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