Methane is emitted during the anaerobic decomposition of the organic content of solid waste and wastewater. There are large uncertainties in emissions estimates, due to the lack of information about the waste management practices employed in different countries, the portion of organic wastes that decompose anaerobically, and the extent to which these wastes will ultimately decompose.
About 20-40 Mt CH4 (110-230 Mt C), or about 10% of global CH4 emissions from human-related sources, are emitted from landfills and open dumps annually. Ten Annex I countries represent abouttwo- thirds of global CH4 emissions from solid-waste disposal, with the United States representing about 33%, or around 10 Mt (SAR II, 220.127.116.11).
CH4 emissions from domestic and industrial wastewater disposal are estimated to be 30-40 Mt (170-230 Mt C) annually, again about 10% of total global emissions from human sources. Industrial wastewater, principally from the food processing and pulp and paper industries, is the major contributor, with domestic and commercial wastewater making up 2 Mt CH4 annually. Unlike solid-waste emissions, the majority of wastewater emissions is believed to originate in non-Annex I countries, where domestic sewage and industrial waste streams often are unmanaged or maintained under anaerobic conditions without CH4 control (SAR II, 18.104.22.168).
8.2. Technical Options for Controlling Methane Emissions
CH4 emissions may be reduced through source reduction or through CH4 recovery and/or reduction from solid waste and wastewater.
8.2.1. Source Reduction
The most important technical option for source reduction is decreasing the use of materials that eventually turn up in the waste stream. This section, however, focuses on solid waste after it has been generated (consistent with SAR II, 22.214.171.124). The amount of organic solid waste may be reduced by recycling paper products, composting, and incineration. Paper products make up a significant part of solid waste in Annex I countries (e.g., 40% in the United States) and in urban centers of upper-income non-Annex I countries (typically 5-20%). A variety of recycling processes, differing in technical complexity, can often turn this waste into material indistinguishable from virgin products. Composting -- an aerobic process for treating moist organic wastes that generates little or no CH4 -- is most applicable to non-Annex I countries, where this type of waste is a larger fraction of the total, although there is also potential in Annex I countries (SAR II, 126.96.36.199). As a secondary benefit, the residue can be used as fertilizer. Reduced land availability and the potential for energy recovery are increasing use of waste incineration in many countries: 70% of Japan's solid waste is incinerated. Stack air pollutant emissions and ash disposal are still issues, however, and characteristics such as moisture content and composition may make incineration more difficult and costly in non-Annex I countries.
The technical complexity of these source reduction options can vary significantly, although this does not greatly influence their effectiveness. In non-Annex I countries, where labor is cheap compared to equipment costs, labor-intensive recycling and composting are common. Annex I countries typically use more complicated, labor-saving machinery requiring higher operating skills.
Costs will depend on the type of system, the size of the facility, and local factors. Capital costs for solid-waste composting facilities can range from $1.5 million for a 300 ton per day (TPD) plant to $45 million for a more complex 550 TPD plant that also composts sewage sludge; associated operating costs can range from $10-90/t, but generally average $20-40. Yard waste facilities are typically smaller and less complex; capital costs range from $75,000-2,000,000 in the United States for plants handling 2,000-60,000 t/yr of waste; operating costs are roughly $20/t. Capital costs for incineration can be quite high, ranging from $60-300 million for 10-80 MW facilities, or approximately $125,000 per TPD capacity (SAR II, 188.8.131.52).
8.2.2. Methane Recovery from Solid-Waste Disposal
Source reduction is applicable to future solid-waste generation. CH4 may be recovered from existing as well as future landfills, since organic materials in dumps and landfills continue to emit CH4 (often called landfill gas) for 10-30 years or more. Frequently, more than half of the CH4 can be recovered and used for heat or electricity generation, a practice already common in many countries (SAR II, 184.108.40.206). Landfill gas also can be purified and injected into a natural gas pipeline or distribution system; there are several such projects in the United States. In Minas Gerais, Brazil, purified landfill gas has been used to provide power for a fleet of garbage trucks and taxicabs.
