This project evaluated the cost savings and performance of innovative bioslurper system designs that provide pre-pump separation of fuel from groundwater. Bioslurping, involving simultaneous vacuum extraction of fuel, soil gas, and groundwater from air-tight wells, has been shown to be most effective for removing subsurface light non-aqueous phase liquids (LNAPL). However, conventional bioslurper operation and maintenance costs can be high due to extensive post-pump treatment needed to remove effluent emulsions, foams, and volatile organic compounds (VOC). Pre-pump separation is required to prevent the mixture of fuel with groundwater within the vacuum pump, which creates stable emulsions and foams as well as excessive release of volatile off-gas contaminants.
Two pre-pump separators were evaluated alone and in series: an in-well "dual drop tube" and an innovative aboveground knockout tank. The "dual drop tube" consists of two vacuum drop tubes separated by a fuel isolation sleeve (i.e., rigid tubing slightly larger than the primary drop tube into which the primary drop tube end is inserted). This design prevents fuel extraction with the larger primary drop tube while groundwater enters the drop tube from below and soil gas from above the isolation sleeve ends. A smaller (e.g., quarter-inch diameter) vacuum drop tube removes accumulating fuel. The fuel isolation sleeve should extend one to two feet both above and below the end of the primary drop tube to prevent the movement of LNAPL over or under the sleeve ends. The driving force for fuel removal is the establishment of both hydraulic and pneumatic gradients as in conventional bioslurping. A simplified vacuum knockout tank design, placed immediately before the vacuum pump, was also evaluated for fuel-water separation efficiency and as a liquid surge protector for the pump.
The two pre-pump separator designs were evaluated for 2 to 5 weeks using a single well at eight field sites contaminated with different fuels. A long-term (15-week) demonstration, using five wells, was conducted at a JP-5 jet fuel spill site at Naval Air Station Fallon, Nevada. The 2- to 6-day, continuous short-term demonstrations used the following configurations: conventional (initial); in-well "dual drop tube"; pre-pump knockout tank; and conventional (final). The "dual drop tube" eliminated emulsion and foam formation and reduced pump effluent total petroleum hydrocarbon (TPH) levels by greater than 99 percent at most sites, compared to conventional bioslurping. The pre-pump knockout tank separator did not completely remove emulsions and eliminated about half the pump effluent water TPH. Fuel and groundwater extraction rates were not affected by either separation technique. System off-gas data indicate about a one-third decrease with the in-well separator and no significant decrease when the knockout tank separator was used alone. The multi-well, long-term demonstration produced similar results.
In-well separation with the "dual drop tube" resulted in significant potential savings and performance improvement compared to conventional bioslurping. At sites with high LNAPL extraction rates, increasing the fuel extraction tube size can result in a higher fuel to groundwater recovery ratio. The knockout tank separator functioned well as a surge suppressor, served as a source for long-term pump effluent contamination, and represented a LNAPL discharge control during ineffective "dual drop tube" operations. The low off-gas contaminant removal rates during pre-pump separation may, in part, represent vapor transport with conventional bioslurping foams. For an average bioslurping site (i.e., 2-acre LNAPL plume, 50 extraction wells to 15-foot depth, 2-year recovery period), the savings would be 40 to 45 percent in the range of $200,000 to $250,000, resulting in a total potential Department of Defense savings of more than $200 million. Capital costs with and without in-well separation are essentially the same. (Project Completed 2003)