10.4.2 CH4 management at landfills
Global CH4 emissions from landfills are estimated to be 500–800 MtCO2-eq/yr (US EPA, 2006; Monni et al. 2006; Bogner and Matthews 2003). However, direct field measurements of landfill CH4 emissions at small scale (<1m2) can vary over seven orders of magnitude (0.0001– >1000 g CH4 /m2/d) depending on waste composition, cover materials, soil moisture, temperature and other variables (Bogner et al., 1997a). Results from a limited number of whole landfill CH4 emissions measurements in Europe, the US and South Africa are in the range of about 0.1–1.0 tCH4/ha/d (Nozhevnikova et al., 1993; Oonk and Boom, 1995; Borjesson, 1996; Czepiel et al., 1996; Hovde et al., 1995; Mosher et al., 1999; Tregoures et al., 1999; Galle et al., 2001; Morris, 2001; Scharf et al., 2002).
The implementation of an active landfill gas extraction system using vertical wells or horizontal collectors is the single most important mitigation measure to reduce emissions. Intensive field studies of the CH4 mass balance at cells with a variety of design and management practices have shown that >90% recovery can be achieved at cells with final cover and an efficient gas extraction system (Spokas et al., 2006). Some sites may have less efficient or only partial gas extraction systems and there are fugitive emissions from landfilled waste prior to and after the implementation of active gas extraction; thus estimates of ‘lifetime’ recovery efficiencies may be as low as 20% (Oonk and Boom, 1995), which argues for early implementation of gas recovery. Some measures that can be implemented to improve overall gas collection are installation of horizontal gas collection systems concurrent with filling, frequent monitoring and remediation of edge and piping leakages, installation of secondary perimeter extraction systems for gas migration and emissions control, and frequent inspection and maintenance of cover materials. Currently, landfill CH4 is being used to fuel industrial boilers; to generate electricity using internal combustion engines, gas turbines or steam turbines; and to produce a substitute natural gas after removal of CO2 and trace components. Although electrical output ranges from small 30 kWe microturbines to 50 MWe steam turbine generators, most plants are in the 1–15 MWe range. Significant barriers to increased diffusion of landfill gas utilization, especially where it has not been previously implemented, can be local reluctance from electrical utilities to include small power producers and from gas utilities/pipeline companies to transport small percentages of upgraded landfill gas in natural gas pipelines.
A secondary control on landfill CH4 emissions is CH4 oxidation by indigenous methanotrophic microorganisms in cover soils. Landfill soils attain the highest rates of CH4 oxidation recorded in the literature, with rates many times higher than in wetland settings. CH4 oxidation rates at landfills can vary over several orders of magnitude and range from negligible to 100% of the CH4 flux to the cover. Under circumstances of high oxidation potential and low flux of landfill CH4 from the landfill, it has been demonstrated that atmospheric CH4 may be oxidized at the landfill surface (Bogner et al., 1995; 1997b; 1999; 2005; Borjesson and Svensson, 1997b). In such cases, the landfill cover soils function as a sink rather than a source of atmospheric CH4. The thickness, physical properties moisture content, and temperature of cover soils directly affect oxidation, because rates are limited by the transport of CH4 upward from anaerobic zones and O2 downward from the atmosphere. Laboratory studies have shown that oxidation rates in landfill cover soils may be as high as 150–250 g CH4/m2/d (Kightley et al., 1995; de Visscher et al., 1999). Recent field studies have demonstrated that oxidation rates can be greater than 200 g/m2/d in thick, compost-amended ‘biocovers’ engineered to optimize oxidation (Bogner et al., 2005; Huber-Humer, 2004). The prototype biocover design includes an underlying coarse-grained gas distribution layer to provide more uniform fluxes to the biocover above (Huber-Humer, 2004). Furthermore, engineered biocovers have been shown to effectively oxidize CH4 over multiple annual cycles in northern temperate climates (Humer-Humer, 2004). In addition to biocovers, it is also possible to design passive or active methanotrophic biofilters to reduce landfill CH4 emissions (Gebert and Gröngröft, 2006; Streese and Stegmann, 2005). In field settings, stable C isotopic techniques have proven extremely useful to quantify the fraction of CH4 that is oxidized in landfill cover soils (Chanton and Liptay, 2000; de Visscher et al., 2004; Powelson et al., 2007). A secondary benefit of CH4 oxidation in cover soils is the co-oxidation of many non-CH4 organic compounds, especially aromatic and lower chlorinated compounds, thereby reducing their emissions to the atmosphere (Scheutz et al., 2003a).
Other measures to reduce landfill CH4 emissions include installation of geomembrane composite covers (required in the US as final cover); design and installation of secondary perimeter gas extraction systems for additional gas recovery; and implementation of bioreactor landfill designs so that the period of active gas production is compressed while early gas extraction is implemented.
Landfills are a significant source of CH4 emissions, but they are also a long-term sink for carbon (Bogner, 1992; Barlaz, 1998. See Figure 10.1 and Box 10.1). Since lignin is recalcitrant and cellulosic fractions decompose slowly, a minimum of 50% of the organic carbon landfilled is not typically converted to biogas carbon but remains in the landfill (See references cited on Figure 10.1). Carbon storage makes landfilling a more competitive alternative from a climate change perspective, especially where landfill gas recovery is combined with energy use (Flugsrud et al. 2001; Micales and Skog, 1997; Pingoud et al. 1996; Pipatti and Savolainen, 1996; Pipatti and Wihersaari, 1998). The fraction of carbon storage in landfills can vary over a wide range, depending on original waste composition and landfill conditions (for example, see Hashimoto and Moriguchi, 2004 for a review addressing harvested wood products).