Psychrophilic and Mesophilic Anaerobic Digestion of Food Waste: Influence of the Rheological Properties of the Substrate and Inoculum

Abstract Rheological properties, such as apparent viscosity (μ) and consistency index (K), of food waste (FW), inoculum, and substrate/inoculum (S/I) mixtures play a critical role in anaerobic digestion (FW–AD). At psychrophilic temperatures (< 20 °C), μ and K increase, limiting mass transfer and reducing methane yield. This study analyzes the influence of μ and K on FW–AD at 10, 15, and 27 °C with different particle sizes (PS1: 11 – 8 mm; PS2: 8 – 6 mm; PS3: 6 – 4 mm; PS4: 2 – 1 mm). Rheological properties were evaluated for substrate/inoculum ratios (S/I: 0.20, 0.30, 0.40, 0.50 gVS substrate ·gVS inoculum –1 ) at the start and end of biochemical methane potential (BMP) tests (days 0 and 60). The lowest μ and K values were at 27 °C with PS2, while within the psychrophilic range, the best performance occurred at 15 °C (PS2).”. Inoculum’s μ and K were lower than FW, indicating that inoculum properties had a greater influence on rheology at lower S/I ratios (0.20; 0.30), whereas substrate rheology dominated at higher S/I ratios (0.40; 0.50). Statistical analysis indicated S/I had the greatest effect on BMP (54%), followed by temperature (28%), while particle size and K contributed minimally (< 1%). K increased proportionally with S/I, which was also the main factor affecting K in response surface models (37% and 32% at days 0 and 60, respectively).The highest BMP (359.43 mL/gVS) occurred at 27 °C with PS2 and an S/I of 0.20, while the maximum BMP at 10 and 15 °C were 38.14 and 90.13 mL/gVS, respectively.