MTBE: A Four Letter Word in Water Pollution
MTBE: Background and History
It is ironic: a decision to improve air pollution has ended up causing water pollution problems. In 1979, the U.S. Environmental Protection Agency (EPA) allowed the use of methyl tertiary butyl ether (MTBE) as an octane enhancer in gasoline, replacing lead as an octane booster and allowing for cleaner burning fuels and lower emissions from vehicles. In the past several years, however, an unplanned side effect of MTBE has occurred through water contamination. Due to its high solubility, low adsorption to soil and poor natural biodegradation, MTBE is a highly mobile and persistent contaminant in groundwater. As a result, over the past two decades of its use, MTBE has found its way into lakes and groundwater supplies in all 50 states.
To respond to MTBE contamination, EPA issued a drinking water advisory level of 20 to 40 parts per billion (ppb) for MTBE several years ago and placed MTBE on its Contaminant Candidate List for further evaluation. Several states, deciding not to wait for a federal regulation, have enacted their own drinking water limits for MTBE. An excellent summary of state regulations can be found on the EPA Web site at www.epa.gov/swerust1/mtbe/mtbemap.htm. . In some cases, health-based standards and taste/odor-based standards have been set.
Meanwhile, the issue of whether to ban MTBE from gasoline in the future is being hotly debated. The state of California has already banned the use of MTBE in gasoline by the end of 2003; several suppliers have announced that they will switch to ethanol as an oxygenate before that date.
While this controversy over the risks of MTBE and its future continues, existing contamination of drinking water supplies needs to be addressed. In an effort to force cleanups of contaminated water supplies, impacted municipalities have filed several lawsuits against the major oil companies in recent years. Some of the higher profiled lawsuits (Lake Tahoe and Santa Monica in California) have recently reached settlements out of court, in which several major oil companies will pay cash settlements and/or costs to operate water treatment facilities. While many regulatory and legal battles revolving around MTBE still lie ahead, attention will now begin to intensify on the treatment of MTBE contaminated waters.
Treatment of MTBE: A Difficult Problem
The same properties that make MTBE a prevalent contaminant in groundwater also make it a difficult contaminant to control compared to other common organic contaminants. Several technologies, including air stripping, activated carbon, advanced oxidation, resins and biological processes, are available to treat MTBE. Treatment may occur at centralized facilities that treat water for either reinjection or immediate potable use or at point of entry (POE) locations at individual residential or business locations. Due to its ease of installation and use, and because it has been historically been the technology of choice for organic contaminant removal, activated carbon is often considered for MTBE applications.
With MTBE's poor adsorption characteristics and the fact that many drinking water applications involve MTBE at very low ppb concentrations, applying activated carbon to MTBE removal applications is a challenge. To properly remove MTBE, an activated carbon requires a relatively high percentage of "high energy" adsorption pores in its pore structure, which have the strongest adsorption forces to remove low concentrations of MTBE from water. Many traditional activated carbon products have difficulty removing MTBE because they do not possess the proper type of pore structure that is required.
Applying Activated Carbon to MTBE Removal: Characterization Methods
Traditional methods of characterizing an activated carbon do not provide any relevant measure of performance as far as MTBE is concerned. In particular, Iodine Number, which has been used as an industry standard for decades to characterize the adsorption properties of activated carbons, has no correlation at all with low concentration MTBE adsorption. Thus, the use of Iodine Number values to predict a carbon's adsorption capacities for trace MTBE levels, which directly relates to how long a bed of carbon can remain on-line before MTBE breakthrough occurs and the carbon must be exchanged, is not recommended.
Fortunately, new characterization methods for carbon that provide for a much better indication of trace level removal of MTBE and other contaminants are in development by the activated carbon industry. The Trace Capacity Number (TCN) measures a carbon's capacity to adsorb acetoxime, a more realistic surrogate for low concentration contaminants than iodine. A comparison of TCN with MTBE capacity yields a much better correlation. New activated carbon products that specify a TCN and thus exhibit more consistent performance for MTBE removal have begun to appear in the marketplace.
Applying Activated Carbon to MTBE Removal: Adsorption Kinetics
Optimizing an activated carbon's capacity is only part of the challenge in applying this technology to MTBE removal. Kinetic factors related to the movement of contaminants from bulk solution to the adsorption sites within the carbon's structure also play a role. Before contaminants can adsorb onto activated carbon, they need to migrate from the bulk solution into the carbon's structure. This rate of diffusion depends on the physical characteristics (solubility, molecular structure) of the contaminant and the properties (particle size, pore structure) of the carbon itself. The portion of the carbon bed where diffusion occurs is called the mass transfer zone (MTZ). A more difficult to remove contaminant, such as MTBE, has a relatively long MTZ compared to other contaminants. An improperly designed carbon system that does not contain this MTZ will exhibit rapid breakthrough of MTBE even if the carbon itself has a high adsorption capacity.
