Introduction to ThermNet Technology

Mike Watson Dan Wibew Ray Kasevich

Mike Watson is a senior scientist with DAHL G Associates, Inc., St. PauL Minnesota, and bas been with DAHL since 1991. He manages the TbemthTet program and concentrates in the development and implententation of innovative, low-cost remediation technolo- gies for the environmen- tal industry. Dan Wiberg is a princZpa1 scientist with BAHL G Associates, Inc. He is a registered professional geologist, specfallzing in innova- tive and cost-enective project Ufe cycle minimi- zation strategies. He has been with DAH.. since 1988. Ray K a s d h is President of KAI Technologies, Ports- mouth, N e w Hampshire. He bas developed n u w o u s applications for radiofiequency heating and holds several patents in the fie& including that utilized with TbermNet.

The Introduction to ThermNet technology is an overview of bow radio frequency (RF) heating can be implemented to enhance conventional remediation technologies. Included in the article is a case study of aproject conducted in March 1PYG at a gasoline release site. The project consisted of a “hot spot ’’ application to reduce BElX concentrations to achieve site closure. The application consisted of RF heating, soil vapor extraction, and groundwater ventilation. A bench scale study was also conducted to measure the effects that RF heating and vapor extraction had on the removal of PAH constituents from a coal tar sludge. Up to 100percent reductions were obserued for some of the constituents in the study. Also included is agraphical representation of temperature versus vaporpressure for contaminants typically encountered at remediation sites, illustrating that as temperature increases, the removal rate increases.

DAHL & Associates, Inc. (DAHL) and KAI Technologies have teamed to develop ThermNet, an in-situ remediation service that enables acceler- ated removal of subsurface contaminants from soil and groundwater environments. ThermNet combines radio frequency (RF) heating with conventional technologies such as soil vapor extraction, air sparging, pump and treat, and bioremediation. RF heating causes chemical, physical, and biological changes in subsurface media, all of which combine to make contaminants more responsive to remediation efforts.

ThermNet delivers heat to the subsurface via electromagnetic radiation and is not reliant on thermal conduction or convectional limitations within the matrix being heated. Heat transfer occurs at the molecular level and can result in temperatures up to 300” C. A radio frequency generator, housed in a modular trailer, emits up to 25 kilowatts (KW) of power via subsurface applicators located in boreholes equipped with special fiberglass liners. Energy is delivered at one of four authorized industrial, scientific, or medical frequencies: 6.78 MHz, 13.56 MHz, 27.12 MHz, or 40.68 MHz. The dielectric properties of the soil, as well as the contaminants present, determine the operating frequency, power level, and antenna design for optimum heat transfer.

CCC 1051 -5658/98/0803073-07 0 1998 John Wiley & Sons, Inc.

73

MIKE WATSON DAN WIBERG RAY KASEVICH

ThermNet accelerates the remedial process through direct volatiliza- tion, or simply enhances natural microbial activity with uniform, low temperature applications. ThermNet can also provide an avenue for mobilizing heavier-end, free-phase product held in the subsurface. The thermally induced viscosity changes can result in a more controllable and recoverable free-phase plume. All ThermNet applications are designed to provide greater predictability of remediation timeframes and costs.

TEMPERATURE DIRECTLY AFFECTS C 0 N " A . N " REMOVAL The most common ThermNet application involves hirect, in-situ

heating of the soil resulting in an accelerated phase change for volatile and many semi-volatile contaminants. Increasing ambient temperatures by just 10" C can increase vapor pressures, and thus removal rates of many contaminants by as much as 50 percent. Exhibits 1 and 2 list several contaminants typically encountered at remediation sites. Each graph represents the influence that temperature has on each respective contaminant's removal rate. By increasing the temperature roughly 90" C from ambient conditions, significant removal rates can be achieved.

