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

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 per capita environmental loadings, we conclude that DBF extraction is environmentally beneficial.