Comparative study of different pretreatments to increase methane production of ensiled sorghum forage
Résumé
Sorghum represents a suitable substrate for anaerobic digestion in agricultural biogas plants. However, its complex structure composed mainly of lignin, cellulose and hemicelluloses justifies the use of pretreatments to increase the anaerobic biodegradability and therefore the methane production of sorghum. The aim of this study was to compare the effects of different types of pretreatment (mechanical, thermal, chemical and biological) on chemical composition and anaerobic biodegradability of ensiled sorghum forage. For that purpose, Biological Methane Potential (BMP) tests were performed on untreated (994 μm particle size) and pretreated sorghum. Ensiled sorghum forage, milled into 471, 269 and 169 μm particle sizes did not showed any significant methane yield, compared to untreated sample (0.28±0.00 LCH4/gVS). Moreover, BMP tests revealed that no improvements in methane production of sorghum were observed after a thermal (0.28±0.01 and 0.27±0.01 LCH4/gVS at 100 and 160°C, respectively), and enzymatic pretreatment (0.23±0.01 LCH4/gVS), compared to untreated sample (0.28±0.00 LCH4/gVS). A significant methane production increase was observed after sodium hydroxide pretreatment (up to 26%) and combined sodium hydroxide (at 10% gNaOH/gTS) and enzymatic pretreatment (up to 35%). Chemical and infra red spectroscopy (FTIR) analysis were performed to characterize the samples and explain these results.