CASE STUDY: "HOT SPOT" APPLICATION ThermNet was implemented for a "hot spot" application in March 1996

at a gasoline release site near St. Paul, Minnesota. The site had been under active remediation via pump and treat and traditional vapor extraction for several years; however, elevated petroleum hydrocarbon concentrations existed in the capillary fringe (extending approximately 1.5 feet above

Exhibit 1. Vapor Pressure as a Function of Temperature

300

200 h

0 W V

0

-100 0 100 200 300 400 500 600 700 80(

Vapor press- (mm HG)

* Tetrachloroethylene (PCE)

* Vinyl Chloride

.+. Benzene

8- Toluene

9 Ethylbenzene

* pXylene

* m-Xylene

+ 0-Xylene

8 Naphthalene

)

74 REMEDIATION/SUMMER 1998

INTRODUCTION TO THEWET TECHNOLOGY

Exhibit 2. Influence of Temperature on Vapor Concentrations

PCE 56 I 27 I 2 2 1 22 I 2 2 1 11 I 41 9

I Toluene b v b e m e d p Xvleme I m-Xvlene I 0-Xylene INaphthalene 3

static water) thus contributing to the persistence of elevated concentrations of dissolved hydrocarbons in groundwater.

The objective of the application was to target this "hot spot" with RF heating and vapor extraction to accelerate removal of the contaminants within the capillary fringe, relative to conventional soil vapor extraction (SVE). One RF well, two SVE wells, three soil vapor probes, and one groundwater sparging structure were positioned in the area of greatest impact remaining at the site. The SVE wells were turned on-line and effluent vapor concentrations were measured. Once concentrations de- creased and reached asymptotic levels, RF heating at 27.12 MHz and 5 KW was initiated. Subsurface temperatures were measured periodically at various monitoring points as a function of depth and time.

Soil temperatures increased from an ambient 8" C to 100" C in the vicinity of the applicator and to 40" C 5 feet from the applicator at a depth of 8.5 feet below grade. Effluent vapor concentrations increased by an order of magnitude as soil temperatures increased. Concentrations re- mained elevated for approximately 10 days before beginning to decrease. Groundwater sparging activities were then initiated; immediately, vapor concentrations began to increase. Due to the additional moisture intro- duced into the vadose zone as a result of the groundwater sparging, temperatures quickly dropped; however, they began to increase again within 24 hours. The study was completed and power to the W applicator was shut down after 21 days of operation. Data collected after the RF power was shut down indicated that residual heat in the soil resulted in elevated vapor concentrations for approximately one week. During the study, a total of 2,300 KW hours of energy was delivered to the soil at power levels ranging between 5 and 10 KW at 27.12 MHz.

Soil and groundwater samples were collected from the target remediation

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WATSON DAN WIBERG RAY KASEVICH

Via ThermNet’s integration of RF heating, soil vapor extraction, and groundwater ventilation, accelerated remediation was accomplished at the subject site.

area before and after the study. Most soil samples collected demonstrated a 90 percent reduction in contaminants, the most dramatic of which was observed in a soil sample collected from 10 feet below grade, approxi- mately 15 feet from the RF well. Gasoline Range Organic (GRO) concen- trations were reduced from 3,200 parts per million (ppm) to 71 ppm (an approximate 98 percent reduction). Most of the groundwater samples collected indicated a contaminant reduction by one or two orders of magnitude. GRO concentrations were reduced from 29,000 parts per billion (ppb) to less than 100 ppb in a sample collected approximately 15 feet up-gradient from the RF well.

Via ThermNet’s integration of RF heating, soil vapor extraction, and groundwater ventilation, accelerated remediation was accomplished at the subject site. For this application, increasing the soil temperatures up to 100” C resulted in more rapid volatilization of the gasoline constituents and significant removal of volatile organics. The subject site is now closed. (Kasevich et al., 1997.)

BENCH SCALE STUDY A bench scale study was conducted to evaluate the effectiveness RF

heating has on the removal of BETX and polynuclear aromatic hydrocar- bons (PAHs) from a sludge matrix. The sludge resides in a former clarifier tank at a former Manufactured Gas Plant (MGP) site.

