Circular-Economy-Oriented Medium-Chain Fatty Acid Production from Organic Wastes via Chain Elongation: Artificial Regulation and Resource-Loop Pathways
Medium-chain fatty acids (MCFAs), including caproate and caprylate, are promising biobased products with high energy density, hydrophobicity, and chemical conversion value, serving as key intermediates linking organic waste valorization to circular economy development. Compared to conventional anaerobic digestion for methane, chain elongation (CE) via reverse β-oxidation (RBO) converts short-chain intermediates (e.g., acetate, lactate, ethanol) into higher-value carboxylic acids, offering a novel route for resource recovery from sewage sludge, food waste, agricultural residues, livestock manure, and high-strength organic wastewater. However, natural mixed-culture CE systems face constraints from substrate composition fluctuations, electron donor competition, methanogenic carbon diversion, insufficient product selectivity, product toxicity, and high separation costs, hindering stable, efficient, and targeted MCFA production. This review systematically summarizes the metabolic mechanisms, artificial regulation strategies, and engineering bottlenecks in CE-based MCFA production, emphasizing directed recovery of carbon and electron resources from organic wastes. Future directions include stable continuous-flow operation, product separation and recovery, techno-economic assessment, and life cycle evaluation. The review aims to provide insights for high-value organic waste utilization and synergistic optimization of carbon and energy recovery.