Below is the Executive Summary from the "Current Anaerobic Digestion Technologies Used for Treatment of Municipal Organic Solid Waste" document written for the California Integrated Waste Management Board. Download the PDF to read the complete report.
Anaerobic digestion (AD) is a bacterial fermentation process that operates without free oxygen and results in a biogas containing mostly methane and carbon dioxide. It occurs naturally in anaerobic niches such as marshes, sediments, wetlands, and the digestive tracts of ruminants and certain species of insects. AD is also the principal decomposition process occurring in landfills.
AD systems are employed in many wastewater treatment facilities for sludge degradation and stabilization, and are used in engineered anaerobic digesters to treat high-strength industrial and food processing wastewaters prior to discharge. There are also many instances of AD applied at animal feeding operations and dairies to mitigate some of the impacts of manure and for energy production.
AD of municipal solid waste (MSW) is used in different regions worldwide to:
Over the past 20 years, AD of MSW technology has advanced in Europe because of waste management policies enacted to reduce the long-term health and environmental impacts of landfill disposal. This has led to relatively high landfill tipping fees (compared with California or the U.S.), which, in combination with generous prices paid for renewable energy, has created an active commercial market for AD and other MSW treatment technologies in Europe. Installed AD capacity in Europe is more than 4 million tons per year.
In some parts of Europe, source separation of the organic fraction of municipal solid waste (OFMSW) is common and even mandatory, which contributes to the growth of biological treatment industries. Regions outside of Europe are also enacting more stringent waste disposal regulations, leading to the development of new AD and other MSW conversion plants.
Although the U.S. has a long history of treating agricultural and municipal wastewater with anaerobic digesters, no commercial-scale solid waste digesters are operating despite several favorable (though economically marginal) feasibility studies and laboratory findings.
Generally in the U.S. and most of California, landfills continue to be the lowest-cost option for managing MSW, since unlike Europe and Japan, space for new landfills is not as scarce, waste management policies are less rigorous, and full life-cycle costs and impacts are not accounted for. Furthermore, the energy market and regulatory mechanisms for licensing MSW AD and other conversion facilities in California have not been developed to easily accommodate commercial systems.
Composting of the OFMSW has increased significantly over the past 15 years, particularly for source-separated wastes, but by far the majority of the yard and food waste generated in the U.S. still goes to landfills. AD facilities are capable of producing energy and reducing the biodegradable content of the organic waste prior to composting, which reduces emissions of pollutants and greenhouse gases. However, these environmental and public health benefits have not been adequately internalized economically, especially considering the lack of familiarity with the technology. Investors and city planners will be more likely to adopt AD of MSW if additional revenues are provided initially. These revenues can come from supports for the energy produced (i.e. tax credits and guaranteed markets), increased tipping fees and, potentially, green or carbon credits.
Many European countries have passed laws mandating that utility companies purchase green energy, whereas in California few of the farms or wastewater treatment facilities that produce excess electricity from biogas have secured contracts with the utilities. Additionally, while European Union directives have called for mandatory pre-treatment and decreased disposal of biodegradable material in landfills, no equivalent regulations exist in federal or state codes. However, waste diversion requirements or targets exist in California and many other states in the U.S., and reducing OFMSW disposal has been a focus of waste managers and municipalities attempting to achieve the targets.
Nonetheless, interest in AD of MSW is growing, and several California jurisdictions are investigating landfill alternatives that include AD. The technologies have been used successfully for over ten years in Europe where the industry continues to expand. Facilities were also built recently in Canada, Japan, Australia and several other countries.
The European market has shown a large preference for single-stage over two-stage digesters and a slight preference for dry digestion systems over wet systems. However, the choice of AD technology depends on the composition of the waste stream, co-product markets, and other site-specific requirements. The design of any new digester facility should be based on a thorough feasibility study, and special attention should be paid to all aspects of the treatment process, including waste collection and transportation, pre-treatment processing (i.e. pulping, grinding, and sieving), material handling, post-treatment processing (i.e. aeration and wastewater treatment), public education, and strategic siting of the system.
Novel technologies are being developed, and several U.S. institutions hold patents on promising high-rate AD technologies. Many U.S. landfills are being built or modified to enhance biological degradation of the OFMSW and collect the resulting biogas, which may provide a stepping stone to full industrial “out-of-ground” AD of MSW. Landfill bioreactors may merit further consideration in their own right, but special attention should be paid to their performance and air/water emissions. In addition to electricity, other value-added product streams from AD systems could provide revenue to help improve the economic viability of organic waste treatment technologies. For example, technologies for upgrading biogas to natural-gas quality biomethane are available, as are technologies that utilize lignocellulosic materials which include residues from digesters. However, regulatory and definitional barriers need to be minimized in order to fully capitalize on these technologies and product streams.
The public desire for change in waste management practices will lead to a reduction in landfill availability. AD and other conversion technologies have the potential to minimize the environmental impact of waste disposal by reducing the amount of biodegradable materials in landfills. Public policies that encourage organic solid waste disposal reduction will help to facilitate the adoption of such technologies. In addition, as the technologies advance, their installation costs should decrease. However, as development of MSW AD facilities in the U.S. proceeds, it would be wise to use the wealth of past experience available in order to reduce potential problems and expedite the development of organic waste treatment. AD technology developers need to work closely with waste collection and management companies in order to develop and implement appropriate digester system designs and material handling strategies and achieve successful enterprises.