The study consisted of five tests. Each test consisted of heating 350 grams of sludge at a specified temperature, maintaining that temperature for a two-hour period. The temperatures were 50” C, 100” C, 150” C, 100” C with airflow, and 353.4“ C. A control sample was submitted to an independent laboratory for analysis to establish baseline concentrations. All samples were analyzed for BETX and PAHs via EPA Method 8020 and 8270, respectively.

The RF energy for the study was simulated utilizing microwave energy, and a fiber optic thermometer was used for temperature readings. A Variac transformer was used for the adjustment of the output power so that the set temperature of the sludge sample could be maintained. Fiber optics were utilized to measure the temperature because it does not interfere with the deposition of microwave or RF energy.

Exhibits 3 and 4 illustrate the target chemicals of the study, the analytical results, and the percent reductions for each chemical per test. The “ASL” listed in Exhibit 3 is the any-use soil level, which represents the maximum concentration of a chemical that will always be acceptable in the soil, regardless of the future land use. The ASL list is published by the State of Missouri as a guideline for cleanup objectives. The “#1#2 As Is” sample listed in Exhibit 3 is the control sample, which was analyzed at ambient air temperatures.

There were minor reductions observed from the 50°C test, and in some cases concentrations increased. In those instances, the higher value between “#1 #2 As Is” and the 50°C sample was used as the baseline concentration in the percent reduction calculations. Averages of 91.16 percent, 90.36 percent, and 96.05 percent of overall contaminant reduc-

76 REMEDUTION/SUMMER 1998

INTRODUCTION TO THERMNET TECHNOLOGY

Exhibit 3. Analytical Results of Sludge Samples (Former MGP Site)

Analytical Parameter

Benzene Ethylbenzene Toluene Xylenes-m,-p Xylene, -0

Acenaphthene Acenaphthylene Anthracene Benzo(a)anthracene Benzo(a)pyrene Benzo(b) fluoranthene Benzocg, h,l)perlyene Benzo(k)fluoranthene Chrysene Dibenzo(a,h)anthracene Fluoranthene Fluorene Indeno( 1,2,3-cd)'pyrene l-Methyl-naphthalene A2-Methyl-naphthalene Naphthalene P henanthrene Pyrene Nitrobenzene-d5 2-Fluorobiphenyl Terphenyl-dl4

Explanation:

ASL

170 380 490 480 480

3400

17000 4.5

0.68 4

34 160

0.62 2300 2300

12

230

1700

#1*2 As Is

38 39 83 64 93

29 150 98 54 57 25 22 27 50

4 0 100 120 20

250 300 640 280 150 NA NA NA

Analvtical Results

50" C

22 21 52 42 65

31 230 140 79 83 39 33 37 69 13

150 170 28

340 420 650 400 210 NA NA NA

100" c

0.19 0.25 0.22 0.22 0.18

7.2 5.7 11 12 10

6.1 4

5.5 11 1.7 20 13

3.8 11 14 17 49 27

NA NA NA

150" C

0.048 0.077 0.078 0.055 0.048

0.9 1.6 8.9 16 8

10 5.7 7.7 16

2.6 28 10

5.6 7.5 8.4 9.2 53 23

NA NA NA

looo c with air

<0.025 <0.025 C0.025 <0.025 0.029

0.35 0.55 3.3 3.3 2.5 1.7 1.2 1.7 31

0.52 6.7 2.6 1.1 2.2 2.2 2.3 13

7.9 0.076

0.06 0.069

Full power

ND ND ND ND ND

ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND NA NA NA

All results are reported in milligramsper kilogram (mg&@ which b equivalent topartspr million @pm>. ASL = Any-Use Soil Level that represents the maximum concentration of a chemical that will always be acceptable in the soil, regardless of future land use. ND = nondetected C = degrees Celsius #I #2 As Is = sample that was analyzed at ambient air temperature.

tions were observed from the 100" C, 150" C, and the 100" C with air flow tests, respectively. Analytical results from the sample exposed to full power (353.4" C) indicated nondetected (ND) concentrations for all contaminants.