Proper carbon selection can help alleviate this mass transfer issue in MTBE applications. Most domestically made coal-based carbons on the market today are manufactured using a process called reagglomeration, in which fine coal particulates are mixed with an inert binder to form the carbon granule prior to activation. The reagglomeration process opens up transport pore structure in the carbon, providing inherent pathways to facilitate diffusion into the carbon particle. Many imported carbon materials, including coal and coconut-based products, are made using what are known as direct activation processes, in which the reagglomeration steps are skipped, thus reducing the diffusion pathways into the carbon's structure.
Also, the activated carbon's particle size can be selected to shorten the mass transfer zone and better utilize the carbon's inherent adsorption capacity. Traditional groundwater applications utilize 8x30 or 12x40 U.S. mesh carbons (nominal particle size 1.0-1.5 mm); contact times required to contain the MTZ in these sized carbon particles range from seven to 10 minutes typically for MTBE applications. Use of a smaller 20x50 mesh granule (nominal particle size 0.5 mm) can dramatically collapse the MTZ, allow for shorter contact times (four to five minutes) and thus smaller carbon beds to be used, provide for more efficient usage of the carbon and prolong the life of the carbon bed. In sensitive applications where trace level breakthrough cannot be tolerated, use of a 20x50 mesh carbon may be beneficial, provided that the system design can tolerate the higher pressure drop due to the smaller carbon particles. For a trace capacity optimized carbon, the cost is approximately $0.45/1000 gallons to remove 100 ppb MTBE from a 700 gallons per minute (gpm) stream. Standard carbons will vary from $0.50 to 0.80/1000 gallons or more.
Applying Activated Carbon to MTBE Removal: Recommendations
While MTBE removal is a challenging application for activated carbon, following a few simple guidelines can optimize the carbon usage and allow for a more economical operation:
- Utilize an activated carbon product that has been optimized for trace removal. An activated carbon that not only specifies an iodine number (which assures enough total adsorption volume within the carbon's structure), but also specifies a trace capacity number (which assures the proper distribution of high energy adsorption pores within this adsorption volume) will provide the best capacity for MTBE removal.
- Ensure that the product will work consistently from one bed of carbon to the next. Request data on specifications and MTBE performance from several lots of production from the activated carbon manufacturer to ensure consistency of performance.
- Size the carbon bed with the proper amount of contact time to contain the mass transfer zone and give the MTBE time to diffuse into the carbon structure where it can then adsorb.
- Consider the use of smaller mesh size particles (20x50) when possible, to allow for better containment of the MTZ and more efficient utilization of the carbon. In cases where the higher pressure drop due to the smaller particle size is unacceptable, use of a 12x40 mesh size product is preferred.
The MTBE problem is a persistent one and will remain with us for years to come. Activated carbon is an established technology that can perform well in MTBE applications. By selecting and correctly applying an optimized activated carbon, water treatment professionals can achieve consistent MTBE removal from drinking water supplies.
What is Activated carbon?
Activated carbon is a crude form of graphite. Although it is an almost pure form of carbon like graphite, it differs from graphite by having a random imperfect structure that is highly porous over a broad range of pore sizes from visible cracks and crevices to molecular dimensions. This porous, graphitic structure gives activated carbon a very large surface area; activated carbon can have a surface of greater than 1000m²/g. This means five grams of activated carbon can have the surface area of a football field.
This large surface area lends itself extremely well to the process of adsorption, in which liquid or gaseous molecules are concentrated on the carbon's surface. Activated carbon has the strongest physical adsorption forces or the highest volume of adsorbing porosity of any material known to humans.
Activated carbon can be manufactured from many substances containing a high carbon content, such as coal, wood and coconut shells. The raw material has a very large influence on the characteristics and performance of the finished activated carbon. "Activation" refers to the development of the adsorption structure of the finished carbon from its raw material. Raw materials such as coal and charcoal do have some adsorption capacity, but this capacity is greatly enhanced by the activation process.
What about TBA?
Tertiary butyl alcohol (TBA) is a byproduct of some MTBE production processes, and is also a breakdown product of MTBE. TBA can be found in fuel grade MTBE and is nearly always found in contaminated waters where MTBE is present. As the regulatory and legal focus on MTBE has intensified, attention has been drawn to TBA as well. The California Department of Health Services recently lowered its discharge limit for TBA to 12 ppb.
Removal of TBA is even more difficult than MTBE; TBA is infinitely soluble in water, and its use of air stripping and activated carbon treatment methods are even more limited than for treatment of MTBE. Advanced oxidation processes for TBA may generate byproducts, such as acetone, that are not desired in drinking water supplies.
Fortunately, new processes involving biological treatment, in which activated carbon is utilized as a biological support medium, have shown great promise to effectively remove TBA from water sources. In these systems, a bed of activated carbon is "seeded" with bioorganisms that are engineered to break down TBA and MTBE after they are adsorbed onto the activated carbon. The breakdown of the TBA and MTBE essentially regenerates the carbon bed, allowing for much longer bed lives. Test installations of this technology have demonstrated consistent MTBE and TBA removal to below discharge limits, with systems operating over one year without carbon changeout.
This article originally appeared in the October 2002 issue of Environmental Protection, Vol. 13, No. 9, p. 37.
This article originally appeared in the 10/01/2002 issue of Environmental Protection.