Estimate of EHC™ Longevity

When first brought to the market, Adventus hypothesized that EHC longevity was in the 3-5 year range for subsurface, saturated environments…

When EHC was first brought to the market, Adventus hypothesized that its longevity was likely in the range of three to five years in the subsurface, saturated environment. As the technology has been in use in the laboratory for approximately three years, and in the field for approximately two years, data are now available validate these initial estimates.

In short – EHC will last at least 5 years in most groundwater environments.

EHC Background: EHC is a patented^1,2 combination of controlled-release carbon and micro-scale (<45 micron) zero valent iron (ZVI) particles used for stimulating reductive dechlorination of otherwise persistent organic compounds in groundwater. Following placement of EHC into the saturated subsurface environment, a number of physical, chemical and microbiological processes combine to create very strong reducing conditions that stimulate rapid and complete dechlorination of organic solvents and other recalcitrant compounds.

There are at least three primary mechanisms through which EHC treats impacted groundwater:

1. enhanced bioremediation through the utilization of volatile fatty acids (VFAs) and hydrogen that are released from the controlled-release, hydrophilic complex solid organic carbon and the ZVI;
2. direct chemical reduction of the constituents by the ZVI particles via beta-elimination reactions; and
3. creation of enhanced thermodynamic conditions for more complete destabilization and destruction of oxidized compounds due to the very low localized redox potential (Eh) that is created by the combination of biological activity and iron corrosion.³

Given the composition of EHC, both the longevity of the controlled-release carbon, as well as the micro-scale ZVI particles are of interest. Unfortunately, it is somewhat difficult to separate the two given that they are always present together. However, some discussion of this is presented herein.

Estimate of Carbon/ZVI Longevity from Column Study #1

The longevity of EHC has been demonstrated in a long-term bench-scale study that evaluated the treatment of chloroethenes in groundwater. Approximately 3 years ago Adventus initiated a continuous-flow, laboratory column study to monitor the treatment of tetrachloroethylene (PCE) in groundwater. During the first phase of the study the influent PCE concentration was set at 16,000 ppb and the flow rate was maintained at 500 mL/day. This corresponded to a contact time of approximately 0.3 days. The second phase of the study was initiated on day 450 to evaluate the performance of EHC at a PCE concentration and groundwater flow rate that were more representative of typical field conditions. During the second phase of the testing the feed concentration was decreased to 1,600 ppb and the flow rate was reduced to 150 mL/day. This corresponded to a contact time of approximately 1 day. In all cases, the columns were operated at room temperature.

Volatile organic compound (VOC) concentrations were monitored in the influent and effluents over time. During the initial phase of the study where the influent PCE concentration was maintained at 16,000 ppb, the EHC column supported between 50 and 80% removal of total VOCs (Figure 1).

The VOC sampling on day 461 revealed a reduction in PCE from 1,499 ppb in the feed to 355 ppb in the EHC column. The PCE concentrations in the EHC column were further reduced (<50 ppb) in the four subsequent sampling events and PCE was non-detect (<20 ppb) on day 972. Trace concentrations of trichloroethylene (TCE) (<120 ppb) and cis-1,2-dichloroethene (cis-1,2-DCE) (<10 ppb) were detected during phase two.

After approximately 1,000 days of continuous-flow column operation, the EHC column reduced the PCE concentration from 1,180 ppb in the feed to <20 ppb, corresponding to 99% removal. The TCE and cis-1,2-DCE concentrations were non-detected (<10 ppb). The control system showed no change in the PCE concentration (1,210 ppb).

Being a hydrophilic substance, the physical sorptive capacity of the organic portion of EHC was estimated to be approximately 48 mg of PCE. Based on the initial PCE concentration of 16 mg/L and a flow rate of 500 mL/day, the adsorptive capacity of the column would have been exhausted after 6 days. Thus, the decrease in PCE concentrations after day 6 can be attributed to either chemical or biological transformations and not temporary physical sequestration (a common “removal”: mechanism exhibited by emulsified oils and other hydrophobic carbon sources).

Given that the EHC has been effective for approximately 2.8 years at room temperature supports at least the lower bound for the EHC longevity estimate. Assuming the carbon portion continues to be a significant contributor to the treatment, we would expect the longevity of the carbon to be greater at cooler groundwater temperatures because the rate of biological activity4, and thus consumption of the carbon, is expected to be lower. Using a temperature activity coefficient of 1.07 and a ten degree Celsius drop in temperature, the rate of biological activity would be cut in half. Thus, one can hypothesize that given the EHC has remained effective for 2.8 years at room temperature, it may remain effective for 5.6 years at cooler groundwater temperatures.

