Six Phase Soil Heating at the U.S. Department of Energy's Savannah River Site, M Area, Aiken, South Carolina; and Hanford Site, 300-Area, Richland, Washington

Site Name:

Savannah River Site

Location:

Aiken, South Carolina

Period of
Operation:

October 1993 to January 1994

Cleanup
Type:

Field demonstration

Technical
Information:

Theresa Bergsman, PNL, (509) 376-3638
Phil Gauglitz, (509) 372-1210
Bill Heath, (509) 376-0554
Harry Burkholder (Licensing), PNL, (509) 376-1867

Technology:
Six Phase Soil Heating (SPSH)

- SPSH splits conventional three-phase electricity into six separate electrical phases, with each phase delivered to a single electrode.
- The six electrodes are placed in a hexagonal pattern, with the vapor extraction well located in the center of the hexagon.
- At SRS, the diameter of the hexagon was 30 ft, and 1 to 2 gals/hr of water with 500 ppm NaCl was added at each electrode to maintain moisture. Electrical resistivity tomography (ERT) was used to monitor heating progress.

Cleanup Authority:
State: Air discharge and underground injection control (UIC) permits for the SRS are in place with the South Carolina Department of Health and Environmental Control (SCDHEC).

SIC Code:
9711 (National Security)
3355 (Aluminum Forming)
3471 (Metal Finishing)
Point of Contact:
Kurt Gerdes, DOE EM-50, (301) 903-7289
Dave Biancosino, DOE, (301) 903-7961
Jim Wright, DOE, (803) 725-5608

Waste Source:
Chlorinated Aliphatics -
- Trichloroethene (TCE) and tetrachloroethene (PCE)
- TCE concentrations in the sediments ranged from 0 to 181 mcg/kg (ppb), and PCE from 0 to 4,529 g/kg

Type/Quantity of Media Treated:
Soil and Sediment

- The contaminated target zone was a ten-foot thick clay layer at a depth of approximately 40 feet, underlain by a thick section of relatively permeable sands with thin lenses of clayey sediments.

Purpose/Significance of Application:
SPSH was demonstrated as an alternative technology for enhancing removal of contaminants from clayey soils during an SVE application

Regulatory Requirements/Cleanup Goals:
- The demonstration was covered by permits issued by the SCDHEC, including an air quality permit and a UIC permit (because of the addition of NaCl-bearing water to the electrodes).
- No specific regulatory requirements or cleanup goals were identified for the SPSH demonstration.

Results:
- Temperature in the clay zone increased to 100 C within 8 days and held at 100-110 °C for 25-day demonstration.
- 19,000 gallons of water were removed from the soil as steam; approximately 5,000 gallons of water were added to maintain electrode conductivity.
- Median removal of PCE from the soil was 99.7%
- 180 kg of PCE and 23 kg of TCE were removed from the soil within the heated zone.
- SPSH at the Hanford site was conducted in 1993 on an uncontaminated area.
- Results from Hanford were used to improve process understanding, refine system design (e.g., of electrodes), and address scale-up issues.

Cost Factors:
- No data are provided on the capital or operating costs for the two demonstrations.
- An analysis of the capital and operating costs comparing SPSH and SVE technologies was made based on the following assumptions: a plume 100 ft in diameter; depth from 20 to 120 ft; energy demand 200 kW-hr per yd3; target contaminants are VOCs and semi-VOCs.
- SPSH was shown to have a lower cost than SVE ($86/yd3 compared with $576/yd3) and to require less time for remediation (5 yrs compared with 50 yrs).

Description:
From 1958 to 1985, Savannah River Area M conducted manufacturing operations including aluminum forming and metal finishing. Process wastewater from these operations containing solvents (TCE, PCE, and TCA) was discharged to an unlined settling basin at Savannah River, which lead to contamination of ground water and vadose zone soils. Treatment of vadose zone soils has been the subject of several demonstrations (e.g., in situ air stripping), including this investigation of the technical and economic feasibility of six phase soil heating (SPSH) technology.

At SRS, SPSH was used to increase the removal efficiency of SVE for a clayey soil contaminated with TCE and PCE. At Hanford, SPSH was demonstrated on an uncontaminated site to improve process understanding, refine system design (e.g., of electrodes), and address scale-up issues. SPSH splits conventional three-phase electricity into six separate electrical phases, with each phase delivered to a single electrode. The six electrodes are placed in a hexagonal pattern, with the vapor extraction well located in the center of the hexagon.

Results from the SRS demonstration showed that SPSH increased the temperature in the clay zone to 100 C within 8 days and maintained it at 100-110 °C for a 25 day demonstration. In addition, there were 19,000 gallons of water removed from the soil as steam, and approximately 5,000 gals of water added to maintain electrode conductivity. The median removal of PCE from the soil was 99.7%, with overall results showing that 180 kg of PCE and 23 kg of TCE were removed from the soil within the heated zone. Operating difficulties included drying out of the electrodes and shorting of the thermocouples. The system design was improved to overcome these difficulties.