Environmental and Spatial Factors Affecting Microbial Ecology and Metabolic Activity During the Initiation of Methanogenesis in Solid Waste
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Date
2009-08-10
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Abstract
Anaerobic decomposition of organic matter occurs in both natural (e.g., soil, peat bogs, digestive tracts) and engineered (e.g. landfills, anaerobic digesters) ecosystems. The primary end-products of anaerobic decomposition are methane (CH4) and carbon dioxide (CO2). Upon landfilling, rapidly degradable materials within the refuse anaerobically decompose resulting in an accumulation of volatile fatty acids (VFAs) and a commensurate drop in pH to a minimum ranging between 5.5 and 6. These low pH, high carboxylic acid conditions have been shown as inhibitory to methanogenic Archaea in analogous ecosystems such as peat and the rumen. In contrast to these findings, methanogenesis initiation occurs under these conditions, indicating the mechanism by which methane production begins in refuse is poorly understood.
There are two theories for how methane production initiates in landfills. One is that methanogenic Archaea (i.e. methanogens) tolerant to the low pH, high VFA conditions consume acids until the bulk pH is suitable for the establishment of methanogens that grow under pH-neutral conditions. The second theory is that spatially isolated areas of neutral pH exist while bulk pH is acidic and these localized regions of neutral pH act as initiation centers for methanogenesis. The goal of this study was to test these two theories and validate their importance relative to methanogenesis initiation in refuse.
To evaluate methanogen acid tolerance in decomposing refuse, three liquid inocula were derived: (1) refuse just entering active decomposition, (2) well-decomposed refuse and, (3) peat. Under high VFA concentrations, results showed methanogenesis initiation occurred at pH minima of 6.25, 5.75 and 5 for actively decomposing refuse, well-decomposed refuse and peat, respectively. The hydrogenotrophic Methanoculleus genus facilitated methane initiation in actively decomposing refuse (pH 6.25) while Methanosarcina triggered methane production in well-decompose refuse (pH 5.75). In peat, methanogenesis was facilitated by an uncultured Methanosarcinales. This is the first study to fully characterize methanogens responsible for methane initiation under low pH, high VFA conditions and suggests acid tolerance (pH 5 – 6.25) is relatively common provided sufficient acclimation time. However, methane production rates at lower pH were found to be 3 to 6 fold lower than those at neutral pH.
To evaluate the spatial influences on methanogenesis initiation, fresh refuse was placed into triplicate laboratory scale reactors, decomposed to the anaerobic acid phase, and destructively sampled when methanogenesis initiated. Large differences were observed spatially in refuse pH, moisture content and VFA concentration. No pH neutral niches were observed in reactors prior to methanogenesis. RNA clone library results showed most bacterial activity was attributed to the Clostridiales order. Methanogenic Archaea activity at low pH was catalyzed by Methanosarcina barkeri. After methanogenesis, pH neutral conditions developed in high moisture content areas containing substantial populations of M. barkeri. These areas expanded with increasing methane production, forming a unified reaction front that advanced into low pH areas. In the absence of pH neutral niches, this study suggests methanogens tolerant to low pH, such as M. barkeri, are required to overcome the low pH, high VFA conditions typically present during the anaerobic acid phase of refuse decomposition.
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Keywords
landfill, refuse, microbial ecology, T-RFLP, cloning, methanogenesis, decomposition
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Degree
PhD
Discipline
Civil Engineering
