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MICROBIAL COMETABOLISM OF RECALCITRANT CHEMICALS IN CONTAMINATED AIR STREAMS
Impact/Purpose:
The objectives are the evaluation of process feasibility and the relative merits of the various technologies, as well as the development of appropriate design bases. We propose a three-year study that will evaluate two model chemicals (trichloroethylene (TCE) and methyl tert-butyl ether (MTBE)), three classes of organisms (methanotrphs, propane degraders, and BTEX degraders), and three bioreactor technologies (hollow fiber membrane reactors, biotrickling filters, and suspended growth reactors). Our patented methanotroph, Methylosinus trichosporium OB3b, will be studied for TCE cometabolism. Ongoing research will select and characterize the kinetics of MTBE-degrading cultures before the proposed research starts. The proposed project expands our extensive experience with cometabolism of chlorinated solvents in contaminated water to contaminated air streams. It also extends the range of recalcitrant chemicals treated by our cultures and bioreactor technologies to the fuel oxygenate, MTBE, which is becoming a major environmental concern and is an excellent candidate for cometabolism. We selected MTBE for study because it is, by far, the most popular fuel oxygenate, is produced in large quantities, and is recalcitrant to conventional biodegradation.
This research seeks to develop and demonstrate at laboratory scale a biological treatment process for cometabolizing chlorinated solvents and MTBE in contaminated air streams. Hollow fiber membrane bioreactors are being used to effect this treatment because environmental conditions for the organisms can be very well controlled, thereby maximizing degradation rates. The objectives are the evaluation of process feasibility and the relative merits of the various technologies, as well as the development of appropriate design bases. We propose a three-year study that will evaluate two model chemicals (trichloroethylene (TCE) and methyl tert-butyl ether (MTBE)), three classes of organisms (methanotrphs, propane degraders, and BTEX degraders), and three bioreactor technologies (hollow fiber membrane reactors, biotrickling filters, and suspended growth reactors). Our patented methanotroph, Methylosinus trichosporium OB3b, will be studied for TCE cometabolism. Ongoing research will select and characterize the kinetics of MTBE-degrading cultures before the proposed research starts. The proposed project expands our extensive experience with cometabolism of chlorinated solvents in contaminated water to contaminated air streams. It also extends the range of recalcitrant chemicals treated by our cultures and bioreactor technologies to the fuel oxygenate, MTBE, which is becoming a major environmental concern and is an excellent candidate for cometabolism. We selected MTBE for study because it is, by far, the most popular fuel oxygenate, is produced in large quantities, and is recalcitrant to conventional biodegradation.
This research seeks to develop and demonstrate at laboratory scale a biological treatment process for cometabolizing chlorinated solvents and MTBE in contaminated air streams. Hollow fiber membrane bioreactors are being used to effect this treatment because environmental conditions for the organisms can be very well controlled, thereby maximizing degradation rates.
Organic chemicals that are recalcitrant to biodegradation are common ground water and soil contaminants at hazardous waste sites. Development of biological treatment technologies for recalcitrant chemicals is of importance because, if successful, such technologies completely destroy the chemicals, which offers an advantage over some competing physical-chemical technologies that merely transfer the chemicals from one medium to another. Also, biological treatment processes typically are less costly than other processes. Contaminated air streams arise from the treat
Description:
Chlorinated Solvents: The treatment system consists of a laboratory-scale hollow fiber membrane (HFM) module containing a center baffle and a radial cross-flow pattern on the shell side of the fibers. The shell and lumen fluids are contacting in a counter-current fashion. Mass transfer from the contaminated fluid to the bacteria occurs in the HFM module, along with partial biode-gradation of the contaminant via cometabolism. Biodegradation is completed in an external polishing reactor. Effluent from the polishing reactor is returned to a growth reactor which serves as the source of microorganisms for the process. The growth reactor operates as a chemostrat with a side stream that recirculates through the HFM module and the polishing reactor. For chlorinated solvent cometabolism, Methylosinus trichosporiumOB3b PP358 was used. The bioreactor system can be applied to the treatment of either contaminated air or water streams. The HFM bioreactor system was shown to effectively remove and degrade trchloroethylene (TCE) from a gaseous influent stream. Long-term operation with stable pseudo-first-order degradation rates was demonstrated. A new method to measure the pseudo-first-order degradation rate also was developed and was shown to give more representative experimental degradation rate constants than in previous research. A computer model for designing aqueous treatment systems was modified for the air treatment system and was able to simulate experimental data well, demonstrating its