Life Cycle Assessment of Biofertilizer Production and Use Compared with Conventional Liquid Digestate Management

Abstract

Handling of digestate produced by anaerobic digestion impacts the environment through emission of greenhouse gases, reactive nitrogen and phosphorus. Previous life cycle assessments (LCA) evaluating the extraction of nutrients from digestate using struvite precipitation and ammonia stripping did not relate synthetic fertilizer substitution (SFS) to nutrient use efficiency consequences. We applied an expanded LCA to compare the conventional management of 1 m3 of liquid digestate (LD) from food waste against the production and use of digestate biofertilizer (DBF) extracted from LD, accounting for SFS efficacy. Avoidance of CH4, N2O and NH3 emissions from LD handling and enhanced SFS via more targeted use of nutrients in the versatile DBF product could generate environmental savings of up to 0.129 kg Sb eq., 4.16 kg SO2 eq., 1.22 kg PO4 eq., 33 kg CO2 eq. and 20.6 MJ eq. per m3 LD, for abiotic resource depletion, acidification, eutrophication, global warming and cumulative energy demand burdens, respectively. However, under worst-case assumptions, DBF extraction could increase global warming and cumulative energy demand by 7.5 kg CO2e and 251 MJ eq. per m3 LD owing to processing inputs. Normalizing these results against wider environmental loadings, we conclude that DBF extraction is environmentally beneficial.