Carbon footprint of organic fertilizers derived from cattle slurry and grass-clover: A life cycle assessment

Summary

Anaerobic digestion is a widely used method for treating liquid manure (slurry) and other biomasses, producing biogas as a renewable energy source for heating and electricity generation. The digestate from the biogas plant can be further processed to produce alternative organic fertilizers and enhance nutrient recycling, but the impact of digestate post-treatment on the environment needs to be further investigated. In this study, using a life cycle assessment approach, we explored how fertilizers derived from cattle slurry and grass-clover co-digestion treatment (digestate, solid and liquid fractions from digestate separation, and an enriched liquid nitrogen-sulfur product derived from post-processing of biogas and drying of the solid fraction) may influence the greenhouse gas (GHG) balance compared to the baseline scenario with untreated cattle slurry. To perform this analysis, results from experiments were used together with data from existing literature and databases. The chosen functional units were I) 100 kg of total nitrogen in the final organic fertilizer and II) 1 ton of spring barley dry matter (DM). The respective system boundaries were cradle-to-processing gate and cradle-to-field application, respectively. The global warming potential (GWP) of production and storage of the alternative organic fertilizers were 13 to 25% of the baseline scenario. When considering the second functional unit, the carbon footprint were 25 to 140% of the baseline scenario. The main hotspots in the carbon footprint of the fertilizers were GHG emissions from storage and field application of the organic fertilizers, and production of the grass-clover, whereas the biogas produced by anaerobic digestion greatly reduced the GWP. The present study shows that there is significant potential for climate impact mitigation in the replacement of untreated slurry with alternative fertilizers derived from anaerobic digestion, mainly due to reduced emissions during storage, and the production of biogas.