Fact sheet: Multi-phase Extraction Systems for non-aqueous phase liquids (NAPL)

From: Public Services and Procurement Canada

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Description

Les systèmes d'extraction multiphase (EMP), aussi appelés systèmes d'extraction sous vide ou système de récupération de produit en phase libre sous vide, comprennent toutes les technologies qui appliquent un vide à l'intérieur d'un puits avec une pression négative afin de pomper le liquide en phase non aqueuse ( LPNA) et/ou des eaux souterraines contaminées combinées à l'extraction des vapeurs. Des systèmes d’EMP sont souvent installés sur des sites où il est possible de simultanément récupérer les LPNA, contrôler la pollution des eaux souterraines ou assainir les sols au-dessus de la nappe phréatique (la zone « vadose »). L’EMP est une technologie énergivore et coûteuse qui est souvent abandonnée au profit d'autres mesures lorsque le produit en phase libre a été récupéré et traité.

Cette technique possède une terminologie très variée; cependant, les deux principales configurations utilisées sont : 1) l'extraction sous vide avec un seul tuyau d’aspiration aussi appelé bioaspiration (« bioslurping »), extraction des deux phases (« Dual-phase extraction ») ou récupération de l’ensemble des fluides (« Total Fluids Recovery »), et 2) l'extraction sous vide à l’aide de deux pompes (généralement une pompe submersible et une pompe à vide), également appelée extraction double phase.

La configuration en utilisant un seul mode de pompage consiste à installer un tuyau d’aspiration dans le puits, au niveau de l’interface air-liquide, pour récupérer les liquides contaminés et extraire les vapeurs. L'extraction est effectuée à partir d'une pompe à pression négative (à vide) située à la surface. Les liquides et les vapeurs contaminées sont récupérés simultanément et un séparateur air-liquide est requis. En raison de la nature du vide appliqué, la profondeur du liquide pompé est généralement limitée à environ 10 m (30 pieds), mais certaines applications sont possibles à plus grande profondeur. Parce que cette technique permet une aération forcée de la zone vadose et améliore ainsi la biodégradation aérobie, elle est également connue sous le nom de bioaspiration.

Un système d'extraction à double phase consiste à installer des conduits séparés pour le liquide et le gaz, une pompe submersible dans le liquide contaminé (LPNA et eau souterraine) et une seconde pompe dans la zone vadose pour extraire l'air uniquement. Une pression négative est créée dans le puits et le liquide et les vapeurs contaminés sont pompés séparément.

Liens Internet:

Implementation of the technology

A multi-phase extraction system involves the use of wells, trenches, permeable drains or other structures to extract:

  • Vapour (air) and a mixed liquid stream containing both groundwater and NAPL (drop tube extraction or bioslurping)
  • Vapour and groundwater and a NAPL separated stream (dual-phase extraction system).

Extracted phases are separated for subsequent treatment or disposal. Phase separators include “knockout drums” (air/water separators) and oil-water separators. Vapour treatment systems commonly use combustion processes, such as thermal oxidation or catalytic oxidation, or filtration/sorption processes, such as activated carbon filtration or biofiltration.

Multi-phase extraction system projects may include:

  • Mobilization, site access and temporary facilities
  • Extraction well installation
  • Collection trench installation and/or permeable drain installation
  • Pump and conveyance pipe installation
  • MPE unit installation (container or small building) including vacuum pump(s), phase separation equipment, air and water treatment equipment and controls
  • Accumulation tanks for non-aqueous phases (free product, recovered fuel, recovered solvent) for off-site disposal of groundwater treatment equipment is presented in the “pump and treat” factsheet.
  • Treated water discharge installations (for example, to ground, to injection wells, to infiltration or “leach” fields, to local storm water system, to local sanitary sewer, or to surface water)
  • Extraction wells and treatment unit decommissioning

Systems are frequently test-implemented on a “pilot” scale.

Materials and storage

  • Multi-phase extraction systems rely on traditional water well, drainage, water works and utility methods and require commonly available construction equipment.
  • Treatment plants may be constructed on-site. Pre-assembled units in trailers, shipping containers or on skids, are commonly available.
  • Extraction and conveyance requires energy input and maintenance chemicals to periodically remove scaling or fouling.
  • Treatment systems will vary and may include a range of oxidants (such as hydrogen peroxide), biological substrates, sorbents, anti-foulants or anti-scaling compounds, emulsion breakers, or auxiliary fuels. 
  • Unless an extensive trench system is required, construction activities are typically low-impact, with minor on-site storage. Treatment systems typically stock fresh reactants and process chemicals as well as collected residuals, such as collected free products and spent sorption media.

