From the EPA web site:
Methane emissions occur whenever animal waste is managed in anaerobic conditions. Liquid manure management systems, such as ponds, anaerobic lagoons, and holding tanks create oxygen free environments that promote methane production. Manure deposited on fields and pastures, or otherwise handled in a dry form, produces insignificant amounts of methane. Currently, livestock waste contributes about 8 percent of human-related methane emissions in the U.S. Given the trend toward larger farms, liquid manure management is expected to increase.
AgSTAR is an outreach program designed to reduce methane emissions from livestock waste management operations by promoting the use of biogas recovery systems. A biogas recovery system is an anaerobic digester with biogas capture and combustion to produce electricity, heat or hot water.The AgSTAR Program has been very successful in encouraging the development and adoption of anaerobic digestion technology. Since the establishment of the program in 1994, the number of operational digester systems has grown to more than 125 systems across the United States. This has produced significant environmental and energy benefits, including methane emission reductions in 2007 alone of approximately 80,000 metric tons and energy generation of about 275 million kWh. The graph below shows the historical use of biogas recovery technology for animal waste management.
The development of anaerobic digesters for livestock manure treatment and energy production has accelerated at a very fast pace over the past few years. Factors influencing this market demand include: increased technical reliability of anaerobic digesters through the deployment of successful operating systems over the past five years; growing concern of farm owners about environmental quality; an increasing number of state and federal programs designed to cost share in the development of these systems; increasing energy costs and the desire for energy security; and the emergence of new state energy policies (such as net metering legislation) designed to expand growth in reliable renewable energy and green power markets.Since 2003, methane reductions from operational digester systems have increased almost four fold.
Comment: As with landfill gas, the beauty of the manure digesters is that they not only trap a potent greenhouse gas, but they use that gas to generate electricity--electricity that might otherwise be generated by burning coal.
While the EPA is on the right track, at 125 systems they have just gotten started. According to their own numbers, an 80,000 metric ton reduction represents only about .2% of the 41 million metric tons of methane emissions that come from manure every year. But by some estimates, digesters would be cost-effective on about 7000 dairy and pig farms. These would have a generating capacity of about 722 MW, enough to power about 144,000 homes, and would reduce greenhouse emissions by about 30 million metric tons--the same as removing 4.7 million cars from the road.
One policy point: Given how important the agriculture sector is to tackling climate change, it is especially important that the environmental movement give farmers our support when our interests align. We will need their good will to push more far-reaching changes such as conservation tillage. We should strongly support greatly increased subsidies for biogas digesters--we will be giving farmers another source of income while at the same time preventing water and air pollution as well as methane emissions.