Issue No. 10, September 1997

 

 

Measurement and Modeling of Methane Fluxes from Landfills

  

Contributed by :
K.A. Smith, University of Edinburgh, UK, and J. Bogner, DOE Argonne National Laboratory, USA

 

Methane is an important and relatively long-lived greenhouse gas. Its atmospheric concentration has grown from about 700 ppbv (parts per billion by volume) in pre-industrial times to over 1700 ppbv today. It is the only long-lived gas that shows chemical feedback effects &endash; increases in atmospheric methane reduce the concentration of the hydroxyl radical, OH, and thus increase the methane lifetime, and also result in increases in tropospheric ozone.

The global annual input of methane to the atmosphere is estimated to be 535±125 Tg (IPCC, 1995a), of which about half is considered to be both anthropogenic and originating from biospheric processes, particularly anaerobic bacterial fermentation. Decomposition of refuse in municipal landfills is believed to be one of the major components of this biogenic methane, but past estimates of the emissions from this source have varied greatly, from 9 to 70 Tg per year. More reliable estimates are clearly needed, but it appears that landfills are the largest anthropogenic source of atmospheric methane in the United States and European countries. This source has been targeted in many countries as one which is capable of control by recovery of the methane and using it as a fuel, thus potentially providing a way of reducing current greenhouse gas emissions.

In contrast, in developing countries, urban refuse disposal is often in open dumps, which do not result in much methane emission even though they create a range of other environmental problems. However, as these dumps are replaced in the future by covered landfills it is likely that methane production will increase, and in most cases this will not be recovered for use as fuel or flared, but released to the atmosphere.

Landfills characteristically have two contrasting microbial ecosystems, often with sharp gradients between them: anaerobic methanogenic zones occur in the upper refuse layer, and methanotrophic zones in aerated cover soils. Rates for both methane production and oxidation can exceed observed rates for other terrestrial ecosystems by large factors. Field flux measurements (net emissions) vary over 7 orders of magnitude, from less than 0.0004 to about 4,000 grams per sqaure meter per day (Bogner et al., 1997). These net emissions, of course, are the result of methane production, oxidation, and gaseous transport processes in the cover soil. The various pathways into which landfill methane is partitioned are shown in Fig. 1, which illustrates that both methanotrophic oxidation and engineered control systems (pumped gas recovery) may reduce emissions.

Figure 1. Landfill methane balance (Adapted from Bogner and Spokas, 1993).

Compared with more well-studied sources, mechanistic understanding of how specific physical and biochemical controls affect net methane emissions from landfills is poor. Hence, it is difficult to predict emission rates at sites with various cover types, climatic regimes, and management practices. methane oxidation in cover soils requires further study to determine its impact on net emissions. Oxidation rates in these soils range up to over 100 grams per sqaure meter per day, among the highest for any biological system, and in some cases, the landfill can be a net sink for atmospheric methane oxidation. Emissions of other greenhouse gases, such as nitrous oxide, and aromatic and chlorinated compounds of environmental concern, also occur, but not much is known about flux rates.

Up to now, "top-down" approaches have been used to estimate methane fluxes from landfills. The quantities and types of decomposable refuse deposited have been calculated, and multiplied by assumed rates of methane generation. However, such estimates have not taken account of many factors which affect net emissions, and there is a need to be able to quantify these emissions by field measurements, to validate the top-down approach. As a contribution to this objective, IGAC's Trace Gas Exchange: Mid-Latitude Ecosystems and Atmosphere (TRAGEX) Activity organised a joint North American-European Workshop at the Argonne National Laboratory, USA, on October 21-24, 1996, to establish the state of the art in field measurement and modeling of emissions, and to identify major research and scaling issues that have to be tackled to improve global estimates for input to climate models. The workshop involved participants from 9 countries, and was sponsored by the European IGAC Project Office, NASA, Argonne National Laboratory, and the U.S. Environmental Protection Agency Landfill methane Outreach Program.

At the workshop, an overview of current work was given through invited presentations and a poster session. The presentations dealt with current global estimates, measurement methods, particularly chamber methods and micrometeorological techniques, oxidation studies, and isotopic techniques for characterizing microbial methane processes. The remainder of the workshop consisted of three working sessions focusing on measurement strategies; on ancillary soil studies; and on modeling, scaling, and inventory issues. The principal conclusions are summarised below. A full report of the workshop has recently been published (Smith and Bogner, 1997) and is available on request from the IGAC Core Project Office.

Emission data can be obtained by chamber, inert tracer, and micrometeorological methods. All these methods have inherent advantages and disadvantages, but are not uniformly applicable to all landfill types, and preferably different methods should be used in combination. To date, most measurements have been by static chamber methods, which have also been used to determine net uptake of atmospheric methane (Whalen and Reeburgh, 1990), and occasionally to measure emissions of nitrous oxide and non-methane hydrocarbons from landfill surfaces.

Tracer methods involve the release of an inert tracer gas, most commonly sulphur hexafluoride (SF6), from points along the upwind edge of the emitting surface, to simulate gas emission. If the released tracer is well mixed in a source "plume" and if the methane concentration in the plume differs sufficiently from background atmospheric methane, then the emission rate can be obtained directly, using a ratio method (Fig. 2).

Figure 2. Use of tracer (SF6) for field measurement of landfill methane emissions. (Czepiel and Mosher, unpublished).

