The major emphasis in the SAR was on the reduction of greenhouse gases associated with industrial recycling in the metals, glass and paper industries. These topics are addressed in the industrial Section 3.5 of this chapter. There was a less systematic account of the methane emissions from landfills, or of the consumer dimension of recycling, both of which will be emphasized here.
Waste and waste management affect the release of greenhouse gases in five major ways: (1) landfill emissions of methane; (2) reductions in fossil fuel use by substituting energy recovery from waste combustion; (3) reduction in energy consumption and process gas releases in extractive and manufacturing industries, as a result of recycling; (4) carbon sequestration in forests, caused by decreased demand for virgin paper; and (5) energy used in the transport of waste for disposal or recycling. Except for the long-range transport of glass for reuse or recycling, transport emissions of secondary materials are often one or two orders of magnitude smaller than the other four factors (Ackerman, 2000).
Worldwide, the dominant methods of waste disposal are landfills and open dumps. Although these disposal methods often have lower first costs, they may contribute to serious local air and water pollution, and release high GWP landfill gas (LFG). LFG is generated when organic material decomposes anaerobically. It comprises approximately 50%-60% methane, 40%-45% CO2 and the traces of non-methane volatile organics and halogenated organics. In 1995, US, landfill methane emissions of 64 MtCeq slightly exceed its agricultural sector methane from livestock and manure.
Methane emission from landfills varies considerably depending on the waste characteristics (composition, density, particle size), moisture content, nutrients, microbes, temperature, and pH (El-Fadel, 1998). Data from field studies conducted worldwide indicate that landfill methane production may range over six orders of magnitude (between 0.003-3000g/m2/day) (Bogner et al., 1995). Not all landfill methane is emitted into the air; some is stored in the landfill and part is oxidized to CO2. The IPCC theoretical approach for methane estimation has been complemented with more recent, site-specific models that take into account local conditions such as soil type, climate, and methane oxidation rates to calculate overall methane emissions (Bogner et al., 1998).
Laboratory experiments suggest that a fraction of the carbon in landfilled organic waste may be sequestered indefinitely in landfills depending upon local conditions. However, there are no plausible scenarios in which landfilling minimizes GHG emissions from waste management. For yard waste, GHG emissions are roughly comparable from landfilling and composting; for food waste, composting yields significantly lower emissions than landfilling. For paper waste, landfilling causes higher GHG emissions than either recycling or incineration with energy recovery (US EPA, 2000).
Recycling involves the collection of materials during production or at the end of a products useful lifetime for reuse in the manufacturing process. The degree of treatment varies from simple remelting of glass, aluminium, or steel, to the breaking apart and reconstitution of paper or other fibres (e.g., textiles or carpets), to depolymerization of plastics and synthetic fibres to monomers, which are then used instead of petrochemicals to synthesize new polymers.
In many cases, manufacturing products from recycled materials is less energy intensive and associated with fewer GHG emissions than making products from virgin materials. This is especially true for aluminium and steel, which are energy intensive and release significant process GHGs during production (CO2 and PFCs). A US EPA analysis finds lower GHG emissions over the product life cycle from recycling than from virgin production and disposal of paper, metals, glass, and plastics under typical American conditions (US EPA, 2000).
Overall energy consumption is lower for recycled paper than for virgin paper, yet there is some debate over life cycle GHG emissions between paper recycling (Blum et al., 1997; Finnveden and Thomas, 1998; US EPA, 1998) and paper consumption with energy recovery (Bystroem and Loennstedt, 1997; Ruth and Harrington, 1998; IIED, 1996). These conflicting analyses make different underlying assumptions concerning the fuel displaced by energy released from paper incineration, the energy source for the electricity used in paper production, how the recycled paper is utilized, and how much carbon sequestration can be credited to uncut forests because of recycling. In all studies, landfilling of paper clearly releases more GHGs than either recycling or incineration.
The life cycle environmental impact and GHG emissions from recycling are usually higher than reusing products. This may not hold true if the used materials have to be transported over long distances. To address this issue, some countries such as Germany, Norway, Denmark, and other European countries have standardized bottles for local reuse.
Composting refers to the aerobic digestion of organic waste. The decomposed residue, if free from contaminants, can be used as a soil conditioner. As noted above under landfilling, GHG emissions from composting are comparable to landfilling for yard waste, and lower than landfilling for food waste. These estimates do not include the benefits of the reduced need for synthetic fertilizer, which is associated with large CO2 emissions during manufacture and transport, and N2O releases during use. USDA research indicates that compost usage can reduce fertilizer requirements by at least 20% (Ligon, 1999), thereby significantly reducing net GHG emissions (see Section 3.6).
Composting of yard waste has become widespread in many developed countries, and some communities compost food waste as well. Small, low-technology facilities handling only yard waste are inexpensive and generally problem-free. Some European and North American cities have encountered difficulties implementing large-scale, mixed domestic, commercial and industrial bio-waste collection and composting schemes. The problems range from odour complaints to heavy metal contamination of the decomposed residue. Also, large-scale composting requires mechanical aeration which can be energy intensive (40-70 kW/t of waste) (Faaij et al., 1998). However, facilities that combine anaerobic and aerobic digestion are able to provide this energy from self-supplied methane. If 25% or more of the waste is digested anaerobically the system can be self-sufficient (Edelmann and Schleiss, 1999).
For developing countries, the low cost and simplicity of composting, and the high organic content of the waste stream make small-scale composting a promising solution. Increased composting of municipal waste can reduce waste management costs and emissions, while creating employment and other public health benefits.
Anaerobic digestion to produce methane for fuel has been successful on a variety of scales in developed and developing countries. The rural biogas programmes based upon manure and agricultural waste in India and China are very extensive. In industrial countries, digestion at large facilities utilizes raw materials including organic waste from agriculture, sewage sludge, kitchens, slaughterhouses, and food processing industries.
Incineration is common in the industrialized regions of Europe, Japan and the northeastern USA where space limitations, high land costs, and political opposition to locating landfills in communities limit land disposal. In developing countries, low land and labour costs, the lack of high heat value materials such as paper and plastic in the waste stream, and the high capital cost of incinerators have discouraged waste combustion as an option.
Waste-to-energy (WTE) plants create heat and electricity from burning mixed solid waste. Because of high corrosion in the boilers, the steam temperature in WTE plants is less than 400 degrees Celsius. As a result, total system efficiency of WTE plants is only between 12%24% (Faaij et al., 1998; US EPA, 1998; Swithenbank and Nasserzadeh, 1997).
Net GHG emissions from WTE facilities are usually low and comparable to those from biomass energy systems, because electricity and heat are generated largely from photosynthetically produced paper, yard waste, and organic garbage rather than from fossil fuels. Only the combustion of fossil fuel based waste such as plastics and synthetic fabrics contribute to net GHG releases, but recycling of these materials generally produces even lower emissions.
Methane emissions from domestic and industrial wastewater disposal contribute about 10% of global anthropogenic methane sources (30-40Mt annually). Industrial wastewater, mainly from pulp and paper and food processing industries, contributes more than 90% of these emissions, whereas domestic and commercial wastewater disposal contributes about 2 Mt annually. Unlike methane emissions from solid waste, most of the methane from wastewater is believed to be generated in non-Annex I countries, where wastewater is often untreated and stored under anaerobic conditions (SAR).
Other reports in this collection