Remediation Technologies - Environmental Investigation and Remediation - page 325

An air-stripping optimization test was performed at the U.S. Air Force Plant 44 (AFP 44), located
in Tucson, Arizona, to determine whether adjustments to the operating parameters for the large-scale
treatment system could effectively reduce the inl uent 1,4-dioxane concentrations (10-15 μ g/L) to

meet a target level of 6.1 g/L (Earth Tech, 2004). The AFP 44 groundwater treatment plant utilizes

three parallel trains of two-stage air-stripping towers (primary and secondary stripping towers) with
a design capacity of ~5000 gallons/min. At an air:water ratio of seven, the 25-feet-tall primary
towers, in combination with GAC treatment of the off-gas, are capable of removing as much as 85%
of the trichloroethene (TCE) and 1,1-dichloroethylene. At an air:water ratio of 25, the 42-feet-tall
secondary towers are designed to remove the remaining TCE and 1,1-dichloroethylene from the
water; the stripped vapors are then vented to the atmosphere. This system had no measurable effect
on 1,4-dioxane removal. Optimization of the existing treatment system to remove 1,4-dioxane was
accomplished by modifying the system to allow higher air-l ow rates, thereby increasing the
air:water ratios. The air:water ratio in the primary tower was set at 69, the maximum air:water ratio
possible, and the secondary tower's air:water ratio was varied from 183 to as high as 291. The study
indicated that the maximum removal rate for 1,4-dioxane was ~10%, even with air:water ratios as
high as 291, which is more than 10 times the designed air:water ratio for the removal of TCE and
dichloroethene. Chlorinated VOC removal was complete regardless of the air:water ratio. The
results of the optimization test indicated that utilizing the air-stripping system as the primary means
to remove 1,4-dioxane would not achieve the target level of 6.1

μ

μ g/L.

7.1.2 A CCELERATED R EMEDIATION T ECHNOLOGIES , LLC I N -W ELL S YSTEM
A more focused and comprehensive mass-transfer approach—combining air stripping, air sparging,
soil-vapor extraction, enhanced bioremediation, and underground circulation—has been demon-
strated to be effective for 1,4-dioxane removal under specii c circumstances (Odah et al., 2005). The
Accelerated Remediation Technologies, LLC (ART) In-Well System is designed to operate in a
4-inch or larger well, screened both above and below the water table. The system uses several con-
current physical, chemical, and biological processes to reduce contaminant levels in groundwater
( Figure 7.1 ). Air is forced into the lower part of the well to remove some levels of contamination in
the groundwater through sparging. Additionally, the aeration of the groundwater reduces its density,
which causes a drop in hydraulic head in the well. The lower head causes cleaner, low-density
groundwater to be pushed out from the upper part of the well and contaminated, high-density
groundwater to be drawn into the lower part of the well to create what the inventor refers to as
dynamic subsurface circulation. The circulated water effectively l ushes soluble contaminants from
the lower vadose zone. Water in the well bore is also pumped to the top of the well and sprayed
downward through the rising air column, creating an air-stripping effect without using a packing
medium to increase the surface area available for phase transfer. The In-Well stripping requires
multiple stripping passes, to compensate for 1,4-dioxane's general resistance to stripping from
groundwater. Vapors and air are extracted from the wellhead, creating a soil-vapor extraction effect
for the screened section of the well above the water table. Lastly, the increased movement of air
from all this sparging and extraction leads to higher levels of dissolved oxygen in the groundwater
and more oxygenation of the vadose zone, potentially stimulating aerobic biologic processes that
may otherwise be absent under natural conditions. The radius of inl uence of the ART In-Well
System has been documented at ~10 times the water-column height, both up gradient and down
gradient from the well (Odah et al., 2005). The ART In-Well technology was applied at a major
aerospace manufacturing facility in North Carolina, where the subsurface was saprolitic soil over
fractured bedrock. A single ART well was installed, and monitor wells were positioned 10 and 20 ft
down gradient (MW-1 and MW-2, respectively) to gauge the system's effectiveness. 1,4-Dioxane
concentrations in that part of the aquifer were as high as 43,000

g/L, and high levels of chlorinated
VOCs were also present. Within 90 days, levels in MW-1 declined from 25,000 to below 7400

μ

μ g/L,

and those in MW-2 declined from 28,000 to 2400 μ g/L. These reductions represented more than