Lactate-based chain elongation : Poduction and separation of medium-chain carboxylates from complex organic residues

Abstract

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 carboxylates (MCC) 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 carboxylates, mainly n-butyrate and n-caproate. The highest n-caproate carbon selectivities (mmol C•mmol Ccarboxylates -1 ) and production rates were 38% and 4.2 g COD•L -1 •d -1 , respectively. Hydroxide input was reduced over time to a minimum of 0.47 mol OH -•mol MCC -1 or 0.79 mol OH -•kg CODcarboxylate -1 . 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.