Natural attenuation gets a boost
Microbes can now be relied on to clean up sites contaminated with chlorinated solvents.
The use of microbes for cleaning up chlorinated compounds has recently gained momentum, largely because of the growing use of mixed cultures of Dehalococcoides bacteria that can reliably transform chlorinated contaminants into nontoxic ethene. Researchers are currently trying to extend the technology to target carbon tetrachloride, chloroform, and trichloroethane.
Since 1999, the U.S. EPA has approved “monitored natural attenuation” as a remedy for many classes of contaminants but not for chlorinated solvents. That is because some naturally occurring microbes can transform chlorinated contaminants like perchloroethylene (PCE) and trichloroethylene (TCE) into vinyl chloride, which is a human carcinogen.
This is a serious concern because “there are probably tens of thousands of contaminated chlorinated solvents sites” in the world, estimates David Major, a principal in the Toronto offices of GeoSyntec. Chlorinated solvents are used in dry cleaning, by automotive service shops, in the fabrication of electronics and pharmaceutical products, and by the military.
Bioaugmentation, or the addition of microbes for remediation purposes, moved rapidly for chlorinated solvents because “there are not a lot of good alternatives out there... . People are willing to try even something that’s quite early on in the development stage,” explains Elizabeth Edwards of the University of Toronto’s department of chemical engineering and applied chemistry. “The entire application moved pretty quickly from the laboratory to the field,” agrees Frank Löffler of Georgia Institute of Technology’s school of civil and environmental engineering.
Both Edwards and Löffler played key roles in the technology’s speedy development. Edwards first discovered a consortium of microbes that naturally broke down PCE and TCE into ethene without accumulating any vinyl chloride more than a decade ago, with colleagues now at GeoSyntec. She named the consortium KB-1 “after my mother’s red pickup truck [Kick Butt] that surprisingly had a lot of ‘zip’—just like the culture.” In 2002, KB-1 became the first commercially available culture for bioaugmentation of chlorinated solvents.
A year later, Löffler and his colleagues were the first to report their success in isolating an organism, Dehalococcoides strain BAV1, capable of metabolizing vinyl chloride to produce ethene. The finding “meant that reductive dechlorination could be productive because organisms that use vinyl chloride and convert it to ethene exist. We could also show that [the Dehalococcoides BAV1] organisms were present at some sites but not at all sites. That’s the key information that was needed to show that at some sites bioaugmentation was needed to make this process work,” Löffler explains.
A few environmental engineering firms and commercial laboratories now offer tests to determine whether vinyl-chloride-metabolizing Dehalococcoides organisms are already present at sites with chlorinated contaminants. When they are, the sites are generally treated with “biostimulation”, which involves adding compounds that can serve as electron donors to inspire the microbes to go to work on the contaminants. Materials being used as biostimulants include emulsified vegetable oil, lactate, molasses, and chitin from shrimp exoskeletons.
When Dehalococcoides BAV1 is not found, cultures can be added, such as Shaw Environmental’s Shaw Dechlorinating Culture, SDC-9, the Bio-Dechlor Inoculum from Regenesis, and GeoSyntec’s KB-1. Once any of these cultures are added to sites with chlorinated contaminants, they “do really well—there’s something going on in the real world that makes them extremely happy,” Löffler says. “There are fewer failures with this technology than any other technology for the same group of contaminants,” he adds. Bioaugmentation also tends to cost less than the conventional alternatives of thermal stripping and in situ chemical oxidation, says Rob Steffan, director of Shaw Environmental’s biotechnology development and applications group.
All of these benefits have resulted in bioaugmentation’s use throughout the U.S. as well as in Denmark and Sweden. GeoSyntec has plans to apply the technology in the U.K. and Switzerland, says Phil Dennis, technology manager of the company’s SiREM division.
“Generally there are no regulations against the use of naturally occurring organisms in the U.S., [although] depending on the state, you may need a permit to bioaugment,” Major notes. “Regardless, you need to show regulators that what you are adding is beneficial, necessary, and safe.”
“It’s an interesting time because of a confluence of things that are happening,” Edwards summarizes. “We’re understanding more about the microbiology and biochemistry of this unique metabolism, and recently several Dehalococcoides genomes have been sequenced. We can now prove that specific bacteria—the ones we inject—are proliferating at a site, establishing beyond any doubt that the bacteria are doing the job... . At the same time, companies are getting more interested in trying new technologies.”
