We would all like to think that methane emissions during composting are negligible, but it appears that as we increase the amount of foodwaste in our composting processes, the potential for methane emission increases.
If we think that we can avoid potential methane emissions from organic waste by processing through anaerobic digestion, a recent German study indicates that methane emission from the digestate following anaerobic digestion may be even higher than methane emitted during green waste / food waste composting.
The US Composting Council suggests that very little methane is actually emitted during composting, based on information from the US EPA (http://compostingcouncil.org/admin/wp-content/uploads/2010/09/composting-and-carbon-credits.pdf), and therefore assigns no CH4 emission value. The IPCC suggests that CH4 emissions during the composting process are 4 g per kg of waste (range is 0.03 to 8 g/kg wet weight basis) (http://www.ipcc-nggip.iges.or.jp/public/2006gl/pdf/5_Volume5/V5_4_Ch4_Bio_Treat.pdf).
The Quantification Protocol for Aerobic Composting Projects (http://environment.gov.ab.ca/info/library/7905.pdf) uses the same factor as the IPCC protocol. This protocol does not apply to animal manures because of uncertainty around nitrous oxide emissions.
A recent report in Germany reviewed GHG emissions from a number of organic waste processing plants ( http://hss.ulb.uni-bonn.de/2012/3002/3002.pdf), and found that the CH4 emission from windrow composting ranged from 4000 g/tonne with greenwaste to 10,000 g/tonne with anaerobic digestate blended with greenwaste. In their review of 8 research publications, methane emission ranged from 6 to 4939 g/tonne of greenwaste or organic waste. It appears that the research concludes that methane emission from windrow composting of green waste alone is higher than zero used by the US protocol and consistent the 4 g/kg (4000 g/tonne) used by Canada and the IPCC. When food waste was incorporated (by adding digestate from an anaerobic digester), the methane emission rate more than doubled.
In the German study, CO2e emissions of CH4 from windrow composting of greenwaste was approximately 125 kg/tonne. When digestate from an anaerobic digester was added to the greenwaste, CH4 emissions increased to 300 kg CO2e per tonne. In a study of GHG emissions from wet or dry anaerobic digestion plants, the net CH4 emission ranged from 50 to 457 kg CO2 equivalent per tonne of organic waste. Most of the CH4 emission occurred during the composting process after digestion. Aerated composting resulted in lower CH4 emissions than windrow composting. It was also important to note that significant CH4 emissions were measured even after 47 weeks of composting.
It appears that significant methane emissions occur during composting. Methane emission rates increase with increasing pile size, and with increasing amounts of food waste. Aeration during the composting process reduces methane emission. Managing curing piles is important as methane emission may occur for a very long period following the active composting process.
Although methane is captured during anaerobic digestion, post digestion management is very important to minimize methane emissions. The German study found that methane emissions were up to 0.5 tonnes CO2 equivalent per tonne of digestate during active post digestion management.