Effect of cycle digestion time and solid-liquid separation on digestate recirculated one-stage dry anaerobic digestion: Use of intact polar lipid analysis for microbes monitoring to enhance process evaluation

One-stage dry anaerobic digestion with solid-liquid-separated digestate recirculation was applied to treat synthetic organic solid waste at Digestion Time (DT) of 15, 25, 35 days for several cycles. The highest methane yield of 0.360 ± 0.045 m3/kg-VSadd with lowest accumulation of hydrolytes was achieved under DT of 15 days. The analysis of intact lipid profiles, including phospholipid fatty acid for bacteria and phospholipid ether lipid for archaea, indicated that the inoculum breakdown occurred, mainly during the start of the process. A significant decline of hydrolytic bacteria was observed during the granular breakdown, which was likely related to the lower methane yield in subsequent cycles. In contrast, the amount of methanogens was still stable even after granular breakdown occurred. The accumulated ammonia in the liquid digestate was partially removed by solid-liquid separation before digestate recirculation, which relieved possible inhibition to some extent with minor microorganism loss. Hence the levels of ammonia, which was highly possible to be the inhibitor causing the decline of methane production, were lower in DT15 than in DT25 and DT35. In this case, it could be implied that the microorganism community reconstruction in DT15 may face less challenges comparing to the other two setups.