HiPOx technology effectively removes 1,4 dioxane to below three parts per billion from groundwater and drinking water supplies.
Advances in water analysis and regulatory changes have brought new toxic compounds to the attention of water authorities and industry experts.
One of many newly emerging and most recalcitrant compounds plaguing groundwater and drinking water supplies, 1,4-dioxane is an industrial solvent and a stabiliser frequently added to chlorinated solvents such as 1,1,1-trichloroethane (1,1,1,-TCA) and trichloroethylene (TCE) to neutralise hydrochloric acid, thus lengthening the solvent's useful life. A toxic substance, 1,4-dioxane is considered a potential threat to human health, and the US Environmental Protection Agency (EPA) has classified it as a probable human carcinogen.
Its high solubility and low affinity for adsorption by soil enables it to travel quickly through groundwater aquifers. The compound is often present at sites contaminated by other chlorinated solvents and is frequently found to extend beyond previously determined VOC plume boundaries. 1,4-Dioxane does not degrade biologically, and remediation methods, such as traditional air stripping, carbon adsorption treatment methods or biological reduction cannot remove enough of it to satisfy environmental regulations.
Not much was known about 1,4-dioxane a few years ago, primarily because the standard EPA Method 8260 analysis had difficulty in detecting levels below 100 parts per billion (ppb). In addition, the EPA list of target compounds did not include 1,4-dioxane. The removal of the compound was not considered economically feasible using standard methods; it was not detected at many contaminated sites for this reason.
More recently, the EPA has designated method 8270M for 1,4-dioxane analysis, and commercially available analytical methods have improved significantly, allowing 1,4-dioxane to be detected at levels of 1 or 2 ppb. In addition, regulatory agencies have tightened water quality requirements. The EPA issued a health-based advisory level of 3 ppb for drinking water but has yet to establish the Maximum Contaminant Level (MCL).
Recently, the US states of California, Florida, Maine, Massachusetts, Michigan and North Carolina adopted action levels ranging from 1 ppb to 50 ppb, establishing a range for water quality standards nationwide. In July, California's San Francisco Bay Area Regional Water Quality Control Board (RWQCB) amended the general permit for solvent site discharges (National Pollutant Discharge Elimination System, NPDES) to require testing for the presence of 1,4-dioxane and assessment of best available technologies for treatment if detection above 3 ppb recurs. Consequently, 1,4-dioxane is being detected at many chlorinated solvent sites.
Since air stripping and carbon adsorption systems installed on existing chlorinated solvent spill sites cannot remove 1,4-dioxane (nor can biological reduction), the industry has been searching for the most economical solutions to remove it from the water. HiPOx technology, an advanced oxidation process (AOP) that uses ozone and hydrogen peroxide, consistently demonstrates effectiveness in removing 1,4-dioxane levels to meet water quality requirements. Developed by Applied Process Technology, Inc. of Pleasant Hill, California, USA. HiPOx technology reduced 1,4-dioxane influent concentrations of seven to 43,000 ppb to effluent concentrations below the three ppb California action level at several treatment sites. In many cases, this technology has been integrated with existing air stripping and carbon remediation systems and serves as a pre- or post-treatment to remove 1,4-dioxane. In this type of configuration, the AOP removes 1,4-dioxane to below three ppb and also reduces or eliminates many of the other chlorinated solvents present, especially alkenes such as TCE, tetrachloroethylene (PCE), and dichloroethylene (DCE) and vinyl chloride. With the majority of the VOCs removed from the water, the air stripper or carbon system is then able to address any remaining contaminants that cannot be destroyed by oxidation. Where Granulated Activated Carbon (GAC) is used, the frequency of carbon change outs is reduced thus greatly extending the carbon life.
HiPOx uses ozone and hydrogen peroxide to form hydroxyl radicals, which react rapidly to oxidise VOCs into non-hazardous compounds, including carbon dioxide and water. This oxidation process occurs in the aqueous phase and does not increase the temperature or pressure of the water because it usually occurs at very low concentrations (<1%). The system uses high-precision distribution of ozone and hydrogen peroxide, high ozone concentrations (8-10% wt.), high operating pressure (35-45 psig versus ambient pressure) and efficient, in-line mixing to maximise mass transfer and reaction efficiency. Hydrogen peroxide and ozone are injected at 20-45 psig in a series of injection modules. After reagents are injected, the dosed fluid flows immediately through the module's mixing section followed by a reaction zone specifically designed to allow sufficient residence time for contaminant destruction. Low, local concentrations of ozone and hydrogen peroxide are a critical design feature of this distributed-injection approach.
