Use of biochars in anaerobic digestion

This study investigated the behavior of biochars from pyrolysis (pyrochar) and hydrothermal carbonization (hydrochar) in anaerobic digestion regarding their degradability and their effects on biogas production and ammonia inhibition. A batch fermentation experiment (42 °C, 63 days) was conducted in 100 mL syringes filled with 30 g inoculum, 2 g biochar and four levels of total ammonium nitrogen (TAN). For pyrochar, no clear effect on biogas production was observed, whereas hydrochar increased the methane yield by 32%. This correlates with the hydrochar’s larger fraction of anaerobically degradable carbon (10.4% of total carbon, pyrochar: 0.6%). Kinetic and microbiota analyses revealed that pyrochar can prevent mild ammonia inhibition (2.1 g TAN kg−1). Stronger inhibitions (3.1–6.6 g TAN kg−1) were not mitigated, neither by pyrochar nor by hydrochar. Future research should pay attention to biochar–microbe interactions and the effects in continuously-fed anaerobic digesters.