Thermal Remediation Coupled with Bioremediation

In situ thermal remediation (ISTR) is an aggressive and well-established technology for contaminant mass removal, particularly in source areas with residual non-aqueous phase liquids (NAPL) and adsorbed contaminant mass. The elevated temperatures associated with ISTR accelerate contaminant dissolution, desorption, volatilization, and removal as well as increase abiotic degradation rates. ISTR is usually part of a combined treatment approach that includes bioremediation. A few ways bioremediation can complement thermal treatments are described below:

High Temperature ISTR Combined with Downgradient Bioremediation: An ISTR system operates at elevated temperatures (>100°C) in the source area to optimize contaminant removal and abiotic degradation. During ISTR, biodegradation is the primary treatment mechanism for the downgradient dissolved plume and may be enhanced by moderate heating.
Low Temperature Heat Enhanced Bioremediation: Electrical resistance heating (ERH) or thermal conduction heating (TCH) operates at temperatures of 30 to 40°C to maximize biodegradation rates along with contaminant desorption.
MNA or Bio-Polishing: Following shut-down of source zone ISTR, MNA or bio-polishing is commonly employed to reach closure levels. Heating subsurface sediments to temperatures of >100°C has been shown to significantly increase the release of dissolved organic matter, which in turn may serve as electron donors promoting microbial activity after ISTR is complete and the groundwater has cooled.

Molecular Biological Tools for Thermal Remediation

QuantArray®-Chlor

QuantArray®-Petro

CENSUS® (qPCR) ER

Thermal Treatment and MBTs

When assessing any treatment approach, examining multiple lines of evidence, including site chemistry, geochemistry, and microbiology, provides the most complete picture. A direct line of microbial evidence is quantification of the concentrations of contaminant-degrading microorganisms present in the subsurface. For example, CENSUS^® qPCR and QuantArray^®-Chlor have become an important component of performance monitoring for sites impacted by chlorinated solvents. Similarly, CENSUS^® qPCR and QuantArray^®-Petro are routinely used to quantify functional genes responsible for the aerobic and anaerobic biodegradation of BTEX, naphthalene, and other petroleum hydrocarbons.

Frequently asked questions

Do the high temperatures associated with thermal remediation limit microbial activity?

Yes, high temperatures can limit microbial growth and activity in the treatment zone during operation. However, microorganisms will persist in subsurface strata that do not reach the extreme temperatures, and recent evidence suggests that moderate heating (30–40°C) may actually enhance biodegradation. The influx of groundwater flow may also transport microbial populations from upgradient areas as temperatures decrease post-treatment.

Is it necessary to collect baseline microbial samples before starting the thermal treatment?

Since treatment outside of the source area often relies on biodegradation, collection of baseline microbial samples from downgradient wells is highly recommended. Microbial analysis of baseline samples from the ISTR zone can be useful for evaluating a return to pre-treatment conditions and the potential for biodegradation after shut-down

Resources for THERMAL REMEDIATION

Tech Bulletin – Thermal Treatment and MBTs

Electrical Resistance Heating Webinar

Heat Enhanced Biodegradation Webinar

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