Dissolved Oxygen Enhancement*

Full Oxygen Saturation of Aquifers With No Chemical Cost

Dissolved oxygen enhancement (DOE) is a simple, patent-pending technology that saturates the groundwater with oxygen, using air as the oxygen source. It uses recirculating wells to treat large capture widths.

Unlike Oxygen Release Compound® and other chemical-based oxygenation approaches, DOE uses air as the oxygen source, making the oxygen essentially free.

Also, unlike chemical-based approaches, which rely on the passive mechanisms of diffusion and dispersion to distribute the oxygen in the aquifer (typically unsuccessfully), DOE thoroughly mixes the oxygen into all of the water within a large and wide treatment cell by pumping all of the water through recirculating wells several times, oxygenating the water during each pass through the wells.

Important features of dissolved oxygen enhancement:

• No blower system or other aboveground equipment. DOE uses no blower, compressor, or air pump of any kind. Unless the very low-flow of off-gas (typically on the order of 0.1 cfm) requires treatment, DOE involves no aboveground equipment other than a control panel.

• Very low power consumption. Air sparging wells, which are often as useful for the aeration of the aquifer as for stripping contaminants, typically operate with positive displacement (PD) blowers. In a typical PD blower system operating at 15 psig, more than 75% of the energy used is wasted, largely in generation of useless heat. In DOE, compression of the air is isothermal, eliminating the generation of waste heat.

Typically DOE systems run on a fraction of a horsepower per well.

• Free oxygen. Oxygen from ORC® or similar compounds can cost over one hundred dollars per pound. Using ORC® can cost tens of thousands to hundreds of thousands of dollars, essentially to purchase oxygen, which instead can be obtained free from the air.
  
  Using DOE, the cost of the oxygen delivered to the aquifer and dispersed thoroughly throughout the aquifer is simply the cost of running a small submersible pump

The groundwater is circulated several times through a DOE well. A large treatment zone is established around the well, and is aerated to full oxygen saturation, and even greater than full saturation, 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 small manhole cover and a power pole with a power meter and a small control panel. DOE systems can be located virtually anywhere a drill rig can drill a well, 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. Figure 1 shows the basic configuration.

• The DOE 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 DOE 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 water is pumped by a submersible pump (or other means) to a point above the static groundwater level, where its direction reverses and it begins to travel back down the well toward the outlet screen.

• As the water flows downward, a partial vacuum is formed in the down pipe.

• At a point along the downward path, a metered amount of air is admitted to the down pipe, where it mixes vigorously and thoroughly with the water.

• [Other gases than air can be used. Pure oxygen, for example can be used, for those situations that would benefit from the small ($0.25 per pound) added cost. Hydrogen can be added to the water very efficiently and effectively, to aid in anaerobic biodegradation of some contaminants. Other gases can be added as well, either alone or in combinations.]

• The water and air (bubble) mixture travel downward to the outlet portion of the well. As the water and air (bubble) mixture descend in the down pipe, the pressure increases to above atmospheric pressure, which increases the saturation concentration of oxygen in water, resulting in the water being oversaturated with oxygen (and other air components) when it reaches the outlet screen.

• The water and air (bubble) mixture separates below the packer. Any excess air returns to the surface through a third pipe in the well. Excess air returns to the surface bearing contaminants stripped from the water. At the surface, the air is either released to the atmosphere or treated for removal of the contaminants.

• At the exit screen, higher than normal pressures are formed from the release of the water back into the aquifer, resulting in higher head values near the well.

• The treated water, containing the dissolved oxygen (and other components of air), 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. Because aquifer materials are typically anisotropic, allowing horizontal flows more readily than vertical flows, the flows are almost always more outward than upward.

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

• The water cycles through the treatment zone and the well several times, on average, before escaping downgradient. On each pass through the well, the water is saturated or super-saturated with oxygen. (Less than saturation levels can also be used, where that would be beneficial.)

• Unless the off-gas requires treatment, there are no aboveground systems or equipment. Typically, the only aboveground expression of a DOE system is a manhole with an adjacent power pole that has a utility meter and a small control panel.
  
  If the off-gas requires treatment, a small vapor-phase carbon treatment system is used. .

While the basic configuration and process are straightforward, even for this simplest case there are many considerations in designing and installing dissolved oxygen enhancement (DOE) 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, other pumping methods, 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.

The major advantages of dissolved oxygen enhancement technology are discussed below. An example of the cost advantages over ORC® is shown in the following table.

Figure 1 - Dissolved Oxygen Enhancement - Click to see a larger image (Patent pending)

Major Advantages Of Dissolved Oxygen Enhancement
With Re-Circulating Wells

No Surface Equipment

• Constructed entirely belowground, DOE systems take up no aboveground space. The only aboveground equipment is a small control panel that operates the submersible pump.

• Silent operation.

Faster

• Faster than air sparging or ORC®. Pumping the water in an established treatment cell around the well and oxygenating it on several passes through the well is much more thorough than the largely unknowable treatment process of an air sparging approach or the slow and uncertain dispersion and diffusion of ORC®.

• More vigorous than traditional air sparging or pump-and-treat approaches. 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. While the interaction between the air and water in a traditional air sparging system is not well understood, or subject to modeling or calculation, the exact opposite is true for recirculating wells. The pumping and aeration processes are thorough and rapid, affecting all of the water in the treatment cell.

• Active rather than passive treatment. ORC® is a passive approach that relies on natural aquifer flow to bring water to a well for treatment and relies on the passive mechanisms of dispersion and diffusion to distribute the evolved oxygen in the water. DOE actively pumps the water and thoroughly mixes the oxygen into the water.

Cheaper

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

• DOE wells are typically three-inch PVC construction, not much more expensive than traditional air sparging wells or wells that might be used to introduce ORC®. But, the equipment in the well costs only a small fraction of the cost of a blower system in an enclosure required by an air sparging approach.

• Fewer wells than air sparging. 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 energy costs. Because DOE involves pumping an incompressible fluid (water instead of air), and because the air that is compressed is compressed isothermally, energy costs are much lower than for traditional in-well sparging.

• The large savings of DOE over ORC® or other chemical-based approaches to oxygenating the groundwater are in the avoided costs of purchasing the chemicals that supply the oxygen. Rather than tens to hundreds of thousands of dollars to purchase what is instead available for free, oxygen, DOE uses a small amount of electricity and uses the free oxygen available from the air.

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 as traditional in-well stripping systems can be, re-circulation patterns are enhanced by these typical real-world features.

• The pumping rate can be adjusted after installation to match actual aquifer response. Pumping rates also can be changed to meet changing conditions during cleanup.

• Can be installed in most existing wells, including Schedule 40 and Schedule 80 4-inch wells (up to 40 gpm) and Schedule 40 2-inch wells (up to 9 gpm).

• 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.

• Does not add anything to the groundwater that is not already a natural component of the aquifer.

* Dissolved Oxygen Enhancement is marketed under the trade name FreeOx™