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Air Lift/Air Stripping Combine to Clean Aquifers {short description of image}

From Ground Water Currents, January 1996, Issue No. 14

Air Lift/Air Stripping Combine to Clean Aquifers

By Michelle Simon, EPA National Risk Management Research Laboratory

The Weston/IEG UVB technology is an in situ ground water remediation technology that combines air-lift pumping and air stripping to clean aquifers contaminated with volatile organic compounds. The UVB technology, in a Superfund Innovative Technology Evaluation (SITE) program demonstration, has removed trichloroethene (TCE) and 1,1 -dichloroethene (DCE) from ground water at Site 31, March Air Force Base, California.

A UVB system consists of a single well and two hydraulically separated screened intervals installed within a single permeable zone. The air-lift pumping occurs in response to negative pressure introduced at the wellhead by a blower. This blower creates a vacuum that draws water into a well through the lower screened portion of the well. Simultaneously, air stripping occurs as ambient air (also flowing in response to the vacuum) is introduced through a diffuser plate located within the upper screened section of the well, causing air bubbles to form in the water pulled into the well. The rising air bubbles provide the air-lift pump effect that moves water towards the top of the well and draws water into the lower screened section of the well. This pumping effect is supplemented by a submersible pump that ensures that water flows from bottom to top in the well at a rate of approximately 20 gallons a minute. As the air bubbles rise through the water column, volatile compounds are transferred from the aqueous to the gas phase. The rising air transports volatile compounds to the top of the well casing where they are removed by the vacuum blower. The lower effluent is treated before discharge using granular activated carbon.

The transfer of volatile compounds is further enhanced by a stripping reactor located immediately above the air diffuser. The stripping reactor consists of a fluted and channelized column that facilitates the transfer of volatile compounds to the gas phase by increasing the contact time between the two phases and by minimizing the coalescence of air bubbles.

Once the upward stream of water leaves the stripping reactor, the water falls back through the well casing and returns to the aquifer through the upper well screen. This return flow to the aquifer, coupled with inflow at the well bottom, circulates ground water around the UVB well. The extent of the circulation pattern is known as the radius of influence, which determines the volume of water affected by the UVB system.

The UVB technology demonstration evaluated not only the reduction of TCE and DCE concentrations in the ground water discharged from the treatment system, but also, over the course of the study, the radius of influence of the system and the reduction (both vertically and horizontally) of TCE and DCE concentrations in the ground water within the radius of influence. Demonstration results indicate that TCE concentrations were reduced by greater than 94% in ground water discharged from the system. TCE concentrations were reduced from a mean of approximately 54 micrograms per liter (ug/L) in the system's influent to approximately 3 ug/L in the system's effluent. A meaningful estimate of the system's ability to remove DCE could not be made due to the low (less than 4 ug/L) influent concentration of DCE. Based on the results of the dye tracer study, the radius of influence was estimated to be at least 40 ft.

The developer claims that the technology can also clean up aquifers contaminated with other organic compounds, including volatile and semivolatile hydrocarbons. Additionally, the developer claims that in some cases the UVB technology is capable of simultaneous recovery of soil gas from the vadose zone.

For more information, call Michelle Simon at EPA's National Risk Management Research Laboratory at 513-569-7469. To get on the mailing list for the Capsule Report and the Innovative Technology Evaluation Report of the demonstration, send a FAX to Michelle Simon at 513-569-7676.

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