Comparison of different pre-treatments to increase methane production from two agricultural substrates

To enhance the methane production from ensiled sorghum forage and wheat straw, thermal, alkaline and thermo-alkaline pre-treatments were performed in batch mode. Alkaline pretreatment was conducted at 40°C for 24 h with the addition of 1 and 10% gNaOH/gTS; thermal and thermo-alkaline pre-treatments at 100°C, and 160°C for 30 min, with and without the addition of NaOH solutions at the same dosages previously mentioned. All the pre-treatments tested led to a solubilisation of the Chemical Oxygen Demand (CODs), with a maximum concentration (around 30-40% for both substrates) obtained at 40 and 100°C with 10% NaOH. Furthermore, a reduction of fibrous fractions was observed for both substrates. The highest lignin reduction, compared to untreated samples, was found at 100°C with 10% NaOH dosage (53% and 72% for wheat straw and sorghum, respectively). Under this pre-treatment condition a high hemicelluloses reduction yield was also found (63% for both substrates). The highest increase in methane yield (up to 32%), compared to untreated substrate was observed at 40°C with 10% NaOH for sorghum. As for wheat straw, significant increases in methane yield were observed at 40°C with 10% NaOH (43%) and at 100°C with 1-10% NaOH (48 and 67%, respectively). According to the results of the preliminary economic analysis, the pre-treatments seemed to be sustainable for both substrates, especially for wheat straw, due to a higher methane yield increased, compared to sorghum.