Anaerobic digestion (AD) is a valuable process for managing food waste (FW) and recovering energy in the form of biomethane. Recent studies have explored the addition of carbon materials (CMs) to enhance biomethane yield, yet the precise mechanisms remain unclear. A study published in Scientific Reports by Michał Kozłowski, Bernard Papaj, Karolina Sobieraj, Kacper Świechowski, Katarzyna Kosiorowska, and Andrzej Białowiec investigates how the specific surface area (SSA) and functional groups of different CMs influence biomethane production from food waste. The CMs analyzed were produced via torrefaction (TP, 240∘C/60 min), pyrolysisPyrolysis is a thermochemical process that converts waste biomass into bio-char, bio-oil, and pyro-gas. It offers significant advantages in waste valorization, turning low-value materials into economically valuable resources. Its versatility allows for tailored products based on operational conditions, presenting itself as a cost-effective and efficient More (BC, 600∘C/60 min), and hydrothermal carbonization (HC, 240∘C/60 min/6-10 Bar).

The research challenged the prevailing assumption that a large specific surface area is the primary driver for improved biomethane yield. Surprisingly, the study found that CMs with the lowest SSA, specifically TP (7.72 m2g−1) and HC (5.46 m2g−1), resulted in the highest biomethane potentials. TP yielded 407 mL gVS−1 and HC yielded 394 mL gVS−1, which were approximately 13% and 9% higher than biomethane production from food waste alone. In contrast, biocharBiochar is a carbon-rich material created from biomass decomposition in low-oxygen conditions. It has important applications in environmental remediation, soil improvement, agriculture, carbon sequestration, energy storage, and sustainable materials, promoting efficiency and reducing waste in various contexts while addressing climate change challenges. More (BC), which had the highest SSA (115.00 m2g−1), actually decreased the first-order biomethane rate constant by 16.4% compared to food waste without CMs.

Further analysis using Fourier Transform Infrared (FTIR) spectroscopy provided insights into these findings. The FTIR analysis confirmed a greater abundance of diverse organic functional groups on the surface of TP and HC. These organic residuals, acting as thermal degradation products, likely served as an additional source of carbon for the microorganisms involved in AD. This suggests that the chemical composition and surface functionalities of CMs might be more critical for enhancing biomethane production than their mere surface area.

The study also investigated the kinetics of biomethane production. The theoretical biomethane potential for the food waste used was 457.03 mL gVS−1. Food waste alone achieved about 79% of this theoretical potential (360 mL gVS−1). The addition of TP and HC significantly improved this efficiency to 89% and 86% of the theoretical potential, respectively. However, the addition of BC did not improve the ultimate biomethane potential (363 mL gVS−1), and even showed a statistically significant decrease in the first-order biomethane rate constant. This negative impact from BC could be linked to the presence of more complex or potentially harmful organic compounds on its surface.

The research emphasizes the inconsistency in findings across various studies regarding the influence of CMs on biomethane formation. This highlights the need for more systematic investigations into the qualitative and quantitative characteristics of organic compounds on the surface of CMs to fully understand their impact on microbial activity and biomethane yields. The findings from this study suggest that for enhancing biomethane production from food waste, torrefied products and hydrochar, with their rich functional groups, hold greater promise than biochar with a high specific surface area.

Source: Kozłowski, M., Papaj, B., Sobieraj, K., Świechowski, K., Kosiorowska, K., & Białowiec, A. (2025). The effect of different carbon materials’ addition on the biomethane production from food waste. Scientific Reports, 15(18728).