The Tucson International Airport Area (TIAA) Superfund Site is located in the Tucson Basin in Pima County, Arizona and includes the Tucson International Airport as well as residential areas of the city of Tucson. As early as 1942, metals, chemicals and other wastes were disposed of in the region, leading to aquifer contamination.
Volatile organic compounds (VOCs) including trichloroethylene (TCE) and 1,1-dichloroethene (1,1-DCE) have been detected at this site, along with 1,4-dioxane, a chemical stabilizer commonly used in solvents.
In 1981, the United States Environmental Protection Agency (USEPA) and the City of Tucson sampled the municipal water wells within the TIAA zone. Unsafe levels of TCE were identified, and a total of eleven drinking water wells were shut down. As a result of the contamination, the Tucson Airport Remediation Project (TARP) was established and a groundwater treatment system was commissioned in 1994. However, continued monitoring of the groundwater detected 1,4-dioxane, a contaminant not easily removed through the air-stripping treatment system used at the original treatment plant. As a result, UV-oxidation was incorporated in the Advanced Oxidation Process Water Treatment Facility immediately adjacent to the TARP to remove 1,4-dioxane.
1,4-Dioxane is a chemical stabilizer commonly added to chlorinated solvents including TCE and tetrachloroethylene (PCE) to prevent their acidification and breakdown.
Due to its high solubility and limited sorption to soils, natural degradation of 1,4-dioxane in water is limited and as a result, 1,4-dioxane will travel farther and remain in areas of groundwater contamination for longer periods of time than TCE and PCE, the solvents it originally was meant to stabilize.
Further, its low Henry’s Law constant makes 1,4-dioxane resistant to air stripping, a treatment method commonly used for the removal of other VOCs.
1,4-dioxane was included in the USEPA’s third Unregulated Contaminant Monitoring Rule (UCMR3) and levels were monitored throughout the United Stated to evaluate whether federal regulations will be required to control concentrations in drinking water.
Results released in 2017 suggested that over 20% of treatment plants tested had at least one sample measure above the minimum reporting limit of 0.07 ppb (μg/ L) and over 7% of sites had at least one sample measure above 0.35 ppb, the USEPA’s established 1 in 1,000,000 cancer risk concentration of 1,4-dioxane in water.
Treating 1,4-Dioxane at TARP
Pilot studies were carried out in 2010 to evaluate UV-oxidation as a possible remedy for 1,4-dioxane contamination. Specifically, evaluation was based on three independent criteria: treatment capabilities, by-product formation and residual hydrogen peroxide quenching.
1,4-Dioxane removal of greater than the targeted 2-log reduction was observed during pilot testing with the highest level of 1,4-dioxane reduction calculated to be approximately 2.8-log. In addition, VOCs including TCE and 1,1-DCE were simultaneously reduced to equal or greater levels than 1,4-dioxane (Figure 1).
A significant by-product of concern was bromate. However, effluent testing after UV-oxidation treatment showed no bromate formation. Equivalent pilot testing experiments carried out with ozone-hydrogen peroxide (O3-H2O2) systems showed increases in bromate to over 50 μg/ L, 5 times the regulated limit (Figure 2).
Quenching residual hydrogen peroxide (H2O2) with granular activated carbon (GAC) was favored over other established methods of quenching due to the high efficiency of GAC and its ability to remove H2O2 with both high loading rates and minimal empty bed contact time. This limited concerns associated with head loss and additional pumping requirements.
The TrojanUV Solution
TrojanUV supplied 6 TrojanUVPhox™ D72AL75 UV units organized in three treatment trains with each train containing two UV units. Each unit contains 144 low- pressure high-output (LPHO) UV lamps designed for high efficiency UV output with minimum energy requirements.
Along with an advanced H2O2 dosing system, the UV-oxidation installation is capable of removing 1.6-log (> 97%) 1,4-dioxane and treating a maximum capacity of 5,800 gallons per minute (gpm). GAC is applied after UV-oxidation treatment to remove residual H2O2 exiting the UV reactors.
System Design Parameters
|Peak Flow Capacity||5,800 gpm (1317 m3/hr)|
|Secondary Contaminants||TCE, 1, 1-DCE|
|Design Treatment Requirement||1.6-Log 1,4-Dioxane Removal|
Commissioned in January 2014, the UV-oxidation installation at the Advanced Oxidation Process Water Treatment Facility treats 1,4-dioxane and produces water that is blended and then treated at the neighboring TARP facility. This purified water is supplied to nearly 50,000 end users in the Tucson area.