In their article, “The Influence of Biodegradability and Inoculum-to-Substrate Ratio on the Anaerobic Digestion Performance and Microbial Diversity,” published in GCB Bioenergy, Marvin T. Valentin and colleagues investigated the interplay between a substrate’s biodegradability, the inoculum-to-substrate ratio (ISR), and the addition of biochar in continuous-flow anaerobic digestion (AD) systems. The study utilized three substrates with varying biodegradability—glucose (GL, high), food waste (FW, medium), and wheat straw (WS, low)—at two different ISRs, 2.0 (optimal) and 0.5 (overloaded), to simulate different stress conditions in a continuous digestion environment.

The research highlights a critical challenge in AD: managing the organic load relative to the microbial community, especially when using materials with different degradation characteristics. Highly biodegradable substrates like glucose and, to a lesser extent, food waste, can rapidly decompose, causing a harmful accumulation of volatile fatty acids (VFAs) that overwhelms the methanogenic archaea and leads to system failure, as observed with the GL0.5 reactor. Conversely, hardly biodegradable wheat straw (WS) degrades slowly, meaning it requires higher organic loads to achieve effective methane production without being inhibited by acid build-up. For the high organic load condition (ISR 0.5), the continuous system reactors fed with glucose (GL0.5) and food waste (FW0.5) showed signs of stress, accumulating total VFA concentrations of 9.70 g/L and 16.57 g/L, respectively, leading to low conversion efficiency and reduced specific methane production (SMP).

The most significant finding concerns the use of biochar (BC) under stressful, low-biodegradability conditions. When biochar was introduced into the continuous-flow system, it had a marked impact on the wheat straw reactor at the high organic load (WS0.5). During the BC phase, the average specific methane production (SMP) at WS0.5 increased to 140.70 mL/(g−VS), which is about 40% higher than the 100.10 mL/(g−VS) observed in the phase immediately preceding BC addition. This suggests that biochar is highly effective for stabilizing and enhancing the AD of challenging, low-biodegradability feedstocks at low ISRs.

Microbial analysis provided a mechanistic explanation for this improvement. The addition of biochar to WS0.5 caused a significant increase in the relative abundance of the mcrA gene, which is a marker for methanogens, indicating an enriched functional microbial community. Specifically, the archaeal community shifted from being dominated by Methanomicrobiaceae to a dominance of stress-tolerant Methanosarcinaceae in the WS0.5 reactor following biochar addition. Methanosarcinaceae are known to be more resilient to stress conditions that often accompany high organic loading rates and VFA accumulation. This shift, coupled with an increase in bacterial diversity, provided metabolic resilience to the system. Furthermore, the study suggests that biochar likely provided essential trace metals like Nickel (Ni) and Cobalt (Co), which act as cofactors for key enzymes like methyl-coenzyme M reductase (MCR) needed for methane production. The biochar likely also helped by adsorbing inhibitory compounds like VFAs, which contributes to overall system stability. Overall, the results emphasize that the beneficial effect of biochar is most pronounced when the AD system is stressed by a high organic load of a difficult-to-degrade substrate, providing a strategy to optimize biogas production from refractory feedstocks.

Source: Valentin, M. T., Kosiorowska, K. E., Siedlecka, A., Świechowski, K., Demeshkant, V., Wiercik, P., Ashikhmina, S., Strzała, T., & Białowiec, A. (2025). The Influence of Biodegradability and Inoculum-to-Substrate Ratio on the Anaerobic Digestion Performance and Microbial Diversity. GCB Bioenergy, 17(e70090).