2.3 Techniques for Recovering LNAPL
Conventional pumping techniques such as pump-and-treat are no longer considered
to be the best course of action to treat groundwater as they require too much time to
reach the cleanup thresholds, and are simply not effective enough (Moyers et al.
1997; Illangasekare and Reible 2000; Bayer et al. 2002; Khan et al. 2004). However,
they can still be useful for recovering pure products (free products) before
implementing other remedial actions.
The various LNAPL recovery techniques include:
• Groundwater extraction and skimming (downwelling)
• Free product recovery with skimming
• Slurping (dual phase extraction)
• Groundwater extraction and vapor extraction
Pumping–skimming is the most popular technique for the recovery of free
product in permeable aquifers with shallow water tables (USEPA 1996; FRTR
2007; ADEME and Ernst & Young 2014). Generally, the efficacy of these recovery
techniques is limited in the case of non-homogeneous and/or fractured media; they
are best suited in the most water permeable zones (USEPA 1996; Nadim et al. 2000).
LNAPL recovery techniques are often interrupted once the recovery of pure products
reaches a plateau, which corresponds to a free product thickness of a few
centimeters.
The following sections describe the main remediation techniques for LNAPL free
product recovery.
2.3.1 Free Product Recovery with Groundwater Extraction
and Skimming (Downwelling)
Pumping groundwater produces a cone of depression. Free product follows the
hydraulic gradient, producing a flow that migrates toward the lowest zone
(s) where it accumulates. This accumulation of free product can then be readily
recovered (Fig. 2.10) (Hyman and Dupont 2001; Sale 2001; U.S. Army Corps of
Engineers 2003; Charbeneau 2007; Charbeneau and Beckett 2007).
The free product is recovered by pumping it toward surface storage units (tanks)
using skimming pumps, before being removed (incinerated, hydrocarbon recovery,
etc.). Skimmers are positioned where the free product accumulated to recover
hydrocarbons, and are designed to minimize groundwater recovery as much as
possible. The most commonly used recovery systems include pneumatic hydroejectors, skimmers (or floating strainers), and oleophilic/hydrophobic mesh
(USEPA 1996).
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S. Colombano et al.
Conventional pumping techniques such as pump-and-treat are no longer considered
to be the best course of action to treat groundwater as they require too much time to
reach the cleanup thresholds, and are simply not effective enough (Moyers et al.
1997; Illangasekare and Reible 2000; Bayer et al. 2002; Khan et al. 2004). However,
they can still be useful for recovering pure products (free products) before
implementing other remedial actions.
The various LNAPL recovery techniques include:
• Groundwater extraction and skimming (downwelling)
• Free product recovery with skimming
• Slurping (dual phase extraction)
• Groundwater extraction and vapor extraction
Pumping–skimming is the most popular technique for the recovery of free
product in permeable aquifers with shallow water tables (USEPA 1996; FRTR
2007; ADEME and Ernst & Young 2014). Generally, the efficacy of these recovery
techniques is limited in the case of non-homogeneous and/or fractured media; they
are best suited in the most water permeable zones (USEPA 1996; Nadim et al. 2000).
LNAPL recovery techniques are often interrupted once the recovery of pure products
reaches a plateau, which corresponds to a free product thickness of a few
centimeters.
The following sections describe the main remediation techniques for LNAPL free
product recovery.
2.3.1 Free Product Recovery with Groundwater Extraction
and Skimming (Downwelling)
Pumping groundwater produces a cone of depression. Free product follows the
hydraulic gradient, producing a flow that migrates toward the lowest zone
(s) where it accumulates. This accumulation of free product can then be readily
recovered (Fig. 2.10) (Hyman and Dupont 2001; Sale 2001; U.S. Army Corps of
Engineers 2003; Charbeneau 2007; Charbeneau and Beckett 2007).
The free product is recovered by pumping it toward surface storage units (tanks)
using skimming pumps, before being removed (incinerated, hydrocarbon recovery,
etc.). Skimmers are positioned where the free product accumulated to recover
hydrocarbons, and are designed to minimize groundwater recovery as much as
possible. The most commonly used recovery systems include pneumatic hydroejectors, skimmers (or floating strainers), and oleophilic/hydrophobic mesh
(USEPA 1996).
78
S. Colombano et al.