Liquid–liquid extraction of volatile fatty acids from anaerobic bioprocesses: Solvent systems, selection framework, and sustainability

The valorisation of organic waste through anaerobic digestion (AD) and arrested methanogenesis (AM) offers a renewable route for producing volatile fatty acids (VFAs), key intermediates for bio-based chemicals. However, VFA accumulation inhibits microbial activity, and their recovery from dilute, complex fermentation broths remains a major economic challenge. Liquid–liquid extraction (LLE) has emerged as a promising recovery strategy due to its ability to selectively transfer VFAs into an organic phase under mild conditions while reducing product inhibition. This review critically evaluates solvent systems for VFA extraction, including conventional extractants (oxygen-bearing solvents, organophosphorus compounds, and reactive amines) and emerging alternatives such as ionic liquids (ILs), deep eutectic solvents (DESs), hydrophobic deep eutectic solvents (HDESs), and bio-based solvents. Reactive amines demonstrate superior performance for dilute short-chain VFAs due to strong acid–base interactions, whereas emerging solvents offer improved tunability and sustainability, but face challenges related to cost and viscosity. The effects of key process parameters, including pH, temperature, and solvent-to-feed (S/F) ratio, are analysed, highlighting their role in controlling extraction efficiency. A decision-making framework is proposed to guide solvent selection based on feed characteristics, acid type, and process constraints. Overall, solvent selection is identified as a multi-objective optimisation problem integrating performance, economics, and sustainability.