Case Study of Separated Liquid Digestate Treatment for Water Recovery and Production of a Nutrient-Rich Solid Fraction

Separated liquid digestate management is a major challenge for anaerobic digestion facilities because of the digestate’s high water content, nutrient load and potential environmental impacts when land application is not adequately controlled. This preliminary single-batch case study evaluated the application of Adiabatic Sonic Evaporation and Crystallisation (ASEC) treatment to an animal-derived separated liquid digestate, focusing on water recovery and the production of a dried nutrient-rich solid fraction. The raw liquid digestate, the ASEC-recovered water and the recovered solid fraction were characterised using physicochemical and microbiological indicators. The recovered water stream showed a strong reduction in electrical conductivity from 11,289 to 42 µS/cm, with turbidity <1 UNF, TSS < 5 mg/L and ammoniacal N < 0.15 mg/L; microbiological indicators were also favourable (no detection of Salmonella spp. or Legionella spp., absence of intestinal nematode eggs and Escherichia coli). The recovered solid fraction contained 95% dry matter (dm), 53.5% organic matter, 3.87% total N, 36,090 mg P2O5/kg dm and 32,800 mg K2O/kg, supporting its preliminary classification as candidate organic-mineral NPK fertilising material. Metal(loid)s were below the contaminant thresholds used as references in the EU for solid organic fertilisers. For a 100 m3/d treatment scenario, the estimated NPK nutrient replacement value was 63,543 USD/year; however, this value should be interpreted as a scenario-based estimate rather than a measured process-scale economic return.