Fact sheet: Reductive Dechlorination - Biological process - in situ

From: Public Services and Procurement Canada

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Description

Reductive dechlorination is a biologically based in situ remediation technology used for the treatment of soil and groundwater contaminated with chlorinated organic compounds. This technology consists of anaerobic biodegradation of chlorinated compounds. The implementation of this technology requires the injection of an organic substrate (electron donor) into the contaminated media to stimulate microbial growth and generate hydrogen through fermentation reactions. Nutrients sometimes must also be injected in order to meet nitrogen and phosphorus requirements of the microorganisms involved.

During reductive dechlorination, the chlorine(s) on the contaminant is (are) replaced by a hydrogen molecule. The hydrogen molecule is generated by the fermentation of a naturally occurring or of an imported electron donor, including lactate, acetate, vegetable oil, methanol and molasses. Hydrogen can also be supplied directly.

The reaction may be performed using chlorinated organic compounds (the contaminant) as either a primary substrate (dehalorespiration) or by cometabolism. When the chlorinated organic compound is the primary substrate, it is used as an electron acceptor to generate energy and act as a carbon source.

Reductive dechlorination by cometabolism is the reduction of a chlorinated organic compound by an enzyme or co-factor produced during the metabolism or another substrate. Since the contaminant reduction reaction doesn’t directly benefit microorganisms, a primary substrate must be present in the environment as a carbon and energy source. As contaminant is not the stimulator of microbial activity, cometabolism allows the remediation of low contaminant concentrations, thus achieving undetectable concentrations.

The transformation of chlorinated organic compounds by cometabolism is more common and is more effective in the transformation of highly chlorinated organic compounds, which explains why the resulting less-chlorinated compounds tend to accumulate in the contaminated matrix. Dehalorespiration is more effective in the biodegradation of less-chlorinated organic compounds. In both cases, reductive dechlorination produce intermediate metabolites (less chlorinated compounds) and a localized increase in chloride concentrations.

Implementation of the technology

Projects implementation may include:

  • Bench scale, pilot scale or laboratory studies
  • Mobilization, site access and temporary facilities
  • Reagent delivery, which could entail such measures as:
    • injection well installation
    • infiltration trench/drain construction
    • injection or infiltration of aqueous treatment solutions
    • injection of substrates in liquid, slurry or solid forms
    • injection of gasses below the water table
    • deep soil mixing with solid or slurry reagents
    • groundwater extraction, amendment and re-injection
  • Monitoring
  • Decommissioning of injection equipment

Injected materials vary widely according to contaminant, general groundwater composition and practitioner. Common generic compounds include urea (as a nitrogen source), ammonium nitrate (as a nitrogen source), lactate, acetate, methanol, ethanol, vegetable oils, etc.

Materials and storage

  • On-site storage is primarily a function of the compounds being applied to the groundwater systems and the manner of application
  • Projects using periodic injections of material may bring materials to the site on an as-needed basis and avoid on-site storage
  • Injected materials vary widely according to contaminant, general groundwater composition and practitioner. Common generic compounds include urea (as a nitrogen source), ammonium nitrate (as a nitrogen source), lactate, acetate, methanol, ethanol, vegetable oils, etc.

Waste and Discharges

  • If treatment is successful, the primary residual is microbial biomass (which decays over time). Excess reagent typically cannot be recovered, and is generally consumed in situ
  • System installation typically requires drilling or excavating in contaminated areas, resulting in the handling and disposal of contaminated soils, typically disposed of off-site

Recommended analyses for detailed characterization

Biological analysis

  • Total heterotrophic and specific bacterial counts (according to the contaminants of interest)

Chemical analysis

  • pH
  • Oxidation reduction potential (Eh)
  • Organic carbon content
  • Metals concentrations
  • Chloride concentrations (chlorinated solvents)
  • Metabolite concentrations
  • Contaminant concentrations present in the following phases:
    • adsorbed
    • dissolved
    • free
  • Nutrient concentrations including:
    • ammonia nitrogen
    • total Kjeldahl nitrogen
    • nitrates
    • nitrites
    • total phosphorus
  • Electron acceptor concentrations/reaction by-products including:
    • dissolved oxygen
    • nitrate
    • sulfate
    • ferric and ferrous iron
    • methane
    • dissolved manganese

Physical analysis

  • Dissolved oxygen concentration
  • Temperature
  • Soil water content
  • Soil granulometry
  • Evaluation of biological conditions and ecological factors

Recommended trials for detailed characterization

Biological trials

  • Microcosm mineralization trial

Hydrogeological trials

  • Tracer tests

Notes:

Tests examining the effect of temperature change on hydraulic conductivity and establishing the zone of freezing with a pilot scale tubing system are recommended to properly design the full-scale containment system.

