Mitigation of greenhouse gas emissions by anaerobic digestion of cattle slurry

Abstract Biogas treatment of animal manures is an upcoming technology because it is a way of producing renewable energy (biogas). However, little is known about effects of this management strategy on greenhouse gas (GHG) emissions during fermentation, storage, and field application of the substrates compared to untreated slurries. In this study, we compared cattle slurry and cattle slurry with potato starch as additive during the process of fermentation, during storage and after field application. The addition of potato starch strongly enhanced CH 4 production from 4230 l CH 4 m −3 to 8625 l CH 4 m −3 in the fermenter at a hydraulic retention time (HRT) of 29 days. Extending the HRT to 56 days had only a small effect on the CH 4 production. Methane emissions from stored slurry depended on storage temperature and were highest from unfermented slurry followed by the slurry/starch mixture. Gas emissions from untreated and fermented slurry during storage were further analyzed in a pilot-scale experiment with different levels of covering such as straw cover, a wooden lid and no cover. Emissions of greenhouse gases (CH 4 , N 2 O, NH 3 ) were in the range of 14.3–17.1 kg CO 2 eq. m −3 during winter (100 day storage period) and 40.5–90.5 kg CO 2 eq. m −3 during summer (140 day storage period). A straw cover reduced NH 3 losses, but not overall GHG emissions, whereas a solid cover reduced CH 4 and NH 3 emissions. After field application, there were no significant differences between slurry types in GHG emissions (4.15–8.12 kg CO 2 eq. m −3  a −1 ). GHG emissions from slurry stores were more important than emissions after field application. Co-digestion of slurry with additives such as starch has a large potential to substitute fossil energy by biogas. On a biogas plant, slurry stores should be covered gas-tight in order to eliminate GHG emissions and collect CH 4 for electricity production.

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