COLUMBUS, Ohio - Trash in a municipal landfill could decompose nearly 10 to 20 times faster than it normally does through a system that keeps the trash continuously wet, new research suggests.
Landfills are normally dry environments, and the lack of adequate moisture doesn't allow biodegradable trash to decompose as quickly as it should, say researchers at Ohio State University.
In fact, keeping a landfill saturated means it could stabilize in five to 10 years, instead of taking the average 100 years or longer to do so, said Ann Christy, an assistant professor of food, agricultural and biological engineering at Ohio State. In a stabilized landfill, the majority of trash has decomposed.
"Quicker decomposition rates mean more room for more trash in the same landfill, which would cut down on the need for additional landfill space," she said. "This also feeds into recycling - once the biodegradable material decomposes, we can extract recyclables from the landfills, then the landfills aren't filling up at as quickly."
Christy is currently experimenting with moisture levels in two laboratory-scale wet-tomb bioreactors. A wet-tomb bioreactor is a self-contained unit with water purposely pumped in - the water creates an environment suitable for bacteria to actively decompose waste. The water is recirculated throughout the system.
Christy's research appears in a recent issue of the journal Applied Engineering in Agriculture. She co-authored the study with Olli Tuovinen, a professor of microbiology at Ohio State, and Michael Myers, an engineer with the North Carolina Department of Environment and Natural Resources.
Christy and her colleagues monitored the experimental bioreactors for 15 months. Each bioreactor - or bin - was filled with approximately 3,300 pounds (1.5 metric tons) of non-shredded municipal solid waste collected from a local sanitary landfill. (While many small-scale landfills require waste to be shredded in order for it to fit, shredding is not economically feasible for a full-scale landfill, Christy said.) The bins were 3 feet long, 6 feet wide and 3 feet tall. The researchers could watch the decomposition through a 2-by-2.5 feet Plexiglas observation window installed in each bin.
The waste in one bin was covered with a single layer of sludge - sewage already decomposed by bacteria. Sludge has been used in landfills to help increase the rate at which trash decomposes. At the beginning of the study, researchers poured distilled water into each bin, until they saw the water draining into the liner under the bins. The used water - or leachate - was continuously pumped through the bins again via leachate recirculation pipes.
While the researchers did not get the decomposition results they had hoped for - the mass in the bin with the sludge layer decreased by 1 percent (to 3009 pounds) in 15 months, and the mass of the other bin decreased by 1.3 percent (to 2989 pounds) - they are confident that keeping a relatively high level of moisture in a landfill would increase the rate of decomposition, Christy said.
She attributed the lower-than-expected decomposition rates in this study to the lack of adequate amounts of bacteria and also the lack of heat production. Paper and plastic comprised 70 percent of the solid waste in these bins, while the two most readily biodegradable products, yard waste and food waste, comprised less than 5 percent of the total mass.
"The disproportionate amount of recyclables (paper and plastic) in the bins were undoubtedly a hindrance to the bacteria's success in breaking down the trash," Christy said. "In a full-size landfill, the types of trash would be more evenly distributed."
Christy is continuing the experiment, and says the next step is to take the technology to a full-scale landfill.
Because it is a self-contained system, constructing a full-scale wet-tomb bioreactor would be costlier at the outset, Christy said. Unlike current landfills, the bioreactors need the machinery, such as pumps and pipes, to recirculate leachate. But they would save money in the long run, because there would be no need to collect leachate and take it to a treatment facility, as is necessary with current landfills.