Beyond Anaerobic Digestion: New Perspectives for the Development of a Biorefinery Platform for the Simultaneous Production of Medium-Chain Fatty Acids by Chain Elongation and Biogas from Food Wastes

The medium chain fatty acids (MCFAs) obtained via chain elongation of short chain fatty acids are becoming a trend topic in the field of biorefinery research. Their synthesis can be implemented in the over 150 000 full-scale anaerobic digestion plants around the world, increasing the economic revenues derived from the mere biogas production. This paper offers a critical insight into the needed actions for the creation of biorefinery model for the simultaneous production of the MCFAs and biogas from the organic fraction of municipal solids wastes and the most abundant food wastes, such as brewery wastes, grape pomace, and cheese whey. A three-stage chain elongation process was suggested with the aim to optimize the operational parameters and the pretreatments for the production and purification of all the compounds involved in the process: (i) the electron donor, (ii) the electron acceptor compounds, and (iii) their reaction into a separate reactor for the synthesis of MCFAs. The remaining organic matter from the fermentation steps for the electron donor and the electron acceptor molecules can be sent to the already-existing AD plant for biogas production. Specifically, it was emphasized that the molar ratio between the electron donor (ethanol or lactate) and electron acceptor compounds (volatile fatty acids, VFAs) in the ranges of 3:1 and 6:1 can optimize the MCFAs yield. Regarding the reactor configuration, batch mode was not able to ensure high yield, while the continuous mode operating at hydraulic retention time (HRT) of 5-10 days and at neutral or slightly acidic pH led to high conversion of food wastes in MCFAs (25-50% (w/w)). Nanofiltration and pertraction were the best techniques to recover the MCFAs from the other organic compounds, with very high yields in the range of 75%-95%. The applications for MCFAs involved different fields: from the production of jet and diesel fuels to polyhydroxyoctanoate and polyhydroxydecanoate biosynthesis. Finally, the main challenges for a full-scale implementation of the chain elongation process were discussed and emphasized: among them, the low concentration of the MCFAs in the reaction medium and the need to increase the technology readiness level of the recovery and purification techniques were considered to be priorities.