Chemistry: molecular biology and microbiology – Process of utilizing an enzyme or micro-organism to destroy...
Reexamination Certificate
1999-08-10
2001-10-16
Redding, David A. (Department: 1744)
Chemistry: molecular biology and microbiology
Process of utilizing an enzyme or micro-organism to destroy...
C435S262500, C435S863000
Reexamination Certificate
active
06303366
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method for converting undesirable environmental contaminants into environmentally acceptable materials. More particularly, the present invention relates to a biological method for converting organic compounds which are water and soil contaminants into innocuous compounds.
The field of the present invention will be described initially in connection with the contaminant methyl-tert-butyl ether (hereafter also referred to as “MTBE”). It should be understood that the present invention has applicability to the treatment of other “ether” contaminants, as will be described below.
MTBE has been used in “premium” gasoline since 1979 as a high octane additive which functions as an oxygenate. Its use has replaced lead and other additives such as benzene, toluene, ethylbenzene and xylenes, which are often referred to as “BTEX” and which are considered highly contaminating materials. More recently, for areas of the country with relatively high air pollution, the 1990 Clean Air Act requires that oxygenates be used in all grades of gasoline to reduce vehicle emissions which constitute air toxics, for example, carbon monoxide and volatile organic compounds (VOCs). Oxygenates cause fuel to burn more cleanly, reducing the amounts of ozone, carbon monoxide, toxics and other pollutants present in vehicle emissions. The current goal of gasoline reformulation is to reduce gasoline's benzene content by 33% and other contaminating organics by at least 15%.
MTBE is the most widely used oxygenate in the United States. In 1992, more than 1.8 billion gallons of MTBE was used in gasoline. Its use has continued to increase each year since 1992 (Anderson, “Health Studies Indicate MTBE is a Safe Gasoline Additive,”
Chemical and Engineering News,
Sep. 20, 1993). MTBE producers have invested billions of dollars into plants already in operation or planned. More than 29 companies now produce MTBE in the U.S. And in 1993, production of MTBE exceeded 24 billion gallons, making it second on the list of organic chemicals produced in the U.S. (M. S. Reisch,
Chemical & Engineering News,
Apr. 11, 1994; p. 12-15).
The toxicity of MTBE is still in question. A recent Italian study suggested that MTBE poses a significant cancer risk (Trenton Times, Nov. 13, 1994). Other studies have suggested that MTBE is not very toxic to humans (Anderson, “Health Studies Indicate MTBE is Safe Gasoline Additive,”
Chemical and Engineering News,
9-18, Sep. 20, 1993).
Without regard to whether MTBE is or is not toxic, it is a fact that as an ether, it has relatively low odor and taste thresholds compared to other organic compounds. MTBE's odor threshold in water is about 45 to about 95 ppb. Its taste threshold in water is about 134 ppb (American Petroleum Institute 1993). This means that MTBE can be detected in drinking water through odor and taste at relatively low concentrations. The Maximum Drinking Water Levels for MTBE are between 540 and 700 ppb (Gilbert and Calabrese, “Developing a Standard for MTBE in Drinking Water,”
Regulating Drinking Water Quality,
231-252). Based on rat model studies, the no-observable-adverse-effect-level (NOAEL) is 100 mg/kg/day (Robinson, M., R. H. Bruner, and G. R. Olson, “Fourteen and ninety day oral toxicity studies of methyl tertiary butyl ether in Sprague-Dawley rats,”
J. Am. Coll. Toxicol.,
9:525-540 (1990)).
The greatest human exposure routes of MTBE are through drinking contaminated water, use of the water in cooking, and inhalation during bathing.
