Floating Product Removal

Two-Pump LNAPL Removal With Re-Circulating Wells

Floating Product Removal (FPR) is a simple technology that removes LNAPL contaminants from groundwater using a two-pump approach, but without removing groundwater from the aquifer.  High efficiency removal of the floating product and treatment of the groundwater for dissolved contamination are accomplished in a single borehole. FPR uses In-Situ Groundwater Remediation (ISGR) wells to depress the groundwater surface around each well, resulting in greatly increased flow of the floating product to the well for removal by a skimmer pump.

Dissolved contaminants (e.g., BTEX, MTBE) are removed by carbon adsorption or other suitable adsorptive or reactive medium. The groundwater is pumped more vigorously than in a pump-and-treat or air sparging approach, circulated several times through a treatment zone established around the FPR well, and treated for removal of contaminants during each of several passes through the well.

The entire process is completed below ground, with the only aboveground expression of the system being a manhole cover and a power pole with a power meter and a control panel. FPR systems can be located virtually anywhere a drill rig can drill a hole, even in an active driveway or the middle of a busy street.

While there are numerous possible configurations, each optimized for a different set of geologic conditions, the most basic approach is also the most commonly used. The Figure 1 shows the basic configuration.

• The ISGR well penetrates to the maximum depth of the dissolved contamination, or to a depth chosen to achieve a desired capture width. An inlet screen is set at or near the top of the groundwater.

• The ISGR well incorporates a second screen, an outlet screen, usually at or near the bottom of the well.

• The inlet portion of the well is separated from the outlet portion by a packer.

• The size of the well casing increases just below ground surface to a diameter of a few feet, to accommodate placement of an adsorptive or reactive treatment medium in the well. A central pipe is located in this larger-diameter portion of the well. The central pipe serves to collect the water after treatment and convey it back to the lower, smaller-diameter portion of the well.

• The water is pumped by the submersible pump to the larger-diameter portion of the well where it is pumped through the treatment medium.

• The treated water exits the treatment medium and is conveyed by the central pipe back to the smaller-diameter portion of the well. From this point, the water is conveyed through the smaller-diameter portion of the well to the exit screen. The water then flows, under gravity and/or applied pressure, through the outlet screen and then flows back into the aquifer through the sand pack and the aquifer materials.

• At the exit screen, higher than normal pressures are formed, resulting in higher head values near the well.

• The treated water flows outward from the well and upward under the influence of the vertical gradients created by the extraction process at the top of the well and the increased pressure at the outlet screen. Because aquifer materials are typically anisotropic, allowing horizontal flows more readily than vertical flows, the flows tend to be more outward than upward.

• A recirculation zone is created that typically returns the majority of the treated water to the inlet screen. The shape and size of the treatment zone are largely determined by the treated thickness, the hydraulic gradient, the hydraulic conductivity, the anisotropy of the aquifer, and the pumping rate.

• The water cycles through the treatment zone several times, on average, before escaping downgradient. The treatment process does not have to achieve final cleanup levels in a single pass, since the water will return for additional treatment. However, using activated carbon as an adsorptive medium, treatment is usually 100% in a single pass through the well.

• A skimmer pump is incorporated in the ISGR well, in an adjacent well in the same borehole, or in a nearby borehole.

• The ISGR treatment for dissolved contaminants and the skimmer pump for floating product removal can be operated independently or simultaneously at any time.

• There are no above-ground systems or equipment. Typically, the only above-ground appearance of an FPR system is a manhole with an adjacent power pole that has a utility meter and a control panel. A drum or tank to collect the recovered product can be located any convenient distance from the well.

While the basic configuration and process are straightforward, even for this simplest case there are many considerations in designing and installing Floating Product Removal (FPR) systems. Well diameter, optimal pumping rate, number of wells and well placement, length of the inlet and outlet screens, special development procedures, controls and instrumentation, in-well plumbing configuration, constructability, and many other factors must be addressed in developing a complete design. For more complex or challenging geology, there are additional considerations such as confined aquifer configurations and multiple rows of wells.

Figure 1 - Floating Product Removal - Click to see a larger image

Major Advantages Of Floating Product Removal
With Recirculating Wells

No surface equipment

• Constructed entirely below ground, ISGR systems take up no aboveground space.
  The only above ground equipment is a drum or other container for the free product.

• Silent operation.

Faster

• Faster than a simple skimmer pump approach. The drawdown induced by the recirculating well results in much faster and more complete flow of the floating product to the fpr well, where it pools at an increased thickness for faster and more complete removal by the skimmer pump.

• Faster treatment for dissolved contaminants than pump and treat or air sparging. Much less than the 30 to 50 years commonly estimated for pump-and-treat systems. With floating product, pump-and-treat or air sparging/soil vapor extraction systems can run for decades before cleanup is completed.

• More vigorous than pump-and-treat or air sparging approaches. Pump-and-treat is passive in nature, extracting the water that is easiest to extract and waiting for contaminants to diffuse from lower permeability zones. Air sparging flows air through paths of least resistance, often treating only a portion of the water that flows through the treatment zone. However, re-circulating wells induce vertical gradients to vigorously circulate and treat all of the water in the aquifer multiple times.

Cheaper

• Lower initial capital costs, lower maintenance costs, and faster cleanups result in lower life-cycle costs.

• Fewer wells. Well spacing typically 2 to 5 times depth of contamination. At a site with 50 feet of saturated zone, well spacing can be 200+ feet.

• Lower pressures than air sparging systems, typically just the pressure required to pump the water to near ground surface. Because FPR involves pumping an incompressible fluid (water instead of air), energy costs are lower than for air sparging.

More flexible

• Large well spacings at many sites allow great flexibility in placing wells. Placing wells at a gas station site, for example, can be quite flexible.

• Tolerant of variable geology. Rather than being impeded by thin silt lenses and discontinuous clay layers, re-circulation patterns are enhanced by these typical real-world features.

• Pumping rates (skimmer pump and submersible pump) can be adjusted after installation to match actual aquifer response. Pumping rate and even screen placement can be modified to meet changing conditions during cleanup.

• Does not affect adjacent plumes. Because groundwater is not extracted, adjacent plumes are not drawn toward a re-circulating well. Specific plumes or parts of a plume can be targeted.

• Compatible with soil vapor extraction systems.

Regulatory advantages

• No extraction of groundwater. Does not lower groundwater levels beyond the immediate vicinity of the wells. No re-injection problems. Eliminates the need for water treatment at the surface, with the attendant routine monitoring and reporting.

• No air emissions. Recirculating wells can be operated with zero discharge.