Waste generation is closely linked to population, urbanization and affluence. The archaeologist E.W. Haury wrote: ‘Whichever way one views the mounds [of waste], as garbage piles to avoid, or as symbols of a way of life, they…are the features more productive of information than any others.’ (1976, p.80). Archaeological excavations have yielded thicker cultural layers from periods of prosperity; correspondingly, modern waste-generation rates can be correlated to various indicators of affluence, including gross domestic product (GDP)/cap, energy consumption/cap, , and private final consumption/cap (Bingemer and Crutzen, 1987; Richards, 1989; Rathje et al., 1992; Mertins et al., 1999; US EPA, 1999; Nakicenovic et al., 2000; Bogner and Matthews, 2003; OECD, 2004). In developed countries seeking to reduce waste generation, a current goal is to decouple waste generation from economic driving forces such as GDP (OECD, 2003; Giegrich and Vogt, 2005; EEA, 2005). In most developed and developing countries with increasing population, prosperity and urbanization, it remains a major challenge for municipalities to collect, recycle, treat and dispose of increasing quantities of solid waste and wastewater. A cornerstone of sustainable development is the establishment of affordable, effective and truly sustainable waste management practices in developing countries. It must be further emphasized that multiple public health, safety and environmental co-benefits accrue from effective waste management practices which concurrently reduce GHG emissions and improve the quality of life, promote public health, prevent water and soil contamination, conserve natural resources and provide renewable energy benefits.
The major GHG emissions from the waste sector are landfill CH4 and, secondarily, wastewater CH4 and N2O. In addition, the incineration of fossil carbon results in minor emissions of CO2. Chapter 10 focuses on mitigation of GHG emissions from post-consumer waste, as well as emissions from municipal wastewater and high biochemical oxygen demand (BOD) industrial wastewaters conveyed to public treatment facilities. Other chapters in this volume address pre-consumer GHG emissions from waste within the industrial (Chapter 7) and energy (Chapter 4) sectors which are managed within those respective sectors. Other chapters address agricultural wastes and manures (Chapter 8), forestry residues (Chapter 9) and related energy supply issues including district heating (Chapter 6) and transportation biofuels (Chapter 5). National data are not available to quantify GHG emissions associated with waste transport, including reductions that might be achieved through lower collection frequencies, higher routing efficiencies or substitution of renewable fuels; however, all of these measures can be locally beneficial to reduce emissions.
It should be noted that a separate chapter on post-consumer waste is new for the Fourth Assessment report; in the Third Assessment Report (TAR), GHG mitigation strategies for waste were discussed primarily within the industrial sector (Ackerman, 2000; IPCC, 2001a). It must also be stressed that there are high uncertainties regarding global GHG emissions from waste which result from national and regional differences in definitions, data collection and statistical analysis. Because of space constraints, this chapter does not include detailed discussion of waste management technologies, nor does this chapter prescribe to any one particular technology. Rather, this chapter focuses on the GHG mitigation aspects of the following strategies: landfill CH4 recovery and utilization; optimizing methanotrophic CH4 oxidation in landfill cover soils; alternative strategies to landfilling for GHG avoidance (composting; incineration and other thermal processes; mechanical and biological treatment (MBT)); waste reduction through recycling, and expanded wastewater management to minimize GHG generation and emissions. In addition, using available but very limited data, this chapter will discuss emissions of non-methane volatile organic compounds (NMVOCs) from waste and end-of-life issues associated with fluorinated gases.
The mitigation of GHG emissions from waste must be addressed in the context of integrated waste management. Most technologies for waste management are mature and have been successfully implemented for decades in many countries. Nevertheless, there is significant potential for accelerating both the direct reduction of GHG emissions from waste as well as extended implications for indirect reductions within other sectors. LCA is an essential tool for consideration of both the direct and indirect impacts of waste management technologies and policies (Thorneloe et al., 2002; 2005; WRAP, 2006). Because direct emissions represent only a portion of the life cycle impacts of various waste management strategies (Ackerman, 2000), this chapter includes complementary strategies for GHG avoidance, indirect GHG mitigation and use of waste as a source of renewable energy to provide fossil fuel offsets. Using LCA and other decision-support tools, there are many combined mitigation strategies that can be cost-effectively implemented by the public or private sector. Landfill CH4 recovery and optimized wastewater treatment can directly reduce GHG emissions. GHG generation can be largely avoided through controlled aerobic composting and thermal processes such as incineration for waste-to-energy. Moreover, waste prevention, minimization, material recovery, recycling and re-use represent a growing potential for indirect reduction of GHG emissions through decreased waste generation, lower raw material consumption, reduced energy demand and fossil fuel avoidance. Recent studies (e.g., Smith et al., 2001; WRAP, 2006) have begun to comprehensively quantify the significant benefits of recycling for indirect reductions of GHG emissions from the waste sector.
