From Waste to Process Enhancer: Pyrolysis-Engineered Biochar as a Booster in Anaerobic Digestion

Anaerobic digestion (AD) is a key technology for organic waste valorization into biogas, but its efficiency is often limited by process instability. The accumulation of inhibitory compounds, such as ammonium and heavy metals, together with pH fluctuations, can impair microbial activity and reduce methane production. Biochar has emerged as a promising additive due to its ability to modulate the physicochemical environment of AD systems. This study investigated the role of biochar properties in enhancing AD performance, focusing on (i) biomass type and (ii) pyrolysis temperature. Four biochars were produced: one from spent coffee grounds (SCG) at 500 °C and three from tomato plant residues (TP) at 500, 700, and 900 °C. This approach enabled comparison of different feedstocks at the same pyrolysis temperature and assessment of the effect of pyrolysis temperature on a selected biomass. A physicochemical characterization was performed to elucidate the functional role of biochar in AD. Point of zero charge (PZC) and BET analysis were used to assess buffering behavior and surface area, respectively. Heavy metal and ammonium release and adsorption tests were conducted to evaluate both potential inhibitory effects and mitigation capacity. The materials were tested in batch anaerobic digesters using livestock digestate fed with glucose. Process performance was evaluated through gas production and composition (CH4, H2, CO2), with the CH4/CO2 ratio as an indicator of biogas quality. Results demonstrated that feedstock plays a key role, with TP-biochar outperforming SCG-based material. Among TP-biochars, 500 °C maximized methane production, while 900 °C yielded the highest CH4/CO2 ratio, indicating improved biogas purification. These findings highlight the importance of tailoring biochar properties to enhance AD stability and efficiency.