Initiation of MTBE Biotreatment in Fluidized-Bed Bioreactors

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.     

Biodegradation Kinetics of MTBE in Laboratory Batch and Continuous Flow Reactors

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.     

Role of Natural Attenuation in Life Cycle of MTBE Plumes

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.     

Comparison of Liquid and Gas-Phase Photooxidation of MTBE: Synthetic and Field Samples

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 TiO2 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 Fe2+, 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.     

Aerobic and Cometabolic MTBE Biodegradation at Novato and Port Hueneme

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.     

Effects of Ethanol versus MTBE on Benzene, Toluene, Ethylbenzene, and Xylene Natural Attenuation in Aquifer Columns

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.     

Multiple Methods for Determining Stability of Attenuating MTBE Groundwater Plume

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.     

Degradation of MTBE and Related Gasoline Oxygenates in Aqueous Media by Ultrasound Irradiation

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 (O2, N2, and Ar). The observed decomposition rates for the oxygenates were similar under Ar and O2 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 N2 and O2 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 O2 saturation with half-lives of 21–25 min.     

Degradation of MTBE Intermediates using Fenton’s Reagent

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.     

Impact of Ethanol on Benzene Plume Lengths: Microbial and Modeling Studies

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.     

Use of Membrane Bioreactor for Biodegradation of MTBE in Contaminated Water

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.     

Co-Occurrence of MTBE and Benzene, Toluene, Ethylbenzene, and Xylene Compounds at Marinas in Large Reservoir

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.     

Comparative Analysis of Chlorine Dioxide, Free Chlorine and Chloramines on Bacterial Water Quality in Model Distribution Systems

Drinking water utilities may be required to change disinfectant to improve water quality and meet more stringent disinfection regulations. This research was conducted to assess and compares chlorine dioxide to free chlorine and chloramines on bacterial water quality monitored within model distribution systems (i.e., annular reactors). Following colonization with nondisinfected water, annular reactors containing either polycarbonate or cast iron coupons were treated with free chlorine, chlorine dioxide or chloramines. Two disinfectant doses (low/high) were tested for each disinfectant. Under specific environmental conditions, bacterial inactivation varied as a function of the disinfectant type and dose, sample type (bulk water versus biofilm bacteria) and coupon material. The ranking by efficiency was as follows: chlorine dioxide > chlorine > chloramines. On preformed biofilms of 106–107?cfu/cm2, the continuous application of a disinfectant led to a log removal of heterotrophic bacteria concentrations for suspended and biofilm bacteria ranging from 1.1 to 4.0, and from 0.2 to 2.5, respectively. Doubling the amount of disinfectant doses led to an additional log inactivation of 1–2.5 of heterotrophic bacteria levels. This study demonstrates that bacterial inactivation in distribution systems is governed by various inter-related parameters. The data indicate that chlorine dioxide represents a viable alternative for secondary disinfection in distribution systems.     

Novel Bifunctional Aluminum for Oxidation of MTBE and TAME

The transformation of methyl tert-butyl ether (MTBE) and tert-amyl methyl ether (TAME) using bifunctional aluminum in the presence of dioxygen (O2) 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.     

Evidence for Phytodegradation of MTBE from Coupled Bench-Scale and Intermediate-Scale Tests

This paper presents methodologies and demonstrates the need to couple bench-scale and intermediate tree-scale experiments, to fully understand the transport and fate of organic contaminants, specifically methyl tert butyl ether (MTBE), in mature trees. Bench-scale experiments showed MTBE to be optimally taken up by small poplar saplings with a transpiration stream concentration factor of approximately 1, little or no degradation in soils and, nearly 100±20% recovery in the coupled water-plant-air system, indicating no measurable phytodegradation at the bench-scale. A large 14?ft tree chamber was designed to evaluate MTBE transport and fate through intermediate-scale (12?ft tall) poplar trees. Abiotic MTBE volatilization tests conducted in the tree chamber showed 100±20% MTBE mass recovery, thereby demonstrating the integrity of the large chamber and its air monitoring technique. In contrast, replicate intermediate-scale experiments conducted with large (12?ft) trees irrigated with a known mass of MTBE showed a deficit of MTBE mass recovery (65±20%) in replicate soil-tree-air systems monitored over a 2-week period. More significantly, tert butyl alcohol (TBA), a degradation product of MTBE, was detected in increasing concentrations in leaf biomass while MTBE concentrations in leaf biomass decreased as the experiment progressed. The MTBE mass recovery deficit, coupled with the detection of increasing TBA in leaf biomass, provides preliminary evidence of MTBE degradation in mature trees.     

