Co-digesting swine manure with crop residues shows great promise in boosting methane generation in anaerobic digesters.

The problem with using swine manure only for anaerobic digestion is its low carbon/nitrogen (C:N) ratio (6:1 to 8:1, normal range), while good digestion re-quires a C:N ratio between 16:1 and 25:1.

University of Minnesota agricultural engineer Jun Zhu says the use of crop residues can be an effective way to enhance methane production, and at the same time reduce the volume of crop residue materials for disposal.

In his research, the addition of the crop residues at all C:N ratios increased the total daily gas volume produced (Figure 1). Wheat straw didn't achieve the same increase in gas production compared to corn stalks and oat straw during the first two weeks of production, but surpassed corn stalks later for the C:N ratio of 25.

Corn stalks and oat straw performed equally well in increasing gas production when compared to the control group (Figure 1).

When the C:N ratio was increased from 16:1 to 20:1, there was a clear increase in gas volume produced for all of the materials tested (Figure 1). However, continuing to increase the C:N ratio to 25:1 didn't produce the same increase in gas production seen in the increase in the C:N ratio to 20:1.

Figure 2 shows the methane (CH4) content in the gas from reactors using different crop residues and C:N ratios. Not much difference was observed in methane production between wheat straw and the control. However, the difference was quite significant for the other two crop products tested. Oat straw showed quick success, achieving a 44.4% digestion rate on Day 1 for a C:N ratio of 20:1, and a 58.8% digestion rate and C:N ratio of 16:1 by Day 3.

Corn stalks also reached 45% methane content on Day 5 for C:N ratios of 16:1 and 20:1.

The increase in C:N ratio doesn't seem to correlate with an increase in methane production for wheat straw treatment as compared to the control when the digester enters into a steady state.

But methane content mushrooms for both oat straw and corn stalks during the same period at a C:N ratio of 20:1 with 57% for oat straw, 68.2% for corn stalks and just 46.5% methane digestion rate for wheat straw.

“Since the quantity of pure methane is a product of total gas volume and the methane concentration in the off-gas, the higher the methane content and gas volume, the more methane is produced,” Zhu explains.

“Therefore, it appears that oat straw and corn stalks perform better than wheat straw when co-digested with swine manure, from the perspective of pure methane productivity,” he concludes.

Researcher: Jun Zhu, University of Minnesota Southern Research and Outreach Center at Waseca. Contact Zhu by phone (507) 837-5625, fax (507) 835-3622 or e-mail Zhuxx034@umn.edu.