Influence of Origin and Post-treatment on Greenhouse Gas Emissions After Anaerobic Digestate Application to Soil

Anaerobic digestion is a beneficial organic waste management technology that, in addition to biogasused for energy production, produces a by-product called anaerobic digestate, which can be used as a fertilizer or as an amendment as long as it has no harmful effects on the environment. The objective of the research described in this article was to assess one of these possible harmful effects, associated with the release of greenhouse gas emissions (GHG). Four anaerobic digestates were subjected to phase separation, and some of them also to composting, drying or reverse osmosis. Carbon dioxide (CO2) and nitrous oxide (N2O) emissions were measured during incubations of soil-digestate mixtures under controlled conditions. The mineralization of organic carbon reached 28–58 % of digestate organic carbon after 3 months in the presence of the solid digestates, and was lower (18–42 %) for the liquid digestates. The raw digestates had intermediate intensity of organic carbon mineralization to CO2. Drying and composting reduced CO2 emissions by stabilizing the digestate organic matter. N2O emission factors varied between 0.11 and 2.10 % of total digestate N depending on the origin and state of the digestates (raw, solid, liquid, composted). The highest emissions were measured with the raw digestates, and the lowest generally with the liquid ones. The study showed that in addition to phase separation, composting also reduced GHG emissions whereas drying and reverse osmosis considerably increased these emissions. Compostedand dried digestates can be used as organic amendment leading to potential carbon storage larger than GHG emission, while for raw digestates, the GHG emissions always exceeded potential C storage