Microorganisms used to cut toxins in groundwater

In 2002, Bruce Rittmann, director of the Biodesign Institute’s Center for Environmental Biotechnology, received a patent for an innovative way to use nature to lend society a hand. He invented a treatment system, called the membrane biofilm reactor (MBfR), which uses naturally occurring microorganisms to remove contaminants from water.

Now Rittmann and his research team, which includes Rosa Krajmalnik-Brown and Jinwook Chung, recently published a paper in the journal Environmental Science & Technology for a new application that removes a problematic contaminant that has made local headlines.

The chlorinated solvent trichloroethene (TCE) has been found to be an increasingly problematic contaminant in groundwater. The detection of TCE recently forced the shutdown of the water supply for the greater Phoenix metropolitan area municipalities of Paradise Valley and Scottsdale.

TCE has been used as a cleaning agent and solvent for many military, commercial and industrial applications. Its widespread use, along with its improper handling, storage, and disposal, has resulted in frequent detection of TCE in the groundwater. TCE has the potential to cause liver damage and malfunctions in the central nervous system, and it is considered a likely human carcinogen.

“As with other elements, the chlorine cycle is becoming a key concern to many environmental pollution scientists,” says Krajmalnik-Brown, a researcher in the Biodesign Institute’s Center for Environmental Biotechnology and an assistant professor in the Ira A. Fulton School of Engineering’s Department of Civil and Environmental Engineering.

Transforming the chlorinated solvent to a harmless product is the best way to eliminate the harmful effects of TCE. In the case of TCE, Mother Nature is the best helper. Scientists have discovered microorganisms that can replace the chlorine in the chlorinated molecules with hydrogen, a process called reductive dechlorination. While other methods are possible, they often are more costly, and many do not transform TCE into a harmless end product.

In the paper, the Rittmann team used the MBfR and a naturally occurring group of microorganisms able to remove TCE from water. These microorganisms, called dehalogenerators, have an affinity for chlorinated organics and can be found all throughout nature, even in clean water supplies, the soil and groundwater.

“These bacteria respire TCE – that is, they can use TCE like we use oxygen to breathe,” Krajmalnik-Brown says. “They take in the TCE and they start removing the chlorines, step by step. In the ideal case, the dehalogenators remove all the chlorines, converting TCE to ethene, which is harmless.”

With this knowledge in hand, the challenge for the research team was to adapt their existing MfBR system, which can remove other water contaminants, to see if it could now handle TCE. A version of the reactor that addresses perchlorate, a byproduct of rocket fuel, is already in the commercialization pipeline.

“A key challenge with using these bacteria is that, if they don’t dechlorinate all the way, the TCE can be converted to vinyl chloride, which is a known human carcinogen,” Krajmalnik-Brown says. “In other words, if you don’t have complete dechlorination, you can end up having something worse than what you started with. So it is critical to have the right mix of microorganisms.”

Their approach was simple in execution. They took an existing MBfR that was handling perchlorate removal and then introduced TCE into the system.

Rittmann’s MBfR works by delivering hydrogen gas to the bacteria through tiny hollow tubes submerged in water. In the right environment, the tubes become coated with a biofilm containing microorganisms. The system provides the microorganisms with hydrogen gas, which must be present for the microorganisms to change the chemical composition of a contaminant and render it harmless. Their results indicated that the MBfR could be an incredibly versatile system, quickly adapting to handle TCE.

By assessing the MBfR community, they found the special dehalogenating bacteria that can take the hydrogen supplied by the MBfR and reduce TCE all the way to harmless ethene. Using the latest molecular techniques, they could identify the bacterial population to handle TCE, and also the genes within these populations that make enzymes that detoxify TCE to ethene.

The team found one particular organism: a new type of Dehalococcoides, the bacteria known to dechlorinate TCE all the way to ethene. The group also was the first to grow these dehalogenating bacteria in a biofilm in the lab.