Although full power revealed ND results for all constituents, the energy costs required to achieve this temperature may not be desirable, especially

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WATSON DAN WIBERG RAY KASEVICH

Exhibit 4. Analytical Results of Sludge Samples (Former MGP Site)

Analytical Parameter

Benzene Ethylbenzene Toluene Xylenes-m,-p

Xylene, -0

Acenaphthene Acenaphthylene Anthracene Benzo(a)anthracene Benzo(a)pyrene Benzo(b)fluoranthene Benzo(g, h,l)perlyene Benzo(k)fluoranthene Chrysene Dibenzo(a, hlanthracene Fluoranthene Fluorene Indeno(l,2,3-cd)'pyrene 1-Methyl-naphthalene A2-Methyl-naphthalene Naphthalene Phenanthrene Pyrene Nitrobenzene-d5 2-Fluorobiphenyl Terphenyl-dl4

Base Line

38 39 83 64 93

31 230 140 79 83 39 33 37 69 13

150 170 28

340 420 650 400 210 NA NA NA

Reduction

100" c 99.50% 99.36% 99.73% 99.66% 99.81%

76.77% 97.52% 92.14% 84.81% 87.95% 84.36% 87.88% 85.14% 84.06% 86.92% 86.67% 92.35% 86.43% 96.76% 96.67% 97.38% 87.75% 87.14%

NA NA NA

150" C

99.87% 99.80% 99.91Yo 99.91% 99.95%

97.10% 99.30% 93.64% 79.75% 90.36% 74.36% 82.73% 79.19% 76.81% 80.00% 81.33% 94.12% 80.00% 97.79% 98.00'Yo 98.58% 86.75% 89.05%

NA NA NA

Explanation:

All results are reported in milligrams per kilogram (rnflgl which is equivalent to parts per million (Ppm). C = degrees Celsius

100' C (with air)

100.00% 1 00. OOYO 100.00% 100.00% 99.97%

98.87% 99.76% 97.64% 95.82% 96.99% 95.64% 96.36% 95.41% 55.07% 96.00% 95.53% 98.47% 96.07% 99.35% 99.48% 99.65% 96.75% 96.24%

NA NA NA

if cleanup objectives are not ND for all contaminants. Data from the other three tests indicate significant reductions in contaminant concentrations and each would likely achieve the cleanup criteria. However, due to the nature of the contaminants involved, off-gas odor and vapor treatment is a major concern. For this application, vapor extraction and thermal destruction of the vapors would also be implemented as part of the corrective actions, therefore, the 100°C with air flow test appears to be the most practicable application for this situation. (Watson et al., 1997.)

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INTRODUCTION TO THERMNET TECHNOLOGY

CONCLUSIONS ThermNet can be applied in many different settings to target a wide

variety of volatile and semi-volatile contaminants. By accelerating remediation efforts, be it increased volatilization, enhanced product removal through temperature-induced viscosity changes, or bioaugmen- tation, results can become more predictable and remediation costs and timeframes can be reduced.

REFERENCES 1. Kasevich, R.S, Wiberg, D., Johnson, M.A., Price, S.L., Watson, M. 1997. "Enhanced Removal of Gasoline Constituents from the Capillary Fringe Utilizing Radio Frequency Heating," in Calabrese, et al., eds. ContumtnatedSofk, Vol. 11, Amherst Scientific Publishers.

2. Watson, M.P., Wiberg, D., Kasevich, R.S. 1997. Bench Scale Study to Demonstrate Enhanced Removal of BETX and PAHs from Sludge Matrix. Performed by DAHL and KAI for confidential client, October.

REMEDIATION/SUMMER 1998 79