Effect of Acid/Ethanol Ratio on Medium Chain Carboxylate Production with Different VFAs as the Electron Acceptor: Insight into Carbon Balance and Microbial Community

Abstract

Medium chain carboxylates (MCCs) are important precursors for biodiesel production. Using chain elongation to produce MCCs is an emerging bioenergy technology. In this study, batch tests were conducted to investigate fermentative MCC production through chain elongation from acetate, propionate, n-butyrate, and ethanol. The effect of the acid/ethanol ratio on MCC production by mixed culture was investigated. Better MCC production, especially n-caproate production, was achieved at optimal acid/ethanol ratios of 1:4, 1:3, and 1:2 with acetate, propionate, and n-butyrate as the electron acceptor, respectively. The n-caproate concentration was high, up to 41.54 mmol/L, and the highest n-caproate production efficiency was 57.96% with the n-butyrate/ethanol ratio of 1:2. The higher concentration of ethanol might stimulate the growth of chain elongation bacteria to promote chain elongation. The highest MCC production efficiency with different electron acceptors corresponded to less carbon loss and a higher chain elongation degree. In addition, with the optimal acid/ethanol ratio, the substrate was maximally utilized for chain elongation. The microbial community analysis confirmed the carbon balance analysis with the maximum relative abundance of 52.66–60.55% of the n-caproate producer Clostridium_sensu_stricto_12 enriched by the optimal acid/ethanol ratios with different volatile fatty acids (VFAs) as electron acceptors.

Lactate and Ethanol Chain Elongation in the Presence of Lactose: Insight into Product Selectivity and Microbiome Composition

“…Decrease of the pH to around 4–4.5 hindered ethanol chain elongating communities, and the metabolism was directed only toward anaerobic ethanol oxidation to acetate, and homoacetogenesis of produced gases to acetate (Figure ) regardless of the initial lactose concentration. CE from ethanol and acetate was described in several other studies; however, the reaction required a high ethanol/acetate substrate ratio. ,, Recently, Candry et al reported that caproate production from ethanol and acetate is mostly affected by pH with higher concentrations of caproate obtained at pH 7 rather than 5.5 . Thus, in this batch group, the concentration of carboxylates with a longer chain was scarce.…”

“…CE from ethanol and acetate was described in several other studies; however, the reaction required a high ethanol/acetate substrate ratio. 16,37,38 Recently, Candry et al reported that caproate production from ethanol and acetate is mostly affected by pH with higher concentrations of caproate obtained at pH 7 rather than 5.5. 39 Thus, in this batch group, the concentration of carboxylates with a longer chain was scarce.…”

The occurrence and ecology of microbial chain elongation of carboxylates in soils

“…2 , 1 mol ethanol: 1 mol acetate). To that extent, a number of studies showed that higher ethanol:acetate mol ratios favor elongation to hexanoate or longer carboxylates whereas lower ethanol:acetate ratios mainly stall at butyrate [ 54 , 55 ].…”

Different Electron Donors Drive the Variation in the Performance of Medium-Chain Fatty Acid Production from Waste-Activated Sludge

“…Several 150 mL bottles were divided into five groups (Eth, Lac, CoED1, CoED2, and CoED3) with different ED supplies. Previous studies have reported that the ratio of ED and EA greater than 2:1 was more suitable for MCFA production. ,, WAS degradation was generally around 25–30%. , Thus, based on the WAS characteristics (available SCOD as EA) and the optimal range of ED/EA, the amount of ED added was determined. The COD in each group was all controlled to be the same, and the final added amount of ED in each group was 133 mmol/L.…”