Costs of recovering CH4 from solid-waste disposal facilities are highly dependent on technology and site characteristics. For a landfill with 1 million tons of waste (serving a population of about 50,000-100,000), collection and flare capital costs will be approximately $630,000, increasing to $3.6 million for a 10 million-ton landfill. Annual operating costs could range from less than $100,000 to more than $200,000. Energy recovery capital costs (including gas treatment) can range from $1,000-1,300 per net kW. Direct use is typically less expensive, with pipeline construction representing the principal cost. Overall, typical electric generation costs for a complete system (gas collection and energy recovery) range from 4-7¢/kWh. These costs are based on equipment and labor costs in the United States, and may vary over a wider range in other countries. Also, in many countries, some landfills and other solid-waste disposal sites already collect their CH4 and either vent or flare it (often for safety reasons). For these sites, the cost of electric generation would be lower than stated above (SAR II, 220.127.116.11; SAR III, 9.4.1).
8.2.3. Methane Recovery and/or Reduction from Wastewater
CH4 emissions can be virtually eliminated if wastewater and sludge are stored and treated under aerobic conditions. Options for preventing CH4 production during wastewater treatment and sludge disposal include aerobic primary and secondary treatment and land treatment. Alternatively, wastewater can be treated under anaerobic conditions and the generated CH4 can be captured and used as an energy source to heat the wastewater or sludge digestion tank. If additional CH4 is available, it can be used as fuel or to generate electricity. As a last resort, the gas may be flared, which converts the CH4 to CO2, with a much lower global warming potential.
Wastewater treatment costs are highly dependent on the technological approach employed and site-specific conditions. Capital costs of aerobic primary treatment can range from $0.15-3 million for construction, assuming a range of 0.5-10 million gallons (2,000-40,000 m3) of wastewater flow per day; annual operation and maintenance costs are estimated to range from $20,000-500,000 for these volumes. Costs of aerobic secondary treatment can be moderately high because of the energy and equipment requirements, and depend to a great extent on the daily volume of wastewater flow into the facility. Costs can range up to $10 million depending upon the technology selected and volume requirements, with the high-end handling approximately 100 million gallons (0.4 x 106 m3) per day. Finally, costs for anaerobic digestors of wastewater and flaring or utilization can range from $0.1-3 million for construction and $10,000-100,000 for operation and maintenance, assuming wastewater flows of 0.1-100 million gallons (400 to 0.4 x 106 m3) per day (SAR II, 18.104.22.168).
High-rate anaerobic processes for the treatment of liquid effluents with high organic content (e.g., sewage, food processing wastes) can help reduce uncontrolled CH4 emissions and are particularly suited to the warmer climates of most developing countries. Both Brazil and India, for example, have developed extensive and successful infrastructure for these technologies, which have lower hydraulic retention times than aerobic processes and therefore are much smaller and cheaper to build. More importantly, unlike aerobic processes, no aeration is involved and there is little electricity consumption.
For upflow anaerobic sludge blanket reactors of 4,000-10,000 m3 capacity (capable of handling a chemical oxygen demand of 20-30 kg/m3/day), capital costs have been estimated to be in the range of $1-3.5 million, with annual operating costs in the range of $1-2.7 million. At these costs, the total CH4 production cost would fall in the range of $0.45-1.05/GJ, with values at the upper end for Europe and at the lower end for Brazil. Using these estimates, all of the costs would be recovered, as CH4 would be produced at a price lower than that of natural gas almost anywhere in the world (SAR II, 22.214.171.124).
8.3. Measures for Methane Reduction and Recovery
In many countries, future actions that reduce CH4 emissions from solid-waste disposal sites and wastewater treatment facilities are likely to be undertaken for environmental and public health reasons; CH4 reductions will be seen as a secondary benefit of these actions. In spite of the benefits, however, a number of barriers prevents CH4 recovery and source reduction efforts described above from tapping more than a small portion of the potential, especially in non-Annex I countries. These barriers include the following (SAR II, 22.5.3):
For the successful implementation of CH4 control projects, these barriers need to be addressed through appropriate measures. In general, the measures are not specific to technology options (see Table 18). The following measures are arranged in the sequence that they would need to be invoked in a country with little or no current waste management infrastructure (more advanced countries and regions would start at a later step):
8.3.1. Institution Building and Technical Assistance Policies
The prior existence of an adequate waste management infrastructure, including a legal framework, is a prerequisite to any measure to control CH4. Where such infrastructure is weak or missing, it needs to be strengthened either within countries (e.g., from more developed areas to less developed ones) or internationally through multilateral or bilateral assistance. For instance, the Interamerican Development Bank gives priority to building waste management infrastructure as part of its developmental assistance programs. Support for institution building may include both financial and technical assistance. Technical assistance and financing are available from the U.S. Country Studies Program, joint implementation initiatives22 and the Global Environment Facility.
