Freeze dredging technology can effectively remove contaminated sediments.

Toxic contamination is a problem at hundreds of thousands of sites on land and in the sediments beneath lakes and streams all over the world. Choosing a treatment technology is usually involves trade-offs among pollutant characteristics, technical and economic conditions, and requirements on how thorough the decontamination needs to be. Sweden’s FriGeo gives property owners and regulators an interesting new addition to their clean-up arsenal: soil freezing.

When she was a graduate student at Luleå University of Technology during the early 2000s, Susanne Rostmark developed a technique for freeze dredging to safely decontaminate polluted waterways by circulating a refrigerant fluid under pressure in units called freezing cells, which may consist of tubes inserted into the sediment, or a plate placed on surface. When the material is frozen, it can be lifted and transported away from the site for further processing. The freezing cell is relocated and the procedure begins again.

Artificial ground freezing is not a new concept; it was used in Wales as early as 1862, and was patented in Germany in 1883. Since then, the most common applications for the technology have been in freezing the ground to stabilize it during construction or while lining underground gas tanks. In the Arctic, artificial freezing prevents the thawing of permafrost near heated buildings and pipelines.

Mercury clean-up

Mercury pollution is an unfortunate remnant of industrial history. Contaminated sediments in lakes and bays can allow the highly toxic element to leach into the food chain.

At a plant owned by the Swedish paint and coatings manufacturer AkzoNobel in the town of Stockvik, mercury contamination in sediment under a nearby bay was judged too difficult to clean up by digging or suction dredging, so the contractor SAKAB chose a different technology. “This was a perfect case for our freeze dredging technology,” says Susanne Rustmark at FriGeo. “The contaminated sediment was only 10 to 30 centimeters deep, and the turbulence from a mechanical digger would have spread pollution in the water.”

The technical solution involved laying out freezer elements in rows on the bottom of the bay, where the contaminated sediment would stick to the units. The frozen sediment was then lifted out, loaded into a sealed container and transported to a hazardous waste landfill. A total of some 320 tons of contaminated sediment were treated in the Stockvik project.

By varying the time and temperature, the operator can control the depth of the freeze and select the treatment area with almost surgical precision. The risk of spreading pollution is minimal, and water samples taken during the Stockvik cleanup showed no increased mercury levels.

Great potential

“There are any number remediation sites where freeze dredging technology has great potential,” said Mats-Johan Rostmark, CEO of FriGeo. Rostmark cites the example of Lake Ala Lombolo outside Kiruna in Sweden’s far north, where industrial contamination of sediment includes several hundred kilos of mercury, along with zinc, lead, molybdenum and other toxic substances. “Worse, the lake’s bottom holds some 17,000 military grenades dumped in the 1950s,” Rostmark explains. “We can do something about all this.”
Refrigeration units are constructed depending on the unique requirements of each project, reducing the amount of capital FriGeo has to keep tied up in storage equipment. Operators freeze as little of the sediment volume possible, lowering the cost of post-treatment and reducing overall impacts on the waterway.

Crashed airplane

In 1952, a Swedish military DC3 aircraft disappeared over the Baltic Sea, where it spent more than 50 years beneath 125 meters of water. Long suspecting that the plane had been had been shot down by the Soviet Union, Swedish authorities were finally able to stage a recovery operation by the 2000s. Debris was scattered over a wide field around the fuselage and embedded in bottom sediments, making it difficult for accident investigators to get to the smallest objects. But freeze dredging technology made it possible not only to pick up the objects, but to do so without changing their position.
In total, FriGeo extracted about 200 cubic meters of sediment, which was kept frozen to await further forensic investigation. The technology was an important element of the technical investigation into the crash.
Article published in March 2011