Environ. Sci. Technol. 2006, 40, 1997-2003
Bioremediation of Groundwater Contaminated with Gasoline Hydrocarbons and Oxygenates Using a Membrane-Based Reactor MAHER M. ZEIN,† M A K R A M T . S U I D A N , * ,† A N D ALBERT D. VENOSA‡ Department of Civil & Environmental Engineering, University of Cincinnati, 765 Baldwin Hall, ML0071, Cincinnati, Ohio 45221-0071, U. S. Environmental Protection Agency National Risk Management Research Laboratory, Cincinnati, OH 45268 U.S.A.
The objective of this study was to operate a novel, fieldscale, aerobic bioreactor and assess its performance in the ex situ treatment of groundwater contaminated with gasoline from a leaking underground storage tank in Pascoag, RI. The groundwater contained elevated concentrations of MTBE (methyl tert-butyl ether), TBA (tertbutyl alcohol), TBF (tert-butyl formate), BTEX (benzene, toluene, ethyl benzene, and xylene isomers), and other gasoline additives (tert-amyl methyl ether, di-isopropyl ether, tert-amyl alcohol, methanol, and acetone). The bioreactor was a gravity-flow membrane-based system called a Biomass Concentrator Reactor (BCR) designed to retain all biomass within the reactor. It was operated for six months at an influent flow rate that ultimately reached 5 gpm. The goal was to achieve a removal of all contaminants to 99%) were purchased from the Fisher Scientific Co., Pittsburgh, PA. ETBE (99%), TAME (97%), tert-amyl alcohol (TAA, 99%), DIPE (>99%), tert-butyl alcohol (TBA, >99.5%), and tertbutyl formate (TBF, 99%) were obtained from Aldrich Chemical Co., Milwaukee, WI. All other chemicals used in this study were of the highest purity commercially available. Analytical Methods. Influent and effluent samples were collected three times daily for the oxygenates and hydrocarbons of interest and dissolved organic carbon (DOC) analysis. In addition, pH, temperature, dissolved oxygen, and flow rate were monitored on-site on a daily basis to ensure optimum conditions for biological activity. The pH of the activated sludge inside the reactor was measured using a Thermo Orion Model 260A portable pH meter, while the dissolved oxygen and temperature in the reactor were monitored with a Thermo Orion Model 810 Aplus dissolved oxygen meter (Orion Research Co., Boston, MA). Influent and effluent BTEX, MTBE, its biodegradation byproducts (TBA and TBF), and the other gasoline oxygenates (ETBE, TAME, TAA, and DIPE) were measured using a HewlettPackard 6890 Series II GC/FID equipped with PTA-5 column (30 m, 0.53 mm i.d., and 3µm film thickness) (Supelco, Bellefonte, PA) and coupled with a 70 °C heated purge and trap (11 min at 40 °C of purging followed by 4 min at 225 °C of desorbing) consisting of a Tekmar Dohrmann 3100 sample concentrator and a Tekmar Dohrmann AquaTek 70 liquid autosampler (Tekmar Dohrmann, Cincinnati, OH). The initial oven temperature of the GC was held at 35 °C for 6 min, followed by a ramp of 12 °C/minute to 190 °C held for another 6 min. Volatile organic compound (VOC) samples were preserved by adding 10 N sodium hydroxide solution to raise the pH to 11. Acid preservation was avoided to minimize chemical hydrolysis of the ethers. Influent and effluent DOC concentrations were determined using a Shimadzu TOCVCSH analyzer (Shimadzu, Kyoto, Japan). The BCR contents were sampled weekly to measure microbial growth as total suspended solids (TSS) and volatile suspended solids (VSS) throughout the study. This analysis was performed by drying VOL. 40, NO. 6, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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a Whatman 934-AH glass microfiber filter (Clifton, NJ) at 550 °C for 1 h, filtering the sludge sample, drying at 105 °C for 2.5 h, and finally taking the difference in filter mass after baking at 550 °C for 2 h (22). All analyses were performed in triplicate for quality assurance purposes. Preserved VOC, DOC, and biomass samples were shipped on ice to the University of Cincinnati laboratory twice per week for immediate analysis.
Results and Discussion Reactor Performance. Following inoculation, the BCR was fed contaminated groundwater at a flow rate of 3.8 L/min. Although the bioreactor was seeded with bacterial cultures already acclimated to MTBE and BTEX, contaminant removal effectiveness initially was not satisfactory and did not reach the desired effluent goal. Effluent MTBE concentrations were higher than the targeted 5 µg/L due to a combination of frequent well failures and problems with the air stripping/ GAC pumping control system that hampered operation for the first month of the study. Since the influent and effluent pumps of the BCR were controlled by the same computer system that regulated the operation of the extraction wells, air stripper, and GAC vessels, any malfunction of this control system resulted in a shutdown of the feed from the bioreactor. Also, the recurrent failure of the extraction wells to provide sufficient contaminated groundwater flow for both the BCR and the air stripping/GAC system contributed to prolonging the time period before the project flow objectives were achieved. Once the above problems were resolved and steadystate flow conditions achieved, the BCR was able to produce consistent results. The flow rate was ramped rapidly upward until the design flow rate of 19 L/min was reached. The BCR was operated at this flow for the last two months of the study. As the biomass acclimated to the operating conditions, effluent concentrations of all the contaminants were observed to gradually decline despite the huge fluctuations in the influent contaminant concentrations. MTBE concentrations were especially variable, ranging between 0.7 and 13 mg/L (see Figure A, Supporting Information). Higher levels occurred earlier in the study before the 19 L/min flow was achieved, after which a lower concentration range (1-5 mg/ L) predominated. This variability was due to the fact that the groundwater feed was supplied by five different extraction wells, each pumping water at different flow rates and each having different contamination levels. Despite the high variability in flow and influent VOC concentrations, MTBE in the effluent decreased to less than 5 µg/L on day 142 and remained below this level until the end of the study (Figure 2). In contrast, the existing air stripper that shared the contaminated water feed was observed to produce effluents of significantly higher MTBE concentrations. On day 165 of operation, for example, the air stripper effluent MTBE was approximately 1300 µg/L relative to an influent level of 3500 µg/L. The air stripper discharge was further passed through three 910 kg GAC beds in series where the MTBE concentrations finally declined to less than 5 µg/L (204 µg/L f 90 µg/L f 2 µg/L). In a previous laboratory study with a covered bench-scale version of the BCR where the influent MTBE concentration was 5000 µg/L, gas-phase concentrations averaged 0.89 ( 0.38 µg/L, representing approximately 2% of the MTBE feed stripped into the headspace (21). Since MTBE and other volatile contaminants are present within the reactor at very low aqueous concentrations, as in the case of the BCR (
