Anaerobic digestate readily reacts with CO2 in pyrolysis to produce inorganic solid residues that exhibit higher microbial enrichment than biochar in anaerobic digestion

The increasing use of anaerobic digestion (AD) is creating challenges in the proper treatment and utilization of digestate as a residue. Given the limitations in terms of safety and transportation, alternative methods of conversion of digestate into valuable products are needed. This study demonstrated that the presence of CO2 in the pyrolysis of digestate significantly enhances the conversion of carbon in digestate to CO at lower temperatures than other types of biomass, i.e., between 700 °C and 800 °C. As a result, the properties of the biochar change significantly. In a CO2 atmosphere, the ash content of the biochar increases in the 650–800 °C range, reaching 100 % at 800 °C. This reaction simultaneously stimulated hydroxyapatite crystal formation, and the resulting ash promoted microbial flocculation and sedimentation in AD. This enhanced microbial retention at high concentrations, accelerating methane production through repeated cultivation. Due to these characteristics, inorganic residues derived from digestate showed higher performance as AD additives than biochar derived from woody biomass, which has higher conductivity and increases Geobacter.