The HiPOx AOP process overcomes several challenges faced by other treatment systems. The waste-free process destroys contaminant on-site. It can be installed at locations where community acceptance is important because it has a low profile, operates quietly, and does not emit toxins into the air. It is based on an exact science that allows predictable results when scaling up from low-flow on-site tests or laboratory experiments to full-scale commercial installations. Most important, it economically removes 1,4-dioxane, while displacing part or all of the existing site treatment costs.
The San Gabriel Basin Water Quality Authority has been using a HiPOx system to treat 1,4-dioxane contaminated groundwater in South El Monte, California, since 2001. The authority had used a granular activated carbon system to remove chlorinated solvents; however 1,4-dioxane was detected at levels of 20 ppb in the water in 1999 and could not be removed by the carbon system. A 10 gallons-per-minute (gpm), mobile HiPOx test system operated on site for one month. Data from site tests indicated that the AOP system would cost-effectively reduce 1,4-dioxane concentrations at the site, so a full-scale unit was constructed and installed as a continuous pre-treatment step to the carbon system. The addition of the AOP not only took care of the 1,4-dioxane problem, but also extended the life of the carbon system from quarterly replacement to annual replacement. The HiPOx system operated at a flow rate of 500 gpm during the first five months of operation until a second well brought flow rates at the site up to 1,000 gpm. The HiPOx system has been operating at 1,000 gpm ever since.
The City of Industry in California reports a similar success at its 70-gpm commercial installation where an air stripper removed chlorinated solvents. The air stripper could not remove the newly discovered 1,4-dioxane, so a HiPOx system was installed for pretreatment to the air stripper. Consequently, the HiPOx consistently reduces approximately 600 ppb 1,4-dioxane to effluent levels of less than three ppb. This installation operated continuously from February 2001 until May 2002. Since then, the installation operates only when the water table at that site increases to sufficient levels, usually every other month for a month at a time.
In 2003, a client of the environmental consulting firm Locus Technologies selected HiPOx to treat an EPA Superfund Site in Mountain View, California, where air stripping and carbon equipment had been installed several years earlier to remove chlorinated solvents. The RWQCB required the responsible party to analyse for 1,4-dioxane, resulting in detected levels of 15 ppb. Locus screened several potential technologies to treat the 1,4-dioxane and presented its findings in a Basis of Design Report that determined the construction cost and 15-year total present value for each system being considered. The client selected a HiPOx AOP system because of its strong pilot test results (reduction of 1,4-dioxane to below two ppb) and guaranteed capital costs and operations and maintenance costs. The HiPOx system has been performing exactly as specified since it was commissioned in December 2003.
Analytical and regulatory changes have lead to a recent surge in 1,4-dioxane treatment activity. Most of the sites have lower flows (five to 50 gpm) and are addressed with portable systems similar to the system used at the Locus-managed EPA Superfund Site. This increase in regulation and treatment represents another burden to responsible parties, but HiPOx technology provides a viable and economic solution to groundwater contamination in the form of ozone-peroxide advanced oxidation.
Author's Note
Chuck Borg was the vice president of business development of Applied Process Technology, Inc. (APT) in Pleasant Hill, California, USA. APT can be reached at (916) 779-7651, or by e-mail at [email protected].
References:
California Regional Water Quality Control Board, San Francisco Bay Region. Tentative Order NPDES No. CAG912003. General Waste Discharge Requirements for Discharge or Reuse of Extracted and Treated Groundwater Resulting From the Cleanup of Groundwater Polluted by Volatile Organic Compounds.
Draper, W.M., J.S. Dhoot, J.W. Remoy and S.K. Perera. 2000. Analyst. 125:1403.
Francis, A.J., C.R. Iden, B.J. Nine and C.K. Chang. 1981. Nucl. Technol. 50:158.
IRIS (Integrated Risk Information System), U.S. Environmental Protection Agency. Summary: 1,4-Dioxane. Last updated: July 8, 2004. http://www.epa.gov/iris/subst/0326.htm
Jackson, R.E. and V. Dwarakanath. 1999. Groundwater Monitoring & Remediation. 19(4):102.
