A big advance in cleaning up small MTBE levels
It was supposed to be a good thing. In 1979, the U.S. Environmental Protection Agency (EPA) reasoned that replacing lead in gasoline with the fuel oxygenate methyl tertiary butyl ether (MTBE) would result in cleaner vehicle emissions. They were right. Added to gasoline, MTBE succeeded in increasing octane ratings and decreasing air pollution. What they didn't foresee was that in the process, MTBE would contaminate drinking water supplies around the country. Today, in just two decades of use, MTBE has found its way into lakes, underground aquifers and urban wells in 49 states. As public awareness of MTBE spreads, it threatens to become what one popular television news magazine, 60 Minutes calls, "the biggest environmental crisis of the next decade."
Its high solubility, low adsorption to soil and aquifer material and poor natural biodegradation relative to other hydrocarbons, make MTBE highly mobile and persistent. Most of the groundwater contamination can be attributed to leaks from underground storage tanks (USTs) and pipelines. Non-point sources, such as recreational watercraft, are the likely source of surface water effluence.
Recent media attention has caused public awareness of MTBE to skyrocket. Despite the fact that MTBE has never been declared a regulated drinking water contaminant, many states and municipalities are moving to set limits. In 1991, the California Department of Health Services (DHS) established an interim "action level" of 35 parts per billion (ppb) for MTBE in drinking water. As a result, Santa Monica lost 80 percent of its local groundwater supply to MTBE contamination in 1996. The city must now spend $3.3 million annually to buy alternative water supplies. In early 2000, a California DHS health-based drinking water standard of 13 ppb is expected.
The federal government is also focusing attention on MTBE. In the process of conducting research on the health effects of MTBE exposure, EPA has tentatively classified it as a possible human carcinogen, primarily based on inhalation studies. An EPA drinking water advisory has been issued to water suppliers to keep MTBE levels in the range of 20 to 40 ppb or lower to prevent taste and odor problems and to protect against potential health effects. EPA has also included MTBE on a list of contaminants (the Contaminant Candidate List) that may require regulation based on their known or anticipated occurrence in public drinking water systems.
Even at extremely low concentrations (2.5 ppb), MTBE can be detected by its turpentine-like odor. This led the California DHS to finalize a taste and odor-based standard for MTBE of 5 ppb. This problem is exacerbated when MTBE-contaminated water is used for cooking or bathing.
In March, EPA announced it would ask Congress to remove the section of the 1990 Clean Air Act requiring gasoline in areas with serious air pollution to contain at least 2 percent oxygen by weight and replace it with a requirement that all gasoline contain components made from renewable resources, thereby phasing out MTBE use.
In seeking to remove MTBE from groundwater, researchers have explored the effectiveness of traditional technologies including air stripping, advanced oxidation, biological treatment and activated carbon adsorption. While all are capable of removing medium or high levels of MTBE to some degree, their results in treating the trace levels present in groundwater to the discharge goals has proven to be questionable, unpredictable and expensive.
Traditional technologies are capable of removing medium or high levels of MTBE to some degree, but their results in treating the trace levels present in groundwater to the discharge gaols have proven to be questionable, unpredictable and expensive.
To air strip MTBE from groundwater, the required air-to-water ratio is typically two to four times higher than for average compounds. As air stripping is not a destruction technology, further treatment of the off-gas may also be required. This involves the use of vapor phase activated carbon or high-temperature thermal oxidation, which adds significant cost and complexity to the overall treatment.
Advanced oxidation technology (AOT) is considered the treatment of choice for higher MTBE concentrations and large systems. It has been successfully applied for source control and remediation, wastewater and tank bottom water treatment. AOT systems rely on the hydroxyl radical (OH) to chemically oxidize MTBE to less toxic secondary products. The OH is created by using high-energy ultraviolet (UV) light to photodissociate hydrogen peroxide (H2O2)into two OHs in a UV reactor. Once formed, the OHs act as extremely powerful oxidants to destroy organics such as MTBE. This is the same natural degradation process caused by sunlight in nature. The AOT process is labeled advanced because it speeds up the natural process by several orders of magnitude, but it is not an affordable option for treating trace MTBE levels. Its cost-effectiveness remains limited to remediating wastewater and source waters.
Bioremediation remains a marginal technology for MTBE due to the contaminant's low biodegradability and the problems associated with achieving very low discharge limits on a consistent basis. Although biodegradation has not yet been widely tested for treatment of MTBE, it is known that ethers, in general, biodegrade poorly.
Traditional granular activated carbons (GAC) have difficulty adsorbing MTBE. The adsorbability of a contaminant is dictated by its chemical characteristics and concentrations and by the carbon's pore structure (the volume between the graphitic plates that make up the activated carbon's skeletal structure). MTBE does not adsorb to regular GAC as well as other hydrocarbons because of its higher solubility and typically low concentrations.
The magnitude of the adsorption forces is directly related to the amount and orientation of the graphitic plates surrounding the pore. Adsorption pores surrounded by a larger number of graphitic plates have high adsorption forces and are termed "high-energy." Adsorption pores that are surrounded by fewer graphitic plates have low adsorption forces and are termed "low-energy." The distribution of high-energy pores versus low-energy pores relates to the quality of the raw material (coal) and the activation process.
Most traditional activated carbons, made from lignite coal, coconut shell or bituminous coal, don't consistently exhibit enough high-energy pores to effectively adsorb MTBE.
Lignite coal-based carbons typically offer a very low percentage of high-energy pores and are not recommended for most trace-removal applications, including MTBE.
Coconut-based GACs often have good pore distribution for trace removal and had been thought to be the best solution for MTBE. Unfortunately, coconut-based carbons are notorious for the wide variation in raw materials and the activation process under which they are produced. Coconut carbons typically vary significantly from one sample to the next, and their performance is extremely unpredictable.
Bituminous coal-based carbons are known to exhibit more consistency than coconut carbons, but most standard bituminous carbons have moderate to low high-energy pore distribution.
A new kind of activated carbon
Industry researchers realized that cost-effective MTBE-removal might be achieved by the development of a specialty GAC that could consistently exhibit the required amount of high-energy pores. That realization, combined with the collaboration of public health officials and oil companies, led to one of the newest solutions for treating MTBE trace-removal carbon. Made from a select, high-grade bituminous coal, trace-removal carbon is optimized through its activation process to produce more of the high-energy pores essential for MTBE removal. It offers a longer bed life between carbon exchanges and reduced down time and operating costs.
The amount of high-energy pores in trace-removal carbon can be measured by a new method that determines a GAC's trace capacity number. This measurement directly relates to the carbon's ability to adsorb organic contaminants at low concentrations.
Traditional characterization methods, such as the iodine number, are not effective at gauging or controlling a carbon's performance at adsorbing trace levels. The trace capacity number measures a carbon's capacity to adsorb acetoxime, a more realistic surrogate for low-level contaminant concentrations than iodine. Trace-removal carbon products consistently exhibit high trace capacity numbers, meaning they are ideally suited to treating low-levels of contaminants.
The Clean Air Act of 1990 brought MTBE into our gas tanks. The challenge today is to get it out of our water.
For high concentrations of MTBE in source control, wastewater and tank bottom water treatment, advanced oxidation technology systems are often preferred for their efficiency and predictable results. For the low-level MTBE contamination that has been detected in drinking water supplies from New York to California, trace-removal activated carbon offers a reliable, cost-effective treatment option.
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This article appeared in Environmental Protection magazine, May 2000, Vol. 11, No. 5, p. 20.
This article originally appeared in the 05/01/2000 issue of Environmental Protection.