Methane production performance of semi-continuous anaerobic fermentation with rice straw pretreated by microbial community

Abstract: China is a large agricultural country with the most abundant straw resources in the world, and about 1.04 billion tons of straw is produced annually. Using anaerobic fermentation technology to transform straw into high-yielding calorific value of biogas is one of the effective ways to solve the environmental pollution caused by incineration, and is also an important means to improve the total quantification and resource utilization of agricultural waste. Straw is composed of cellulose, hemicellulose and lignin, which are tight association and intertwined by chemical bonds. This dense structure will prevent straw from contacting with microorganisms. Therefore, pretreatments are necessary to facilitate biogas production by overcoming hydrolysis limitations. In the present study, rice straw were pretreated by microbial community for semi-continuous anaerobic fermentation, after taking out the samples in the fermentation stage of acid production, the degradation of rice straw by microbial community for acid production effect was studied by high-throughput sequencing and metagenomic technology, and the structure and function of microbial community were analyzed, which was then applied to semi-continuous anaerobic fermentation for methane production. The acid production fermentation temperature was (35±1)℃, and semi-continuous anaerobic fermentation was (35±1)℃. The inoculation amount of the microbial community in the fermentation stage of acid production was 3%, after fermentation for 3 days, the volatile fatty acid (VFAs) and alcohol produced by biological pretreatment was centrifuged at 2 000 r/min for 10 min, and then 285.7 mL of which was inoculated into the semi-continuous anaerobic fermentation, the fermentation lasted for 40 days, and the hydraulic retention time (HRT) was 7 days. In the anaerobic fermentation tank, the biogas slurry of pig manure fermentation with the inoculation volume ratio of 20% was used as the inoculation material. The degradation of rice straw by microbial community in the acid production fermentation was found mainly occurred in the first 3 days, and the degradation rate of rice straw reached 64.05%, the degradation rate of rice straw by microbial community was 89.02% after 12 days of pretreatment, and the degradation rates of hemicellulose, cellulose and lignin were 88.73%, 80.51% and 62.68%, respectively. The metabolites of rice straw pretreated by microbial community were mainly acetic acid and butyric acid, which accounted for 54.51% and 29.02% of the total VFAs, respectively. Microbial community is mainly composed of Cellulosilyticum, Prevotella, Pseudomonas, Mobilitalea, Lysinibacillus, Clostridium and Oscillibacter etc, all of them have a relative contribution to carbon metabolism of more than 45%, among them, Prevotella and Cellulosilyticum on metabolism of fructose, mannose and pentose transformation have the largest relative contribution, reaching 20.90% and 11.98%, respectively. Pseudomonas has the largest relative contribution to lignin degradation, reaching 7.5%. Semi-continuous anaerobic fermentation was carried out on rice straw pretreated by microbial community, the daily gas yield, methane content and daily methane yield increased by 11.34%, 25.24% and 34.10% respectively compared with the control, and the daily methane yield was always maintained at about 200 mL/d. We conclude that rice straw pretreatment by microbial community can effectively improve the efficiency of rice straw anaerobic fermentation to produce methane.