Original Date: 09/14/1998
Revision Date: 01/18/2007
Best Practice : Dual Phase Extraction Concept for Remediation
Dual phase extraction (DPE) is a means for effectively extracting large quantities of TCE and other volatile solvents from below the water table at former solvent disposal sites. DPE works hundreds of times faster than other pumping methods in removing solvents from the contaminated groundwater plume. Since these solvents have very high vapor pressures but are relatively insoluble in water, ordinary soil vapor extraction is not effective.
Raytheon Missile Systems Company (RMSC) developed DPE as an enhancement to its pump-and-treat remediation system. The technique (Figure 2-6) involves evacuating air (vapor extraction) from the casing of a pumping groundwater extraction well. The pumping lowers the water level in the well and exposes solvent-contaminated soils to the air. The vacuum also causes the solvents to evaporate. As the air is sucked from the well, the vapor is pulled out and captured within granular activated carbons (GACs). RMSC’s cost of adding pumps to its existing extraction wells was approximately $15,000 per pump.
In the northwest section of RMSC’s facility lies a 65-acre shallow groundwater zone. Here, a 20- to 40-foot layer of perching clay (reddish brown clay/sandy clay) runs laterally at a depth of 75 to 80 feet below the ground surface. This clay is overlain with sand and clayey sand. The shallow groundwater occurs in and above the perching clay at depths of 55 to 98 feet. The water levels in the shallow groundwater zone fluctuate seasonally in response to surface water recharge. The upper zone of the aquifer extends from 100 to 140 feet below the ground surface to a depth of 200 to 220 feet. This zone is underlain by a thick sequence of clayey sediments (known as the Aquitard Unit), which extends to a depth of 350 feet. Below the Aquitard Unit, the sediments consist of thin lenses of sand and gravel within predominantly clayey sand and sandy clay. The reclamation well field consists of 21 upper zone extraction wells, four lower zone extraction wells, nine shallow groundwater zone extraction wells, and 21 recharge wells.
Geologic conditions favor DPE at this site due to the existence of the clay bed at the top of the aquifer, which traps non-aqueous phase liquids. The heavy/clay soils also favor this method. Although TCE is denser than water and relatively insoluble, the 20 parts per million solubility exceeds the drinking water limit of five parts per billion. Operating at a pressure of 12 inches of mercury, the pump vacuum removes TCE at a rate of 120 pounds per month. When the water table level of the aquifer is drawn down below the location of the TCE, the DPE process is triggered to begin.
Vapor is discharged into primary carbon vessels until the vapor concentration of the vessel effluent matches the concentration of the vapor entering the vessel. This ensures maximum saturation of the GACs. Next, the primary vessel is removed and the partially-saturated secondary vessel is moved into the primary vessel’s position. A new carbon vessel is placed in the secondary position and the process is repeated. The spent primary carbon vessel is shipped off-site for regeneration. The carbon supplier replaces the saturated units with dry ones, freeing RMSC from the liability of this material. Activated carbon is used in roll-offs of 10,000 and 20,000 pounds. At a cost of $1.25 per pound, carbon is the best alternative despite being the highest, single-cost material for operations and maintenance. RMSC experimented with various-sized pumps and synthetic resin filters, and determined the best arrangement consisted of two medium-sized pumps with activated carbon to remove chlorinated hydrocarbons. The company’s success with DPE is indicated by the following sites’ removal rates: 33,844 pounds (TCE) at Site 2/E-16; 3,308 pounds (TCE) at Site 2/E-20; 1,714 pounds (TCE, DCE, trichloroethane, freon 113) at Site 3/E-19; 4,724 pounds (TCE, DCE) at Site 5/E-12; and 1,605 pounds (TCE, DCE) at Site 14/shallow groundwater zone.
Figure 2-6. Dual Phase Extraction Concept
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