Extraction of volatile fatty acids from fermented wastewater

• A detailed study on extraction of volatile fatty acids (VFAs) from fermented wastewater.
• Salts negatively influenced VFA distributions, both for traditional solvents and ILs.
• Mechanisms of extraction include both ion exchange and extraction by hydrogen bonding.
• Cross-current extractions concentrated 0.25 wt% aqueous butyric acid to 7 wt% in the solvent.

Valorization of wastewater streams can be done by fermentation to produce volatile fatty acids (VFAs) which are applied as platform chemicals for synthesis of value-added chemicals. Since VFA concentration in fermented wastewater is very low (∼1 wt%) and fermented wastewater contains considerable amounts of dissolved salts, recovery of VFAs from fermented wastewater is challenging. To study the potential of some ionic liquids compared to traditional solvents for extraction of VFAs from fermented wastewater, a detailed study on the effects of various salt-originating ions on VFA extraction was performed. Ion exchange and intermolecular interactions (e.g. hydrogen bonding) were found to be responsible for extraction of VFAs. The presence of salts (e.g. KCl) resulted in extraction of acidic forms of salt-originating anions (e.g. H+ + Cl−). 20 wt% trioctylamine (TOA) in n-octanol (representing conventional solvents) and [P666,14][Phos] (an ionic liquid) were found to be the most promising VFA-extracting solvents. Their maximum VFA loadings while being in equilibrium with an artificial fermented wastewater were determined by performing cross-current extractions. [P666,14][Phos] achieved a higher maximum VFA loading which enables it to deliver a much more concentrated VFA stream at a lower solvent to feed ratio (S/F).