Landfill gas contains numerous speciated organic compounds (SOCs) including alkanes, aromatics, chlorinated aliphatic hydrocarbons, alcohols, ketones, terpenes, chlorofluoro compounds, and siloxanes. The source, rate and extent of release of these compounds are poorly understood. The objective of this study was to characterize the release of SOCs and the regulated parameter, non-methane organic compounds (NMOCs) during the decomposition of residential refuse and its major biodegradable components [paper (P), yard waste (YW), food waste (FW)]. Work was conducted under anaerobic conditions in 8-L reactors operated to maximize decomposition. Refuse and YW were also tested under aerobic conditions. NMOC release during anaerobic decomposition of refuse, P, YW, and FW was 0.151, 0.016, 0.038, and 0.221 mg-C dry g(-1), respectively, while release during aerobic decomposition of refuse and YW was 0.282 and 0.236 mg-C dry g(-1), respectively. The highest NMOC release was measured under abiotic conditions (3.01 mg-C dry g(-1)), suggesting the importance of gas stripping. NMOC release was faster than CH4 production in all treatments. Terpenes and ketones accounted for 32-96% of SOC release in each treatment, while volatile fatty acids were not a significant contributor. Release in aerobic systems points to the potential importance of composting plants as an emissions source.

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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.

Refuse decomposition in landfills is a microbially mediated process that occurs primarily under anaerobic conditions. Because of limited moisture conditions, hydraulic transport as a means of cellular translocation within the landfill appears limited, especially during the initial stages of decomposition. Thus, microbial communities within the incoming refuse serve as a primary source of facultative and obligate anaerobic microorganisms that initiate refuse decomposition. Fresh residential refuse was collected five times over 26 months, and microbial communities in these samples were compared with those in individual refuse components and decomposed refuse. Bacterial and archaeal community structures were determined using T-RFLP. The Bacterial microbial community richness was correlated (r(2) = 0.91) with seasonal differences in ambient air temperature. Analysis of the results shows that fresh refuse is most likely not the source of methanogens in landfills. Microbial communities in the solid and leachate phases were different, indicating that both matrices must be considered when characterizing microbial diversity within a landfill.