Biogas and CH(4) productivity by co-digesting swine manure with three crop residues as an external carbon source

Co-digesting swine manure with three agricultural residues, i.e., corn stalks, oat straw, and wheat straw, to enhance biogas productivity was investigated in this study. A 3x3 experimental design with duplicates was adopted (3 crop residuesx3 carbon/nitrogen ratios) to examine the improvement of batch digestion in terms of biogas volume produced, CH(4) content in the biogas, and net CH(4) volume. The crop residues were first cut into small sections and then ground into fine particles smaller than 40 mesh size (0.422mm) before being added to digesters. All the digesters were run simultaneously under controlled temperature at 37+/-0.1 degrees C. The length of experiment was 25days. The results showed that all crop residues significantly increased biogas production and net CH(4) volume at all C/N ratios, among which corn stalks performed the best with increase in daily maximum biogas volume by 11.4-fold as compared to the control, followed by oat straw (8.45-fold) and wheat straw (6.12-fold) at the C/N ratio of 20/1, which was found to be the optimal C/N ratio for co-digestion in the present study. In addition, corn stalks achieved the highest CH(4) content in the biogas ( approximately 68%), which was about 11% higher than that of oat straw ( approximately 57%), whereas wheat straw and the control both had produced biogas with approximately 47% CH(4) content. Wheat straw demonstrated a lower biogas productivity than corn stalks and oat straw even it had a higher carbon content (46%) than the latter two residues (39%).