Waste and Discharges

  • System installation typically requires drilling or excavating in contaminated areas, resulting in the handling and disposal of highly contaminated soils, which are typically containerized and disposed of off-site. 
  • Treatment systems may generate extensive solid, liquid and gaseous residuals. Appropriate storage, management and disposal of treatment residuals are part of proper treatment system operation. Spent sorbent (for example, activated carbon) and collected solids (sludge) require collection and off-site transport. The nature of treatment processes frequently concentrates contaminants in these solid residuals. Under specific conditions, solid residuals may be flammable, corrosive and/or produce toxic leachate.
  • Ideally, treated groundwater meets all applicable criteria for release and doesn’t constitute a high-risk discharge. However, inadequately treated discharge, discharge containing by-products and discharge containing excess reactants or at unacceptable pH levels may constitute a hazard to downstream receptors.
  • Free product (NAPL) is typically collected in drums or tanks for eventual shipping off-site.
  • MPE systems typically include a vacuum component and require monitoring and/or treatment of vapours discharged from the vacuum pump. Volatilization from accumulated free product and/or highly contaminated groundwater can also create areas of high vapour concentrations within the treatment system enclosure.

Recommended analyses for detailed characterization

Biological analysis

  • Total heterotrophic and specific bacterial counts (according to the contaminants of interest)

Chemical analysis

  • pH
  • Organic matter content
  • Contaminant concentrations present in the following phases:
    • adsorbed
    • dissolved
    • free
  • Nutrient concentrations including:
    • ammonia nitrogen
    • total Kjeldahl nitrogen
    • nitrates
    • nitrites
    • total phosphorus

Physical analysis

  • Dissolved methane concentration
  • Soil granulometry
  • Contaminant physical characteristics including:
    • viscosity
    • density
    • solubility
    • vapour pressure
  • Measurement of NAPL surface tension under site conditions
  • Presence of non-aqueous phase liquids (NAPLs)

Recommended trials for detailed characterization

Biological trials

  • Microcosm mineralization trial
  • In situ respirometry trials
  • Biodegradation trial

Physical trials

  • Gas permeability trials
  • Vapour survey
  • Evaluation of the radius of influence
  • Evaluation of operating pressure/vacuum

Hydrogeological trials

  • Pneumatic trials
  • Permeability test
  • Pumping trials

Other information recommended for detailed characterization

Phase II

  • Contaminant delineation (area and depth)
  • Presence of receptors:
    • presence of potential environmental receptors
    • presence of above and below ground infrastructure
    • the risk of off-site migration

Phase III

  • Soil stratigraphy
  • Identification of preferential pathways for contaminant migration
  • Conceptual site model with hydrogeological and geochemical inputs

Applications

  • Primarily suitable for treatment of NAPL contamination (free phase)
  • May also apply to residual contamination within the vadose zone, dissolved contamination within the groundwater, and contaminant vapours within the vadose zone
  • Most frequently used for light NAPL such as diesel and gasoline, which are lighter than water.
  • Use for dense NAPL such as chlorinated solvents has also been successful; however, inducing DNAPL flow to an extraction point may be infeasible depending on the density and profile of underlying stratigraphic units
  • Applicable in soils with permeabilities ranging from 10-5 to 10-3 cm/s
  • May be used in lower permeability soils (above 10-3 cm/s), but in this case special design considerations must be considered such as more closely spaced extraction wells and a sealing layer installed at the ground surface
  • Enhance the volatilization of some contaminants in the residual phase
  • May be used as a form of enhanced Soil Vapour Extraction (SVE). Decreasing the groundwater level dewaters soils such that SVE becomes effective

Applications to sites in northern regions

Intensive multi-phase extraction may not be appropriate for remote northern sites without access to utilities or local operations and maintenance labour. Possible alternatives include source area excavation, passive skimming, passive reactive barriers, and bioventing as alternatives. Northern systems require climate-appropriate design, including consideration of deep frost, permafrost, seasonal changes in ground conditions and long periods without operator intervention, fuel supply or collected product removal.  

