Combined ERH Processes
Electrical Resistance Heating for Heat Enhanced Bioremediation of Chlorinated Solvent Sites
As the most experienced Electrical Resistance Heating (ERH) Technology provider, Thermal Remediation Services, Inc. (TRS) has developed an extensive database on the effect of subsurface heating on the abiotic and biotic remediation of chlorinated solvents. This experience, data and “know how” has been gathered on over 50 of the estimated 65 ERH projects ever implemented.
The on-going remediation at Fort Lewis, Washington, is the largest and the most complex ERH remediation of non-aqueous phase liquids (NAPLs) performed by any ERH provider. At this site, TRS has captured the most extensive base line data set and we are capturing the most extensive operational data parameters for the evaluation of the impact of heat enhanced biotic and abiotic mechanisms for the remediation of chlorinated hydrocarbons ever gathered on an ERH project.
Figure 1. Effects of Heat on Petrophilic Bacteria Before and After ERH
TRS is taking this Heat Enhanced Bioremediation experience from the Fort Lewis project and the 50+ other TRS ERH site remediation projects and transferring this experience and “know how” on data collection, remediation monitoring, near-real-time decision making processes, hydraulic control, heat enhanced multi-phase extraction, and heat enhanced in situ destruction of TCE (both abiotic and biological) directly to other remediation projects.
Based on laboratory studies and TRS’s industry-leading work at other sites heating the saturated zone to 37°C is likely to increase the rate of anaerobic dechlorination by a factor of 3 to 8. TRS has also developed patent-pending synergies between ERH and bioremediation that can be used to further enhance bioremediation rates. Similarly, heating the vadose zone from 17°C to 37°C will double the contaminant recovery rate due to increased VOC vapor pressure.
TRS’s remediation design includes unique electrodes that have the capability of injecting or extracting groundwater and/or injecting or extracting air and injecting biological or chemical amendments to enhance degradation rates.
During the first applications of ERH, conventional wisdom was that the total indigenous bacteria population within an ERH treatment area suffered overwhelming losses as a result of increased subsurface temperature. However, as ERH was applied at an increasing number of sties, antidotal evidence began to emerge suggesting that in situ reactions, including biological reactions, could be a significant factor in VOC reduction.
Figure 2. Results of DNA Analysis at Temperature NAPL Area 1 Groundwater, Ft.
A limited data set from an early ERH pilot test on petroleum hydrocarbons showed that plate counts of indigenous petrophilic (hydrocarbon degrading) bacteria taken before and after ERH remediation contained more bacteria after heating than before. Samples taken from a depth of 8 to 10 feet bgs contained 100 times the number of bacteria three months after heating, while samples from 12 to 14 feet bgs contained 46 times the starting number (see Figure 1).
In order to further understand the effects of heat on indigenous bacteria, including the TCE degrading bacteria Dehalococcoides, groundwater samples were collected from Fort Lewis NAPL Area 1 in May 2004 at varying locations, depths and temperatures. The samples were collected from 5 wells across a range of temperatures and filtered for DNA analysis. Analyses included DNA extraction and quantification, quantitative PCR for Dehalococcoides spp. and T-RFLP. Figure 2 shows the results of the DNA analysis of the Dehalococcoides preferential to a temperature range of 30-60 °C.
Additional microbe identification, sampling, and testing are being conducted during the remediation of NAPL Area 2 and three to develop a better understanding of the effects of ERH as an enhancement to bioremediation.
After ERH remediation, a relatively clean source zone can be cooled and its heat transferred to surrounding regions by any of several methods:
- Air is sparged into the source area saturated zone. The saturated zone evaporatively cools and the warm, humid air is pulled up into the surrounding vadose zone by the action of the vapor recovery system. This enhances the VR system operation.
- Air is injected into the source area vadose zone. The vadose zone evaporatively cools and the warm, humid air is pulled laterally into the surrounding vadose zone by the action of the VR system. This enhances the VE system operation.
- Treated groundwater is injected into the remediated source area and flows horizontally to existing groundwater extraction wells. This cools the source zone to the optimal bioremediation temperature and likewise heats the surrounding saturated regions to enhance their bioremediation rates.
- Low VOC content hot groundwater is extracted from the treated source region and infiltrated into the surrounding regions. This warms the surrounding perched water to enhance its bioremediation rates.
- The pumped liquid streams can incorporate biological amendments to further enhance dechlorination rates.
Figure 3. Expected Subsurface Temperatures in a Contaminant
Source Zone and a Heat Enhanced Bioremediation Zone
The exact details of the extraction/injection modes, flow rates, and durations are developed during the design; however, all possible injection and recirculation is designed to maintain overall hydraulic and pneumatic control of a site. Expected source zone temperatures without cooling and conceptual temperatures of a thermally enhanced region are shown on the graph in Figure 3. Source area temperatures with cooling are not shown because they are dependent on flow rates and cooling mechanisms that will be determined during system design.
TRS uses multiple electrode designs during ERH remediation. Some electrodes are optimized to aid in the extraction of soil vapor or groundwater during ERH operation and some electrodes are optimized to aid in polishing the site and thermally enhancing the treatment of surrounding zones.