heating was primarily developed in order to enhance vapour extraction remediation technologies in low permeability soils. Soil and ground water are heated by passing an electrical current through saturated and unsaturated soil between electrodes (not by conductive heating from the electrodes). The resistance to the flow of electric current results in increased sub surface temperatures. ERH has been used to treat source zone contaminated with VOC’s, CVOC’s, LNAPL or DNAPL, pesticides, PAH and creosote.

 A new technology, six phase heating (SPH) has successfully remediated DNAPLS from the subsurface. It uses in situ electric resistive heating and steam stripping to achieve sub surface remediation. SPH transformers convert regular three phase energy system into 06 phases which are delivered throughout the treatment volume through electrodes. This raises the sub surface temperatures to boiling point of water and is equally effective in all soil types including fracture rock, under both vadose and saturated conditions. It is commercially available for the following treatment applications.

• Low permeability soils
• DNAPL and LNAPL remediation
• Cold regions treatment
• Heavy hydrocarbon Mobilisation
• Rapid Clean up
• Enhanced Biological remediation
• Aquifer heating.

Radio frequency Heating

Radio frequency antenna heating has been safely utilised to accelerate remediation of contaminated sites. It has been used satisfactorily with soil vapour extraction, ground water aeration (air sparging), product recovery and bio remediation. Heating causes changes in the physical, chemical, end biological properties of soils, ground water and contaminants making them more amenable to remediation efforts. The commercial radio frequency heating system designed to heat the vadose zone consists of two 2.4 KW continuous wave solid state R.F. generators operating at 27.12 MH3 and supplying energy to the vadose zone through 04 Antennas deployed in wells.

Initial pilot tests had indicated (Alaska, USA; cold climates) that a bio degradation rate increase of 2 to 10 times is possible with the application of R.F. and nutrients. In the USA, the RF antenna heating increased the off-gas soil vapour extraction contaminant concentrations substantially and helped complete the clean up within 06 months against the normal time of 2 to 3 years required at ambient soil temperatures.

Micro wave heating has been studied and found successful in heating the soil but it has limited penetration in the soil.

In situ Thermal desorption

In situ thermal desorption is a process for soil remediation where heat and vacuum are applied simultaneously to subsurface soils either through surface heater blankets or with an array of vertical heater/vacuum pumps. It has been used to remediate a wide variety of contaminants from low boiling VOC’s and CVOC’s to high boiling PAH’s, PCB’s and dioxins in both the vadose and saturated zones as well as fractured media (clay and rock).  As soil is heated, volatile contaminants in soil are vaporised by a number of mechanisms including evaporation into the air stream; steam distillation into the water vapour stream; boiling; oxidation and pyrolysis. The vaporised water, contaminants and natural organic compounds are drawn by the vacuum pump in counter current to the heat flow into the vacuum source in the heat blankets or wells. The soil can be heated to high temperatures (750⁰C) for prolonged times. It is a highly efficient removal technology. In practice most of the contaminants are destroyed in the soil before reacting the surface. Those contaminants that have not been destroyed are removed from the vapour steam through Air Pollution Control System.

Ex situ Thermal desorption

Thermal desorption removes harmful chemicals from soil and other materials (like sludge and sediments) by using heat to change the chemicals to gases. It uses an oven like large desorber where excavated soil is added and contaminants allowed to evaporate. The collected gases are cleaned and the treated soil returned to site. Clean up can take only a few weeks at small sites with small amounts of chemicals. If the site is large and the chemical levels are high, cleanup can take years. It works well at sites with dry soil and contaminants like fuel oil, coal for, chemicals that preserve wood and solvents.

Thermally enhanced Bio-degradation

Biodegradation techniques utilise the contaminants as the food source. They are typically implemented at low costs. Contaminants are destroyed and no residual treatment is required. Some compounds however can be transformed into more toxic compounds (TCE to Vinyl Chloride) which may be mobilised in ground water if proper steps are not taken.

Bioremediation techniques have been successfully utilised to remediate ground waters contaminated with petroleum hydrocarbons, solvents, pesticides, wood preservatives and other organic chemicals.

Heating the bioremediation site such as use of warm water injection may speed up the remediation process. Too high temperatures may be detrimental. Temperature also effects the abiotic loss of contaminants through evaporation.

In situ soil mixing

Source zone volatile organic compounds, semi volatile organic compounds and petroleum hydrocarbons contaminating soils and ground water can be removed effectively by in situ soil mixing with the injection of hot air and steam simultaneously. During active mixing (to depths of about 70ft) the permeability increases permitting the soil and ground water to be treated evenly. Steam heats the ground water and soil to about 75⁰C, thermally desorbing the VOC’s and volatilizing the non-adsorbed VOCs. The air carries the volatilized off gases to the surface for capture and treatment. The technology may remove 90% to 97% of the VOC’s and 50% to 90% of the SVOC’s.

Chlorinated DNAPL source zones can be remediated by injection of zero valent iron which continues the remediation after the thermal treatment has stopped. Removal efficiencies of 99% are achieved along with cost savings as compared to the use of the thermal treatment alone.

