Heat Enhanced In-Situ Alcoholysis for Reductive Bioremediation
Gary Birk, P.E., Managing Partner, Tersus Environmental, LLC
Anaerobic in-situ remediation has emerged in recent years as a viable and cost-effective remediation strategy for recalcitrant contaminants. The process is used to modify chemical, physical, and biological conditions in soil and groundwater to facilitate degradation of a broad range of contaminants under anaerobic conditions to harmless end products. Emulsified vegetable oil (EVO) is often used as an assimilable carbon source as it slowly ferments and can act in the subsurface as an organic carbon and hydrogen source that stimulates organohalide-respiring bacteria that in turn mineralize chlorinated solvents. Although emulsifying vegetable oil allowed overcoming limitations of pure vegetable oil injection and minimize field interventions by using a long-lasting electron donor, hundreds of EVO injection events over the past years has demonstrated that EVO effects are limited to the area in the immediate vicinity of the injection point. This is evident through low TOC values measured even tens of meters downgradient to injection points, and the TOC is composed predominantly of acetic acid, as opposed to a more diverse fatty acid composition. Another inconvenience of EVO injections is biofouling of the matrix and particularly screened injection wells. This phenomenon is typically attributed to biomass developing in the aerobic vicinity of injection wells due to hydrophobic oils creating a film (residual electron donor) that stimulates biomass growth. In many cases, biofouling or permeability losses could very well be attributed to geochemical incompatibilities between EVO and cations in the subsurface, or EVO’s intrinsically high retention to soils. Enhanced reductive bioremediation using In-Situ Alcoholysis overcomes two of the main challenges associated with EVO injection: poor fatty acid subsurface distribution and biofouling. The method is generally directed towards transesterification of vegetable oils to improve formation and distribution of slowly fermenting and soluble electron donors that are favorable to anaerobic reductive bioremediation. Recent field work in the US and Australia has demonstrated that In-Situ Alcoholysis reactions can within 90 days reduce over 90% of the contaminant mass. Because the typical aquifer temperature is about 15°C, adding heat further enhances this process as the reaction temperature significantly influences the transesterification reaction. Based on temperature, the time needed for the transesterification reactions drops to hours from the expected months. Heat also enhances the degradation rates. Löffler et al. 2013 identified an optimal range of 25-30°C for neutrophilic, strictly hydrogenotrophic Dhc strains. Heat sources include steam, electrical resistance heating, thermal conduction heating, gas thermal heating or residual heat from an in-situ thermal remediation project. Lower cost options are under development. This presentation will discuss methods of heating the amendment mixture and injecting hot water. Using hot water offers many advantages. In general, hot water dissolves fewer gases (like oxygen or carbon dioxide) but more solids (sugars) than cold water. Temperature modeling temperature at the injection of water heated to 90°C into 3 injection wells at a flow rate of 150 m³/d was able to maintain temperatures of greater than 30°C for 10 days in the vicinity of the injection wells. Remediation results presented will show the benefits of this approach.
Gary is a founder and Managing Partner of Tersus Environmental based in North Carolina. He has a bachelor’s degree in chemical engineering from North Carolina State University and holds registrations as a Professional Engineer in North Carolina, Virginia, and Florida. Gary has 39 years of experience in the development and implementation of technologies for remediation of contaminated soil and groundwater. In 2011, Gary commercialized EDS-ER™ the first and most widely used water-mixable vegetable oil based organic substrate to provide a lasting source of carbon and hydrogen for enhanced reductive dechlorination and other bioremediation processes. Gary has authored Design Tools for in-situ bioremediation that have advanced the state-of-the-practice in estimating substrate application rates. He also is an author of several U.S Patents for Remediation of Soil and Groundwater.