Advantages Of Density-Driven Convection
Over Air Sparging
In-well stripping is frequently compared to air
sparging because both remove contaminants via air stripping
without bringing the groundwater to the surface. However, there
are differences. The two technologies are actually
more dissimilar than similar.
Point of stripping.
In-well stripping
accomplishes the stripping step within the well and then
releases the stripped water to the aquifer to recirculate
and bring additional contaminants back to the treatment well. The
air stream with the stripped contaminants remains within
the well, from which it is easily recovered for treatment.
Air sparging performs the stripping step outside
of the well. The injected
air and the stripped water do not move in any predictable patterns, and the nature
and effectiveness of the interaction between the sparging air and the groundwater
is largely unknown. The air, with the stripped contaminants,
is allowed to drift upward to the vadose zone, where recapture
is uncertain and typically incomplete.
Pumping and groundwater circulation.
In-well stripping
pumps water through the well, drawing water from one level
of the aquifer, treating it, and releasing it to another
level of the same aquifer. The result is
that the water moves outward from the well in a torroidal circulation pattern. The
water moves both horizontally and vertically through the aquifer under the influence
of strong vertical gradients induced by the draw down and mounding created by
the pumping and release. The torroidal treatment zone has a much greater
radius of influence than air sparging can create. The radius of influence
has to be determined for each site, but often is 2 to 5 times the distance between
the inlet screen and the outlet screen. For a 50-foot thickness of contamination,
well spacings of 300 to 500 feet are theoretically possible, though more conservative
designs (200+ feet) are typically used. A typical maximum
radius of influence cited in the literature for sparging
wells is 20 feet.
Air sparging does not circulate the groundwater
effectively. Consequently,
the zone of influence of an air sparging well is restricted to the area near
the well, through which the air moves upward. Flows, if any, created by
the upward drift of air through the aquifer are impossible to predict. Various
patterns are envisioned for the zone of influence of an air sparging well; most
commonly, it is envisioned as a fractious pattern of air paths from which the
water has been excluded. The radius of influence is generally thought to
decrease with depth, allowing untreated water to flow between adjacent wells
and escape treatment. To counter this, very close well
spacings are required, often on the order of 25 feet, and
many more wells are required than for in-well stripping systems.
Fewer wells.
For the reasons given immediately above,
DDC systems require fewer wells than air sparging systems.
And DDC wells are typically not expensive. They have
an extra section of screen and an air line down the center. Other
than that, they are similar to air sparging wells and are
constructed of off-the-shelf, typically PVC, components.
More flexible well placement.
Because fewer wells are
required and the distances between wells can be so much larger
than for air sparging, there is greater flexibility in placing
wells. DDC systems are not as sensitive
to where the wells are placed. This allows locating
wells out of the way of surface obstructions, utilities,
roadways, residences, etc.
Lower pressures.
Direct
sparging works best when it is possible to inject air into
the formation at the full depth of the contamination. There is necessarily a high pressure in the well, if
the contamination extends very far below the water table. With no resistance
in the aquifer, such as a large gravel and cobble aquifer might approximate,
there is a pressure of approximately 0.43-psi per foot below static groundwater
level. Given an additional pressure requirement due to flow resistance
in the aquifer, the pressures in a direct sparging system can be quite high. For
a 50-foot depth of contamination below the water table, the required air pressure
can easily be 35 to 50 psi. This results in a need
for expensive compressors and ultimately results in large
energy costs, significant noise problems, and large quantities
of heat that must be handled.
In-well stripping works at low pressures. It is only necessary to inject
air a few feet below the water table, just far enough to facilitate creating
a stripping zone in the well of ten to fifteen feet of bubbles. And, there
is no need to push air against any resistance in the formation; all flows (air
and water) occur in the smooth, open well casing. Thus, in-well stripping
systems typically run at 3 – 7 psi. Electrically
driven blowers are used instead of compressors, keeping energy
costs and noise to reasonable minimums, and eliminating smoke
from diesel-driven compressors.
More targeted remediation.
DDC systems can treat just
the portion of the aquifer that is contaminated. If
the contamination does not extend to the surface of the groundwater,
the upper screen can be placed below the water table, and
only the water that is contaminated is treated.
Air sparging systems must inject air at the full
depth of the contamination, even if the contamination is
restricted to a limited zone deep in the aquifer. As
an example, the hydrostatic pressure at the maximum depth
of contamination at a site in Massachusetts would have been
100 psi, although the plume thickness was only 120 feet (52
psi). The energy cost for air sparging at such a
site would be quite large. DDC systems treat only the
contaminated portion of the aquifer and the energy costs
reflect that. An
in-well stripping system at MMR ran at approximately 4 psi.
Capturing the stripped contaminants
Capturing the stripped contaminants is easier
and less uncertain. When
stripping volatile contaminants in the formation using air
sparging, the flow patterns of the air are not well known,
even in theory. This results in
reduced ability to capture the stripped contaminants. To
the extent that capture fails, contaminants are only moved
from one medium, the groundwater, to two others, the vadose
zone vapor and the atmosphere.
DDC treats the contaminants in
the well, where they are stripped from the water. The
contaminant-laden air is not allowed to disperse beyond the
well. By confining the stripping
process to the well, the contaminants are readily available
for capture and treatment.
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