Using On-Site Biological Methods to Clean Up Soil
Decontamination companies and researchers in France are working to restore polluted soil using treatments that range from biological methods to soil washing and incineration. The problem of polluted soil was dramatically illustrated prior to construction of the Stade de France soccer stadium built for the 1998 World Cup. The location of the site, on industrial wasteland just north of Paris, caused immense difficulties for the developers. Tens of thousands of cubic meters of earth contaminated by a gasworks had to be cleaned before the stadium was built.
The company — Gaz de France — contracted the restoration work to ICF Environment of Gennevelliers, which used a variety of pollution removal techniques. It incinerated in an accredited plant more than 388,000 cubic feet of tar, bitumen, and other material containing more than 20 grams of aromatic hydrocarbons per pound of dry matter (50,000 ppm). It also treated, using biological methods, 530,000 cubic feet of earth contaminated at concentrations between 500 and 50,000 ppm. Moved into a biological mound, the soil was decontaminated by activating the naturally occurring microbes in the soil. Finally, 918,000 cubic feet of soil with concentrations from 30 to 500 ppm were left in place but laid out in layers separated from the surface and the water table. Throughout the area of the stadium, tests confirmed a concentration of aromatic hydrocarbons lower than the permitted level of 500 ppm.
Biological treatment in situ
According to Laurent Sohier, site restoration manager of the Krebs company: “The removal of pollution from the Stade de France demonstrates the interest in biological treatments in situ.” Krebs made use of research from INRA (National Institute for Agronomic Research) to degrade pollutants with the help of filamentous fungi. This innovative technology is being evaluated on soil polluted by Pyralene, a chlorinated compound liable to give off dioxins. It should reduce the degradation time to a period of months, which otherwise would have taken hundreds of years.“Biological pollution removal, which on the face of it is relatively simple, nevertheless requires case by case investigations,” notes Sylvain Durecu, research department manager at Tredi in Paris.
Environmentally friendly and relatively inexpensive, in situ biological treatment is also favored by the French firm BRGM (Geological and Mining Research Office) located in Orléans. In the north of France, this company has restored a water table polluted by industrial waste containing nitrates and ammonia. For reasons of cost, it chose in situ pollution removal by the addition of carbon and the stimulation of indigenous flora.
Washing the soil
On land that is highly polluted, BRGM carries out washing, a technique drawn from its mining experience. “Washing is inexpensive, from $45 to $90 a metric ton,” observes Georges Morizot, a researcher at BRGM. “It reduces the amount of contaminated soil that has to be treated off site or sent to a waste disposal plant.”
The French environmental company Geoclean, based in Chassieu, has treated in situ, in a similar way, the soil at a former oil depot, where concentrations of hydrocarbons varied between 500 and 4,000 ppm. “In order not to prevent use of the site during the treatment, we moved all the moderately contaminated soil into mounds,” explains engineer Franck Leclerc. The mounds were fitted with drains and connected to a pump. It was then possible to remove the pollutants by creating a vacuum of 5,800 psi with a flow rate of 35,000 cubic feet per hour.
When the contamination is severe, in situ treatment is not sufficient. It is necessary to transport the soil and remove the contamination in a suitable reaction vessel. In the Lorraine region of eastern France, Geoclean has installed a unit that treats no less than 15,000 metric tons per year, with an optimum throughput of 15 metric tons per hour. This is accomplished using a physico-chemical technique of its own invention known as Ulis. The polluted soil is passed along pipes by means of endless screws and is cleaned in a countercurrent of a dichloromethane solvent, with residual traces of solvent removed by steam. Then the solvent is regenerated and the pollutants disposed of at accredited plants. “The technique is suitable for soil containing organic pollutants up to 100,000 ppm,” advises Leclerc. “On a clay soil, for example, we have reduced the concentration of PCBs from 170 milligrams per pound of soil to less than five milligrams per pound.”