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. [Display omitted] •Hydrothermal pretreatment of safflower straw was studied for biogas production.•The pretreatment solid and liquid fractions were subjected to anaerobic digestion.•Hydrothermal pretreatment of safflower straw significantly improved biogas yield.•Maximum biogas yield was obtained at low severity pretreatment conditions.