Sammendrag
In common with other European countries, the Norwegian agriculture sector is responsible for the greatest emission of greenhouse gases, methane and nitrous oxide, as well as ammonia into the atmosphere. The treatment of animal manure by anaerobic digestion for the production of biogas has been identified as a priority measure by the government as a means of mitigating greenhouse gas emissions with possibilities of up to 30% of manure being managed to produce biogas by 2020 (Meld. St. 39, 2008-09; Meld. St. 21 (2011-2012)). In order to evaluate the benefit of manure management by anaerobic digestion it is necessary to document the release of greenhouse gases from digestate and compare these with untreated slurry. This report focused on a review of the literature available on the release of greenhouse gases from stored digestate produced from the anaerobic digestion of manure during the process of biogas production.
Different properties of digestate compared to untreated slurry resulted in a specific emission pattern for the relevant gases. Variability in the published data made it difficult to extract patterns but generally methane emissions are greatly reduced whereas the effects on nitrous oxide and ammonia emissions are more variable. The use of a surface cover or a cover over the storage tank can reduced methane and ammonia emissions during digestate storage but can result in higher nitrous oxide emissions in some cases. Research on N2O potential and production mechanisms in digestate is lacking. Further research in the form of large scale measurement programs, possibly in cooperation with other Scandinavian countries or the expansion of an existing GHG model are possibilities. Regardless, some level of field measurement data of GHG emissions and other parameters relevant for Norwegian conditions are deemed necessary.
Different properties of digestate compared to untreated slurry resulted in a specific emission pattern for the relevant gases. Variability in the published data made it difficult to extract patterns but generally methane emissions are greatly reduced whereas the effects on nitrous oxide and ammonia emissions are more variable. The use of a surface cover or a cover over the storage tank can reduced methane and ammonia emissions during digestate storage but can result in higher nitrous oxide emissions in some cases. Research on N2O potential and production mechanisms in digestate is lacking. Further research in the form of large scale measurement programs, possibly in cooperation with other Scandinavian countries or the expansion of an existing GHG model are possibilities. Regardless, some level of field measurement data of GHG emissions and other parameters relevant for Norwegian conditions are deemed necessary.