Estimate of Carbon Longevity from Column Study#2

A continuing evaluation of the ability of EHC to treat perchlorate in groundwater showed that greater than 99% removal continues to be achieved after 815 days of continuous-flow operations.(influent perchlorate concentration was approximately 100,000 ppb and the flow rate through the columns was maintained at 150 mL/day).

Following a short acclimation period, the EHC column reduced the perchlorate concentration from 120,000 ppb in the influent to 9,400 ppb, corresponding to a 92% removal (Figure 2). The perchlorate concentration was further reduced in the first soil microcosm to non-detect (detection limit = 200 ppb). This suggests that the removal mechanism for perchlorate is biologically controlled. The control system showed a slight (17%) decrease in the perchlorate concentration.

The most recent data from day 815 of the study show that the EHC column continues to support complete reduction of perchlorate (detection limit = 2 ppb) while the control column shows little, if any, reductions. If we again adjust the estimated longevity of the EHC to more typical groundwater temperatures, we would estimate the longevity to be at least 4.5 years. Given that perchlorate is being treated biologically, this is a reflection of the controlled-release carbon rather than the iron. The study is continuing, so the longevity estimate will increase assuming the column remains capable of treating the perchlorate.

Estimate of Carbon/ZVI Longevity from an Early PRB Field Project

Adventus supplied EHC for a site that has groundwater contaminated with a mixture of chlorinated solvents; primarily chloroethenes. Two areas of contamination were targeted for treatment. The first was a dissolved plume migrating from a source zone through a thin permeable soil unit overlying bedrock at approximately 12 feet below ground surface. The second was an open excavation containing groundwater, after contaminated soil had been removed.

A permeable reactive barrier (PRB) was constructed to intercept the dissolved groundwater plume (Figure 3). The PRB was constructed with an excavator by digging a 3-foot wide trench down to the top of the bedrock, followed by filling the bottom foot with a mixture of 10% EHC and sand. Only the bottom foot required filling because the groundwater table was less than a foot above the bedrock, and the permeable soil unit was also at this depth. The remainder of the PRB was backfilled with pea gravel.

Two pairs of upgradient and downgradient monitoring wells are being used to evaluate the effectiveness of the PRB. The data for the first upgradient-downgradient pair are provided in Figure 4, with the upgradient data on the left-hand side, and the downgradient data on the right-hand side. Large reductions of all of the constituents present continue to be observed after 21 months of field data. Data for the second well pairing is shown in Figure 5, with lower overall reductions potentially due to flow heterogeneity, but consistent performance over time. These results support a minimum field longevity of nearly two years, and this may increase as time passes and more data are collected.

Conclusions

The bench studies suggest that, with an estimated temperature correction, the predicted longevity of EHC in the field of three to five years is reasonable, and may even be an underestimate. Field data have shown a longevity of nearly two years, and counting. Certainly there will be many factors that can influence the actual longevity observed at a given site. Factors such as temperature, groundwater flow velocity, electron acceptor demand, mass applied, installation method, and inorganic chemistry are just a few of the factors that could affect the actual longevity. However, we can state with confidence that the EHC will last at least 5 years in most subsurface environments. As always, more data are generated, we will openly share all available information with you.

Figure 4: Influence of EHC on groundwater chloroethene concentrations for upgradient and downgradient well pairing #1.

Figure 5: Influence of EHC on groundwater chloroethene concentrations for upgradient and downgradient well pairing #2.

¹     Seech et al. May 1995. U.S. Patent 5,411,664. Method for Dehalogenation and Degradation of Halogenated Organic Contaminants.
²     Gillham R.W. Nov 1993. U.S. Patent 5,266,213. Cleaning Halogenated Contaminants from Groundwater.
³     Dolfing, J., van Eekert, M., and Mueller, J. 2006. Thermodynamics of Low Eh Reactions. Battelle’s Fifth International Conference on Remediation of Chlorinated and Recalcitrant Compounds. May 22-26, 2006, Monterey, California.
⁴     Tchobanoglous G., and Burton, F. L. 1991. Metcalf and Eddy, Inc. Wastewater Engineering, 3^rd Edition. Irwin McGraw-Hill.

EHC™ is a trademark of Adventus Intellectual Property Inc.

Interested In A Detailed Review Of Your Contaminant Removal Options? Contact Us For A Free Site Evaluation