DUDEK - UV Treatment for Groundwater Contaminants

Infrastructure Development

UV Treatment Corrects Unexpected Contaminants in Groundwater
Treating and testing groundwater for known chemical contaminants – such as trichloroethane (TCA), trichloroethylene (TCE) and other volatile organic compounds (VOCs) -- had become routine at a project site in Stockton, California. Then one day, an unexpected contaminant showed up.

Testing revealed it was 1,4 dioxane, a substance whose physical and chemical properties make it difficult to treat. For instance, it is miscible – completely soluble in water – which makes it a poor candidate for air-stripping treatment. And with its limited biodegradability, it cannot be eliminated by in situ bioremediation treatment. Finally, it has a low affinity for carbon, which rules out the use of granular-activated carbon treatment.

So what do you do when your existing pump and treatment system consists of air stripping and granular-activated carbon treatment processes?

Above ground treatment has long been the standard for treating groundwater. Over the years, underground treatments such as in situ chemical oxidation, in situ bioremediation and natural attenuation have been touted as the “latest and greatest” technologies. These methods cut costs and require minimal maintenance. But now there are new technologies that are bringing above ground treatment back into the limelight as an equally effective and cost-efficient way to treat groundwater that cannot be treated in situ.

The Innovative Spin: Ultraviolet/Oxidation
Several treatment technologies that could be used to cost-effectively treat 1,4 dioxane were considered. For this project, the most effective treatment technology for 1,4-dioxane contamination was determined to be ultraviolet/oxidation. So how does this treatment work?

“Ultraviolet/oxidation destroys contaminants by breaking chemical bonds between molecules,” explained Glenna McMahon, Dudek’s hydrology/hazardous waste project manager. “This is achieved by adding hydrogen peroxide to the groundwater followed by exposing it to high-intensity ultraviolet light. The water is exposed to ultraviolet light by flowing through a chamber that contains an array of ultraviolet light bulbs.”

Two ultraviolet/oxidation (UV/OX) systems were evaluated for the Stockton site. Each system used slightly different light sources. Disparate wavelength light responds differently to varying water chemistry. The water at this site had slightly elevated nitrate concentrations. Nitrate interferes with the effectiveness of ultraviolet light dosing.

The Dudek team selected the system that was effective for the particular condition and had lower operating cost. “This system was most cost-effective for the particular conditions at this site, resulting in an approximately 50 percent saving in energy consumption over the other system,” McMahon said.

The ultraviolet/oxidation unit was added to the existing treatment train. This change in the treatment system required a modification to the permit issued by California. The approval process included a “prove-out phase.”

During prove-out, the system was operated in various configurations and its performance was evaluated. The prove-out phase was closely monitored by state officials. Dudek’s hydrogeology/hazardous waste team successfully completed the prove-out phase and is now operating the treatment system at full capacity.

What Is UV-Oxidation?
The ultraviolet/oxidation (UV/OX) process unlocks the power of hydrogen peroxide using the energy of UV light. Together, they act to both disinfect and remove organic contaminants from water. The technology can be applied in a wide range of applications, from the treatment of small industrial discharge streams to the treatment of several hundred million gallons per day at a municipal water treatment facility.

UV/OX begins with splitting the hydrogen peroxide molecule, which is made up of two oxygen and two hydrogen atoms. When exposed to UV light, hydrogen peroxide splits into two hydroxyl radicals by a process known as UV-photolysis. UV-photolysis is the photochemical process by which the chemical bonds of a molecule are broken by the energy associated with absorbed UV light.

The resulting hydroxyl radicals are highly reactive molecules that react unselectively with any organic compound in the water through a process known as oxidation. This process breaks down contaminants such as methyl tertiary butyl ether (MTBE), pesticides, 1,4-dioxane, taste and odor-causing compounds, and pharmaceuticals into their harmless component forms.

In addition, while the hydroxyl radical continues its reaction with the chemical contaminant, photolysis of the contaminant will also occur. This parallel process breaks down the contaminant into its harmless components. The eventual products of this reaction are carbon dioxide, water, and other ionic components.

UV/OX is one of the most efficient oxidation processes and the most widely used commercially. It is increasingly recognized as a cost-effective, multi-functional part of a multi-barrier treatment system.

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