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| Resource Library > Technology Transfer > Programs and Initiatives > Enhanced In Situ Anaerobic Bioremediation > System Configurations |
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Enhanced in situ anaerobic bioremediation can be implemented to provide source area or dissolved plume treatment or containment, or a combination of
source area and dissolved plume remediation can be used. Enhanced bioremediation and conventional source treatment or containment approaches (e.g.,
chemical oxidation or groundwater extraction) will be subject to the same difficulties associated with mass transfer limitations of a continuing source and
preferential flow paths in heterogeneous formations. The single largest difference between conventional remedial technologies and enhanced bioremediation may
be that enhanced bioremediation, if properly implemented, can maintain effectiveness over a longer period of time. This is a cost-effective approach because
in many cases it may be necessary continue remediation over a long period of time due to the substantial challenges of significant contaminant source removal.
Typical system configurations and associated remedial action objectives which engineered anaerobic bioremediation may be used to address include the following:
System Configurations
Typical system configurations and associated remedial action objectives which engineered anaerobic bioremediation may be used to address include the following:
- Source Zone Treatment: Remediation of source zones where good substrate/contaminant contact is possible.
- Plume Containment using a Biologically Reactive Barrier: Reduction of mass flux from a source zone or across a specified boundary.
- Plume-Wide Restoration: Total treatment of an entire dissolved plume.
Anaerobic reductive dechlorination is a process which takes place in the aqueous phase and does not directly attack DNAPL sources (except perhaps indirectly by
enhanced dissolution). Therefore, enhanced bioremediation may be limited in its ability to treat DNAPL source zone areas. Plume-wide applications where
substrate is delivered to the entire plume may be cost-prohibitive for large plumes or cost-inefficient for low-level contaminant plumes. In such cases,
several approaches may be combined. For example, a small source area may be targeted for remediation using a grid configuration, combined with a linear
barrier configuration upgradient from a downgradient point of compliance (Figure 1).
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| Figure 1: Schematic of Source Area and Biobarrier Injection Configurations |
The appropriate application of enhanced anaerobic bioremediation will be site-specific and based on a strategy that takes into account final remedial
objectives, feasibility of the application, and regulatory issues. Ultimately however, there will be an economic limit to the size of a plume that can be
treated with a complete plume-wide application of enhanced bioremediation. For plume sizes greater than 10 to 20 acres, use of containment strategies combined
with other remedial approaches may be the only feasible way to attain regulatory compliance.
Source Zone Treatment
Enhanced in situ anaerobic bioremediation may be used to address source zones by either removing the source mass, or by limiting mass flux as a containment
measure. As discussed above, removal or destruction of a DNAPL source using enhanced bioremediation is a slow process. On the other hand, treatment to reduce
mass flux and at the same time to perhaps increase the rateof dissolution and treatment as compared to natural attenuation or pump and treat may be more
achievable. Fluid substrates can be injected directly into a source area using injection wells (Figure 2) or direct-push technology.
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| Figure 2. Direct Injection Configuration |
Enhanced in situ anaerobic bioremediation may be more effective than conventional pump and treat for source remediation due to the chemically and biologically
facilitated dissolution or desorption that results from addition of an organic substrate. For example, an increase in the rate of dissolution of DNAPL to the
aqueous phase may occur due the presence of a greater concentration gradient induced by degradation in the aqueous phase or by reductions in interfacial
tension of DNAPL in water. Injection of a low solubility, persistent carbon source such as vegetable oil into a source area may serve to reduce mass flux
and to effectively sequester the source. However, while degradation of dissolved constituents may be stimulated, this may not accelerate destruction of
DNAPL or sorbed source mass.
Plume Containment using Biologically Enhanced Barrier Systems
For large plumes having poorly defined, widely distributed, or inaccessible source areas, enhanced bioremediation systems may be configured as permeable
reactive barriers (biobarriers) to intercept and treat a contaminant plume. For example, biobarriers may be employed at a property boundary or upgradient
from a point of regulatory compliance in order to prevent plume migration to potential receptors. Biobarriers typically consist of either rows of substrate
injection wells or a solid-substrate trench located perpendicular to the direction of groundwater.
Passive biobarriers typically utilize slow-release, long-lasting substrates (e.g., HRC®, vegetable oils, or mulch) that can be either injected or
otherwise placed in a trench, and that are designed to remain in place for long periods of time in order to maintain the reaction zone. Contaminant mass is
delivered to the treatment zone via natural groundwater flow. Capital and operating costs for a passive biobarrier configuration are typically lower than for
plume-wide configurations because fewer injection locations are required. However, life-cycle costs could be significantly higher if the source of the CAHs
upgradient of the biobarrier is not addressed.
Semi-passive or active biobarriers are similar to passive biobarriers except that a soluble substrate is typically injected periodically (semi-passive) or a
recirculation system (active) is employed. Soluble substrates migrate with groundwater flow, are depleted more rapidly, and require frequent addition. However,
these systems offer the advantage of being able to adjust the rate or type of substrate loading over time, and soluble substrates may be easier to distribute
throughout larger volumes of the contaminant plume. Injection and/or extraction wells are typically installed in rows perpendicular to the groundwater flow
direction. Recirculation results in increased retention time for treatment, but the overall groundwater flux downgradient of the system does not change.
Plume-Wide Restoration
In situ enhanced bioremediation remedial systems may be configured to treat dissolved CAHs across an entire contaminant plume. Creating an anaerobic reaction
zone across broad areas of a plume is an aggressive approach that can reduce the overall time frame for remediation. Plume-wide delivery systems will typically
be configured as a large injection grid, or a recirculation well field may be employed to increase the effective area of substrate distribution. Higher initial
capital and operating costs of recirculation systems may be offset by shorter remedial time frames and resulting potential for lower monitoring and total
long-term operating costs.
For smaller sites (less than an acre), pilot testing can be used to define achievable field-scale degradation rates and radii of influence from injection
points. A grid of injection wells can then be installed across the entire footprint of the plume, and remediation completed perhaps over a period of 2 to 4
years. At sites where larger plumes are present (greater than several acres), or the depth of the plume makes installing injection wells difficult and
expensive, multiple treatment lines can be established perpendicular to the direction of groundwater flow, typically separated by 6 to 12 months of groundwater
travel time. A recirculation approach may not be practical or cost effective at a large scale due to the large volumes of groundwater to be processed and
ineffective in situ mixing in heterogeneous environments.
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