Hydrothermal pretreatment of safflower straw to enhance biogas production

Safflower straw, an abundant, inexpensive, and renewable lignocellulosic waste, was used as a substrate of anaerobic digestion to produce biogas. Hydrothermal pretreatment was carried out on the straw at 120, 150, and 180 °C for 1, 2, and 5 h to increase the biogas yield. The pretreatment resulted in a solid (mainly cellulose) and a liquid (mainly hemicellulosic monomers). The highest biomethane yield from solid fraction (191.4 NmL/g VS) was obtained at the least severe pretreatment conditions (120 °C for 1 h), which showed 98.3% improvement in comparison to the untreated straw. The maximum methane yield of 406.9 NmL/g VS was attained from the liquid fraction of pretreatment at 180 °C for 1 h. Overall, at the optimum pretreatment conditions (120 ^°C for 1 h), 148.4 m³ methane was produced from each ton of pretreated safflower straw, whereas the methane production from the untreated straw was 86.9 m³ methane. Furthermore, enzymatic hydrolysis was carried out on solid fractions. The results revealed that the most severe pretreatment conditions (180 °C for 5 h) led to the highest released sugar concentration of 25.1 g/L using the enzyme loading of 10 FPU/g substrate, while it was only 4.5 g/L for the untreated one.