The best way to clean up most contaminants --- even the most toxic pollutants such as PCBs or dynamite --- is to let microbes that are already there do the job for you, according to a Northwestern University researcher.
The microbes usually have to be fed, and the site has to be carefully monitored, but these microscopic one-celled creatures can do an effective job of breaking down the pollutants into harmless by-products, according to Bruce Rittmann, John Evans Professor of Environmental Engineering at Northwestern.
Rittmann detailed the chemical processes involved in breaking down a wide variety of pollutants on Wednesday, Nov. 6 at the Eco-Informa '96 conference on Global Networks for Environmental Information at Lake Buena Vista, Fla.
"Microbes get a lot of bad press," Rittmann said, "but the fact is the vast majority of microbes are 'good microbes' that help sustain the earth's ecosystem."
In most cases the local microbes have to be fed and watered, much like growing plants, Rittmann said. In one example of a process called "in situ (or in place) bioremediation," air from a compressor is pumped into the groundwater under the spill and supplies the microbes with oxygen. At the same time, the nutrients nitrogen and phosphorus can be fed into the contaminated soil to stimulate growth of the bateria. If the spill is above the water table, water is added also to keep the microbes moist. The well-supplied microbes than go about feeding on the contaminant, breaking it down into harmless byproducts.
From a chemical point of view, what is happening is that the microbes break chemical bonds in the contaminant and transfer electrons to an "electron acceptor," such as oxygen. This process gives off energy, which the microbes use, with carbon, to build new cells.
Rittmann described case studies of three different kinds of bioremediation.
1) Denver, Colo., 1989. A temporary holding tank under a garage used to service vehicles had leaked crankcase oil, diesel fuel and gasoline into the groundwater below. The ground water was treated by circulating oxygen, phosphorus, and nitrogen through the area. After three years of treatment, tests showed that the plume of contaminants had been virtually eliminated.
2) Bemidji, Minn., 1979. An oil pipeline burst open, spilling 100,000 gallons of crude oil into the surrounding soil and ground water. In the years following the spill, portions of the crude oil dissolved in the flowing ground water and moved 200 meters from the site. The undissolved crude oil moved 30 meters in the direction of ground water flow, and crude oil vapors moved 100 meters in the soil. But detailed monitoring since than has determined that the containment plume has not moved since 1987, and that the oil was slowly disappearing. This process is called "intrinsic bioremediation," since there was no intervention in the natural process.
3) Moffett Naval Air Station, Calif., 1989. Researchers from Stanford University conducted a field demonstration of in situ bioremediation of chlorinated solvents. These solvents cannot support microbial growth on their own, but if supplied with methane, a special class of organisms can destroy the contaminants through a process known as "cometablism," in which enzymes involved in normal microbe metabolism break down the solvents. In this case the researchers added oxygen and methane, and most of the solvents were destroyed.
Rittmann noted that in all three cases, tests were conducted to determine that the bioremediation had in fact taken place. There was a loss of contaminants; laboratory tests demonstrated that the microbes had the potential to degrade the contaminants; and evidence was gathered that determined that this process was in fact happening on the site, he said. These three criteria must be met to determine that in situ bioremediation is taking place, he stressed.
Rittmann said that, in most cases, bioremediation is safer and cheaper than alternative treatments, such as incineration or burial in special landfills.
Rittmann did much of the basic research that has led to the establishment of theDepartment of Energy's Natural and Accelerated Bioremediation Research program, according to Jay Grimes, co- manager of the program.
"The main obstacle to bioremediation right now is the question of public acceptance," Grimes said. He said he was optimistic that this approach would help solve some of the Department of Energy's most intractable problems, such as the Hanford site in Washington State and the toxic waste problems at Oak Ridge National Laboratories, Tenn.