Tracer methods circumvent the problem of spatial heterogeneity by integrating the whole area flux and are therefore a favored method for estimating emissions for whole landfills. However, their high cost, dependence on meteorological conditions, and potential for interference from other sources of methane limit their applicability. Only two micrometeorological methods -- eddy correlation and flux gradient -- have been applied so far. These methods can be used to evaluate whole-landfill methane emissions, and because they are more automated, they are especially useful for the study of diurnal and seasonal flux variations. However, they require complex instrumentation and calculations, and also have surface constraints (relatively level terrain) that may limit their application.

New methods have been proposed (e.g., Fourier transform infrared methods with dispersion modeling), but have not yet been applied to landfill studies. Because previous investigations have shown significant spatial variability at a given site, major research needs include effective screening tools &endash; simple portable gas analysers -- to aid experimental designs; a systematic comparison of various methods under both controlled conditions and full-scale field conditions; and basic studies on the variables controlling gaseous emissions.

Considerable attention was given to the effect of methanotrophic methane on net emission vs. gross production, and possible isotopic approaches to quantify this relationship. Important variables include soil texture, gas-filled and total porosity, tortuosity, dynamic water content and moisture-holding capacity, clay mineralogy, and nutrient and organic matter content. For example, in landfill soils containing organic matter with a low C/N ratio, methane oxidation can be suppressed because of increased nitrogen turnover. Soil cover design and management practices are also important.

Isotopic methods (both carbon-13 and deuterium (D)) are especially attractive for quantification of methane oxidation in landfills. As methane is oxidized, the lighter isotopes are used preferentially, leaving residual methane enriched in both carbon-13 and D. The del-carbon-13 for methane in the anaerobic zone is about -50 to -60 ä and the del-D about -285 to -325ä (Bergamaschi and Harris, 1995). The isotopic shift is proportional to the fraction of methane that is oxidized and the degree of preference of the microbes for the lighter isotope. Measurements of the shift have been used with success to estimate the fraction of methane oxidized in wetlands, and have an obvious application in analogous landfill studies. Identifying the depth of maximum methane oxidation would assist with determination of a minimum cover thickness and other properties needed for optimum oxidation.

The workshop also addressed issues associated with modeling landfill methane emissions at various scales, including the development of improved global inventories for input to climate models. In particular, the problem of scaling up from specific site studies was discussed with reference to suggested protocols for future site classification and inventory purposes. Three methods are currently being used: (1) a U.S. EPA system using current estimates of per capita refuse generation and landfill disposal in a first-order kinetic model for methane generation, without methane oxidation (Doorn and Barlaz, 1995); (2) the UK approach (Aitchison et al., 1996), also based on a first-order model, which considers numerous factors shown to be important for methane emissions over time (pumped gas recovery; refuse composition; methane oxidation); and (3) the current IPCC approach (IPCC, 1995b), which assumes steady-state methane generation on the basis of the degradable organic carbon content of landfilled refuse.

The improvement of models to estimate global emissions depends on the development of more refined methods, as well as improved inventories for waste generation rates, waste composition, organic carbon conversion, and methane recovery. The extrapolation of results from small-scale studies to estimates of national or global emissions is difficult. The current models used for global estimates have not been validated by field measurements for either net methane flux or methane oxidation rates. Recommendations for scaling up include the direct use of available methane flux or oxidation data where available. For many locations, national estimates could be improved through development of algorithms inclusive of specific management practices (above-ground or below-ground sites; gas recovery or no gas recovery), landfill size (gross size and surface-to-volume ratio), and realistic rates for methane oxidation. An improved methodology was suggested (Fig. 3) that incorporates these factors for countries where solid waste statistics are available. As such approaches are adopted and field measurement programs are completed, there is reason for optimism that "top-down" and "bottom-up" approaches may be reconciled.

Figure 3. A conceptual model for improved quantification of country-based landfill methane emissions. This model is for developed countries with available solid waste and landfill management statistics. The primary criterion is wet vs. dry sites (based on the moisture content of the bulk landfilled waste), the second tier criterion is the presence or absence of pumped gas recovery, the third criterion is size (small vs. large), the fourth criterion is fractional methane oxidation, and the fifth criterion relates to site construction (above-ground vs. below-ground at small sites).

References

  1. Aitchison, E.M et al., 1996. A methodology for updating routinely the annual estimate of methane emissions from landfill sites in the UK. ETSU Rept. RYWA/18678001/R/4.
  2. Bergamaschi, P., and G.W. Harris, 1995. Measurement of stable isotope ratios (13CH4/12CH4; 12CH3D/12CH4) in landfill methane using a tunable diode laser absorption spectrometer, Global Biogeochem. Cycles, 9, 439-447.
  3. Bogner, J.E., and K.A. Spokas, 1993, Landfill methane: rates, fates, and role in global carbon cycle, Chemosphere, 26 (1-4), 369-386.
  4. Bogner, J.E., M. Meadows, and P. Czepiel, 1997, Bidirectional fluxes of methane between landfills and the atmosphere: natural and engineered controls, Soil Use Management (in press).
  5. Doorn, M., and M. Barlaz, 1995. Estimate of global methane emissions from landfills and open dumps, USEPA Rept. EPA-600/R-95-019, Washington, D.C.
  6. IPCC, 1995a. Climate Change 1994, Cambridge Univ. Press, Cambridge, UK.
  7. IPCC, 1995b. Greenhouse gas inventory reporting instructions. Notebook and Reference Tables, IPCC/OECD, Paris.
  8. Smith K.A and Bogner, J.E., 1997. Report of joint North American-European workshop on measurement and modeling of methane fluxes from landfills. IGAC, Cambridge, Mass.
  9. Whalen, S., and W. Reeburgh, 1990. Consumption of atmospheric methane by tundra soils, Nature, 346, 160-162, 1990.

 

 

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