Description and assessment of mitigation technologies and practices, options and potentials, costs and sustainability

Existing waste management technologies can effectively mitigate GHG emissions from this sector – a wide range of mature, low- to high-technology, environmentally-effective strategies are commercially available to mitigate emissions and provide co-benefits for improved public health and safety, soil protection, pollution prevention and local energy supply. Collectively, these technologies can directly reduce GHG emissions (through landfill CH₄ recovery and utilization, improved landfill practices, engineered wastewater management, utilization of anaerobic digester biogas) or avoid significant GHG generation (through controlled composting of organic waste, state-of-the-art incineration, expanded sanitation coverage). In addition, waste minimization, recycling and re-use represent an important and increasing potential for indirect reduction of GHG emissions through the conservation of raw materials, improved energy and resource efficiency and fossil fuel avoidance. For developing countries, environmentally responsible waste management at an appropriate level of technology promotes sustainable development and improves public health (high agreement, much evidence) [10.4].

Because waste management decisions are often made locally without concurrent quantification of GHG mitigation, the importance of the waste sector for reducing global GHG emissions has been underestimated (high agreement, medium evidence) [10.1; 10.4]. Flexible strategies and financial incen-tives can expand waste management options to achieve GHG mitigation goals – in the context of integrated waste management, local technology decisions are a function of many competing variables, including waste quantity and characteristics, cost and financing issues, regulatory constraints and infrastructure requirements, including available land area and collection/transportation considerations. Life-cycle assessment (LCA) can provide decision-support tools (high agreement, much evidence) [10.4].

Landfill CH₄ emissions are directly reduced through engineered gas extraction and recovery systems consisting of vertical wells and/or horizontal collectors. In addition, landfill gas offsets the use of fossil fuels for industrial or commercial process heating, onsite generation of electricity or as a feedstock for synthetic natural gas fuels. Commercial recovery of landfill CH₄ has occurred at full scale since 1975 with documented utilization in 2003 at 1150 plants recovering 105 MtCO₂–eq/yr. Because there are also many projects that flare gas without utilization, the total recovery is likely to be at least double this figure (high agreement, medium evidence) [10.1; 10.4]. A linear regression using historical data from the early 1980s to 2003 indicates a growth rate for landfill CH₄ utilization of approximately 5% per year. In addition to landfill gas recovery, the further development and implementation of landfill ‘biocovers’ can provide an additional low cost, biological strategy to mitigate emissions since landfill CH₄ (and non-methane volatile organic compounds (NMVOCs)) emissions are also reduced by aerobic microbial oxidation in landfill-cover soils (high agreement, much evidence) [10.4].

Incineration and industrial co-combustion for waste-to-energy provide significant renewable energy benefits and fossil fuel offsets at >600 plants worldwide, while producing very minor GHG emissions compared with landfilling. Thermal processes with advanced emission controls are a proven technology but more costly than controlled landfilling with landfill gas recovery (high agreement, medium evidence) [10.4].

Controlled biological processes can also provide important GHG mitigation strategies, preferably using source-separated waste streams. Aerobic composting of waste avoids GHG generation and is an appropriate strategy for many developed and developing countries, either as a stand-alone process or as part of mechanical-biological treatment. In many developing countries, notably China and India, small-scale low-technology anaerobic digestion has also been practised for decades. Since higher-technology incineration and composting plants have proved unsustainable in a number of developing countries, lower-technology composting or anaerobic digestion can be implemented to provide sustainable waste management solutions (high agreement, medium evidence) [10.4].

For 2030, the total economic reduction potential for CH₄ emissions from landfilled waste at costs of <20 US$/tCO₂-eq ranges between 400 and 800 MtCO₂-eq. Of this total, 300–500 MtCO₂-eq/yr has negative cost (Table TS.14). For the long term, if energy prices continue to increase, there will be more profound changes in waste management strategies related to energy and materials recovery in both developed and developing countries. Thermal processes, which have higher unit costs than landfilling, become more viable as energy prices increase. Because landfills continue to produce CH₄ for many decades, both thermal and biological processes are complementary to increased landfill gas recovery over shorter time frames (high agreement, limited evidence) [10.4].

For wastewater, increased levels of improved sanitation in developing countries can provide multiple benefits for GHG mitigation, improved public health, conservation of water resources and reduction of untreated discharges to water and soils. Historically, urban sanitation in developed countries has focused on centralized sewerage and wastewater treatment plants, which are too expensive for rural areas with low population density and may not be practical to implement in rapidly growing, peri-urban areas with high population density. It has been demonstrated that a combination of low cost technology with concentrated efforts for community acceptance, participation and management can successfully expand sanitation coverage. Wastewater is also a secondary water resource in countries with water shortages where water re-use and recyling could assist many developing and developed countries with irregular water supplies. These measures also encourage smaller wastewater treatment plants with reduced nutrient loads and proportionally lower GHG emissions. Estimates of global or regional mitigation costs and potentials for wastewater are not currently available (high agreement, limited evidence) [10.4].