Journal of Environmental Engineering

The feasibility of photooxidation treatment of methyl tert-butyl ether (MTBE) in water was investigated using two systems: (1) a slurry falling film photoreactor and (2) an integrated air stripping with gas phase photooxidation system. Methyl tert-butyl ether-contaminated synthetic water and field samples from contaminated sites were used for these studies. Using a ${\mathrm{TiO}}_2$ slurry (0.1 g/L; Degussa P25) flowing down at a rate of up to 0.26 L/min over the inner surface of a glass tube surrounding a 1-kW medium pressure mercury lamp, more than 99% of MTBE in the synthetic samples, initially at 1 mg/L, was degraded within 90 min. The major degradation products from MTBE were tert-butyl alcohol, tert-butyl formate, and small amounts of acetone. However, the degradation of MTBE and its byproducts in contaminated groundwater samples was hindered significantly by dissolved metals such as ${\mathrm{Fe}}^{2+}\text{,}$ chloride ions, and aromatic organic species. Integrating air stripping with gas-phase photocatalysis is an an effective alternative that would not be affected by the water chemistry. The reaction rates for MTBE degradation in the gas phase are orders of magnitude faster than in aqueous solution.

The transformation of methyl tert-butyl ether (MTBE) and tert-amyl methyl ether (TAME) using bifunctional aluminum in the presence of dioxygen $\left(O_2\right)$ has been examined. Bifunctional aluminum, prepared by sulfating zero-valent aluminum with sulfuric acid, is an innovative extension of zero-valent metal technology. It has a dual functionality of simultaneously decomposing both reductively and oxidatively degradable contaminants. Bifunctional aluminum is capable of utilizing dioxygen through a reductive activation process to degrade oxygenates at ambient temperature and pressure where oxygenates are stable. The reductive activation of dioxygen is a new concept for oxygenate treatments for which most of oxidative technologies require strong oxidants. Results indicate that aluminum serves as a reductant to create favorable reducing conditions while sulfur-containing species, generated by the sulfation of aluminum at the metal surface, are considered to act as active sites. MTBE and TAME underwent similar parallel reaction pathways where the oxidation occurred on both sides of ether linkage. The oxidation of MTBE produced primarily tert-butyl alcohol, tert-butyl formate, methyl acetate, and acetone while tert-amyl alcohol, tert-amyl formate, methyl acetate, methyl ethyl ketone, and acetone accounted for 71.7% of the TAME lost. A postulated mechanism rationalizing the oxidation of oxygenates by bifunctional aluminum is proposed.

In a previous study, the chemical oxidation of methyl tert-butyl ether (MTBE) at low concentrations in water using Fenton’s reagent (FR) was investigated. At certain reaction conditions the process achieved 99.99% degradation of MTBE but it did not result in complete MTBE mineralization. In the present study, the major intermediate by-products generated during the reaction, such as tert-butyl formate (TBF), tert-butyl alcohol (TBA), methyl acetate, and acetone were separately used as parent contaminants and treated under the same reaction conditions initially used for MTBE (i.e., pH of the water, molar ratio of pollutant to FR) in order to compare their degradability by hydroxyl radicals generated from Fenton’s reaction. The results were compatible with the second order reaction rate constants for the reaction of hydroxyl radicals with each contaminant commonly available in the literature. The comparison of the degradation kinetics for each intermediate by-product provided information that aims at unveiling the limiting step(s) of the entire MTBE degradation pathway. In this context, it was found that (1) TBA was generated by reactions subsequent to those that produced TBF, (2) acetone was originated by at least three independent pathways involving direct hydroxyl radical attack on MTBE, TBF, and TBA, and (3) methyl acetate was formed exclusively from MTBE.

Gasoline oxygenates [methyl tert-butyl ether (MTBE), di-isopropyl ether, ethyl tert-butyl ether, and tert-amyl methyl ether] in aqueous media are readily decomposed upon ultrasonic irradiation (665 kHz). The optimal instrumental settings for the production of reactive radical species employing a specific reactor were determined using dosimetry. Since hydroxyl radical is generally believed to be the predominant reactive species leading to the ultrasonically induced degradation of organic substrates in aqueous media, a terephthalate dosimeter was employed to selectively determine the yields of hydroxyl radicals. The decomposition of the gasoline oxygenates and the primary decomposition products of MTBE were compared in the presence of different saturating gases $(O_2\text{,}$ $N_2\text{,}$ and Ar). The observed decomposition rates for the oxygenates were similar under Ar and $O_2$ saturated conditions, but significantly slower under nitrogen saturated conditions. The observed decomposition rates for a number of the primary decomposition products of MTBE are significantly slower under $N_2$ and $O_2$ saturation relative to Ar saturation. All the gasoline oxygenates in this study were degraded by more than 98% within 3 h of ultrasonic irradiation under $O_2$ saturation with half-lives of 21–25 min.

