Enhanced Anaerobic Bioremediation for Chlorinated Aliphatic Hydrocarbon Contaminants in Groundwater
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.