A predictive simulation framework for the design of an integrated reverse osmosis - nanofiltration process to concentrate volatile fatty acids mixtures

Volatile fatty acids (VFAs) from waste-derived fermentation streams are promising precursors for biofuels, bioplastics, and other chemicals. Reverse osmosis (RO) and Nanofiltration (NF) are key technologies for VFA concentration prior to final downstream upgrading. A novel integrated RO–NF process is proposed, where RO ensures near complete VFA rejection, while NF overcomes RO osmotic pressure limitations, maximizing VFA concentration at moderate pressures; recycling NF permeate to RO enhances overall recovery. A multiscale simulation framework was developed for VFAs mixtures, comprising acetic to hexanoic. At its core lies a local membrane transport model with easily obtainable mechanistic parameters, accounting for the effects of membrane type, temperature, pH, VFAs concentration and composition. The framework was validated against batch literature data in terms of flux and permeate concentrations at 30 bar and pH≥ 6 and then applied to predict steady state membrane unit performance across varying operating conditions. As a preliminary design, the RO–NF process was evaluated for a 20 g/L feed targeting 100 g/L VFA concentration, resulting in 98.9% recovery and an energy consumption of 1.7 kWh/m³ of feed at 40 bar, 30 °C, and pH 7. Overall, the framework paves the way for advanced techno-economic process design and optimization.