Through 1998, methyl tertiary-butyl ether (MTBE) was the most commonly used fuel oxygenate in Reno, Nevada. Winter-use of oxygenated gasolines is required in areas of the country that exceed carbon monoxide air quality standards. MTBE has not been detected in Reno’s raw water sources, but treatment alternatives must be assessed to fully prepare for possible contamination events. In this research, bench-scale studies using activated carbon and air stripping were conducted to evaluate the treatability of a high concentration of MTBE in Truckee River water, which is the primary surface supply for the Reno area. Results indicated that neither method appears practical for treating MTBE-laden water for one day at a $1.14\times {10}^{8}L/\mathrm{day}$ (30 MGD) treatment plant. The capital costs estimated for full-scale application of these processes are approximately $5 million each. Estimated treatment costs for activated carbon and air stripping are approximately $0.043/L ($0.161/gal) and $0.047/L ($0.177/gal), respectively. Temporary closure of treatment facilities may be the best response to an accidental spill.

Methyl tert-butyl ether (MTBE) biodegradation was investigated using a continuously stirred tank reactor with biomass retention (porous pot reactor) operated under aerobic conditions. MTBE was fed to the reactor at an influent concentration of 150 mg/L (1.70 mM). An identical reactor was operated as a killed control under the same conditions. Operation of these reactors demonstrated that removal of MTBE was biological and suggests that biomass retention is critical for effective degradation. MTBE removal exceeded 99.99% when the volatile suspended solids concentration in the reactor was above 600 mg/L. Batch experiments conducted using mixed liquor from the porous pot reactor indicated that the individual rates of biodegradation of MTBE and tert-butyl alcohol (TBA) increase with increasing initial concentration. When batch tests were later repeated, the MTBE degradation rates were found to have increased while the TBA degradation rates remained constant. All batch tests confirmed that the degradation rate of TBA governed the overall degradation rate (degradation rate of both MTBE and TBA). The presence of TBA at lower concentrations did not affect the rate of MTBE degradation; however, higher concentrations of TBA did reduce the rate of MTBE biodegradation.

Methyl tert-butyl ether (MTBE) contamination in groundwater often coexists with benzene, toluene, ethylbenzene, and xylene (BTEX) near the source of the plume. Tertiary butyl alcohol (TBA) is a prevalent intermediate of MTBE degradation. Therefore, there is a significant potential for interference of MTBE and TBA degradation by the presence of BTEX whether treatment is in situ or ex situ. In this study, the effect of BTEX on the degradation of MTBE and TBA was examined using a mixed bacterial culture enriched on MTBE and BTEX. In batch studies, the presence of BTEX did not have a significant effect on MTBE degradation, but did have a slight effect on TBA degradation. Under continuous flow conditions, all compounds degraded simultaneously. Normalizing rates to the MTBE loading to the reactor indicates that BTEX may assist in the development of the biomass for TBA and overall MTBE degradation. Using denaturing gradient gel electrophoresis, several diverse organisms were identified, two of which showed very high similarity with PM1, a known MTBE degrader.

An ultrafiltation membrane bioreactor was evaluated for biodegradation of methyl tert-butyl ether (MTBE) in contaminated water. The system was fed 5 mg/L MTBE in granular activated carbon (GAC) treated Cincinnati tap water containing ample buffer and nutrients. Within 120 days the culture had adapted to membrane operational conditions and was consistently achieving greater than 99.95% biological removal of both MTBE and tert-butyl alcohol. This condition was steadily maintained for the next 200 days of study. Effluent dissolved organic carbon values remained at or below concentrations of the feed GAC treated tap water alone. An increase in biomass concentration as measured by volatile suspended solids was observed to correlate with an increase in MTBE removal efficiency. Some operational observations, including fouling, recovery from an accident, and overall performance, are described.

