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 and 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.

Oxidised starch is currently produced from native starch using sodium hypochlorite as an oxidising agent. The use of hypochlorite has undesired side reactions and produces stoichiometric amounts of waste (salt), thus alternative oxidation methods are desired. In this study, the potential of two catalysed starch oxidation methods to reduce the environmental impact (EI) of oxidised starch production are assessed. We compared the EI of oxidation with molecular oxygen (heterogeneously catalysed) and hydrogen peroxide (homogeneously catalysed) to hypochlorite oxidation through life cycle assessment (LCA). The results confirm that hypochlorite oxidation is the main environmental hotspot in the current process of oxidised starch production, and that both hydroperoxide oxidation and molecular oxygen oxidation can significantly lower the EI of the process. The impact reduction is most significant in the categories of freshwater eutrophication (6 7 %), ozone depletion (~66 %), climate change (35-60 %) and resource use (40 %-78 %) for peroxide and molecular oxygen oxidation, respectively.

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