Cleaning Soil With Steam: a Hot New Method

Carpets are steam cleaned. Upholstery too. Now engineers and industrial site managers are adding soil and ground water to the list of substances amenable to steam-cleaning technology.

By applying the same concept used to get greasy dirt out of fabric, engineers at Southern California Edison have been vaporizing, then vacuuming up, industrial chemicals at a Visalia site.

From the 1920s through 1980, utility poles were treated with chemicals there to keep the wood from rotting. More than 110,000 gallons of coal-tar creosote and pentachlorophenol, a chlorinated hydrocarbon that was dissolved in diesel fuel, leaked into the soil and contaminated the ground water.

One senior research scientist at Edison said the compounds at the pole yard, a U.S. Environmental Protection Agency Superfund site since 1989, were “about as nasty a compilation of chemicals [as] you’ll ever find.”

What made them particularly tough to remove was their density and low solubility in water, which allowed them to sink through water rather than float. The contaminants seeped to about 115 feet below the ground, 50 feet lower than the surface of the ground water.

The site managers at Visalia were having little luck getting the compounds out through a conventional cleanup method. Under that technique, used for about 20 years at a cost of $1 million annually, ground water was pumped up to the surface, where contaminants could be filtered out.

“We were getting 10 pounds [of contaminants] a week,” said Craig Eaker, an environmental engineer at Edison and project manager at Visalia. “It was a Band-Aid. It controlled. It didn’t remove.”

Sixteen months ago, Edison took a new approach.

Under the new system, boilers shoot pressurized steam through pipes that surround the contaminants. As the steam circle closes in, the soil is heated to about 100 degrees Celsius, hot enough to boil water. Along the way the heat loosens the contaminants’ grip on the soil.

The steam corrals the liquid and vapor toward extraction pipes in the middle of the polluted area, where they are vacuumed up to the surface to be treated.

Inspiration Came From Oil Industry

Inspiration for the technique actually came from the oil industry. For decades, oil companies have been injecting steam into the ground to loosen oil and boost petroleum production.

By the late 1980s, Kent Udell, a professor of mechanical engineering at UC Berkeley and an Edison consultant, had begun applying the technique to industrial contaminants. He found that periodically shutting the steam off actually enhanced the recovery process by creating a low-pressure environment that forces contaminants to vaporize at lower-than-normal boiling points.

Scientists at Lawrence Livermore National Laboratory developed a technique called dynamic underground stripping, which uses steam and electrical currents to heat layers of soil, such as clay, that would otherwise be difficult to penetrate.

Since the steam project began in Visalia, engineers have removed 902,000 pounds of contaminants. It would have taken more than 1,000 years to achieve the same results using conventional pump-and-treat methods.

“I’m tickled pink about this technology,” Eaker said. “This is certainly a breakthrough.”

The ability to treat contaminants without disturbing the ground has great appeal.

“The other option is to excavate the soil, chemically treat it, dump it in a landfill or burn it,” said environmental consultant George Baggett.

A bonus to heating the soil seems to be that the chemicals are oxidized into such benign substances as carbon dioxide and water while still in the ground.

“Most chemicals want to oxidize in the presence of water,” said Robin Newmark, geophysicist at Lawrence Livermore. “It happens slowly in ambient temperature, but as things heat up, the process speeds up tremendously.”

The process continues even when steaming shuts off, because the ground stays hot for years. Engineers at the Visalia site estimate that this effect accounts for at least 15% of the contaminants already removed.

The dramatic results have caught the attention of state and federal officials. Although they say it is too early to know how many industrial waste sites could be cleaned using steam, they are considering dynamic underground stripping at two other wood-treating sites.

“I think we were all surprised by the volume of material that came out,” said Richard Hume of the state EPA’s Department of Toxic Substances Control. “I’ve been real happy with this.”

Researchers expect the steam to work just as well at other pole yards in the country and on other substances.

The process already has successfully cleaned smaller fuel leaks. In January, the effectiveness of dynamic underground stripping on trichloroethylene, or TCE, will be tested at a Department of Energy site in Ohio.

TCE, commonly used to clean rocket motors and in computer chip manufacturing, is one of the most common contaminants at Department of Energy sites. It is easier to vaporize than the Visalia chemicals, because of its higher volatility.

Experts say, however, that using heat to remove waste is only viable if you can control where the steam goes. Cold spots in the ground could mean incomplete extraction. Contaminants could also be pushed farther down or into neighboring land.

Several methods of monitoring the steam flow exist. Although surgically accurate monitoring is nearly impossible, having some type of surveillance in place is crucial, engineers say.

The steam technology has been used at a few other sites in the last decade with limited success. But not until the process was tried at the Visalia pole yard was there such a large-scale demonstration.

Some Worry About Potential Drawbacks

Many regulators and site managers, however, are not yet ready to jump on board the steam train. Hume cautions that there may be limitations to applying the technology to other hazardous waste sites, particularly near areas where the steam could inadvertently spread the contaminants or heat nearby water sources.

“Visalia was unique in that it was not located right next to a large water body,” he said.

Newmark adds that steam heating the ground would be more worthwhile for contaminants at lower depths.

“It’s easier to steam deeper, because you can run the steam at higher pressure and temperature,” she said. “It’s more difficult to control the steam at very shallow depths. In those cases, if you can dig a site up, it’s probably cheaper to do that.”

The reception from companies and site managers to steam technology is still somewhat cool. Wary of snake-oil cures, many site owners are not yet convinced that their investment in the method would pay off.

Steam extraction could save a tremendous amount of money in the long term. Most of the cost, however, is borne upfront, which may be prohibitive for companies that have a limited budget for environmental cleanups. The Visalia project is estimated to be costing Edison $20 million.

“The weakness of this is that it has a high sticker shock,” Eaker said. “It took me four months to sell this to management.”

The reward for Edison’s investment, he said, is the likely removal of the Visalia site as a liability for the company. Steaming is scheduled to continue for four to six months, with the goal of bringing the water back to drinking standards by 2001.

“Keep in mind we were looking at 3001 with pump-and-treat,” he said. “I think this is a slam dunk for organic solvents.”

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Steam-Cleaning Ground Water

Engineers first fenced in the contaminated area with a series of injection wells, or pipes. Boilers shoot pressurized team through pipes which extend deep into the ground. The steam exits through slotted screens in the pipes and heats the soil to about 100 degrees Selsius, hot enough to boil water.

The heat helps the contaminants loosen their grip from the soil, much like hot water helps loosen grease from dirty dishes or laundry. working from the outside in, the steam corrals the compounds to extraction wells in the middle of the polluted area. The vacuum and pumps then transport the liquid and vapor to the surface to be treated.

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Site of pole yard: Visalia

The top of the ground water begins 65 feet below the surface.

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Source: Southern California Edison Environmental Affairs