Particle Size Effect on Biodegradability and Kinetics During Anaerobic Digestion of Fruit and Vegetable Waste

This study examines the anaerobic digestion (AD) of fruit and vegetable waste (FVW) and digestates to assess the effect of particle size on anaerobic biodegradability (AB) and process rate at different stages of digestion progress. Batch assays were conducted with FVW mixtures and digestates from 5, 10, and 15 days of digestion, using four particle size ranges: Ø1 < 1.8 µm, 1.8 < Ø2 < 500 µm, 500 < Ø3 < 1000 µm, and Ø4 > 1000 µm. While AB and specific methanogenic activity (SMA) showed no significant differences among FVW mixtures, particle size significantly influenced these variables. Methane yields were 298.2 and 309.8 mL CH4·g−1VS for Ø4 and Ø3 particles, exceeding the 186.7 and 161.8 mL CH4·g−1VS obtained for Ø2 and Ø1, respectively. These results indicate that particle size reduction enhanced methane production; however, reducing the particle size below 500 µm inhibits methanogenesis due to intermediate accumulation. Compared to FVW, digestates exhibited a 23% reduction in AB, a 73.9% decrease in SMA, and methane yields of 55.8–294 mL CH4·g−1VS. Additionally, the surface-based kinetic constant (KSBK) decreased from 0.4523 g·dm−2·d−1 for FVW to 0.0437 g·dm−2·d−1 for digestates. These differences are attributed to the rapid consumption of easily biodegradable fractions within the first 5 days of digestion.