GHG emissions and savings (credits) are attributable to various stages of a waste management system. Figure 2 shows a simplified schematic of a municipal waste management system with the predominant climate impact sources.
The general suite of activities – collection, separation, treatment, transfer, and disposal – applies to all waste types (i.e. MSW, C&I, C&D, hazardous), with varying levels of sophistication, with the possible exception of agricultural waste. In many rural areas, agricultural waste is dealt with in-situ, through uncontrolled burning, burial, or simple land dumping.
Evidently, not all sources of emissions are indicated in the diagram: there are further environmental burdens associated with manufacture of waste receptacles, vehicles, andtreatment facilities, as well as the transfer of residual waste materials from intermediate stationsand treatment facilities to landfill.
Methane emissions from landfill are generally considered to represent the major source of climate impact in the waste sector (this impact is quantified in later sections). It is worth noting that, if a broader view of waste management were taken, which included materials management, landfill methane would no longer be the largest source of GHG in the sector.
Waste contains organic material, such as food, paper, wood, and garden trimmings. Once waste is deposited in a landfill, microbes begin to consume the carbon in organic material, which causes decomposition.
Under the anaerobic conditions prevalent in landfills, the microbial communities contain methane-producing bacteria. As the microbes gradually decompose organic matter over time, methane (approximately 50%), carbon dioxide (approximately 50%), and other trace amounts of gaseous compounds (< 1%) are generated and form landfill gas.
In controlled landfills, the process of burying waste and regularly covering deposits with a low permeability material creates an internal environment that favours methane-producing bacteria. As with any ecological system, optimum conditions of temperature, moisture, and nutrient source (i.e. organic waste) result in greater biochemical activity and hence greater generation of landfill gas.
The gradual decay of the carbon stock in a landfill generates emissions even after waste disposal has ceased. This is because the chemical and biochemical reactions take time to progress and only a small amount of the carbon contained in waste is emitted in the year this waste is disposed. Most is emitted gradually over a period of years.
Methane from wastewater management is the second largest source of GHG emissions from the waste sector as a whole, according to IPCC inventories (Bogner et al 2008). As previously stated, wastewater is not discussed within the scope of the present report, but certainly merits global attention.
Additional, comparatively minor sources of GHG from the waste sector at the global scale include combustion of waste, and biological treatment. Uncontrolled burning of waste is largely obsolete in developed countries, but continues to be practiced in developing regions, causing release of CO2 . Some landfills in developing countries, such as the Smokey Mountain site in Manila, smoulder continuously.
Controlled burning, in waste incinerators, also generates CO2 emissions. Where incinerators generate energy, GHG may also be credited – this is discussed in the following section. Where incinerators do not generate energy, they will be net energy users, which will also contribute to their total GHG emissions. Advanced thermal treatment technologies, such as gasification and pyrolysis, may emit fewer emissions compared to mass-burn incineration. However, these are emerging technologies and cannot be considered ‘established’ technologies for the treatment of bulk mixed waste.
Aerobic composting processes directly emit varying levels of methane and nitrous oxide, depending on how the process is managed in practice. Closed systems, such as enclosed maturation bays or housed windrows, reduce emissions through use of air filters (often biofilters) to treat air exiting the facility.
Compost plants require varying, but usually small, amounts of energy input (with associated ‘upstream’ GHG emissions). Further GHG emissions occur ‘downstream’, depending on the application of the compost product – CO2 will be gradually released as the compost further degrades and becomes integrated with soil-plant systems.
Anaerobic digestion (AD) systems are enclosed in order to capture and contain the biogas generated by the digestion process. GHG emissions from AD facilities are generally limited to system leaks from gas engines used to generate power from biogas, fugitive emissions from system leaks and maintenance, and possible trace amounts of methane emitted during maturation of the solid organic output.
Such systems also consume energy, however plants are generally self-sustaining if appropriately operated (i.e. a portion of the biogas output generates energy for use in-plant). ‘Downstream’ GHG emissions will depend on the application of the matured digestate (as per aerobic compost product).
Mechanical biological treatment (MBT) encompasses mechanical sorting of the mixed residual waste fraction, with some recovery of recyclable materials (limited due to contamination), and separation of a fine, organic fraction for subsequent biological treatment. The biological component may include anaerobic digestion with recovery of biogas for energy/heat generation, or aerobic composting to produce a biologically stable product for either land application (limited applicability) or use as refuse-derived fuel (RDF) to substitute fuel in industrial furnaces (i.e. coincineration in cement kilns).
MBT facilities vary considerably in terms of sophistication, configuration, scale, and outputs. GHG emissions associated with MBT are due to energy inputs (although AD systems may be self-sustaining), direct process emissions (this will depend on the air protection control system, such as a biofilter, attached to the aerobic composting component), gas engine emissions (for AD), and use of the composted organic output (disposed of to landfill or applied to land). There is some use of composted MBT output to remediate contaminated land, however most OECD countries strictly regulate the use of compost derived from mixed waste, and the majority is disposed of in landfill, or used as cover material for landfill operations.
Waste to Energy Framework
• Position waste management as an area requiring urgent action, and call for policy and decision makers to take such action.
• Expand the concept of ‘waste management’ to become ‘waste and resource management’, including waste prevention and minimization and also aspects of resource efficiency and sustainable consumption and production (SCP).
• Demonstrate the relation of waste and resource management to other global challenges such as sustainable development, water and energy balance and security, sound chemicals management, climate change, food security, resource scarcity and security and poverty alleviation; establish the links to wider health and environmental policy challenges.
• Identify policies and governance strategies for sound waste management, considering the varying levels of economic and human development between countries, their needs and the practices in use; provide a critical overview of what instruments have been deployed towards which goals and have worked under which circumstances.
• Examine the available approaches to waste management financing and set out a framework for establishing a sustainable financing model in a particular local situation; consider the direct costs and revenues, the costs of inaction and the indirect benefits of environmentally sound waste management; examine how to raise sufficient revenue to cover the net costs of service provision, and examine investment financing.
• Assemble sets of standardized performance indicators on waste management that allow benchmarking exercises and facilitate better analysis of the state of waste management around the world and provide a standardized means for monitoring progress.
Source: UNEP
