Environmental▼News Salty incinerator fly ash affects soil microbes’ activity
µg N2O-N/g of dry soil/day
(a) 2.0 1.5 1.0
0.9
0.88 0.62
0.5 0.0
W
(b)
BA Fly ash leachate leachate
2.0E-10 1.6E-10 1.5E-10 1.2E-10 1.0E-10
7.4E-11
5.0E-11 0.0E+00
W
BA Fly ash leachate leachate
(a) Denitrifying activity and (b) nitrogen fixation measured in bare soil samples supplemented with water (W), municipal solid waste incinerator (MSWI) bottom ash (BA) leachate, and MSWI fly ash leachate.
with MSWI fly ash leachate compared with soils to which only water was added. In addition, they observed a change in the overall composition of the bacterial community. They observed decreased activity both in the laboratory columns and in bare soil in the field, but not in soils in the field under permanent grass cover, presumably because grass layers protect soil bacteria against direct leachates better than bare soil, Jocteur-Monrozier says. In contrast, the researchers saw no change in denitrifying activity in soils supplemented with MSWI bottom ash leachates. They did, however, see a decrease in nitrogen fixation with the bottom ash leachate, although it was not as great as with the fly ash leachate.
448 A ■ ENVIRONMENTAL SCIENCE & TECHNOLOGY / DECEMBER 1, 2002
(ENVIRON. SCI. TECHNOL. 2002, 36, 4729–4734)
are important parts of the nitrogen cycle and changes in them may have consequences for the supply of nutrients to plants. After 30 days, the researchers saw a significant decrease in the denitrifying activity and nitrogen fixation of bacteria in soils supplemented
moles C2H4/g of dry soil/h
Although concerns about using municipal solid waste incinerator (MSWI) ash in construction materials have centered on heavy metals and organic chemicals, such as dioxins, leaching into aquatic environments when it rains, research in the November 15 issue of ES&T (pp. 4729–4734) suggests that high levels of salts in the ash may actually cause more of a problem, particularly for nitrogen-fixing and denitrifying bacteria in soils. MSWIs equipped with air pollution control devices produce two types of ash—bottom ash, which is ash that remains at the bottom of the incinerator, and fly ash, which is produced in the smokestack from processes used to clean the gases before they are released into the atmosphere. Bottom ash is typically considered safe for the environment; however, fly ash is rich in heavy metals and must be stabilized with cement or other hydraulic binders before it can be used in construction materials. To assess the impact of runoff from solidified/stabilized incinerator ash on soil bacteria, as part of a large program on solid-waste ecocompatibility set up by the French Agency for Energy and Environment Protection, Lucile Jocteur-Monrozier of Centre National de la Recherche Scientifique and colleagues at Université Claude Bernard Lyon 1, Université de Savoie, and Ecole Nationale des Travaux Publics de l’Etat in France, added MSWI bottom ash and stabilized fly ash leachates to soils in laboratory columns and in field plots, and monitored the resulting changes in bacterial activity and community composition. Both leachates contained high levels of salts, but relatively low levels of heavy metals. By analyzing gene pools, the researchers obtained information about the diversity of microorganisms in the soil communities. They then compared that information with changes in two key bacterial functions—denitrifying activity and nitrogen fixation. Both processes
The heavy metal content of both MSWI fly ash and bottom ash leachates is very low and cannot explain the reaction of the microorganisms, Jocteur-Monrozier says. “In the short term, the content of salts—sulfate, chloride, sodium, and calcium—appear to be more important in the contamination of the environment than the heavy metals,” she claims. “It may be true that the salts are a bigger problem than the heavy metals,” says Isabel Baur Keller of the Swiss Federal Institute for Environmental Science and Technology (EAWAG), who previously investigated the leaching behavior of cement-stabilized MSWI fly ash (Environ. Sci. Technol. 2001, 35, 2817–2822). In most cases, cement stabilization immobilizes heavy metals; however, it does not stabilize salts such as NaOH, KOH, or chloride salts, she says. Even so, it is difficult to generalize, because the solubility of metals and some salts is strongly dependent on pH, she adds. Still, many scientists believe that MSWI ash is safe to use in construction materials and may actually have environmental benefits. Researchers at Oregon State University (OSU) recently completed a project for the U.S. National Research Council, addressing the environmental impact of construction materials on surface and groundwaters. According to Jim Lundy of OSU, much of the research focused on waste-modified construction materials, including MSWI bottom ash. The environmental impacts were judged using biological indicators, including algal and daphnia toxicity tests. “Our findings suggest that when MSWI bottom ash is incorporated in an asphalt or portland cement concrete mix, there are no significant impacts,” Lundy says. However, the findings were based on a limited number of MSWI ashes and should not be extrapolated to include all MSWI ashes, he says. —BRITT E. ERICKSON
