Researchers have long known that landfills produce methane, but had a hard time figuring out why – since landfills do not start out as a friendly environment for the organisms that produce methane. New research from North Carolina State University shows that one species of microbe is paving the way for other methane producers.
Specifically, the researchers found that an anaerobic bacterium called Methanosarcina barkeri appears to be the key microbe.
“Landfills receive a wide variety of solid waste, and that waste generally starts out with a fairly low pH level,” says Dr. Francis de los Reyes, an associate professor of civil engineering at NC State and co-author of a paper describing the research. “The low pH level makes it difficult for most methanogens – methane-producing organisms – to survive. We started this project in hopes of better understanding the mechanism that raises the pH level in landfills, fostering the growth of methanogens.”
What the researchers found was M. barkeri – a hearty methanogen that can survive at low pH levels. M. barkeri consumes the acids in its environment, producing methane and increasing the pH levels in its immediate area. This, in turn, makes that area more amenable for other methanogens.
As moisture leaches through the landfill, it disseminates those high pH levels – making other parts of the landfill habitable for M. barkeri and other methane-producing microbes. M. barkeri then moves in and repeats the process, leaving neutral pH levels – and healthy populations of other methanogens – in its wake.
Since M. barkeri and its methanogen cousins produce large quantities of methane, and methane is a powerful greenhouse gas, this could be bad news for the environment. But not necessarily. Methane can be, and often is, collected at landfill sites and used for power generation. Furthermore, methanogens break down solid waste as they go, compacting it so that it takes up less space.
“The research community can use our findings to explore ways of accelerating the methane-generation process,” de los Reyes says, “creating methane more quickly for power generation, and making additional room in the landfill for waste disposal.”
The paper, “Effect of Spatial Differences in Microbial Activity, pH, and Substrate Levels on Methanogenesis Initiation in Refuse,” will be published in the April issue of Applied and Environmental Microbiology. The paper was co-authored by Dr. Bryan Staley, who did the work while a Ph.D. student at NC State; de los Reyes; and Dr. Morton Barlaz, professor and department head of civil, construction and environmental engineering at NC State. The research was funded by Waste Management, Inc. and the Environmental Research and Education Foundation.
NC State’s Department of Civil, Construction and Environmental Engineering is part of the university’s College of Engineering.
Note to Editors: The study abstract follows.
“Effect of Spatial Differences in Microbial Activity, pH, and Substrate Levels on Methanogenesis Initiation in Refuse”
Authors: Bryan F. Staley, Environmental Research and Education Foundation; Francis L. de los Reyes III, Morton A. Barlaz, North Carolina State University
Published: April 2011, Applied and Environmental Microbiology
Abstract: The initiation of methanogenesis in refuse occurs under high volatile fatty acid (VFA) concentration and low pH (5.5 to 6.25), which generally are reported to inhibit methanogenic Archaea. One hypothesized mechanism for the initiation of methanogenesis in refuse decomposition is the presence of pH-neutral niches within the refuse that act as methanogenesis initiation centers. To provide experimental support for this mechanism, laboratory-scale landfill reactors were operated and destructively sampled when methanogenesis initiation was observed. The active bacterial and archaeal populations were evaluated using RNA clone libraries, RNA terminal restriction fragment length polymorphism (T-RFLP), and reverse transcription-quantitative PCR (RT-qPCR). Measurements from 81 core samples from vertical and horizontal sections of each reactor showed large spatial differences in refuse pH, moisture content, and VFA concentrations. No pH-neutral niches were observed prior to methanogenesis. RNA clone library results showed that active bacterial populations belonged mostly to Clostridiales, and that methanogenic Archaea activity at low pH was attributable to Methanosarcina barkeri. After methanogenesis began, pH-neutral conditions developed in high-moisture-content areas containing substantial populations of M. barkeri. These areas expanded with increasing methane production, forming a reaction front that advanced to low-pH areas. Despite low-pH conditions in >50% of the samples within the reactors, the leachate pH was neutral, indicating that it is not an accurate indicator of landfill microbial conditions. In the absence of pH-neutral niches, this study suggests that methanogens tolerant to low pH, such as M. barkeri, are required to overcome the low-pH, high-VFA conditions present during the anaerobic acid phase of refuse decomposition.