Post-consumer waste is a significant renewable energy resource whose energy value can be exploited through thermal processes (incineration and industrial co-combustion), landfill gas utilization and the use of anaerobic digester biogas. Waste has an economic advantage in comparison to many biomass resources because it is regularly collected at public expense (See also Section 18.104.22.168). The energy content of waste can be more efficiently exploited using thermal processes than with the production of biogas: during combustion, energy is directly derived both from biomass (paper products, wood, natural textiles, food) and fossil carbon sources (plastics, synthetic textiles). The heating value of mixed municipal waste ranges from <6 to >14 MJ/kg (Khan and Abu-Ghararath, 1991; EIPPC Bureau, 2006). Thermal processes are most effective at the upper end of this range where high values approach low-grade coals (lignite). Using a conservative value of 900 Mt/yr for total waste generation in 2002 (discussed in Box 10.1 below), the energy potential of waste is approximately 5–13 EJ/yr. Assuming an average heating value of 9 GJ/t for mixed waste (Dornburg and Faaij, 2006) and converting to energy equivalents, global waste in 2002 contained about 8 EJ of available energy, which could increase to 13 EJ in 2030 using waste projections in Monni et al. (2006). Currently, more than 130 million tonnes per year of waste are combusted worldwide (Themelis, 2003), which is equivalent to >1 EJ/yr (assuming 9 GJ/t). The biogas fuels from waste – landfill gas and digester gas – typically have a heating value of 16–22 MJ/Nm3, depending directly on the CH4 content. Both are used extensively worldwide for process heating and on-site electrical generation; more rarely, landfill gas may be upgraded to a substitute natural gas product. Conservatively, the energy value of landfill gas currently being utilized is >0.2 EJ/yr (using data from Willumsen, 2003).
An overview of carbon flows through waste management systems addresses the issue of carbon storage versus carbon turnover for major waste-management strategies including landfilling, incineration and composting (Figure 10.1). Because landfills function as relatively inefficient anaerobic digesters, significant long-term carbon storage occurs in landfills, which is addressed in the 2006 IPCC Guidelines for National Greenhouse Gas Inventories (IPCC, 2006). Landfill CH4 is the major gaseous C emission from waste; there are also minor emissions of CO2 from incinerated fossil carbon (plastics). The CO2 emissions from biomass sources – including the CO2 in landfill gas, the CO2 from composting, and CO2 from incineration of waste biomass – are not taken into account in GHG inventories as these are covered by changes in biomass stocks in the land-use, land-use change and forestry sectors.
Figure 10.1: Carbon flows through major waste management systems including C storage and gaseous C emissions. The CO2 from biomass is not included in GHG inventories for waste.
References for C storage are: Huber-Humer, 2004; Zinati et al., 2001; Barlaz, 1998; Bramryd, 1997; Bogner, 1992.
A process-oriented perspective on the major GHG emissions from the waste sector is provided in Figure 10.2. In the context of a landfill CH4 mass balance (Figure 10.2a), emissions are one of several possible pathways for the CH4 produced by anaerobic methanogenic microorganisms in landfills; other pathways include recovery, oxidation by aerobic methanotrophic microorganisms in cover soils, and two longer-term pathways: lateral migration and internal storage (Bogner and Spokas, 1993; Spokas et al., 2006). With regard to emissions from wastewater transport and treatment (Figure 10.2b), the CH4 is microbially produced under strict anaerobic conditions as in landfills, while the N2O is an intermediate product of microbial nitrogen cycling promoted by conditions of reduced aeration, high moisture and abundant nitrogen. Both GHGs can be produced and emitted at many stages between wastewater sources and final disposal.
Figure 10.2: Pathways for GHG emissions from landfills and wastewater systems:
Figure 10.2a: Simplified landfill CH4 mass balance: pathways for CH4 generated in landfilled waste, including CH4 emitted, recovered and oxidized.
Note: Not shown are two longer-term CH4 pathways: lateral CH4 mitigation and internal changes in CH4 storage (Bogner and Spokas, 1993; Spokas et al., 2006) Methane can be stored in shallow sediments for several thousand years (Coleman, 1979).
Figure 10.2b: Overview of wastewater systems.
Note: The major GHG emissions from wastewater – CH4 and N2O – can be emitted during all stages from sources to disposal, but especially when collection and treatment are lacking. N2O results from microbial N cycling under reduced aeration; CH4 results from anaerobic microbial decomposition of organic C substrates in soils, surface waters or coastal zones.
It is important to stress that both the CH4 and N2O from the waste sector are microbially produced and consumed with rates controlled by temperature, moisture, pH, available substrates, microbial competition and many other factors. As a result, CH4 and N2O generation, microbial consumption, and net emission rates routinely exhibit temporal and spatial variability over many orders of magnitude, exacerbating the problem of developing credible national estimates. The N2O from landfills is considered an insignificant source globally (Bogner et al., 1999; Rinne et al., 2005), but may need to be considered locally where cover soils are amended with sewage sludge (Borjesson and Svensson, 1997a) or aerobic/semi-aerobic landfilling practices are implemented (Tsujimoto et al., 1994). Substantial emissions of CH4 and N2O can occur during wastewater transport in closed sewers and in conjunction with anaerobic or aerobic treatment. In many developing countries, in addition to GHG emissions, open sewers and uncontrolled solid waste disposal sites result in serious public health problems resulting from pathogenic microorganisms, toxic odours and disease vectors.
Major issues surrounding the costs and potentials for mitigating GHG emissions from waste include definition of system boundaries and selection of models with correct baseline assumptions and regionalized costs, as discussed in the TAR (IPCC, 2001a). Quantifying mitigation costs and potentials (Section 10.4.7) for the waste sector remains a challenge due to national and regional data uncertainties as well as the variety of mature technologies whose diffusion is limited by local costs, policies, regulations, available land area, public perceptions and other social development factors. Discussion of technologies and mitigation strategies in this chapter (Section 10.4) includes a range of approaches from low-technology/low-cost to high-technology/high-cost measures. Often there is no single best option; rather, there are multiple measures available to decision-makers at the municipal level where several technologies may be collectively implemented to reduce GHG emissions and achieve public health, environmental protection and sustainable development objectives.