Extraction of volatile fatty acids from wastewater anaerobic digestion using different extractant-diluent mixtures

Biobased volatile fatty acid (VFA) production from anaerobic digestion (AD) using extractive fermentation is a promising approach to control the inhibitory effects of acid accumulation in active digesters while producing a second valuable product stream. Five extractants (tri-n-octylamine, tri-n-butyl phosphate, tri-n-octylphosphine oxide, Aliquat 336 and trihexyl(tetradecyl)phosphonium bis-2,4,4-(trimethylpentyl)phosphinate) in combination with oleyl alcohol, lamp oil or canola oil as diluents were investigated based on (i) extraction capacity, (ii) biocompatibility and (iii) feasibility of VFA back extraction. Liquid-liquid extraction with VFA solutions revealed solvents with the capacity to extract acids within pH ranges suitable for AD. Bench-scale biogas experiments demonstrated that biocompatible extractant-solvent systems allow the co-production of biogas and VFAs while enhancing biogas productivity. Solvents were back-extracted using NaOH(aq) to recover VFAs and regenerate the solvent. The novel methodology implemented here could be considered in the development of pH control strategies for the partial removal of VFAs from AD systems while improving the stability of methane production, promoting resource recovery, integrated waste management, waste reduction and utilisation of renewable energy.