Chain elongation is an open-culture biotechnological process which converts volatile fatty acids (VFAs) into medium chain fatty acids (MCFAs) using ethanol and other reduced substrates. The objective of this study was to investigate the quantitative effect of CO2 loading rate on ethanol usages in a chain elongation process. We supplied different rates of CO2 to a continuously stirred anaerobic reactor, fed with ethanol and propionate. Ethanol was used to upgrade ethanol itself into caproate or to upgrade the supplied VFA (propionate) into heptanoate. A high CO2 loading rate (2.5 LCO2∙L 1∙d 1) stimulated excessive ethanol oxidation (EEO; up to 29%) which resulted in a high caproate production (10.8 g∙L 1∙d 1). A low CO2 loading rate (0.5 LCO2∙L 1∙d 1) reduced EEO (16%) and caproate production (2.9 g∙L 1∙d 1). Heptanoate production by VFA upgrading remained constant (~1.8 g∙L 1∙d 1) at CO2 loading rates higher than or equal to 1 LCO2∙L 1∙d 1. CO2 was likely essential for growth of chain elongating microorganisms while it also stimulated syntrophic ethanol oxidation. A high CO2 loading rate must be selected to upgrade ethanol (e.g. from lignocellulosic bioethanol) into MCFAs whereas lower CO2 loading rates must be selected to upgrade VFAs (e.g. from acidified organic residues) into MCFAs while minimizing use of costly ethanol.

The production of biochemicals from renewables through biorefinery processes is important to reduce the anthropogenic impact on the environment. Chain elongation processes based on microbiomes have been successfully developed to produce medium-chain fatty acids (MCFA) from organic waste streams. Yet, the sustainability of chain elongation can still be improved by reducing the use of electron donors and additional chemicals. This work aimed to couple lactate production and subsequent chain elongation to decrease chemicals input such as electron donors and hydroxide for pH control in repeated-batch food waste fermentation. Food waste with adjusted pH was used as substrate and fermentation proceeded without pH control. During fermentation, lactate was first formed through the homolactic pathway and then converted to fatty acids (FA), mainly n-butyrate and n-caproate. The highest n-caproate carbon selectivities (mmol C/mmol CFA) and production rates were 38% and 4.2 g COD/L-d, respectively. Hydroxide input was reduced over time to a minimum of 0.47 mol OH-/mol MCFA or 0.79 mol OH-/kg CODFA. Lactate was a key electron donor for chain elongation and its conversion was observed at pH as low as 4.3. The microbiome enriched in this work was dominated by Lactobacillus spp. and Caproiciproducens spp. The high abundance of Caproiciproducens spp. and their co-occurrence with Lactobacillus spp. suggest Caproiciproducens spp. used lactate as electron donor for chain elongation. This work shows the production of n-caproate from food waste with decreased use of hydroxide and no use of exogenous electron donors.

Current challenges for microbial electrosynthesis include the production of higher value chemicals than acetate, at high rates, using cheap electrode materials. We demonstrate here the continuous, biofilm-driven production of acetate (C2), n-butyrate (nC4), and n-caproate (nC6) from sole CO₂ on unmodified carbon felt electrodes. No other organics were detected. This is the first quantified continuous demonstration of n-caproate production from CO₂ using an electrode as sole electron donor. During continuous nutrients supply mode, a thick biofilm was developed covering the whole thickness of the felt (1.2-cm deep), which coincided with high current densities and organics production rates. Current density reached up to −14 kA m⁻³_electrode (−175 A m⁻²). Maximum sustained production rates of 9.8 ± 0.65 g L⁻¹ day⁻¹ C2, 3.2 ± 0.1 g L⁻¹ day⁻¹ nC4, and 0.95 ± 0.05 g L⁻¹ day⁻¹ nC6 were achieved (averaged between duplicates), at electron recoveries of 60–100%. Scanning electron micrographs revealed a morphologically highly diverse biofilm with long filamentous microorganism assemblies (~400 μm). n-Caproate is a valuable chemical for various industrial application, e.g., it can be used as feed additives or serve as precursor for liquid biofuels production.

Introduction: Medium chain fatty acids (MCFAs), such as n-caproate, are potential valuable platform chemicals. MCFAs can be produced from low-grade organic residues by anaerobic reactor microbiomes through two subsequent biological processes: hydrolysis combined with acidogenesis and chain elongation. Continuous chain elongation with organic residues becomes effective when the targeted MCFA(s) are produced at high concentrations and rates, while excessive ethanol oxidation and base consumption are limited. The objective of this study was to develop an effective continuous chain elongation process with hydrolyzed and acidified food waste and additional ethanol. Results: We fed acidified food waste (AFW) and ethanol to an anaerobic reactor while operating the reactor at long (4 d) and at short (1 d) hydraulic retention time (HRT). At long HRT, n-caproate was continuously produced (5.5 g/L/d) at an average concentration of 23.4 g/L. The highest n-caproate concentration was 25.7 g/L which is the highest reported n-caproate concentration in a chain elongation process to date. Compared to short HRT (7.1 g/L n-caproate at 5.6 g/L/d), long HRT resulted in 6.2 times less excessive ethanol oxidation. This led to a two times lower ethanol consumption and a two times lower base consumption per produced MCFA at long HRT compared to short HRT. Conclusions: Chain elongation from AFW and ethanol is more effective at long HRT than at short HRT not only because it results in a higher concentration of MCFAs but also because it leads to a more efficient use of ethanol and base. The HRT did not influence the n-caproate production rate. The obtained n-caproate concentration is more than twice as high as the maximum solubility of n-caproic acid in water which is beneficial for its separation from the fermentation broth. This study does not only set the record on the highest n-caproate concentration observed in a chain elongation process to date, it notably demonstrates that such high concentrations can be obtained from AFW under practical circumstances in a continuous process.