Treatment of MTBE-Contaminated Water in Fluidized Bed Bioreactor

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.     

Bioremediation of Mixed Wastes (BTEX, TPH, TCE, and cis-DCE) Contaminated Water

Together with industrialization, more sites have become contaminated with mixed wastes of organics. Contamination of groundwater with benzene, toluene, ethylbenzene, and three isomers of xylene (BTEX) and chlorinated aliphatic hydrocarbons (CAHs) allows for the application of aerobic bioremediation to achieve mineralization of both types of compounds. In this study, the aerobic bioremoval of mixtures of these compounds [BTEX, BTEX/trichloroethylene (TCE), BTEX/cis-1,1-dichloroethylene (cis-DCE), BTEX/total petroleum hydrocarbons (TPH)/TCE, and toluene and ortho-xylene (ToX/TCE] was assessed under different environmental conditions (pH and temperature) by using indigenous microorganisms isolated from potentially contaminated regional sites. The highest TPH bioremoval efficiencies (<50%) occurred at 25°C and under neutral or alkaline conditions. TCE was cometabolized with toluene and o-xylene provided as growth substrates, and the highest bioremoval efficiency occurred at ToX/TCE, and pH and temperature impacted the mineralization of compounds. This study would help enhance the applicability of bioremediation technology to the mixed-wastes contaminated sites.     

Carbon Adsorption and Air-Stripping Removal of MTBE from River Water

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 1.14×108?L/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.     

Aerobic Biodegradation of Methyl Tert-Butyl Ether in Gasoline-Contaminated Aquifer Sediments

Intrinsic biodegradation of methyl tert-butyl ether (MTBE) in aquifer sediments under oxic conditions was investigated using laboratory microcosms. Aquifer samples were collected from three different areas (source area, upgradient, and downgradient) of a shallow gasoline-contaminated aquifer within the Atlantic Coastal Plain Province located in Virginia. Biodegradation of MTBE was observed in the source-area microcosms in which MTBE declined from a starting concentration of 2.7 to 0.28 mg/L over a 58-day period, following an initial lag period of 20 days. The same set of microcosms was respiked with MTBE to an initial concentration of 4.8 mg/L and MTBE concentrations declined to 0.20 mg/L over a 52-day period with no lag in biodegradation. First-order MTBE biodegradation rates for the first and second periods were 0.037±0.003 and 0.063±0.003 day?1, respectively. When another set of source-area microcosms was spiked with MTBE (5 mg/L), toluene and ethylbenzene (1 mg/L each), the initial lag period increased to 33 days but there was no significant change in the MTBE biodegradation rate (0.065±0.026 day?1) and MTBE was not detected after 134 days. Biodegradation of MTBE was also observed in the microcosms constructed using aquifer sediment with only limited exposure to MTBE but the degradation rate was lower and statistically different (0.022±0.005 day?1) than the source area microcosms. Biodegradation of MTBE ceased when oxygen was depleted. Methyl tert-butyl ether did not biodegrade in the uncontaminated, upgradient microcosms; however, rapid biodegradation of toluene was observed. Methyl tert-butyl ether biodegradation appears to be limited in the absence of dissolved oxygen and in aquifer sediments where petroleum hydrocarbons including MTBE were not previously observed.     

Effect of Uncertain Hydraulic Conductivity on the Simulated Fate and Transport of BTEX Compounds at a Field Site

Monte Carlo simulations were conducted to investigate the effect of uncertain hydraulic conductivity (K) on the natural attenuation of BTEX compounds (benzene, toluene, ethyl benzene, and xylenes) through aerobic respiration, denitrification, Fe(III) and sulfate reductions, and methanogenesis observed at a field site on Hill Air Force Base, Utah. First, the uncertainty in the ln?K field was represented by multiple realizations of spatially correlated random fields. The simulated BTEX plumes resulting from a light nonaqueous phase liquid source were analyzed for mass distributions and the relationships among various factors such as dissolved BTEX mass, plume spreading, and depletions of electron acceptors or productions of Fe2+ and methane. Second, additional K realizations with the same mean but different variances and correlation lengths were used to determine how the model responds to varying degrees of uncertainty in the K field. The methodology and insights are of general interest and applicable to fuel-hydrocarbon natural-attenuation sites.