THE OIL CATCHERS

HUNDREDS OF THOUSANDS OF LITERS OF OIL HAVE SEEPED INTO THE SOIL AT THE FORMER REFINERY SITE. NOW SEPARATORS ARE EXTRACTING IT FROM THE GROUNDWATER.

Once upon a time the citizens threw their rubbish onto the streets — and were overrun by rats. The mayor promised 100 gold ducats to anyone who could rid the city of the plague. Then a stranger appeared on the scene. When he played his flute, the rodents came out of their lairs and followed the man down to the river where the fast-flowing water washed them away.

In the Austrian version of the fairy tale, the Pied Piper catches his rats in Korneuburg, a small town on the Danube north of Vienna where an updated version of the story is currently taking place — but it takes more than a flute to rid the town of the effects of pollution.

In World War II the oil refinery on the edge of town was a target for allied bombers. The blind eye turned over decades of operation and even after the plant closed in 1961 did the rest. Hundreds of thousands of liters of oil found their way into the subsoil. When residents living in nearby residential areas voiced increasing concern that their drinking water tasted oily, the federal environmental service launched an investigation of the polluted “Tuttendorfer Breite” site: An area of 18 hectares proved to be contaminated with hydrocarbons to a depth of six meters — the remnants of the site’s oil-refining past.

FILTERS AND SOIL BACTERIA
BALSA, the federal agency charged with cleaning up toxic waste sites, issued an invitation to tender to resolve what was becoming an urgent problem that drew ever-louder protests from the people of Korneuburg. Bilfinger Berger Baugesellschaft, an Austrian subsidiary, presented an innovative clean-up concept and in 2008 the company was awarded the contract for the project.

Engineers built a 1,200-meter long containment wall at right angles to the direction of groundwater flow and reaching deep down into an impervious layer of clay. A series of “gates” were installed in the underground wall. These perforated glass fiber pipes nearly two meters in diameter are filled with activated carbon granules. “The gates were developed by Bilfinger Berger and we have a patent on them,” says geotechnical engineer Thomas Pirkner. The groundwater flows through the filters, leaving the hydrocarbons trapped inside. Once the activated charcoal is saturated, it is sucked out and replaced with fresh granules, thereby preventing any further contamination of the adjacent areas.

However, the polluted refinery site also had to be cleaned up. Inside the protective underground barriers, a thick film of oil floated on top of the groundwater. Bilfinger Berger first sank eight wells in which oil separators are now busily extracting the oily residue. The second step is both unusual and unique in Austria on this scale: a microbiological rehabilitation of the soil. This process relies on natural soil bacteria to do the cleaning. A total of 40 shafts were dug on the site through which the soil organisms that degrade the toxins are now being actively cultivated.

“All soils contain bacteria that break down hydrocarbons naturally,” explains environmental engineer Rainer Adami who is overseeing the project. “In order to help these microorganisms cope with such large quantities, we are adding nutrients and oxygen.” Extractors draw off air from the base of the shafts, creating a negative pressure below ground. The soil compensates by absorbing air on the surface, thereby providing the bacteria with fresh oxygen. The environmental technicians are using some of the other shafts to introduce artificial fertilizers dissolved in water: The nitrates give the bacteria a healthy appetite.

THE GOAL IS TO ACHIEVE DRINKING WATER QUALITY
Staff from Bilfinger Berger will be monitoring the site for a period of ten years. “We are confident that we will achieve our goal,” says Michael Zorzi, General Manager of BALSA, the client. “When the clean-up is completed, the groundwater in the Tuttendorfer Breite should be fit to drink once again.”

Text: Bernd Hauser, Photos: Eric Vazzoler
Bilfinger Berger Magazine 1/2010