By Gerard E. Spinnler, Joseph P. Salanitro, Paul M. Maner and Karen A. Lyons

Introduction

The removal of MTBE dissolved in groundwater has turned out to be a difficult task.  Conventional remediation methods used successfully for other petroleum constituents have proven difficult, ineffective or financially prohibitive to remediate MTBE. We have demonstrated an in-situ biological method that degrades gasoline oxygenates to CO2 and water that overcomes many of the hurdles encountered with conventional MTBE remediation approaches.

There are several strategies for in situ bioremediation of MTBE.  The most straightforward is natural or intrinsic bioremediation, where MTBE biodegrades as a result of natural microbes present in soil.  So far, the evidence indicating intrinsic bioremediation of MTBE is inconclusive. In fact, the extreme length of some MTBE plumes implies intrinsic bioremediation does not occur at rates capable of controlling plume migration.

Biostimulation is the process of adding oxygen and/or nutrients to stimulate natural MTBE degraders present (but inactive) in the soil.  We have successfully demonstrated biostimulation at Port Hueneme Naval Base in California (see Soil Sediment & Groundwater MTBE Special Issue-March 2000) by adding oxygen to the aquifer.  MTBE was present at low PPM concentrations and biodegraded after a substantial lag period. Other researchers have claimed similar success in low concentration MTBE plumes using oxygen stimulation.  Microcosm experiments done in our laboratory from soil collected at MTBE contaminated sites indicated MTBE-degrading microorganisms were present at few sites.  The presence of natural degraders does not, by itself, insure biodegradation will be successful. Degraders need to be present in sufficient quantities and respond to stimulation.

Adding MTBE degraders as well as O2 and/or nutrients to degrade MTBE in the aquifer is termed bioaugumentation.  We have demonstrated MTBE and TBA degrading to CO2 and water by adding a mixed culture of MTBE-degrading microorganisms and oxygen into the aquifer at Port Hueneme as well as at retail gasoline stations throughout the U.S.

This report will detail MTBE bioremediation using bioaugmentation at two retail gas stations.

TECHNOLOGY OVERVIEW

A leak from a UST resulted in an MTBE plume in groundwater.  At the leading edge of the plume, an oxygen gas injection well is supplying oxygen to the local surrounding area.  MTBE-degrading microbes have been placed in the aquifer (indicated by the green rectangle) and form a continuous “biobarrier” that the MTBE plume will flow through.  MTBE-contaminated water flowing through the microbe and oxygen-rich zone will contact the microbes and be degraded to CO2 and water.

MTBE-degrading microbes are introduced to the subsurface through special injection techniques or through infilling a trench.  Oxygen is supplied to the subsurface either by pulse injecting oxygen gas or air.  Unlike other systems, BioRemedy does not require the addition of co-substrates such as other hydrocarbons to degrade MTBE.  All that is necessary in the BioRemedy process is oxygen and proprietary MTBE-degrading microbes.

MTBE-degrading culture

Microorganisms capable of degrading MTBE are not so ubiquitous as BTEX degraders.  As previously mentioned, few soils collected from MTBE contaminated sites showed any MTBE-degrading activity when stimulated with oxygen.  We identified a proprietary mixed culture (MC) capable of aerobically degrading MTBE as well as the other common gasoline oxygenates, including TBA.  This mixed culture has been sustained and is currently being grown in commercial quantities for MTBE remediation purposes.

The soil alone had no activity for degrading MTBE, even though it was oxygenated.   Soil with mixed culture showed various rates of MTBE degradation depending on the amount of mixed culture applied.  Thus, using soil from a contaminated site we did not observe natural MTBE degraders, however, MTBE did degrade in soil amended with MC.

MTBE Remediation at Retail Gas Stations

The enhanced MTBE bioremediation process, BioRemedy, was applied at a retail gas station in California.  The site had a history of MTBE concentrations greater than 20 mg/L.  The tanks had been excavated along with surrounding soil, yet MTBE concentrations persisted downgradient of the former tanks and dispensers.  A biobarrier approach was implemented to reduce the MTBE concentrations.

Eighteen oxygen-injection well points were installed in a line along the downgradient property boundary and perpendicular to the flow direction. Existing subsurface information and pilot test results determined the location and depth of the wells. The oxygen points were installed at five-foot lateral spacings.  Mixed culture was injected at one-foot centers laterally and vertically throughout the saturated zone (~3-10 ft. bgs) along the same line as the oxygen points

MC was also added to an existing groundwater-extraction trench located about 10 feet upgradient of the biobarrier.  Since the trench already existed, we decided to add MC to enhance MTBE degradation. The trench was retrofitted with an oxygen supply system using the perforated drain pipe located in the base of the trench.  MC was injected using similar methodology as in the biobarrier (Fig. 3a).

The oxygen generation system consists of a commercial air compressor connected to an oxygen generator.  The output of the oxygen generator fills a 120-gallon storage tank, and that in turn is connected to various injection points through solenoids.  The sequencing of the unit is controlled electronically and has a telemetry system capable of diagnosing and communicating its operating conditions.

Rapid reduction of high MTBE concentration was accomplished using BioRemedy at this site.

Another BioRemedy remediation system is located at a retail site in Connecticut.  This site had MTBE concentrations as high as 100 mg/.  Pump and treat had been implemented at the site without success.  A biobarrier approach was chosen to curtail the offsite migration of dissolved MTBE. Pilot test data indicated the need for oxygen injection wells spaced approximately 5 ft. laterally and at two depths. Monitoring wells were placed upgradient and downgradient of the biobarrier to monitor performance.  MC was injected on two-foot centers both laterally and vertically across the site using direct push tools and specially designed injection tips and an injection pump. The oxygen system was improved to maximize oxygen distribution. Solenoids control the flow to individual wells.  The system is controlled by an electronic sequencer and has telemetry.  A fax is sent when an operational interruption occurs and the status of the system can be determined remotely via a modem connection.

Site Selection and Installation Process

BioRemedy is commercially available and may be an appropriate remediation technology for MTBE and TBA at a variety of sites.  The process of implementing BioRemedy at a site starts with a review of existing site assessment data.  Suitable sites generally are characterized by unconsolidated sediments, a constant or near constant groundwater flow direction, water level fluctuation less that 10 feet, and depth to groundwater less than 25 feet bgs.  Heterogeneous or low permeability soils may require a biobarrier-type trench. A pilot test may be performed to determine gas distribution.  Additional lithologic data and vertical delineation of the MTBE plume may also be necessary.

Oxygen-injection wells are usually installed prior to the microbe injection to allow for sufficient oxygenation of the subsurface.  Groundwater flow velocity and MTBE concentration determine the needed amount of microbes.  Microbes are shipped to the site and injected, generally using direct push methods and specialized injection equipment.

Summary

BioRemedy has been successful at controlling the offsite migration of MTBE plumes at retail gas stations.  BioRemedy offers several advantages over conventional MTBE remediation such as pump and treat.  BioRemedy is an in situ process, eliminating the need for handling water above ground.  No discharge permits or fees are necessary.  No air permits are required.  Since it is an in situ process, degradation occurs in the ground, and the system has a very low surface profile and visual impact.  The contaminants are truly destroyed not just changed to another phase.

BioRemedy is commercially available.  For more information check www.bioremedy.com.

Gerard E. Spinnler, Joseph P. Salanitro, Paul M. Maner and Karen A. Lyons all work at Westhollow Technology Center in Houston TX.  BioRemedy is marketed by Shell Global Solutions (US) of Houston, TX.

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