Enhanced Anaerobic Bioremediation for Chlorinated Aliphatic Hydrocarbon Contaminants in Groundwater

 

Dr. Al W. Bourquin

Camp Dresser & McKee, Inc.

 

            Enhanced anaerobic bioremediation (EAB) is the newest technology in our continuously evolving biological treatment for chlorinated solvent contaminated groundwater.  Developments in our understanding of the biodegradation of chlorinated aliphatic hydrocarbon compounds (CAHs) in the past five years now allows us the option to stimulate in-situ anaerobic biological dechlorination reactions. 

            EAB relies on the addition of sufficient organic compounds into contaminated groundwater and soil to induce highly reducing or methanogenic conditions necessary to achieve complete and rapid reductive dechlorination of many CAH compounds. Thus, naturally occurring biodegradation rates of CAH compounds are potentially enhanced by the addition of organic substrates (i.e., electron donors), particularly at sites where available electron donor is limited.  By inducing highly reducing conditions, EAB may improve both the extent and rate of biological dechlorination reactions leading to complete detoxification.

            A large number of field demonstrations and projects are being conducted using various organic substrates to enhance the reductive dechlorination of CAHs.  These systems involve the introduction of acetate, lactate, methanol, ethanol, or molasses into the aquifer as means of supplying electron donor necessary for driving dechlorination reactions. Recent work includes the successful addition of vegetable oils, compost and milk or cheese whey to groundwater as electron donor substrates. 

            The economics of EAB is somewhat dependent upon site groundwater having low levels of alternative competing electron acceptors (e.g., oxygen, sulfate, iron, and nitrate), such that a minimal addition of organic compound is necessary to achieve and sustain a highly reduced environment.  However, competing electron acceptors can be overcome through the addition of organic substrate.  The presence of indigenous microorganisms capable of the complete dechlorination of CAH compounds to innocuous ethene and ethane prevents the accumulation of toxic intermediates. The complete dechlorination process is complex involving multiple species of organisms at different redox conditions.  However, any microbial limitation can be overcome through the addition of specific cultures that are many times lacking from natural sites. 

Recent advances (past year) in EAB technology indicate the injection of high concentrations of soluble organic compounds (e.g.- lactate) combined with enhanced microbial activity causes an increase in dissolved contaminant concentrations.  This increased soluble contaminant phase increases the potential for reductive dechlorination and subsequently reduces sorbed contaminant mass (i.e.- DNAPL).  This process is critically important in reducing the time to meet remediation goals and costs.

            In summary, EAB of CAH compounds is a very effective remediation technology that has been proven in over three-hundred field sites. CDM has a number of systems in the field with excellent results.  Substrates used by CDM are the most prevalent in the industry and include lactate and Hydrogen Release Compound (HRC®).  This presentation will present basic information critical for implementation and the mechanisms involved.  Data from field sites will be used as examples.