In this study, a reduced total volatile fatty acid (VFA) yield (VFA/tCOD, P = .003) and accumulated fractions of butyric acid (BA) (BA/total VFA, P < 0.001) was observed after feeding the arrested anaerobic digestor with thermal hydrolysis pretreated food waste. Further analysis through meta-omics revealed a reduction in the diversity of acidification communities (Shannon index, P = .03), indicating a sterilization effect of THP on the indigenous microbes in the feedstock. Additionally, although both the community-level acidification genetic potential and transcriptional activities were reduced (DNA and mRNA RPKM log2 fold change = 1.77 and 0.11, respectively) after the inclusion of THP, the remaining microbial communities exhibited an elevated mRNA/DNA RPKM ratio in acidification-related genes, especially within the AA and BA metabolic networks. This suggested that the elevated BA fraction might stem from improved chain-elongation activities using AA as a precursor. Furthermore, the inclusion of THP in the feedstock changed the main functional microbial groups of BA production (via the gene crt-buk2) from Selenomonas and Bacteroides to Prevotella and Megasphaera. Overall, these results reveal a complex shift in the acidification community and provide new insights to evaluate THP technologies behind the formed black box of VFA production performance.

In this study, two arrested anaerobic digestion bioreactors fed with food waste operated under different hydraulic retention times (HRTs) exhibited long-term stable volatile fatty acid (VFA) production performance including similar total yields (p = .085) with propionic acid (PA) being the most abundant VFA. Meta-omics analysis revealed distinct microbial structures (p = .02) at the two HRTs while there were no differences in potential and extant functionality as indicated by the whole-genome and whole-transcriptome sequencing, respectively. The highest potential (relative abundance of DNA sequence reads) and extant (relative abundance of mRNA sequence reads) functionality corresponded with PA production compared to other acids. The most abundant genus Prevotella produced PA mainly through the acryloyl-CoA pathway. Based on our results, the mechanistic basis for the similar VFA production performance observed under the HRTs tested lies in the community-level redundancy in convergent acidification functions and pathways, rather than trends in community structure.