Methyl tertiary-butyl ether (MTBE) biodegradation was evaluated in a laboratory-scale granular activated carbon (GAC)-based fluidized bed bioreactor system. The reactor was operated in seven distinct phases during which the MTBE loading rate, hydraulic retention time, cocontaminant loading [butyl, toluene, ethylbenzene, and xylene (BTEX) and tertiary-butyl alcohol (TBA)] and temperature were varied. The reactor was able to treat MTBE to less than 20 ug/L at 25°C and total organic carbon (TOC) loading rates between 0.01 and 1.1 kg/m³ of expanded GAC bed per day (kg/m³ day). Net biomass yield in the reactor under high loading conditions was approximately 0.55 g of total suspended solids (TSS) per gram of TOC consumed. This high yield under the higher loading rates necessitated that biomass be removed from the reactor to control bed expansion. At a loading rate of 1.5 kg/m³ day, MTBE effluents exceeded 20 ug/L. Reactor performance decreased as the reactor temperature was reduced from 25 to 15°C, but even at the lower temperatures MTBE removal efficiency exceeded 99%. Methyl tertiary-butyl ether treatment efficiency was not affected by the addition of TBA or BTEX under the conditions evaluated. Results of this study demonstrate that fluid bed bioreactors inoculated with an appropriate microbial culture can efficiently treat MTBE-contaminated water.

Methyl tert-butyl ether (MTBE) is one of the most common ground water pollutants in the United States. Although MTBE has been characterized as a recalcitrant pollutant, it is now established that MTBE is biodegradable. A few bacteria that can grow on MTBE as a carbon and energy source have been identified and a host of bacteria that can cometabolize MTBE are known. There is very little information available concerning the biological treatment of MTBE contaminated ground water, despite the strong interest in applying biological treatment to the decontamination of MTBE laden water. In this paper we examine the treatment of contaminated ground water using a fluidized-bed bioreactor. Field studies demonstrated that the initiation of MTBE biotreatment was unpredictable, with one reactor starting to degrade MTBE immediately and a second reactor never degrading any MTBE. Laboratory studies were conducted to determine if a cosubstrate could be used to reliably enrich MTBE metabolizing microorganisms from a variety of environmental samples. It was determined that a number of compounds could enrich MTBE degrading populations, but that iso-pentane was the most reliable cometabolite of the compounds tested. Iso-pentane was used to initiate MTBE biotreatment in a laboratory fluidized-bed bioreactor. It was found that MTBE biotreatment continues even after iso-pentane addition was halted, suggesting that bacteria can gain maintenance energy from MTBE degradation. The reactor started with iso-pentane was as efficient as MTBE biotreatment as a reactor that started MTBE degradation without cosubstrate addition.

The increased use of ethanol as a replacement for the gasoline oxygenate, methyl tert-butyl ether (MTBE), may lead to indirect impacts related to natural attenuation of benzene, toluene, ethylbenzene, and the three isomers of xylene (BTEX compounds). Ethanol could enhance dissolved BTEX mobility by exerting a cosolvent effect that decreases sorption-related retardation. This effect, however, is concentration dependent and was not observed when ethanol was added continuously (at 1%) with BTEX to sterile aquifer columns. Nevertheless, a significant decrease in BTEX retardation was observed with 50% ethanol, suggesting that neat ethanol spills in bulk terminals could facilitate the migration of pre-existing contamination. MTBE (25 mg/L influent) was not degraded in biologically active columns, and it did not affect BTEX degradation. Ethanol (2 g/L influent), on the other hand, was degraded rapidly and exerted a high demand for nutrients and electron acceptors that could otherwise have been used for BTEX degradation. Ethanol also increased the microbial concentration near the column inlet by one order of magnitude relative to columns fed BTEX alone or with MTBE. However, 16S-ribosomal ribonucleic acid sequence analyses of dominant denaturing gradient gel electrophoresis bands identified fewer species that are known to degrade BTEX when ethanol was present. Overall, the preferential degradation of ethanol and the accompanying depletion of oxygen and other electron acceptors hindered BTEX biodegradation, which suggests that ethanol could increase the length of BTEX plumes.

Recent legislation in several states has called for the removal of methyl tert-butyl ether (MTBE) from gasoline. In order to comply with Federal Clean Air Act requirements for carbon monoxide and ozone attainment, ethanol is being considered as a replacement for MTBE. The objective of this study is to evaluate the potential impact of ethanol on benzene plume lengths in subsurface environments following accidental spills of ethanol-blended gasoline. Two types of studies were conducted here. First, laboratory studies were performed using a pure culture indigenous to a gasoline-contaminated aquifer to evaluate the effect of ethanol on the rate of benzene biodegradation under aerobic conditions. Results from microbial studies showed that the biodegradation of 25 mg/L benzene was severely inhibited in the presence of 25 mg/L ethanol. While the enzymes responsible for benzene biodegradation by the culture were inducible, ethanol degradation appeared to be constitutive. Second, a two-dimensional model was developed to quantify the impact of ethanol on benzene plume lengths using weighted-average aerobic and anaerobic biodegradation rates for benzene in the presence and absence of ethanol. Model simulations indicated that benzene plume lengths are likely to increase by 16–34% in the presence of ethanol.