In cold climates, freeze-thaw cycles can cause the remobilization of residual NAPL. As wet soil freezes, its volume increases because ice has a larger volume than liquid water. The increased volume results in material transport through frost heave and related phenomena. 

Treatment type

Treatment type
Treatment typeApplies or Does not apply
In situ
Applies
Ex situ
Does not apply
Biological
Applies
Chemical
Does not exist
Control
Does not exist
Dissolved contamination
Applies
Free Phase
Applies
Physical
Applies
Residual contamination
Applies
Resorption
Applies
Thermal
Does not exist

State of technology

State of technology
State of technologyExist or Does not exist
Testing
Does not exist
Commercialization
Exist

Target contaminants

Target contaminantsApplies, Does not apply or With restrictions
Aliphatic chlorinated hydrocarbons
Applies
Chlorobenzenes
With restrictions
Explosives
Does not apply
Metals
Does not apply
Monocyclic aromatic hydrocarbons
Applies
Non metalic inorganic compounds
Does not apply
Pesticides
Does not apply
Petroleum hydrocarbons
With restrictions
Phenolic compounds
With restrictions
Policyclic aromatic hydrocarbons
With restrictions
Polychlorinated biphenyls
Does not apply

Treatment time

Treatment time
Treatment timeApplies or Does not apply
Less than 1 year
Applies
1 to 3 years
Applies
3 to 5 years
Does not apply
More than 5 years
Does not apply

Notes:

Free product recovery rates typically decline quickly, in a matter of weeks to months. When further free product recovery becomes uneconomic, the system is often replaced with an alternative residual impact management strategy, such as monitored natural attenuation or groundwater pump and treat.

Long-term considerations (following remediation work)

As discussed above, multi-phase extraction systems are commonly taken off-line and replaced once recoverable free product has been exhausted. At the time of replacement, environmental clean-up criteria typically have not been met. Rebound of LNAPL can occur some time after system shutdown. Monitoring of LNAPL over a few weeks to months depending on the type of soils is recommended.

Secondary by-products and/or metabolites

Multi-phase extraction systems can stimulate contaminant biodegradation because of increased air circulation and oxygen concentration within the vadose zone. Biodegradation of certain chlorinated aliphatic hydrocarbons may produce toxic metabolites (e.g. biological transformation of dichloroethene forms vinyl chloride). By-products and metabolites produced in the substrate will typically be captured by the extraction well system and can thus be removed by the treatment unit. In general, the potential for by-product production in situ is very limited.  

MPE systems include treatment systems for the extracted vapour and liquid (NAPL or mixed NAPL and groundwater). Incomplete reaction within a given treatment system may result in hazardous degradation products. Management of treatment system by-products is a part of normal treatment system operation.

Limitations and Undesirable Effects of the Technology

  • Generally, this technology is less effective in very high permeability or very low permeability formations, depending on the treatment objectives. The ideal permeability range is 10-5 to 10-3 cm/s. 
  • The pumping rate of liquids is reduced when the ground water level is deep (> 10 m).
  • Temperature can limit biodegradation activity and influence the volatilization of organic contaminants.
  • Heterogeneous material, impermeable sub-layers or preferential pathways can reduce effectiveness of the technology, limit NAPL recovery rates and affect soil aeration.
  • Volatilization may be reduced in soils with high organic matter content.
  • In some cases, the multi-phase extraction of free phase generates a significant volume of water for treatment.
  • Phase separation processes (water/NAPL/vapour) and/or effluent treatment can be complex, depending on the nature of the contaminants.
  • Significant fluctuations in the water table may complicate contaminant extraction.
  • MPE, by design, creates large, short-term changes in subsurface vapour pressure and hydraulic gradient. As a result, groundwater levels may be depressed and the tension-saturated zone above the water table (the “capillary fringe”) may be substantially altered.
  • Large changes in water table can “smear” free product across the vadose zone. Biosplurping systems are specifically intended to mitigate this type of contaminant infiltration in the vadose zone. 
  • Inadequate or inappropriate treatment may expose receptors downstream of the system discharge point to contaminants or by-products.
  • Free product and its vapours can create serious fire or explosion hazards. Because of its flammability and/or toxicity, it is usually handled as a hazardous material/waste or dangerous good. 

