Study on optimal technology of methane production by sequencing batch dry anaerobic digestion with corn straw and cattle manure

Abstract: The resources of agricultural waste are huge in China. With the improvement of agricultural intensive level, the collection resources of straw, livestock manure are relatively concentrated, which creates appropriate conditions for the application of sequencing batch dry anaerobic digestion (SBD-AD) technology to produce methane. However, the development of the SBD-AD full scale biogas plant is slow and rare in China, and the technology is still immature, especially the influence of key factors on the fermentation characteristics and their interaction relationship are still unexplored, which limits the effect of process optimization. On the basis of previous studies, the key factors affecting methane production in the SBD-AD, such as spray frequency, inoculum concentration and feedstock ratio, were determined. In this paper, the characteristics of methane production and the formation of intermediate products by the SBD-AD using corn stalk (CS)-cow dung (CD) mixture under the three key factors were studied. The result showed that the effect of spray frequency and inoculum concentration on methane production was different under different feedstock ratio. When the total solid content (TS) ratio of CS to CD was 3:7, there was a significant negative correlation between spray frequency and methane yield (P<0.05). Increasing spray frequency could increase significantly methane production, which occurred in the 5:5 of TS ratio of CS to CD. When the TS ratio of CS was further increased (CS:CD of 7:3), methane production could be significantly promoted by increasing inoculum concentration and decreasing spraying frequency. The contribution rate of feedstock ratio in the SBD-AD process reached 61%, and the TS ratio of cow dung was increased (CS:CD of 3:7). The spraying frequency was controlled at 6 h interval, the inoculum concentration was 30%, and the methane yield was 135.7 L/kg. The total content and components of intermediate products were significantly affected by the changes of key factors (P<0.05) during the period of rapid increase of methane production (the 7th-20th days). The accumulation of propionic acid was above 350 mg/L resulted in the decrease of methane production in the SBD-AD system, while the increase of ammonia nitrogen (NH4+-N) concentration was beneficial to the increase of methane production. The highest concentration of NH4+-N reached 854.37 mg/L, which did not cause ammonia inhibition. It is necessary to optimize organic acid components, accelerate its transformation, and improve the buffer capacity and the number of microorganisms to increase methane production in the SBD-AD system. This study provides theoretical guidance for large-scale farms to treat agricultural wastes via the batch dry anaerobic digestion technology.