Arrested anaerobic digestion for sustainable chemical production

Abstract

Arrested anaerobic digestion (AAD) shows promising potential for producing fatty acids (FAs) from organic residues. However, the optimal operating parameters for achieving desired FAs yields remain unclear. These uncertainties might stem from unidentified interactions among operating conditions affecting FA production and microbial communities. This work was aimed to advancing foundational understanding of microbial processes and interactions between operating conditions in AAD. The approach involved investigating how fixed and interactions effects between operating parameters (e.g., pH, feedstock, temperature, inoculum source, inoculum-to-substrate ratio (ISR), and methanogenesis inhibition) shape microbiomes and production of FAs. First, full factorial experiments were executed to address fixed and interaction effects of pH (5.0, 7.0, and 9.0), feedstock (food waste and manure), temperature (35°C and 45°C), and microbial inoculum source (anaerobic sludge and bison rumen) on FAs and microbiomes. The results revealed significant interactions among the four operating parameters influencing FA concentrations and microbiome structure. Feedstock and pH were the most critical factors affecting FA concentrations. Inoculum source significantly affected medium-chain-FA (MCFA) production: anaerobic sludge maximized pentanoic acid, whereas bison rumen enhanced hexanoic and heptanoic acids at 45°C and pH 5.0. Functional predicted pathway analysis linked MCFA production to sugar degradation and lactic acid-driven chain elongation pathways. Key microbial genera such as Megasphaera, Prevotella, and Lactobacillus were associated with MCFA production in food waste. Next, the microbial ecology of feedstocks and inoculum was investigated by evaluating the effects of the ISR (0–3) on FAs and microbiomes. Results indicate that extreme ISRs (0 and 3) exhibited distinct microbial structures; however, both showed increased caproic acid concentrations, driven by distinct mechanisms involving Lactobacillus (ISR of 0) and Clostridium sensu stricto 1 (ISR of 3), respectively. Notably, higher ISRs (up to 3) significantly enhanced total FA and odd-chain FA (propionic and valeric acid) production, with taxa such as Prevotella, DMER64, and Cloacimonadales identified as potential contributors to odd-chain FA production. Conversely, lower ISRs (0.1–0.5) resulted in microbial communities specialized in butyric acid production, influenced by Clostridium sensu stricto 1. A deeper mechanistic understanding of FA-producing metabolism was performed by investigating the effects of iodoform (a methanogenesis inhibitor), on microbiome structure and function. Methane was produced in controls but not in bioreactors amended with iodoform. The control bioreactors produced higher concentrations of the shorter-chain FAs (acetic and propionic acid), while concentrations of longer-chain FAs (butyric, pentanoic, hexanoic, and heptanoic acids) were higher in iodoform-treated bioreactors. Amplicon and metagenomic sequencing analysis revealed that iodoform initiated substantial shifts in microbiome composition and genomic potential. Hydrogen- and butyric acid-producing bacteria such as Prevotella, Peptostreptococcaceae, and Clostridium sensu stricto 1 dominated iodoform-treated bioreactors, while methanogenic archaea and bacteria including Paraclostridium and Romboustia were prevalent in controls. The presence of iodoform led to a reduction in oxidative phosphorylation genes and genes encoding vitamin B12-dependent enzymes in methanogenesis and odd-chain FA pathways. In contrast, iodoform led to increased abundances of genes for substrate-level phosphorylation, sulfur metabolism, reverse β-oxidation, and fatty acid biosynthesis pathways. Finally, a life cycle assessment of producing butyric acid via AAD was conducted, addressing the effects of organic loading under different waste collection distances and electricity source scenarios. The results showed that increasing total solids (%TS) reduces the global warming potential (GWP) at short waste collection distances (10 miles). Conversely, at medium to long distances (≥50 miles), increasing %TS does not reduce GWP. Renewable electricity scenarios (hydro and solar) at high %TS (≥12%) and shorter distances (10 miles) resulted in lower GWP (negative) compared to petrochemical production routes, meeting or surpassing benchmarks for alternative biowaste-derived processes. AAD plants should be located near waste sources (≤10 miles), with renewable-powered and high-solids (>12%) operations prioritized to minimize climate change impacts.