Medium-Chain Carboxylates Production from Co-Fermentation of Swine Manure and Maize Silage Via Lactic Acid : Without External Electron Donors

Environmental issues caused by untreated animal manure and crop straws require the development of resource recovery from waste through a circular economy approach. Swine manure and maize straws as the main agricultural biomass can be a renewable resource to produce biofuels and chemicals, and they have been successfully converted into methane via anaerobic digestion. However, the revenue from methane has been relatively low. This study aimed to convert swine manure and maize silage into a high-value medium-chain carboxylic acid (MCCA) through a temperature-phased two-stage bioprocess. The carbohydrates in these streams, which were first converted to lactic acid (LA) at thermophilic condition, were then fermented to MCCA by mesophilic chain elongation without the addition of external e-donors. In this study, thermophilic condition enhanced the LA fermentation and shifted the dominant product spectrum from volatile fatty acid (VFA) to LA. The average LA concentration was 8.4 gCOD/L at thermophilic, which was improved by 3.4 times compared with that achieved at mesophilic condition. For the chain-elongating bioreactor, the LA-rich broth was successfully converted into MCCA without any external e-donors. Moreover, the maximum productivities of MCCA and n -caproate were 1890 and 1290 mg COD/L/d, respectively, at pH 6.0 with an LA loading rate of 2.1 g COD/L/d. Maintaining an appropriate LA/VFA ratio and mild pH was necessary to overcome the acrylate pathway and drive LA conversion toward n -caproate instead of propionate. In addition, Pseudoramibacter and Caproiciproducens became the dominant community, with a relative abundance of 45.6% and 9.7%, respectively