Superfund Innovative Technology Evaluation (SITE)
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Demonstration of a Two-Stage Methanotrophic Bioreactor for Biodegradation
of Trichloroethene in Groundwater
BioTrol, Inc., Eden Prairie, Minnesota, developed an above-ground biological remedial system for water contaminated with halogenated volatile organic compounds, including trichloroethylene (TCE) and related chemicals. The Methanotrophic Bioreactor is innovative because it has design features that circumvent problems peculiar to treatment of this class of chemicals. BioTrol uses a specific organism (Methylosinus trichosporium OB3b, or simply OB3b) that oxidizes TCE rapidly and utilizes methane, an abundant and inexpensive source of carbon, as its growth substrate. BioTrol also incorporates a two-stage reactor to first grow the microorganisms (culture vessel) and then supply them to a second stage contact reactor (plug flow) where conditions allow volatile TCE to remain in the liquid phase. The technology is applicable to water contaminated with halogenated aliphatic hydrocarbons, including TCE, dichloroethylene isomers, vinyl chloride, dichloroethane isomers, chloroform, dichloromethane, and others. The first stage of the Methanotrophic Bioreactor involves the propagation of OB3b on methane in a 400-L continuous-flow suspended growth culture vessel. Methane transfer efficiency to the culture vessel was improved by addition of a circulating gas-saturation device. Cells grown in the culture vessel are supplied to the plug flow contactor where the culture medium is mixed with the contaminated water. OB3b microorganisms are capable of degrading TCE in the absence of methane. This is important because TCE is extremely volatile, and bubbling of methane through the reaction vessel would likely result in stripping of TCE from the water. The plug flow bioreactor consists of 100 ft of 2-in stainless steel pipe fitted with stainless steel sample ports every 20 ft. The specific objectives of the pilot demonstration were to test the scale-up potential of a bench system and determine the economic feasibility of the technology based on operating costs at pilot scale. In 1993, six thousand gallons of air-stripped (previously TCE-contaminated) groundwater were shipped to, and stored at, BioTrol's pilot-testing facility in a stainless steel tank trailer. Water was piped into a 500-gal polyethylene surge tank, and then pumped at a rate of 1 L/minute into the plug-flow reactor. TCE was metered into the ground water within the enclosed reactor system to achieve an influent TCE concentration of 2 ppm. OB3b cell culture was fed from the culture vessel to the plug flow reactor at a rate of 0.1 L/minute. The reactor was periodically sampled throughout its length and the water was analyzed for TCE. The results from this pilot test show that TCE could be reduced within the reactor by 87.6% to less than 0.155 ppm. Estimated methane costs for the study were $0.33/1000 gal of water treated. BioTrol claims that system modifications could potentially reduce this component of treatment.