Blowerless In-Well Stripping
In-Well Stripping With No Aboveground Blower System
Blowerless In-Well Stripping (BIWS) is a simple patent-pending
technology that strips the groundwater with air (or other
sparging fluid) in the well, as in traditional
in-well stripping. BIWS
uses recirculating wells to accomplish the compression of
the air and the stripping of the water. Recirculating
wells enable BIWS to treat large capture widths.
There are two differences between blowerless in-well stripping
and traditional in-well stripping:
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No blower system or other aboveground equipment. BIWS
uses no blower, compressor, or air pump of any kind;
unless the off-gas requires treatment, it uses no aboveground
equipment other than a control panel.
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Power consumption is a small fraction of the power
consumption of traditional in-well stripping. In-well
stripping wells typically operate with positive displacement
(PD) blowers. In a typical PD blower system operating
at 15 psig, more than 75% of the energy used is wasted,
largely in generation of useless heat. In BIWS,
compression of the air is isothermal, eliminating the
generation of waste heat.
Typically an in-well stripping system uses 5 to 20 hp per
well. Blowerless in-well stripping uses approximately
one fifth the power that a traditional in-well stripping
system uses.
The groundwater is circulated several times through the BIWS
well and a large treatment zone established around the well,
and treated for removal of contaminants (and saturation of
the water with oxygen) during each of several passes through
the well.
Dissolved volatile (and some semi-volatile) contaminants
(e.g., BTEX, MTBE) are removed from the groundwater by the
stripping action of the air, which occurs entirely inside
the well.
The entire process is completed belowground, with the only
aboveground expression of the system being a small manhole
cover and a power pole with a power meter and a small control
panel. BIWS systems can be located virtually anywhere
a drill rig can drill a well, even in an active driveway
or the middle of a busy street.
While there are numerous possible
configurations, each optimized for a different set of geologic
conditions, the most basic approach is also the most commonly
used. Figure
1 shows the basic configuration.
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The BIWS well penetrates to the maximum depth of the
dissolved contamination, or to a depth chosen to achieve
a desired capture width. An inlet screen is set
at or near the top of the groundwater.
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The BIWS well incorporates a second screen, an outlet
screen, usually at or near the bottom of the well.
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The inlet portion of the well is separated from the outlet
portion by a packer.
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The water is pumped by a submersible pump (or other means)
to a point above the static groundwater level, where its
direction reverses and it begins to travel back down the
well toward the outlet screen.
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As the water flows downward, a partial vacuum is formed
in the down pipe.
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At a point along the downward path, a metered amount
of air is admitted to the down pipe, where it mixes vigorously
and thoroughly with the water.
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The water and air (bubble) mixture travel downward to
the outlet portion of the well. As the water and
air (bubble) mixture descend in the down pipe, the pressure
increases to above atmospheric pressure, which increases
the saturation concentration of oxygen in water, resulting
in the water being oversaturated with oxygen (and other
air components) when it reaches the outlet screen.
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The water and air (bubble) mixture separates below the
packer. The air, laden with stripped contaminants,
returns to the surface through a third pipe in the well. At
the surface, the air is either released to the atmosphere
or treated for removal of the contaminants. If
the air is treated, it is returned to the well to serve
again as the stripping fluid.
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At the exit screen, higher than normal pressures are
formed, resulting in higher head values near the well.
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The treated water, containing the dissolved oxygen (and
other components of air), flows outward from the well
and upward under the influence of the vertical gradients
created by the extraction process at the top of the well. Because
aquifer materials are typically anisotropic, allowing
horizontal flows more readily than vertical flows, the
flows tend to be even more outward than upward.
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A recirculation zone is created that typically returns
the majority of the treated water to the inlet screen. The
treated depth, the hydraulic gradient, the hydraulic
conductivity, the anisotropy of the aquifer, and the
pumping rate largely determine the shape and size of
the treatment zone.
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The water cycles through the treatment zone and the
well several times, on average, before escaping down
gradient. On
each pass through the well, the water is stripped of
contaminants.
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Unless the off-gas requires treatment, there are no
aboveground systems or equipment. Typically, the
only aboveground expression of a BIWS system is a manhole
with an adjacent power pole that has a utility meter
and a small control panel.
If the off-gas requires treatment, a small vapor-phase carbon
treatment system is used.
While the basic configuration and process are straightforward,
even for this simplest case there are many considerations
in designing and installing Blowerless In-Well Stripping
(BIWS) systems. Well diameter, optimal pumping rate,
number of wells and well placement, length of the inlet and
outlet screens, special development procedures, controls
and instrumentation, in-well plumbing configuration, other
pumping methods, constructability, and many other factors
must be addressed in developing a complete design. For
more complex or challenging geology, there are additional
considerations such as confined aquifer configurations and
multiple rows of wells.
The major advantages of Blowerless
In-Well Stripping technology are discussed below.
Figure 1 - Blowerless In-Well
Stripping - Click to see a larger
image (Patent pending)
Major Advantages Of Blowerless In-Well
Stripping
With Re-Circulating Wells
No Surface Equipment
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Constructed entirely belowground, BIWS systems take
up no aboveground space. Unless the off-gas requires
treatment, the only above ground equipment is a small
control panel that operates the submersible pump.
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Silent operation.
Faster
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Faster than air sparging. Pumping the water in
an established treatment cell around the well and treating
it on several passes through the well is much more thorough
than the largely unknowable treatment process of an air
sparging approach.
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More vigorous than traditional air sparging or pump-and-treat
approaches. Air sparging flows air through paths
of least resistance, often treating only a portion of the
water that flows through the treatment zone. However,
re-circulating wells induce vertical gradients to vigorously
circulate and treat all of the water in the aquifer multiple
times. While the interaction between the air and
water in a traditional air sparging system is not well
understood, or subject to modeling or calculation, the
exact opposite is true for recirculating wells. The
stripping and aeration processes are thorough and rapid,
affecting all of the water in the treatment cell.
Cheaper
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Lower initial capital costs, lower maintenance costs,
and faster cleanups result in lower life-cycle costs.
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BIWS wells are typically three-inch PVC construction,
not much more expensive than traditional air sparging
wells. But,
the equipment in the well costs only a small fraction
of the cost of a blower system in an enclosure required
by an air sparging approach.
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Fewer wells. Well spacing typically 2 to 5 times
depth of contamination. At a site with 50 feet
of saturated zone, well spacing can be 200+ feet.
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Lower energy costs. Because BIWS involves pumping
an incompressible fluid (water instead of air), and because
the air is compressed isothermally, energy costs are
much lower than for traditional in-well stripping.
More Flexible
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Large well spacings at many sites allow great flexibility
in placing wells. Placing wells at a gas station
site, for example, can be quite flexible.
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Tolerant of variable geology. Rather than being
impeded by thin silt lenses and discontinuous clay layers
as traditional in-well stripping systems can be, re-circulation
patterns are enhanced by these typical real-world features.
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The pumping rate can be adjusted after installation
to match actual aquifer response. Pumping rate
can be modified to meet changing conditions during cleanup.
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Does not affect adjacent plumes. Because groundwater
is not extracted, adjacent plumes are not drawn toward
a re-circulating well. Specific plumes or parts
of a plume can be targeted.
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Compatible with soil vapor extraction systems.
Regulatory Advantages
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