Principle

For in-situ remediation of soil contaminated with organic compounds like volatile hydrocarbons e.g. aromates (BTEX), PAH, mineral oil like petrol, diesel and fuel oil, volatile chlorinated solvents like e.g. tetra- en trichloroethene, use is made of AC current. The most important problems with these kind of pollutants are the so-called smear-zones around the groundwater table and tight soils like clay and peat layers as well as hot spots with high concentrations and/or free product. Conventional groundwater remediation, which is nothing more than flushing the aquifer, will in most cases not be sufficient, because the effect even during very long extraction periods is relatively minor. Contaminants, present as droplets between and films around the soil particles will but slowly dissolve and it will take ‘forever’ before all free product has dissolved. An effective way of accelerating desorption and dissolution is increasing soil temperature. This can be achieved by using Joule heating of the soil with alternating current (AC). Temperature increase is fastest and most easily to achieve in the saturated zone where transition resistance is lowest. The rising heat from the saturated zone will also heat up the overlying layers of the unsaturated zone. Above the capillary fringe temperature increase per meter of soil attains an average of 15 °C. As a result of temperature increase, a number of physical and chemical parameters in soil and groundwater are changed:

• Decrease of the specific mass of the heated groundwater and he dissolved components therein. 
• Increase of the solubility of the contaminants. 
• Increase of the rate of solubility of the contaminants. 
• Increase of the vapor pressure of the different solutions and solution mixtures. 
• Increase in permeability for water as well as hydrocarbons in tight soils. 

Permeability is almost doubled for each 25 to 30 °C temperature increase. Decrease of the solubility of dissolved gasses in the ground water. The temperature difference between the warm soil/water volume between the electrodes and the surrounding cold soil/water complex results in an upwards pressure, whereby a "natural" convection system develops. By deploying additional in-situ techniques like selective groundwater pumping (1.5m³/hr) soil vapor extraction, and in some cases compressed air injection, this upward movement is even more intensified. Moreover, the elevated temperature has a pronounced effect on microbial activity in the soil. Through periodic injection of nutrients into the ground, biorestoration is further enhanced. Investigation during cleanup projects regarding the behaviour of microorganisms subjected to a slow and even warm up has shown that there is considerable bioactivity at temperatures of 60°C and higher. Microorganisms adapt themselves selectively to gradual temperature increases. It appears that those colonies remain that are capable of staying active under these specific circumstances. These positive effects remain noticeable during a relatively long period.

Application

To heat the soil, electrodes are inserted into the ground in a fixed pattern. In general, the electrodes are placed so that they coincide with the lowest point at which groundwater decontamination is to take place. Likewise, extraction filters and, if appropriate, (fixed) injection points for compressed air and nutrients are installed between the electrodes according to a specific pattern. During the process of electro(bio)reclamation the following phases are generally envisioned:

1. Intensive phase

AC-heating of soil and groundwater in the high concentration areas (free product zones and hotspots). Direct heating of the saturated zone and Indirect Heating of the unsaturated zone (capillary fringe and smear zone) by rising heat from the saturated zone. Heating is combined with groundwater and soil vapor extraction and periodical injection of nutrients oxygen and/or electron donors/acceptors.

2. Extenuated phase

During this phase attention is focused on biodegradation in the remaining source areas and adjoining groundwater plumes. Electrical heating is stopped. Groundwater and soil vapor extraction continue as well as periodical injection of nutrients oxygen and/or electron donors and carbon sources.

3. Monitoring phase

Monitoring and control on the basis of periodic sampling and analyses, if clean-up criteria have not yet been met completely or there is risk of possible recontamination.

Execution of phase 2 and/or 3 depends on the results of the foregoing phase(s). It might be that most of the remediation targets have been reached, but that the remediation efficiency of the intensive phase is getting too low or even negative because of the electrical heating. Phase 3 might be necessary when there are still some remaining low concentration spots left and regulators want to be certain these do not increase in size.

The purification equipment for groundwater and soil vapor as well as the energy supply is housed in containers. If necessary, electricity cables, extraction ducts and pipes can be installed underground.

Site characteristics

Applicable to in-situ cleanup of soil and groundwater polluted with BTEX aromatics, chlorinated hydrocarbons (DNAPL), diesel and fuel oil, light PAH, phenol, and similar pollutants. Applications for other types are being investigated. The technology is primarily applicable to pollution source areas with high concentrations and/or a free product. Areas with lower concentrations are taken on at a later stage. The technology can be applied to clay, sand, and peat soils. In addition, the technology can be applied in the form of an electrokinetic (bio)fence as a passive in-situ cleanup method for containment and remediation of contaminated groundwater plumes.

Combination with other techniques

In combination with direct current electro-reclamation for removing cocktails of inorganic and organic contaminants. Additionally in combination with selective groundwater pumping, soil vapor extraction and compressed air and nutrient injection.

Advantages

Independent of the lithological composition of the subsoil, and applicable to relatively great depths (>>10m) and under buildings. No disturbance of the groundwater flow regime occurs. In addition, biological activity is not disturbed by strong radicals; on the contrary, biological degradation is stimulated. Suitable for heavily polluted sites.

Limitations

Not economically applicable to heavy oil types or solid oil particles such as tar.

Supplementary Provisions

Availability of electric power is required. If no connection via the local grid is possible, a generator can be used. Because of high noise levels,. a generator can only be used outside residential areasTemperatures > 40 °C can have an adverse effect on some coatings of subsurface cables. The presence and location of these cables must be known so that they may be insulated.

Duration of cleanup

Depends primarily on lithology (sand, clay, peat) as well as on the nature, concentration, and extent of the pollution. Duration of the intensive phase varies from a few months to a few years. Duration of the extensive phase varies, in general, from one to three years.

Costs

Depend on specific factors such as the nature and extent of the contaminants, geohydrologic situation, specific soil resistivity, nature of the site (developed or undeveloped), availability of electric power, and other related factors.