Unveiling biochar’s role in Anaerobic Digestion: myths and facts for enhanced biogas production and sulfate inhibition mitigation

Anaerobic digestion (AD) is a key facilitator of circular bioeconomy, fulfilling the pressing need for sustainable energy sources and effective management of the continuously growing organic waste streams (Magnusson et al., 2022). In pursuit of economic enhancement, biogas facilities are actively exploring strategies to maximize biogas yield from processed waste materials. Recently, the addition of biochar, a low-value biobased carbon material, has been presented as a viable strategy to significantly improve digestion and increase methane production. Its physicochemical properties enable it to serve as an adsorbent of inhibitory compounds, pH regulator, promoter of microbial immobilization, facilitator of microbial interactions, and stimulant of direct interspecies electron transfer (DIET) (Wang et al., 2022). While literature highlights the potential of biochar addition to enhance digestion and substantially increase methane yield (Kumar et al., 2021), the involved mechanisms remain unclear. This study aimed to assess the role of biochar in the AD process and provide a comprehensive understanding of the mechanisms involved.

To this end, this study investigated the effect of biochar addition in anaerobic digestion systems, starting with a model substrate, cellulose, and using biochars from various feedstocks and production methods. Biochars were evaluated in terms of methane production during mesophilic digestion of cellulose, and residual biodegradable organic content, followed by a thorough characterization of their physicochemical properties (CHNOS, BET-specific surface area, QSDFT porosity, SEM, pH, EC). Production temperatures ranged from 400 to 950 oC to assess the effect of biochar formation temperature on the AD process. The influence of biochar dosage and the microbial population distributions during AD cultivations were also examined.

Tested biochars presented great variations within their physicochemical properties. Although some biochars appeared to have favorable to digestion properties and led to an increase in the relative abundance of the methanogenic community, results indicated that, regardless of the added concentration, tested biochars had a rather neutral effect on methane production, contradicting existing literature. An inhibitory to the process biochar was also observed, with inhibition being attributed to its raw material and high production temperature (Godlewska et al., 2021). Microbial results also reflected this inhibitory behaviour, showing a poorer microbial population. An increase in methane potential of up to 40% was attributed to the residual biodegradable organic matter in biochar produced at a low temperature (400 oC) and under incomplete anoxic conditions. Thus, it is clear that beyond reaching stoichiometric methane potential, no additional methane production should be expected from the addition of biochar, unless biochar itself acts as a co-substrate.

Finally, this study evaluates the role of biochar in the digestion of sulfate-rich wastes and its possible mitigation mechanisms in terms of process efficiency and recovery of methanogenic activity. Overall, this project contributes to clarifying the function and impact of biochar as a biobased conductive material in the anaerobic digestion process.