Complementary technologies that improve treatment effectiveness

  • Thermal treatment (electrical resistance heating, thermal conduction heating, hot air injection, or steam injection) improves the mobility of NAPL as well as the volatilization of semi-volatile organic compounds and enhances biodegradation activity when the operational temperatures are below 40 °C.
  • Mechanical or pneumatic fracturing processes increase soil permeability of less permeable soils. The creation of new transport pathways by soil fracturing may increase contaminant recovery rates.
  • Sealing of the ground surface increases the zone of influence of each well’s multi-phase extraction system and reduces the risk that vapour emissions will migrate from the ground to the surface.
  • Multiphase extraction can be operated in different configurations such as with or without drawdown to target different treatment objectives (LNAPL recovery, soil vapour extraction, bioremediation).
  • When further free product recovery becomes uneconomic, the system is often replaced with an alternative residual impact management strategy, such as monitored natural attenuation or groundwater pump and treat.

Required secondary treatments

  • Phase separation systems such as air-liquid separators and NAPL-water separators are required.
  • A groundwater treatment system is usually required. Water treatment processes vary significantly depending on the type of contamination present. The following list presents examples of treatment and is not exhaustive.
    • Air or vapour stripping to treat VOCs;
    • Adsorption onto matrices, such as activated carbon, to treat SVOCs;
    • Bioreactor treatment to treat non-halogenated VOCs, SVOCs, and petroleum hydrocarbons;
    • Chemical and/or UV oxidation to treat volatile and semi-volatile organic compounds (VOCs and SVOCs, respectively), some hydrocarbons and pesticides;
    • Membrane separation to treat VOC and SVOC hydrocarbons.
    • Coagulation and flocculation;
    • Filtration
    • Electro coagulation;
    • Phytoremediation (treatment wetlands);
    • etc.
  • A vapour treatment system is required. Vapour treatment systems commonly involve combustion processes, such as thermal oxidation or catalytic oxidation, or filtration/sorption processes, such as activated carbon filtration or biofiltration. 
  • Separated NAPL must be treated or prepared for disposal.

Application examples

  • Multi-phase extraction systems are most suitable for recovering LNAPL and treating volatile organic compound (VOC) contamination in the free phase. It is also applicable for treating dissolved contamination within the groundwater and residual contamination within the vadose zone.
  • This technology allows for removal of LNAPL while minimizing the amount of groundwater which is pumped.
  • In soils with low or moderate permeability, this technology can be used to lower the water table and increase volatilization or biodegradation of organic compounds within the capillary fringe.
  • The operation of the system can be modified to promote a desired sequence of treatments (such as the removal of LNAPL followed by a drawdown of the water table to favor bioventilation).

The following sites provide sample applications:

Multiphase Extraction (Bioslurping) of JP-4 Impacted Soil and Groundwater, Yokota, Japan—Shih & Griffin 1996—Carbtrol pdf

Multi-Phase Extraction and Dual-Pump Recovery of LNAPL at the BP Former Amoco Refinery, Sugar Creek, MO March 2005—CluIn—US EPA pdf

Risk Reduction using Innovative Vacuum-enhanced Plume Controls—Solc & Botnen 2009—US Dept. of Energy

Performance

Multi-phase extraction is a proven technology used, for over 20 years, to recover LNAPL when LNAPL is above residual range. In general, multi-phase extraction technologies are more cost-effective than other active LNAPL technologies (ITRC, 2009).

A report published by the U.S. EPA (2005) includes a comprehensive discussion of the performance and costs of multi-phase extraction systems:

Measures to improve sustainability or promote ecological remediation

  • Pump type and size optimization
  • Schedule optimization for resource sharing and minimization of mobilization days
  • Use of renewable energy, such as geothermal or solar energy, and of energy efficient machinery
  • Wastewater process optimization to reduce wastes and consumables, such as activated carbon
  • Re-cycling of recovered free product for use as a fuel
  • Optimization of air and water flow rates to reduce size of treatment equipment and energy consumption
  • Use of biofilters for air and water treatment
  • Implementation of cycling operation mode rather than continuous operation to improve recovery
  • Minimizing site visits by the use of telemetry for remote monitoring of site conditions.

Potential impacts of the application of the technology on human health

Unavailable for this factsheet

References

Author and update

Composed by : Martin Désilets, B.Sc., National Research Council

Updated by : Jennifer Holdner, M.Sc., Public Works Government Services Canada

Updated Date : April 29, 2014

Latest update provided by : Marianne Brien, P.Eng., Christian Gosselin, P.Eng., M.Eng., Golder Associés Ltée

Updated Date : March 31, 2018

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