The natural life cycle of a plume of methyl tert-butyl ether from a spill of gasoline is controlled by the rate of attenuation of the source (due to partitioning from the residual gasoline to the flow of groundwater) and the rate of attenuation in the plume (due to dispersion and natural biodegradation). Rates of attenuation were extracted for plumes in California, Florida, North Carolina, New York, and New Jersey. The maximum rate of attenuation of the source was 0.75 per year. The rates of attenuation in the plumes varied from 0.56 to 4.3 per year. In all cases, the rate of attenuation of the plume exceeded the rate of attenuation of the source. As these plumes progress through their life cycle, they should recede back toward their source.

Methyl tert-butyl ether (MTBE), a gasoline oxygenate additive, is present in groundwater aquifers at the Department of Defense Housing Facility, Novato, Calif. (Novato), and the Naval Base Ventura County, Port Hueneme, Calif. (Port Hueneme). A microcosm study was conducted to examine and compare the potential for and performance of aerobic, anaerobic, and aerobic cometabolic MTBE biodegradation processes using soils and groundwater collected from the Novato and Port Hueneme sites. Propane and butane were tested as the cometabolic growth substrates. Nitrogen requirements were tested by preparing microcosms with and without nitrate as a nitrogen source. The results of this study demonstrated the potential for aerobic MTBE biodegradation and mineralization at both sites. In the commingled, or upgradient, portion of the Novato plume, nitrate enhanced aerobic MTBE biodegradation; in the absence of nitrate or under anaerobic conditions, MTBE degradation was insignificant. Downgradient, where the groundwater was impacted only by MTBE, the MTBE was readily degraded with and without nitrate addition and without other external nutrient amendments. Mineralization studies showed that MTBE was mineralized at both sites, with maximum recoveries approaching 80% of the radiolabeled carbon added to the microcosms. In the downgradient, MTBE-only portions of both sites, the addition of propane and butane to stimulate cometabolic MTBE degradation provided negligible improvement over direct oxidation under aerobic conditions. Furthermore, when nitrate was not present, propane and butane were not degraded and the residual propane and butane in the bottles appeared to inhibit the MTBE degradation; this inhibition was most pronounced in the Port Hueneme microcosms where MTBE degradation all but ceased in the presence of residual propane and butane. In the upgradient, commingled Novato plume, propane plus nitrate-fed microcosms outperformed the aerobic, nitrate-fed microcosms; this was the only condition where cometabolism enhanced MTBE degradation over direct aerobic oxidation.

A gasoline release site at the Dept. of Defense Housing Facility (DODHF) in Novato, Calif., resulted in dissolved methyl tertiary-butyl ether (MTBE) in a shallow, thin, heterogeneous aquifer. Multiple methods were used to evaluate the stability of the MTBE plume, including concentration versus time plots, isoconcentration contour maps, total dissolved MTBE mass and center of mass (COM) estimates, total plume extent changes, and parametric and nonparametric statistical techniques. The COM evaluation revealed a small upgradient shift in COM relative to both total plume extent and total MTBE mass reduction over the period examined. The total mass removal received substantial contributions from both the southern and northern portions of the plume, and the only removal mechanisms active in the northern plume were natural attenuation mechanisms. The results indicate that there exists at DODHF Novato a dissolved MTBE plume at the approximate balancing point between groundwater advection, which is carrying MTBE downgradient, and natural attenuation processes including aerobic biodegradation, which are reducing MTBE concentrations at about the same rate.

Methyl tert-butyl ether (MTBE) is released into the environment as one of some gasoline components, not as a pure compound. Benzene, toluene, ethylbenzene, and xylene (BTEX) compounds are major volatile constituents found in gasoline and are water soluble and mobile. This study focused on the occurrence of MTBE with BTEX compounds in several marinas in Lake Texoma, which is a large reservoir located on the Oklahoma and Texas border. During a monitoring period from June 1999 to July 2001, MTBE and BTEX were detected in 28 and 5% of samples analyzed, respectively. Methyl tert-butyl ether co-occurred with BTEX compounds in 15% of lake water samples when detectable MTBE was present. The relatively low co-occurrence (15%) of MTBE with BTEX compounds is primarily due to the volume percentage in gasoline mixtures and physicochemical properties such as water solubility and Henry’s law constant. Toluene was the most commonly co-occurring BTEX with MTBE. Values of the ratios of the BTEX concentration to the MTBE concentration generally increase with depth of water.