Self-sustaining technology for Active Remediation (STAR)

The technology relies on smouldering combustion processes and involves slow combustion of a liquid or solid fuel within a porous matrix and may apply to sites that are heavily contaminated with NAPL’s that have low volatility and significant fuel value, like longer chain petroleum hydrocarbons, coal tar and creosote. The combustion processes are primarily dominated by pyrolysis and oxidation which are initiated by ignitions at high temperatures (250 to 400⁰C) and sustained by the heat of combustion and injection of air. The process destroys the higher boiling point NAPL components in situ and gives off Co₂, Co and water vapor. Lighter components of the NAPL like Benzene, toluene, ethyl benzene and Xylene (BTEX) are generally volatilised before the combustion front reaches them. The combustion process may be terminated by stopping the injection of air.

Vitrification

Vitrification is a process that permanently traps harmful chemicals in a solid block of glass like material. This keeps them from leaving the site. Vitrification can be done either in situ or ex situ. It uses electric power passed through electrodes to create heat with temperatures of 1600-2000 degrees Celsius to melt the soil. Melting starts near the ground surface and moves down. As the soil melts, the electrodes sink deeper into the soil causing more soil to melt. When the power is turned off, the melted soil cools and vitrifies turning into a solid mass of glass like material and the electrodes become part of the block. When vitrified, the original volume of soil shrinks. This causes the ground surface in the area to sink slightly. To level it the sunken area is filled with clean soil. The heat used to melt the soil can also destroy some of the harmful chemicals and cause others to evaporate. The evaporated chemicals rise through the melted soil to the ground surface. Here, a hood, which covers the heated area, collects the chemicals. Those chemicals are sent to a treatment system where they are cleaned up. Any harmful chemicals that remain under ground become trapped in the vitrified block, which is left in place. This prevents rainfall, ground water flow and wind from transporting the chemicals off site. If offers faster clean up times. Often weeks to months instead of years.

Suggested Readings

1. 1. Siegrist, R.L., Urynowicz, M.A. and West, O.R., 2000. An overview of in-situ chemical oxidation technology features and applications. In “Abiotic in-situ technologies of ground water remediation”, Conference Proceedings, August 2000, EPA 625/-R-99/102 P. 61-69.
   2. Baker R.S., Nielsen, S.G.. Heson, A. and Ploug M. 2016. How effective is thermal remediation of DNAPL source zones in reducing ground water concentrations. Ground Water Monitoring and Remediation 36(1):38-53.
   3. Davis Eva L., 2023. Ground Water Issue Paper – In situ Thermal Remediation. U.S. Environment Protection Agency. EPA/600/R-23/062 June 2023. P.1 to 46.
   4. Grant, G.P., Major, D., Scholes, G.C., Horst, J., Hill, S., Klemmer, M.R. and Couch, J.N. 2016. Smoldering combustion (STAR) for the treatment of contaminated soils. Examining limitations and defining success. Remediation, 27-51, summer 2016.
   5. Kinsgston J.T., Dahlen Part R., Johnson Paul C., Foot Eric and Williams Shane 2010. Final Report-Critical evaluation of State of the Art In situ Thermal treatment Technologies for DNAPL Source Zone Treatment. ESTCP Project ER-0314 p.p. 1 to 1270.
   6. Mechanisms in In situ Thermal Remediation Davis, EVA, L. 2000 in “Abiotic Insitu Technologies of ground water remediation”, Conference Proceedings, August 2000, EPA/625-R-99/012.
   7. Price S.L and Kasevich S. 2000. R.F. Heating Technology for soil remediation.  In “Abiotic insitu technologies of ground water remediation,” Conference Proceedings, August 2000, EPA 625/-R-99/102 PP 27-29.
   8. Remediation Technologies Screening Matrix and Reference guide, Version 4.0, 3.9. Insitu Biological Treatment for ground water surface Water and leachate FRTR 2012.
   9. Stegmeier, G.L. and Vinegar, H.J. 2000. Thermal conduction heating for insitu thermal desorption. In “Abiotic Insitu Technologies of ground water remediation”, Conference Proceedings, August 2000, EPA 625/-R-99/102, p.30.
   10. Stewart Lloyd. D. 2000. Field Demonstrations of Thermally enhanced Extraction. In “Abiotic insitu technologies of ground water remediation”, Conference proceedings, August, 2000, EPA/625-R-99/102 P.25.
   11. Trowbridge Brett, 2000. Insitu DNAPL Remediation using six phase heating. In “Abiotic Insitu Technologies of ground water remediation,” Conference proceedings, August, 2000, EFA/625-R-99/102 P.26.
   12. U.S. Environmental Protection Agency 2001. A Citizens guide to thermal desorption, EPA SR-F-01-003, April 2001.
   13. Udell Kent.S., 2000. Remediation of soils and Ground Waters using steam enhanced extraction. In “Abiotic insitu Technologies of Ground Water remediation, “Conference Proceedings, August 2000, EPA/625-R-99/102.
   14. United States Environmental Protection Agency 2001, A. Citizens guide to vitrification. Office of Solid Waste and Emergency Response (5102G). EPA 542-F-01-017, December 2001.
   15. United States Environmental Protection Agency, 2001, A citizens guide to Insitu thermal Treatment Methods, Office of Solid Waste and Emergency Response (5102G), EPA 542-F-01-012, May 2001.

Also See:

1. Soil Vapour Extraction and Air Sparging
2. Remediation of Contaminated Sites- Permeable Reactive Barriers
3. Remediation of Contaminated Sites-The Pump and Treat System
4. Remediation of Contaminated Sites
5. Remediation of Chromium Contaminated Groundwater