Other information recommended for detailed characterization

Phase II

  • Contaminant delineation (area and depth)
  • Presence of receptors:
    • presence of potential environmental receptors
    • presence of above and below ground infrastructure
    • the risk of off-site migration

Phase III

  • Soil stratigraphy
  • Identification of preferential pathways for contaminant migration
  • Conceptual site model with hydrogeological and geochemical inputs
  • Characterization of the hydrogeological system including:
    • the direction and speed of the groundwater flow
    • the hydraulic conductivity
    • the seasonal fluctuations
    • the hydraulic gradient

Applications

  • Allows treatment of residual and dissolved contamination within saturated zones
  • Applies to chlorinated organic contaminants
  • Hydrogeological conditions must allow adequate distribution of the substrate
  • Soil must be sufficiently permeable and homogeneous to allow efficient distribution of the substrate and nutrients

Applications to sites in northern regions

Reductive dechlorination, using campaign-based injection is potentially applicable to remote northern sites where impediments to material transport and injection equipment mobilization can be overcome. Extreme cold can hamper biodegradation and volatilization in shallow material, but deep soil temperatures (below permafrost) are relatively constant over the course of the year.

Treatment type

Treatment type
Treatment typeApplies or Does not apply
In situ
Applies
Ex situ
Does not apply
Biological
Applies
Chemical
Does not exist
Control
Does not exist
Dissolved contamination
Applies
Free Phase
Does not exist
Physical
Does not exist
Residual contamination
Applies
Resorption
Applies
Thermal
Does not exist

State of technology

State of technology
State of technologyExist or Does not exist
Testing
Exist
Commercialization
Exist

Target contaminants

Target contaminantsApplies, Does not apply or With restrictions
Aliphatic chlorinated hydrocarbons
Applies
Chlorobenzenes
With restrictions
Explosives
Does not apply
Metals
Does not apply
Monocyclic aromatic hydrocarbons
Does not apply
Non metalic inorganic compounds
Does not apply
Pesticides
With restrictions
Petroleum hydrocarbons
Does not apply
Phenolic compounds
With restrictions
Policyclic aromatic hydrocarbons
Does not apply
Polychlorinated biphenyls
With restrictions

Treatment time

Treatment time
Treatment timeApplies or Does not apply
Less than 1 year
Does not apply
1 to 3 years
Applies
3 to 5 years
Applies
More than 5 years
Applies

Long-term considerations (following remediation work)

Follow-up monitoring may be required to verify that the remediation objectives as well as applicable regulations are met once the groundwater system normalizes; after stimulation is withdrawn and excess biomass dies off. 

Secondary by-products and/or metabolites

Reductive dechlorination of certain chlorinated aliphatic hydrocarbons could produce metabolites of greater concern than the parent compound. The formation of chloroethane or vinyl chloride may warrant the use of an aerobic biostimulation step. Bench scale and/or pilot testing, as well as strict quality control for injected materials, are typically required. 

Limitations and Undesirable Effects of the Technology

  • Saturated zone treatment only
  • NAPL must be removed before apply reductive dechlorination technology
  • Soil with low permeability can limit the application of the technology. Ideally, the hydraulic conductivity must be above 10-4 cm/sec
  • Fractures and a heterogeneous soil matrix may create preferential pathways which prevent homogeneous injections
  • High contaminant concentrations may inhibit reductive dechlorination
  • High metal concentrations may inhibit reductive dechlorination
  • Low nutrient concentrations can limit contaminant biodegradation
  • The soil pH should be between 6 and 8
  • Understanding and monitoring of reductive dechlorination processes can be complex
  • The application of the technology requires a detailed characterization of the contaminated site, which increases treatment costs
  • By design, reductive dechlorination will have large effects on parameters like oxidation-reduction potential, pH and total organic carbon
  • Injected fluids may displace (or “push”) contaminated pore water ahead of the injection front, leading to short-lived but dramatic changes in the distribution of groundwater contamination
  • Treatment is relatively passive: accident scenarios are relatively limited (with the exception of systems employing toxic co-metabolites)
  • If the treatment areas experience accident or upset conditions, contaminated groundwater may escape untreated

Complementary technologies that improve treatment effectiveness

Bioaugmentation may be required if there is an insufficient number of dehalogenators present. Fracturing injection methods or soil mixing could enhance substrate delivery in the subsurface.

Required secondary treatments

Because by-products and metabolites produced during reductive dechlorination are in general more easily biodegraded under aerobic conditions (e.g. chloroethane or vinyl chloride), aerobic biostimulation sometimes follows reductive dechlorination.

Application examples

Application examples are available at these addresses:

Performance

The treatment time for site remediation using reductive dechlorination varies according to the contaminant types and properties, the indigenous microbial population and the physical and chemical characteristics of the site.

A large variety of different substrate can be used including proprietary compounds. Performance can vary significantly with the type of substrate used.

Measures to improve sustainability or promote ecological remediation

  • Schedule optimization for resource sharing and fewer days of mobilization.
  •  Use of renewable energy and energy efficient equipment (e.g., geothermal or solar energy for reagent delivery)
  • Select substrates and amendments with lower energy requirement for production
  • Encourage substrate and amendment supply by local producers
  • Use of groundwater recirculation to maximize the use of amendments and lower number of injection wells
  • Use of rainfall for amendments/nutrients mix for injection
  • Use of recyclable bulk solution containers

Potential impacts of the application of the technology on human health

Non disponible pour cette fiche

References

Author and update

Composed by : Karine Drouin, M.Sc., National Research Council

Updated by : Karine Drouin, M.Sc., National Research Council

Updated Date : April 1, 2008

Latest update provided by : Marianne Brien, P.Eng., Christian Gosselin, P.Eng., M.Eng., Golder Associés Ltée

Updated Date : March 31, 2018

Version:
1.2.1