Effect of Particle Size on Methane Production of Raw and Alkaline Pre-treated Ensiled Sorghum Forage

In Italy, sorghum is a suitable substrate for anaerobic digestion in agricultural biogas plants. However, its complex structure justifies the use of a pre-treatment to improve its anaerobic biodegradability. Thus, the aim of this study was to investigate the impact of particle size on structural changes and methane production from ensiled sorghum forage, with and without the addition of sodium hydroxide (NaOH). Sorghum samples were ground into particles with mean diameters of 2, 1, 0.5 and 0.25 mm. Then, samples milled into 1 and 0.25 mm particle sizes were soaked in a NaOH solution at 10 gNaOH/100gTS dosage and maintained at 55°C for 12 h. The study revealed that no significant differences in terms of chemical composition were observed between 2 and 0.25 mm particle sizes. Moreover, after the addition of NaOH a similar reduction of lignin (around 30%), cellulose (around 30-40%), and hemicelluloses (around 40-45%), on the solid fraction separated after the pretreatment, was observed for both particle sizes (1 and 0.25 mm). Similar results were confirmed by the Infrared spectroscopy technique which revealed that the crystallinity of cellulose seemed to be not significantly affected by the particle size reduction with and without the addition of NaOH. Finally, milling did not improve the methane production (275.3 ±3.5 mLCH4/gVS) and the kinetic constants (0.11 ± 0.01 d-1) between 2 and 0.25 mm. On the contrary, by adding the NaOH solution, an increase in both methane yield (by 324.5 ± 0.7 mLCH4/gVS) and kinetic constants (by 0.16 ± 0.00 d-1) was observed, but these results were not significantly influenced by the particle size.