Treatment of groundwater, polluted by chlorinated solvents, by permeable reactive barrier utilising the Keops Process.
Two distinct sources of DNAPL (dense non aqueous phase liquid) situated below the factory are the origin of significant groundwater pollution by a series of dissolved phase chlorinated solvents. SITA-Soletanche Bachy proposed a unique solution to protect the groundwater, by providing a barrier equipped with filter gates and utilising the Keops® process.
This process provides an efficient solution for the treatment of chlorinated solvents dissolved in groundwater.
It results from the combination of 2 original and patented methods:
- The Panneau-drain® method allowing installation of filter gates equipped with easily removable cartridges (Soletanche Bachy Patent).
- The use of zero valent iron with precious metal catalysts, which is approximately fifteen times more efficient than zero valent iron alone, for the reduction of chlorinated solvents (SITA Remediation patent).
The Keops process, together with associated maintenance, allows the guarantee in the long term of the quality of the outflow at the exit from the gates.
Design of the Barrier
The design of the permeable reactive barrier was carried out in stages:
- Initially column tests were performed to determine the half-life of the chlorinated solvents under the specific site conditions. This allowed the calculation of the dimensions of the filters to accommodate the estimated flows at the site.
- Subsequent modelling (Feflow) allowed final design of the barrier (location of the gates, size of the filters in this case 3 etc…).
Execution of the Works
The works, carried out by Fontec, with the technical support of Soletanche Bachy, have comprised the construction of filter gates by the panneau-drain process and the installation of a drainage network to guide the groundwater flow through the filter gates.
The stainless steel gates were installed in 2 elements (main body and an extension tube) allowing the collection of water to 12.00m depth). The separate elements were connected on site during installation into a fresh slurry panel.
The drains were installed with the help of a Mastenbroek 50-60 trencher.
The filters inside the gates were subsequently installed by SITA Remediation who were also responsible for the maintenance associated with the guarantee.
The attached graph shows the performance of gate 2. After an initial "bedding-in" period of about 6 months for both the groundwater regime and the reactive material, the results have been excellent. The results in March 2003 demonstrate the efficiency of this treatment. For example, the concentration of vinyl chloride has reduced by a factor 500 at the gate outlet which clearly meets the set objective (< 5µg/l). The first maintenance operation was carried out 18 months after commissioning.
After cleaning of the gate, and after re-establishment of groundwater flow through the gate, the oulet values again become lower than the set objectives.
The flows passing through the gates vary between 0.01 and 0.13 m3/h.
The guarantee associated with the maintenance is for 10 years.
Groundwater is one of the most complex issues to be resolved in geotechnics. During the start phase of a project, the proper techniques have to be chosen for building any cut-off walls that might be required, pumping (wells, filter points, etc.) and monitoring (piezometers) resources.
Soletanche Bachy, and its subsidiary Solenvironment, provide solutions for pollution issues, involving, for example: inertisation of waste, rehabilitation and treatment of polluted sites, cleaning of discharge to required environmental control levels, construction of waste storage facilities, protection of groundwater.
Through his subsidiary Sol Environment, Soletanche Bachy provides solutions to various pollution problems: stabilisation of waste, rehabilitation and treatment of polluted sites, cleaning of discharge to required environmental control levels, construction of waste storage facilities, protection of the water table.
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