Effects of liquid fraction of digestate recirculation on system performance and microbial community structure during serial anaerobic digestion of completely stirred tank reactors for corn stover

Several completely stirred tank reactors (CSTRs) connected in series for corn stover anaerobic digestion was devised to obtain more methane yield and increase conversion rates. Liquid fraction of the digestate (LFD) was recirculated from the second-stage reactor to first-stage reactors to reuse LFD and improve system performance. LFD recirculation didn't inhibit methane production of serial digestion, and methane and biogas production were increased by 2.3% and 10.8%, respectively. Moreover, LFD recirculation increased pH and alkalinity concentration (AC) and decreased volatile fatty acids (VFAs) concentrations and the ratio of VFAs to AC, which means a significant increase in system stability of anaerobic digestion (AD). Ammonia concentrations gradually increased with LFD recirculation, but was far lower than the inhibition concentration. Microbial analysis indicated that the recirculation increased the richness, but decreased the diversity of both bacterial and archaeal community in the first and second stage of serial system. More specifically, LFD recirculation enriched the dominant bacterial phyla (Bacteroidetes and Firmicutes), but had little influence on the dominated archaeal genus (Methanobacterium). The results showed that the recirculation of LFD during serial AD was technically suitable by minimizing both discharge of LFD and possible pollution related with discharging LFD.