Ammonia emissions after field application of anaerobically digested animal slurry: Literature review and perspectives

Emission of greenhouse gases from liquid animal manure (slurry) can be reduced if slurry is treated through anaerobic digestion. The effluent from the reactor (digestate) can be utilized as a nutrient source for crop production if applied in the field. Physical and chemical properties of digestate vary considerably due to large variation in digester feedstock and operation, and these differences can affect ammonia emission following field application. Generally, there is a scarcity of literature assessing the downstream effects of anaerobic digestion of slurry on ammonia emission in the field, and different results have been reported. The present work summarizes current knowledge on ammonia emission after field application of digested slurry through a systematic literature review. Furthermore, implications of how the literature data are interpreted is evaluated with a focus on the current situation in Denmark, where the biogas industry is rapidly expanding. The potential effects of differences in pH and dry matter between raw slurry added to the digester and the digestate was evaluated with the ALFAM2 model. Overall, no consistent difference in emissions from digestate and undigested slurry was observed in literature data, likely due to opposing effects of consistent decreases in dry matter and increases in pH. However, due to the use of high dry matter co-substrates for biogas production in Denmark, the literature digestates are not representative of the current Danish conditions, where digestates may have high dry matter contents. Predictions from the AFLAM2 model show that the trend towards increasing dry matter contents of the digestates coupled with nearly universal elevated pH increases the risk of high ammonia emission. Better empirical documentation of emission is urgently needed, especially for high dry matter and high-viscosity digestate, along with new methods to mitigate emissions from land application of digestates with high emission potential.