Effect of Feeding Ratio on Anaerobic Co-Digestion of Rice Straw and Cow Manure

Mesophilic (37°C) anaerobic co-digestions of rice straw and cow manure were carried out based on different VS ratios (0:1, 1:2, 1:1, 2:1 and 1:0). Results indicated that co-digestion has a better performance than mono-digestion of rice straw; both biogas and methane yield were significantly increased. Biogas yield was 363.33, 402.45, 390.25, 388.50 and 389.33 mL/gVS for RS: CM ratios of 1:0, 2:1, 1:1, 1:2 and 0:1, and the corresponding methane yield was 175.01, 194.91, 185.13, 194.59 and 189.09 mL/gVS, respectively. Average methane content was 48.17%, 48.43%, 47.44%, 50.09% and 48.57%, respectively. No volatile fatty acids and ammonia inhibition was observed in the experiment. The pH values were in the range of 6.8-7.5. By comparing the ratios of actual to theoretical biogas yield (Y_a/Y_m), it was found that the ratio increased when carbon to nitrogen (C/N) ratio was 15.10-28.85; the further C/N ratio increase led to Y_a/Y_m decrease. In actual application, the ratio of rice straw to cow manure of 2:1 and HRT of 18 days was recommended.