effectiveness for planning experiments and system design. In a subsequent series of experiments, the hollow fiber membrane bioreactor system was operated successfully at high flow rates of approximately an order of magnitude larger than in all previous experiments. These experiments demonstrated the potential economic feasibility of the system in comparison to GAC adsorption. Along with the high flow rate experiments, several bioreactor experiments investigated specific transformation. Specific transformation is defined as the mass of TCE degraded divided by the mass of bacteria leaving in the waste. This parameter is important because the microorganisms have a finite capacity for degrading chlorinated solvents and once that capacity is reached, the bacteria are inactivated. Specific transformation is a parameter that gives insight into the amount of TCE degraded by each unit mass of bacteria in a continuous flow system. Transformation capacity is a parameter defined as the mass of TCE degraded divided by the mass of organisms in batch experiments conducted until degradation ceases. The specific transformation experiments showed that the hollow-fiber membrane bioreactor system could be operated with specific transformations as large as 48 µg/mg, which is approximately 50% of the transformation capacity. These results are extremely encouraging because they indicate that the system can operate stably at large values of specific transformation, which is of considerable economic importance. Since the conception of this technology, it was envisioned as treating multiple chlorinated solvents simultaneously, however, early research was not directed toward the treatment of multiple chemicals. The most recent experiments were directed toward this objective. The system was modified to deliver TCE and chloroform simultaneously and biological experiments were undertaken. With influent TCE concentrations ranging between 100 and 180 µg/L, an average of 65% of the TCE transferred the membrane and 93% of the transferred TCE was degraded. For chloroform, influent concentrations ranged between 50 and 70 µg/L and an average of 58% of the chloroform transferred the membrane and 65% of the transferred chloroform was biodegraded. The computer model was used to simulate the experimental conditions for TCE and chloroform, and model simulations compared closely to the experimental data. With data from these experiments demonstrating the feasibility of treating multiple chemicals simultaneously, the hollow-fiber membrane bioreactor system can be positioned as an attractive technology for situations were multiple chemicals are present.
MTBE:Five bacterial cultures have been screened for their ability to cometabolize MTBE. Out of five bacterial cultures tested, Arthrobacter sp. (ATCC 27778) was identified as the best candidate for bacterial cometabolism of MTBE, with a transformation capacity of 19.9 µg MTBE/mg TSS , and a pseudo-first-order degradation rate constant of 0.2 L/mgTSS/day. This degradation rate was measured with the culture grown on a basal salt media with ammonia as the nitrogen source and butane as the sole carbon and energy. The MTBE biodegradation rate was not affected by the purity of the growth substrate. Arthrobacter cultures were able to cometabolize MTBE at the same rate when grown on either pure n-butane (99.9%) or lighter fluid (mixed butane). However, cultures grown on either n-butanol or glucose were not able to biodegrade MTBE to any appreciable extent. Batch kinetic assays of Arthrobacter have shown that this organism biodegrades MTBE at low concentrations. Unique Monod kinetic parameters for Arthrobacter degrading MTBE were determined by simultaneously fitting data from six kinetic experiments, each with a different initial concentration, to the Monod equation. The kinetic parameters, kmax (maximum specific substrate utilization rate) and Ks (half saturation coefficient), were determined to be 0.43 mg/mg-d and 2.1 mg/L, respectively. The Ks of Arthrobacter is lower than any that have been recently reported, and highlights the potential effectiveness of this organism in treating MTBE at low concentrations.We concluded our characterization of Arthrobacter degradation of MTBE with several radiochemical kinetic studies. We conducted a degradation study using radiolabeled MTBE. Arthrobacter produced 14CO2 when given uniformly labeled [14C]-MTBE, thus confirming its ability to mineralize MTBE. Radio labeled MTBE was degraded within 2 hours of the experiment. Approximately 30% of the 14C was recovered as 14CO2 after 10 hours of incubation. The remaining 14C was recovered as dissolved radio labeled compounds, including tert-butyl alcohol (TBA). Mass balance calculations from GC measurements showed that approximately 35% of the TBA formed from MTBE was also degraded. Lastly, inhibition experiments were conducted to gain additional information on the nature of the MTBE degrading enzyme in Arthrobacter. It was determined that MTBE the degradation rate was negatively impacted in the presence of acetylene. Likewise, cultures exposed to acetylene and TBA degraded TBA at a lower rate compared to the control that was given only TBA. The presence of butane also had a negative impact on MTBE degradation rate. Although the project has officially ended, experimentation will continue through the spring of 2001 examining the performance of Arthrobacter for MTBE cometabolism in the hollow fiber membrane bioreactor system.