Remediation technology: Dual Phase Extraction (DPE)
Dual Phase Extraction (High vacuum extraction) is a methodology for decontamination “in situ” of aquifers affected by free phase of light hydrocarbons (light free phase) and contaminated water. Also known as multiphase extraction (MPE, Multi Phase Extraction) because it allows to simultaneously extract liquid phase and gas phase.
Consists of the application of a high vacuum to the ground via small diameter pipes (slurpers) installed in extraction wells in order to recover both free phase from contaminated water and extract underground vapours from the unsaturated area (vadose zone). This technique principally extracts the gaseous phase (contaminated air and vapour) and the gas stream drags the liquid phase as droplets within the vacuum air current. The free phase, contaminated water and subsurface extracted gases are treated in a physical and/or chemical treatment system.
During the recovery of free product, contaminated water and extracted vapours from the vadose zone, the forced ventilation generated in the contaminated area can promote biodegradation, and is often referred to as Bioslurping
Applicability of Dual Phase Extraction
This technique can be used in shallow aquifers contaminated with free phase (light non-aqueous phase liquid, LNAPL) in the groundwater. Generally used in middle or low permeability soils (silts and silty sands) where there is not enough water available to apply the technique of pumping.
In order to obtain an optimal performance using Dual Phase Extraction, it is recommended that the depth to the groundwater is between 7 and 8 metres. It is considered, that this technique loses its effectiveness when the groundwater level is below 10 metres in depth, although other factors may come in to play. It is therefore advisable to perform pilot trials in order to verify the viability of this technique as well as determining the number of extraction wells, radius of action, type of vacuum pump, etc.
Among the main advantages of treatment using Dual Phase Extraction are as follows:
- Allows you to treat simultaneously the vadose zone and hydrocarbon impacted groundwater both in the form of free phase and/or dissolved phase.
- Is applicable in low-permeability soils where pumping will be inefficient.
No pumps are required to be installed in each of the extraction wells.
- Installation costs are cheaper and maintenance of the equipment is easier.
- No risk of deflagration at the point of extraction.
The treatment is suitable for installations in use, generating little interference with the day-to-day operations.
Some of the limitations of this technique are as follows:
- High cost in deployment, in relation to the number of extraction wells to install.
- High cost in electrical consumption during the treatment.
- The monitoring process is complex.
- Radius of influence is low.
- Limited application depending on the depth and distance.
- Noise mitigation of the treatment system is required.
- The treatment system needs to be ATEX certified. This is important due to the fact that the system receives inflammable gases and liquids from the extraction wells.
As with all other ground remediation techniques, the effectiveness DPE decreases with time. Therefore, once the objectives of remediation are 85-95 % completed, cheaper and less intrusive remediation techniques can be employed.
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Publicado el 30/05/17