Influences of different technological strategies on performance of anaerobic co-digestion of pig manure with straw in solid-state

Abstract: Solid-state anaerobic digestion has advantages of water conversation, convenient management and low energy consumption in winter. After the digestion, substrate contains low water and can be easily treated, thereby achieving zero emission. Under the condition of increasingly scarce water resource in the world, this technology conforms to the demand of resource utilization and water conversation. However, the process of solid-state anaerobic digestion was very complex and the solid-state anaerobic digestion of pig manure could easily lead to serious accumulation of volatile fatty acids (VFAs), which was the main inhibiting factor in solid-state anaerobic digestion of pig manure. Due to the low water content, the reaction of solid-state anaerobic digestion could not proceed normally, or even be ceased. Several measures could be used to minimize the accumulation of VFAs. Leachate recycling was one way to accelerate mass transfer rate, and proper recirculation rate could optimize the digestion process and improve the quantity and quality of biogas. Stratified inoculation was an alternative method for solid-state anaerobic digestion, in which inoculum was separated with substrate layer by layer. The reaction hypothesis suggests that stratified inoculation could be used to decrease accumulation of VFAs, however, only a few studies had investigated its effect to decrease VFAs accumulation. This study was mesophilic batch fermentation under the same temperature (37℃) and water content (80%) condition. To investigate the most efficient treatment of starting the fermentation and the variations of methane production and VFAs, co-digestions of pig manure with maize straw, leachate recycling and stratified inoculation were carried out with the solid-state anaerobic digestion of pig manure as control. Ammonia nitrogen content and soluble chemistry oxygen demand were analyzed. The performances of VFAs production and methane production in the treatments of co-digestion with dried maize straw, leachate recycling, and stratified inoculation in solid-state anaerobic digestion of pig manure were compared. The results were as follows: The performances in stratified inoculation reactors were the best, characterized by no lag time, the highest methane yield of 9.4 mL/(g·d) and the highest cumulative methane yield of 139.2 mL/g; leachate recycling could retain TVFAs at a low level of around 0.66 mg/g, and its cumulative methane yield was 16.7% lower than the stratified inoculation treatment; the concentration of TVFAs in the co-digestion and mono pig manure digestion reactors reached 19.08 and 19.83 mg/g, respectively, and methane yield was less than 0.1 mL/(g·d) in the first 15 days in both reactors. The most efficient treatment with the highest methane production and the quickest initiating can be obtained by contrasting different treatments, which thereby provides the reference for environmental disposal and utilization of the wastes with high solid and high organic matter content.