Effect of particle size on alkaline pretreatment and methane production of ensiled sorghum forage.

Residual and dedicated energy crops, such as sorghum, are suitable substrates for anaerobic digestion in agricultural biogas plants. However, their complex structure justifies the use of a pretreatment to improve their anaerobic biodegradability. Thus, the aim of this study was to investigate the impact of particle size reduction on structural changes and methane production from ensiled sorghum forage, with and without the addition of sodium hydroxide. Sorghum samples were ground into particles with mean diameters of 994, 471, 269 and 169 μm. Milled samples with 471 and 169 μm particle sizes were soaked in a NaOH solution at 10% gNaOH/gTS dosage and maintained at 55°C for 12 h. BMP tests were performed on all samples. All results are discussed both in terms of specific methane production and kinetic constants. The main conclusion is that milling did not improve methane production (275.3± 3.5 mLCH4/gVS) nor kinetic constants (0.11±0.01 d-1) between 994 and 169 μm. On the contrary, by adding sodium hydroxide, an increase in both methane yield (324.5±0.7 mLCH4/gVS) and kinetic constants (0.16 ± 0.00 d-1) was observed, but these results were not influenced by the particle size. Chemical and infra red spectroscopy (FTIR) analysis were performed to characterize the samples and explain these results.