Valorization Strategies for Liquid Fractions of Anaerobic Digestates: A Review on Volatile Fatty Acids Recovery and Circular Economy Integration

The valorization of liquid fractions from anaerobic digestates (LFDs) represents a promising avenue for enhancing the sustainability and resource efficiency of anaerobic digestion (AD) beyond biogas production. This review examines current advances in the extraction and recovery of volatile fatty acids (VFAs), with a particular focus on electrodialysis (ED) and membrane-assisted techniques, including in situ recovery using polytetrafluoroethylene (PTFE) membranes coupled with ED. A comparative analysis of these technologies is provided regarding recovery efficiency, energy consumption, and scalability. Additionally, the potential for nutrient recovery, especially ammonium via struvite precipitation, is discussed in the context of circular economy principles. To illustrate practical implications, a case study is presented where methanogenesis inhibition is employed to maximize VFA accumulation, followed by integrated PTFE+ED recovery. The review highlights current challenges, including membrane fouling, cost-effectiveness, and process integration, and emphasizes the need for systematic techno-economic and environmental assessments to enable large-scale implementation.