8.3.2. Voluntary Agreements
Voluntary agreements also can be used to overcome the barriers to waste management projects. In the United States, a landfill outreach program encourages state agencies (who permit projects) and utilities (who frequently purchase landfill energy) to voluntarily promote and participate in landfill projects. This type of program can be quite low-cost and flexible in targeting key barriers and providing effective information and assistance to overcome them. The U.S. program, for example, provides a variety of tools, including detailed descriptions of candidate project sites, and software to assess economic and technical potential.
8.3.3. Regulatory Measures
A major regulatory measure to reduce the quantity of solid waste through recycling is requiring separation at source (e.g., into paper, glass, metal, and plastics). Regulations also can include setting standards for recycled paper use or recycled material content. In the United States, for example, many states have recycling goals, often included in mandatory programs. For existing dumps and landfills, regulatory measures can range from the mandatory recovery and combustion of CH4 to actions aimed at clarifying existing regulations and ensuring that they are supportive of CH4 recovery. The United States recently enacted a mandatory regulation to require CH4 recovery and combustion at the largest landfills, which will result in annual CH4 reductions of about 60% (or ~6 Mt CH4 in 2000) (SAR II, 126.96.36.199).
8.3.4. Market-Based Programs
Once an appropriate infrastructure as well as technical awareness exists, market-based programs may be helpful to reduce perceptions of risk or high up- front capital costs. Domestic actions can include providing tax credits or low-cost financing. In the United States, for example, landfill gas energy recovery projects are eligible for an "unconventional gas" tax credit worth approximately 1¢/kWh of electricity generated. International financial support also may be provided through mechanisms such as the Global Environment Facility or other similar funds. The Global Environment Facility currently is funding a landfill gas-to-energy project in Pakistan, which should demonstrate the potential of this technology for CH4 reduction throughout the region.
8.4. Comparison of Alternative Measures and Policies
Most of the technical options for CH4 emissions reduction are independent of each other, and not mutually exclusive. Recycling of some solid waste and composting of others can occur simultaneously. The remainder may be placed in landfills where land disposal costs are low, or incinerated. CH4 from landfills may be used for energy where possible, and flared if recovery costs are not competitive with alternative energy sources. Overall, 30-50% reductions in CH4 emissions are economically feasible (SAR II, 188.8.131.52; SAR III, 9.4.1). Using the range of emissions estimates in the IS92 scenarios, this implies equivalent carbon reductions of about 55-140 Mt in 2010, 85-170 Mt in 2020, and 110-230 Mt in 2050.
Wastewater CH4 removal options involve a choice between traditional aerobic treatment and recently improved anaerobic processes. The latter appears to have a cost advantage (both capital and operating costs).
The associated environmental impacts of CH4 reduction alternatives are generally positive. Indeed, CH4 reduction may be a secondary benefit of processes that reduce water and air pollution and improve health. Difficulties in quantifying these primary economic benefits make it difficult to estimate the cost-effectiveness of CH4 reduction. For solid wastes, costs for recycling are expected to be low, for composting medium (as a consequence of land disposal costs), and for incineration relatively high (as a consequence of high investment and operational costs); the feasibility of specific applications depends on local circumstances. Costs for CH4 recovery from landfills are expected to be low to medium. Aerobic treatment of wastewater is expected to have medium to high costs, while anaerobic treatment costs will be in the low to medium range.
Macro-economic consequences also are generally favorable. The waste stream is a source of raw material for the production of recycled products, compost, or energy recovery -- contributing to economic production and creating jobs, while providing health and air pollution benefits that can make major contributions to development for lower income countries. Acquiring knowledge in some technologies may imply foreign exchange costs for those non-Annex I countries that do not have them. For this reason, technical assistance is an important measure from a developmental and environmental perspective for lower income non-Annex I countries.
Equity considerations are also generally favorable, within and across countries, as well as across generations. The poor suffer more the consequences of improper waste management, and are also more likely to benefit from the jobs created. Future generations will benefit insofar as today's waste stream is a considered a resource, reducing the consumption of primary raw materials.
As with the technical options, the measures are not mutually exclusive. The
choices involved depend on the circumstances within a given region or country.
Institution building and technical assistance may be starting points for
non-Annex I countries, while voluntary and regulatory initiatives may be more
appropriate for Annex I countries. In countries with well-developed waste
management infrastructures, opposition to regulatory measures could be
expected from the affected industry, although U.S. experience indicates that
this opposition can be surmounted. Regulatory programs may be hardest to
implement successfully in most countries, while market-based programs will
depend both on national priority given to waste management and on
international financing sources available.
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