Conclusions trends and broader perspectives

Landfill CH4 is a small contributor to global anthropogenic greenhouse gas emissions. The broader implementation of cost-effective landfill gas recovery and utilization systems can achieve additional reductions in landfill CH4 emissions. Moreover, landfill gas utilization can assist local communities by providing renewable energy benefits to offset fossil fuel use. Methanotrophic CH4 oxidation in cover soils, in association with the transport limitations provided by the cover materials, provides a secondary biological control on emissions that is, of course, limited by the capacity of soils to develop and continuously maintain methanotrophic consortia under field conditions. Enhanced ('biocover') methanotrophic oxidation in cover soils can be assisted

Figure 11.4 Model simulations for landfill CH4 emissions with and without CH4 oxidation using the model described in Spokas et al (2009): (A) variability in the seasonal methane emissions for a hypothetical Midwestern US landfill (Chicago, IL) with and without methane oxidation; and (B) the resulting percentage of CH4 oxidized in the cover material

Figure 11.4 Model simulations for landfill CH4 emissions with and without CH4 oxidation using the model described in Spokas et al (2009): (A) variability in the seasonal methane emissions for a hypothetical Midwestern US landfill (Chicago, IL) with and without methane oxidation; and (B) the resulting percentage of CH4 oxidized in the cover material

Note: The cover was assumed to consist of 60cm clay loam with an active gas recovery system. Methane concentration at the base of the cover was assumed to be 45 per cent. The spike in the emissions at day 57 was due to soil thawing. Source: Unpublished author's data by site-specific designs to promote oxidation in cover soils. Other important considerations to limit emissions include landfill design, operations and maintenance practices including, where feasible, the implementation of economical horizontal gas collection systems concurrent with filling. In both the EU and the US, annual national landfill CH4 emissions have decreased since 1990 (Deuber et al, 2005; US EPA, 2008). This is due to increased landfill CH4 recovery and utilization as well as policy measures to reduce the mass of biodegradable waste landfilled (such as the EU landfill directive 1999/31/EC). For developing countries, however, rates of landfill CH4 emissions are expected to increase concurrently with increased landfilling. This increase could be mitigated through increased rates of CH4 recovery via incentives such as the current CDM and anticipated post-Kyoto policies and measures expected to be implemented during the next two to three years.

In addition to the direct reduction of landfill CH4 emissions via engineered gas recovery systems, the retardation of emissions by engineered cover systems, and the optimization of in situ rates of methanotrophic CH4 oxidation, one must also be aware of complementary waste management practices to reduce emissions. These include: (1) alternative waste management practices such as composting or incineration that reduce landfill CH4 generation; and (2) recycling, reuse and waste minimization practices that reduce waste generation. In the context of integrated waste management, it is important to preserve choices for local officials to make informed decisions regarding local waste management practices. Those decisions can benefit from consideration of multiple technical and non-technical issues, including waste quantities and characteristics, local costs and financing issues, regulatory constraints, and infrastructure requirements including available land area, collection and transport considerations. Most waste sector technologies are mature, cost-effective and bestow significant co-benefits for public health and environmental protection (Bogner et al, 2009).

In addition to CH4 transport and oxidation, there are numerous other C and N cycle processes in landfill cover soils which influence observed soil gas profiles and measured fluxes. These include aerobic respiration (resulting in CO2 production and flux), plant photosynthesis (CO2 uptake) and, especially where N is abundant, nitrification and denitrification processes that can produce gaseous intermediates of soil N cycling (N2O, NO, N2). Although these processes have been infrequently studied in landfill settings, nevertheless, they further complicate field measurement and the understanding of gaseous fluxes and CH4 oxidation in landfill settings. At vegetated sites with an engineered gas recovery system and low CH4 fluxes, the observed CO2 flux may be dominated by root zone respiration rather than CH4 oxidation or the direct transport of landfill gas (Bogner et al, 1997a, 1999). For field studies where it is important to quantity the various CO2 fluxes, one can choose among a variety of applicable techniques (for example, see Panikov and Gorbenko, 1992).

Field studies to date have quantified landfill CH4 emissions at a limited number of humid, temperate, semi-arid and subtropical sites, but extensive emission measurements in tropical environments are generally lacking. In addition, there have been relatively few comprehensive field campaigns over complete annual cycles, none of which have used multiple methods for emissions. Existing data have shown that emission and oxidation rates vary spatially and temporally over several orders of magnitude. Therefore, additional field data and improved modelling tools, including models currently under development that incorporate seasonal meteorological and soil microclimate variability (Spokas et al, 2009), are needed for better understanding and predictability of emissions over various spatial scales.

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