Biological stability of multi-component agri-food digestates and post-digestates.

The use of digestate in agriculture has been an efficient way to mitigate greenhouse gas emissions through the recycling of organic materials. However, harmful effects can arise if the organic matter is unstable. The goal of this study was to determine the biological stability (4-day oxygen demand for degradation of readily biodegradable organic matter (AT4), 21-day anaerobic biogas potential (GP21), and organic matter (VS) content) of six digestates after mesophilic digestion, and that of the corresponding post-digestates after psychrophilic post-digestion. Moreover, the kinetics of the changes in biological stability during post-digestion were determined. Mesophilic digestion of six multi-component agri-food feedstocks consisting of maize silage, bovine manure, mallow silage, pig slurry, glycerin, and spent wash from distillation was carried out at an organic loading rate of 2-3 kg VS/(m·d), and at a hydraulic retention time of 45-60 days. Digestates were left in stirred reactors, imitating storage digesters, and kept for the next 120 d under anaerobic psychrophilic conditions (20 ± 1 °C) for further stabilization. The additional biogas yields during post-digestion (50.9-114.9 dm/kg TS) accounted for 8.5-27.4% of the biogas productivity of the feedstocks and 40-80% of that of the digestates. The efficiency of the loss of organic matter content was 22.5-40.2%. The decrease in the values of AT4, GP21 and VS content made the post-digestates more biologically stable than the digestates (digestates: AT4 = 13.7-67.0 mg O/g TS, GP21 = 71.5-130.1 dm/kg TS; post-digestates: AT4 = 6.6-37.4 mg O/g TS, GP21 = 15.7-79.2 dm/kg TS). For digestates and post-digestates, AT4 values strongly correlated with GP21 values.