Concept In-situ thermal remediation

This sheet is part of the BOSS application.


Technique 1: Injection of steam into the soil in order to generate heat to volatilise pollutants – combination necessary with soil air extraction/multi-phase extraction 

Steam injection can take place in zones that may, or may not, be saturated with water. The working mechanism remains the same: Injected steam condenses in the formation around the injection point, whereby a large quantity of latent heat is released – which heats the formation. The warm water spreads (warm water front), which causes the cold water to be forced away (cold water front). Once the temperature at the injection point has risen sufficiently, the steam spreads itself in the formation. The border area between the steam and water is called the steam front (Vito report nr. 2004/MPT/R/042, 2004).

In case a pollutant is present in a concentration which greater than the residual content, the pollutant is also pushed away with the flowing liquid (first cold, then warm water). Upon being heated, the viscosity of the polluting liquid falls as do the capillary forces that fix the pollutant to the pores.  The residual saturation level is therefore reduced. When the temperature is rising, volatilisation also increases; for pollutant mixes with low boiling points, a distillation process will come into effect at a given moment (boiling of pollutant mix). Heating also leads to a rise in the solubility of the pollutant in water and to desorption of pollution absorbed by solid soil particles.

The area within the steam front is heated most intensively, whereby the less volatile compounds (boiling points up to 300°C if the steam injection is continued for long enough) are also volatilised (stripping effect); the influence zone of the steam phase is less than the influence zone of the warm water front.  The quantity of residual product that is left behind in the zone that has only been flushed with cold water and/or warm water, is determined by the capillary properties of the soil (soil texture), the surface tension properties of the pollutant and the pressure gradient which causes the displacement. In the water-saturated soil layer, instead of cold ground-water, soil air is displaced from the injection zone.

For pollutant liquids with greater viscosity (e.g. certain oils) it can be generally expected that migration will not take place in waves, but that particular channels with be followed (“product fingering”).

Steam injection is always implemented in combination with soil air extraction.


Implementation area and implementation conditions

In steam stripping, substances can in principle be removed that have a vapour tension of at least 100N/m² at 100°C (pure product) or a Henry coefficient greater that 10-5 atm.m³/mol.

Steam injection is primarily interesting for the remediation of deeper pollutants (depths in excess of 30 m can be reached). For shallow pollutants, which are relatively limited in size and where there are no buildings present, excavation remains cheaper than steam injection/vacuum extraction.

Another important point of note is that steam injection causes a “liquid wave” of pure pollutant. If this liquid has a density greater than 1, downward migration may take place because the viscosity reduces the capillary forces as a result of the increasing temperature. If there is a clay layer under the pollutant, this problem will occur.

Another term is steam quality. This is defined as the quantity of fluid water present in the damp phase. At 100% steam quality, there will be no fluid water in the damp phase. The remediation efficiency will increase with higher steam qualities (Vito report nr. 2004/MPT/R/042, 2004).



Because there is no experience with this technique on a large scale, no concrete prices were found for this technique.

However, we are able to state that in comparison with air-sparging and bio-sparging, this installation requires more energy and other materials (steel/inox) for piping than  installations for air-sparging and bio-sparging (HDPE).


Environmental burden and measures to be implemented

Costs lie between 31 and 280 euros per tonne, with an average cost of 40-120 euros/tonne (KVIV study day 6 May 2004, Marcel Kolle).


Technique 2: Current injection, applying an electric field in the soil for heat generation to volatilise pollutants – combination necessary with soil air extraction/multi-phase extraction


Besides current injection, there are other ways to heat the soil to mobilise the pollutants within it. One of these methods is the use of the soil’s electrical resistance. By passing current through the soil, the soil is heated. The current (3 or 6 phase alternating current) is passed into the soil via vertical, inclined or horizontal electrodes (applied via normal boring methods); the heat energy that is thus created is equal to R x I² (thermal capacity developed in a resistance R; I is the current strength in amperes).

Because the electrodes are part of one phase, the electrical current flows from one electrode to all surrounding electrodes and vice versa. This results in the soil being heated, and can be generated in the saturated as well as the unsaturated zone. Because the current finds the path with the least resistance, the soil layers related to that path will be heated to a greater extent. This will cause in-situ steam to be generated, whereby the contaminants are mobilised as described above. The main advantage of this approach is that even (and primarily) difficult to permeate soil zones are cleaned.

As with steam injection, in the implementation of current injection a closed network of (multi-phase) extraction filters must be in place to contain released pollutants and to counter undesired dispersion (vertical, lateral and into the atmosphere).


Implementation area and implementation conditions

Pollutions type

This technique is only suitable for the volatilisation and mobilisation of organic compounds. In contrast to soil air extraction, less/non-volatile compounds (such as household fuel) can be cleaned using this technique.

In addition, electro-reclamation can support the removal of the floating layer, which leads to the viscosity of the oil being lowered as a result of the rise in temperature.

Soil type

In order to guide the electrical current, the soil must be highly saturated with water, which means that electro-reclamation cannot be implemented in the unsaturated zone of well-permeable soil. Limited heating of the unsaturated zone can be attained if the saturated zone below is treated with electro-reclamation.

Just as in other in-situ cleaning techniques, the soil must be sufficiently permeable in order to transport the pollutants via the water phase and the air phase. Because the rising temperature leads to a change in the properties of pollutants, the process speed is increased via this approach and the implementation in less permeable soils is improved.

By using current injection, volatile oil pollutants can be fully removed. Even non-volatile pollutants (diesel oil, household fuel) can be removed at an economically viable cost, up to a level of 100 to 200 mg/kg.

Due to the increased temperature, the process speed is greatly increased, which means the technique can achieve results quicker than soil air extraction, for example.



The cost of full-scale implementation of current injection, in the various projects, amounts to 75 to 325 euro/m³, with an average of 125 euros/m³. A prior laboratory-scale investigation to determine the attainability of this technique costs around 3200 euros. The electricity costs represent approximately 20% of the total cost (Vito report nr.  2004/MPT/R/042, 2004).


Environmental burden and measures to be implemented

The energy use is determined by the desired end result and amounts to circa 50 to 100 kWh/m³, with an average yield of 85 to 95%. To obtain complete removal and reach the background values, energy uses of 200 to 600 kWh/m³ is required.

As a result of the high temperatures and electrical charges and currents, appropriate safety measures need to be taken.