VertaseFLI Limited
Number One
Middle Bridge Business Park
Bristol Road, Portishead
Bristol
BS20 6PN
Registered England & Wales No: 3944198
Dual-Phase Extraction (DPE) is an in-situ remediation method involving the combined extraction of liquids, water, free-phase product, and vapour from the vadose and phreatic zones of the contamination plume. Although “dual-phase” is commonly used, the term “multi-phase” may be more appropriate depending on specific site objectives and conditions.
Generally, the three phases extracted are:
Vapour and liquid can be withdrawn together under a vacuum using a down-well stinger (a process called “slurping”) or separately, extracting the liquid with a submersible pump and the vapour with a soil vapour extraction unit. In recent times, the “stinger” approach has been used and is generally more efficient.
Additional benefits of DPE are that the system can be used to lower the water table within the contamination plume, causing contaminants in the newly exposed capillary fringe to be accessible to soil vapour extraction (SVE). Each system must be designed and operated on a site-by-site basis, with the technique being particularly well-suited to medium-permeability horizons with generally volatile or semi-volatile contaminants or free-phase products.
Following abstraction, water and free-phase products are separated; the water is treated and either returned to the site or disposed of via sewer. The free-phase product is collected and disposed of off-site at a recycling facility, while abstracted vapours are treated to remove VOCs.
The technique also encourages the ingress of air and, thus, oxygen into the plume, which can enhance microbial activity and support in-situ biological treatment. This is often termed bio-slurping or bioventing and can be useful in reducing dissolved-phase contaminants following product removal.
Where depths to the water table are significant or formations are relatively impermeable, more powerful units can be used to generate higher vacuums, further enhancing recovery. This method is known as high vacuum extraction.
A contaminant with a vapour pressure greater than 1.0 mm Hg at 20°C and a Henry’s Law constant greater than 0.0001 atm/m³/mol is typically suitable for extraction by their volatile nature. In addition, LNAPL (less dense than water) separate-phase hydrocarbons can be recovered through DPE, and the technique can also be effective for certain DNAPL (dense non-aqueous phase liquid) plumes. It’s essential to consider explosion hazards at the design stage when applying vacuum conditions to free-phase contaminants.
A typical DPE plant setup generally includes:
The vacuum pump, knockout pot, control systems, and manifolds can be housed in a container for ease of transport. The oil/water separator and vapour treatment unit are situated within a bunded area on a suitable site section. All equipment can be automated and remotely monitored via telemetry.
VertaseFLI has designed and built a number of DPE units for both high and low vacuum applications.
A high vacuum extraction unit was designed and constructed in-house to provide a multi-phase solution for a former manufacturing site undergoing reconstruction. Approximately 70 wells were installed around the groundworks contractor as contamination was discovered during the construction phase. All wells and pipework were then buried, and a system was mobilised to the site. All pipework was incorporated into the construction, allowing the project to continue whilst the treatment was undertaken. The plant was located remotely from the construction area and monitored via telemetry. An online FID (flame ionisation detector) and free product flow meter were used to monitor the abstracted vapour and free product, enabling a robust examination of the remediation and effective regulatory sign-off.
Another simpler system comprised 15No. transferable, top-loading submersible pneumatic pumps used to abstract water and free-phase product into an oil-water separator. The product was separated and collected, ready for off-site recycling. Water was stored in a separate tank, pumped through a GAC filter, and discharged into cleaned water drainage under a trade effluent licence.
A vacuum pump and air-water knockout pot system was used to abstract vapours from abstraction wells simultaneously with the submersible water-free phase abstraction. Vapours were treated with granular activated carbon and liquids from the knockout pot were treated with the abstracted free-phase liquids.
The system operated for approximately 30 weeks, treating 26 wells and removing over 6,000 litres of free product, with the majority removed during the first 18 weeks of operation. Levels of free product in the wells were reduced from over 600 mm to effectively zero during the works.
Daniel Baker
Contracts Director, Brown and Mason Group Limited
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Talk to usNumber One
Middle Bridge Business Park
Bristol Road, Portishead
Bristol
BS20 6PN