Comprehensive characterization of the liquid fraction of digestates from full-scale anaerobic co-digestion

Waste management by anaerobic digestion generates a final byproduct, the digestate, which is usually separated into solid and liquid fractions to reduce the volume for transportation. The composition of the solid fraction has been recently studied to allow its valorization. However, full composition of liquid fraction of digestate and its size fractionation are less considered in the literature for efficient post treatment and valorization purposes. Therefore, here we characterized in detail liquid fraction of digestate obtained after solid-liquid separation from 11 full-scale co-digestion plants. The liquid fraction has a high concentration in organic matter with Chemical Oxygen Demand (COD) from 9.2 to 78g/L with 60-96% of COD in suspended particles (>1.2μm), 2-27% in colloids (1.2μm to 1kDa) and 2-18% in dissolved matter (<1kDa). Besides, it contained from 1.5 to 6.5g/L total nitrogen and high ions concentrations (0.5-3.1g/L NH₄⁺, 1.05-5.48g/L K⁺, 0-2.13g/L PO₄^3-). In addition, liquid fraction of digestate has poor biodegradability due to presence of humic substances making aerobic treatment inefficient. Only physico-chemical post treatment can be proposed for organic matter removal.