Enhanced biogas production from sorghum stem by co-digestion with cow manure

• The biogas yield for sorghum stem was 413 mL/g VS with the C/N ratio of 25.
• Co-digestion of sorghum stem and manure had the highest biogas yield, 478 mL/g VS.
• Co-digestion enabled a more stable AD process and avoided extreme acidification.
• Certain trace elements in manure may help to enhance the biogas production.

In order to achieve high biogas yield and make clear the effects of carbon to nitrogen (C/N) ratio on digestion to further understand the co-digestion performance, this study investigated the high-solid anaerobic digestion (HSAD) of sorghum stem using urea to adjust its C/N ratio and the co-digestion of sorghum stem and cow manure. The HSAD of sorghum stem possessing high energy potential and high C/N ratio was easy to become acidified, even with addition of urea, leading to generation of plenty of hydrogen during the first 3 days of digestion. When the C/N ratio was adjusted to 25 with urea, the biogas yield was 413 mL/g VS, 26% higher than that of raw sorghum stem. With the C/N ratio of 25, the co-digestion of sorghum stem and cow manure generated the highest biogas yield of 478 mL/g VS. Co-digestion enabled stable pH value and high digestion efficiency and avoided extreme acidification. On one hand, the improvement on biogas production by co-digestion was partially attributed to the adjusted C/N ratio. On the other hand, trace elements in cow manure such as Fe, Ni, Zn, Ca, etc may also contribute to the enhancement in biogas production.