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Innovative process for the on-site decontamination of soils (ECO-SOIL)

Final Report Summary - ECO-SOIL (Innovative process for the on-site decontamination of soils)

The purpose of this project was to develop a process for the on-site decontamination of soils. The test site of the project was situated in the centre of the city of Linköping, Sweden. On the test site there was an operating dry-cleaning. As dry-cleaning agent, PCE (Tetrachloroethene, also called Perchloroethylene), which is a chlorinated organic compound was being used. Due to improper use and inadequate disposal, PCE and its degradation products contaminated soil and groundwater on the site.

The degradation products of PCE belong to a group of compounds generally called DNAPL (dense non-aqueous phase liquids). The characteristics of DNAPLs are that they have a low solubility in water and are heavier than water. These properties makes the compounds sink through water and through the soil until they reach impermeable parts of the soil. As a consequence of the low solubility, the compounds can stay in the soil for long periods of time, up to decades, slowly leaking contaminants to the surrounding area.

Mineral oil was also found within the area. It was assumed, that the oil's source of origin was a car repair shop nearby. An additional hint may be the fact that Dichloromethane (DCM) was found in the soil as well. DCM is not used in dry-cleaning facilities but is common in metal processing to clean parts and surfaces. In contrast to the chlorinated hydrocarbons described above, oil has a lower density than water and tends to accumulate on the water surface.

The contamination analysis delivered an extreme contamination with PCE. Taking into account a limiting value (Poland) of 0,1 mg PCE/kg soil - the maximum concentration found was ~10 000 mg/kg. For groundwater the situation was similar. The limiting value was 10 µg/litre (Germany) respectively 40 µg/litre (Dutch level limits) while the concentrations on-site were far above with a maximum of 13 250 mg/litre.

DCM was found with increased concentrations in the soil layers close above the very dense layer at 3 m. The heavy metal concentrations found in the soil can be neglected due to the very small amount.

The remediation of the site would be done by use of adsorption mechanism. Horizontal drillings would be placed in the soil and filled with so called oil booms, which include sockets filled with sorbents. Thus, an analysis searching for the optimum sorbent was carried out.

The most commonly used sorbent materials for the removal of different contaminants from soils were activated carbon, peat and synthetic polymers. The research efforts were intensified during the last years of the project for the development of alternative materials to replace the use of activated carbon. Lignite seemed to be the best among the examined sorbents.

After the ECO-SOIL system was successfully installed at the test site, it was in used for almost one year. However, the sorbents were changed over after about six month in order to get information on the handling of the system as well as to get the contaminated sorbents analysed. The following sorbents were tested and analysed: modified pine bark, peat, e-clay and standard active carbon. Active carbon was only used to have a comparable material since active carbon was very well known in market of sorbents.

Furthermore, the issue of the reutilisation was investigated. Since the modified pine bark was proven as the best performing sorbent these investigations where only done for pine bark. The results were that for mineral oil hydrocarbons pollution the sorbents could be easily reused after biodegradation under controlled conditions, however, for chlorinated hydrocarbons it wasn't an efficient procedure since they are highly volatile and must be also degraded as a co metabolite by adding other pollutants to the sorbent.

Since it was realistic that volatile chlorinated hydrocarbons should be taken into consideration at least sometimes, further investigations have shown that the only realistic way of getting rid of the contaminated sorbents would be a special waste combustion plant, which had permission for burning such a waste.

The work progress was divided into four work packages. The objective of the first package was to define the technical specifications of the process technology in accordance with the end-users' needs in terms of soil, type and degree of contamination, legal requirements for the final quality of the soil and economical and administrative circumstance.

The geo-technical characterisation of the Swedish test site was done as well, providing information about the site's particle size distribution, water permeability, grain density and plasticity, organic matter content and so on.

The study of soil and groundwater contamination, as well as the analysis of microbial degradation in the soil were finished delivering important information for layout of the ECOSOIL system.

The aim of the second work package was to design and manufacture the ECO-SOIL system and to choose and set up a drilling method, which creates the adequate infrastructure for its installation on-site.

Most important point was the selection of the appropriate drilling location. Based on the results gathered in work package 1, the drilling location was fixed. In case the concrete reinforcement disturbed the communication between the steering equipment and drilling head, an alternative drilling would be done.

The development of the hydraulic model started with the analysis of the geotechnical characterisation regarding important boundary conditions of the soil, e.g. thickness of soil layers, conductivity inside the layer etc. Based on these analyses a model for computational fluid dynamics (CFD) was set up to simulate the plume dispersion under the building. Because of the fact that the model did not deliver suitable results, it was decided during the mid-term meeting to reject this simulation and try to find a solution by using the programme 'Mudflow' instead of 'CFX'.

The aim of the third work package was to get the system installed on the test site and to demonstrate its performance under real conditions. The equipment that was required for the installation procedure and the components used for the ECO-SOIL systems consisted of the drill, which was mounted to some kind of carrier system and the lubricant, which has the function of cooling the drill, fixing the walls of the drilled hole and to build up the slurry (lubricant plus soil), which then can be pumped out of the holes.

Furthermore, there are the components of the ECO-SOIL system. Inside the hole, a filter was placed to allow any ground water to drain through as well as to secure sufficient space for the sockets filled with the different sorbents. Inside the filters, the sockets were used to carry the different types of sorbents. For the tests a socket of 25 metre length was used, which could be pulled in and out of the filters by means of a rope. The sockets were produced from the material Polyproylene. The sockets were filled with the different types of sorbents. Each sorbent took about 5 metre length of the socket.

After the ECO-SOIL system was successfully installed at the test site the system was in use for almost one year. However, the sorbents were changed over after about six month in order to get information on the handling of the system as well as to get the contaminated sorbents analysed in the laboratories of WUT and the TTZ. The following sorbents were tested and analysed: modified pine bark, peat, e-clay and standard active carbon.

The aim of the fourth work package was to assess the efficiency of the ECO-SOIL system in removing or decreasing the contamination of the ground, by means of new analysis of the decontaminated soil and groundwater. The results of this evaluation would help to identify necessary modifications in order to optimise the systems performance. Finally, if the technical, environmental and safety objectives were reached, the system would be validated.

On a regular basis samples were taken from the sorbents, the soil and the groundwater. These samples analysed in order to find their content of mineral oil and chlorinated hydrocarbons. Furthermore, microbiological investigations were performed of micro-organisms inhabiting the sorbents used for decontamination with the ECO-SOIL system. Also, the issue of the reutilisation was investigated.

Concerning the soil reconditioning it was originally planned to refill the drilled hole after use with the ECO-SOIL system again with soil. However, during discussions within the consortium a more logical approach was discussed and finally agreed on. This was to cap both ends of the hole off. This would stop any rain water to carry unwanted soil into the boring, which, by that time, would be blocked and therefore not functional for any further use in the future. It also would stop any rats getting into the boring. This method also presents from a financial point of view the highest efficiency, even more since a hole left behind empty does not implement any dangerous situation concerning the stability of the ground itself. 'Soil reconditioning' by capping both ends off was the best alternative.