Liquid purification or separation – Processes – Treatment by living organism
Reexamination Certificate
1998-10-27
2002-02-26
Barry, Chester T. (Department: 1724)
Liquid purification or separation
Processes
Treatment by living organism
C210S611000, C210S617000, C210S908000
Reexamination Certificate
active
06350381
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the aerobic biodegradation of alkyl ethers by employing biological organisms in which microbial growth of the organisms has been enhanced by fatty acids. More particularly, this invention relates to the biodegradation of ether-based compounds, such as methyl-tertiary-butyl ether (MTBE), in aqueous systems by treatment of the aqueous systems with microorganisms the growth of which has been stimulated by the presence of a fatty acid such as lactate.
BACKGROUND OF THE INVENTION
Methyl-tertiary-butyl ether (MTBE) is a fuel oxygenate that is added to gasoline to enhance octane levels, increase burning efficiency and reduce the emission of atmospheric pollutants. Oxygenated compounds such as this have become common in recent years as air quality standards have become more stringent. MTBE is especially popular due to its low production cost, high octane rating, evaporative qualities and solubility with other gasoline components. In general, ether-based compounds such as MTBE, especially those compounds which have only one ether linkage and without other functional groups, are chemically stable and thus not easily degradable. These compounds are also soluble in water and absorb poorly to soil. One characteristic of MTBE, for example, is that it rapidly migrates to ground water through soil. Thus, due to its increased usage and chemical characteristics, MTBE is showing up in greater quantities in drinking water, groundwater, surface waters, and waste water. Although the effects of MTBE on humans are still under study, in the very least, when present in large enough quantities, MTBE in drinking water can result in unpleasant taste and odor. Therefore, as the use of MTBE continues to increase, there is a heightened need to develop processes that can effectively, but cost efficiently, reduce levels of MTBE on a large enough scale to be used in water purification.
The four most common methods for removing MTBE from water are air stripping, absorption, advanced oxidation and hydrophobic membrane separation. Each has inherent drawbacks. For example, since MTBE is a highly water soluble polar compound, high air to water ratios are required in the air stripping process, and, consequently, the use of air stripping may not be economical, especially on large scales as is necessary for water purification. Likewise, membrane separation is considered too expensive for use in general water treatment applications. Notwithstanding these economic drawbacks, there are environmental drawbacks as well. Although air stripping and absorption to particles such as granular activated carbon (GAC) result in remediation of the target contaminant, these methods suffer from the fact that they simply transfer the contaminant to another medium, either to the atmosphere (air stripping) or to solid medium (absorption) which must then be disposed of as hazardous waste. In other instances, such as in some advanced oxidation processes that utilize ozone and ozone/peroxide, the processes result in undesirable by-products such as bromate.
The drawbacks associated with the most common methods of hydrocarbon removal from water have led to the investigation of other purification processes. Biological degradation (biodegradation) is widely used for the treatment of many pollutants. If the target compound is biodegradable, biological treatment is recognized as being the most cost effective treatment method. For this reason many recent processes for removing MTBE and similar ether-based compounds from water have focused on biodegradation techniques. As mentioned above, alkyl ethers such as MTBE are not easily biodegraded. However, it is known that other compounds exhibiting similar biodegradation characteristics to alkyl ethers, such as trichloroethylene (TCE) which is a chlorinated solvent, can be degraded through a process known as “cometabolic degradation” or biodegradation. Cometabolic degradation, also referred to as “co-metabolism”, is a process in which enzymes that serve a primary metabolic function in a microorganism can be used to break down a target compound that otherwise would not be biodegradable. In such cometabolic processes, the target compound undergoes metabolic transformation while a second compound serves as a primary energy or carbon source.
Often, the success or effectiveness of biodegradation is dependent upon key factors being simultaneously present. First, microorganisms must be present, either naturally or by addition. Second, there must be oxygen, water and trace minerals present available to permit the microorganisms to be metabolically active. Third, there must be available sufficient quantities of biologically utilizable nutrients or “food” to enable the microbial population to grow and survive. In the industry, these nutrients are known as the “growth substrate” or “energy source”. In many cases, the growth substrate or energy source is the target compound itself. In other cases, the target compound will not support bacterial growth by itself and an additional growth substrate must be added to allow the degrading microorganism to grow and cometabolize the target compound. Alkyl ethers, in particular MTBE, have been utilized in both instances. Specifically, in some instances MTBE is merely the target of a biodegradation process, while in other instances, not only is MTBE the target of a biodegradation process, but MTBE is also the growth substrate utilized in the biodegradation process.
U.S. Pat. No. 5,750,364, illustrates use of alkyl ethers directly as growth substrates. This patent teaches a bacterial culture prepared from an ether containing a t-butyl moiety, such as MTBE, and an activated sludge. Specifically, a solution is formed by combining a mineral nutrient solution with biosludge recovered from a waste water treatment plant. The resulting solution is then subjected to an enrichment process that typically lasts from about one month to about one year during which the resulting solution is flushed with O
2
; combined with a branched alkyl ether; stirred mechanically; skimmed to remove a portion of the supernatant; and replenished with additional mineral solution and ether. The patent teaches that following this enrichment process, the culture becomes active in consistently degrading MTBE concentrations in the supernatant. One drawback to the prior art method of growing microorganism directly on alkyl ethers is that the growth of the microorganism is sporadic and processes dependent on the growth of microorganisms directly on the alkyl ether are inherently unreliable. Often, the result is identification of microbes that can be grown and are effective on a laboratory scale in small quantities, but are not easily reproduced in large enough quantities to be viable for large scale water purification operations, such as is used by municipalities. Further, although the microorganisms might be effective in degrading the target compound, the microorganisms do not grow prolifically in the presence of the target compound. Thus, growth and maintenance of such microorganisms, when possible, is typically time consuming and expensive. For example, in U.S. Pat. No. 5,750,364, referenced above, the process for growing an initial colony of microorganisms requires a minimum of four months.
The prior art has focused on the cometabolism of poorly degraded compound such as chlorinated solvents and alkyl ethers. For example, U.S. Pat. No. 4,713,343 discloses a method and apparatus for treating ground water contaminated with halogenated aliphatic hydrocarbon compounds. In the process, bacteria that can aerobically degrade low molecular weight chlorinated solvents utilizing monooxygenase are combined with the chlorinated solvents in the presence of a low molecular weight alkaynes and oxygen.
In another example, U.S. Pat. 5,814,514 teaches the use of propane-oxidizing bacteria that produce propane monooxygenase (PMO). Specifically, the patent teaches that since PMO is the enzyme primarily responsible for degradation of ether-based compounds, those bacteria that produc
Kilkenny Scott Tracey
Stringfellow William Thornton
Barry Chester T.
Bracewell & Patterson L.L.P.
Kinder Morgan Energy Partners, L.P.
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