What is dual phase extraction?
Dual phase extraction, sometimes known as multi-phase extraction or high vacuum extraction, is an in-situ technology which is similar to soil vapour extraction, except that a higher vacuum tends to be applied. This causes the physical removal of several phases of contamination including vapour phase, free phase (also known as free product) and aqueous phase. Dual phase extraction tends to be deployed on highly contaminated sites that contain large amounts of free product hydrocarbon contamination.
Abstracted water and free product are passed through above ground treatment equipment which physically separates and processes the various phases of contamination. The treated water is either returned to the site or disposed to the sewer and the free product is temporarily stored in a bunded tank before transporting to an off-site recycling facility.
Advantages and limitations of dual phase extraction
The main advantages of dual phase extraction are:
- In-situ technology so there is no excavation of contaminated materials and minimal disruption to site activities.
- Simultaneous removal of free product, contaminated groundwater and hydrocarbon vapour.
Another benefit of dual phase extraction is that the system can be used to lower the groundwater level within the contamination plume causing contaminants in the newly exposed capillary fringe to be available to soil vapour extraction.
Site geology and contaminant type and distribution have a big influence the effectiveness of dual phase extraction. Each system is designed and operated on a site specific basis with the technique being particularly well suited to medium permeability ground with free product and volatile or semi volatile hydrocarbon contamination.
Design and correct installation of the abstraction wells are important factors in the effectiveness of the system. Dual phase extraction is difficult to apply at sites where the water table is less than 1m below the ground surface. This is due to the high potential for air-flow short circuiting due to large vertical air-flow rates in the immediate vicinity of extraction wells within highly permeable soils. On the other hand the maximum depth to groundwater is typically 7-8m below ground level due to physical limitations of vacuum pumps to lift liquids more than this vertical distance.
A case study of one of our remediation projects incorporating dual phase extraction can be viewed here.