Improving acid-stressed anaerobic digestion processes with biochar - towards a combined biomass and carbon management system

Interest in biochar as an additive to enhance anaerobic digestion (AD) has grown in the context of biomass cascading use and the 2050 net-zero goal. However, few studies have investigated the effects of biochar on AD from a biochar production perspective, including biomass feedstocks and pyrolysis temperatures. To valorise biomass and better understand the mechanisms and environmental implications of using biochar in AD, this study investigated the effects of distinct biochar types on AD under acid stress-induced process inhibition using batch tests. The results demonstrated that biochar can mitigate acid stress and enhance the methane production rate. The kinetic rate constant of methane production is positively related to the buffer capacity of the tested biochars (R ² = 0.88). The choice of feedstocks is a crucial factor (P = 0.003), particularly the best-performing biochars derived from raw grass silage. In contrast, the pyrolysis temperature effect was less significant (P = 0.18). Furthermore, the analysis of biochar indicates that the alkali (K) and alkaline earth (Ca, Mg) metals contained in biochar may be one of the important factors contributing to buffer capacity (R ² = 0.82 to 0.86). Hence, buffer capacity is a crucial quality criteria when evaluating biochar for AD applications. Raw grass silage biochars are promising for acid stress mitigation due to their high buffer capacity, while carbon-rich woody biochars have high CO₂ sequestration potential. A compromise between mitigating acid stress and sequestering carbon is the use of pre-treated grass biochar. Overall, the use of biochar-enriched digestate offers a potential way to close material loops and complete the biomass-to-biochar value chain.