Control strategies for ethanol-based chain elongation processes

Abstract

Chain elongation with open cultures is an emerging biotechnological process for the conversion of residual biomass to precursors for fuels and chemicals. Like anaerobic digestion, chain elongation is catalyzed by an anaerobic open‑culture (i.e. reactor microbiome). The open culture chain elongation process upgrades low‑value volatile fatty acids (VFAs from e.g. acidified organic waste) with an electron donor (such as ethanol) into high‑value medium chain fatty acids (MCFAs), such as n‑caproate. Although fermenting with open‑cultures has many advantages, they do typically bring also undesired competing processes which degrade substrates and products. A selective control (i.e. inhibition) of these competing processes will lead to a more effective chain elongation process. The goal of this thesis was to control competing processes in ethanol‑based chain elongation. Special attention was given to control the competing process excessive ethanol oxidation (EEO). EEO degrades ethanol, which is a valuable substrate, but does not contribute directly to chain elongation. In this thesis, it was shown that EEO is dependent on hydrogenotrophic methanogenesis. The overall reaction can be referred to as syntrophic ethanol oxidation. By limiting the CO2 loading rate to a chain elongation process, syntrophic ethanol oxidation was also limited. Next to CO2 loading rate, it was found that a long HRT in a continuous chain elongation process also resulted in a limited rate of syntrophic ethanol oxidation. A major advantage of this strategy over a limited CO2 loading rate is that the n‑caproate concentration can become very high. Later on, with inhibition assays, it was shown that these high n‑caproate concentrations were inhibitory to syntrophic ethanol oxidation. As such, accumulation of n‑caproate in chain elongation bioreactors inhibits syntrophic ethanol oxidation which leads to a more selective and ethanol‑efficient chain elongation process. In this thesis, also the discovery of granular sludge formation in a chain elongation process was presented. The granules did contribute to MCFA production; though the formation of these granules seemed to coincide with high‑rate syntrophic ethanol oxidation. Although chain elongation can effectively produce n‑caproate from organic residues, ethanol and base use can be further reduced to lower operational costs and environmental impact. An outlook is provided, therefore, on how to further minimize ethanol‑use and base‑use to further increase the effectiveness of chain elongation processes.