Keeping dioxins down in the dumps.

Keeping dioxins down in the dumps

When the contents of a household trash can--an unsavory melange that may include chicken bones and food scraps, empty cans and bottles, plastics and foils, worn clothing and rags, and lots of paper products carrying a wide range of inks and coatings--burn up in a municipal incinerator, the process creates hundreds of compounds, which get trapped in fly ash or escape into the air. This noxious mixture spewed out by incinerators includes about 200 compounds known as polychlorinated dibenzodioxins and polychlorinated dibenzofurans, many of which are toxic and some of which are potentially cancer-causing.

For the last decade, researchers throughout the world have been studying how dioxins and furans are generated and how to reduce the levels of these compounds in emissions from municipal incinerators. Because incineration is a major contributor of dioxins to the environment, fears of contamination have slowed the building of incinerators to solve urban garbage problems.

Two recent reports, however, show that incinerators can be operated under conditions that minimize dioxin and furan emissions and provide clues about the conditions under which dioxins form.

One study, conducted at an incinerator facility in Pittsfield, Mass., concerned the role of combustion in generating and destroying dioxins and furans. The research was initiated by the New York State Energy Research and Development Authority (NYSERDA) in Albany and supervised by members of the Dioxins Committee of the American Society of Mechanical Engineers (ASME), based in New York City.

The researchers looked at how a wide range of combustion conditions and refuse quality affected the amount of dioxins and furans formed and destroyed during combustion. They found that neither the amount of polyvinyl chloride (PVC) plastic found in trash nor the wetness of the garbage is related to the level of dioxins or furans produced under good combustion conditions. Some scientists had suspected PVC in trash as a major contributor to the formation of dioxins.

The level of carbon monoxide and the incinerator operating temperature, however, were found to be related to dioxin levels. According to the study, by monitoring carbon monoxide amounts or tracking temperature, incinerator operators can minimize dioxin production. For the Pittsfield plant, carbon monoxide levels had to be below 100 parts per million and the temperature between 1,500|F and 1,800|F.

"Minimizing conditions will be different for different plants,' says NYSERDA's Joseph R. Visalli, project manager for the incinerator test. "You have to know the incinerator, and you have to do some testing.'

This research and other studies also hint that burning garbage at temperatures higher than a certain level may be counterproductive. "There's been this feeling that the higher the temperature, the better off you are,' says Visalli. Higher combustion temperatures may, in fact, lead to greater carbon monoxide production because of incomplete combustion and to the vaporization of larger quantities of heavy metals, which become part of the fly ash and act as catalysts for the production of dioxins.

Chemist Francis W. Karasek of the University of Waterloo in Waterloo, Ontario, reports in the Aug. 14 SCIENCE that fly ash plays a major role in the production of dioxins. "We found that [fly ash] is indeed a very strong catalyst,' says Karasek, "which causes dioxins to form from almost anything.' That process seems to occur most readily after the fly ash has cooled to about 300|C, often in a pollution control device such as an electrostatic precipitator. The chief culprits are the heavy metals present in the fly ash particles.

Karasek suggests that one way to solve the dioxin emissions problem is to add a substance that "poisons' the catalyst, preventing it from contributing to the formation of dioxins. "We've been doing quite a bit of experimental work in which we are able to introduce compounds that render the fly ash completely inactive,' he says. "It's possible to completely block the formation of dioxins.'

Controlling combustion is still important, says Visalli. "Whatever pollution control you put on, you still want to minimize the amount [of pollutants] that goes into it,' he says. "The less that's produced and the more that's destroyed, the less you have to worry about it in the ash.'

Another alternative is to eliminate metals from garbage before it is incinerated. "We need to know more about where some of these chemicals are coming from in the waste,' says Visalli. Pigments in printing inks, for example, may contribute heavy metals. The Pittsfield researchers also found traces of dioxins (but not furans) in the garbage even before incineration. "There are a whole host of things that should be removed from refuse and recycled,' Visalli says.

Much remains to be learned. Still unclear is the role that metals play in the formation of compounds other than dioxins. More full-scale tests of incinerators should be done to confirm the Pittsfield results. And researchers need a better understanding of how to take samples and analyze them for dioxins so that the results are reliable and accurate.

Nevertheless, says Floyd Hasselriis of ASME's dioxin committee, the Pittsfield study "is the first clear road map [for incinerator operating conditions] that we've had. We really understand it pretty well now.'

"To me,' says Visalli, "the dioxin controversy, if it's done anything positive, has inspired research into learning how to better combust [garbage] and better control emissions. I think we're achieving that.'