Influence of pig manure and rice straw mass ratio on its biogas production and archaeal communities in dry anaerobic co-digestion system

Abstract: Compared with wet anaerobic digestion, dry anaerobic digestion has the advantages of water conversation, low requirements on the composition of raw materials, less biogas slurry drainage, and low treatment cost of residues. With the increasing scarcity of water resources in the world, this technology is in line with resource utilization and water resources management. However, dry anaerobic digestion also has problems such as ammonia suppression, acid inhibition, and material flow problems. Studies have shown that compared with sole raw material anaerobic digestion, co-digestion can improve methane production performance and gas production rate, advance gas production peak time, balance daily gas production and avoid large fluctuations. To obtain better gas production performance, it is necessary to study the ratios of raw materials. With the rapid development of intensive and large-scale agriculture, livestock manure and crop straw have become China's most important biomass energy. The annual output of livestock manure and crop straw in China reached 3.8 billion tons and 1.04 billion tons respectively, and the total amount of pig manure discharge is the first in several livestock manure. The direct discharge of livestock manure and the burning of crop straw are serious harm to the environment. Pig manures and rice straw were used as raw materials to study the effects of ratios of pig manure to rice straw on gas production performance and archaea. The gas production process was simulated using index of the accumulation VS methane yield by modified Gompertz kinetic model. Therefore, in order to obtain the optimal proportion of raw materials in dry anaerobic batch digestion, the various substrate ratios of pig manure to rice straw at 1∶0, 3∶1, 2∶1, 1∶1, 1∶2, 1∶3, and 0∶1 were tested at 37 ℃and 90 g/(L·d) organic loading rate. The results showed that the cumulative methane yields on volatile solids reached 188.8, 204.0, 213.4, 198.1, 168.5, 169.6 and 124.7 mL/g from ratios of pig manure to rice straw at 1∶0, 3∶1, 2∶1, 1∶1, 1∶2, 1∶3 and 0:1, respectively. The highest accumulative methane yields on volatile solids was from pig manure∶rice straw = 2∶1, which increased by 13.0% compared to that from the sole pig manure as substrate. Synergistic effect analysis showed that synergistic effects existed between pig manure and rice straw at the different mass ratios. The highest synergistic effect was from pig manure∶rice straw = 2∶1, and increase rate was 27.5%. The analysis of archaeal communities showed that the decline of the Shannon index before and after fermentation was consistent with the change of methane production. When the substrate ratio of pig manure and rice straw was 2∶1, the Shannon index decreased the most, reaching 29.1%. Methanosphaerula was the dominant genera in our anaerobic digestion, and the abundance of Methanosphaerula was positively correlated to methane production. For practical project application, the substrate ratio of pig manure to rice straw at 2∶1 is recommended, and the retention time is suggested for 36 d. The results provide theoretical and technical guidance for large scale dry anaerobic co-digestion of pig manure and rice straw biogas project.