The approach the U.S. Environmental Protection Agency (EPA) uses to estimate greenhouse gas emissions from agricultural anaerobic lagoons that treat manure contains errors and may underestimate methane emissions by up to 65 percent, according to scientists from the University of Missouri.
A 2007 U.S. Supreme Court ruling mandated the EPA consider greenhouse gases a pollutant. This led the EPA in 2009 to approve greenhouse gas reporting requirements for any facility that annually releases 25,000 metric tons or more of carbon dioxide equivalents to the atmosphere. The objective of these reporting requirements is to quantify emissions as a first step towards developing strategies to reduce greenhouse gas losses.
Direct measurements of methane emissions from anaerobic lagoons are technically difficult and very expensive, so the EPA adopted a calculation method to estimate methane emissions from anaerobic digesters. They relied on the method used by the International Panel on Climate Change (IPCC) in their 2006 worldwide estimate of greenhouse inventories.
An interdisciplinary team of scientists from the University of Missouri evaluated the EPA and IPCC approach to estimate greenhouse emissions from anaerobic lagoons. They reported the results of their analysis in the May-June issue of Journal of Environmental Quality, published by the American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America.
The team documented errors in the approach, which the EPA and IPCC adapted from a method used to estimate methane production from anaerobic digesters.
A literature review of the performance of uncovered anaerobic lagoons indicated that there are important difference between anaerobic lagoons and anaerobic digesters that were not accounted for in the EPA and IPCC approach. They found that uncovered anaerobic lagoons were more efficient at converting waste to methane than predicted using literature based on digesters. The team also found mistakes the equations that the EPA and IPCC used.
John Lory, a member of the team that reviewed the EPA rule, says "Our calculated estimates of methane emissions from anaerobic lagoons indicated that the EPA approach could substantially underestimate methane emissions from these facilities, perhaps by as much as 65 percent."
The report also suggests that some other operations currently excluded under the rules may in fact produce emissions beyond the threshold. Manure storage facilities are the only on-farm source required to report under the current rules. The most likely manure storage facilities to meet the current reporting requirements are anaerobic lagoons. EPA projected that operations with more than 3,200 dairy cows or 34,100 pigs would likely meet the reporting requirement.
Lory emphasizes that there have been few direct measurements of methane emissions from anaerobic lagoons and the few measurements that exist indicate that both the University of Missouri calculated estimate and the EPA calculated estimate of methane loss from these facilities may be high.
Though the research team outlined a different approach to measuring methane emissions, but pointed out that understanding of anaerobic lagoons is still evolving. "More research is needed before we can provide accurate estimates of methane losses from anaerobic lagoons," adds Lory.