Effects of Gasoline on Aerobic Methyl tert-Butyl Ether Biodegradation in Fluidized-Bed Bioreactors Nicholas C. Kordesch Abstract Since the1990 Clean Air Act, methyl tert-butyl ether (MTBE) has been used commonly as a gasoline oxygenate to increase fuel efficiency and lower vehicle emissions. It has become one of the most common pollutants of ground water and surface water. Because of its undesirable effects on drinking water and ecologically harmful effects, MTBE removal has become a public health and environmental concern. Pure MTBE has been found to be biodegradable in ex-situ treatments, but in pollution sites, it is found with gasoline. This study examines the effects of gasoline’s presence on MTBE degradation. In this study, two up-flow fluidized-bed reactors were fed pure MTBE as a sole carbon source for 124 days, and then gasoline was added for 13 days. MTBE concentration was measured for influent and effluent samples of each reactor by gas chromatography. Degradation was determined by MTBE removal. MTBE was degraded to non-detectable levels without gasoline present. After gasoline was added, MTBE degradation was significantly impeded. Nine days after gasoline was no longer added, the reactors began degrading MTBE at pre-gasoline levels. Results suggest that treatment of MTBE contamination is possible under controlled environmental conditions.Introduction Methyl tert-butyl ether (MTBE, CAS no. 1634-04-4) was introduced in the 1970s to replace lead and other toxic fuel components that were used to increase engine efficiency. More recently, the 1990 Clean Air Act Amendments required the addition of fuel oxygenates in order to lower carbon monoxide emissions. Compared to the most common fuel oxygenates, which include ethyl tert-butyl ether (ETBE), tert-amyl methyl ether (TAME), benzene, toluene, ethyl-benzene, and xylene (BTEX), MTBE is the most cost-effective. As a result, MTBE is currently added to over 30% of gasoline sold in the U.S. (Steffan 1997). It makes up 11% of gasoline by volume. Oxygenates are potentially beneficial to the environment. Gasoline with MTBE has significantly helped reduce vehicle emissions and increase engine combustion efficiency (Deeb 2000). Although gasoline oxygenates are used to lessen environmental damage and improve public health, they pose indirect human and environmental health risks. MTBE has become the second most common water pollutant in the US (Stringfellow 2001). Widespread gasoline spills and storage tank leaks have contaminated groundwater systems. These water sources supply 60% of American drinking water (Bradley 1999). The undesirable taste and odor of MTBE contaminated water is detectable by consumers at low concentrations. The US Environmental Protection Agency (EPA) has established a drinking water advisory level of 20-40µg/L (Fayolle 2001). Fuel oxygenates pose a particularly difficult environmental problem because of their high water solubility, allowing them to move quickly and easily through the water column and makes them difficult to remove. Once these contaminants move below the surface, they can have even longer life spans due to anaerobic conditions (Zogorsky 1999). Gasoline oxygenate contamination is a cause for concern due to various human health risks. The long-term and short-term effects of human exposure to MTBE are still uncertain. Introduction of MTBE in various regions of the U.S. has coincided with health complaints including symptoms included headache, nasal, throat, or ocular irritation, nausea or vomiting, dizziness, and sensations of “spaciness” or disorientation (University of California Toxic Substances Research and Teaching Program 1998). However, a study by Prah et al. (1994) that exposed humans to MTBE at 1.39 ppm (typical exposure during vehicle refueling) for one hour showed that MTBE inhalation and dermal absorption had no immediate adverse effects on humans. Evidence remains anecdotal. MTBE is recognized as an animal carcinogen. The body metabolizes MTBE to another carcinogen, tert-butyl alcohol (TBA, CAS no. 75-65-0), whichremains in the body longer than MTBE (National Toxicology Program 1995). Studies of MTBE exposure in rats showed that incidences of tumors, lymphomas, and leukemia increased (International Agency for Research on Cancer 2000). One experiment tested the inhalation of MTBE by mice and rats. It increased the incidence of hepatocellular adenomas in female mice and that of renal tubular tumors in male rats at 8000 ppm (University of California Toxic Substances Research and Teaching Program 1998). Such studies have led the U.S. EPA to classify MTBE as a possible human carcinogen. The Material Safety Data Sheet (MDL 2000) lists MTBE as a moderate central nervous system toxin by ingestion and inhalation. Ecologically, MTBE is toxic to aquatic organisms only at high concentrations. 50% of fish exposed to MTBE at 1,000,000 µg/L for 96 hours died. At the same concentration, exposure was fatal to 50% of aquatic invertebrates. Although MTBE was once considered recalcitrant, recent studies have shown that it is biodegradable by an assortment of aerobic microbial cultures (Deeb 2000, Salintro 1994, Wilson 2001). Both in-situ and ex-situ treatments have been explored. In-situ treatment consists of injection of an engineered MTBE-degrading strain into the contaminated site, then augmenting conditions to favor the desired strain (Salintro 2000). Subsurface in-situ treatments have been ineffective because degrading cultures are sensitive to a variety of unpredictable environmental conditions including nutrient and substrate availability and a lack of oxygen (Finneran 2001). Studies have shown that ex-situ treatment in fluidized-bed reactors (FBR) is effective in biological degradation of MTBE. In a laboratory-scale system, biofilm-bound microorganisms were been observed to convert 97% of influent MTBE to CO2 (Fortin 1999). Kharoune et al. (1998) observed 99% removal efficiency in a laboratory scale fluidized-bed reactor. The FBR system uses a granular activated carbon (GAC) substrate for culture growth to simulate the attached microbial growth that is predominant in soil and groundwater (Kharoune 2001). This study uses fluidized-bed reactors because they accurately replicate field-scale reactors while providing controlled environments for monitoring degradation. While numerous past studies have observed degradation of pure MTBE in controlled environments, recent studies have
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