Reduction in methane and nitrous oxide emission from animal slurry through anaerobic digestion

Biogenic emissions of methane (CH4) and nitrous oxide (N2O) occur during handling, storage and after field application of animal manure. The emissions are linked to decomposition of volatile solids (VS), which provide energy for microorganisms. During anaerobic storage, VS drives the microbial processes, which lead to CH4 production. Also, turnover of VS in slurry applied to fields will consume oxygen and can thereby stimulate N2O production. Anaerobic digestion of manure and organic wastes for biogas production removes VS prior to storage and field application, therefore, this treatment also reduces the potential for CH4 and N2O emissions. A model has been developed to account for the effect of anaerobic co-digestion of animal manure and organic waste on CH4 and N2O emissions. The model estimates the reduction in VS during storage and digestion, and an algorithm for prediction of CH4 emissions from manure during storage relates the emission to VS, temperature and storage time. Nitrous oxide emissions from field-applied slurry are calculated using VS, slurry N, soil water potential and application method as input variables, thus linking C and N turnover. The amount of fossil fuel that is substituted by CH4 produced during digestion is also calculated in order to estimate the total effect of anaerobic digestion on greenhouse gas emissions from slurry. The experience from application of the model to different scenarios is that the emission of greenhouse gases and their reduction must be calculated with dynamic and integrated models. Specifically, the results indicate that digestion of slurry and organic wastes could reduce Danish greenhouse gas emissions by as much as 3%.