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. Dual-phase vacuum extraction is not a strictly accurate description and multi-phase may be a better suited term dependent on site objectives and conditions. The three phases that are generally extracted are:
- Free phase liquids
Vapour and liquid can be withdrawn together under a vacuum using a down well stinger (called slurping) or separately; extracting the liquid with a submersible pump and the vapour with a soil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 is used and generally more efficient.
Additional benefits of DPE are that the system can be used to lower the water table within the contamination plume, this causes 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.
Abstracted water and free phase products are separated, the water is treated and either returned to the site or disposed of to sewer. The free phase product is collected and disposed off-site to a recycling facility 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 thus in-situ biological treatment. This is often termed bio-slurping or bioventing and can be useful to reduce dissolved phase contaminants following product removal.
Where depths to water table are significant or formations relatively impermeable, more powerful units can be used to generate higher vacuums to enhance recovery further. This is termed high vacuum extraction.
A contaminant with a vapour pressure of greater than 1.0mm Hg at 20°C and a Henrys Law constant greater than 0.0001atm/m 3 /mol will be suitable for extraction by their volatile nature. In addition, LNAPL (less dense than water) separate phase hydrocarbons can be recovered this way as well as the technique being useful for some DNAPL plumes. It is important that the potential for explosion is considered at design stage when applying vacuum conditions to the free phase contaminants.
DPE plant generally comprises of:
- Vacuum pumps, (rotary vane, lobe or liquid ring)
- Inlet separators (air-water knockout pot)
- Control systems
- Vapour treatment unit (typically granular activated carbon or cat-ox)
- Oil/water separators
- Pipework to wells
- Abstraction wells
The vacuum pump, knockout pot, control systems and manifolds can be located within a container for transport to site. The oil/water separator and vapour treatment unit are then located within a bunded area within a suitable area of the site. All equipment can be automated and even controlled telemetrically.
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 to a former manufacturing site that was undergoing re- construction. 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 mobilised to site. All pipework was incorporated into the construction allowing the project to continue whilst the treatment was undertaken. The plant was located remotely to the construction area and monitored via telemetry. An online FID (flame ionisation detector) and free product flow meter was 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 of 15No. transferable, top-loading submersible pneumatic pumps which were 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 and pumped through a GAC filter and into a 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 a granular activated carbon, and liquids from the knockout pot were treated with the abstracted free phase liquids.
The system operated for approximately 30 weeks, 26 wells were treated and over 6000 litres of free product was removed; the majority during the first 18 weeks of operation. Levels of free product in the wells was reduced from over 600mm to effectively zero during the works.
Advantages and Limitations
- Proven track record on UK contaminated sites
- In-situ (only required a small area for unit setup)
- DPE is particularly suited to medium open textured granular soils but can be adapted to a wide variety of soil types and ground conditions Can address free product, vapours and dissolved phase
- Cost effective in-situ approach
- Maximum water table of 7-8m below ground level ‘slurping’ (deeper is limited physically by vacuum capabilities and suffers design constraints)
- Explosion hazards must be assessed