Variations of carbon and nitrogen during anaerobic-aerobic fermentation for co-production of biogas and organic fertilizer using animal manure and straw

In the co-production of biogas and organic fertilizer in traditional biogas engineering, biogas production by anaerobic fermentation and organic fertilizer production by aerobic fermentation are normally separated. Over-consumption of nutrients in the fermented residue often occurs during the biogas production stage while higher biogas yield is required, which consequently influences the subsequent aerobic fermentation stage, resulting in long fermentation period and low qualities of the final products in treating the fermented residue. To shorten the production period of biogas and organic fertilizer and improve the efficiency and quality of biogas and organic fertilizer production, a series of dry anaerobic fermentation and aerobic fermentation tests with the mixed materials of pig manure, chicken manure and straw, were carried out in this study. During the anaerobic fermentation stage, tests of 15-day and 30-day fermentation periods were carried out. During the aerobic fermentation stage, tests of 65% and 70% moisture contents of the initial fermented staff, and 15-day fermentation period were set. The results showed that the daily biogas yield increased gradually from the 1st day to 8th day during the anaerobic fermentation stage, then decreased to less than 50% of the peak value on the 15th day. On the 15th day, the cumulative biogas yield reached 71% of 30-day anaerobic fermentation and the average daily volumetric gas yield reached 1.91 m3/(m3∙d), which was 41.5% higher than that of 30-day anaerobic fermentation. During the aerobic fermentation stage, concentrations of total organic carbon (TOC) in each treatment decreased continuously, and concentrations of total nitrogen (TN) decreased firstly and then increased slightly. Based on electrical conductivities, humification degrees and germination indexes in the final fermented products, higher organic fertilizer qualities were gained when using 15-day anaerobic fermented biogas residues. Moreover, concentrations of TOC and TN were also higher in the final fermented products made from 15-day anaerobic fermented biogas residues than those made from 30-day anaerobic fermented biogas residues. Moisture contents of the initial aerobic fermented staff had little influences on the qualities of final fermented products. In conclusion, adjusting the periods of both anaerobic fermentation and aerobic fermentation for 15 days, can shorten the fermentation period, increase the biogas production efficiencies and the concentrations of carbon and nitrogen elements significantly in final fermented products, and improve the operation efficiency and cost-effectiveness of biogas industry. © 2019, Editorial Department of the Transactions of the Chinese Society of Agricultural Engineering. All right reserved.