Background: In a previous study, single-stage processes were compared with two-stage processes, using either food waste alone or mixed with thin stillage as substrate. Overall methane yield increased (by 12%) in two-stage compared with single-stage digestion when using food waste, but decreased when food waste was co-digested with thin stillage (50:50 on VS basis). The obtained difference in methane yield was likely caused by a higher acetate level in the first stage reactor operating with food waste alone (around 20 g/L) compared to the reactor also treating thin stillage (around 8 g /L). The present study sought to shed additional light on possible causes of the large difference in methane yield by scrutinizing the microbial community in the first- and second-stage reactors, using a combined Illumina sequencing and qPCR approach.

Results: In the first-stage process, acid-tolerant Aeriscardovia and Lactobacillus formed a highly efficient consortium. For food waste with high levels of acetate (20 g/L, equal to 0.14 g acetate/g VS) was produced but when thin stillage was added the pH was lower (<4), resulting in lactate production exceeding acetate production. This difference in hydrolysate composition between the reactors resulted in development of slightly different communities in the second-stage, for both hydrolysis, fermentation, and acetogenesis. High acetate concentration appeared to promote proliferation of different syntrophic consortia, such as various syntrophic acetate oxidizers, members of the genus Syntrophomonas and candidate phylum Cloacimonetes, likely explaining the higher methane yields with two-step compared with single-stage digestion of food waste.

Conclusion: Using food waste as sole substrate resulted in enrichment of Lactobacillus and Aeriscardovia and high acetate yields in the first-stage reactor. This was beneficial for biogas yield in two-stage digestion, where efficient acid-degrading syntrophic consortia developed. Addition of thin stillage contributed to low pH and higher lactate production, which resulted in decreased methane yield in the two-stage process compared with using food waste as sole substrate.

背景：在先前的研究中，将单阶段过程与两阶段过程进行了比较，分别使用食物垃圾或与稀釜馏物混合作为底物。与单级消化相比，使用餐厨垃圾时，两阶段的甲烷总产量提高了（12％），但是当餐厨垃圾与稀釜馏物一起消化时（以VS为50:50），则甲烷总产量下降。与仅处理稀釜馏物的反应器（约8 g / L）相比，仅与食物残渣一起运行的第一步反应器中较高的乙酸盐水平可能导致了甲烷收率的差异。本研究试图通过结合Illumina测序和qPCR方法，仔细研究第一阶段和第二阶段反应器中的微生物群落，从而进一步揭示甲烷产量差异巨大的可能原因。

结果： 在第一阶段的过程中，耐酸 Aeriscardovia 和 乳杆菌组成了一个高效的财团。对于产生高乙酸盐含量（20 g / L，等于0.14 g乙酸盐/ g VS）的食物垃圾，但是当添加稀釜馏物时，pH较低（<4），导致乳酸产量超过乙酸盐产量。反应器之间水解产物组成的这种差异导致第二阶段在水解，发酵和产乙酸两方面都产生了稍微不同的群落。高乙酸盐浓度似乎促进了不同的同养菌群的增殖，例如各种同养乙酸菌氧化剂，是该属的成员。食肉单胞菌 和候选门类梭菌，很可能解释了与单阶段消化食物垃圾相比，采用两步法可产生更高的甲烷产量。

结论： 使用食物残渣作为唯一的底物可导致 乳杆菌 和 Aeriscardovia在第一步反应器中乙酸盐产率高。这有利于沼气在两阶段消化中的产生，在此阶段开发出了有效的酸降解合成营养菌群。与使用食物残渣作为唯一底物相比，添加稀釜馏物会降低pH值并提高乳酸产量，从而导致两步工艺的甲烷产量下降。