Elucidating the drivers of butyric acid degradation and production in anaerobic digestion systems

Butyric acid is a highly valuable chemical due to its wide range of applications. Its production and recovery from organic waste offers greater economic benefits and reduced environmental impacts, as it is currently mainly derived from fossil fuels. Although the accumulation of butyrate is a recurrent phenomenon in anaerobic digestion, there has been limited research on the optimal conditions for butyrate accumulation and the underlying accumulation mechanisms. This thesis aimed to investigate the factors influencing butyric acid degradation and production, with the goal of identifying the optimal conditions for its accumulation in anaerobic digestion systems. The specific objectives of this PhD research were: 1) to elucidate the effects of ammonia (TAN) concentration and pH on butyrate degradation; 2) to investigate the influence of butyrate concentration and temperature on butyrate degradation and its associated metabolic pathway; 3) to identify the best conditions (pH, temperature, and inoculum to substrate ratio) for maximizing butyric acid yields and to explore the underlying mechanisms governing volatile fatty acid (VFA) distribution under varying conditions. The results showed that at pH 7.5, butyrate degradation experienced remarkable inhibition when TAN exceeded 8.0 g N/L, while no discernible impacts were observed at pH 7.0–8.0 and 4.0 g TAN/L. Additionally, the lag phase for butyrate degradation extended with increasing TAN concentration. NH4+ contributed more to inhibition than NH3 at TAN concentrations of 2.0–20.0 g N/L. Notably, the activity of butyrate-degrading bacteria was able to be fully recovered from severe ammonia inhibition (TAN of 20 g N/L or NH3 of 779.2 mg N/L), provided a prolonged adaption time. Complete butyrate degradation occurred in the range of 2.0 to 20.0 g COD/L, regardless of temperature (37 oC or 55 oC); however, degradation kinetics differed in the two temperature conditions. At 37 oC, iso-butyrate production was observed with butyrate concentrations from 2.0 to 20.0 g COD/L, while it only occurred with 20 g COD/L butyrate at 55 oC. The kinetic analysis of butyrate oxidation showed that the production of iso-butyrate was important for butyrate degradation. Metagenomic analysis revealed that the key enzymes such as enoyl-CoA hydratase (EC 4.2.1.17) and 3-hydroxyacyl-CoA dehydrogenase (EC 1.1.1.35) involved in iso-butyrate metabolism correlated positively with efficient butyrate degradation. The optimal condition for efficient butyrate accumulation from glucose was pH 5.5, a temperature of 37°C, and an ISR of 1:3. Different experimental conditions significantly shaped the microbial community dynamics. The genera playing significant roles in butyrate production included Clostridium, Caproicibacter, Caproicibacterium, Sporolactobacillus, and Ethanoligenens. The relative abundance of genes encoding key enzymes involved in reverse β-oxidation significantly increased under optimal conditions, suggesting that the enhancement of butyrate production was driven by carbon chain elongation, using intermediate metabolites like ethanol or lactate as electron donors and acetate as the electron acceptor. These findings demonstrate that chain elongation can be sustained without the need for external electron donors or additional chemicals, effectively promoting both the yield and purity of butyrate. This PhD research provides deeper insights into the biochemical and microbial mechanisms of butyrate accumulation in anaerobic digestion, and provides practical solutions for enhancing butyrate production from wastes, thereby supporting sustainable resource recovery and advancing biorefinery technologies.

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University of Galway

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Attribution-NonCommercial-NoDerivatives 4.0 International

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University of Galway Theses (PhD Theses)