The chances of such exposure are not insignificant since vast amounts of MTBE-containing gasoline are stored in underground storage tanks, including tanks which leak. Seepage of MTBE from leaky tanks into groundwater and spillage of MTBE during tank filling operations and transfer operations at distribution terminals have led to considerable contamination of groundwater near these tanks. Because MTBE is highly soluble in water (43,000 ppm), it is now often found as plumes in groundwater near service stations, related storage facilities and filling terminals throughout the United States (American Petroleum Institute, Chemical Fate and Impact of Oxygenates in Groundwater: “Solubility of BTEX from Gasoline-Oxygenate Mixtures,” Pub. No. 4531, 1991). A market survey by The Jennings Group (1993) estimated that there are greater than 234,000 federally regulated contaminated underground storage tank (UST) sites in the United States and greater than 42,000 hazardous sites.
The recalcitrance of MTBE relative to other gasoline components makes it particularly resistant to inexpensive biological treatment approaches such as bioventing or biosparging. Conversion or “remediation” of the contaminated media to innocuous, environmentally-acceptable compounds, therefore, has been particularly difficult. Furthermore, MTBE can be difficult to air strip from ground water and trap on activated carbon, thereby limiting air sparging/soil vapor extraction (AS/SVE) approaches to remediation. In a recent study of 15 sites, stripping efficiencies of as low as 56% were observed (American Petroleum Institute, supra). And yet this method has been deemed to be the most effective method for remediating contaminated groundwater.
There are other ether-based compounds that are also widely used and that are considered contaminants. Examples of such ether-based compounds include cycloaliphatic compounds, for example, tetrahydrofuran, a widely used solvent. Examples of other aliphatic ethers which are considered contaminants are ethyl-tert-butyl ether (“ETBE”), tert-amyl methyl ether (“TAME”) and diisopropyl ether (“DIPE”), which are used as gasoline oxygenates.
As production of such ether-based compounds continues to grow, it can be expected that the incidence and severity of spills will increase and that the threat to the water supply will become more severe. The present invention is related to the biological treatment of ether compounds to counter such a threat by providing means to efficiently remediate contaminated sites.
Reported Developments
It appears that relatively little work has been done to develop means for biodegrading ethers of the aforementioned type. In one study, an aerobic consortia isolated from acclimated sludge was maintained on MTBE which served as the sole source of carbon for the consortia (Salanitro et al., “Isolation of a Bacterial Culture that Degrades Methyl t-Butyl Ether,”
Applied and Environmental Microbiology,
July 1994). MTBE was degraded to tertiary-butyl alcohol (“TBA”) which was also degraded by the enrichment culture. The consortia is described as comprising at least 6 different uncharacterized bacteria. The physiology of the individual organisms is not reported. It is reported that the consortia appear to have a significant population of nitrifying bacteria.
Another recent study reported on the isolation from soil and sludge of several aerobic organisms that were able to degrade MTBE (Mo et al. 1995; Abstracts Ann. Meet. Am. Soc. Microbiol., Q51), but the degradation was relatively slow and inefficient, and the characteristics of the degradative organisms were not reported.
It appears that in situ degradation of MTBE in aquifers also has not been studied extensively. However, recent unpublished studies by researchers at Mobile Oil Corporation have provided evidence, based on historical concentrations of MTBE in groundwater, that natural attenuation of MTBE may occur over very long periods of time in aquifers. Apparent degradation occurs after the concentrations of benzene, toluene, ethylbenzene or xylene (BTEX) are reduced to low levels. The identity of the organisms responsible for the decline in MTBE were not reported. In further studies, it was observed that MTBE was partially transformed in only one of several anaerobic sediment samples tested (Mormile et al. “Anaerobic Biodegradation of Gasoline Oxygenates: Extrapolation of Information to Multiple Sites and Redox Conditions,”
Environ. Sci. Technol.,
28:1727-1732, 1994). Transformation of MTBE in the one active sample required m
Condee Charles Whitman
DeFlaun Mary F.
McClay Kevin Rock
Michelson Jennifer Diane
Steffan Robert Jon
Envirogen, Inc.
Redding David A.
Synnetstvedt & Lechner LLP
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