Simultaneous high volatile fatty acids concentration and ethanol extraction using nanofiltration, reverse osmosis and forward osmosis

Mixed culture fermentation is a promising approach for producing volatile fatty acids (VFAs), which can replace petrochemical sources for plastics, fuels, and organic acids generation. However, biological VFAs production generates by-products like ethanol and results in dilution within the culture medium, posing challenges for downstream recovery. To address this, 2 nanofiltration (NF), 2 reverse osmosis (RO) and 2 forward osmosis (FO) commercial membranes were tested for VFA concentration and ethanol separation. All tested membranes successfully concentrated VFAs up to 50 g·L⁻¹ under energy-efficient conditions compared to previous works. pH played a significant role in the process. At low pH (4), membranes exhibited lower VFA rejection but extracted more water due to a reduced solution osmotic pressure. In contrast, high pH (close to 7.0) significantly increased VFA rejection thanks to electrostatic repulsion. However, NaOH used for neutralisation and dissociations of VFAs at pH led to a severe increase of conductivity (related to osmotic pressure), limiting water extraction and, consequently, VFA concentration. Regardless of pH, approximately 70 % of ethanol was removed, with considerably better ethanol/VFA separation at higher pH levels. The Aquaporin FO membrane outperformed others, recovering over 95 % of VFAs while removing 80 % of ethanol. These findings highlight the potential of RO and FO membrane technologies as effective solutions for simultaneously concentrating VFAs and separating ethanol, offering an efficient solution for processing biologically produced VFAs.