Envisan applies a wide range of remediation techniques for the treatment of polluted soils and groundwater. Thanks to many years of experience and extensive expertise in soil and groundwater remediation and also thanks to the support from its mother company Jan De Nul Group, Envisan is able to execute multidisciplinary projects. As for the treatment of soil contamination, Envisan has experience in both in-situ, on-site and off-site remediation techniques. When using in-situ techniques, the soil is not excavated but treated by using specific injection/extraction methods. In on- and off-site techniques, the soil is excavated and either treated on site or transported to a soil treatment centre.
Selective excavation (including stability-supporting civil engineering measures)
The contaminated soil is excavated selectively according to layer-specific excavation plans and either treated on site or transported to a treatment centre. If needed, Envisan also takes stability-supporting civil engineering measures.
Pump & Treat
This is one of the best known and most commonly used techniques for the remediation of groundwater pollution. The groundwater is pumped up and treated in a groundwater treatment plant. Usually, this technique is part of an overall remediation project in which first the contamination in the soil is tackled by excavation works, which is then followed by extra pumping in order to extract the (residual) pollution from the groundwater.
Soil air extraction
In this soil remediation technique, the soil air is extracted from the unsaturated area. By extracting the soil air, volatile compounds are removed from the unsaturated area. By lowering the concentrations in the gas phase, the balance between the liquid phase (groundwater) and the gas phase (soil air) is restored. Volatile compounds migrate from the water phase to the gas phase and the concentrations in the water phase are lowered. By refreshing the soil air time and again, both the soil and the upper part of the groundwater are treated.
Bioventing / Airsparging
Through air sparging or compressed air injection, air is injected under pressure into the groundwater. Following the injection, volatile components in the water turn gaseous. The polluted gas phase ascends and is captured through soil air extraction.
In biosparging, oxygen (air) is injected in the saturated soil area as well, be it under limited pressure, thus stimulating the aerobic biodegradation of pollutants.
Using a high-vacuum pump a vacuum is created in the soil, after which groundwater, soil air and, if applicable, pure product (Light Non Aqueous Phase Liquids or LNAPL) are extracted. After a phased separation process the different phases are treated above-ground in a treatment centre.
Selective floating layer removal
The floating pure product (Light Non Aqueous Phase Liquids or LNAPL) is removed through a selective process. This is done using an open trench or by installing so-called skimmers. When using the open-trench method, the upper soil is excavated up to groundwater level, after which the affluent pure product is removed using a mobile suction apparatus or pump system.
Skimmers pump up pure product in a selective manner and convey it to an above-ground storage tank. The pumped up product is then removed for further treatment or recycling.
In-situ simulated biodegradation
For degrading the pollution, in-situ bioremediation makes use of micro-organisms that may or may not be already present in the soil. Ideal conditions for promoting this biodegradation are created. Depending on the type of pollution, this can be done by creating an aerobic or anaerobic environment, through the injection of oxygen or nutrients. If needed, grafting with special micro-organisms can be executed.
Injecting an oxidant into the soil, mostly in the saturated area, activates the in-situ decomposition of (organic) components. Fully oxidised, the components are decomposed in situ into CO2 and water.
In-situ thermal treatment
For soil polluted with coal tar, hydrocarbons, cyanides, mineral oil, chlorinated solvents, sulphur or mercury, thermal desorption is a good treatment option. The present pollutants evaporate through heating. As a result, they are absorbed in the gas flow, which in turn is captured through soil air extraction and treated above-ground (usually through catalytic oxidation). In-situ thermal desorption can be executed using several heating techniques. With thermal conduction, metal pipes are placed into the soil and heated using hot air. This way, temperatures up to 300 to 600 degrees are within easy reach. Electrical resistance heating entails applying an electric current into the soil causing a resistance to heat the soil. Finally, one could also apply radio-frequency heating, with radio waves creating frictional heat (dipole rotation) in the soil. The technique to be used depends on the required temperature, the available public utilities and the exact location of the pollution (soil and/or groundwater).
Groundwater control (hydraulic barriers, reactive screens, etc.)
This process keeps the groundwater pollution under control. This can be done by installing a so-called hydraulic barrier; this is a row of pumps ('screen') that are placed crosswise to the groundwater's direction of flow in order to capture (pump up) all groundwater passing this screen and treating it above-ground in a groundwater treatment plant.
A second technique entails a so-called biological screen whereby oxygen or a reductant is injected in the pollution plume, whether or not in combination with nutrients, and the biodegradation is stimulated and monitored.
Also a diaphragm wall or reactive screen (funnel and gate) can be applied. A diaphragm wall isolates the pollution whereas a reactive barrier provokes a chemical reaction when the groundwater flows across the screen. In the 'funnel and gate' technique, water is sent across a limited area (gate) in which a reaction is provoked by adding chemicals or adsorption media into this so-called gate.