Recovery of volatile fatty acids from water using medium-chain fatty acids and a cosolvent

The use of industrial waste streams as a source of biomass is beneficial, not only to reduce environmental problems, but also to save valuable resources. Waste streams containing volatile fatty acids (VFAs) are one such example. However, separation from these streams is challenging because of the low concentrations of VFAs found in aqueous solutions. In this study, medium-chain fatty acids (MCFAs) diluted in an organic solvent are used as extractants for VFAs from dilute aqueous solutions. MFCAs have the advantage of forming double hydrogen bonds, which makes them very interesting for the recovery of VFAs. Liquid-liquid extractions (LLE) are carried out using hexanoic, octanoic or decanoic acid as the MFCA extracting agents, diluted with either hexane or toluene for economy. The various VFAs extracted consist of either acetic, propionic or butyric acid. In this manner, the effect of parameters such as the alkyl chain length of the VFAs and MCFAs and the nature of diluent have been investigated. In addition, the influence of the concentration of MCFAs in the diluent (n-hexane and toluene) on the extraction efficiency of the various VFAs has been determined. Results suggest that the decrease in extraction efficiency by diluting an MCFA with a conventional solvent is not at all a linear function of volume fraction. Therefore, a diluted MFCA has the potential to be the more economic extractant rather than a pure MCFA, as very little extraction efficiency is sacrificed over relatively large ranges of dilution.