Effect of Grinding on Anaerobic Digestion of Green Waste: the Role of Hydrolysis in Methane Production

Abstract

This study assessed the impact of grinding on the hydrolysis stage of anaerobic digestion (AD) of green waste (GW), correlating particle size reduction and CH4 production profiles. For this purpose, GW was used in two forms, ground (GGW) and cut (CGW), which exhibited particle sizes of 0.36 and 20 mm, respectively. Both water retention capacity and water solubility were enhanced upon grinding, which was associated with an increase in soluble organics concentration in the medium. Although no statistically significant difference (p > 0.05) was observed between the cumulative CH4 yield of the AD of CGW (118.4 NmL CH4/gVS) and GGW (138.7 NmL CH4/gVS), the CH4 production profiles differed notably. The AD of GGW showed typical behavior, while that of CGW showed a diauxic pattern. The kinetic study, considering the modified Gompertz and logistic models, revealed that the AD of CGW was slower, characterized by a more prolonged lag phase (∼ 19 d) compared to GGW (∼ 1 d). Furthermore, the time required to reach 90% of the maximum CH4 yield was longer for CGW (23 d) compared to GGW (10 d). These results reveal that although grinding of GW did not lead to an increase in the CH4 yield, it facilitated the hydrolysis of GW, potentially decreasing the hydraulic retention time of AD process.