New device removes drinking water contaminants

A Northwestern University environmental engineer has received a U.S. patent for a treatment device that renders perchlorate — a thyroid-damaging ingredient of rocket fuel and a drinking water problem — harmless. The applications extend beyond the safety of drinking water and this one pollutant.

Bruce E. Rittmann, John Evans Professor of Environmental Engineering at the Robert R. McCormick School of Engineering and Applied Science, received U.S. Patent No. 6,387,262 for a hollow-fiber membrane biofilm reactor, that, through a natural biochemical process of electron transfer, turns perchlorate into innocuous chloride.

The cost-effective and environmentally friendly system also works on nitrate, a contaminant from agricultural fertilizers that can cause methemoglobinemia, or blue-baby syndrome, in infants, and is expected to be successful with other oxidized pollutants, such as bromate, selenate, heavy metals, radionuclides, and a range of chlorinated solvents, including trichloroethylene, a problem in the semiconductor industry.

Currently there is no effective clean-up solution for perchlorate, which was discovered in the water supplies of a large number of states in the late 1990s, and existing methods are not always successful when dealing with other contaminants.

“Many emerging pollutants are difficult to treat with conventional methods,” said Rittmann. “These methods do not destroy the contaminants but simply move them from place to place, from the water to a solid resin to a nasty brine that still contains the contaminants. Our simple method, which destroys the contaminant, should work for almost every oxidized pollutant, which means it has an incredible range of applications, including being used on more than drinking water.”

Rittmann has teamed up with the environmental engineering firm Montgomery-Watson-Harza Engineers, Inc. to conduct a pilot study in La Puenta, Calif., treating groundwater that is highly contaminated with perchlorate and nitrate. Results have shown that the biofilm reactor can effectively treat 0.3 gallons of water per minute, removing perchlorate and nitrate at the same time.

The decontamination process takes advantage of a community of microorganisms that lives as a biofilm on the outer surface of the membranes in the system. The microorganisms, found naturally, act as catalysts for the transfer of electrons from hydrogen gas to the oxidized contaminant, such as perchlorate or nitrate. Chemically speaking, the oxidized contaminants are eager to receive electrons, which reduces them to harmless products. The hydrogen gas supplies the electrons, and the biofilm microorganisms are the agents for the transfer.

A bundle of 7,000 hollow-fiber membranes are in one of the pilot-study biofilm reactors, a column approximately 5 feet tall and 18 inches in diameter. Each membrane is like a long, very thin straw, only 280 micrometers in diameter (the width of a thick sewing thread). Hydrogen gas is fed to the inside of the membrane fibers, and the hydrogen diffuses through the membrane walls into the contaminated water that flows past the fibers. At this meeting point, on the outside of the membrane, bacteria attach to the surface because they gain energy from the process of transferring electrons and can grow and thrive. The contaminants are reduced to harmless end products — perchlorate to chloride and nitrate to nitrogen gas — while the hydrogen gas is oxidized to water.

“We are exploiting nature,” said Rittmann. “Life is all about transferring electrons. We have an extraordinarily efficient system for bringing hydrogen and its electrons to oxidized pollutants, such as perchlorate, and reducing them to innocuous substances.”

Hydrogen gas is an ideal electron donor for biological drinking water treatment as it is non-toxic and inexpensive, and Rittmann’s system has been shown to be safe. Another advantage is that the performance of the reactor can be controlled simply by adjusting the pressure of the hydrogen gas.

Rittmann also is conducting research on the microbial ecology of the bioreactor system in order to understand how it works. Which microorganisms are doing the work? How fast do they work? How do they achieve the essential reaction of electron transfer?

“By looking at the details of what’s going on in the biofilms, we can make the system even more reliable and efficient in cleaning up some of the most dangerous and newly discovered contaminants in drinking water, ground water and wastewater,” said Rittmann.

The current research is supported by a grant from the U.S. Environmental Protection Agency and administered by the American Water Works Association Research Foundation.